Twin Screw for Ski/Wake Boat

[MMH]

Would a 20' to 24' ski boat have same benefit of twin screws as a larger boat? I have to believe that the docking, reverse, etc. would benefit, but what about high speed? How would the boat handle? When the boat leans over one of the props would not be submerged as deep - would that be a problem? I am working on a transmission with one engine input and two outputs (the two outputs would rotate in opposite directions).

What about prop sizing? Should the two props be the same size & spin slower or should they be smaller. I am under the impression that larger, slower props are more efficient. Is this true? Smaller props would be nice as the boat could go in shallower water.

How difficult would it be to modify an existing v-drive boat to have two screws come out of the bottom, both from a engineering & building perspective? I really talking about the hull & prop shaft modifications, not the engine/trans mods.

I am not very familiar with boats and needs lots of help. I do want to develop my transmission, and need much boat design help, advice, etc. Hopfully you guys can bear with me & help me out!

Thanks in advance!

[charmc]

I'm not that familiar with ski applications, but there must be a good reason why every ski boat I've ever seen was single screw.

Does the transmission allow:
Independent gear selection for each screw?
Independent speed selection for each screw?

These would be important for maximum manueverability.

Just some random thoughts.

[charmc]

Originally Posted by MMH

I am not very familiar with boats and needs lots of help. I do want to develop my transmission, and need much boat design help, advice, etc. Hopfully you guys can bear with me & help me out!

Thanks in advance!

After thinking some more, I believe this is a pretty challenging first project for someone who doesn't know much about boats. Not a criticism, just a thought that smaller projects would build up your skills before challenging a tricky one like this.

More thoughts on your questions:

Twin screws will create more drag and give you less top speed and more fuel consumption. Twin screw boats are normally so because more power is needed than a single engine can provide economically/reliably, because of the reliability factor of two engines, and better low speed manueverability. Twin screws might allow a shallower draft.

Overall, it seems like a costly project without much benefit. That's just my opinion, of course. There may be benefits I'm not seeing, and others may have some ideas.

[MMH]

Agreed that it would be a challanging project. I would not be doing the boat/prop design but would obtain the services of those who know how to do this. At this point, I'm just starting a 'feasibility' study. My objective is developing the transmission. But, I just want to make sure that a disease exists before I invent the cure, thus the feasibility study.

The way I see it:

Disaddvantages:
- Drag of two screws vs. one
- complexity/cost

Advantages:
- low speed maneuverability
- strait tracking/reverse maneuverability
- shallower draft due to smaller props

more on the transmission:
- it would offer independent gear selection between the two shafts
- it would offer infinitely variable speed variation between the two shafts

As far as not seeing it on any ski boats to date - I think that this has not been seen because of a lack of any practical way to get two engines/shafts in these smaller ski boats. The same limitation exists on all smaller boats. A smaller fishing boat can be used for the prototype as well.

As stated above, I agree that this would be a big project. I'm trying to get my arms around how big a project it would be (i.e. determine how much $$$ I need). The objective is to develop the transmission. That expertise I have, the boat expertise I need help on.

I would appreciate any dialog, constructive criticism, feedback etc. If anyone cares to send me PM or e-mail, that would be great as well.

Thanks!

[charmc]

Thanks for the details; now I understand the picture somewhat.

Straight inboard ski boats have a particular hull shape and inboard engine, single screw configuration mainly to insure consistency of wake for competition. They are all the same, as I understand it, because skiers need to practice on the same conditions they will see in competition. There is something about the flat bottom, single screw inboard that appeals to the skiing fraternity. Penetrating that market would take creation of a new class or worldwide replacement of the existing standard ski boat, I believe.

Small pleasure boats might be a market, but there you're up against single screw stern drives and outboards, which both allow manuevering at low speeds and steering in reverse by moving the prop. So you are left with shallow draft as a benefit. Hmmmmmm.

The transmission concept is interesting, however. The properties you describe might provide benefits in other applications. Good luck!

[Kay9]

20-24' Boats that want 2 props will allmost allways find it more affordable and praticale to install 2 outboard engines, over 2 inboard engines or 1 inboard with a trans that moves 2 shafts. The cost of the 2 outboards will be 1/2 of 2 inboards and 5 times more effective then a single inboard with 2 shafts via transmission.

Given the fact that the smallest inboard engine you could install would be a 4 cylinder 95 HP I really think 2 110 Yamaha's at 1/2 the cost would be the way to go.

My $0.02