Hello
I am using 1250kv motors (only two) in my 40" PT 109 with 37 mm 3 blade props.
Running on 2S 4000 mah Lipo batteries with 80 Amp 2-4S car ESCs.
No water cooling used.
The boat is a bit over scale speed.
Canabus

Hi Chris, I'm starting with this last hull; if necessary, I'll make another one with a different shape and proportions.

I want to start studying the positions for the engine and the propeller shaft, as well as its inclination (for now, we're using a single-propeller solution).

I was thinking about installing a three-bladed propeller with a diameter of 42 or 40 mm.
Be careful, it's clear that if I set up the propeller shaft for a 40 mm propeller, I can experiment with smaller but certainly not larger diameter propellers. So for now, I'm considering the larger diameter. I wouldn't go beyond 42 mm. What do you think?

Returning to the motor, in a previous message, you recommended an Overlander Tornado Thumper V3 3542/05 1250 KV.

This is very important to me because I already have a starting point.

Keep in mind that the advice I'm asking you and other ship modelers for right now isn't aimed at immediately purchasing an electric motor, but at studying its size and weight to accommodate it in the project.

Ideally, I'd like to find a position for the propeller shaft and the hull mounting arrangements that allows me to easily try out different types of motors and experiment with them.


BRUSHED MOTORS (should we just eliminate them?)
I know you're not hesitant about choosing brushless motors over brushed ones, but I'm asking you to make an effort to answer this question.
Given that when it comes to fast boats, brushless motors are almost a must for many reasons (high power and RPM that brushed motors can't reach, and low weight despite high power and RPM), I'd like to know if it's still possible to use a brushed motor for a ship model with a planing hull with these characteristics.
The reason is essentially economic, as brushless motors and their ESCs are much more expensive.
I wanted to draw your attention to two types of motors in particular.
How would you evaluate the choice of a 775 class brushed motor at 12V-16.8V powered by 4S LiPo or 12V NiMH, combined with a 42-45mm two-blade plastic or brass propeller (fully submerged)?
To save money, instead of the Graupner, I would choose a lower-cost equivalent. Or I could opt for an even more powerful brushed motor such as an 800 (or 850) class motor, a Graupner Speed ​​800 12V, or a high-performance industrial equivalent, powered above 12 volts.

BRUSHLESS MOTORS
Speaking of brushless motors, I think the choice should fall on outrunners.
In short, if you agree, I would discard any research into inrunner motors.
The high RPM of these motors, I believe (but correct me if I'm wrong) is incompatible with the choice of a submerged propeller, as in my case. I would have to install a surface propeller, which isn't a solution to my liking and isn't suitable (at least I don't think so) for the hull I've designed.

Well, staying on the subject of outrunner motors, I wanted to ask you: besides the Overlander Tornado Thumper V3 3542/05 1250 KV motor, do you have any other options?
Specifically, how do you rate these outrunner motors?
Brushless Outrunner class 3542 or 4240 (approximately 1000 - 1200 KV)
On Aliexpress, I'm leaning toward a 1200KV 4240 outrunner motor (Surpass Hobby or GARTT).
In this case, is it better to power it with a 3S LiPo (11.1V) to achieve approximately 11,000 - 13,000 rpm?

WATER COOLING JACKET AND OTHER COOLING SYSTEMS
Having always worked on displacement boats (with low speeds), I've never needed any cooling system, also because I always used oversized motors that weren't at their maximum power.
In the case of the motor you recommended (Overlander Tornado Thumper V3 3542/05 1250 KV) and the motors I mentioned, do you think a cooling system is mandatory?
If so, which one? Should I use it, since we're talking about outrunner motors, not inrunners?
I don't think a cooling jacket can be used for an outrunner motor. What method do you use?


Sorry for the many questions, but I invite you to answer me slowly and calmly.
If you'd like, please send me the commercial links for your options directly.

Looking good Alessandro. I think the revised bow is more suitable for a naval craft. The bow will cut through waves at lower speeds until it gets on the plane and will look better with a naval superstructure.

The immersed volume is a good result.

Chris

Hi All
I found the best hull design for straight line and turning is the Spearfish hull.
Built in chine rails and two spray rails each side which finish short of the stern.
Also a curve up in the stern keel which I think helps with the turning.
We clocked my boat at 37KPH !!!
Harry

You may ask how you press the wrong button, easy if you have fat fingers on a smart phone, deadly combination

Good evening to all naval modelers, I've been able to draw very little since last time.

This is the new hull.
I'm a little more convinced than the others, but I don't know if it will be the final one to work on.

It has the following characteristics:
- The ratio between width and length is 1:3.48 (so we're back to the value of 3.5)
- The slope of the bow has been accentuated.
- The transom has a reverse slope (this means the total length is greater than the length at the waterline)
- The shape of the hull (at the waterline it has a wedge shape). That is, it widens towards the stern while the lines on the deck become narrower.

The keel is parallel to the waterline, while the gunwale is sloped and rises slightly towards the bow.

I temporarily imagined a scale of 1:32 for this model.
So the actual hull would have been about 24 meters long.

