Help with prop sizing.

JohnD · Post by **JohnD** » Mar 15th, '07, 10:02

The motors (420hp 3126's for my B35) are at the rebuilder and I'm still deciding on what to do with trans. and props. They pacakge came with 2:1 gears and using the prop calc on boatdiesel.com I come up with roughly 25x32 and that seems like a lot of prop.

So I've asked for a quote on new 1.5:1 gears which should bring the prop size down to something more reasonable.

I would love to find real world references as to which tranny and props are bing used in B35's with 380-450hp diesels.

Thanks,
JohnD

Capt. Mike Holmes · Post by **Capt. Mike Holmes** » Mar 15th, '07, 10:05

With that much power, 1.5:1 gears will probably be best. The prop calculater on Boatdiesel.com is ridiculous.

Doug Crowther · Post by **Doug Crowther** » Mar 15th, '07, 11:06

John- call Black Dog Propellors on Kent Island/Stevensville. They have a realistic computer program that will tell you what you need to know.

JohnD · Post by **JohnD** » Mar 15th, '07, 11:44

Capt Mike, I agree on the gear ratio, though it looks like not mater which ratio I go I'll have to install trolling valves to keep a manageable fishing/docking speed. I rember how Doug's boat would kick around when shifting in/out of gear at the dock.

Doug, I'll give them a call next week. I'm off to go camping with the family this weekend. It figures, the past couple of days have been over 70 degrees and now that we're going camping it's going to drop below freezing and snow.... Ahh, such is life in Maryland.

I'm also trying to find out the prop size of the 40-Ocean the motors came out of but just have had time to stop by the yard.

Thanks All,
JohnD

CaptPatrick · Post by **CaptPatrick** » Mar 15th, '07, 11:53

John,

Don't waste time on finding out about the wheels on the Ocean. Apples and oranges... Two totally different hull types, weights, wetted surface, etc, etc.

Br,

Patrick

scot · Post by **scot** » Mar 15th, '07, 12:12

Capt Mike,

Are you saying the calculators on Boatdiesel.com are not much help? I have run some numbers by hand on props and that software "appears" to be basing it's results on a 30% rate of slippage. Actual readings and calculations on my current boat show around 20% slippage. I realize the heavier the boat....the more the slippage "might" be.

Does that software show your 31 moving faster...or slower than it actually does? :shock:

Capt. Mike Holmes · Post by **Capt. Mike Holmes** » Mar 15th, '07, 12:52

I think it showed I need like a 22 x 56 prop, or something ridiculous like that. Vic also got a crazy number. He went backwards, also, giving it his actual props, hp, and speed, told him he had a50 foot boat or somethign like that. It's garbage, as are a lot of the other prop programs.

Bruce · Post by **Bruce** » Mar 15th, '07, 13:23

There is absolutly no substitute for an experienced propeller shop.
Period.

Don't waste your time on any computer program. I haven't seen one yet that worked.

scot · Post by **scot** » Mar 15th, '07, 22:22

Don't waste your time on any computer program. I haven't seen one yet that worked.

Bruce...What about the computer programs the prop shops use?
[/quote]

Bruce · Post by **Bruce** » Mar 16th, '07, 01:53

The prop shop I use has a old worn out set of tables taped to a desk that is used with a ruler.

He is 70% right on the money. 30% within an inch of change.

They have a very expensive computer program that he says dont' work. While back he calculated a set of props for me and I made him fire up the computer to check. Computer said a much different size.

Paper calculation was right on the money.

I imagine if the program allowed you to go in and tweak the parameters, he said he might be able to work it eventually. But his didn't.

Al C · Post by **Al C** » Mar 16th, '07, 08:30

There is a 36Bertram w/435hp cats in the vessel section of boatdiesel. The data on that boat is 22x28 4 blades w/cup and 1:51 twin disc gears.

bertram31.com

Help with prop sizing.

Help with prop sizing.

Props

Props

Who is online