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.

@zooma

Hi Bob, thanks for the explanation.

Now I understand (at least I think so) which chine you're referring to.
Is it the side where the two hull wings meet, as shown in the two images?

If that's the case, I understand that my hull can be defined as sharp-edged.
Okay, now to better understand, could you show me some images of a hull with a soft chine? That would be very helpful.

Hi Alessandro,

Here is a definition of a hard chine hull taken from an online description:-

“A hard chine is an angle with little rounding, where a soft chine would be more rounded, but still involve the meeting of distinct planes. Chine log construction is a method of building hard-chine boat hulls. Hard chines are common in plywood hulls, while soft chines are often found on fiberglass hulls”.


My own thoughts are that when a plywood skin is placed over a frame formed by a series of fairly regularly spaced flat or slightly curved surfaced bulkheads (also called “formers” by some) of a typically standard plywood model boat construction, the sides and under-surfaces are usually more flat that curved……and where the bottom and side skins join a hard edge is adorned, and is how I define a “hard chine” model boat.construction.

Bob.

This how I

Hi Alessandro,

I have been building and driving Classic Model Power Boats since the early 1960s and have clocked up hundreds of enjoyable hours running them (and at one time racing them off-shore too).

Most of these have been hard chine models - but not all.

Almost every boat I have owned and driven has been built or restored by myself, but I have also been fortunate enough to be invited to drive other peoples power boats for lots of different reasons over the years as well, and this has greatly added to the number and variety of different types I have been able to enjoy and experience.

I have scratch built a TID tug and restored other larger tugs and trawlers so I have also been able to work with them to know how the different types of hull forms react.

If any of this 60+ years of running (mainly) r/c power boats can be used to assist you in any way, I would be only too pleased to help in any way I can.

Bob.

@zooma

Hi Bob, your opinion, like Chris's, is very important to me, and I will take your advice into full consideration.

You wrote:
"Your hull drawings can all be classified and built as simple “hard chine” hulls that should work very well indeed."
I'm very pleased to hear this, but I ask you: what does “hard chine” hull mean?
Could you explain it to me better, perhaps with a drawing?

You wrote:
"A deep vee-shaped hull (as used on several LesRo and Aerokits powerboat designs) gives good stability and generally gives a dry hull and is less likely to roll."
Very good. As I told Chris, this is pleasing and saves me from having to completely revise the stern.

You wrote: "As a matter of interest, I have never found the angle of the prop shaft to be that critical in real-life performance. I did steepen the angle on my Rapier when I restored it in an attempt to drive the bows a little lower when under power, but I'm not convinced that it made any difference."
This observation is also very important and useful to me, because perhaps I place too much emphasis on the inclination of the propeller shaft.

You've been very helpful, and I hope to be able to bother you again with other questions.

Logically, messages aren't compartmentalized, and each person can comment on the messages addressed to the other.

@ChrisF

Hi Chris.

Please note that your opinion on this matter carries a lot of weight with me.

You wrote: "Your vee at the stern is fine; some of my Faireys have a greater vee—they are better in rougher seas than a flat stern. So both will work; what won't work, of course, is a rounded hull, as used in displacement boats. So a vee is a good start—the overall look of your hull is reminiscent of my Fairey Huntsman 28. Some of my Faireys have parallel hull sides, but some also have narrower sides toward the stern, which I think looks better and gives that classic rounded look on plan."

This is very helpful to me, as well as pleasing. I was thinking of creating other variations with flatter hulls (or less pronounced Vs), but after what you wrote, I can leave the V hull as is.
You've cleared up a big doubt for me, and I thank you.

You wrote: "For my water-jet project, I started with the hull from the Lesro Rapier, which has parallel sides, and modified it so that it was narrower at the stern. I also have designs where the stern is even narrower and the sides are more rounded."
In the Lesro Rapier, the bow line is clearly convex.
I initially designed a convex bow (from the side, I mean), but then I opted for a hull with a straight bow (although very steep), as you can see in the attached drawings.
Which do you prefer aesthetically? Are there any differences in dynamic behavior between these two types of bows, or is it the same?

You wrote: "You don't really have to worry much about weight in a planing hull unless you're thinking about using a lead-acid battery in a small boat! In my experience, they are very buoyant and forgiving."
On the drawing, I set an overall length of about 70 cm.
With this length, the immersed volume is only about 1.8 kg, so the entire speedboat should weigh only 1.8 kg.
I could increase the immersed volume by increasing the scale to a maximum of 90 cm (I can't go any higher).
Lead-acid batteries are certainly out of the question in this type of boat, but the limited weight worries me a bit.

You wrote: "There's a question mark regarding my Huntsman 31 as they tend to sit nose down (but plane easily) due to the lack of support from the flared bow, so I might have to use some ballast in the stern - a bath test soon to check that out."
Is this Huntsman 31 also jet-powered or propeller-driven? If it is propeller-driven, I could suggest the cause.
Besides ballast, there could be another system. This system is used for real speedboats, but I don't know how effective and feasible it is for scale models. It consists of fins at the stern that adjust the trim.