The proportions are as follows:

Height 120 mm
Length 756 mm
Width 217 mm
Maximum draft 50 mm
Minimum draft 40 mm

The deadrise angle is 21 degrees.

The maximum immersed volume (with 50 mm draft)
is just over 3 dm3.
This is a satisfactory value, considering Chris's valuable information.

The red hull represents the immersed volume.

Here are the first lines of the other prototype.

Hi Alessandro

It is difficult to see if the chine does rise upwards at the bow but I think I can make it out in Picture 4, so yes it is quite subtle. Are the chines rounded or as Bob said a soft chine?

The chines don't rise much on my Fairey Swordsman I suppose - see photo.

For interest also see the photos showing the comparison between my Swordsman 33 and Huntsman 31 hulls. The Swordsman is only 50mm longer but has a wider beam and almost parallel sides. The model is much bigger and heavier which is evident when just moving them around whilst I'm painting.

Chris

@ChrisF

Hi Chris

you wrote:
"""""""""""""Evening Alessandro - the drawing was primarily for interest and to show how twin prop shafts are often positioned rather than suggesting it was the best hull design for your needs.""""""""
Perfect, that's what I meant and thank you very much, you understood exactly what I needed.


you wrote:
""""""""""Whilst I have provided detailed information on some aspects, for example the vee hull and narrowing on plan as you move towards the stern I've been conscious of not trying to impose my own design preferences on you - not that, that would happen! Like me you ask for and research information and then make your own decisions based on what you want. Quite right!""""""""""""""
We're on the same page.



you wrote:
"""""""""""You don't need to worry about it planing. Anything from a flat bottom to a deep vee will do that, just that the deep vee will do it at a lower point below the water surface than a flatter hull.
I like this about your advice. The very direct approach based on experience. Clear and concise answers. Thank you so much. They are infinitely helpful. I'm taking advantage of your experience (I'm new to planing hulls). You can't understand how useful this simple statement of yours is for me to continue.

You wrote:
""""""""""I know that the drawing is to show the submerged volume, but don't forget that the chine and chine rail will curve upwards at the bow.""""""""""""
So I ask you: why does the drawing seem to you like there's no upward curvature? Is it too subtle?

Evening Alessandro - the drawing was primarily for interest and to show how twin prop shafts are often positioned rather than suggesting it was the best hull design for your needs.

Whilst I have provided detailed information on some aspects, for example the vee hull and narrowing on plan as you move towards the stern I've been conscious of not trying to impose my own design preferences on you - not that, that would happen! Like me you ask for and research information and then make your own decisions based on what you want. Quite right!

You don't need to worry about it planing. Anything from a flat bottom to a deep vee will do that, just that the deep vee will do it at a lower point below the water surface than a flatter hull.

I know that the drawing is to show the submerged volume but don't forget that the chine and chine rail will curve upwards at the bow.

Chris

Hello all the ship modelers, hello Chris and Bob.

Chris, I drew another hull as soon as I had time.
Of course, I'm not referring to the drawing you sent me, because I could faithfully trace it if I wanted, but I prefer to find a different shape. Your drawing was very helpful, however.

You can see the images of the new shape and the comparison with the other one (in blue).
Here's a summary of the changes:
I further reduced the length-to-width ratio, now the ratio is 3.2.
I redid the keel, which is now parallel to the waterline.
At the same time, the deck line rises toward the bow with a slight slope (less than one degree).
The bow has the same slope, but the waterline (the profile at the waterline) is less sharp and therefore has a larger surface area, resulting in more underwater volume.
I raised the freeboard.

The resulting hull is more likely than the other to plane, even at moderate speeds (at least I think so). It has a submerged volume exceeding 2 dm3 (2 kg of weight), but its shape is not very slender, a bit squat. I'm still not convinced by the result I have in mind.
I'll have to find a compromise between the hull with the aesthetic lines I like (very slender with an aggressive bow and a sharp profile, which, however, could be difficult to control when turning, could have difficulty planing immediately, or could lose control and capsize) and a hull with proportions more suited to motorized navigation at high scale speeds.
It's clear that the chosen prototype will then have to pass practical tests and still prove a failure.

I'll design other prototypes until I find the one I'm most satisfied with (and which will in any case be put to the test).
My next hull will have a steeper bow, a reverse-rake stern, and perhaps I'll return to the 3.5 length/beam ratio. To compensate for the inevitable reduction in submerged volume caused by these changes, I might increase the draft.
In the meantime, any comments or suggestions on the images I've attached are welcome.

I said previously that the weight of my Huntress 28 is 2.3kg. I've just calculated what the weight of my 1:12 model "should be" based on the weight of the full-sized boat and it works out to 2.15kg. So I'm pretty pleased with that especially as the weight of the full-sized one would increase once passengers, provisions and crates of champagne are taken into consideration.

Chris

Bob - it was the drawing but as we've said there shouldn't be much difference between builds from the drawing or kit. You know what, looking at the hull whilst painting it I wouldn't be surprised if it did need some ballast! Though it it only sits a bit high I won't bother. As we know for it's length of 33" (838mm) it has a wide beam of 11.4" (290mm) and a vee angle of 24 degrees - it gets away with this because it is so wide! This angle was often used in race boats. Will take me sometime to finish painting the hull as it's two-tone but I'm aiming to get it on the water soon - about 3 weeks I hope.

