gearboxes on inboard engines?

[badman]

got a 140 bhp v8 car engine here which i wouldn't mind throwing into a boat for a laugh.
What is the situation with auto gearboxes and boats?
are they used? or are the gear changes not smooth enough for an outdrive... perhaps too 'jolty' at the gear changes?
I always thought that just using the throttle of the engine to create thrust was a bit like revving the guts out of it and perhaps gearing should be used?

Also, it wold need to be centre mounted in the boat with a prop shaft running to the out drive intermediate housing. Does this sound ridiculous or is it sometimes done?
cheers

 

[Bondo]

Re: gearboxes on inboard engines?

Does this sound ridiculous

Ayuh,.... For various reasons,.. Yes it Does....

 

[Bronc Rider]

Re: gearboxes on inboard engines?

Unless you were going to set up a manual tranny you would never get an automatic to downshift. In the end, it would be pure madness to stick a cars gearbox in a boat.

 

[Billdc]

Re: gearboxes on inboard engines?

Why? I am new to boating and can change the oil in my car and that is about it but why don't they have transmissions in boats?

 

[haulnazz15]

Re: gearboxes on inboard engines?

I have a feeling that the torque placed on the prop by the water would make shifting damn near impossible without grinding gears. The prop would slow down too quick to keep the synchros aligned. I wouldn't want to imagine the issues with an auto tranny. At some point, your prop would become inefficient spinning so fast in 3 or 4th gear, so you'd like be cavitating.

 

[guyaverage]

Re: gearboxes on inboard engines?

....but why don't they have transmissions in boats?

In very, very simplified terms, because cars require a lot of power to get them up to speed, and very little to keep them there. Boats require a lot of power to get up to speed, and a hell of a lot to keep them there.

Take your average car up to 50 mph, drop it into neutral, and you may coast for 1/2 mile before coming to a stop. Take a boat up to 50 mph, chop it back to idle, and you will be at a dead stop in 150 feet (these numbers are skewed, but you get the point).

You car needs 1st gear only to get rolling. Your boat needs 1st gear pretty much all the time.

 

[mkast]

Re: gearboxes on inboard engines?

Automotive transmission is used to keep the engine RPM low while keeping the drive wheels turning at a cruising speed. All you have to do to make this work is convince the propeller to act like the engine is turning 4800 RPM while it's really only turning 1800 RPM.
How fast is your boat at 1800 engine RPM?

 

[badman]

Re: gearboxes on inboard engines?

some bloody excellent replies there.
like it.
Ok, so it seems an automoutive engine and lets say, automatic gearbox in a boat would probably act like a car going up a hill, all the time, due to the massive friction involved between the hull and the water.
Perhaps even a steep hill.
My car weighs two tonnes. With its sound system and tool box, fluffy dice (heavy man) and two inches of farm clay on my spare boots.
I wonder what the equivalent weight of a 750 kilogram boat is in water at a given speed?
My truck tends to end up in third gear on a hill when aproached with some momentum. If i am going slowly and i am towing 2 or 3 tonne then second.....
But this is heavy oil, rapeseed oil or diesel which has that bottom end power,.

The engine i am going to play with is a petrol (less bottom end) but a V8.

Like one of you said, it will probably stay in first in a boat... perhaps jump to second when on the plane?

You know i might just have to find a hull and strap this lot to it just to see!

Have you seen the miltary amphibious vehicles?
The aerican one... DUKW has an auto gearbox which converts power from the engine to at least one, i think two props. I don't know what kind of ride it gives but i will ask on HMVF tonight.
Thew Alvis Stalwart is 9 tonnes but uses jets to force water in/out.... but i think it still uses the same transmission and engine.

Still think if there was enough momentum versus friction in the boat the auto gearchange wouldn.t be pleasant for splines, key-ways and ball gears.
nice one
matt

 

[Billdc]

Re: gearboxes on inboard engines?

Thanks for the descriptions. I had not thought of the constant pressure on the prop. Honestly I could have thought about that all winter and never came up with the right answer. I guess that is why I keep coming back here and reading threads that I know nothing about.