Prop selection for twin AD31P-A and DP-E drives.

I'm looking for duoprop recommendations for a twins setup with AD31P-A, 150hp each, and DP-E drives, 2.32:1 gear ratio.

The boat is a 28' Skipjack Flybridge. 7000lbs dry weight (added half tower), 164 gallons fuel, 40 gallons water. I'm guessing, fully loaded, she's around 10,000lbs.

Here's a link to the boat info: http://www.skipjack-boats.com/history_28fb.html

Re: Prop selection for twin AD31P-A and DP-E drives.

I have a 1986 Seasport 2700 pilothouse (8'6" beam) that may match the specifications of your boat pretty closely. I have twin 1994 AD31-P Volvo Penta 150 hp diesels with Duo-prop 290 outdrives. I do not know the gear ratio. Here is what I do know:
The boat came with 2 sets of props A-8 and A-9. The A-9's were installed when I bought the boat. I was disappointed with the time that it took to get the boat on plane. It seemed like it would hang at 2000 RPMs for a long time before it would plane out. (45 seconds?) Also with only one engine running the boat would not climb over 2000 RPMs. The amazing thing is that this tub would do 40mph+ on the GPS. I changed the props to the A-8 thinking I would lose fuel economy and speed and gain hole-shot. The hole-shot became respectable once the engines are warmed up. The speed did not appear to go down at all (in my experience there appears to be a block at 40MPH for many watercraft where it takes a substantial amount of extra HP to go faster, 40 is more than fast enough for me anyway) The boat will still show 40.3MPH on the GPS. I did not measure the fuel econonmy with the A-9s but with the A-8s I did an 80 mile run on the GPS and got 2.6 MPG which I thought was very respectable. Dw

http://www.itns.com/boatrepair/

Re: Prop selection for twin AD31P-A and DP-E drives.

Wow, that's excellent news!

I have several sets of A8s that came with one of the DP-E outdrives I purchased. I was led to believe I should be looking at A5 or A6 props. I think I'll give these props a try, before I settle on something else.

Do you recall your WOT RPM?

Re: Prop selection for twin AD31P-A and DP-E drives.

I would bet you can't use the A8 props at all.
Hope you have the right gear ratio for the drives.
Your rpms should be 4250?4350 in neutral at WOT under load you should be seeing 4100 under load.
You really don't want to lug those engines, 4000 to 4100 WOT rpm under load. The closer to 4100 the better.

Re: Prop selection for twin AD31P-A and DP-E drives.

According to Volvo's literature and my mechanic, max RPM is 4000. My mechanic says between 3900-4000 would be acceptable. He's the one that was recommending the A5-A6s, but he wasn't sure, since he's never worked with this engine/drive setup in a boat like mine. I got a little excited when I read the prior post about the A8s ... I did think it was a little too good to be true ... lol!

Yes, the 2.32:1 gear ratio is the correct ratio for the 4 cylinder diesel. Volvo sells this engine/drive ratio as a package, so I'm confident of that.

So, what do you think? A5s or A6s? I don't want to make a mistake and get the wrong props ... that would be an expensive mistake!

Re: Prop selection for twin AD31P-A and DP-E drives.

I have paperwork that shows that the engines and drives were sold as a package and a separate flyer that shows the package as having a drive ratio of 2.3:1. I know the RPMs run over 4100 (from memory) when both engines are running. It may be lugged down when only one engine is running. (with A8s the boat only gets on half plane at about 10 MPH with only one engine running) I have receipts from a previous owner that show he sold a set of A7s and bought two sets of A9s. I will not be able verify the actual RPMs until spring. I also have some hand written fuel flow charts that show a top speed of 39 MPH at 4400 RPM. I am thinking that this is with the A7s. My guess is that he went for the A9s to get the RPMs down, but of course this is just a guess.

Re: Prop selection for twin AD31P-A and DP-E drives.

Very interesting information.

I would like to hear back from you, after you take the boat out this spring.

I'm going to get the engines/drives installed in the next couple of months (fingers crossed), so I'll give the A8s a try first.

Thanks for the input!

Re: Prop selection for twin AD31P-A and DP-E drives.

Every now and then I do a search for AD31p and I see this thread. As I was rereading it I was thinking that maybe twin AD31Ps are too much power for this size of boat and that is why the engines can spin the larger props. With a full load of fuel my Seasport sits a little low in the back. Did you ever get your engines installed? Do you have any pictures on the web?

Re: Prop selection for twin AD31P-A and DP-E drives.

No, I haven't finished the project yet. Unfortunately, my father took sick and I've been tied up with him. I'm hoping that I get it done fairly soon, since he is doing a bit better.

Here is a link to the boat I have: http://www.skipjack-boats.com/history_28fb.html