Alessandro - this actually is an interesting hull for you as it it very similar (though bigger) than your original design in that the beam doesn't narrow much as it approaches the stern and why it has a high submerged volume helped by the very deep vee hull. It has an S shaped bow as there is some flare which extends a small distance along the hull. I didn't really mention this one as I haven't even tried it in the bath yet so don't know how it sits.

Chris

@zooma
Thanks, Bob, but calculations are a big word, just a few measurements here and there.
I'm still deciding on the hull shape.

Hi Chris.

Don't worry about the provenance too much, I am sure that this advice (given at the time) would apply equally to either of the 33" versions of the Swordsman.

I think we need to get our own 33" replica's into the water this year and find out for ourselves..............

Bob.

Hi Bob - It's from the magazine article but can't remember if it related to the kit or drawing, I'll check tomorrow. But obviously the volume of each would be the same and the weight around the same even with the different constructions. I'm still amazed at the quoted ballast, it's not like it's a tug or something and you certainly don't want to be lugging that around!

It's inspiring me to get mine on the water to see how it sits and performs.

@ChrisF Thats an interesting artefact Chris.

Did that little gem come with the original Aerokits Swordsman kit (I don't remember it) or did it come in the Model Boats review of their same size plans that were printed in the 1960's ?

It does seem like a lot of dead weight to "heft" around, but we forget just how light our Classic Model Power Boats were with just a (typical) 10cc glow engine and a small fuel tank of fuel to carry in the hull.

Our current brushless motors and LiPo battery packs with suitable ESC probably weight quite a lot more and should make quite a dent in the suggested additional ballast weight suggested?

Bob

I am looking forward to seeing all of your calculations transformed into a working power boat hull that planes beautifully Alessandro.

Bob.

Just checked and my length/beam ratio, 654/217mm at the waterline, is actually just over 3.0 which does surprise me as the H28 doesn't look particularly short and fat! Out of interest I will have to look at the Huntress which is basically the same hull which was extended by 5 feet for the H28. I'll look at the Swordsman 33 as well as that does have a wide beam. All interesting stuff!

I know you like doing your calculations and nothing wrong with that but don't get hung up on them. Whatever you design, being a planing hull, can't differ that much from all the designs and kits that have already been designed over the years which don't have a problem with weight. It tends to be small displacement models like destroyers that you have to be careful with isn't it? As I've said from my own experience, planing hulls are very buoyant and I don't think that even doubling up on components will make much difference. In fact I read the story that came with the release of the 1:12 Swordsman 33 model drawings years ago and it states that 7.5 pounds (3.4kg) of ballast is needed to get it down to the waterline!!! See text. I find that very hard to believe and look forward to trying mine in the bath! I have a feeling it will be fine without any ballast!

@ChrisF
Sorry, you're right, I didn't post the photos and I wrote the numbers wrong.
The rush.
30 mm and 70 mm, not 300 and 700, of course.

Evening Alessandro - No photos! But 15 degrees makes sense, as I said you could increase to 20 degrees if you want to increase the submerged volume.

Details given of only one motor, but that is much too small - I'm using a bigger motor in my little 420mm long Faun! You need to be looking at something like a 3536/06 1300kV on 3S. Or similar with a minimum of 1100kV to give the required rpm.

https://overlander.co.uk/product/3536-06-1300kv-tornado-thumper-v3-brushless-outrunner-rc-motor/

"Here, with 300 mm of immersion (the 700 mm you mentioned seemed excessive), I would also like an immersed volume of at least 2 kg. That seems like a good compromise with these dimensions." I'm confused by what you mean with those figures?

Chris

@ChrisF
Hi Chris, this photo shows the angle measurements. Since I'm not sure which one you were referring to, I included both.
One is 15°, the other 150° (obviously, since the plane angle is 180°).
Anyway, I think you wanted to know the 15° one.

Here are photos of the only two motors I currently own. I think they're a bit too small for this boat. I didn't buy them for this purpose, and I don't intend to burn them needlessly. Please let me know what you think.
Instructions at the end.

Thanks for sending me the photos of your model. Here, with 300 mm of immersion (the 700 mm you mentioned seemed excessive), I would also like an immersed volume of at least 2 kg. That seems like a good compromise with these dimensions.

Electric motor characteristics
A2212: Internal stator dimensions. Indicates a diameter of 22 mm and a height of 12 mm.
13T: Number of turns. This means the copper wire makes 13 turns around each pole.
1000KV: Speed ​​constant (RPM/V). The motor theoretically rotates 1000 revolutions per minute for every applied volt at no load.
Outrunner configuration: The outer metal case rotates with the shaft.
Power requirements: Supports 2S to 3S LiPo batteries.
Operates at nominal voltages between 7.4V and 11.1V.
Current draw: Supports maximum loads of approximately 12A for 60 seconds.

Interested to see what they are.

I keep referring to my Huntsman 28 as it's not much longer at 711mm and the hull has similarities to the one you are designing. Also it's been on the water so I know how it sits. The draught is 30mm.

