Gear box in boat motors

[polydriver]

This has been bugging me for years
Why dont boat motors have gearboxes similar to cars to reduce the motor rpm but maintain the prop rpm. Surely this would reduce fuel consumption. I would have thought that with todays technology an auto box would have been invented. I know there is a gear box of sorts.

Anyone know? would be interesting to find out. Have done a search but havent found anything on the net about it.

[Big_Kev]

Its called cost. The current gearbox as it is refered to is only a simple drive unit.
Stern drive legs do come in two speed coupled with a inboard engine.I once had a drive of a boat that had an electric button to up shift and was powered by a 390hp diesel pushing a 25ft hull.
The performance was awsome.
But the price of these units far surpasses the budget of the average boatie.
Beside that engines have been developed to perform very well and for a long time doing the job they do in current applications.
This is my insight toward your question anyway.
Cheers Kev.

[freefish]

Think it could have a bit to do with size. Small enough gearbox may not be tough enough.

[Big_Kev]

I don't think that size would worry to much as I think that a box could be installed in the current configuration without to much increase.
It would add some weight for sure. Up to 10kg I would think.
Maybe some clever little Jap engineering group will develop some sort of a variable V/belt drive set up that seems to be catching on in the car scene for the smaller rice burners.
Maybe this will be the advent of the next step in motor technology.
Cheers Kev.

[littlejim]

Other possibility is variable pitch propellor like aeroplanes. Remember the fixed pitch wooden ones?
Once again cost would come into the picture.

Until something affordable happens we're stuck in 1st.

[Zeeke]

you can get variable pitch props for boats ;) there is a propellar on the market that you take off and has a couple of bolts you undo and then you just slot the blades out and slot them back in on a different angle, also handy if you bust up a blade, you can replace the blade and not the whole prop


Tim

[Big_Kev]

Thats great Timmay, but what I think he means is a prop that actualy changes its pitch when the motor is working.

[blaze]

yep
I will have a couple Kev, even if they are prota types
cheers
blaze

[Zeeke]

oh.. nevermind then *crawls back to his cave*

Tim

[littlejim]

Zeeke,

no harm in putting it up. I'd never heard of it, suspect many others in the same boat(?).
Could be handy to know about of you are boating in a spot where prop busting is steady risk. have been on a river up near Childers where rocky outcrops pop up from the bottom with little or no warning.

[Oldyella]

I think it may be a bit more technical than just putting a gearbox on and dropping the revs of the motor. It takes quite a bit of horsepower to get a boat up to a constant speed and keep it there. Gearing would allow higher revs at the prop and lower revs at the motor. This may compromise torque ? You would think that if it was a benefit, then a big liner like the QE2 would have a transmission rather than direct drive.

[littlejim]

8055,

don't know the technicalities, but planes like the Hercules have 'constant speed' props. Turn the wick up, and the pitch changes to transfer the maximum power for that throttle setting. maybe someone with aeronautical knowledge could explain it further. Water and air are both fluids so you'd think that a similar application would work on a boat. maybe they have and found it is a WOMBAT. (waste of money brains and time. (ABC Newsradio wordwatch))
I'm surprised someone isn't playing round with it for marine purposes and telling us about it. Must admit the number of times the water is flat enough to change out of first gear can be counted with the ends of Kerry's double ended dipstick.

[aido]

thats a pretty good basic idea of what needs to be achieved.
in flight, engine speed is constant and if more power is required,
more fuel is added.
to extract this extra power, prop pitch increases to absorb it.
on the ground, low power is called for and in this case, the
prop pitch varies with power lever movement. (beta range).

i guess a constant speed prop on an outboard motor might be
of use if you want to stay constantly in the powerband, like
5,000rpms, lol.
for my liking the fixed pitch beasty does it just fine.

[Oldyella]

http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=17;t=037704

[littlejim]

How did you find that needle in the haystack.
Looks as though some other boaties want to get out of first too.