A replacement of 1-2 diesels of a power boat with a small gas turbine

You are right , jet is good at high speed and I would use thippose it will move only forward = s property when at the surface . On the other side under the water it will work at max 5 knots and as you suppose it will move only forward , for side movement will have additional thrusters .

 

I understand . Life equation for normal planing as I am informed is around 20 kg per 1 horse power , what do you think about that ?

 

I think it's a minimum for a light weight, good design, planing hull to achieve plane.
You are asking for much, much more, so 5 - 10 kg per HP may be more appropriate...

 

Thanks for your opinion . I'll check about these 5-10kg and come back soon to the matter .

 

Waterjets work just as well under water as they do above, but realize that jets provide higher efficiency as speed increases.

At really low speeds (like 3 or 4 knots) I have no idea how efficient jets are at relatively low thrust levels. If the thrust was high then of course the efficiency would be low, but at low speeds and low power levels they act more like props so the efficiency may not be that bad. At low speed and high power the propulsive efficiency is low because the exhaust velocity is high and that results in low propulsive efficiency. At low thrust levels, as you may find in a sub at low speeds, is another case and perhaps Beckmo can chime in as to what the efficiency looks like at that condition.