I've weighed it and it's 2.3kg.

Chris

@ChrisF
Very interesting thoughts, Chris.
Tonight, in addition to measuring the angle (I don't think I'll be able to make any progress with the project), I'll show you the only two brushless motors I have (hoping to find their construction characteristics as well). I have a lot of brushless motors.

One motor, or two or even more? Warning, most of what I'm going to say isn't based on scientific research! Just my thoughts based on logic and commonsense (I hope!)

Unless your planing model is really big, say over 4 foot (1200mm) then for good performance you don't need more than one brushless motor as that will provide all the speed that you need - for a brushed motor you are probably looking at a smaller boat but why go brushed at this size?

So, only reason for two or more motors is for differential steering, or more likely because it's a scale model and the full-size boat has multiple engines.

Twin motors will give you more acceleration (if you need it!) but little more in top speed (unlike with full-size), well usable anyway, as the models we are considering, as opposed to race boats, will be faster than we need and possibly well over scale speed anyway even with one motor if brushless.

Size/power of motors. I'm sticking with brushless here as I don't know much about brushed and information on specific motors seems more readily available like the power output for a specific voltage.

My thoughts are that whatever power a single suitable motor has then twin motors should have say 75% of that each to allow for the additional weight of another motor, prop shaft and rudder. But that's assuming that the suitable single motor isn't providing too much power in which case the % could be reduced!

Reason I say this is that motors are usually chosen for their kV i.e. 1000 revs/volt rather than the power output and I make my choices based on using 3S (11.1v) and around 1100/1300 kV to give sufficient rpm. for good planing performance.

One motor I use is an Overlander 3542/05 1250 kV which has a quoted output of 710w which is quite high compared with some other motors and it could therefore be argued that 50% of this for each twin motor would be perfectly adequate?

Alessandro - perhaps in the interests of scientific evaluation you should build your model with single and twin prop shafts!

Me, I just over-engineer!

Chris

Hi - the angle won't affect how it sits on the water, difficult to know if it will sit exactly parallel to the waterline but I've found that they are usually not far off - in any case ballast can always be used.

Advantage, I think for drawing is that you can see how the sheer and the superstructure will actually look if it's at the right angle rather than bow down a bit.

Plenty of the old kit boats planed Ok on brushed motors. The killer was if they needed a big lead acid battery as often they would then struggle. I have a couple of ready-to-run race boats and a plastic Club 500 that use brushed (that in the Club 500 gets very hot though - battery is a 7.2 volt NiMh).

But I'm sold on brushless and use them in all my builds, even the slow models. I like the engineering and the fact they are more flexible for voltage used - for better performance you can just use a higher voltage battery. Also they will drive a bigger prop.

Many folks who refurbish and upgrade their old models replace the brushed motors with brushless as they found the performance poor.

@ChrisF
Hi Chris, as soon as I open my PC tonight I'll measure the angle and let you know.
I drew it by eye without measuring it.
Now the fixed point is the 1:3.5 ratio between width and length (or a little closer to 3).
My dilemma now is the keel angle: whether to make it parallel or leave it that way.

But do you think it can't plane with a brushed motor?
I have two brushless motors I can use (I'll show you later), but I wanted to start experimenting with my own brushed motor. I know you're a fan of brushless motors, but do you think brushed motors should be avoided at all costs?

What angle are you using for the vee? If you need more submerged volume you could increase to 20 degrees if it isn't already.

As my round bottomed Faun has a brushless motor I have had a mad idea to see if I could get it to plane! But I've shelved it as I think it could end in tears!

Hi Chris, as soon as I finish version 3.5 I'll show it to you.
I'll try to highlight some differences.
It's not finished, but I'm still not entirely convinced.
Then I'll try making a version with a parallel keel (as you said, the submerged volume will benefit).

Yes, unfortunately. It used to be quite rare and often after rain so the car would be a right mess. Last few years it's been happening more frequently and especially annoying if you have washed the car the day before!

A ratio of 3.5 sounds good and could be the sweet spot. Surprisingly the Huntsman 28 is around 3.1 but the water-jet project is 3.4.

Inclination - I think it will make a small difference to the submerged volume (slightly worse) but the main thing is that it will more accurately indicate the angle of the chine rail and how the superstructure looks. The line of the deck isn't normally parallel to the waterline.

Sheer (spelt it wrong before) - this is the line between the deck and hull sides running from bow to stern viewed from the side.

@ChrisF

Hi Chris, I didn't think desert sand would reach you in the British Isles.
It sometimes comes here with the Sirocco, but we're facing Africa; you're much further away. Who would have thought?

Thanks for the info on the propellers. I'm thinking of creating space for a 50 mm diameter propeller and then installing one between 35 and 45 mm.
With the finished hull, I could do some empirical testing, which is the fun part.

