squirt hitting rev limiter

Outboard designs up to 14'

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Hercdrvr
Posts: 346
Joined: Mon May 11, 2015 12:52 pm
Location: McKinney TX

squirt hitting rev limiter

Postby Hercdrvr » Fri Sep 18, 2015 12:03 pm

Just put on a new 20 hp Yamaha on Squirt. Hole shot is great, but at top speed engine hits rev limiter . Prop is a 10 pitch. I know I need more prop but the highest pitch I've found available is 11", not sure that will slow engine down enough. 4 blade maybe. BTW, she's running 28 mph coupe inches of throttle travel left but if I push it up any more engine hits rpm limiter
Prop experts wade in.
Matt

Hercdrvr
Posts: 346
Joined: Mon May 11, 2015 12:52 pm
Location: McKinney TX

Re: squirt hitting rev limiter

Postby Hercdrvr » Fri Sep 18, 2015 12:06 pm

Pic

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vupilot
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Re: squirt hitting rev limiter

Postby vupilot » Fri Sep 18, 2015 1:02 pm

That's a lot of engine for a Squirt. You may not be able to get a prop with enough pitch or enough clearance not to over rev on a boat so light. You could buy longest prop with the most pitch and have a prop shop put a little more twist in it but they can usually only do so much too. That's all I could think to try.

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galamb
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Re: squirt hitting rev limiter

Postby galamb » Fri Sep 18, 2015 8:42 pm

Michigan Wheel's Vortex line has props up to 14 pitch for Yamaha outboards (under 35 horses).

They are also available in 3 or 4 blade configurations.

So assuming the diameter is the same as your current prop (10 1/8") - because you can also increase/decrease rpms by changing the diameter (everyone knows about pitch effects, but diameter has the same effect), if you went from your current 10 pitch to a 14 AND went from 3 to 4 blade you could drop your rpms by as much as 700 rpm, which is "significant".

When you are trying to "tweak" your rpms using a prop to do so, some general rules of thumb are:

One Inch of Pitch = 200 rpm difference

One Inch of Diameter = 200 rpm difference

3 blade to 4 blade (or vice versa) = 100 rpm difference

Cupped blades = 50 rpm "loss" compared to non-cupped

Composite props (plastic under a number of patent names) slip more than Aluminum, which slips more than Aluminum alloy, which slips more than Stainless Steel.

The easier a prop slips (fails to really bite the water due to the material it's made from - so it "bends" ultimately reducing it's effective pitch) the higher the resulting rpms all else being equal - so while I have no actual measured data on it, a stainless prop should reduce your rpms a little (20-50 rpms would be my guess) compared with an aluminum prop - all else being equal.
Graham

Yes, Plywood is "real" wood :)

A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)


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