Horsepower vs Prop Shaft Size

[nhschneider]

I am seriously considering replacing the Volvo MD40A that was originally installed in my Contest 48 with a new, higher horsepower engine. The boat is equipped with a 35mm prop shaft. The current engine is 85 horsepower and the one I'm considering is 110 (the Volvo D3-110). At what point is a prop shaft too small for the output of the engine? There's probably some engineering reference book that has a chart of some sort - does anyone know where to find it? Is it possible to put a larger shaft in without going through extensive remodeling of the deadwood?

Thanks. Fair winds and calm seas.

[James S]

On my boat, changing the shaft to a larger size requires changing the cutlass bearing and the packing gland.
This is possible on my boat with out major work because the O.D. Dia. of the larger cutlass bearing is the same size as the original.
You’ll notice when you start looking at them that the same O.D. has 2 or 3 I.D.s available.
It was some time ago when I looked at this but I think my original is 1-1/2” and I can go up to 1-3/4”

My original engine was 74HP and my new one is 85 my prop is three blade 23” Dia. and I think 18” pitch.

[çemberci]

For Calculating for size here is a lotus program Welcome to Hull Body Plans on DXF Format-Achieving Best Marine Propeller is our goal ŞAFT HESABI (Shaft Calculation) Lotus Wk4Oktay Çemberciİstanbul/turkey

[Keough]

Well theres a way to figure out the size thats needed. Lloyd's Rules for shipping uses a formula to determine the size of shaft required. You said it was a prop shaft (not an intermediate shaft), so use this:

Shaft Diameter = 100K x (3rd root of)[(P/R)*(560/(rho + 160))]

Where:
K = 1.22 for a keyless propellor, or 1.26 for a keyed propellor.
P = Engine Power (this power is in kW, use conversion 1hp = 0.746kW).
R = RPM of engine, or shaft after reduction gear if there is one.
rho = Material strength, Assuming the shaft is made of mild steel, use a value of 550 N/mm2, if stainless use around 750.

The answer overall comes out to be in mm, and since your wondering about if you need a bigger shaft, I would see how much difference there is in the size of shaft needed and the size already there. These shafts are picked with a safety factor of about 500% (5x), so if there isnt much difference in size required don't worry too much about it... unless your using the boat in icy conditions.

[Weyalan]

I don't know about the shaft diameter thing really (and posters above obviously do). What I do know, however, is that there are plenty of much better (i.e. stronger) steels than the most commonly used 316 grade. 2205 grade duplex stainless, or even better, Aquamet 22 grade.

What I am saying is, that if your current shaft is "marginal" for stranght, maybe using the same diameter but a better steel might be an option to consider.

[markpierce]

Seems to this ignoramus that unless the new engine has significantly higher potential revolutions per minute, there will be little effect unless a different propeller is used which can move greater quantities of water at similar shaft speeds.

[djmarchand]

It is torque that can snap a propeller shaft while accelerating. If there is a significant difference in the torque curve between the new and old engine, this can be a problem.

Volvo's web site has data on the new engine and also the old engine. The MD40A peaks at about 195 Nm and the D3 at 315 Nm. That is a big difference. But what you really need to do is to change the transmission ratio which will reduce the torque on the prop shaft.

But as others have said switching from stainless steel to Aquamet 22 can make up a lot of difference.

But do you really want to do this. You are going from a big iron, normally aspirated engine to one that has a much smaller displacement and that generates its horsepower with a high turbo charge boost pressure and a sea water aftercooler.

This would not be my choice for a sailboat, but fine for a planing power boat.

Unfortunately there aren't a lot of good choices that will fit. The John Deere 4045 is a modern "big iron" engine, but it may not fit. An older engine that might fit is a remanufactured 135 hp Perkins 6.354. Check out Trans Atlantic Diesel who offers Perkins remans.

