Forward drive propeller

My name is João and I am a member of Tecnico Solar Boat Team.

We are on the quest to build a foilborne trimaran and we want to make it as efficient as possible. To achieve this we are considering a forward drive propeller because we think it would be more efficient. Is that true? If it is then we dont know what kind of design would work better.
We are considering 2 options, notice the diference would be in the position of the propeller relative to the engine structure as follows:

1-

In this case we would have the advantage that we know the engine shaft can withstand the torque from the propeller, since it was built inicially for this purpose, and that it would be easier to build.

2-

We think this design might be more efficient once almost every front facing proppeller is disigned this way. On the other hand it would require more torque rigidity from the engine shaft once it would have to rotate a bigger mass.

Which design should work better and why?

I hope you can help us!
Best regards!

Ps: Sorry for my paint skills and that I had to use paint instead of proper pictures, I hope you can understand the meaning of the drawings.

 

All things being equal the forward facing prop should be better. The shaft etc... should be pretty much the same. And in reality, I don't think you will see much difference between one and other. I have used electric trolling motors that are reversible and haven't noticed any difference. I think it is far important to match the prop to motor and boat configuration.

 

A forward propeller can be more efficient. It will not be affected by the turbulence of the gearbox, leg, etc.

 

Yes but will it make a quantifiable difference at 5mphs with a propeller 5 inches in diameter?

 

That boat is going to have a much higher speed than 5MPH to be able to foil.

 

I am not convinced. Perhaps there is hard evidence somewhere. I guess it also depends on hull and general design. In a rear facing prop, the water in front of it is disturb. In a front facing prop the water behind it is disturb. I believe I rather disturb the water in front than behind it. So this does not take into account rudders, which may not be needed if prop pod turns. Anyway, airplanes benefit from a front prop because it helps with the airflow over the wing. No such advantage in a boat. I would rather have a prop behind a boat than under it. The clearance with the hull in of itself causes turbulence. I guess the only way to really know is to build it both ways on a particular hull and try it.

 

Because the boat will have foils the prop will have a significant distance from the hull, so I guess the hull turbulence would not affect the prop in a different manner. But I agree that it probably is more efficient because the turbullence the engine structure would be behind not in front of the propeller. I think in the case of airplanes the props are in front more for stabillity than efficiency. In fact in the case of super efficient planes propellers are normally on the back of the wings because you can keep a laminar flow in the wing (no turbullence for the propeller anyway) and in the other way you will not have such flow on the wing. Not so much in water because it is way harder to keep the flow in laminar regime.

In case we decide for a front facing prop, would it be equal for eficiency both in situation 1 and 2?

Thanks for your answers.

 

Volvo likes forward pointing props so it must be OK.

 

The skeg appears to be deeper than a conventional drive, I suppose that provides more grounding protection.

 

An appropriately designed and sized forward facing propeller will be more efficient because of cleaner flow into the prop. Efficiency defined as thrust produced by the propeller * forward speed of vessel (or alternatively average water inflow velocity) / mechanical power into propeller.

A forward facing propeller may result in increased drag of the appendage supporting the propeller and possibly hull due to interaction with the propeller wake.

 

Thus it may be a wash... pardon the pun.

 

wrong post

 

There are three peculiarities with a front propeller:
None or less wake
Recovering rotational energy from the strut.
Less waved cavitation effect, essentially less EAR