Has anyone ever put a transmission in a boat?

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PSG-1

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I'm curious why boats don't use transmissions, like a car or a motorcycle. It would save on fuel, not to mention on the additional wear and tear of the engine from running at a reduced RPM through the use of a transmission, instead of high RPM's with direct drive.

Has anyone ever experimented with this idea? If so, how did it work out?
 
I'm not too well versed in this subject, so everything here is going to fairly theoretical, but the overall concepts should be sound.

The short answer is that some do (over and above the F-N-R transmissions most have). However, they are generally 2 speed deals that are there for one of two purposes - to slow the in gear idle speed of a high speed rig down to a level more comfortable for close quarters maneuvering, or to assist a heavy load on plane quicker, such as yanking a heavy skier out of the hole while the boat is loaded down. You don't want to put on a lower pitch prop, as you still need the top end for the skier, but it is tough to have to push water for a length of time before finally coming out of the hole.

But, as far as for fuel efficiency, you aren't going to gain much, as your gear ratio is determined by the prop. Think of a boat as a car going up a never ending hill, due to the higher resistance of the water at planing speeds. Up a steep hill, you aren't going to be upshifting to 4th or 5th, but instead climbing in 3rd, at a higher RPM. You try to go to 4th, and you lug the engine, and probably aren't saving any gas, while at the same time overburdening the engine.

Well, why the lack of 1st and 2nd, then? Back in the 1st paragraph, we have the practical uses for the lower gear, but in more common circumstances, the prop provides enough slip to get out of the hole. In a car, you can make 3rd gear starts if you slip the clutch long enough. In a boat, the prop slips, instead of the clutch, but the overall (very generalized) concept is similar.

As far as maximum fuel efficiency, that is done with prop selection. Generally, one would experiment and determine what prop is most efficient on a given boat at the optimal point in the power curve (provided fuel efficiency is the goal) or at WOT, if top speed is the ultimate goal.
 
I'd think of it like this . . .
Have you ever driven a car on wet ice? It really doesn't matter what gear your in, you're going to have to feather the accelerator to get anywhere, so what difference does any amount of gears make.
Since a boat prop running in water does not have near the resistance of a dry road on car tires, whatever the output gear ratio of the boat motor will be enough since you don't have any rolling resistance to overcome. The water itself causes slippage on the prop.
Anyway, my 2 cents.
 
OK, I guess that answers it. Does make sense, the part where you mention trying to shift to higher gears like 4th or 5th, while trying to go up a hill in a car. Over-lugging the engine, which can cause failure. I never thought about it that way, but I'd say that theory sounds about right.

So, this is why we don't see transmissions in boats. If the concept worked, a manufacturer would have already done it by now!
 
Sounds like you need to mix in a variable pitch prop or gearing like a 10 speed.
 
Actually I think a boat has more resistance than rolling tires. Much more. Boat resistance continues until you go fast enough to get on plane then part of the boat lifts out of the water and reduces resistance some which is why you can usually throttle down a little after getting on plane.

Just try coasting in a 30 mph car vs a 30 mph boat to get an idea of the resistance you are overcoming in each.

I don't think most engines would have enough torque to keep a boat on plane if you could shift to a second or third gear like a car. Plus lower rpm's do not necessarily mean better mileage, most efficient torque and hp use usually result in the best mileage and least wear and tear on an engine.

Tim
 
KRS62 said:
Sounds like you need to mix in a variable pitch prop or gearing like a 10 speed.

They do made variable pitch props - they do not help all that much for the cost

https://www.popularmechanics.com/outdoors/recreation/boating/1276976
 
When I purchased my 2005 Polar Kraft with a 90hp Yamaha 2 stroke the rigger at the marina told me that prop selection for a particular engine is dependent on what hull it is being installed and that one size doesn't necessarily fit all. The 1st prop they put on my boat ran 43mph @ 4500rpm and slow to get on plane. The second prop ran 41 mph @ 5000rpm and got on plane quickly. I also understand that the best compromise on speed and MPG is just after you get the boat on plane, lower hull resisitance in the water and a modest rpm. The data in the boat test link below seem to support that. This has been an really interesting thread! :)

https://www.google.com/url?sa=t&rct=j&q=out%20board%20engine%20speed%20vs.%20mpg%20chart&source=web&cd=1&sqi=2&ved=0CCgQFjAA&url=https%3A%2F%2Fwww.trailerboats.com%2Foutput.cfm%3Fid%3D2259109&ei=upzCTuP7JZSUtwfipuipDQ&usg=AFQjCNF9apK49u8HmUkQh43lKa2W_yqcbA&cad=rja
 
The only way i see a transmission working, is if it was a cvt drive. I have always pondered building a electric outboard with a cvt drive built in. And someday try it to find out if it will work. reason being, you could use faster prop, and still have the power to get on plane. well electric may not either way.
 
Bugpac said:
Interesting, video seems as from the 70's or so tho. :)

Yea, it looks old. From memory, I beleive L&S had a factory fire and closed down. I generally heard good things about their props. They did require more periodic maint, especially with moored boats. I think merc had some shifting props, too. I never had one to try. I do like the concept, and see the most potential for a marginally powered boat. I tend to lean the other way, max hp or just a little more. ;)
 
On the note of adjustable props.....there's a company that makes one, I think the name of the company is "Pro-Pulse" The pitch of the prop is adjustable, although, not on the go, you have to take the prop off the engine to adjust it. Still, it's great for experimenting with different pitches, without having to buy all the props.

The blades are also individually removable and replaceable. A GREAT concept.

Except for one thing......the $&^% thing is made out of plastic. That might be OK for a freshwater lake where you never hit bottom, but out here in the intertidal marshes with oyster rocks and sandbars, a plastic prop is as useless as double D breast implants on a boar hog.

If only they would make it out of stainless, heck even aluminum, as long as the individual blades are replaceable, as the hub NEVER wears out......I'd run one on my boat. But as it stands, I'm currently running a Mercury Vengeance 15 pitch prop on my 16 foot Triton with a 50 merc.
 

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