Prop shaft

[Yamanx]

55' boat with 2.2 litre BMC engine, hydraulic drive.

 

I have to weld in another stern gland and replace the prop shaft whatever.

 

At present she has a 1 1/4" prop shaft.

 

The boat yard where she is has recommended a 1 1/2" prop shaft should be fitted.

 

Now this may be ideal but the extra work and cost to do this is significant, over £500 extra cost to me, another days work and I would have to burn out the old steel tube that runs through the hull, not a small job.

 

I would much prefer to stick with 1 1/4" shaft, as I already have some of the parts and could, with a bit of machining, get away with not having to burn out the back of the hull. But am I fooling myself, will it become a problem later? Bear in mind that this boat is over thirty years old and has always had 1 1/4" shaft, albeit I am increasing the prop size.

 

What do you think?

[Richard Bustens]

How much are you increasing the prop by Dave?

[Yamanx]

Difficult to say exactly because I dont know the pitch of the one which was fitted. I can say that it was 13" dia and the new one will be 15" x 12" (Probably). At a guess, say 25%.

[Richard Bustens]

I would have thought it would be ok Dave i have a 2'' prop shaft and a 24 x 24 prop but as you know i have a slow running slow pick up engine. the boat yard were going to put a 1 1/2 prop shaft on but i said it was to be increased to 2''

 

By prop shaft i presume you mean the solid bar that goes through the hull with the prop on one end and is about 18'' long ?

 

Anyway if 1 1/2 was not big enough for the prop you would not be able to buy a prop with that size hole in it

Edited December 16, 2005 by Richard Bustens

[davidandheather]

I would have thought it would be ok Dave i have a 2'' prop shaft and a 24 x 24 prop but as you know i have a slow running slow pick up engine. the boat yard were going to put a 1 1/2 prop shaft on but i said it was to be increased to 2''

 

By prop shaft i presume you mean the solid bar that goes through the hull with the prop on one end and is about 18'' long ?

 

Anyway if 1 1/2 was not big enough for the prop you would not be able to buy a prop with that size hole in it

 

Hi Yamanx

A 200hp ag tractor PTO shaft is only 1.5 in dia sure if it can stand it your boat will be ok. I have known them to shear of but it is not very common. They use the same size shaft from 10hp up to big tractors

David

[John Orentas]

The only real advantage of a large diameter prop shaft is to prevent a whipping or bending action, for the sort of lengths modern narrowboats use even 1.25" can be considered 'over the top'. I suspect the habit of using such big shafts comes from working motor boats were shafts had to be up to 10 feet long.

 

Sea boats are totally different it is common to see a 3 litre engine unit with a 3/4" shaft connected to it.

[Alastair]

Or a 5l engine with a 4" shaft (on the sailing barge). But that is to avoid whip, 'cause the shaft is about 12' long.

 

I've had a prop shaft break, on my motor barge. A weld between the mis-alignment coupling and the shaft from the gearbox went. Real pain it was, had to anchor in a hurry and be towed in.

 

I took it out, shipyard rewelded it. No problems since, and I've done quite a few miles, with a big engine. I think the shaft on my barge is about 1.5" - but the weak point would be the welds. if the shaft is short, then it doesn't need to be big.

[DHutch]

Fro reading what other people have said, can see it being a problem, so i'd be for saving the £500 and a lot of work!

 

Also, to add to the info, we have 26"x32" prop on a 2" shaft, and the shafts about 12foot long.

 

 

Daniel