The width/length ratio of hull no. 5 is 1 to 4, as already mentioned.
I'm designing a new hull with a 3.5 ratio, so on a length of 68/70 cm, I'll have a width of 20 cm.
I went from a length/width ratio of 4 (hull no. 5 you saw in the last attached images) to this one of 3.5 because I didn't want to go all the way to the max right away.
I don't think there are any ratios lower than 3. They range from 3 to 5 for real ones; more rarely with higher values, but almost never below 3.
If you're strongly inclined toward the 3 to 1 ratio, however, let me know.
It's clear that if I want to go from 70 cm in length to 80 cm in length, I can do so by adjusting the scale. Adjusting the scale will also result in a proportional increase in height.
If I want to lengthen it but leave the same height, I'll have to redraw it from scratch.
So, if you're not adamant about the 3 to 1 ratio, let's see how this hull with a 3.5 to 1 ratio turns out. If you'd like, you can give me your personal opinion on this hull, which I'll call "Hull No. 3.5."

You wrote:
"""""Last thing for now. In your drawings, you have the keel angled downwards towards the bow. For this type of boat, the drawings I use/produce usually have the keel parallel to the waterline. This affects how you see the chine, deck/shear, and the submerged volume.

In displacement hulls, I've always preferred the parallel keel (parallel to the waterline).
In planing hulls, however, I've seen that this inclination is very common, and I wanted to try it. Honestly, I don't know what practical differences it will create on a model.
Do you expect substantial differences in behavior, and if so, what?

I'm not sure what you mean here:
""""If you haven't already, something else you need to think about is the shear. Common on planing boats is to run it at an angle along the side of the boat, which then flattens or dips down at the bow. """"""

@zooma

Thanks so much, Bob, for your advice.
If you continue to share your thoughts on the issues surrounding planing hulls, including the messages between me and Chris, I'll be very happy.

If you haven't already, something else you need to think about is the shear. Common on planing boats is to run it at an angle along the side of the boat which then flattens or dips down at the bow. On many of my models I've averaged out these angles to form a straight line to make building slightly easier but more to simplify planking which I usually do off the model - not a problem with naval vessels though as usually painted. And camber of the deck?

As for props, yes, 2 or three blade. All my builds so far have been scale or thereabouts and I have used 3 blade brass on all of them except one. Full-size use 3 blade and I like a bit of polished brass/bling! The exception is the Huntress with the stern-drive which is 2 blade as will be my outboard build. I arrived at the sizes partly by considering the motor size (though outrunner brushless will turn bigger no problem) but also by what looked right - I didn't want a prop that looked too big. So far performance from all of them is very good and I don't want to go much above scale speeds - I've got race boats for that! My props are: Huntsman 28" and 31" - 35mm dia. M4, Swordsman 33" - 40mm M5. The difference in weight between M4 and M5 prop shafts can be significant. I've used M5 in the Swordsman and bought one for the Huntsman 31 but felt it was too big and heavy so changed it for an M4 which I use in most of my builds.

Last thing for now. In your drawings you have got the keel angled downwards towards the bow. For this type of boat the drawings I use/produce usually have the keel parallel to the waterline. This affects how you see the chine and deck/shear and the submerged volume.

Chris

British weather! Few days ago it was a bit chilly to do much outside and now it's too hot! Being doing some work in the garden and washed that pesky Saharan sand (which is getting increasingly common) off my wife's windscreen so that she could go shopping, but it's 33 deg. now some come in to reply!

I agree on your thoughts to reduce the length to beam ration as it is in the area now of that I was advising against as the model, especially the smaller it gets, will have a tendency to rock more and not turn in a smooth controlled manner. Mentioning size, I think it would also be a good idea to have the length towards the upper end of you limit, say 800/840mm, as this will give you increased buoyancy (not that I'm worried about that!) but also more room to fit the components, especially if you do decide to fit twin prop shafts - a narrow hull/transom will make things more difficult. With twin prop shafts these are usually fitted closer to the keel and not mid-way between the chine and the keel - reason being is that on a tight turn/aggressive driving with a deep vee hull there is a danger that one prop will come out of the water. 800/840 provides a nice size model that handles well and is good for storage and transportation.

I see in your drawing that you do have some narrowing towards the stern and was going to suggest making it a bit more narrow so that the line between the maximum beam and the stern is a more natural curve. But reducing the length/beam ratio will have that effect and you don't want a stern that is too narrow for stability and twin prop shafts if used.

To be continued.

Hi Alessandro,

Regarding propellers for your planing hull design.

The size and type of propeller you choose to use should take into account the size of the hull and the amount and type of power available.

Most Classic Model Power Boats use a two bladed propeller and my own favourite power boats that are between 30" and 40" long are mostly driven by an out-runner brushless motor and run on a 45S or a 45X size propeller, but when building or restoring a power boat (of this size) I choose to allow clearance for a 50X propeller as I know this would be the absolute maximum size that I could ever want to use on it....so I keep my options open.

I tend to spend a lot of time testing different propeller types and sizes on my Classic Model Power Boats until I find the one that works best for me on any given model. This is something that I enjoy doing, but I am aware that very few people spend any time doing this - and their boats usually run well enough simply by following the suggested size shown on the plan drawing.

Moulded nylon propellers are inexpensive compared to a custom made brass propeller and so they give the opportunity to experiment with different sizes at an affordable cost. A brass propeller could be added later once the ideal size and pitch has been determined, but I have found that this is not quite as easy as I had hoped, as brass propellers have no movement in the blades as they rotate, and so they work differently to the moulded nylon types that have some "flex" in their blades.

