Prop Location

[bsmit24]

With only performance in mind, where is the best place to locate the prop for a straight drive. Most single inboards have the prop well under the transom so that the rudder is also under the transom. Is this the best place? Most outboards obtain better performance with a set back engine would the same be true for inboards? If I have a swim platform that covers the prop, why not have the prop further aft?

[fcfc]

All I have found is this : http://www.hydrocompinc.com/knowledg...RudderProp.pdf

But does not deal with planning boats.

[bsmit24]

Yes, it is very difficult to find info for single inboards. I would think that with fuel and outboard prices increasing that there would be more interest in economical alternatives and some improvement in the technology.

Thanks, any related info is appreciated.

[yipster]

normal prop(s) under a small angle get easely ventilated so near the surface but surface drives are build as you say.
but wait, normal extended shaft, swimplatform, ruder... i guess that may work well?

[bsmit24]

usually I see both the prop and the rudder completely under the boat. I have seen some with the rudder bolted to the transom and the prop flush with the transom instead of a foot forward. If the performance is the same the point is mute but if there is a performance gain, then I can hide the assembly under the swim platform.

[jehardiman]

For a planning boat, manipulation of the prop and rudder cavitation effects are everything. This is usually handled by controlling the dynamic pressure at the prop/rudder and preventing air ingestion into the prop unless you need it. Additionally, there are porpeller diameter/draft considerations. For the rudder there are also sturctural concerns that go away with the move to an outdrive.

Generally.......

Non-cavatation design props are further inboard than super-cavatating props. This is to take advantage of the higher pressure under the lifting surface. Surface piercing props are mounted aft of the transom to allow for their larger diameter and air ingestion. Rudders are size/placed so that they are in the prop wash at low speed for handling consideration , but still don't overpower the boat at high speeds. Surface piercing props are normally steered with possibly a small fixed rudder for tracking stability.

[yipster]

hmm, usually there are good reasons why things are the way they are.

setbacks related to a long shafti was thinking of. outboards are indeed placed back on a bracket and i belive for planing boats its a good idea having the props (eighter type) back as far as possible.

i belive the idea has some merrit. a quik search did not trow much light on the subject however. now what else did i wanted to ventilate?

[bsmit24]

This is similar to what I was thinking but could not find any specs or comparisons to other setups.

If you go to the rudder page and look at the bottom 3 pics.

[fcfc]

Most easy thing for optimization are already done.

Whatabout of boats with integrated swim platform ? The swim platform can be seen as and add to a transom, or as a recessed cockpit.

Look at http://www.yachtworld.com/core/listi...tform&photo=11

http://www.yachtworld.com/core/listi...1&photo=1&url=

The swim platform IS part of the hull. and the aft cockpit does not go up to the transom. (If the url does not work, the boats are menorquin MY110).

And last, but not least, ALL the engine/prop optimizations you can do on a given boat at ANY cost will NEVER reach the gain you would had with a 10 % lighter boat going 10% slower.

[GeoffSChapman]

I'm building a 32' single engined planing fishing boat with a shallow tunnel.
Due to a combination of factors I've ended up with the prop further forward in the tunnel than I would have liked (about half way) Will this have any bad effect on the boats speed?
The rudder is about 12" in from the transom, then the prop is about another 12" forward of the rudder, the tunnel is 6' long.
Geoff
Uniquest Marine

[Thunderhead19]

It depends on the geometry of the tunnel. It might make no difference at all. You may have to make changes for having moved the line of thrust of the propellor. You might wind up with blade clearance problems

[cyclops]

Solid water gives submerged shaft, prop and rudder the best control. Soft water can cause a prop to stall and a rudder to stall also. That is due to loss of pressure on the back side of either in a turn. Boats turn one way better than the other. They also stall one direction faster than the other. That is why racing OB lower units are made as thin as possible. It raises the speed at which the turn control is suddenly lost. Steering stalls at high speed in a tight turn usually roll the boat over side ways instantly.

[woodboat]

Have you looked at a pulse drive for ideas? http://www.pulsedrive.net/ remember the hull must be of a type that works with this arrangement.
Here is a good angle http://www.pulsedrive.net/images/contact/lg.jpg

[Thunderhead19]

These guys have experienced using the "Pulse Drive", maybe contact them.
orcas@5starwhales.com

[cyclops]

To answer your original question. A straight drive with the rudder behind it so you can dock, using foward and reverse steering of the prop wash has not changed one bit. You can offset the rudder but docking is impossible in wind or current. All improvements means you kiss direct drive good by. It is still that simple. Keep it that way or go to surface drives with a counter balanced 1/2 prop and the out of the prop wash, rudder setup. Final step up in eff. is a pure reaction jet engine. That's about it, till I perfect my Warp III Drive.