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.

"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

"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.

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

In my younger days, when I had more time on my hands and little money, I use
a vertical shaft lawn mower engine and the lower unit off a old Johnson
Seahorse.

I mounted the engine on the keelson with a the shaft going through to the
lower unit, which was bolted to the bottom of the boat. I was using the old
outboard shaft and it ran through a home made stuffing box and a second hole
for the fwd/rev shifter shaft.

It was quite a deal and complex to operate in a 10 foot dingy.. the engine
being between the steer seat and the middle thwart. Lot's of heat on my bare
legs.

Steering was done with the normal sailing dingy rudder.

Had to be careful when running her up on the beach or in shallow water.

Sure was fun to be 'young and dumb and ....'

Steve
s/v Good Intentions


"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.

Hi guys, would it be possible to mount the thrust bearing on the prop
shaft close to the transmission ? I would like to use a prop from a 25
to 40 Hp outboard with a pitch of about 14, will that be to much prop
for the motor as it will have somewheres in the neighborhood of 20 to
30 Hp, it will make this power around 8,000 to 9,000 RPM, I want to use
simple chain and gears to reduce the input(3-1 roughly) to the input of
the tranny, I think with 3-1 it should give a good holeshot. Can the
exhaust be run through a muffler and about 5 ft of pipe into the water,
any chance the water might back up into the engine ? the boat should
weigh in around 650 to 750 lbs fully loaded ( hull,motor,gas,2 adults)
I`m hopeing to achieve the 30 to 40 mph mark, Glen-L has a speedboat
called TNT, it`s the one I`m baseing my modified plans on,it will do 37
mph with a 25 hp outboard pushing it, you guys are really helping me
lots with your feedback and I apreciate this, I`m a little scared of
setting up this engine only to find out I can only get say 20mph, the
specs on everything seem good and possible but theres allways the
chance that I should have say mayby used a little little motor such as
say a 3 cyl. Geo Sprint or 1200cc Toyota,what are your opinions on
this,, thanks Bill--How do we upload pictures into these posts?

rvbilly wrote:

I`m a little scared of setting up this engine only to find out I
can only get say 20mph, the specs on everything seem good and
possible but theres allways the chance that I should have say mayby
used a little little motor such as say a 3 cyl. Geo Sprint or 1200cc
Toyota,what are your opinions on this,,

I don't have it at hand now, but I remember Dave Gerr's book "The Nature of
Boats" gives quite simple calculations to estimate how much speed you
should be able to get out of your boat. Check it out!

- Heikki



P.S.
How do we upload pictures into these posts?

We don't. Many news servers strip them from all newsgroups not specially
marked as binaries. Better find a website where you can post your picture,
and send a link here.

"Heikki" wrote in message
. ..
rvbilly wrote:

I`m a little scared of setting up this engine only to find out I
can only get say 20mph, the specs on everything seem good and
possible but theres allways the chance that I should have say mayby
used a little little motor such as say a 3 cyl. Geo Sprint or 1200cc
Toyota,what are your opinions on this,,

I don't have it at hand now, but I remember Dave Gerr's book "The Nature
of
Boats" gives quite simple calculations to estimate how much speed you
should be able to get out of your boat.


How much speed will this motor get?
http://www.weldingfaq.com/turbine2.asp