Ultimate performance racing boats often use custom made balanced and sharpened brass propellers that suite them perfectly, but these are not worth considering IMHO for most model power boats and I would suggest that the much less expensive moulded nylon types are more than capable of producing the performance that you would enjoy from your planing hull design.

I like my power boats to go well and to make "white water" - just like their full sized types would - and a two bladed propeller tends to be more aggressive and more suitable for this type of boat than a multi-bladed propeller could be.

3 or 4 bladed propellers are generally (but not always) best suited to scale models where a smoother power delivery is preferred to outright performance.

I have used three bladed propellers on more scale like models previously to give a smoother power delivery, and I have used larger sized 4 bladed and 5 bladed brass propellers on my larger sized tugs in the past when a slower turning prop shaft combined with a torquey motor was preferred.

If your model is anywhere near 1 metre in length, I would suggest starting with a 40 - 45mm moulded nylon propeller - if you are going to power it with a brushless motor. An in-runner motor can benefit from the smaller end of this size range as they are not quite so torquey as a similar sized out-runner would be.

Hope this helps.

Bob.

I'll try lowering the length-to-beam ratio.
Mine is currently 4 to 1.
I might have trouble getting it to settle at low speeds.
I should get closer to 3 to 1.
At least 3.5 to 1.
To do this, I won't further modify hull #5 but will create a new one from scratch.

Continued from message 23 (read 23 first)

You wrote:
"""""""For full-size boats, they recommend not exceeding 15 degrees, as vibration can occur. For me, I limit it to 12 degrees, as I think it can look odd anymore. Not on the water, of course, but on display, where it spends most of its time!""""

Very good, I'll try not to exceed this angle.

At this point, I have to ask a question:
What propeller diameter would you recommend (if I choose the single-propeller option)?
Three-bladed or two-bladed?

Thanks for the many tips, including the construction advice.

Continued from message 22 (read that first)

You wrote:
"""""""""I've never had to bother with calculations, as most of my models are based on real boats and their drawings. So far, as I said, they have all sat on their waterlines and therefore performed well."""""""""

I prefer to know the submerged volume because I'll already know how much my ship model will weigh in total. But it's my way of proceeding; I don't want to impose it on anyone, and I'm not saying it's better than others.
This way, before having any unpleasant surprises, I'll be able to know in advance whether the total weight of all the electrical and mechanical components (motor, battery, propeller shaft, rudder, servo, etc.) won't be excessive.
The next step will be to choose the configuration (search for commercial options online), add the individual weights, and compare them with the total weight of the model.
If the total weight of the electrical and mechanical setup is too close to (or even equal to or greater than) the total weight of the model, I certainly can't proceed any further. I'll have to reduce the setup weights or increase the immersed volume.
If I don't do this, the greater weight will naturally create a greater immersed volume (equal to its own), but it will do so by inevitably raising the waterline.
The waterline must instead be a fixed point of the design and (except for a small margin) should not be increased.

You wrote
""""""""Immersed volume. At what position are you calculating this from? The chine is usually the same as the waterline, or very close. At a length of between 700 and 900 mm (similar to many of my models), you really shouldn't have any problem with the hull supporting the weight of the superstructure, two motors, battery, RC installation, prop shaft, and rudder, etc. As I said before, a planing hull is very buoyant unlike a long narrow warship displacement hull.

This worries me because, as you can see in the attached images (they only show the submerged part of the hull (below the waterline), the maximum draft of hull no. 5 is only about 5 cm.

Continued from message 21 (read this first).

You wrote
"""""Flaws - I think it's just how it comes across in the small photos; it probably looks fine on your PC. In the first four photos, it's clear and reminiscent of a motor torpedo boat or similar, and as I said, similar to my Huntsman 28. With the other photos, I'm confused with what I'm looking at, and the bow in photo 13 is the one that looks odd.""""""""

First, I'll attach many more images so you can better understand the shape and proportions.
I've also included some images with the dimensions.
Remember, if the shape and proportions are good, I can increase or decrease the size simply by adjusting the scale factor.
For now, I'm considering hull #5, which is the result of various modifications made so far.

Description of the attached images.

The first seven images (1 through 7)
show hull #5, with all surfaces closed but without the deck. This helps you understand the shape of the bow, stern, bottom, and the hull in general, observing it from various angles.

The following two images (8 and 9)
show hull #5 with the waterline. The horizontal surface is the water surface; below it is the bottom [submerged hull], and above it is the topside [Above-water hull].
This way, you can evaluate whether, in your opinion, it is too high, too low, or just right.

The next two images (10 and 11)
show hull #5 with dimensions.
This way, not only can you visually assess the proportions, but you can also more precisely evaluate them.
The hull is approximately 68 cm long, 17 cm wide, and 10 cm high.

The next four images (12 through 15)
show the hull covering.

The last seven (16 through 22)
show possible profiles of the superstructure. This is only to better understand the vessel's appearance, as seeing only the hull can be misleading.

