Prop sizing and thrust

Diameter is the KEY.

Simply tow a bucket of various diameters and see how much the speed losses bother you.

Then tow a prop of the same diameter and take what ever electric that creates.

If the vessels prop is over 24 inches , be prepaired for HUGE speed loss.

FF

 

you have pizza for show off, but you only eat bread. Then why keep pizza? thats wat Gonzo meant..

 

Jonr,

force and power are not equivalent.

Power = force x speed

So you only need 50hp if you want to make the full 6knots into the wind.
If you happen to have 6 knots of adverse current and a gale off a lee shore then you need the 50hp, or even 60hp. But being in that position doesn't say much for you route planning.

If you have a large enough propeller you can theoretically make headway into any amount of wind with a tiny amount of power. Practical prop size constraints will limit it somewhat. Using controllable pitch prop will help a lot, as Richard pointed out.

If, as you suggest you are willing to accept 1 knot forward speed, and we assume to prop can operate at this speed with reasonable efficiency, and we ignore currents, then the HP needed to propel you at 6knots in still air will be adequate in the gale.

 

Unfortunately those assumptions are very far from reality for typical sail boat propellers and engines. They are designed to give rather good efficiency at 6-8 kn, not at 1 kn. For good efficiency at 1 kn, you would need a much bigger propeller AND much lower RPM.

A much better assumption is that maximum thrust is constant. E.g. you might get 4 kN (900 lbs) thrust from a 40 hp engine at maximum speed (say 7-8 kn). You can assume, that you will get about the same thrust at any speed lower than the maximum all the way to bollard pull.

If you typically cruise at 10 hp, you have 4x margin for bad weather. If you allow your speed to decrease, you have a bigger margin, since resistance will be lower.

I just ran a test for a 8 tn 40' sailboat with my VPP. It had a resistance of 1 kN at 6.6 kn requiring about 10 hp in calm conditions. At 20 m/s (40 kn) headwind the resistance is about 3.7 kN and about 40 hp is needed. At 4 kn the resistance at 20 m/s headwind is only 2 kN, thus you would still have a good margin for getting forward.

Note that my VPP has a model for some average wave drag. In reality it can be much higher or lower. At 20 m/s headwind at 6.6 kn the windage was 1.1 kN and sea wave drag 1.5 kn. Windage may be much higher for boats with higher superstructures, freeboards etc.

 

Motor-sailing

I have been out in a 7.7m trailer-sailer in 38knots and 1.5-2.0 m waves. We decided we'd had enough of sailing, even with a deep reef in the main and the furling jib down to a very small size.

We went for the outboard motor (a 10hp 4 stroke Honda with a 4 blade prop). We went backwards using 'normal' revs and could only just hold our own against wind and waves on about 3/4 throttle. We were able to maintain 'head-to-wind' as long as the helmsman concentrated. Under normal conditions, a 3/4 throttle setting would have had us plowing along with a huge bow wave.

It would have been more comfortable to heave-to.

We weren't terribly happy and so we decided to sail the boat again. We put the second reef in and furled the headsail in all the way and started motor-sailing to windward at about half throttle. We made slow but steady progress and after getting up to speed, we were able to reduce the revs to about a quarter throttle. I should mention that a quarter throttle is much more than we would use in more benign conditions, even if in a hurry.

On reflection, I think the motor prevented us from being stopped dead by waves while sail power actually moved us over the ground. Sails certainly made the ride much more comfortable.

I guess what I'm saying is that yes, you do need some spare capacity in your motor, but maybe not huge reserves if you are still willing to sail the boat.

I have used a number of outboards types designed for trailer sailers, two and four stroke, 5-10hp. Newer and older designs. This is my opinion:

• Somehow, the ponies seem bigger in newer outboards.
• 'High thrust' props really work for slow motor boats like trailer sailers.
• Three blades are better than two but I'm not sure of the benefits of four blades.
• Good reliable propulsion is about the best insurance you can have.

Peter

 

Apologies Greenhorn,
I should have checked how old this thread was.
Shame, I really worked hard on my response.
Peter

 

I agree for a typical fixed propeller thrust won't increase nicely as speed goes down, but surely a big variable pitch prop is a better solution than an engine four times the size?
Although I have to admit I've never used a VPP myself.

 

I don't think anyone has yet mentioned out the easy way to produce thrust to windward, at least in a sailboat: Use the keel.
Motor and/or sail with wind a bit forward of the beam. The motor then only needs it's 10hp to get to 5-6 knots and the keel produces the lift to make way to windward.
Obviously this is not an option if the waves get to the point where beam to wind isn't a workable position.

 

You would not use a 10 hp engine for a 40 footer. 40 hp would be quite normal size and it then has 4x margin for normal cruising speed.

I don't know much about CPP's, but I don't think you get much more thrust with them either with a typical shaft rpm of 1300 and propeller diameter of 450 mm. Maybe you get 50% more, but certainly not 4x.

If you have a sail boat, you don't want to have a big CPP, which probably costs a fortune and causes a lot of drag while sailing.

 

Very true. I would expect at least 25hp in an average 40 footer and 40hp isn't excessive.

I would assume there is a low drag configuration for CPP's, either a particular pitch, or feathered, but as I say I've never tried one, so I might be talking out of my arse