Prop shaft diameter ?

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ianrichards
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Prop shaft diameter ?

Post by ianrichards » Fri Oct 19, 2018 9:00 am

My hull is now progressing well and as always I have two think two steps ahead.
Considering that my hull is 15’ 3” long and I have a small single cylinder engine, what diameter and material prop shaft should I be looking at.
I do have a propeller that could be suitable and has a 3/4” shaft.
Any advice appreciated.
Thanks
Ian
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fredrosse
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Re: Prop shaft diameter ?

Post by fredrosse » Fri Oct 19, 2018 2:37 pm

A 3/4 inch shaft, common steel, or stainless steel, or bronze will be perfectly adequate. For launch type of a couple horsepower a smaller shaft is probably adequate (although the launch I am currently building is 15 horsepower, and 14 feet long, that will have a 1 inch diameter steel shaft) and the 3/4 inch diameter is only needed to provide generally good insurance against bending the shaft when grounding or hitting a big rock while underway!
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Re: Prop shaft diameter ?

Post by ianrichards » Fri Oct 19, 2018 2:44 pm

Br, thanks for that..
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Re: Prop shaft diameter ?

Post by DetroiTug » Fri Oct 19, 2018 3:26 pm

My two centavos. It depends on a few things. Is it going to be left in the water? Only used occasionaly/rarely, then plain steel is ok, bronze or stainless is better. If it's going to be used a lot, left in the water for extended periods, then use bronze or stainless for sure. If you're using a standard rubber cutless bearing, do not use steel, the shaft will rust and upon removal will destroy the bearing. I used a steel shaft on the tug and after about one season, I had to cut it up to remove it. Regarding size, how is it supported and where? If there is a strut bearing right at the prop, smaller shaft sizes can be used, it's not the torque of the engine that is the issue, its the weight of the hull and the danger of running aground (and you will) and bending the shaft. I use a 1" stainless shaft in the tug which is relatively short and well supported on either end and have had no issues (It has a deadwood and strap to the rudder). I think 1" diameter is really about minimum size.

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ianrichards
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Re: Prop shaft diameter ?

Post by ianrichards » Fri Oct 19, 2018 3:38 pm

Mmmmm ... more then nags to consider.
Many thanks
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Lopez Mike
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Re: Prop shaft diameter ?

Post by Lopez Mike » Fri Oct 19, 2018 7:28 pm

A couple of data points:

My launch is 5 hp at 400 rpm which is 65 ft/lbs on a 1.25" shaft turning an 18 diameter by 24 pitch prop. I think the shaft is oversized.

My 36 foot sailboat has 30 hp at a shaft speed 1400 rpm which is 114 ft/lbs on a 1" shaft turning a 15" diameter by 12 pitch prop. This is typical practice is 20 to 40 ft sailboats.

You ar probably safe with the 3/4" shaft if it's supported properly. I think shaft support is probably more important than torque loads anyway. Don't hit stuff or get a line in the prop and you will be fine.
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ianrichards
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Re: Prop shaft diameter ?

Post by ianrichards » Fri Oct 19, 2018 7:46 pm

Thanks.
Ian
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Re: Prop shaft diameter ?

Post by Kelly Anderson » Sat Oct 20, 2018 1:53 am

What diameter is your crank shaft?

My boat is 23' long, weighs 3,000 lb., has a 6 plus hp engine, and a 1" stainless steel shaft, and is perfectly happy.

My last boat is 20' long, weighs 1,200 lb., has a 2 hp engine, and a 1-1/4" hollow brass shaft about 6' long which would buckle under full steam until I added a third bearing mid-way. Don't forget that the prop shaft is also under compression pushing the boat along.
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Re: Prop shaft diameter ?

Post by barts » Sat Oct 20, 2018 5:04 am

As you might expect, there is lots of detail about this in boat design books.

One handy method is shaft diameter for Aqualoy or Monel should be 1/17.5 3 bladed prop diameter... so if you're using a 14" diameter prop, a 3/4" shaft would be a bit marginal. If you have a Tobin bronze shaft, the divisor is 14.

This is from Gerr "Boat Mechanical Systems Handbook", which is a great book - highly recommended reading.
https://www.amazon.com/Boat-Mechanical- ... s+handbook

Shaft bearing spacing formulae are there as well.

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ianrichards
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Re: Prop shaft diameter ?

Post by ianrichards » Sat Oct 20, 2018 7:32 am

“Boat Mechanical Systems Handbook” ordered, £9.10 here in the UK
Thanks for the pointer.
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