height of prop...

[rayiow]

i have a chrysler striper with a 50hp force us marine(model-507F9B s/n 1254).I have just taken the motor off as i am replacing the transom due to rot and noticed a 1" block of wood on the engine bracket(between transom and mounting bracket)is this bit of wood important and would that extra 1 inch make much difference?.(transom depth is 20.5 inches,engine bracket to centre of prop is 28 inches i also have hydro fins on.is there an ideal depth that the prop should be at to maximise performance?

 

[OBJ]

Re: height of prop...

Ray...the block was there probably to get the engine up a little. The general "rule of thumb" about engine height, refering to the anti-cavitation plate (that's the plate right over the prop)...it should be no more than an inch below or an inch above the keel of the boat. Preferably one inch above. Yes it does effect performance.

 

[rayiow]

Re: height of prop...

thanks OBJ i shall put it back when i reinstall motor.

 

[KCLOST]

Re: height of prop...

It really all depends on your boat set-up as to how far up the motor can go. There is a pressure wave on every boat that affects this as well as how the boat performs in turns, WOT (does it chinewalk), and your rpms at WOT...<br /><br />It's possible to go even higher than 1", but the key is not to loose water intake when the motor is trimmed out to max and also not to loose prop bite on the turns. Raising the motor up on the transom, will increase spped, increase rpms, and reduce chinewalk if you have that problem.<br /><br />My set-up is currently at the max as I'm able to loose water pressure at Max tilt. The plate is about 2" above the bottom of the hull, however it is a stepped hull which will raise up the pressure wave more. This was necessary for me to take care of a chinewalk problem that is now gone.

 

[rayiow]

Re: height of prop...

with the block in place my hydrofins are about 1.5" below the hull and as i am replacing the wood in transom now would be the time to re-drill mounting holes higher in transom as there are only two settings in the engine bracket and it was on its highest setting.with the fins being so low in the water would this explain why the engine does not appear to rev as high as it should(no rev counter)so not sure on actual revs just going on my mates 30 mariner which sounds alot higher in revs(but i will wire up a rev counter),although i get 25knots out of it,the hull being a cathederal should i be getting more knots?(the boat is a chrysler striper 15ft s/nCB057738M79D).would it be best if i made three settings in the transom the middle one having the hydrofins level with the hull thus allowing me to go 1" higher if needed.

 

[KCLOST]

Re: height of prop...

Yes it would... If you can find an affordable jack plate, now is the time to install it. That will make the height adjustments very easy and also allow you to raise the motor even higher since it is located back further... Hence even better performance.