# Prop math 2.0

Discussion in 'Props' started by CBTerry, Nov 23, 2018.

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### CBTerryBetter, is the enemy of good enough.

So I have joined the cult of cherubini and purchased my second 30-foot Hunter. I am mounting a 9.9 Johnson sailmaster on her backside and I purchased a 4 blade prop measuring 10.2 in in diameter and 5 in of pitch. It is one of those four bladed Mickey Mouse ear affairs. I reckon the hull speed is about 6.6 knots or so. The math came out a little bit higher than that but I like to round down just a bit so I will call it six and a half Nots. Please forgive the misspellings as I am using voice wreck ignition.
5 / 12 is equal to about .417, which is the pitch in feet.
The next figure I do not assign too much Credence to as it comes from my addled memory. Seems I read such a prop would be about 85% efficient.
So .417x .85 = about .354, which would be the theoretical movement, in feet, after taking inefficiency into account. Not wanting to rev the motor hi for a sustained amount of time I picked 5300 RPM for max.
5,300 x .354= about 1,872, which would be feet travelled in one minute. 1,872x60=112,320 feet per hour...divided by 5,280 = 21.27MPH.
I am sure many others have been sucked into the Vortex I am currently in, in which one goes over there math over and over again, all the while not seeing the plane mistake that they have made. Obviously the boat is not going to go anywhere near that fast but it certainly seems fast for a 5-inch pitch. Help!

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### ondarvrSenior Member

You need use the RPMs the motor is rated for, for longevity of the motor and best performance you need to hit that RPM or be a bit higher at wide open throttle.

The lower unit has a gear reduction of probably 2:1 or a bit less, so with this gear reduction and the amount of slip at low speed you're going to cut your number in more than half.

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### CBTerryBetter, is the enemy of good enough.

Ahhhhh, I forgot about the gear reduction! It is 2.42 /1 for this motor. Dividing my previous number by 2.42 yields about 8.7 mph, which is just above hull speed. Perfect. The motor and the prop have been in storage for a year-and-a-half and I had bought the prop for a different 30 foot boat and thought I had done the math and that that was the best prop and so now it seems that is correct. The rated RPM is not a specific number, but rather a range. I have read the 9.9 being rated between 5000 and 6000 as well as between 4500 and 5500 RPM. The 15, using a larger carburetor initially and then a different exhaust and reed configuration in later years, is rated between 55 and 65 and supposedly as high as 7,000 according to one piece of sales literature. I am comfortable with 5300 and if it did not go above 5000 that would be fine with me. They get rather loud and it is an older motor so I don't feel inclined to rev it all that much. Thank you for bringing the missing piece of the puzzle to the game. Any comments on 85% efficiency/slippage ?

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### ondarvrSenior Member

If you don’t get the RPMs up into the correct range at wide open throttle it can be very hard on the motor, lugging even a new motor will destroy it.

It doesn’t mean you need to run it wide open at high RPMs, just that it needs to be able to reach those RPMs for good health.

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### CBTerryBetter, is the enemy of good enough.

I am familiar with the proposition. If I had so much pitch on it that it would not surpass even 4,200 RPM at Full Throttle, then rings/ringlands and such are potentially injured. Getting to 5000 or 5300 is plenty close enough and within the range at which it is rated (at least according to One Source). Another thing I considered is that non-ethanol gas that I purchase is rated at 90 octane which is superior to the requirements of mid-80s motors. Detonation and pre-ignition are the primary killers of lugging an engine (carbonizing a close secound) and not as likely to occur with a superior octane rating. Besides, throughout my searching I found no prop with a pitch less than that which I purchased. If I need more horsepower I have a 15 carb at the ready to install.

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### ondarvrSenior Member

At those speeds there will be far more slip, so pitch isn't quite as critical.

Last edited: Nov 26, 2018
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### philSweetSenior Member

What you are calling efficiency is actually called the advance ratio. The *efficiency* of that setup would likely be below 40%. I tend to use absolute slip for these kinds of speeds. An absolute slip of 13 to 17 ft per second would be typical of full throttle condition for a 2.4 reduction.

So 6000 rpm * .417 ft/rpm / 2.42 / 60 sec/min = 17.2 ft/sec. If I subtract my middle figure, you are going 1.3 knots. Pushing into a stiff breeze, you'd be lucky to manage that. I'd bump it up to an 8 pitch. You could probably still hit the powerband tied to the dock. The only downside is backing up. The flatter prop will provide better handling in reverse. If that's important, maybe go with a 7. Typically, a four bladed prop makes more sense in the over-square pitch ranges. In flat props, smaller, fewer blades are usually better because of blockage. At a .5 pitch ratio, you really want to be looking for a 2 bladed prop of very modest blade area. A comparable saildrive prop would be 14 x 7 and two bladed. You don't have that kind of diameter, so more pitch and more slip and more blades, but three is plenty. You want to be able to hit the power band.

The bigger carb would do no good whatsoever. It is only of use on a faster boat where you can deliver the power at high rpm and high pitch and not cavitate. You have more power than you can use. Your problem is getting it to the water at low speed with the prop bobbing up and down on the back of your boat.

8. Joined: Jun 2017
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Location: cocoa beach, planet earth

### CBTerryBetter, is the enemy of good enough.

Most All the props and new outboards sold as "high thrust" seem to have 4 blades and modest pitch. In certain Seas it may pull out (exceptionally unlikely on the intercostal Waterway) but it is a 25" shaft motor, mounted very low on the transom.
I've Never seen a two blade for an old Johnson.
I got the 5" pitch 10.2"D
Prop, and in two (ish) weeks when I return home I'll post speed and rpm numbers. I bought a tic tac rpm meter, occasionally referred to as a tachometer. I may purchase the same prop, SOLAS, in 7"pitch just to do a comparison. They are only 70ish \$.
I will bet you a cocktail, payable the next time we bump into each other at The Beach Shack in Cocoa Beach, that the boat goes twice as fast, at least, as your prediction of 1.3 knots.... and @5,300rpm

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### ondarvrSenior Member

Yes, the selection of props for those motors is very limited, plus they aren't that expensive.

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