Prop to use my outboard on yacht

simonafloat · 27 April 2017, 20:11

I'm viewing a 20ft displacement sailing yacht this weekend. It has an outboard well behind the cockpit and the 15hp mariner off my sib would be ideal. However I have no idea what prop to use... I know it'll need changing but what will I need? The yacht probably weighs 2 tonnes and has a 6knot hull speed.

camelgas · 27 April 2017, 20:27

Most sailboats use a longshaft motor , so measure carefully as most SIB's use a shortshaft.

willk · 27 April 2017, 20:29

You'll be needing a high thrust prop of some description:

simonafloat · 27 April 2017, 20:38

Quote:

Originally Posted by camelgas

Most sailboats use a longshaft motor , so measure carefully as most SIB's use a shortshaft.

My sib is longshaft so all good

Poly · 27 April 2017, 22:32

Simon, 15HP is quite big for a 20' yacht. So I think you'll want the smallest pitch prop you can find that will fit. Is the o/b well designed for the weight and power that 15HP will involve? As willk suggests if you can find one that is designed for thrust rather than speed then all the better. Often that will be 4 blades rather than 3.

Working backwards: 6knots = ~437000 inches per hour. Or 7291 inches per minute. If we assume that you want the engine to be running at ~ 4000 rpm (so its not maxed out, but is giving high torque) and that the gear box is roughly 2:1, then the prop would be doing 2000 rpm so in round numbers the prop would need to move about 4" per revolution. Your gearbox probably isn't exactly 2:1, the prop will slip a bit (or a lot!) and 4000 rpm won't be the perfect sweat spot - so we are talking very back of fag packet here. I've never seen a prop as small as that... on an engine as big as 15HP so I think you might be into compromise territory.

Landlockedpirate · 27 April 2017, 23:56

I dont know what yacht you are looking at, but I think your biggest problem maybe size.

My dad had a Leisure 20, it was a squeeze to get a 5hp 2 stroke in the well, there was no chance a 15 would fit.

If its a deal breaker, you really need to try the engine in the boat.

On a separate note, swapping the engine from yacht to dinghy and vice versa will be nearly impossible unless you use the topping lift or outboard crane of some sort.

Pikey Dave · 28 April 2017, 09:37

Quote:

Originally Posted by Landlockedpirate

On a separate note, swapping the engine from yacht to dinghy and vice versa will be nearly impossible unless you use the topping lift or outboard crane of some sort.

I agree, depending on how frequently you swap the engine from boat to boat, you'll soon start looking for another engine for the yacht. Especially if it involves swapping the prop each time.

Fenlander · 28 April 2017, 10:04

I would buy a dedicated OB for the sailing boat... 6hp or 8hp would be fine. 4-stroke better as you will reduce the chance of exhaust smoke "collecting" in the well or cockpit and the reduced noise at lower revs is very welcome on a sailing boat. Get a 4-blade prop of a pitch at the lower end of the range available for the OB as it will give better grip on the water... for stopping and reversing in particular.

Fenlander · 28 April 2017, 10:13

I would buy a dedicated OB for the sailing boat... 6hp or 8hp would be fine. 4-stroke better as you will reduce the chance of exhaust smoke "collecting" in the well or cockpit and the reduced noise at lower revs is very welcome on a sailing boat. Get a 4-blade prop of a pitch at the lower end of the range available for the OB as it will give better grip on the water... for stopping and reversing in particular.

Btw interested seeing you are thinking of a modest size sailing boat. I spend my childhood into my teens on and about such craft and self and Mrs Fenlander wonder if we might enjoy one up to 25ft now. We looked and looked at this ad thinking hard "what if" but decided this year had other priorities... an idea for the future perhaps...

Jaguar 25 Sailing Boat / Yacht - Bilge Keel - on the hard at Portsmouth | eBay

Fenlander · 28 April 2017, 10:15

I would buy a dedicated OB for the sailing boat... 6hp or 8hp would be fine. 4-stroke better as you will reduce the chance of exhaust smoke "collecting" in the well or cockpit and the reduced noise at lower revs is very welcome on a sailing boat. Get a 4-blade prop of a pitch at the lower end of the range available for the OB as it will give better grip on the water... for stopping and reversing in particular.

Btw interested seeing you are thinking of a modest size sailing boat. I spent my childhood into my teens on and about such craft... self and Mrs Fenlander wonder if we might enjoy one up to 25ft now. We looked and looked at this ad thinking hard "what if" but decided this year had other priorities... an idea for the future perhaps...

Jaguar 25 Sailing Boat / Yacht - Bilge Keel - on the hard at Portsmouth | eBay

Fenlander · 28 April 2017, 10:26

Re pitch something like a 6" would be close to ideal for a 6/8hp.

simonafloat · 30 April 2017, 14:31

Quote:

Originally Posted by Fenlander

I would buy a dedicated OB for the sailing boat... 6hp or 8hp would be fine. 4-stroke better as you will reduce the chance of exhaust smoke "collecting" in the well or cockpit and the reduced noise at lower revs is very welcome on a sailing boat. Get a 4-blade prop of a pitch at the lower end of the range available for the OB as it will give better grip on the water... for stopping and reversing in particular.

Btw interested seeing you are thinking of a modest size sailing boat. I spent my childhood into my teens on and about such craft... self and Mrs Fenlander wonder if we might enjoy one up to 25ft now. We looked and looked at this ad thinking hard "what if" but decided this year had other priorities... an idea for the future perhaps...

Jaguar 25 Sailing Boat / Yacht - Bilge Keel - on the hard at Portsmouth | eBay

I had another look at the yacht yesterday and decided not to go for this particular one. I've sailed dinghies a lot but always wanted to step up to a yacht. To be honest I think I'll continue with plan A which is save up for something a bit bigger with full blue water capabilities. We are lucky that moorings are still dirt cheap down here and an old yacht, up to 35 foot is achievable, given a lot of hard saving. My lifes ambition is to do a circumnavigation or at least across the pond. In the meantime the aerotec is definitely the simplest, cheapest way to stay on the water. Will be trying to get aboard yachts and do some crewing. And yes, my 15 would have been massive overkill on the boat I viewed!

simonafloat · 30 April 2017, 14:33

Quote:

Originally Posted by Fenlander

I would buy a dedicated OB for the sailing boat... 6hp or 8hp would be fine. 4-stroke better as you will reduce the chance of exhaust smoke "collecting" in the well or cockpit and the reduced noise at lower revs is very welcome on a sailing boat. Get a 4-blade prop of a pitch at the lower end of the range available for the OB as it will give better grip on the water... for stopping and reversing in particular.

Btw interested seeing you are thinking of a modest size sailing boat. I spent my childhood into my teens on and about such craft... self and Mrs Fenlander wonder if we might enjoy one up to 25ft now. We looked and looked at this ad thinking hard "what if" but decided this year had other priorities... an idea for the future perhaps...

Jaguar 25 Sailing Boat / Yacht - Bilge Keel - on the hard at Portsmouth | eBay

The jaguar is nice. I've viewd a few westerly centaurs recently which are 26ft with twin keels. Really like them, not the best sailing performance but loads of cabin space and are often outstanding value for money.