Prop Performance Difference: 131/4 x17 vs. 133/8x17

[atengnr]

I have a 60hp johnson on a 14 foot fiberglass boat. With the current 131/2x15 prop, it runs about 5800 rpm, however above 5500 rpm, engine is objectionably noisey.

What is the predicted performance difference between these two 17 inch pitch props: 13 1/4 vs. 13 3/8??

Thanks.

 

[funpilot]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

A lot of people here will say that you are dialed in just right, achieving 5800 RPM because the engine is pretty happy at that loading at full throttle. It would indicate that you aren't lugging your engine. You can probably expect a 500- 600 rpm drop by going to one of the 17 inch props. Your top speed may increase 10%. The blade and cup design will affect your performance more than the 13-1/4 vs. 13-3/8 diameters. And we don't know what the blade and cup design of each of the 17s is.

fp

 

[atengnr]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

Yes, thanks for all the input regarding max rpm for this motor. This issue is, this motor is quite loud at 5800 and it seems that going down a few hundred rpm would help...

I was looking at a michigan wheel prop. I dont know any about blade design or that of these props...

What effect does the diameter have (smaller vs. larger)??

 

[ezeke]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

If you check the propeller part of the Iboats Marine Store and look over iin the lower left corner. you will find helpful information that might answer some of the questions.


In the meantime there is a prop forum here that is where the question will get the proper response: http://forums.iboats.com/bbBoard.cgi?a=viewforum;fid=40

 

[G DANE]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

I once swapped a standard OMC prop til a Michegan same size and pitch and dropped 500 rpms. I was dissapointed. But it was on a smaller motor, and maybe the Michegan had better bite, as blade area was larger. If you go with the Michegan, buy it somewhere where they will allow you to test it and swap it to a different pitch, after a testrun. Being close to 5800 - 6000 wot is better for motor than 5300 - 5500 which will carbon it. How much of your operating time do you go wot ?

 

[atengnr]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

I probably spend 25% of operating time at WOT.

 

[Silvertip]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

That small difference in diameter will make very little difference in performance regardless of blade design. Going to a 17 pitch prop may or may not increase speed because the engine may not have enough power to twist it at its recommended WOT throttle RPM on that particular boat. That means the engine is being lugged and as has been suggested, that's not good for the motor. You are in the optimal range for that motor. Don't hurt it just because you don't like the noise.

 

[zzzzz]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

agree with Silvertip...a 1/8 inch diff in diameter is only a 1/16 diff on each blade8)

 

[Texasmark]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

Only way you will really know is to run the prop. Props are so varied in their performance parameters that you can't just second guess them. BTDT

Don't believe me ask Walleyehed.

Mark

 

[nordy]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

I have a 60hp johnson on a 14 foot fiberglass boat. With the current 131/2x15 prop, it runs about 5800 rpm, however above 5500 rpm, engine is objectionably noisey.

What is the predicted performance difference between these two 17 inch pitch props: 13 1/4 vs. 13 3/8??

Thanks.

You may be successful in increasing boat speed a bit by choosing the 13 1/4 diameter in combination with the 17" pitch.
The lower the prop rotates the better prop efficiency is normally with same speed.
You won't loose too much engine speed and engine power because of the smaller diameter which lets the prop turn more easily.
Consider, thrust is proportional to D^3, a rather small variation in size makes an appreciable change in prop load.
Further you have to consider the effect of pitch increase on prop slippage.
The bigger the pitch the bigger is slippage.
Think of screwing a higher pitched bolt in wood, it requires more force.
More prop force means more slippage.
The increase in slippage compensates a bit for speed loss.
Finally better prop efficiency will increase both speeds a bit compensating as well for pitch related speed loss.

Greeting from Germany

Peter

 

[BF]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

since your complaint seems to be only about the noise, you might consider building/attaching a little noise deflector to the boat in front of the motor. I've seen pict's of them the odd time in the "projects boats" section.

Next time you go for a ride, take a passenger and couple flat pieces of different sizes. (e.g. scraps of plywood, or a cooler top or whatever)... Have the passenger hold up one of these pieces in front of the motor (at the leading edge of the splashwell ?). I think it'd make a big difference. You could have a piece with hinges on the front that lays down into the splash well and is out of the way when you are fishing or whatever. Snap it up for faster runs. I'd guess it probably doesn't need to be very big to make a difference in the noise.

Just a thought... ditto what's said above... I wouldn't overprop it to bring the rev's down... better to just leave it as is and cut the throttle back so rpm's drop a bit... at least you're not lugging it then.

that's my $ .02

 

[atengnr]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

Thanks for all the replies.

Additionally, I cannot trim this motor up any, as when I do, the boat immediately begins to porpoise. Boat is definitely a bit stern heavy with this motor. The boat runs about 32 mph currently....

Is there anything I can do to improve this??

 

[funpilot]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

Can you move some of your load to the front? Is the hull waterlogged?

fp

 

[atengnr]

Re: Prop Performance Difference: 131/4 x17 vs. 133/8x17

Can you move some of your load to the front? Is the hull waterlogged?

fp

cannot move more than 50 lbs forward.

Boat is dry, floor and stringers were replaced in 1997. No water inside the hull or soaked foam (boat has no foam).