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  Prop and Motor Height for 50HP Honda on Thirteen Footer

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Author Topic:   Prop and Motor Height for 50HP Honda on Thirteen Footer
Nate posted 04-20-2004 01:51 PM ET (US)   Profile for Nate   Send Email to Nate  
I have just replaced the 1987 Johnson 50 HP on my 1968 thirteen footer with a 2003 Honda 50 HP. The Johnson ran a twenty-one pitch OEM stainless prop which it pulled to 5200 RPM. This motor never slipped or cavitated the prop under any circumstances. Cavitation plate was even with bottom of transom. The boat was returned to me with the motor mounted about 1/2" too low. Using a Honda OEM stainless prop, the propeller slipped a lot just as the boat was leveling onto plane. I raised the motor to the same height as Johnson and performance was better but, propeller still slips at same point. There is also a spray at high speed that appears to be from cavitation plate suggesting to me that motor should still be raised more. This setup pulls an 11X15 prop to 6700 RPM (rev. limiter) and a 10 3/4X16 to 6000. I am wondering if something in the way of an aftermarket prop might be required to solve the problem and what optimum cavitation plate height should be. Any help much appreciated.

Nate

BQUICK posted 04-22-2004 03:00 PM ET (US)     Profile for BQUICK  Send Email to BQUICK     
I'm confused.....why are you raising the motor if you have cavitation?
Nate posted 04-22-2004 05:31 PM ET (US)     Profile for Nate  Send Email to Nate     
Cavitation can be caused by anything that creates air pockets in the vicinity of the propeller. A cavitation plate that is to low can create turbulence as it drags through the water rather than riding at the surface. This turbulence could cause air int he vicinity of the propeller. My understanding is that a cavitation plate would never properly be below the bottom of the boat. The slippage I experience is only when applying a lot of throttle while transitioning to plane. With a conservative amount of throttle the boat pops effortlessly up on plane and then has no further problems when applying full power.
In any case, it acts about the same at the two motor heights I have tried. I am questioning if the prop has enough blade area or a good overall design for my application at this point.

Nate

Perry posted 04-22-2004 06:29 PM ET (US)     Profile for Perry  Send Email to Perry     
You are confusing cavitation with ventilation. Prop cavitation is caused the same way as pump cavitation. Low pressure at the front side of the blade causes bubbles of water vapor to form. It becomes a problem when the leading edge of the next blade cuts into these vapor bubbles rather than water and it loses its grip on the water. The engine then revs high as it "spins its wheel" in the bubbles.

The plate above the prop is called an anti-ventilation plate. Ventilation uccurs when the prop grabs air from above the surface and the engine spins the wheel in air bubbles.

As a rule of thumb, the anti-ventilation plate should be about an inch above the bottom of the hull at transom trimmed in. You might want to try a double cupped performance prop to eliminate prop slip. I still find it odd that a one inch increase in pitch gave you 700 less rpm. Something is not right.

Nate posted 04-23-2004 09:25 AM ET (US)     Profile for Nate  Send Email to Nate     
Perry,
Why do you find it odd that a one inch increase in pitch dropped RPM by 700?

I have now raised motor again so that "anti-ventilation" plate is about 1/2" above bottom of transom but, have yet to try the boat out this way. That pesky workday has been in the way of boating time.

As common as this combination should be, somebody has been through sorting out this rig and getting it right.

I believe, as you suggest, an aftermarket prop may be required. At $300 to $350 apiece, I'd like a pretty good idea about a specific one to try. I saw a twenty-five HP Evinrude on the back of a jon boat yesterday with a four-blade propeller. This would sure increase blade area but, at the expense of another leading edge dragging through the water.

Nate

Perry posted 04-23-2004 12:40 PM ET (US)     Profile for Perry  Send Email to Perry     
Nate, a one inch increase in pitch generally lowers max rpm by about 200.
Nate posted 04-23-2004 01:34 PM ET (US)     Profile for Nate  Send Email to Nate     
Perry,

There is no set amount that a one inch increase in pitch will lower RPM. A motor's "final drive ratio" is a function of the propeller pitch and the lower unit gear ratio. As different motors have different lower unit ratios, the RPM change from a given change in pitch will vary between different models of motors. In addition, a rig that is "under propped" does not work the motor hard enough to restrain it to the correct RPM and it is free to overrev as my combination did with 15" pitch. A propeller should be piched to govern the motor to proper maximum RPM.

Nate

Perry posted 04-24-2004 03:13 AM ET (US)     Profile for Perry  Send Email to Perry     
Nate, I'm confused. You said a 15 pitch Honda OEM prop gave you 6700 rpm. You also said a 16 pitch prop got you to 6000 rpm. I assume you were refering to the same Motor (Honda 50).

I said that a one inch increase in pitch GENERALLY lowers max rpm by about 200. I assumed you understood that I meant 200 rpm change using the SAME motor. There is now way a one inch change in pitch will change rpm by 700 rpm on the same motor unless there is something wrong with the prop or motor height. That's why I said something isn't right.

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