Marinette Boat Forum

37 sedan converted to twin 330hp Cummins. What is minimum shaft size called for. Boat diesel indicates for safety factor of 2 with 1.2 to 1 ratio it is 1.66 inches with stainless 304 shafts.
What is the shaft size with the big gas engines?? What was the standard shaft material back in 1982 Time frame. There is a 3208 Cat. NA listed, about 210 hp what is your shaft size and material? Also a Volvo diesel. Curious about your shaft material and size??

Won't know shaft size till haul out Tues,, and probably won't know material unless surveyors know it. So, hope it is at least 1.5 inches. Thanks

Standard: 1.25"


dave

Thanks, Barkleydave. Hope they changed the shafts, since 1.25 won't be enough. Do you know what the shaft material was or is on the 80s boats?

My 32' boat with twin Chrysler 318 240hp engines had 1 1/4" shafts. If you go for bigger shafts, will the struts accomodate them?

My 32'er '88 has 1 1/4" SS shafts.

Shaft size is not necessarily based on engine HP, although it is a small contributing factor. The four biggest factors are prop size/pitch, shaft speed, shaft length and boat weight. Example: You can use smaller shaft size if you ran a 1:1 trans with a smaller prop diameter. If you ran a reduction gear with larger and/or steeper props, you would need to increase shaft diameter to hold up to the added torque to the shaft.

The 330hp cummins should be the 5.9b series. This engine should turn 2800rpm so you could use an overdrive trans, or a 1:1 and prop it to match. Rule of thumb on planing boats is 100 rpm SHAFT SPEED for every mph of hull speed. Obviously there is slippage to account for.

There is a guy around Sandusky Oh I talked to last year that installed a b cummins in his 28. I don't remember all the details but he seemed very pleased.

I'd worry about an engine with so much torque going through a 1 1/4 inch drive shaft. But that's just me, the eternal worry wart.

Jeff

It is really pretty straight forward. For best performance you prop the diesel to the most prop that will fit diameter wise and than pitch up, but you should make 100 to 150 rpm over max allowable at wot so as to not overload the engine. Those two conditions determine the best combination for the diesel.
The other variable is 3 or 4 blade. The boat diesel calculator will show preferred ratio-- usually 1.2 on up to 1.5 in the 210 to 370 hp range. If you choose to run a smaller shaft and/or mess with ratio you give up performance. The largest prop possible, and than pitch close to it, "square" like 19 x 19 or 19x 20 etc. will be in the ball park for best performance. Now got to the calculator and plug in the hp, rpm etc. and it will give you the shaft size which is dependent upon the type of material.
And Fastjeff, you are right, if you are propped for best performance you better choose your shaft material carefully because 1 and 1/4 shafts are marginal even with the best material. Safety factors run from the bottom of 2 on up to 18. With the 19 inch props on the 37 1and 1/4 stainless shafts, they are too small.

I'm just a backyard mechanic that don't know much, but I see nowhere in all those numbers and equations the weight factor plugged in .
And with 2 engines it seems to me the load on the shafts are divided in half. As light as the boat is my guess is you couldn't get a big enough prop on it to twist a 1 inch shaft. I wonder how many trouble free miles it has travelled with the Current shaft size? I'd be willing to bet If there was ever a problem it would be due to a prop strike and not to much torque on the shaft . Disclaimer : I am a backyard mechanic that don't know much

I think the issue is WOT from a standstill puts the max torque on the props, until the boat accelerates and comes up on plain. That is the time of greatest risk of twisting the shaft. And I'm just an old fighterpilot but I take my quidance from the experts. Remember we are talking 330HP diesel engines, not gas.