Boat propulsion

[dassman]

Can someone explain why boat propulsion engines do not have a gear box with changeable gears? A car left in 1st gear will accelerate to a speed relating to its highest recommended rpm, say 6000rpm. If we now change to 2nd gear the rpm will reduce and the speed will increase, why does a boat propulsion engine have to stay in 1st gear!

 

[tico]

simply put, it doesn't. It stays in top gear. Thats why at WOT the revs should just reach max power.
The difference is the the water acts as a slipping clutch to allow the revs to build.before getting to terminal velocity where slip is at a minimum.

 

[STEVEDUNSTABLE]

simply put, it doesn't. It stays in top gear. Thats why at WOT the revs should just reach max power.
The difference is the the water acts as a slipping clutch to allow the revs to build.before getting to terminal velocity where slip is at a minimum.

if thats the case, wouldnt it be possible to fit "overdrive" ??

 

[sarabande]

if you did that, then the prop would be turning too fast to grip the water properly, and would be always 'slipping' If you watch the wake when you open the throttle very quickly you'll see that lots of water just gets squirted up above the surface . It doesn't really 'work' effectively against the relatively static water until boat speed increases to link more closely with the backwards going prop water.

 

[epervier]

if thats the case, wouldnt it be possible to fit "overdrive" ??

Well,.................yes it is, but at a price

been around a while but I guess you can still buy it

http://www.stepdrive.co.uk/sd/MBY Tony Jones.pdf

I have this same boat, but with two engines and often wondered if it could recoup it's initial cost by fuel savings, but even at 2001 prices (as in the article) 5k today would buy a great deal of fuel.