Question for the experts: Why don't boats have gear boxes

[milanbenes]

Afternoon All,

after going to the boat show at the weekend, one I've never understood,

why don't boats have gears,

I mean, you get in a boat which has an inboard, and give it some and all you are doing is revving the blx out of the engine, why can't boats have gears so that you can keep the prop spinning at the same speed, yet have the engine running at lower revs ?

Is it that if you tried to change gear the friction of the water against the prop would slow the prop down so much that the engine in a higher gear wouldn't be able to get the prop spinning again ?

Milan.

[d000hg]

I'd guess you don't have the same torque issues, you can't stall a propeller as the friction on it spinning is very low.

[VectraMan]

Why do cars have gears? It's because cars have lots of grip. You can't have the car wheels going twice as fast as the car (well you could, but it'd be dangerous and would kill your tyres). It doesn't matter in a boat, the water is effectively one big torque converter.

Also boats generally don't have to worry about hills.

[shoes]

I don't think the propeller is easier to spin the faster you go, there's less of a correlation there versus a wheel on tarmac. Consider currents etc.

With very big boats I think they do have a variable leverage system of some sort.

See, I am an expert.

[d000hg]

That's when the prop spins too fast, the question was about running the engine slower and gearing up the prop to run at the same speed.


I;d also guess marine engines run at much lower revs (maybe again due to no stall issues) so when you really hammer them, they are only at a normal range.

Note also, marine engines are normally designed to run for days or weeks non-stop. I don't think cars are that reliable.

[minestrone]

Momentum is not conserved well in a boat. The faster you go the more work you have to do displacing water, which requires more work. The faster you go the more power you have to use.

[milanbenes]

ok, first of all, VectraMan, focus on not grounding the Honda.

secondly,

when you ride on a boat you must have noticed how to go fast you have to rev the blx out of the engine

in a car, you keep taking higher and higher gears and getting the same speed or faster with less revs than the last gear

why don't boats have the same

maybe because, there is so much friction/resistance from the water that in between gear changes the prop would lose so much speed the engine wouldn't get it up again,

dooohg, clueless get, there's more friction than ever with a prop in the water


shoes, ok, I guess a CVT would be the answer

minestrone, yep


Zeity, nice coat that jacket that's when you have two props

Milan.

[Churchill]

Some screws(props) have a variable pitch.

[Mich the Tester]

Some screws(props) have a variable pitch.

Many props have an unusual itch, which is why I always wear a scrum cap.

[Churchill]

Many props have an unusual itch, which is why I always wear a dutch cap.

FTFY