Duoprop Sizing

[wadouglas]

I have a Duoprop 290 geared for a V-8 with a set of B4 propellers in decent shape. I have a 1979 27 ft. Bayliner Victoria fly bridge with a 260 HP V-8, WOT is 4400 RPM. I think the boat weighs around 9000 lbs wet.

The boat planes quickly and is running 28-29 MPH GPS at about 38-3900 RPM, then when I push the throttle further nothing happens except the engine starts to cut out and run poorly. If I throttle back to 3800-3900 RPM and it smoothes out.

I have seen a few posts where B3's were mentioned for boats a little bigger than mine. I?ve looked at the Volvo Penta website and am a little confused by the chart they provide. B2's are rated for 27-29 mph, B3's 30-34 mph, and B4's 34-38 mph. Are those the top speeds the boats are designed for?

 

[bruceb58]

Re: Duoprop Sizing

You may have a carb issue. I would try to work on that before you start switching props. Also, make sure you are getting full advance on your distributor.

That is an excellent speed for that boat by the way. That is a very heavy boat! My dad had the exact same boat without the duoprop. No way he got the speed you are getting. Is that speed measured with a GPS?

 

[wadouglas]

Re: Duoprop Sizing

Yes, that's GPS speed. Pidot speed is a few MPH less. I consider GPS to be more accurate. Is that correct?

 

[wadouglas]

Re: Duoprop Sizing

I just had the carb rebuilt and before I went to the duoprop my single VP280 went to about 4200 RPM with no problem.
I figure this is a basic thing that I should understand so let me ask this assuming it is only a prop issue;

What is it I'm looking for with regards to over proping, meaning if I should be running B2's and I'm running a set of B4's, does the engine over speed, bog down, Stall? Thanks!

 

[stylesabu]

Re: Duoprop Sizing

Changing prop size, will change rpm's and speed. a prop can't cause you to bogg down when you are moving. this is an engine issue.sounds like you are starving for fuel at higher rpm's. fuel pump, float level, fuel filter or line restriction. easist way to enrich is propance gas cylinder connected to long hose connected to intake vacumme. when engine start to sputter add some fuel, if rpm pick up, fuel problems. if it gets worse, spark problem.

 

[wadouglas]

Re: Duoprop Sizing

So again, and I apologize if I'm not asking this correctly, If I'm running B4 props at 3800 RPM going 27 MPH what would I likely see in RPM and MPH if nothing else changes except I change to B3 props? What would I likely see If I went to B5 Props?

 

[jaxnjil]

Re: Duoprop Sizing

nothing. there is no prop combination that is going to cure a missing staling engine. there is no way your prop combination is causing your motor to cut out and miss at that rpm.
you need to get your motor to run smooth and run out to what ever it will at wot. it should run up to that point and stay steady.
if as you state, you can advance the throttle and nothing happens you need to get that adjusted, and figure out whats wrong there before you go any farther. if every thing is proper you should be able to fire wall your throttle after on plane and gain rpm steadily till it tops out at a-some max rpm.
as stated above your prop size, rpm, speed for your boat weight doesn't sound to bad as it is

 

[wadouglas]

Re: Duoprop Sizing

Thank you!

 

[hwsiii]

Re: Duoprop Sizing

Douglas, if you were running a regular prop I could tell you exactly what prop you need and your prop slip as well. It appears from your writing that you had a normal outdrive on your boat before you changed it to a Duoprop design. You also say that your motor was turning 4,200 RPM before you changed drives and now it has dropped to about 3,900 RPM and you motor states that it should turn right at 4,400 RPM. Unless your motor is turning more RPM than your tachometer is showing and that is ausing the motor to shutdown, then you most likely have a motor problem that is causing the motor to cut out and stumble.
Quite frankly by changing to a B3 prop set you would gain back another 400 RPM, which would put you right at 4,300 RPM. BUT you really appear to have a MOTOR problem and NOT props that are pitched too high for your boat, you can get new B3's but that will only make the motor run worse at higher RPM and throw money out the window, because you have a motor problem in my opinion, especially since you say the carburetor was just rebuilt. The motor didn't have this problem before you had it rebuilt, because it was turning 4,200 RPM without missing and running poorly.

In my opinion you need to rebuild the carburetor or find what is causing the fuel at this time.
Bruce, Jax and Styles are right.

H

 

[wadouglas]

Re: Duoprop Sizing

First, thank you for answering my question about prop sizing first and then talking about a possible engine problem. I think I have learned 2 things.
1. I must not have been clear enough in how I stated the problem/question.
2. I should have started a new thread instead of confusing the issue.

I am not convinced it's an engine problem yet, although I have taken the engine advice onboard. The carburetor was rebuilt about 8 months ago and the engine was running great with regards to that. The single prop VP280 crapped out only 2 months ago but was turning around 4200 RPM WOT. I changed over to a duoprop with B4 propellers and now all I get is 3800 RPM WOT.

 

[wadouglas]

Re: Duoprop Sizing

I just went back and looked at the entire thread. Sorry to everyone who tried to help. I stated things poorly. WOW! It seemed OK when it was in my head.:redface:

 

[hwsiii]

Re: Duoprop Sizing

Douglas, if your are sure your carburetor and everything else is fine then just get a set of B3 props. But I would use a shop tach to verify RPM and make sure the carburetor and timing is fine before spending that many dollars for props.
LOL at your last post. I NEVER mess up, UNLESS I am breathing.


H