# Need help calculating headwind drag in watts from force in newtons

Discussion in 'Hydrodynamics and Aerodynamics' started by Jacques_clue_no, Jul 20, 2012.

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### philSweetSenior Member

I don't know how the anchor rode tension was calculated, but I would be very skeptical of any generic formula for this or trying to relate it to some other set of circumstances. For instance, if anchored in a five-knot current with the wind blowing 90 degrees to current, some boats will pivot to the wind and drag sideways through the current. Others will stay with the current and pull to the side with the wind on the beam. You'd need to make some pretty strong assumptions just to put limits on what a 30 knot wind could do to your anchored boat. I don't see much hope of then relating this to aero drag when underway.

The other problem I see is you are holding the propulsive efficiency constant as aero drag increases. It will plummet. You might figure the prop efficiency for 1. a 14x14 sailor prop at 1200 rpm with a VA of 4 knots and 2. a 14x14 at 1500 rpm with a VA of 4 knots. The amount of drop will depend a lot on your prop selection. E drives can fiddle with the torque curve and one argument for them has been that they can be set up to suffer less under off design conditions than a diesel would. But I haven't heard of anyone who says they can feel this from the seat of their pants. They seem to be less versatile than diesels, probably due to weight and cost constraints.

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### DCockeySenior Member

I assume propulsive efficiency is defined here as:
(thrust x speed through water) / (power into shaft/propeller)

In that case for a particular boat with a particular fixed blade propeller the difference in propeller propulsive efficiency will depend only on the speed through the waters and the external conditions such as wind which are affecting the resistance of the boat. The propeller rotational speed needed to generate the thrust required to offset at a particular resistance and particular speed through the water will be independent of the source of the motive power turning the shaft; whether gasoline IC engine, Diesel IC engine, electric motor, steam engine, etc.

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### philSweetSenior Member

DCockey, I agree as far as a single design point is concerned; but on a small craft, a diesel prop is selected largely due to the diesel's idiosyncrasies. The argument is that you can (should) choose a different prop for an electric drive and let the electric motor/controller deal with excursions from the design power curve. It looks good on paper, but I haven't heard any gushing reviews.

A diesel will hold constant governed RPM with a change in torque. A throttle plate gas motor will adjust RPM and torque. An electric motor will respond however you program the controller to respond. And this, they say, means a more efficient prop can be used and still provide acceptable driveability, or whatever you call that in a boat. As an analogy, think turbo selection in a truck motor. It's the idiosyncrasies that end up costing 5 hp or so out the waste gate. Or you can save the 5 hp but it will be a dog to drive.

So it depends on your goal. If you are trying to maximise efficiency, that's one set of choices. If you are trying to improve throttle response and general ease of operation, you probably end up doing the opposite things.

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### DCockeySenior Member

For a given propeller in a given boat
- speed through the water
- thrust
- shaft speed
- shaft torque
are inter-related such that there are only two independent variables. Pick any variables from the list above and the other two are determined. The variables can be rearranged into different combinations; for example shaft power (shaft torque x shaft speed) can be used instead of shaft torque. The combinations used in propeller charts are also examples.

As noted above different engines and motors will have different types of relationships between shaft speed and shaft torque (or shaft speed and shaft power, etc). They also have different maxium shaft torque/power available as shaft speed varies. This means that how the engine/motor responds when the throttle is changed or the boat's drag characteristics change due to wind, wave, etc changes will be different depending on the engine/motor's characteristics.

But once the boat settles down to a new steady speed the propeller characteristics are what determine both shaft speed and torque, not the engine/motor control characteristics. It appears to me that at least some of the arguments along the lines of "you can (should) choose a different prop for an electric drive and let the electric motor/controller deal with excursions from the design power curve" don't recognize this. Once a propeller is selected at a given boat steady operating point there are no choices to be made in shaft speed and shaft power/torque, irregardless of the control characteristics of the engine/motor.

The differences between various engines/motors in maxium shaft torque/power available as shaft speed is varied as well as variation in engine/motor efficiency with shaft speed and shaft torque/power and other engine characteristics can certainly lead to different choices for propellers.

Perhaps I'm belaboring a fundamental point that is obvious to many reading this. But I believe it is not universally understood.

Last edited: Jul 27, 2012
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What you mean by : lead to different choices for propellers'?

Choices of what?

6. ### FrostyPrevious Member

Stand back boys,-- give en some space, this is going to get messy.

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### DCockeySenior Member

Diameter, pitch, number of blades, etc.

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### DCockeySenior Member

Not necessarily.

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So you're suggesting that as a boat is moving along, the changes in engine power will make the engineer on board whip off the prop installed and change the prop from say a 4 bladed to a 5 (or diameter/pitch etc) because at a lower power its is better?

10. ### FrostyPrevious Member

Oh,--- if only you could.

Variable pitch is an attempt at it.

And 2 speed transmissions.

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### DCockeySenior Member

Of course not. Why would you think I might be suggesting something like that?

Before I saw this message from you I added a phrase to the sentence you questioned to clarify that I was talking about different propellers being selected for different engines/motors.

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### DCockeySenior Member

A 2 speed transmission allows the engine/motor operating point to be changed but not the operating point of a fixed pitch propeller for a given boat speed and drag.

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Because you said so here:

You are suggesting that as the shaft speed is varied the power changes, thus ....lead to different choices for a propeller.

A propeller is designed for a fixed position, unless a CPP. Once the parameters have been set the effects of lower rpm doesn't mean a change of prop mid journey, it just mean lower efficiency. (Unless a CPP).

These effects are considerations during the design of a prop. But once fixed, it is what it is. Yet your suggesting ...different choices of propeller....when there is a change in rpm or power etc.

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### FMSSenior Member

What I thought he was saying is that the prop selection for a conventional engine considers the whole torque curve. A prop selected for maximum efficiency at 3000 rpm wouldn't be the same as for 4000 rpm on a gas engine or to allow some engine and hull combinations to have adequate holeshot.

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### DCockeySenior Member

You have mis-understood what I wrote, apparently because you selected one sentence and incorrectly analyzed it in isolation, rather than in the context of the entire posts and other relevant posts.

You brought up changing the propeller in mid journey or when there is a change in power. Not what I said or intended.

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