Power Sailing - Useful or just fun?

[DaveC426913]

I don't think it's quite correct to say that the boat does not make its own wind. The engine and propeller thrust -will- generate speed for the boat, and wind for the sails. In turn, this wind -will- generate aerodynamic forces on the sail. If, on average, these forces have a component toward the bow, then they will generally help the engine and make the boat go faster (there are other factors, e.g., how close to hull speed, variable vs. fixed pitch prop, etc.). If the average component is toward the stern, then aerodynamics will fight the engine. This just as a general principle. For more details, one needs to do actual calculations -- the article cited has very nice qualitative explanations.

Roberto

I don't see how forward motion of the boat, as caused by motoring, could possibly generate any wind that has a force component toward the bow.

[RobertB]

Motoring causes an increase in the apparent wind - that in turn creates more lift on the sail.

[yukonbob]

Same concept as sailing on a broad reach as compared to running. A very efficient boat can in theory sail faster that the actual wind speed on a broad reach. You'd think the same would apply to zero wind and power sailing...

[BOAT]

I never did get the hang of motoring a boat with the sails up - it's just something I only do to get the sails down. It's a thought I guess.

[Y.B.Normal]

Motorsailing works similar to an airplane flying. The airplane's wing moving forward through the air makes its own "wind", creating lift because the air over the top of the wing moves faster than the air over the bottom of the wing. That's also why a glider can fly - It's not the thrust of the engine, but the relative wind that creates the lift. A sailboat sail also uses its forward movement (or the wind blowing over the sail) to create lift. The rudder and keel or centerboard keeps the boat from moving sideways toward the lift vector, thus moving it forward. Picture the sail as a vertical wing. Clear as mud??

[Russ]

When I motor with no wind and sails up, they just luff because the apparent wind is head on. Maybe I need to tack.

[cptron]

When I motor with no wind and sails up, they just luff because the apparent wind is head on. Maybe I need to tack.

+1

[Catigale]

Something isn't jibing there.....

[DaveC426913]

Motoring causes an increase in the apparent wind - that in turn creates more lift on the sail.

While it may seem plausible that that could translate into forward motion, it does not follow.

For starters, note that it suggests a runaway effect.

Take a sailboat and give it some temporary speed (motor is one way, but ramp or slingshot will accomplish the same thing).
Deploy your sails to take advantage of this apparent wind.
(Optional step: More sail = more motion. Therefore let's scale the sail up by 10x, LOTs of forward motion.)
However you get up to speed, you've now got forward motion and a fully operating sail.
So if, as you say, more apparent wind creates more lift, and therefore more forward motion, then you can now turn off the motor (or run out of ramp/slingshot).
You are now sailing in zero wind - for free. You have "bootstrapped" yourself into motion. And this can continue forever (no reason you'd slow down - wind is already zero).

In fact, we can simplify it further - the implication of "more apparent wind = more forward motion" is that sailing is a runaway effect - the faster I go, the faster I can go.

No, motoring does not generate wind in a way that can be used to increase the speed of the boat. But it does generate wind that can be used for other things, such as increasing stability.

[BOAT]

Yeah, i hear you all taking but I don't understand it - I think I know what dogs feel like when their owners are talking to them.

This is like the lightning thing - I have listened to a LOT of opinions from many skippers on this and from one in particular that many of you know:
Captain Mike Inmon -
Captain Mike told me about using the motor and the sails together to get back from Catalina - he seemed to know how to do it. My dad used the motor sometimes in the early hours of the morning when the fog was still in to cross the San Pedro Channel, but in my recollection it was always at a very slow speed.

I myself have just never got comfortable with a motor when the sails are up - it just bugs me - I see people do it all the time - but for me it just has a real UN-natural feeling to me. I feel like I'm over-running my sails. I don't like it - so for the most part I don't do it and when I do the motor is rarely more than at an idle speed.

[drams_1999]

Wow it seems that I really got a debate going with the original post! Thank you all (and, simultaneously, my condolences hahaha).

So I have gathered and agree that you can't use the engine to "generate" wind under "practically zero wind" conditions, since, as suggested, this is equivalent to sailing in the "no sail" zone.

I have also gathered that if you motor while on a run, what you are actually doing is reducing the apparent wind, which would negate most if not all savings (the fuel saving would become more negligible as the boat speed increased and the apparent wind coming from astern reduces to zero.)

