Power Boat Transmissions

[BBSebens]

So I've been thinking; a dangerous thing, but I do it nonetheless.

I come from a car family, and married into a boat family, so this is a constant process of education.

Why is it that powerboats don't use Transmissions with multiple forward gears? Now I'm sure it would have limited applications, but say its a planing hull... once your up to hull speed, or cruising speed, or whichever speed you like... imagine that you could bump your tranny into 2nd gear. Instead of cruising at 3500 rpm, you can now cruise at 2200. you're all ready up on plane, and don't need as much power to maintain the speed. fuel savings, engine wear, less noise, seems a good deal to me.

Now, I have to imagine that this has been proposed, and even tried before. so, feel free to shoot me down.

[Paul Pless]

Its been done. Mercury offered a 2 forward speed transmission at one time for some of their Mercruiser inboard/outboard packages. I've been out on a large sportfisher that had two speed gearboxes, but I believe it was setup to help keep the revs up on the big diesels when trolling at low speed - lugging a big marine diesel at low low rpm can be bad for the engine. IIRC this boat had John Deere marine engines.

[The Bigfella]

OK

I'd say its because a prop has a best operating speed - and that's what its geared to at max engine torque / best operating speed.

Nothing to be gained by spinning it faster than its best speed - plenty to lose with cavitation, etc.

Oh yeah - and acceleration and hills aren't an issue normally on a boat - which is why cars have gears

[PeterSibley]

A combination of a variable pitch prop and varied engine revs is the ideal .The variable pitch is really the equivalent of the multispeed gear box in a car .Someone else will probably elucidate .

[oldsub86]

An interesting question that I too have wondered about.

I have a V8 I/O in a 22 foot boat. If it will run around 30 mph at 3000 rpm and 45 mph at 4600 rpm then what would happen if I could shift to a higher gear so that I could run faster at lower engine rpm? Would I just find that the engine did not have sufficient power to do that?

If it were true that it could handle the higher gearing at lower rpm then why wouldn't it just have the higher gearing in the first place? Why would I need the lower gear?

I have to assume that the engine just would not have sufficient power to do the job.

If memory serves to be correct, with cars, they say torque gives one acceleration and horsepower gives one high speed. Is this a similar situation in that the the horsepower becomes more important than the torque once one is up on plane and moving forward?

I know that the gear drives for V8 I/O's will be a different ratio than those for smaller engines. I assume this is related and that there is a practical limit to the gearing that works for the power available. Again sort of similar to the size and pitch of the prop. I like the idea of a varable pitch prop. That would likely give the best bang for the buck.

Randy

[BBSebens]

OK

I'd say its because a prop has a best operating speed - and that's what its geared to at max engine torque / best operating speed.

Nothing to be gained by spinning it faster than its best speed - plenty to lose with cavitation, etc.

Oh yeah - and acceleration and hills aren't an issue normally on a boat - which is why cars have gears

OK.. this makes sense. The thought was more that one could lower RPM of the motor, and not the prop. Now, that does lower then power output of the motor, which is why I thought it'd work on something like a planing hull.

Acceleration and hills are why cars have several forward gears (lexus has an 8 spd! :confused: ) but it seems that boats do encounter hills, in the form of large waves. in such situations you wouldn't shift into the "cruising gear".

I would imagine that control for such a thing would be a toggle switch , with one of those red covers. It would have to be really simple and quick to get it back into normal gear.

ok... this is good stuff. maybe we can make this work. ahh.. at least its fun to think about! :D

[The Bigfella]

The other thought that comes to mind is that car tyres (as a rule) don't slip during acceleration - whereas the prop is constantly slipping, so there is no real need to change the speed from the best drive speed for the prop.

[BarnacleGrim]

Wouldn't it make a lot more sense to have a constant-speed propeller? Most propeller aircraft do this. The propeller pitch is set using engine oil pressure controlled by a prop governor with a prop speed lever. A bit like an automotive automatic transmission.

