Is 1" propsshaft enough for 90hp diesel in 23' alumium CC

Hi Folks,

I realize that I ideally one would figure out prop size/ boat design/engine and gear choice all togteher. But, I am now considering an older Volvo Penta, in good condition, with a Hurth 1.65:1 gear and some extras thrown in like 1" shaft, stuffing box, and prop, of which I don't know the size yet.

Main questions

- Is 1" propsshaft enough for 90hp diesel in 23' alumium CC

- are wide range of sized props available in 1" shaft - or do they tend to use large shafts like 1 3/8" etc, when they start getting larger.

A guy on the glen L site runs his similar 25' boat with a 130hp Mercruiser and 2:1 Velvet Drive and has a 16" prop (not sure of pitch)

May buy the engine / trannie anyway, but wanted to know if I would just sell the shaft and look for larger diameter, or if it would be OK.

Any help appreciated!

 

Do you know what the shaft material is?

 

just emailed the seller to find out. The pic of it is small but it looks shiny - so thinking more likely stainelss than bronze.
If it is stainless, would that be OK?

 

I've used 1" shafts, 316 stainless, for VW 1.9 TD diesels with 2:1 reduction. It never crossed my mind to take a larger size.

 

thanks for the info. It is not a deal breaker, but would be a bonus if the shaft, stuffing box and shaft coupler were all usable.

 

confirmed - yes, the 1" sgaft is stainless. Has a couple of bronze props, 13" and 14".

So, do you guys think the shaft will work?

 

Torque is what matters; what is the RPM?

 

> Torque is what matters; what is the RPM?

While I agree, diesel vs gas also makes a difference. Diesels have more torsional vibration. I suppose that even number of cylinders may have an effect (a 25 HP 1 cyl engine may have instantaneous torque values equal to a 200 HP 8 cylinder engine (not that either are common in boats)).

 

In order to help, I attached the Bureau Veritas formulas for calculating the propeller shafts.

 

Thanks for the help so far.
I started into the formula provided, and 20 years without any challenging math practice has taken it's toll.

83hp at 4000rpm(max), 57hp at 3000rpm (continuous rating)

full specs at:

http://www.volvopenta.com/volvopent..._engines/Pages/out_of_production_engines.aspx


model is MD27

Thanks.

 

Using the formula provided (which is the same for ABS) a 1" shaft does not meet the requirement for that power and RPM. If the shaft is some very high strength 17-4PH material (~120 ksi tensile), the required diameter would be 1.1". If the material is of lower strength ~(90 ksi), then the required diameter is 1.2"

Now the rules do provide for a reasonably high factor of safety; so if you have a good situation regarding torsional vibrations and overhung momoent, or don't run very often at full power, it may be OK. But you will be in a situation where the stresses in the shaft are higher than recommended and the probability of fatigue failure exists.

 

Thanks very much for the reply, and thorough explanation! If I decide to buy that engine/trannie and shaft material, I will still look around for a larger shaft and prop, as I will not require them for another few months.

Thanks again!

 

Yes, a 1" shaft is adequate for your application, while still having a reasonable safety margin.

 

Thanks for all of the help!