VetteSteve

I had a new 540 Dart engine built by XXX and after 19 hours it blew a whole in the cylinder head due to low fuel pressure from the builder install fuel pump. Rudy Dryden in Wilmington, DE rebuilt the engine with corrections and it is running great. I now have more power, which is leading me to my prop question.

Pryor to the rebuild I had tested a 26” and 28” Bravo 1 props but now with more power I am wondering if a prop change is in order. I discovered the tach reading was incorrect and installed a new tach and checked it with my digital gauge. So my RPMs are correct and my speed-readings are from my GPS.

The boat is a 27’ Fountain Fever with a single step. The prop is a 28” Bravo 1 that was lab finished by BBlades. For maximum speed I have to trim out to about 6+ on the trim gauge. The hull runs free and handles well. It will run 85.5 mph at 5700 rpms. I do not know the sweet spot for the engine. Rudy had told me that it could run 6200 rpms without any problems.

Online using Mercury Racing prop slip calculator I get a slip of 12% with my current setup.

So I changed figures in the calculator using different pitches and rpms to see how the changes would affect the maximum speed. Using the 12% slip with a 26” pitch there would need to be at least an increase of 300 rpms to have an increase in speed.

I am not sure if the prop slip would change using the 26” pitch therefore receiving an inaccurate MPH reading??????

Is a 12% slip with my current setup the best I should expect or should I be looking for a change??

Thanks for your input?

COME ON SPRING!!!

Griff

5700rpms is probably about where you'd want to spin it. I really don't think you will gain any mph by spinning it faster with a smaller prop. Actaully, you might be better off gong the other way and going to a labbed 30. No way to really tell without some dyno numbers. If its a hyd roller cam, I wouldn't spin it any faster than you are now. Its harder on the valve train.

Philm

That sounds pretty good to me. Sometimes slip will change with changes in pitch, but usually only once you get to 32p and higher.

Rudy is good people, and builds a solid motor. He rebuilt my 598 N/A last winter and did a hell of a job. I guarentee that he used the same hydraulic roller lifters in mine as he did in your motor, and I spin 6000rpm all the time.

VetteSteve

Philm - Is your boat all white and did you go on the Hap run last year? If so I was hanging out with you at Lloyds Creek that afternoon. I was at Rudy's shop yesterday and he did mention your boat.

Philm

No, that is Jdub (john). Mine has the blue and white graphics on it. I am sure I will see you around at some point.

ezstriper

I'm thinking like griff, try a labbed 30..I think you are on the edge of where you want the max RPM, worth a shot..

bbladesprops

We have done many 27 Fountains. Contrary to some opinions, your slip number is right where it should be for that hull. I agree the rpm number sounds good to me as well. But as mentioned prior, a dyno sheet and knowledge of all the internals will determine whether to go in pitch. On the surface, because your slip is good, less pitch will add acceleration and more pitch will add cruise speed and cruise economy with the top speed staying relative with each change (give or take a mph).

Yes going up in pitch can occasionally decrease slip due to the bigger bite of water. Also going down in pitch numerous inches can change slip due to the decrease in stern lift.

It looks like your good where you are now.

Brett

VetteSteve

Thanks all for your responses.