new prop, lower RPM

[mattdd]

took my old 13x17 prop to the repair shop and they said it had 3" worn off the blades. it ran my 90hp yamaha at 5500rpm, the motors range is 45-5500, so i was just right. the new one is a 13.25x17, and, with so much more material runs the boat at 4800rpm. thats still in the range of my motor, but since i want to be in the high end of the recomended range should i go with a 15 pitch? the old prop was the original from the factory i belive. it runs so much better with the new one and i went from 34.5 mph to 38.5 mph (gps). with a 15p prop that will get me higher rpm's will i see more mph?

 

[HighTrim]

Re: new prop, lower RPM

You are definately correct in wanting to be in the upper range during WOT. The correct propeller will prevent the engine from over-revving, yet allow it to reach the minimum RPM where maximum horsepower is produced. Given you are only able to obtain a RPM somewhat lower than the maximum rating given by your engine manufacturer, you would need to decrease pitch. Decreasing pitch would increase your RPM. For every 1" of pitch size, the effect will be approximately 200 RPM.

Once your WOT RPM falls within the recommended range of the engine manufacturer, you have a propeller that is suited correctly for your boat with respect to RPM. If you use your boat for fishing, cruising and skiing, one prop probably won't do all three things equally well. It is best in circumstances like this to have two propellers. One to accommodate one set of circumstances and the other to perform best under the different load. It could, in fact, be that more than one propeller would be suitable for your boat and motor combination depending on your usage. It is imperative, however, that the wide open throttle RPM fall within the range specified by your engine manufacturer

 

[Silvertip]

Re: new prop, lower RPM

Wow -- I can't beleive there was 3 inches worn off the blades unless they spent a lot of time in contact with the bottom. Lowering pitch to bring RPM up does not "theoretically" raise speed. You are merely bringing the engine rpm up. Remember, you've decreased pitch which means the engine must turn faster to achieve the same speed because the prop is taking less bite on the water hence it moves forward less distance with each rev. However, if the engine was really lugging before the prop change, you may indeed see both rpm and speed increase. How much is hard to predict. But before you buy another prop, work with setup (engine height). Check the position of the antiventilation plate in relation to the bottom of the hull. It must be flush to an inch or two (sometimes more can be tolerated) above the bottom. This puts less lower unit in the water and you may see some rpm gain from that alone. That and using proper trim alone may get rpm where it needs to be. You didn't mention what boat you have but a 90 has a 2.0:1 gear set so the 17P may be a tad stout for the application. A 15P apparently would be a better choice.

 

[mattdd]

Re: new prop, lower RPM

the boat is a 18.5' flat bottom jon boat. its 60" accros at the bottom at the back. i think its a pretty big jon boat. plus its decked out for fishing....front and back decks, live wells, etc. it weighs 2220 lbs including the trialer. i have it all the way trimmed up when i reach 4800 rpm. the AVPlate is a little above flush with the bottom of the boat. so i need a 15p prop, but do i have any reason to keep the 17p. or, if i stick with the 17p, cause it is running so much better then with the old prop, will it be bad to only get it up to 4800rpm?

 

[Silvertip]

Re: new prop, lower RPM

I guess it depends on how much you actually run wide open. 4800 is barely into the recommended WOT rpm range and the idea is to prop at or close to upper end of the band. This is easier on the engine since it's not lugging as hard. If it were my decision, I'd try the 15P.

 

[steelespike]

Re: new prop, lower RPM

the boat is a 18.5' flat bottom jon boat. its 60" accros at the bottom at the back. i think its a pretty big jon boat. plus its decked out for fishing....front and back decks, live wells, etc. it weighs 2220 lbs including the trialer. i have it all the way trimmed up when i reach 4800 rpm. the AVPlate is a little above flush with the bottom of the boat. so i need a 15p prop, but do i have any reason to keep the 17p. or, if i stick with the 17p, cause it is running so much better then with the old prop, will it be bad to only get it up to 4800rpm?

If Your not experiencing any ventilation you may be able to raise the motor more. 1" could give you 2 mph or more.And a couple of hundred rpm.Better mpg and a little quicker hole shot.No cost at all.

 

[mattdd]

Re: new prop, lower RPM

if i raise it how do i know when i'm ventilating?

 

[steelespike]

Re: new prop, lower RPM

Ventilation is when the prop loses its bite.Motor revs and boat slows.
Some incorrectly call it cavitation.

 

[pecheux]

Re: new prop, lower RPM

Ventilation is when the prop loses its bite.Motor revs and boat slows.
Some incorrectly call it cavitation.

Wish I would have knowed about this back them ....lol
Had a 115hp Merc with a 15 pitch prop on a 21 footer and it did exactly what you described (cavitation) At that time I thought the prop inner rubber was slipping so I just went ahead and got me a used 21 pitch prop that was available for a low price ... the problem stopped and the Wot Rpm's where in the 5200 range.

 

[Silvertip]

Re: new prop, lower RPM

Believe me, you will know when the prop is ventilating. Engine revs go skyward in a heartbeat and speed drops quickly because the prop has lost its bite on the water. Sort of like having your car in high gear climbing a hill and pushing in the clutch but not releasing the gas.

 

[QC]

Re: new prop, lower RPM

Even though the 17P is at the bottom of the RPM range, my guess is the the 15P will actually be slightly slower in top speed. Probably not much, and the only way to know for sure is to try it, but most data shows that with a good prop they are fastest towards the bottom of the RPM range. Check out Walleyehed's data and it shows that in most cases . . .

 

[pecheux]

Re: new prop, lower RPM

Even though the 17P is at the bottom of the RPM range, my guess is the the 15P will actually be slightly slower in top speed. Probably not much, and the only way to know for sure is to try it, but most data shows that with a good prop they are fastest towards the bottom of the RPM range. Check out Walleyehed's data and it shows that in most cases . . .

Re: my previous post ... I had more speed on the boat with the 21 pitch prop at 5200 rpm than with the 15 pitch prop at 5600 rpm ... so you may be guessing right when it come to speed.