30hp etec prop opinions

[kerry]

quote johnyrude200:

I hadn’t even thought of that, good point with planing vs top speed. I have my motor set with the ventilation plate about 1″ above the bottom of the boat. I’ve set most my motors up this way and it seems to give an extra 2-3 MPH once up on plane. You think this would help avoid the extra drag off a hydrofoil that you are talking about?

I also checked my prop – it is a 10.3 x 12. So switching over to a SST 14 or 15 pitch, I would think, will make a pretty noticeable difference in speed? I top out at 27-28MPH, going 31-33 would be a pretty big difference!

Its possible the height will be enough to overcome the drag issue. Is the hull at the transom flat, like most lumies? Do you have the possibility to raise the motor a bit more if needed? If both answers are yes, you have rested one of my two hates with fins. As for the prop, we still need to know the RPMs at full throttle with your 12" so we get it right.

If you have too many, AND not enough, you're a collector.

[johnyrude200]

The bottom of the hull at the transom has a slight V to it; the boat is only 6 feet wide, and has 1 inch of taper to the V at the transome per foot (seems to be a typical taper for these small starcraft boats). The middle of the boat has a small keel (about 1") so that it tracks better.

The motor has PT/T, so trimming it is a cinch. Usually what happens is after I get up on plane I trim the motor 2-4", otherwise it splashes water back (not into the boat). Once trimmed, I gain 2-3 MPH as it holds the bow up a little better. I’m constantly changing trim based on water conditions and chop to help the ride be a little smoother for me and my passenger at the bow (who’s on a roller coaster ride if it’s rough out). The angle of the bow as I’m fighting through waves makes a big difference.

I can hook up a tach to my motor and run it at WOT on smooth water (I live on a very calm water body) and get us some numbers.

WILLG – what is the optimum RPM at WOT for these motors…5800 RPM?

[willg]

Joel, the range is 5500 to 6000 so 5800 would be fine.

Evinrude also makes props up to 15 pitch in aluminum for your engine as well.

http://www.google.com/url?sa=t&rct=j&q= … 4aDnJIWVBg

The link above will open a PDF of Evinrude’s prop selection guide. Page 14 of the PDF lists props for the 15HO-30 etec.

[johnyrude200]

I used a tach on the motor today, and ran it through about 10 trials of WOT for a 1/2 to 3/4 stretch of water. I found that it was running between 5820 and 5880 RPM, depending on the typical load and configuration I use it in (1 passenger, plus 45lbs of ballast – lost 60RPM with the extra ballast).

Given what I’m reading I’m guessing the highest pitch I should aim for would be a 14" pitch (currently have a 12" pitch).

Opinions? From what I’ve been reading, I’m seeing that for each 1" of pitch you add, you lose 150-200RPM.

What is the effect of diameter as well. I mean, the stock prop on this motor is a 10.3 x 12". The props I’ve been looking at are 10"…so does a 1/3rd of an inch diameter decrease negate some of the RPM loss?

[johnyrude200]

So I found a 10 x 13 prop that I ran on the motor, gained 2 MPH, but also noticed an INCREASE in RPM from my test run with the 10.3 x 12. Load was comparable on both trials, same body of water, same water conditions.

I’m planning on bumping up to a 10 x 14, but was wondering if anyone had an explanation for the slight increase in RPM (I’m thinking that 1/4 inch decrease in diameter?).

I clocked the motor on 6 test runs at 5780-5810 with the 10 x 13. It was an aluminum too. Will and Fleetwin – I’m going to stick with an aluminum prop as per your recommendations for long-term gearcase health.

[fleetwin]

Well, it doesn’t really make sense that the WOT RPM would go up with an increase in prop pitch, unless you can trim the engine out farther with the 13" prop due to more cupping, which I doubt is the case.
The only other thing you might have to look out for here is a rev limiter built into the EMM. I’m sure there is one, but not sure what RPM it is set for, Will can probably access this information.
PS- Don’t get bogged down with the prop diameter, that is designed/engineered by the manufacturer, just worry about the pitch.

[johnyrude200]

Will mentioned that the motor has enough ‘kutz-pah’ regardless of pitch/diameter to make sure it is turning the right RPM. So maybe the EMM just plain gives out more RPM based on load x prop…I’m guessing…some SENSOR reads?

Am I talking out of my arse here?

[Author]

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