I have also gathered (thanks to the earlier post of the article) that when reaching, you CAN successfully power sail, provided that there is enough wind to drive the boat to begin with. In the case of a reach, with wind, the boat is already using wind to propel itself forward and if you use the engine in these conditions to push forward more, there would clearly be additional apparent wind generated, and the force would still be directed toward the bow as long as the forward speed isnt enough to make the apparent wind create a new apparent "no sail zone" at your bow. This could easily be proven, as someone suggested, by power sailing on a beam reach on a windy day, with say, the engine at a low RPM, you can measure the speed using gps, then release the sheets, and measure your new speed (without the sails there will be a drop in speed). As suggested (although not explicitly stated), the benefit would be less time to get to the destination, if you were planning on only sailing, or the benefit would be saving in gas to get to the destination, if you were planning on only motoring.

In either case, I think it can only be useful when you are on a beam reach (or close to it), or slightly beating to windward. Any other use of power sailing would be "for fun" or possibly even "for stability", but in my book anytime I run the engine I am cheating....hahaha.

Great thread!!

[RobertB]

Motoring causes an increase in the apparent wind - that in turn creates more lift on the sail.

While it may seem plausible that that could translate into forward motion, it does not follow.

For starters, note that it suggests a runaway effect.

Take a sailboat and give it some temporary speed (motor is one way, but ramp or slingshot will accomplish the same thing).
Deploy your sails to take advantage of this apparent wind.
(Optional step: More sail = more motion. Therefore let's scale the sail up by 10x, LOTs of forward motion.)
However you get up to speed, you've now got forward motion and a fully operating sail.
So if, as you say, more apparent wind creates more lift, and therefore more forward motion, then you can now turn off the motor (or run out of ramp/slingshot).
You are now sailing in zero wind - for free. You have "bootstrapped" yourself into motion. And this can continue forever (no reason you'd slow down - wind is already zero).

In fact, we can simplify it further - the implication of "more apparent wind = more forward motion" is that sailing is a runaway effect - the faster I go, the faster I can go.

No, motoring does not generate wind in a way that can be used to increase the speed of the boat. But it does generate wind that can be used for other things, such as increasing stability.

Suggest a high school physics course - concentrate on the part about aircraft wings. The faster you fly a plane, the more lift the wing creates. A sail, especially on a reach where air is passing over the sail from bow to stern, creates lift but in the forward direction. Increase the speed of the air moving over the sail and you get more lift. It does not matter where this increase in speed comes from. There is also drag from several areas, this will prevent the runaway effect. No one stated that motor sailing lets you move for free, just that the physics indicates there is some advantage in doing so. Now, lets discuss if this is theoretical physics or applied physics (are you in the Sheldon or the Leonard camp?).

[Kevin]

One real common use for motor sailing which remains consistant with all the principals presented is San Pablo bay.

You often can have a nice west wind so you should have a great downwind run but the current can run 3 - 4 knots from the east. Paddlewheel speed through the water is ~4 knots but GPS speed can be 0 to negative. Use the motor just enough to overcome the current and you sail at the speed over the ground you might expect in a given wind with a speed through the water way above what you'd expect in that wind under sail alone or at that throttle under engine alone.

[DaveC426913]

Suggest a high school physics course - concentrate on the part about aircraft wings.

I am pretty comfortable with the physics. It is a wonderful day when I get to combine my love of physics with my love of sailing.

The faster you fly a plane, the more lift the wing creates.

Yes, lift - not forward motion. While the wings on a plane do lift it (force upward), they do not supply a force forward - that's thrust. Wings in-and-of-themselves still impart drag - the opposite of thrust. Look at any force diagram of a wing. You will see that the lift component is ALWAYS toward the rear of the craft. It might be arbitrarily close to vertical, if the wing is thin enough, but it will never be forward of vertical.

(Again, if a wing didn't act horizontally as a drag component, you would not see faster craft having smaller wings - they do that to reduce drag. If the wings did not produce drag, they could be made with unlimited size - which is of course nonsensical.)

It is worth noting that, while a sail operates similar to an aircraft wing, they are not the same beast. A wing is rigid, and has a camber even when attack angle is zero (and therefore could supply lift). A sail at attack angle zero will luff and has no camber. Which is why, when head-to-wind, a sail provides zero lift. Just something to keep in mind when we're talking about extracting lift from attack angles.

[DaveC426913]

Let me ask you a question:

You are motoring, there is no wind.
Where exactly is the apparent wind coming from? (dead ahead)
How exactly would you set your sails to catch this wind?

And how would your sails, (which are set to catch this apparent wind, coming from dead ahead) provide thrust, as opposed to drag?

No, the key is that there must be enough actual wind to take the apparent wind off your centreline (away from your bow). Apparent wind is off dead-ahead, then and only then can you redirect a portion of it into your sails (in a way that will provide lift in a direction more forward than directly abeam).

And note that it decreases as you throttle up. The faster you motor, the closer the apparent wind will come to dead-ahead ,the less you can extract from it. Once it is dead-ahead (as it will always be if actual wind is zero) you can extract no further forward movement from it.