---continued---

Hi Chris, you've written a lot, and I'm happy about that because I really enjoy reading.
I've read carefully and taken your advice into consideration.
However, I'll have to divide the topics into several posts to avoid creating very long ones.
I won't repeat the ones I've understood and assimilated, but I'll try to memorize them for future work.

You wrote
"""""A lot of what I'm saying you probably already know or are finding in your research, so sorry if I'm stating the obvious or the basics!""""""
You don't need to apologize, "repetita juvant", more information is better than less.

I reiterate my initial goals, for which I'm asking other modelers' opinions.
1. Hull shape and proportions (I'm ignoring the topsides for now)
2. I have to decide whether to go with a single engine (and therefore a single propeller) or a twin engine (therefore with two propellers). I'm very undecided on this point. I'd like to try both.
3. I need to decide whether to use brushed or brushless motors. But at this stage, I'll prepare the internals for both.

...continued...

Hi Chris, to give you a proper answer, I need to consult the project on my PC. I can do that this evening because I still have some questions.
First of all, I wanted to thank you for all the messages you've sent me.

Forgot to mention as regards the prop shaft angle. As well as this being determined by the motor position and size of prop you need to allow for space between the prop and the underside of the hull. For full-size this is usually 15 to 20% of the prop diameter otherwise vibration and cavitation can occur. You said on your other thread that you will be experimenting with different components so you will need to allow for the biggest prop you are likely to try. A bigger gap from using a smaller prop won't have much effect on a model.

I just go by what looks right!

A lot of what I'm saying you probably know already or are finding in your research so sorry if I'm stating the obvious or the basics!

Chris

Woke up this morning and thought I must mention hull shape and construction which are interlinked!

Once you have moved towards finalising the basic hull shape; shape at stern, bow design and plan then you need to start thinking about other aspects of the hull. In its simplest form the hull sides start off vertical at the stern and as they approach the bow angle in towards the keel before joining at the bow itself. This is easy to build using ply bearing on the deck and chine stringers and the keel, is the basis for many kits and favoured by many modellers. But this can look slab sided, particularly as models get bigger, and so looks better if some angle is introduced, usually with the bottom towards the keel, though also the other way as per the Sea Hornet kit which curves out from the top and down to the chine before becoming vertical to help with the transition to the bow.

I notice in your drawing you have some flare at the bow, this is good as it looks better and keeps the deck drier (though the chine rail acts as a spray rail at the bow and diverts most of the water away. The flare is more useful at full-size and in rougher seas. With a model less so, particularly once on the plane. Introducing flare brings difficulties though as I found with my Fairey Huntsman 31 build. The Precedent and SLEC kits have a bit of flare but are limited in that ply doesn't like, hates even, being curved in two directions at once. With my build I wanted to replicate the flare of the full-sized boats. I think this was my second build and I wasn't very experienced then. I managed it but it was a bit of a bodge. I pretty much got the shape required using very thin ply but ended up with a bulge which I then sliced into with a Stanley knife and used some P38 filler. Extra support for the shape was provided by an extra former each side which I screwed into whilst the glue set. It ended up pretty good but if I was building again I'd do another way, more on that later. My Fairey Swordsman also has a bit of flare, similar to your drawing, which I dealt with using a big balsa block which isn't ideal.

Another option for the hull sides is a convex shape which is exemplified by the RTTL boats. But to do this justice you really need to change the construction method and go for double diagonal planking on frame which will require the introduction of a number of stringers. This method will deal with any bow flare as well and if I was building the Huntsman 31 again I'd use this method, certainly at the bow anyway.

I've attached a link for John's build on Mayhem which I think you will find useful and interesting.

Oh, and you need to think about the transom as well! Vertical or angled, flat or curved or a combination.

Chris

https://www.modelboatmayhem.co.uk/forum/index.php/topic,20758.0.html

Alessandro. I'm rather regretting my comment on "don't go too long and narrow"! It was based on my observations of some models that didn't turn nicely into turns but sort of just dropped into them in an unrealistic way. To some extent this can be true of many models due to their light weight and not to scale water and also due to drivers not being smooth on the controls. Not all MTB's, MGB's and similar are like this though, as Bob contests to. I think the smaller models are more prone to this behaviour, as increased size and weight helps models to perform well. Narrow models are, by their nature, inherently unstable.

Prop shaft angle. Yes, ideally an angle of zero degrees but as you say in this type of boat, model and full-size it is unrealistic. It could be achieved by using pullies or similar but that makes it more complicated and with a model a risk of water ingress. A slightly angled prop shaft has negligible affect so no point really. I still stand by my 12 degrees, though as Bob says there is little difference in practise. This is fine as long as you don't go too steep as you will start to waste power, though with brushless motors it's not really an issue - it is with full-size though as performance and fuel consumption will be affected. I think that the stern lifts slightly when power is first applied but at speed and particularly on the plane with the prop being away from the stern and closer to the CoG/the pivot point when most of the hull is out of the water then the upward thrust from the prop has little affect. An outboard or stern-drive would have greater effect being further from the CoG but they can be set level or altered to alter the attitude of the boat. For full-size boats they do recommend not exceeding 15 degrees as vibration can occur. For me, I limit it to 12 degrees as I think anymore can look odd. Not on the water of course, but on display where it spends most of its time!

