No RPM change from lower pitch prop

lutzinmn

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Sep 30, 2009
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I have a 1987 230HP Mercruiser with Alpha One on a 22.5' Thompson boat. I bought the boat which came with an aluminum 19P prop. With the boat trimmed out, I was getting about 4000 RPM and 38MPH. I know the RPM range is between 4200 and 4600 RPM. I just recently tried a stainless steel 17P prop. With the boat trimmed out with that prop, I was getting 3950 RPM and 44 MPH. Also The hole shot was better. The engine has been tuned up, so I know the compression in all cylinders are good. The Engine purrs very smoothly. I liked the stainless prop but, I do not understand why the RPM's did not increase. Is there anything that would cause this? I borrowed this prop from the prop shop where I live. When I returned the prop and explained what my results were, another gentleman said if I liked the way the prop worked I should go woth it. It doesn't matter what the RPM is if it runs the way you want it too. I do not want to do any damage to engine. But if RPM is correct, shouldn't I be able to go even to a lower pitch which will give even more power?

Thanks for any help someone can give.
 

bruceb58

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Mar 5, 2006
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30,610
Re: No RPM change from lower pitch prop

The engine has been tuned up, so I know the compression in all cylinders are good.
Not sure why you think compression would be good becuase you tuned it up.

First question I would have concerning the prop is the design of each prop. If each design was the same, size(area) of blades, number blades, edges cupped...then you would get am RPM change. If the props vary in design, all bets are off.

What was the condition of your original prop? With the new prop, what is your WOT RPM? Were the conditions between the two props exactly the same...same altitude?

As far as gaining power, a different size prop is the equivalent of changing gears. If you want to accelerate faster with a drop in peak speed, you would drop your pitch. You may also drop your fuel economy assuming you weren't overpropped to begin with.
 

QC

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Mar 22, 2005
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22,783
Re: No RPM change from lower pitch prop

Yes and Stainless are often 2 inches apart from Aluminum due to flex, that's why you also got 44 MPH. How you getting your speed? How do you know your tach is right?

BTW, the gentleman is wrong. You need to be within the window, bottom of that window is OK if you like the hole shot, but you need to be able to get there . . . Do you know your gear ratio?

Edit: Tach's wrong, speed's wrong or my assumption of a 1.50 ratio is wrong. Numbers don't compute . . . I always start with the tach. I was a little suspicious because my guess was that a 19 with only 4000 RPM would be a real pig out of the hole . . . You need to use GPS, no current, and verify the tach. Gear ratio too. I am guessing you are (were) already very close to a perfect prop (the SS 17) if your speed numbers are correct.
 

lutzinmn

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Sep 30, 2009
Messages
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Re: No RPM change from lower pitch prop

I think the Compression is good from tuneup because on my receipt they wrote that they check compression and all cylinders were good. Both props were Quicksilver 3 blade props. Not sure how much cup on each. The numbers off the SS are 48 79794 A5 17P and the aluminum are 48 78120 A10 19P. Both tests were done in Duluth harbor Lake Superior. The SS prop test, the water was a little choppier than with the orhter prop.

I guess I do not know if tach is accurate. Speed was measured with my GPS. Gear ration is 1.5. You are right the 19P with 4000 RPM was a pig out of the hole. I had trouble pulling an adult slalom skier.
 

QC

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Mar 22, 2005
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Re: No RPM change from lower pitch prop

Based on what you have posted, your tach is off, I am guessing you were hitting over 4500 RPM from what I have calc'd . . . Get the 17 back, it was sweet ;)
 

lutzinmn

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Sep 30, 2009
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Re: No RPM change from lower pitch prop

QC How do you compute what the RPM was. I suppose the only way to verify tach is incorrect is replace it?
 

QC

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Mar 22, 2005
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22,783
Re: No RPM change from lower pitch prop

Well, if you have the pitch you know how far the boat should theoretically go in one revolution of the propshaft (17"). Water is not a solid, so the boat cannot really go that full 17 inches. The difference between that 17" and how far it really goes is called slip, jut hold onto that for a minute. Then if we know your gear ratio (1.5), then we can determine how may propshaft revolutions we get relative to engine RPM. If we know the speed accurately (GPS or Radar) then we know how fast your combo is really going. So basically engine RPM x gear ratio = propshaft RPM, multiply that by the Pitch, multiply that by slip = speed.

In your case I am assuming the gear ratio is correct, your speed was GPS so I trust that, and I am assuming that we really had a 17 inch pitch prop. You gave me the RPM so I only need slip which is the squishy number. When I cranked all of your numbers into a Prop Calculator that I use instead of doing the calcs myself, I got a negative slip number. Again, slip is the difference between how far your boat goes vs. the theoretically perfect (sorta impossible) distance of 17". From experience with this I know that 10% slip is really good for a recreational boat. What that means is that the boat actually went 17" x .9 (10% slip) or 15.3 inches instead of the full 17 . . . In your case the boat would've had to go farther than the 17 inches which is even more impossible (negative slip). So if I take the one number we didn't know (slip) and assume it is a semi reasonable 10%, again I assume the gear ratio is right, the speed was GPS, then the only bad number has to be engine RPM. If I plug in everything but RPM into the Calculator I get 4555 RPM . . . Voila, tach is bad.

Look for a switch behind your tach that can be turned for 8 cyl, 6 cyl and 4 cyl. There may be another spot too. It is possible that it is just set wrong . . . If not you can use a "shop tach" or other method to verify the bad tach before you replace it. You really need to confirm it is bad before you start buying stuff . . . ;) Good luck!
 
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