need help proping my volvo penta.

Joined
Aug 6, 2013
Messages
8
I have a 1986 bayliner 1950 capri cuddy with Aq125b motor. Had the head redone on it, the engine builder wanted me to put smaller prop to raise my rpm. I bought the boat with a 15x19 prop. At wot my rpms with 2 people on board and full 32 gallons of gas is 4,300 rpm.. book says 4,800 to 5,000 rpm at wot.. So the boat came with another prop 15x17 prop but that took me around 5,300rpm. What should I do next in size to get me were I need to be?
 

Maclin

Admiral
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May 27, 2007
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6,761
That is a puzzle, as the drop from 19 to 17 should have hit the sweet spot. What MPH do you get with each? If you are OK with the speed from the 17p then you may just need to watch the throttle/rpms.
 
Joined
Apr 16, 2015
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Changing from a 19 inch pitch to a 17 inch pitch should result in a RPM change of a couple hundred RPM not 1000. I suspect that the 17 inch pitch prop you have is either a smaller diameter, fewer blades, has significantly less cupping, or one or both of your props are damaged or have had a pitch change. I agree with your mechanic that the 19 inch pitch prop is too much prop for the boat if you can only get 4300RPM WOT with two people on board. I would double check the diameter of your 17 inch pitch prop, if it is less than 15 inches than I would try to find another 17 inch pitch prop with a diameter of 15 inches. If it is 15 inches in diameter than you may have to bring both to a prop shop that can actually look at both props as well as let you try new props without having to buy them. It is possible to find an 18 inch pitch prop, but I really think you should figure out why the two props you have are affecting your RPM the way that they are.
 
Joined
Aug 6, 2013
Messages
8
The 15x17 that was extra that came with the boat looked brand new not a scatch on it with the size stamped. But I guess it could of been reworked and painted.. I will have to take to a shop see what they tell me.
 

Maclin

Admiral
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May 27, 2007
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6,761
Using a 2.0 ratio I am getting a really high slip% on the 17p. The 19p seems about right. I would be suspect of that 17p, either not marked right or something else. Is the 17p a ventilated prop?
 
Joined
Aug 6, 2013
Messages
8
Im not sure I would have to look. I know it's for a short shaft, I had to buy a spacer and plate thru my volvo penta mairena to make it work.
 

steelespike

Supreme Mariner
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Apr 26, 2002
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19,069
If the tach and speedo are original they are both suspect.You really need the speed by gps,
17" indicates 41% slip highly unlikely. You could check the rpm with an induction tach.
Just tell them what its going on.
Is the AQ125 a 120 hp?
 
Joined
Aug 6, 2013
Messages
8
Yep I said gps in my one comment. Thats what I use since my speedo is broke. I did check the tach last october to see if that was good. Yes its 120hp motor
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
25 is like cruising speed and produces 41% slip using 5300 rpm. Perhaps you should retest with the 19.
Simpler and more fun than dragging the 17 to the prop shop for a measure. 1000 rpm
Did you stop the rpm at 5300?
 
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