"rvbilly" wrote in message
oups.com...
Hi I am building a little 11 ft. V bottom plywood epoxy speedboat, I am
really interested in installing a 250cc dirtbike engine in it for
propulsion, I would have to use a stuffing box and shaft supports and
rudder, but I live in Chilliwack BC Canada and nobody locally can
supply parts,has anyone ever done such a install? The engine (dirtbike)
would have about 20 to 40 Hp. be watercooled with a small rad, I was
thinking I could use some kind of tranny with F,N,R from a lawn tractor
to obtain reverse in the boat, I know people have told me to use a
outboard but this is a project and I`m going to make it all work one
way or another, any help would be greatly appreciated.



You stated" I could use some kind of tranny with F,N,R from a lawn tractor"
You need to research the function of a 'thrust bearing' and either add one
to your lawn tractor tranny or use a marine reduction gear which has one
incorporated into the design.

Hi, I am not following what you mean with with the thrust bearing, and
what would be the advantage of the reduction gear? thanks Bill

On 30 Oct 2005 13:20:29 -0800, "rvbilly" wrote:

Hi, I am not following what you mean with with the thrust bearing, and
what would be the advantage of the reduction gear? thanks Bill

It is a strange thing when you first hear of it: but the same HP can
give a few pounds of thrust at high boat speeds and hundreds of pounds
of thrust at slow speeds.
The latter place is where marine transmissions live - so they need
buff thrust bearings if they are to last any time at all.
Water props get inefficient at high speeds.

Brian Whatcott Altus OK

Horsepower is the rate of doing work. Why is it surprising that a small
force at high speed is different than a large force at low speed? It's how
the small displacement car engines at 9,000 rpm are able to compete.

Roger

http://home.earthlink.net/~derbyrm

"Brian Whatcott" wrote in message
...
On 30 Oct 2005 13:20:29 -0800, "rvbilly" wrote:

Hi, I am not following what you mean with with the thrust bearing, and
what would be the advantage of the reduction gear? thanks Bill

It is a strange thing when you first hear of it: but the same HP can
give a few pounds of thrust at high boat speeds and hundreds of pounds
of thrust at slow speeds.
The latter place is where marine transmissions live - so they need
buff thrust bearings if they are to last any time at all.
Water props get inefficient at high speeds.

Brian Whatcott Altus OK

"rvbilly" wrote in message
ups.com...
Hi, I am not following what you mean with with the thrust bearing, and
what would be the advantage of the reduction gear? thanks Bill

When you turn the prop (pushing water aft) the shaft is forced forward. This
forward thrust must be transmitted to the boat through a thrust bearing:
http://science.howstuffworks.com/bearing3.htm

The reduction gear takes the higher input rpm of the engine and reduces it
down to a usable rpm so the prop does not cavitate. It also allows F N R.