Volvo Penta Props RPM too high!

[lgdpt]

Engine is a Volvo Diesel AD41. It has A4 duo props and the engine gets to 4000 RPMs (fast). Im told it should barely get to 3800. What props should I buy? I can only afford to do this once, so appreciate the help! Travis

 

[steelespike]

One thing you need to realize is the Volvo especially diesels are a mystery to most of us.
And your throwing in a duo prop.
The first thing to do to save you some money is to verify your tach is accurate.
What is the pitch of A4 props Do you know the gear ratio?
There are a few here that can give you some specific info.

 

[lgdpt]

Ok thank you. I dont know the pitch or gear ratio. I was told this was the right set of props and spent $700.... Now I find out RPMs are too high, so I need to get props that tops out the RPMs at 3800. I think based on reading... A5's would be right, but I wanted to make sure I wasnt backwards and needed A3's...

Getting the Tach verified is a great idea. Thanks again...

 

[steelespike]

Let us know how you make out.You know you can run your present props and just watch the rpm.
If you often load your boat with lots of people your props may turn out to be just right.

 

[lgdpt]

Good point. I will try it when loaded down and see how high the RPM gets. Thank you

 

[Maclin]

A5 is the next step up in pitch. A3 would be the wrong direction if trying to lower RPM's.

 

[lgdpt]

Thank you Maclin. I appreciate that.

 

[jestor68]

The VP engine data base lists all of the AD41's as having a max rpm of 3900 rpm, with most having a range of 3700-3900 rpm.

You are as close as you're ever going to get with the current prop set.

My advice; leave it alone. Now you have the margin to haul some extra stuff.

Considering that the tachometer can have an error of 100 rpm, this is a non issue.

 

[lgdpt]

Ok thanks Jestor, The reason Im considering is a volvo certified mechanic drove it and said he got to 4000 RPMs "way to easy"... he told me if I got it proped "right" I would gain ~5-6 mph. He didnt know what props I had at the time... thats why I didnt get an answer (from him) on what props I should get....

 

[steelespike]

He was assuming the tach was accurate.If the tach turns out to be correct it is possible a pitch change to lower rpm could result in some more speed.

 

[Outsider]

and said he got to 4000 RPMs "way to easy"... he told me if I got it proped "right" I would gain ~5-6 mph.

What does 'way too easy' mean? If it was WOT, then you're 100 RPM off maximum. If 4000 wasn't WOT, the number (by itself) doesn't mean much other than indicate a potential prop change needed. You're 100 RPM away from maximum (if actually WOT) and he says you can gain 5-6 mph? I bet he sells duoprops. Have you tach checked, then verify for yourself what the RPM gets to (without over-speeding too much), and know what your loading is. If you're light, or maybe medium loaded, I certainly wouldn't fool with 100 RPM. If you're heavy, then think about a prop change. V-P says about +/- 250 per prop set increment, but I doubt a change to A5 will get you 5-6 RPM, A3 is going the wrong way. If you have throttle left when you get to 3900 (how quickly you get there is irrelevant, it's how much throttle you have left, if any, that counts), then a little more investigation is in order. BTW, I don't believe prop slip calculators mean much with duoprops ...