Motorsailing- It seems like magic

[AuChante]

I've only had the boat out a few times, and I've leaned toward the safe side and reverted to motor power at any sign of possible navigational dangers (narrow channels and such). I have always expected the sails to flap about like big flags, but it seems that the engine creates wind.


I'll be sailing as close as possible into the wind and it doesn't seem like enough to stay in the channel. I crank up the engine, and now I can sail 10deg into the wind. Or I'm making way at half a kt, crank the engine, pick it up to 5 kt, and the sails magically stay inflated and seem to be helping propel the boat (even going down wind).


What's happening here? If I'm going close to directly down wind at half a kt and crank the engine... the sails should deflate and flap about right? I'm astounded every time I start to motor.

[danstanford]

Apparent wind is what you are seeing. The sails only react to the wind they feel and motoring creates wind over the sail. The interesting thing is noting the change in wind angle which will move forward on the boat.

[ahun]

It's useful to know how to do arithmetic operations with vectors. Substitute wind and boat velocity with vectors, lay it down on paper and it will clear it up.

A real life example: look up recent America's Cup racing videos. These boat go so fast, they are always go upwind even when they go downwind, hence they don't need spinnaker. They just go faster than wind itself, shifting the apparent wind forward of the beam.

I hope I managed to confuse the matter further.

[AuChante]

Originally Posted by ahun

It's useful to know how to do arithmetic operations with vectors. Substitute wind and boat velocity with vectors, lay it down on paper and it will clear it up.

A real life example: look up recent America's Cup racing videos. These boat go so fast, they are always go upwind even when they go downwind, hence they don't need spinnaker. They just go faster than wind itself, shifting the apparent wind forward of the beam.

I hope I managed to confuse the matter further.

It’s been a long time since I’ve done that, but in my head that doesn’t seem account for it.

[capt jgw]

Ice boats do that too. Some can do over 80kn so even when sailing with the wind they end up close hauled. True wind 20kn on the quarter (they never sail dead down wind, that would be slow, no faster than the true wind speed) plus 80kn boat speed equals 60kn just off the bow. Scary.

[Brioche]

Vectors are two-dimensional measurements that can be added easily on paper. A one-dimensional measurement could be something like distance or velocity. When you add another dimension, especially direction, you have a vector. You could be going 5 knots in any direction - that is one dimensional (a scalar). But if you are going north at 5 knots, you have a vector. You can diagram that as a line, say five cm or inches long, from a dot directing upward. If you have another directional speed, say current or tide, coming from the port beam at 5 knots, you can diagram that as an arrow from left to right, again 5 units long. when you attach the first arrow to the end of the second, you get a right angle that shows the addition of the two vectors. The sum is from the beginning of the first arrow to the end of the second, and it is a 45 degree line, the hypotenuse of a right triangle, with a length you could measure, or calculate as the square root of the sum of the squares of the other sides (25 plus 25), or about 7, which means your motion is the addition of your velocity vector and the tide vector, or a total speed of 7 knots at 45%. The same can be done with your motion plus the speed of the apparent wind.

[StuM]

Originally Posted by ahun

It's useful to know how to do arithmetic operations with vectors. Substitute wind and boat velocity with vectors, lay it down on paper and it will clear it up.

A nice little tool to play with to get the feel of true/apparent wind vectors
https://www.starpath.com/freeware/true_wind.htm

[wpatch]

I have to admit I prefer either power or sail...I don't like powersailing all that much...it feels uncomfortable and I get that nasty pucker feeling especially in higher 15+ wind.

I get that a lot of folks like it, but are there advantages that I am missing, or is it just a fast/easy solution?

[Wolfe10]

Originally Posted by wpatch

I have to admit I prefer either power or sail...I don't like powersailing all that much...it feels uncomfortable and I get that nasty pucker feeling especially in higher 15+ wind.

I get that a lot of folks like it, but are there advantages that I am missing, or is it just a fast/easy solution?

We motorsail all the time when in ICW or making overnight runs and sail only is more than a 1.5 knots below hull speed. Not talking about day sailing, but "making distance".


In 15 knot wind, can't imagine that sail only won't get you to your speed goal. OK, make the 12 knots or so unless DDW.

[Cwens]

All this talk about apparent wind vectors is accurate, but may be missing the point of the original question, which I read as, approximately, "Why, when motor sailing, can you carry the apparent wind at a much-closer-than-close-hauled angle that, when not motor sailing, would leave the sails flapping.



