Need Help Selecting a Duo Prop....

[GilligansIsland]

I have my new 7.4 Volvo and new lower unit. Here's a bit of history...

My old 7.4 was mated to a duo prop lower unit ratio of 1:1.95. I ran C4's and got a WOT of 4400+rpm (not quite 4500)

I replaced that engine with a new 7.4 and replaced the lower with a new 1:1.78. I am running B2's and only hitting 4100+ rpm

I put on the C4's to get some numbers, and only hit WOT of 3800rpm


Michigan Motorz said this engine is spec'd for WOT of 4200-4600rpm. It would appear that each drop in prop sets raises my WOT by about 150rpm (I'm assuming if I put on the B3's, I would get to the middle of the RPM I hit with the 4's and the 2's).

So my question is, if I would like to increase my WOT RPM by 400, what prop selection would you recommend?... Is anything even available below a B1?

 

[GilligansIsland]

Re: Need Help Selecting a Duo Prop....

And I've come across these... any input?

Solas Props Type "B" Duo-Prop Propellers for Volvo Penta 280,290DP

 

[Maclin]

Re: Need Help Selecting a Duo Prop....

Really need to know the speed at the RPM's mentioned to do any prop-calc type comparisons.

To answer one question, there is nothing lower than the B1. The B series aluminum sets are B1-B8 range, from about 15 pitch to 26+ pitch.

Why did you change out the 1.95? Can you switch back?

The 1.78 ratio is for diesels, they have a much lower top rpm range. They have lots more power at lower rpm's but not as much RPM range. In my opinion a (naturally aspirated) gasoline V8 (for marine usage) cannot match the diesel powerband and will suffer some when mated to a "diesel" drive.

 

[GilligansIsland]

Re: Need Help Selecting a Duo Prop....

With the B2 my WOT was 4150 and 35.? mph

With the C4 my WOT was 3800 and about 33 mph

I have the 7.4 and Volvo says the 1.95 isn't stout enough for the big block. I think the Drive gear in the 1.78 is smaller, making it beefier.

 

[Maclin]

Re: Need Help Selecting a Duo Prop....

Are you sure the new 7.4 is putting out what it can? It may be missing some power if it has not been dialed in yet.

B1 is all the farther down in pitch you can go, I would expect your RPM to go up about 200 maybe, probably hit same top speed.

I had not heard anything about the 7.4 torque being able to stress that drive. But if it can then you may want to consider the A series props for diesels, they have thicker blades and have a 4 blade aft prop. They range from A0 thru A10 to match up to varying rpm bands for that 1.78 ratio, I am not sure how the numbers relate to the B and C series regarding pitch. In the Volvo prop guide the charts for A series only show diesel power plants.

 

[GilligansIsland]

Re: Need Help Selecting a Duo Prop....

I've heard mixed things on which drive should be mated to the 7.4, and I've heard different opinions from very knowledgable people. I just figured I'd play it safe and go with the 1.78.

The motor was bought from Michigan Motorz who ran, tested and timed it but I'll double check those numbers including the Total Advance Timing.

As to propping, I think the B1 will get me to a about 4350rpm which would be fine, but considering MM recommends this WOT spec of 4200-4600, I would really like to be as close to 4600 as possible.

...Maybe I could have the B1's tuned down a bit.

The few people I've talked to have no idea how an A series prop relates to a B series and I'd hate to just start throwing money at props.