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

[TimberWolf]

imagine the pilot on the intercom system,

i can't get the bugger into second


Milan.

They had gears in one of the Airplane! movies. On a spacecraft. Did they have trouble shifting gears? I think so, but don't remember exactly. Also they spelled "warp", "worp", which I thought was funny.

Anyway...

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 ?

I imagine the bother of using gears isn't usually worth it. With a car you can't floor the engine in 5th gear from a standstill without slipping and ruining the clutch because you're trying to accelerate a tonne of metal, but with a propeller you only have to accelerate a cross sectional area of water given by the area swept by the blades and a volume determined by their pitch, speed of rotation and the speed of the boat. I suppose it's possible efficiency gains are to be had that outweigh losses incurred in the gears though. Ditto for performance I expect. Perhaps instead of gears they design the engine and propeller such that performance is decent compromise in normal use. One might imagine a huge prop for example, too powerful for a smaller engine to turn without lower gears. I expect in practise this doesn't work out well.

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 ?

Doubt it. When the boat is moving the water will exert a force on the propeller making it want to turn, like a windmill in the wind, at the water speed. Though because the propeller is so relatively small the force on it may not be enough force to overcome transmission/engine losses.

[Peoplesoft bloke]

Not really. It is funny image you admit. Plus, aircraft don't use gears either, that I know of (excluding landing gear). The same principle as boats exaggerated.

Larger Prop (piston and turbo prop) planes have vriable pitch props though.

Smaller Jet engines (and especially apu turbines) can rev to several thousand RPM (as in 25-50K rpm)

The larger ones run at varing speeds with multiple stages

Apart from that, they're just like marine engines.

[Peoplesoft bloke]

Get on the roof of a train travelling at 100mph and try running forward.

On a line with a lot of bridges?

[scooterscot]

I used to work on the Navy's type-23 frigates - there's a whole room dedicated to the gear boxes. Unsurprisingly it's called the gear room.

The vessel is fitted with gas turbines you see, not a massive engine with pistons, which I fear is what your thinking.

If you ever get the chance to get in the engine room of a type-23 you'd think your at the airport.

[scooterscot]

Gosh - the ignorance in some of those posts is amazing, stay in IT people!

Have you ever tried turning a prop even at low revs in salt water? It takes a considerable amount of energy you know. The weight or volume of water that is displaced every second I think may surprise you.

[TimberWolf]

There's a chap here who reckons oars are more efficient than propellers.

[threaded]

Gosh - the ignorance in some of those posts is amazing, stay in IT people!

Yeah, they're making me to laugh muchly.

Some aircraft do have variable speed gearboxes.

There is at least one helicopter with two variable speed gearboxes, one for the main rotor and one for the tail rotor, and they're independent.

Have to say they're not very common due to Jesus bolt issues. But darned helpful if you want to extend that ceiling.

HTH

[milanbenes]

Zeity,

moi ?


Interesting though, a question which on the face of it looks silly, interesting that there does not seem to be a definitive answer.

Funny to see the argument about whether turning a prop in water is easier than a car wheel on a flat road.

I loved that comment, go and run through the swimming pool and see what happens.

Even the jury is out among the CUK experts.

Milan.

[TimberWolf]

Interesting though, a question which on the face of it looks silly, interesting that there does not seem to be a definitive answer.

Funny to see the argument about whether turning a prop in water is easier than a car wheel on a flat road.

I loved that comment, go and run through the swimming pool and see what happens.

You can see directly from the acceleration how great or little force the prop is generating (F=MA). For the most part boats accelerate relatively slowly, a small force.

[milanbenes]

TW,

'most boats' is rather a generalisation what

if we discard under powered fishing boats, marine contractors, ferries and cruisers,

the rest are fast acceleration and top speed machines, think about water skiing

Milan.