You wrote: "It's not the submerged volume you have to think about, but the opposite!"
I don't want to cause any misunderstanding. The submerged volume calculated on the drawing only helps me determine the displacement and therefore the maximum weight of the components (electric motors, batteries, etc.) that I can fit in the hold.

You wrote: "As for flaws, I can't see much wrong with it, though the bow on some drawings looks a bit odd."
This interests me a lot, but it worries me a little.
Since in this initial phase I want to establish a correct, harmonious shape (not only aesthetically pleasing but also effective in terms of handling in the water), I'd like to understand what you mean.

You wrote: "Don't go too long and narrow."
This is also a key point for me. What do you mean?
Is it too long compared to the width, or the opposite?
Would it be better to widen it a bit, or, on the contrary, to taper it?

The model's length ranges from a minimum of 70 cm to a maximum of 90 cm.

You wrote: "but aim for an angle of 12 degrees maximum, though the odd degree more won't hurt."
Okay, that's very specific information I'll take into account.
Let me clarify something. In my opinion, it's best to minimize this angle. The ideal, in my opinion, would be zero. This way, there's no thrust wasted upward (a thrust component that, at least initially, will push the stern up and the bow down).
Unfortunately, in this type of boat, it's impossible to reduce this angle too much due to the engine and propeller layout.
Isn't that the case? Would you prefer to increase this angle?

Sorry for the many questions, but please answer slowly and only when you have time.

Hello everyone.

I was honestly hoping for a response from Chris and Bob.

From your posts so far, I've seen that you have a wealth of knowledge and experience with planing hulls.
So I'm very happy you've commented, and I intend to respond to your messages individually.

Of course, advice and opinions from other modelers are also welcome.

Hi Allesandro,

Apart from Classic Model Power Boats, I also have an interest in Coastal Command power boats and have built/owned and driven quite a few over the years.

All of them have had a hard chine construction and all of them were planing hulls that drive very nicely and can cope well with comparatively rough water.

Your hull drawings can all be classified and built as simple “hard chine” hulls that should work very well indeed.

A deep vee shaped hull (as used on several LesRo and Aerokits power boat designs) gives good stability and generally gives a dry hull and is less likely to roll.

The Vosper and other hull types of Motor Torpedo Boat and Motor Gun Boat that I have driven are all hard chine construction and all of them drive very well and generate the “white water” of the originals when they are under way, and they have all handled very well indeed too.

As a matter of interest I have never found the angle of the prop shaft to be that critical in real life performance. I did steepen the angle on my Rapier when I restored it in an attempt to drive the bows a little lower when under power, but I am not convinced that it made any difference.

I hope this observation is of some value to your design thoughts.

Bob.

I know the prop shaft is a consideration for later but aim for an angle of 12 degrees maximum though the odd degree more won't hurt.

The Huntsman 28 was my first build and I didn't want to cut the prop shaft and tube any shorter so the motor is further forward than it needs to be to achieve the required angle. The angle/attitude of the model on the water is fine though as the weight of the motor is easily offset by the other components.

But hull design first!

Hi Alessandro

I don't do naval craft, so does this exclude me from the discussion! Only joking!

But I do do planing hulls! As you say easier to design than displacement or semi-displacement. As long as the hull has a flat or vee at the stern and sufficient power it will plane!

Your vee at the stern is fine, some of my Faireys have a greater vee - they are better in rougher seas than a flat stern. So both will work, what won't work of course is a rounded hull as used in displacement boats. So a vee is a good start - the overall look of your hull is reminiscent of my Fairey Huntsman 28. Some of my Faireys have parallel hull sides but some also narrow towards the stern which I think looks better and gives that classic rounded look on plan. For my water-jet project I started off with the hull from the Lesro Rapier which has parallel sides and modified it so that it was narrower at the stern - I also have designs where the stern is even narrower and the sides more rounded. Depends on what your overall design for the model is really as to what will work the best.

You don't really have to worry much about weight in a planing hull unless you are thinking about using a lead acid battery in a small boat! In my experience they are very buoyant and forgiving. I was a little worried about the stern drive on my little 23" Huntress but it sat bang on the waterline and performs really well. I don't build light and so far all my models have sat on the waterline. There's a question mark as regards my Huntsman 31 as they tend to sit nose down (but plane easily) due to the lack of support from the flared bow and so I might have to use some ballast in the stern - a bath test soon to check that out. It's not the submerged volume you have to think about but the opposite! As I said, very bouyant!

Motor will go towards the front but then if you distribute the battery in the middle and the rudder and prop shaft obviously towards the rear you can't really go too far wrong.

As for flaws, can't see much wrong with it though the bow on some drawings looks a bit odd. Don't go too long and narrow - I've noticed with a number of MTBs/crash tenders/fireboats models that they can be prone to dropping too quickly into turns and are a bit unstable, partly in some cases due to the operator not being smooth on the rudder and not helped by the relative lightness of the model.

What length are you looking at?

Chris