Decided to have a go at building a Watercat M12 as I have recently brought a couple of 28mm jet drives, yet to arrive! and wanted a craft to fit them in.

The model will be semi- scale and I intend to modify the stern's deadrise to ensure both jet intakes keep in the water when turning a deep v stern may cause ventilation on turns.

upscaled the plans available to make a craft around 35.5in (90cm).

From the enlarged plan, I design the sections and cut them out,
please note, the deck appears to have no camber, but the deck does rise slightly from stern to bow.

Hi There,
I thought I'd better mention the jet drives, they are relatively cheap, compare to the quality German products (loads of money). the output port is about 28mm and suitable for a 36mm diameter motor with 3.17mm shaft.

I have ordered two 4-Pole 3650-1500KV 2-4S inboard Brushless Motors to power them.

regards Roy

Traced Frames from paper patterns to 4mm ply. the stringers will be 4.5mm square and the skins will be 0.8mm birch ply.

The stern (frame) is a little more complex as had to reduce it's width to allow for the 2x 18mm quadrants to represent the curved stern corners.

Please note. There is a 1mm gap to insert a 0.8mm ply stern panel to support the jet drives output ring. as 4mm is likely to be too thick!

As with all scratch builds you have to make it up as you go along!

Slowly I will cut out the rest of the frames,
the middle frame will be solid as a watertight bulkhead and forward of that will most probably be foamed or filled with polystyrene blocks.

regards Roy

Jet Drives Arrive today at 14:30.

I have added a pic with some measurements.

The solid coupling appears to be 4mm to 3.2mm and is 23mm long, this could be changed for a flexible one.
Also noticed a 2mm movement in the shaft this would put pressure on the motor bearings if not adjusted.
The impeller is a normal prop about 25mm if you wish to improve the unit this may be the one of the things to change, for a proper impeller.

Not bad for the money, £22.

regards Roy

First water Jet up-grade is a new coupling as a solid coupling transfers vibrations to the assembly, and I have found a review on Amazon where it has fractured the unit casing.

I looked at a suitable Impeller but it cost £25, more than the entire unit.

Regards
Roy

New couplings and motors arrived today!
Took out prop shaft and put the PTFE washer on the prop end, for some reason it was between the housing and the coupling the prop is a standard dog type.

Unfortunately the coupling could not be used without modifying the length of the Motor shaft, about 5mm needs to be removed from it.

No real problem as motors were cheap 3650 1500kv 2-3cells 7.4-11.1v.

Hi there,
One of my sources of info, was the Model boats Magazine website which has a Watercat M12 build of a commercial kit by Nautical Marine Models (Now out of Business)

https://www.modelboats.co.uk/news/article/watercat-m12--u619/14635

They quoted that 'a scale of 1:16 which gives a model hull length of 35 inches and 10 inches beam.

Unfortunately when I look at the specs of the real boat:
Beam 3.65m as per 'Model boats'
Length 13.10 excluding the waterjet guard
length with Guard 14.25m.

If the model was 1/16 and the model ha a beam of 10in the real boats beam would be 160in =4.064m
the scale is way off.
I think the hull they used may have been an old KMB (Germany) CB90 boat they used to sell.🚤

So back to the Drawing Board😐
So my boat after some re-calculations, is a 1:14 scale / semi scale craft with a beam of 10in (23cm) and a length ex, jet guard of 36.75in (93.37cm)
and with jet guard will extend it by another 3.23in (8.21cm)giving a LOA of 39.98in (1m).

Regards
Roy

Hi Shipmates,

Work on transom, recess the waterjets in 4mm transom then add a 2mm outer skin over the transom
with circular 32mm hole to fix the the unit in place.

Regards Roy