Props and speed

[twonahil]

I have a 1970 B31 flybridge with cummins 5.9 QSB's and 2:1 gear box. Can anyone give suggestions on prop size, pitch, cup and anticipated performance with this setup?

[Tony Meola]

Unless you have changed the shaft angle or put in prop pockets, you are limited to running a 20 to 21 inch 4 blade prop. I am running the 270 6BTA's and I am turning a 20x23 with some cup.

Your performance would be much different than mine due to the higher horsepower you will run.

Hopefully one of the members with the QSB's will jump in.

[twonahil]

I am running the 230 hp version. What are you getting for speed with your props?

[Tony Meola]

I am running light meaning no hard top or tower. At 2100 I am at about 21 knots, at 2200 it is 22 knots, at 2300 just shy of 24 knots and 2400 depending on the tide, she will run between 24.5 and 25 sometimes she does hit 26, but all factors have to right. At 2650 I am just shy of 30 knots.

[twonahil]

Tony, what gear reduction do you have? I am hearing some of my speed problem might be the 2:1 gears I am using. What do you think?

[lobsta1]

I can't help you.However I do know that no one is a mind reader. There are many people here who would jump to help you if you gave them some basic info.
What speed problem do you have. In your initial post you didn't say anything about a problem. What props do you have. New or reconditioned? Is the bottom clean? Is this a new repower? Is there enough air flow?

I apologize if this seems sharp. These are just some of the ??'s your query made me think of.
Al
P.S. HAPPY HOLIDAYS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[Tony Meola]

Lobsta is correct. We do need some additional info. The 2 to 1 might be perfect for your set up. I need to double check, but I believe it is 1.5 to 1.

[twonahil]

I am running Cummins qsb 5.9, 230hp hd's coupled to twin disk 502's at 2:1 gear reduction. I have new 19" bronze props with 28" pitch. The problem I am having is I can't get better than 62% load at WOT. The 230 hp engine is rated at 2600 rpm. I can get to WOT with little effort. Seems like I am under propped. Does this help describe situation? My prop guy is stumped. Hope someone has some suggestions

[Charlie J]

if it helps I am running 1:5 zf with my 230 yanmars

[Tony Meola]

Have you tried squaring the prop off a little more. The 250 and 270 HP Cummins boats run 20 or 21 inch props. Pitch some where between 20 to 25 with cup. But most have 1:5 to 1 trans. Borrow a pair of 4 blade props at 20 to 21 inch play with the pitch. See if that helps.