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

[Northeaster]

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!

 

[gonzo]

Do you know what the shaft material is?

 

[Northeaster]

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?

 

[CDK]

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.

 

[Northeaster]

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.

 

[Northeaster]

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?

 

[johneck]

Torque is what matters; what is the RPM?

 

[jonr]

> 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)).

 

[TANSL]

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

 

[Northeaster]

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.

 

[johneck]

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.

 

[Northeaster]

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!

 

[PAR]

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

 

[Northeaster]

Thanks for all of the help!