Chris

Edit: I see that Clockmaker in Harbour is building a Fast Patrol Launch - the plan view fits with what I consider to be a nice shape and balanced length to beam ratio.

Thanks Chugalone. It will look even slicker when I've finished the painting and started fitting some details!

@ChrisF
That’s a slick looking model Chris.

Immersed volume. At what position are you calculating this from? The chine is usually the same as the waterline, or very close. At a length of between 700 and 900mm (similar to many of my models) you really shouldn't have any problem with the hull supporting the weight of the superstructure, 2 motors, battery, RC installation, prop shaft and rudder etc. As I said before, a planing hull is very buoyant unlike a long narrow war ship displacement hull.

I've never had to bother with calculations as most of my models are based on real boats and their drawings and so far, as I said, they have all sat on their waterlines and then performed well. I didn't even worry about the CoG (centre of gravity), just placed the components as I thought looked right to distribute the load. The Huntsman 31 may be the exception to this! My 2 non-Faireys are based on existing models, though quite extensively modified, but mainly for the water-jet and outboard fitments - I have the utmost confidence though!.

To be honest I've never thought of doing any calculations since I took up this hobby 9 years ago. I used to do structural calculations as part of my work in the building industry, and whilst I enjoyed doing them, I didn't really want to bring calculations into my hobby. I work on the basis of rule of thumb and it will either work or it won't! No big deal, no repercussions and I can always have it as a display only model! Out of interest though, I might do the calculations for my Huntsman 28 and see how it compares with the full-size boat.

Huntsman 31. This is traditional propeller driven - the hull underside is shown in my previous photographs, here I've added another view. The problem with them isn't at speed, as they get up onto the plane easily and perform well. I've noticed in various photos and videos though, that in some cases, when standing still, they have a slight bow down attitude and when coming off the power the bow can drop very quickly, very ungainly! In some part due to driver ham-fistedness. I just feel that the bow, being quite long and slender, doesn't offer much support. It may be that the components aren't distributed correctly to give a CoG of between 30 and 40%, but nearer to 30% for deep vees. I'll position everything as far back as I can and see how it looks in the bath - just need to finish the hull painting.

To be continued!

Chris

Alessandro. I have a feeling that this could be a long, albeit interesting thread! Though no doubt you are keen to get on with the design of the hull!

Your last response is nearly right as a hard chine hull can be flat-bottomed as well as a vee. The chine is simply the result of the angled join between the hull sides and the bottom of the hull as shown in the previous photographs of my Huntsman hulls. For performance/planing hulls a chine rail with a sharp bottom corner needs to run along this joint until it approaches the bow where it turns up. This chine rail is pretty much essential for good turning. We will come onto spray rails/strakes later!

For the type of boat that you envisage I'm taking it as a given that it will be a deep vee hull and as long as you don't go too deep you can't really go wrong and the angle will be what you desire really. I'm going to keep referring to Fairey Marine boats as they are what I know most about and have experience of in model form. In that context the Huntsman 31 was a real success for Fairey and had excellent sea-keeping abilities which were a real asset in off-shore power boat racing. From the horizontal the angle of the vee is 20 degrees which is around the maximum you want to go.

The bow. Visually I prefer the convex design - I really like the look of the hull and the bow of the Rapier which is why I used it as the basis for my water-jet design. Fairey started off with straight bows on the first designs which look Ok though dated. Problem with a straight bow is that water can be directed over the deck and superstructure in heavier seas which isn't very pleasurable for the driver and passengers! This was addressed with the design of the Huntsman 31 which has a sleeker hull and bow which flares to the underside of the deck. This resulted in a much drier boat and as I said better for racing. There is another option of the S shaped bow - see the photo. You asked about dynamic behaviour - as I said it does affect how boats deal with bigger waves, but with models it doesn't really matter as water is rarely that rough and no-one on board and as is usual with models it will soon be on the plane anyway! So go with what you like the look of best!

Flaws - I think it's just how it comes over in the small photos, probably looks fine on your PC. In the first 4 photos it's clear and reminiscent of a motor torpedo boat or similar and as I said similar to my Huntsman 28. With the other photos I'm confused with what I'm looking at and the bow in photo 13 is the one that looks odd.

Time for tea, more later! Chris

@zooma
Hi Bob
Oh, okay, so you mean a sharp chine is simply a V-shaped hull, while a soft chine is a U-shaped hull.
I classify U-shaped hulls as more displacement than planing, but logically it depends on the specific shape.

I believe that, considering only V-shaped hulls, deeper Vs are less planing but more stable, while shallower (almost flat) Vs are more planing but less stable.

If this is true for you, how do you think the profile I drew is? Too deep, too shallow, or okay?

Hi Alessandro,

I no longer have any of my scale model tugs or work boats to picture for you, but any scale model boat that has a rounded section hull like those that have no hard edged chine section can be described as having a soft chine hull.

The profile you have shown in your last post is most definitely a hard chine hull.

A soft “U” shaped section hull would be described as having a soft chine.

Bob.