Volvo Penta aq170 prop

I have a volvo penta AQ170 wit

I have a volvo penta AQ170 with a 280 outdrive on a 22ft fiberform. It is a fairly heavy boat of course but it will only get around 16 mph at 4000rpm. I have a 15in 17 pitch prop and by my calculations I have a lot of slip going on. My outdrive says "D" at the end of the serial number which makes me think it is a 2.15:1 gear ratio. I need some advice on prop selection as my current prop started to give out last time out and needs to be replaced. I have not pushed the power all the way up to see how much is left at the top end but there seems to be a lot of throttle left. Also what rpm should I be running the engine at? 5000 is the limit but what is a good rpm for cruising out to the fishing hole? Thanks.....

Are you running a 15 x 17 High

Are you running a 15 x 17 High Speed or standard?
The Aq 170 normally runs with a 1:95 gear ratio.

"I checked out the prop and ge

"I checked out the prop and gear ratio again and discovered after a bit of scrubbing on the prop that it is a 15x15. It is quite scored up on the face but that is pretty much all I can come up with. I have no idea if it is High Speed or standard seeing I don't know how to tell the differance. It has 3 blades with a lot of surface area on each blade, if that helps. I also turned the engine and found out that my gear ratio is 1.89 and not the 2.15 that the upper gear box says. It must have been changed at one point. My outdrive is the single not duel prop so according to my 280 manual I think the gear ratio is 1.89 (I came up with 1.88 when I turned it) I am still coming up with a prop slip of 45%. I also checked out my butterfly in the carbs and at 4000 rpm they are not even close to verticle. I think I have a lot more power to push from the engine. I am not even sure which way to go on the prop. I was thinking that the prop slip was from to much prop for the engine but I have plenty of engine power left it seems. Thanks for any help you can give..."

"The less pitch in a prop, the

"The less pitch in a prop, the more RPMs you will get. A 2 inch change in pitch will result in a 300-400 RPM change (lower pitch = more RPMs, higher pitch = less RPMs). I would find out how many RPMs you get at full throttle, but don't push it past your engine's maximum RPM for more than a few seconds. If you can get to maximum but not much more, then your prop is probably good. If you can get more than 300-400 RPM past maximum RPM, then you probably need a higher pitch prop. If you can't get to maximum RPM, then you probably need a lower pitch prop. I'll leave potential prop slippage to someone else. "

"Ok, the pitch change makes se

"Ok, the pitch change makes sense. How about the prop size. If I go from a 15 x 15 to a 16 x 15 what is the approximate engine speed loss? Is there a rule of thumb for that? Thanks for the info as I am new to boating and have been learning as I go."

You may end up at a 17 as I be

You may end up at a 17 as I believe mich jumps 2" at a time. Also look into getting a cupped prop. We fixed many of these kind of problems with such a simple fix.

Check the location of you speedo and depth finder senders as I've seen them cause cavitation - and look the hull over for trailer damage. Sometimes a cup in the hull will take a certain speed before it causes you any problems.