My answer is, "I don't think you really can." Try sailing close hauled, no motor. Now head up another 5 or 10 degrees or more. Windward telltale will be flipping all over the place, boatspeed will drop to near zero, but the sails don't literally start flogging until you're almost directly head to wind. Sailcloth is stiff and a sail will hold some shape way above close hauled.

[Wolfe10]

Originally Posted by ChrisOwens

My answer is, "I don't think you really can." Try sailing close hauled, no motor. Now head up another 5 or 10 degrees or more. Windward telltale will be flipping all over the place, boatspeed will drop to near zero, but the sails don't literally start flogging until you're almost directly head to wind. Sailcloth is stiff and a sail will hold some shape way above close hauled.

Correct. Same true wind and more forward speed (engine) will move apparent wind FORWARD.

[Cwens]

Originally Posted by Wolfe10

Correct. Same true wind and more forward speed (engine) will move apparent wind FORWARD.

I don't think the question asked was, "why does the apparent wind move forward when you add engine power?" which I imagine everyone on this thread understands. I think it was "Why don't the sails flog when you apply engine power and bring the apparent wind forward of close hauled?"

[AuChante]

This could very possibly be a lack of experience thing (2 outings so far). Ill describe my thought process.


****
It looks like I won't be able to make it through the pass without tacking... especially since my max speed has been 4kt today. I'm darn near 45deg to the channel so swapping to a starboard tack won't help. I really don't want to tack in such a congested area without more experience on the boat. I think I'll crank the engine and motor thru. Hopefully the sails don't flog me to death.
****


Then I cranked the engine and motored toward what I thought was dead into the wind and the sails stayed happily inflated. Cue confusion.

[Wolfe10]

Originally Posted by ChrisOwens

I don't think the question asked was, "why does the apparent wind move forward when you add engine power?" which I imagine everyone on this thread understands. I think it was "Why don't the sails flog when you apply engine power and bring the apparent wind forward of close hauled?"

If close hauled before starting the motor, it WILL flog or at least luff as motor-driven speed increases as apparent air moves forward.


Said another way, "apparent wind forward of close hauled= luff/flog".


Luff vs flog is pretty much wind speed and sail shape/condition related.

[TrentePieds]

AuChant saied: " I really don't want to tack in such a congested area"

Don't be afraid of doing that. But you do have to understand that to drive the head of the boat reliably through the "eye of the wind" you need to rely on her inertia, and to do that you need to keep your "footing", i.e., moving as fast as possible.

I learned to sail in 21-foot keelboats out of a ship canal that was no more that 100 yards wide, i.e., 15 lengths of the boats we used, and 2 1/2 miles long. That sharpens up your short-tacking :-) So try it this way:

Imagine, now, that you are sailing very "high in the wind", i.e., you are sailing with a TRUE WIND ANGLE" of about 45º on, say, port tack, i.e., the boom is out to starboard. The APPARENT WIND ANGLE will be MUCH less than 45º. How much less will depend on how fast the boat is moving through the water. This is where the business of "vectoring" comes in, but we can come back to that later. Because what YOU feel on your face is the apparent wind, you may FEEL that you are going faster than you really are, which may not be fast enough to bring the boat about, and if you try it, it will seem as if the boat isn't responding to your will.

Therefore, if you now want to come over on the starboard tack, the first thing to do is turn AWAY from the wind, i.e., the "wrong" way. Just a little, 5º or 10º, will do it. This is to build up more speed and therefore more inertia. The apparent wind will feel as if it is shifting further aft than that. Now, the moment the luff of either sail begins to "luff" (Confusing! the fore edge of the sail is named "the luff", the action of the luff collapsing is called "to luff"), you smartly lay the helm "alee", i.e., you turn the boat's head into the wind. Both sails will luff, their bunts (their "bellies") will "shake" (flap about), and as the boat continues to turn on her inertia both sails will fill again on the new side. A rudder deflection of 15º or so should be enuff. If you deflect the rudder more than that, it will act as a brake rather than a steering device.

You can leave the jib sheet set for the part of the turn that takes the boat's head JUST past the eye of the wind. That will backwind the sail and thereby help the boat turn. Hold the jib backwinded for just a brief moment, then let fly the "old" jib sheet and trim up the "new" jib sheet on the new leeward side.

Let the boat come to her footing on the new tack. But don't "pinch" her, i.e., don't try to point too high! As her speed builds you can "come to weather" again, i.e., point her as high as she is comfortably willing to go. The "tell tales" you have, or should have, in your sails will tell you when you've reached your minimal feasible apparent wind angle.

Bonne chance :-)!

TrentePieds