[rbgarr]

The initial expense, complication, space needs and maintenance costs vs. fuel savings probably come out unfavorably in the end (but maybe not anymore?) People are conservative and fewer are likely to experiment on something like a boat versus automobiles, which have a wider market.

[BarnacleGrim]

It would probably be a lot easier to experiment in the US because of higher availability of used aircraft parts. Still, I think a lot of effort has to be made in order to marinize a prop governor. I don't even know what one looks like, I've only flown fixed-pitch propellers.

[The Bigfella]

The marine environment can be a tough place for finicky devices. Especially in places that you can't inspect easily. Barnacles, etc might just stop it working.

[BarnacleGrim]

The easiest way would probably be to have an actual marine variable pitch propeller and use electronics and an actuator instead of a governor. However I can only see this being worth the effort on big boats with a very wide speed range, such as the one Paul Bless describes. After all, compared to air, water has pretty much the same density wherever you go.

[Tylerdurden]

There is an animal called a reeves drive that allows one constant input speed and variable output speed by means of a belt and two ratio changing pulleys. I had considered fabbing one for the rescue minor project in concert with the Kitchen rudder. Would keep the motor in best power curve or economy depending on whats needed.

[Lew Barrett]

The marine equivalent of a variable speed transmission is of course a variable pitch prop, just as it would be on a plane. As has been pointed out, the trick isn't to vary the gearing of the transmission, it is to vary the pitch of the prop.
But in practice, torque and power are the limiting factors that are in operation when driving a boat forward. Props are almost always pitched to reflect maximum top speed rather than compromised to give maximum forward thrust at idle or just above. In gearing (selecting the wheel) Diesels you want to pull redline very near the maximum speed the boat will pull. Over wheel the boat (make pitch too great) and the engine won't pull redline, you'll have reduced fuel efficiency, and added increased wear and thrash, put extra load on the motor, having gained nothing. Put in too little pitch and the combination will give up responsiveness and zoom to redline, lowering top speed and again reducing efficiency. There is a ratio that is correct for every boat and wheel combination. Being off may yield slightly better low speed power or faster acceleration to a lower top speed, but it will not maximize efficiency, wear characteristics and noise. Alternately, pulling too much wheel will not result in the higher speeds hoped for, but will yield additional load on the engine, overheating and general sluggishness. The trick is to find the right ratio, and be happy that the simple two speed with neutral marine gears we use are the right solution for most pleasure boat needs.

Variable pitch propellors are useful on airplanes that need extra thrust for takeoff and greater pitch for speed at cruising because the range of speeds is so great for them, but for most boats, a simple ratio derived to pull redline (or get close) at the maximum speed the boat could make with the available power is plenty adequate. So the simple answer is; "there's no benefit to such an arrangement for pleasure boats."

[Dan McCosh]

Actually, the question is backwards. Cars use a torque converter to create the effect of a prop in the water. The torque converter slips until the car "catches up" and solid gears can be engaged.

There is, however, a variable-pitch prop offered for sailboats that lets the blades find the optimum angle for a given forward speed. This acts like a variable-pitch prop. Don't remember the name, and haven't seen it used on a powerboat, although it may make sense for a displacement hull.

Generally, the slippage and bad efficiency at low speeds doesn't mean much on a boat, as the serious power is needed at higher speeds, when the prop becomes most efficient.

[kc8pql]

There is, however, a variable-pitch prop offered for sailboats that lets the blades find the optimum angle for a given forward speed.


Autoprop

[BBSebens]

So let me see if I can summarize this.

The powertrains of powerboats are designed to run pretty much at optimum efficiency at cruising speed. everything else doesn't really add up to much.

Also, let me see if I understand the physics here. As speed increases, so does drag and resistance, and so does the power needed to maintain that speed. Is this accurate?

If it is, then that makes multiple forward gears an unnecessary complication.

which is kind of what i was afraid of all along.

well this was fun. :D

[Tylerdurden]

You got it.