Horsepower and Shaft Diameter

[Das_Boot]

Apart from the obvious double meanings. Is there a web site or publication where I can look up the relationship. To be more specific will a 30mm vetus shaft be adequate for a 50hp motor running at max 3000rpm.
I have googled till my fingers blead but cant come up with any definitive answers therefore any help would be greatly appreciated.

 

[boatmike]

Two considerations.
Torque. A 30mm shaft will transmit 50HP OK. No problem.
Bending moment and Vibration. This really depends on overhang. If the shaft is well supported by bearings close to the prop 30mm is OK but top end. If doing a new installation I would ideally go bigger. If everything is in good balance though it should be OK.

 

[Das_Boot]

The engine and prop are in direct alignment. I am looking at a beta 50hp but the guy who surveyed for this said the prop shaft would be at the limit as you have just said. I have spoken to other engineers who say that the diameter can diminish if the engine is in direct line (no tilt). Also vetus say their shafts are considerably stronger than standard. I dont want to install a new thru hull. What are the dangers of leaving the 30mm shaft in place, I dont mind buying a new shaft just not a bigger one.

 

[ongolo]

Well, is the shaft running at 3000 or is the engine running at 3000/ the torque and thus the strength of the shaft depend also on the tranmission ratio. If you run a 1:1 the shaft is big eneough for 50HP but say you run one hell of a big prop and a transmission ratio of 1:10, 50 HP will twist a knot into this shaft.

One can calculate the required shaft diameter, but many manufacturers, and if I am not mistaken Vetus too, have a graph with three columns of values, put a ruler across of two values and read of the missing value.

I can look it up, or calculate it for you, but need shaft revs and not engine revs

regards ongolo

 

[Das_Boot]

Cheers Ongolo I will get the info. What I do know is the prop will be 18 inch with 12 pitch the ratio is 2:1. Actualy I think that covers it.
Also there is no incline as the engine is inside the keel.

 

[pvb]

Graph of Vetus shafts...

There's a graph of Vetus shaft sizes for various horsepower/speed combinations in the Vetus catalogue. You can view this online at the Vetus website - select "Digital Media", then "Digital Catalogue". Takes a long time to download if you haven't got broadband. You can click on a page to enlarge it. The 50hp Beta is about 37kW at 2800rpm, so should be fine with a 30mm Vetus shaft.

 

[Das_Boot]

Re: Graph of Vetus shafts...

Thanks for your help I will download it at home as I have broadband there.

 

[pappaecho]

I have a 26 year old Evasion 32 fitted with a Perkins 4108 (48shp) and a 25 mm shaft... yours looks a bit over specificed at 30mm!

 

[eyehavit]

Take a look at the stats on this race ski-boat. Built 1996 still races.
Name: "Status"
Engine: 454 Chevy
HP: 740 at 6,500 rpm
MAx engine rpm: 7,400
Max shaft rpm: 10,400
Max boat speed mph: 108
Propshaft diameter: 31.75mm