new Solas prop same dia and pitch lost 430 rpm and 4 mph

[Larry3215]

Boat is a 17' aluminum riveted weighs about 2400 fully loaded
Evinrude 3 cyl 70 HP 1986

Old prop was beat up aluminum 3 blade 13.25x17 with chunks missing and some dings etc but was giving us 5650 RPM on a new Tiny Tach and 35-36 mph GPS speed. Slip calculated to about 7%. This was an old style prop with the ring at the rear edge with a small gap all around between the prop body and the ring.

Bought a new Solas Amita 3 blade 13.2x17 aluminum and tested it out today. Now Im getting 5150 RPM and 31-32 mph on the GPS. Slip calcs out to the same roughly 7%.

I dont really care too much about the speed but Im worried the new prop will be lugging the motor too much. Should I look for a different prop or get this one re-pitched or nt worry about it?

Thanks!

Larry

 

[Larry3215]

Re: new Solas prop same dia and pitch lost 430 rpm and 4 mph

I also have a secondary question - why did this prop cause me to loose the rpm and speed? Is it cupped or does it have extra rake or is there some other factor that causes it to load the motor so much more?

 

[jestor68]

Re: new Solas prop same dia and pitch lost 430 rpm and 4 mph

All Solas props are cupped.

Was your OMC prop cupped?

What is the WOT operating range for your motor?

 

[steelespike]

Re: new Solas prop same dia and pitch lost 430 rpm and 4 mph

Rpm range is 5000 to 6000. Using a 2.42 gear ratio I get 3% slip at 5580 and 36 mph.
32 @ 5150 is a more believeable 6%
Did you make any other changes? Have you tried different trim settings?
How does it now respond to trim settings?
Its possible the new prop will allow raising the motor higher.
Could add some rpm and speed.

 

[Georgesalmon]

Re: new Solas prop same dia and pitch lost 430 rpm and 4 mph

When you tested the new prop was everything exactly the same? Weather, waves, people and fuel, cans of beer, etc. Loading and conditions can make the small differences you found. I would "nt worry about it" if it was me until you run it some more. Although getting up into the rpm band a little more won't hurt anything either.

 

[Larry3215]

Re: new Solas prop same dia and pitch lost 430 rpm and 4 mph

Thanks for the help gents! I'll answer the questions in order.

First, I goofed on the RPM in the first post. It was actually 5650, so Ive lost even more than I thought.

Im not sure if the old prop was cupped. The blades look flatter than the Solas blades though.

Nothing significant was changed between the tests. The boat was probably slightly lighter (1/2 tank of gas = 6 gals) when tested with the new prop.

Trimmed all the way in I was getting about 29 mph and trimmed out to just before proposing started was the 32-33 mph. It doesn't take much trim change to get there though. We moved weight forward which allowed more trim out but didn't effect speed noticeably.

The comment about the "believable" slip numbers reminds me that I suspect my Tiny tach may be reading a little low. If I substitute 10% slip the rpm calcs out to 5846 with the old prop and 5345 with the new one. (using the lower 35 and 32 numbers)

This new prop does seem to bite and accelerate better and it feels like we stay on plane better at lower speeds - although I havent really documented any of that.

Right now the ventilation plate is roughly 1" to 1.25" below the bottom of the boat when trimmed full down. So it looks like there is room to raise it up for sure.

Allowing for the possible/probable error in the rpm readings makes the new numbers more acceptable. We dont ski or tube with this boat so max speed isnt really an issue. Its strictly a fishing platform and we rarely even use full speed except for short bursts. Better cruising will off set the lower top speed I think.

Still, it sounds like a good idea to raise the motor and re-test.

Thanks again for the help!

 

[steelespike]

Re: new Solas prop same dia and pitch lost 430 rpm and 4 mph

Typically the vent plate ends up to an inch above the bottom.Sometimes this change can net a couple of mph.
Some times no change just the knowledge that your dragging as little as possible through the water.

 

[Larry3215]

Re: new Solas prop same dia and pitch lost 430 rpm and 4 mph

Thanks again. I'll give that a try.

 

[Larry3215]

Re: new Solas prop same dia and pitch lost 430 rpm and 4 mph

Wow! I got to test the raised motor today and what a change! I raised the motor 2 holes which put the anti-ventilation plate about 1" above the bottom of the boat.

Initial acceleration is excellent and there is no slipping at speed even in sharp turns. The boat feels like it stays more solidly on plane at lower speeds too.

Top speed went to 36 mph on the GPS at 5650 RPM. That calcs out to just over 4% slip. That seems like an pretty low slip number. I still suspect my Tiny Tach is off some. If I assume 10% slip, the RPMs calc out to 6013. Either way, I like these numbers much better.

Thanks for the advice and help guys!!

 

[Larry3215]

Re: new Solas prop same dia and pitch lost 430 rpm and 4 mph

I just read the instructions on the prop calculator again and noticed that Im supposed to add 1" to the pitch for cupped props.

Doing that, the slip calcs out to a more believable 9.5%, so maybe my tack isnt off so much after all.

 

[steelespike]

Re: new Solas prop same dia and pitch lost 430 rpm and 4 mph

Isn't great when a plan works, it seems your right on the nose.You might even note a little better mileage because it makes more speed with less throttle.