Motor height?

[aborgman]

I'm in the process of setting up the Mystery Boat (http://forums.iboats.com/showthread.php?t=307360) and I'm wondering about motor height.

Motor is a 1961 Johnson Seahorse 40hp (RDS-23).

I know generally you'd like to have the AV plate about even with keel.

I know with many older motors you need to run them deeper so they can adequately pick up water.

I know the current motor position (as low as possible) puts the AV plate about 2"-3" BELOW the keel.

I know there is only room on the transom to raise the motor maybe 1.5", and that much would be dodgy because the clamps would be right on the upper edge of the transom. Also note that this motor transom clamp is the two hand screws only - no way to actually bolt it to the transom.

Any thoughts? Leave it where it is?

--
aborgman

 

[Sandhammaren05]

Re: Motor height?

Use 1/4"-5/16" aluminum plates (and stainless 5/16" bolts with fiber locknuts) with wood between the plates above the transom, to build the transom height up to 17.5". That will get the cavitation plate above the bottom of the boat, above the water. No problem picking up water, and cup the prop a bit as well. Unless the boat bottom's badly hooked (and most are), the major drag is due to the gearcase submerged in the water. With the same waterpickup, I ran a 1972 25 Johnson at 19" transom height on a very fast 15' boat and still picked up water, but that boat did have a radically offset transom.

I'm in the process of setting up the Mystery Boat (http://forums.iboats.com/showthread.php?t=307360) and I'm wondering about motor height.

Motor is a 1961 Johnson Seahorse 40hp (RDS-23).

I know generally you'd like to have the AV plate about even with keel.

I know with many older motors you need to run them deeper so they can adequately pick up water.

I know the current motor position (as low as possible) puts the AV plate about 2"-3" BELOW the keel.

I know there is only room on the transom to raise the motor maybe 1.5", and that much would be dodgy because the clamps would be right on the upper edge of the transom. Also note that this motor transom clamp is the two hand screws only - no way to actually bolt it to the transom.

Any thoughts? Leave it where it is?

--
aborgman

 

[aborgman]

Re: Motor height?

My preference would be to keep it as original as possible...

What do I stand to lose by leaving it more submerged? Obviously drag is higher - and may require me to go to a lower pitched prop to get up to correct RPM at WOT - but other than the performance hit, is there any issues?

Any good reason to raise it in the manner you suggested as opposed to just going with a jack plate?

--
aborgman

 

[steelespike]

Re: Motor height?

That 1 1/2 inches could get you 1 to 2 mph. under ideal conditions.
with a slight improvement in economy.If you can get 2 to 3 inches 2 mph would be cake and maybe more with an rpm improvement making a possible prop change that could get even more improvement in speed.The faster you go at a given throttle setting the less gas you use per mile.

 

[aborgman]

Re: Motor height?

That 1 1/2 inches could get you 1 to 2 mph. under ideal conditions.
with a slight improvement in economy.If you can get 2 to 3 inches 2 mph would be cake and maybe more with an rpm improvement making a possible prop change that could get even more improvement in speed.The faster you go at a given throttle setting the less gas you use per mile.

So we're talking a really minor improvement then - in the 10% or less range.

I'll probably leave it as is for this summer then, and worry about making a change when I re-do the transom this coming winter.

--
aborgman

 

[steelespike]

Re: Motor height?

That probably makes sense.In rereading your post if it's really 3" below you could end up gaining close to 4 inches.If it also allows a prop change.
you may have a surprising gain.If the prop needs a tune up get it done and give us the test run results.Prop shop can tell what size it is. Give us the prop size,max rpm and speed.