David

[Weyalan]

Originally Posted by Keough

Well theres a way to figure out the size thats needed. Lloyd's Rules for shipping uses a formula to determine the size of shaft required. You said it was a prop shaft (not an intermediate shaft), so use this:

Shaft Diameter = 100K x (3rd root of)[(P/R)*(560/(rho + 160))]

Where:
K = 1.22 for a keyless propellor, or 1.26 for a keyed propellor.
P = Engine Power (this power is in kW, use conversion 1hp = 0.746kW).
R = RPM of engine, or shaft after reduction gear if there is one.
rho = Material strength, Assuming the shaft is made of mild steel, use a value of 550 N/mm2, if stainless use around 750.

The answer overall comes out to be in mm, and since your wondering about if you need a bigger shaft, I would see how much difference there is in the size of shaft needed and the size already there. These shafts are picked with a safety factor of about 500% (5x), so if there isnt much difference in size required don't worry too much about it... unless your using the boat in icy conditions.

Interesting, useful info

Maybe I'm mssing something, but your "strength" figures seem a little high, but maybe I'm splitting hairs.

Also, can you tell me whether the Engine power is the rated maximum power for the engine, or the power at maximum revs used. i.e. My Volvo MD2003 is rated at 20.5kw, but I don't runit above 2500rpm at which it is developing about 17kW

Finally, are you saying that the formula has a 500% Factor of Safety built in; i.e. that if you use the formula and calulate a shaft diameter, the diameter yielded by the formula has the x5 FoS already? It makes sense that is is so, but I just wanted to clarify.

[barnakiel]

Vetus gives a graph in their catalog. Look it up.

If I read it correctly, 35 mm dia is OK till 140 kW and 3000 rpm. (AISI 316)

They quote equivalent Duplex 1-4462 as fine for 180 kW at the same revs.

b.

[SkiprJohn]

This is from Skene's Elements of Yacht Design.

regards,

[barnakiel]

Only when looking at SkiprJohn's post did I realize that the relationship with rpm is reverse for any given hp.

It was not clear in Vetus info as it is construed in a different manner.

THX!
barnie

[SkiprJohn]

Just to be clear. We are talking about shaft RPM and hp at the shaft.
regards,

[Davey1000]

One also needs to consider the shaft taper. The continental or Vetus taper of 1 in 10 is the worst taper that one can get. The Imperial or British 1 in 12 taper is better but in North America they prefer the Society of Automotive Engineers 1 in 16 taper. The reason is that the SAE taper gives the best grip and the greatest surface area of metal in contact. Teignbridge Propellers in the UK also use the SAE taper and I'm informed that they only produce expensive high-end products.

1 in 16 does need more precise machining but nowadays this is not really a problem.

[Jsta_Rebel]

Originally Posted by nhschneider

I am seriously considering replacing the Volvo MD40A that was originally installed in my Contest 48 with a new, higher horsepower engine. The boat is equipped with a 35mm prop shaft. The current engine is 85 horsepower and the one I'm considering is 110 (the Volvo D3-110). At what point is a prop shaft too small for the output of the engine? There's probably some engineering reference book that has a chart of some sort - does anyone know where to find it? Is it possible to put a larger shaft in without going through extensive remodeling of the deadwood?

Thanks. Fair winds and calm seas.

Considering you are only going from 85-110 HP, I don't think the shaft size would make much difference. I have a 130hp engine that uses a 1 3/4". Shaft turning a 26 x24 prop. However a buddy of mine runs his 650hp engines on his cigarette with a 1" shaft. This makes me think that overall tonnage of the boat and prop size has more to do with shaft size than HP of the engine does.

[a64pilot]

Torque is HP X RPM, the shaft only cares about torque, high speed boats turn their props at much higher RPM.
Plus the unsupported length of a shaft matters too, short shafts can take much more torque.
A ridiculous example is a helicopter tail rotor driveshaft, can take hundreds of HP in a tiny, thin hollow aluminum shaft, but its turning maybe as fast as 6600 RPM and has several supporting bearings.