one prop shaft or two?

I'm working on a new project in my head. (hasn't made it to paper yet) I would like to build a small offshore racing boat. When I say small, I mean less than 14 feet.
Plans call for a tunnel hull with a surface drive of my design in each tunnel and powered by a single engine in the middle near the transom.
The plan is using cogged belts to drive the shafts. I have also designed a gear box to reverse one shaft since twin shaft must conter rotate.
The question I have is, what is the advantage of two shafts other than greater "bite" in the water? I know 2 shafts have twice the drag, but is there better performance with two than one?
This is just a head dream at the moment, so it will be easy to erase.

 

Why dont you build a central pod with just 1 shaft and prop Could Vee drive it and bring shaft back from the vee drive back to you surface drive !! worth thinking about and save on a few head aches and making a gear box .The pod could start quite small and shallow and get deeper as it goes aft whick with a tunnel is a good thing to help reduce and compress the air thats just going straight through the tunnel

 

Two props do not provide better performance than a single prop of larger diameter (141% of the twins). Dual shaft are used because: a) it is physically impossible or otherwise undesireable (draft usually) to fit a single large wheel, and b) redundency is a main requirement, such as a combatant or offshore come home capability (note: in this case the whole propulsion systems are seperate, not ganged shafts off a single engine).

 

My experience with 50 knot zodiac style boats is counter-rotating is really nice for take offs. That said, single props can be managed too but requires learned skill (ability, experience, whatever you want to call it) to launch straight. (It simply takes a twist of the wheel at the right moment, the right amount at the right tempo, on take off.)

So, I would go with single screw. Lighter, cheaper and in my opinion, better.

Now, you could go for contra-rotating on the same shaft...$$$$$

-Tom

 

What is important to you is to understand surface propellers. Surface drive is a misnomer. A surface propeller can be attached to a variety of systems, including outboards. A "surface drive" can run a completely submerged propeller efficiently too is designed properly for the application.

 

A 14 foot offshore racer??? Fourteen foot boat and offshore are may not be mutually compatible terms except for ultra adventurous pilots.

 

i will second that. how fast do you think you will go in a fourteen footer at sea.

 

Really fast until you hit the first wave and go airborne.

 

You can also call it a kamikaze pilot.

But, anyways... What is the top speed of the boat and the engine power? Then the torque - if you put a single prop you'll have to evaluate the effect of the torque on the handling and the stability of the boat.

 

Just because I said offshore, doesn't mean ocean. I live on a lake 1 X 5 miles in size and my side of the lake is usually rough with waves 12 to 18 inches high and 5 or 6 feet apart.
Building an off shore style boat is for the looks. Single engine, twin screw is for the challenge, surface drive is because I can build them cheap ( because they're small).

 

fair enough. when you said offshore racing boat , i thought you mean't an offshore racing boat. my mistake. a small lake would suit what you are talking about but i can't see how the twin arrangement will work when the engine is up on the tunnel , won't the belts have to protrude into the tunnel.

 

Correct. The belts would be arranged in an inverted V. The sponsons will be water tight forward of the shaft alley. If I go with my Arneson drive clone, then the shaft sprockets will be within 12 inches of the transom.
I'm thinking of a design simular to the one summitted by Masalai on the trimmable surface drive thread.

 

what sort of engine will you use.

 

I'm thinking of using a Subaru 4 cam motor. It's wide and low to help balance the hull. The exhaust exits out the bottom of the motor from each head. It's mainly aluminum, so it's light, and HP is around 150 stock.

 

So, what way are you leaning, one prop or two?

-Tom