Hi Westward
The Robbe kit is now sold as a Krick Romarin Dusseldorf. Cornwall models sell this kit and it has, I believe, two KRICK MAX GEAR 540 TYPE MOTOR 2.5:1. You can see the specs at:
https://www.hobbies.co.uk/krick-max-gear-540-type-motor-2-5-1
If this is your motor set up then I suspect an SLA 12v 12Ahr was used as you can also fit working fittings which will increase the current required so will benefit from an SLA, which will also act as ballast for this large model.
I suggest you contact Cornwall Model Boats, who sell the Krick Romarin and ask what battery they suggest for your Robbe Dusseldorf.
However, if you search "Boat Harbour" on this site for Robbe Dusseldorf you will find three member's models and lots of details that will help with your model.
Good luck and please post some details of your model so we can see how you progress.
Dave976

Yes it is and the motors are fitted

Hi Westward
The actual motor and ESC details are also needed to answer your question. Are you saying your model is the Robbe Dusseldorf?
Dave976

its two brushed motor with a reduction gear box and the props are 40mm thats the one in a robbie duseldolph having never had anything to with lipo before bit baffled on what to use

Hi Westward
Welcome to the site.
Motors for model boats are very varied and some are economy types that draw low current whilst others are aimed at the sports fast sailor and can be very high current users.
Batteries for the main power are usually of the rechargeable type but dry cells were also used when I started modelling in the 1950's.
SLA (sealed lead acid) types are best suited for larger models as they are heavy and serve as useful ballast also. Typically 6 or 12 volts and amperage in the smaller sizes up to about 12 amp but car motorcle/wheelchair/Leisure types can provide much higher currents.
NiMh (Nickel Metal Hydride) have replaced NiCad types but both offer a range suitable for model use. They are similar in shape to Dry-cells but have a nominal voltage of 1.2 volts and a fully charged voltage of 1.4 volts. They can be made up into packs giving typical values of 4.8v, 6.0v,7.2v etc and typically up to 12v but higher values can be made. Current ranges are from milliamps up to high amps and are good for most models.
LiPo are the latest technology and are both light weight and very powerful. They are popular across all modelling types and offer the best performance.
They do need to be treated with respect and within very strict parameters regarding charging and discharging. This can be achieved by using a suitable balanced charger and an ESC (electronic speed control) with a built in cut off that reduces the power drawn below a set voltage (typically 3.5v per cell on modern batteries). The nominal cell voltage is 3.7v per cell with the fully charged max of 4.2v per cell. Storage voltage should be 3.7v as the battery is in its most balanced and stable state at that voltage.
They should never be left charging unattended and you can buy special charging bags to protect you in the event of any malfunction.
I store my LiPo packs in an ex army metal container with a loose fitting lid.
To answer your question we do need to know what model you are building and the actual motor (brushed or brushless) you intend to use. The size of the prop will have a big effect on the max current drawn.
Hope this helps
dave976

It's not the physical size of the motor that determines the power requirements, but the efficiency and load. Every motor is different, and every installation is different. As to the type of battery needed, it depends on the particular application. More details about the boat in question would be helpful.