Prop sizing and thrust

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.

 

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.

 

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

 

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.

 

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

 

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).

 

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

 

"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?

 

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

-Tom

 

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

 

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...

 

Put the sails up and sail or buy a powerboat.

 

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

 

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

 

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 ?