Why not a gearbox

[Merc805]

One for the JB's of the world... Would there be any benefit of having a multi-speed gearbox on an outboard to keep down the revs, whilst keeping the prop spinning at the required speed ?

 

[kd6nem]

Re: Why not a gearbox

I won't claim to be in the same league JB, but I'll throw out my 3 cents anyway: The prop already sits in a fluid, allowing it to slip enough for the RPM's to increase as the motor simultaneously increases torque, allowing for even better accelleration until the boat starts to settle into an equilibrium between drag and thrust. A multi-speed gearbox would add complexity, weight, and cost. It would also have to find a way to very, very rapidly change gear ratios in response. And it would result in very little benefit for all the bother. Slower boats like mine would see absolutely no benefit at all. Faster boats, maybe. Behemoth displacement vessels- Freighters and tankers- I have no idea. (I know better than to ask a naval personnel..... NOW. ) But the internal gear ratios changing would be far more complex and inefficient compared to a "constant speed" propeller like many airplanes use. Essentially has the same intent anyway. These attempt to keep the engine at constant RPM's (& usually power) by varying the load. They do this by twisting the blade to change the angle it "attacks" the water by hydraulic or electrical actuators controlled by an RPM governor. They work well for propeller airplanes big and fast enough to gain enough benefit to justify the added cost, maintenance & weight. So why don't we use those? A very few actually do. At greater expense, added weight & some (perhaps slight? If avoiding prop strikes, anyway) reduction of reliability. Here is where I need to jump off and allow the old salts take over, but this is a start.

 

[Winger Ed.]

Re: Why not a gearbox

Originally posted by Bearcat Powered:<br /> The prop already sits in a fluid, allowing it to slip enough for the RPM's to increase as the motor simultaneously increases torque, allowing for even better accelleration until the boat starts to settle into an equilibrium between drag and thrust.

Sir,<br />For those who can grasp these fundamentals, you've just said it all.<br /><br />The only thing I could possibly add is to insert 'applied horsepower' in the sentence somewhere near where you state "equilibrium".

 

[kenimpzoom]

Re: Why not a gearbox

I have always believed that a two speed gearbox on some heavier boats would allow a low gear for better holeshots, then a crusing gear for better economy. Shifting would be manual, or speed controlled.<br /><br />Most lighter boats wouldnt see any advantage.<br /><br />There are pitch shifting props that do the same thing.<br /><br />Ken

 

[BoatBuoy]

Re: Why not a gearbox

As to variable pitch airplane props, airplanes fly in a variety of densities of air, unlike water.

 

[jim dozier]

Re: Why not a gearbox

If you had an engine that developed maximum horsepower in a narrow range of high rpm, like say a high performance motorcycle, you might get some benefit with a shifter outboard. But you would be adding some weight so it wouldn't help small boats. Current engine rpm ranges don't really require it, though the range may go up in the future. When I was a kid 3 speed manuals were the norm for Detroit cast iron. I'm currently looking at a new car with a 6-speed manual, go figure. Gimme a 10,000 rpm direct injected Rotary outboard with a gearbox and I might be happy.

 

[JB]

Re: Why not a gearbox

The only things I can add to the excellent information above is that the power loss in a gearbox is significant.<br /><br />The engines would need to be redesigned to operate in a narrower rpm band to take full advantage of a multi-speed gearbox, too.<br /><br />The torque curve in modern outboard engines, including 4 strokes, is so broad that I see nothing to be gained from changing gear ratios.

 

[Chinewalker]

Re: Why not a gearbox

There already exists a propeller that acts as a "gearbox" - called the Torque Shift prop. I've never used one, but I know they're out there. They can vary between a lower pitch for acceleration and a higher pitch for top-end...<br /><br />Some composite props also have a shifting characteristic as the blades flex a bit on take-off, thus effectively producing less pitch/better acceleration, and once the extra load of acceleration is relieved, it returns to more-or-less it's rated pitch.<br />- Scott

 

[mattttt25]

Re: Why not a gearbox

we use variable pitch props on our naval ships. prop pitch changes from a max down to zero, and then to negative pitch, while the shaft remains constant in one direction. allows for amazing deceleration of very large, heavy ships.

 

[mjbrueck]

Re: Why not a gearbox

If you were thinking of something along the lines of an overdrive, so that you can cruise at 2000 rpm instead of 4000, it won't work. Your engine is (or should be) propped close to its optimum cruising efficiency. If you have a 100hp engine in a planing boat, you'll need 70 or 80 of it's hp to maintain your cruising speed. If you could shift to a lower rpm, your motor wouldn't develop enough power to maintain your speed. It's different than a 100hp car that may only need 15 or 20 hp to drive 60 mph.

 

[kenimpzoom]

Re: Why not a gearbox

My idea would be direct drive in low, then overdrive for high i.e. planentary gears. A Lenco transmission would work great. It would handle the power and have low friction losses.<br /><br />The difference in ratios would be small, just enough to reduce the rpms to get better economy.<br /><br />Lets say you had a prop pitched for 40 mph top speed at 4500 rpms (Inboard motor config). You shift into high, you would reduce RPMs to 4000 and get about 38 MPH. Your hole shot would be much improved, and I imagine your WOT economy would be much better.<br /><br />You could use it as a cruise gear. When cruising 20 mph at 2500 rpm, you could put it in overdrive and get 20 mph at 2000 rpm. You would need a motor with a very broad torque curve. I am thinking twin 454 Chevy inboards.<br /><br />But funny thing is, if it were such a great idea, why isnt anyone doing it?<br /><br />Two possible reasons, 1) no market demand 2)it wont work.<br /><br />Ken

 

[Scaaty]

Re: Why not a gearbox

Back in 1975 a dude in Wisconson had a blown BB in a Picklefork with a 2 speed...awesome!

 

[Dhadley]

Re: Why not a gearbox

There are lots of stern drive boats with multi speed transmissions. Mostly air entrapment type hulls.<br /><br />Some newer 4 stroke outboards have a gear reduction right under the powerhead as well as a gear reduction in the lower unit.