Prop sizing and thrust

[jonr]

I've seen a lot of posts about sizing props and engines based on maximum speed achieved and how "hull speed" means that more power is a waste. I have not seen discussion of having a significant amount of reserve thrust to account for windage in storm conditions. Ie, it seems to me that in normal conditions, one should be significantly under-proped and over powered.

Roughly how big a factor is this? Assume a 40' sailboat, perhaps 40 kts wind and a current that is keeping the boat from pointing straight into the wind (ie, increased windage).

I get something like 5x more thrust than you might think you need. But doesn't this also mean that under normal conditions, you are going to be operating at 20% of max HP? This isn't good for some engines.

[mark775]

Hi, Jon. You may be confusing 20% of available power with 20% of RPM. 20% of power is going to put engines I know past the carbon/ glazed cylinder stage that you are worried about. I cannot address your sailboat example; "40' sailboat, 40 kts wind and a current that is keeping the boat from pointing straight into the wind (ie, increased windage)" precisely, but we have all felt big gusts blow us over (turn us from the entended course) and had to give more throttle to compensate. The extra throttle is usually momentary, only needed to turn back up and I would think, with a keel, wind would have less effect than on most powerboats. I think you are talking about the attempt to maintain light airs cruising speed when in a gale and I think you just need to accept the fact that when it is blowing 40 on the nose, you are going to go slower. If it is blowing enuf some points off the bow to blow you around, I submit that a different course may be in order because, while you certainly want reserve power, five times is more than we want to pay for. Now the current - a non-issue, as far as I'm concerned. You have current whether it's windy or not and you account for it, try to run with it - or take longer, burn more fuel. Too many people look at current as a force attacking your boat, IMO, when it should be looked at as the body of water moving with your boat on it. No amount of horsepower changes that - the only thing you can do is use current, live with current and plan for it. It does NOT turn you off course (unless you run across a tideline or such. It only makes your effective COG or SOG different.

[FAST FRED]

First most sailboats will not chose to motor at hull speed, its noisy, fuel wasting and makes big wakes.

SQRT LWL x 1.34 will give hull speed , usually x1.15 is chosen as it has 1/2 the fuel burn and damn near the same speed.

Most aux cruisers ARE operating at perhaps 40% of engine power usually at 1 gph or less.

So there is usually loads of excess power left if you want to slam into big waves , and smash the crew up with high G loadings.

FF

[jonr]

Let's see if I can put some rough numbers on it that others can refine. Say a 40' sailboat will go 6 knots (SQRT LWL x 1) with a 10HP engine in calm conditions. This equates to a thrust of ?? 500 lbs?. You think the prop and engine are sized nicely.

Later you are anchored on a lee shore and a gale comes up and the anchor is dragging. A 40 knot wind is creating 1000+ lbs of force and you need to motor into it. The engine is bogged down and you are being pushed backwards - you wish you had a different prop and a bigger engine.

My point is that the engine and prop should be sized for the extreme conditions, not the normal ones.

[FAST FRED]

A simple rule of thumb is an average prop will produce 20 lbs of thrust for every hp , a really finely matched unit will produce as much as 25 lbs per hp.

Stick it in a Kort ring , vary the pitch to perfection and perhaps 27 might be doable.YRMV

So that "50" hp marinization might be able to make 1000lbs of thrust , although it is really doubtful.

The prop will be optimized for the usual cruising speed of the boat , say 6K , so the prop will expect 6K of water flow to accelerate to push the boat.

Stop the 6K water speed and at rest the prop is hardly optimized for producing the required thrust.

"My point is that the engine and prop should be sized for the extreme conditions, not the normal ones."

If this is done the boat will perform very poorly at ALL times ,,,except the extrime.

The usual compromise is to optimize the cruise,, 99% of the boats use and accept less than sparkeling performance that 1% that a tug boat is required.

FF

[jonr]

Good points - looks like 5x difference in thrust needed from normal conditions to a gale is about right. I agree that 50HP (not 10HP) in a 40' sailboat is in the ballpark for extreme conditions.

So what is the solution - how can I get an engine/prop that will get me out of trouble but also perform efficiently under normal circumstances? An engine that can turn off cylinders and a variable pitch prop?

A couple of things that help. In a gale, I'm willing to accept 1 Kt forward speed and I'm willing to operate at 100% output (vs < 70% in cruise).

[apex1]

Get a CPP and enjoy a efficient and long lasting propulsion.

http://www.boatdesign.net/forums/props/controllable-pitch-propeller-summary-30695.html

Regards
Richard

[TollyWally]

"You have current whether it's windy or not and you account for it, try to run with it - or take longer, burn more fuel. Too many people look at current as a force attacking your boat, IMO, when it should be looked at as the body of water moving with your boat on it. No amount of horsepower changes that - the only thing you can do is use current, live with current and plan for it."

I love the feeling of running with the current even in a powerboat. I call it ride with the glide. I use a fuel flow meter as my go to instrument for monitoring my motor. Between that and a gps I track current pretty accurately. You can nose the boat into or away from currents, the speed and the tach stay the same but the fuel burn is going up or down in tenths or hundredths.

Fred,
Is there any poor boy way of estimating real world thrust?

[Submarine Tom]

Yes, current is a wonderful thing... if you're going with it!

-Tom

[FAST FRED]

Is there any poor boy way of estimating real world thrust?

The rule of thumb already given does well, but D Geere (SP?)has a very good simple book on propellers.

FF

[kistinie]

Would not it be easier to impose the prop-builders to give a normalized power curve ?

Till this moment, all of us, loose time and money to try and guess...

[gonzo]

Put the sails up and sail or buy a powerboat.

[kistinie]

Put the sails up and sail or buy a powerboat.

Do you mean saying this it will be impossible to get Power curve ?

[gonzo]

No, I am saying that having a sailboat and not using the power provided by the wind is bad seamanship.

[kistinie]

I love sailing
I love producing my own electricity when sailing by regeneration from the prop
I love electric boats for the noise comfort

A sailing boat can also be an electric boat


So i need to select a prop reading its 4 quadrant power curve.

Where is the contradiction ?

[FAST FRED]

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

[conceptia]

I love sailing
I love producing my own electricity when sailing by regeneration from the prop
I love electric boats for the noise comfort

A sailing boat can also be an electric boat


So i need to select a prop reading its 4 quadrant power curve.

Where is the contradiction ?

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

[Autodafe]

Let's see if I can put some rough numbers on it that others can refine. Say a 40' sailboat will go 6 knots (SQRT LWL x 1) with a 10HP engine in calm conditions. This equates to a thrust of ?? 500 lbs?. You think the prop and engine are sized nicely.

Later you are anchored on a lee shore and a gale comes up and the anchor is dragging. A 40 knot wind is creating 1000+ lbs of force and you need to motor into it. The engine is bogged down and you are being pushed backwards - you wish you had a different prop and a bigger engine.

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.

[Joakim]

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.

[Dr. Peter]

Put the sails up and sail or buy a powerboat.

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

[Dr. Peter]

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

[Autodafe]

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.


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.

[Autodafe]

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.

[Joakim]

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?


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.

[Autodafe]

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

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.

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 :)