Choosing a prop

[captchuck]

This is more of a brain teaser than a problem but thought the answers might be interesting and informative. I am going to experiment the next time I get a chance to take the boat out but until I get some data let’s see what all your predictions (and reasons for) to this setup;

1) 11 ft home made racing runabout built this winter off plans from 1955 (Hal Kelly JINX). The boat including engine, driver and all rigging, ready to run weighs about 450 lbs.

2) 1974 9.9 hp Evinrude engine, stock, model 10424S

Here’s the question:

9 1/4 x 8 or 10 x 7 prop. Which one for speed and why? Any predictions on speed?

Both are stock Evinrude aluminum 3 bladed.

[dave-bernard]

with that motor I think 35mph won’t be fast enough to catch air to lift. you need a better motor with a race lower. I would say C class.

[outbdnut2]

If your motor has enough oomph to plane it off, you want the higher pitch prop (9-1/4 x 8) for max speed because more pitch = more speed, and on a small boat like that more pitch keeps the RPM from revving too much above the manufacturer’s spec., assuming it doesn’t load the RPM way down too much so it doesn’t plane off. Pitch is the prop’s second number, and it is how many inches the prop pushes the boat with each revolution, assuming ideal case – no slippage, etc, which never really exists in the real world. One inch pitch increase is like going from 3rd to 4th gear on a car.

A motor set up for racing would have a 2-blade prop with small diameter (the first number on the prop) both items reduce drag; along with high pitch, and a higher gear ratio (lower ratio number) to further transmit power with out over-revving. Racing versions also have no gearshift so the lower unit can be made really thin for less drag.

The site below claims 25 MPH with a 9.9, but I’d guess that’s for a 70 pound kid in it. It may not plane off with a 200 pound adult.

http://muskokaseaflea.ca/StaticWeb2/?page_id=259

It was designed to be a class B hydro, and that would be a 20 cubic inch motor (which your 9.9 is), with racing lower unit. putting out all it can – which would be a 15 HP OMC, not the 9.9. Thant’s still 20 cu in, but a bigger carb.
Dave

[captchuck]

First off I have always been under the impression the first number of the prop was the diameter and the second the pitch. ???

Secondly we have run it once with the engine and the 10 x 7 prop. Got 23 mph with me aboard. At 210 lbs I am why the boat weighs in at 450 and not lower as it should. I actually hope the 9 1/4 x 8 will give me the rpms to gain a little speed, provided it gets up on a plane.

And to Dave Bernard we have a carb from a 15 hp that will go on eventually. Too many projects right now around the house have priority.

[outbdnut2]

quote CaptChuck:

First off I have always been under the impression the first number of the prop was the diameter and the second the pitch. ???

.

You are right – a apologize – I must have been brain-dead when I wrote that!…..or one of those "Senior moments"?!

I will go back and correct that post.

Dave

[fleetwin]

Well, what is wrong with the original prop that came with the engine, which is a 10" pitch prop. 8" pitch is going to over rev, and the 7" pitch prop is going to be even worse. Yes, the acceleration/ability to plane the rig will be diminished with the 10" pitch prop, but I’m guessing this little boat is just meant for going fast with one person…
Do you have a shop tach so you can verify the actual WOT PRM with the different props?

[outbdnut2]

On my grandson’s hydroplane (which I drive too) I’ve had from 6 to 15 HP on it – all stock motors without special props. Due to the higher RPMs, I always go one heat range colder on the spark plugs (had to go 2 ranges colder on a 1953 8.4 HP Champion). I increase the oil mix too, like 20 or 25 to one with a 50 to 1 or 100 to 1 motor – actually I never run 100 to 1 in anything, even if that’s the spec and gentle use. Note that the Champion class B "hot rods" made for racing from the factory were spec’d at 4 to 1 gas/oil mix (that 4 is not a typo).
Dave

[thumper]

Get a 2 blade 13 pitch if it exists

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