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

[d000hg]

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

Learn some physics, or at least common sense, you tool. You try turning a propeller in water by hand... easy, the water moves out of the way as the prop is essentially 'skidding' on the water. Now try turning a car wheel while it's touching the road. If you turn off a boat engine, the prop slows down as you watch, it doesn't stop instantly.

Water is less friction than tarmac.

Idiot.

[d000hg]

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.

How's that different than a sports car, air resistance is by far the biggest factor above about 100mph. Above that speed, cars conserve momentum about as well as a boat does at 10knots.

[shaunbhoy]

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, Milan.

I suspect you will find that when you were doing that, the boat you were on was mounted on a plinth of some sort, nowhere near any body of water.
Did some burly men remove you from the Boat Show shortly thereafter whilst fretting First Aiders collected small weeping windswept children from the sides of the arena?

[threaded]

Afternoon All,

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

why don't boats have gears,

I've read this thread with much amusement.

Not being an expert, but having some experience in the issue, the answer is:

Some boats do have a multispeed gearbox, usually electrically operated, and a clutch too.

HTH

[Troll]

somewhere here?

[milanbenes]

doooogh, you clueless get,

when you stop push power to a wheel, it will continue turning assuming the road is flat, in this situation the car will be coasting

stop applying power to a boat propeller, and see how fast the boat stops

now, who did you say the idiot was ?

maybe you've never seen a prop slow down when the power to it is stopped, it's quite simultaneous

still me old fruit, ignorance is bliss eh





Milan.

[milanbenes]

thanks Troll.

Doogh, you'd do well to read the discussion on Troll's link.

toodle-pip,

Milan.

[threaded]

For similar reasons motor-yachts also have gearboxes.

With an engine you normally have two (main) sweet-spots: the most efficient (distance travelled for fuel used) and the top speed (the point at which if you rev any higher you'll have cavitation + a load of other stuff to do with drag and chine positions (which can give you other sweet spots)).

With a gear box you can effectively bring these two sweet-spots together in the rev range of the engine thereby increasing the efficiency of top speed.

[d000hg]

doooogh, you clueless get,

when you stop push power to a wheel, it will continue turning assuming the road is flat, in this situation the car will be coasting

stop applying power to a boat propeller, and see how fast the boat stops

now, who did you say the idiot was ?

maybe you've never seen a prop slow down when the power to it is stopped, it's quite simultaneous

still me old fruit, ignorance is bliss eh





Milan.

Sorry, you're talking tripe. How fast does a car wheel stop when it's not rolling along a road but held in place. e.g put your hand-brake on, spin the wheels and take your foot of the accelerator. You stall instantly. That's the comparison here, the rotating part's interaction with the thing it pushes on. A prop spins in water in the same way a car wheel can skid, like continual wheelspin. But it does this without much effort at all.

Try starting your car in-gear, does it manage it? Now try starting a boat engine in gear.

[d000hg]

thanks Troll.

Doogh, you'd do well to read the discussion on Troll's link.

toodle-pip,

Milan.

It's just another bunch of uneducated folk, why would they know better that CUK? Other than the noteworthy mention of a reduction gear, which is not really relevant, I saw nothing on the first page other than "some engines do, most don't".
I don't think the original question was "are there any marine engines with more car-like gearboxes", it was why most don't.