Motor Sailing Dos and Don'ts

[Bash]

don't make the easy things difficult.

"true wind" is apparent wind minus STW.
"ground wind" is apparent wind minus SOG.

[cwyckham]

Originally Posted by Ziggy

So your boat sails with wind from dead ahead? Could you explain the physics of that?

As Dockhead pointed out, apparently (pun intended), I was misusing the term true wind. I've never heard of ground wind. So yes, the wind over the ground was zero, but we were in a few knots of current, trying to cut across it. This gave us a few knots of true wind (apparent wind minus boat speed) on our beam, and we were able to use that to sail forward at about a knot(ish) which swung the apparent wind forward and increased it to a bit over three knots. We close reached across the current until we popped out the other side. Once we were out of the current, our apparent wind was just opposite our motion, our foresail backed and we stopped.

Chris

[frant]

Originally Posted by cwyckham

This sort of thing is always at risk of degenerating due to arguments about exact wording. It's not clear to me exactly what you mean or how you arrived at your figures. I tried to be pretty careful with my wording in my post to indicate that enough thrust was given to increase the speed to two knots, independent of any extra synergistic apparent wind increases. Therefore, even if apparent wind didn't help your case, you would at worst have 1+1=2, simply by the fact that you increased your thrust until you got 2 knots of boat speed.

After that, you're right that the increased power from the apparent wind due to your boat speed would absolutely be offset at least somewhat by the increased drag from going faster. At two knots, your engine is giving you enough power to overcome all drag, but you have some extra new apparent wind to play with, so you have a surplus of thrust over drag and you will accelerate. As the drag increases, you will find a new equilibrium that will be somewhat greater than two knots where the total thrust due to wind and engine exactly equals the increased drag due to the faster water speed. This exact figure will depend on the precise wind angle, boat type, sail area, etc. It obviously isn't exactly 3, but the point is that it's bigger than 2.




I can't agree with your assertion. The sails feel only the apparent wind. They can't tell that it's the vector sum of true wind and vessel velocity (be it from sail, engine, or current). Don't fall into the trap of trying to think of the wind as a superposition of the true wind and apparent wind. Superposition of winds doesn't work like superposition of forces. You have to do the vector sum of the two winds.

In fact, I have quite recently sailed along quite nicely in a situation where there was zero true wind. We were in a few knots of current and used the apparent wind from that to propel sail at about a knot of boat speed. Took us a while to figure out what was actually going on.

Chris

Chris. Sorry you appear a bit confused. If the motor provides thrust (F) equal to one unit which leads to a speed of one unit (V) and you add thrust from the sails of one unit (F) then you will have a thrust of 2xF. Because V is proportional to the square root of thrust a thrust of 2F will give a speed of sq rt 2 or 1.41. On my boat we will motor sail on passage and if in a hurry when sailing speed falls to 4 or 5 knots. We use engine revs which would drive us at 4 knots and presto our speed jumps to 6 knots (NEVER 12 knots I am afraid).

You might need to revise your vector physics. All forces and wind can be broken down to vector sums. The forward motion component of apparent wind is always from dead ahead. Conventional sails can not extract energy from the wind and provide thrust in the opposite direction to drag. ie you cannot sail forward directly to windward. There are sailing craft which can do this ie a wind turbine driving a water propellor. these craft can sail directly to windward and therefore are a different case. A conventional sail cannot and does not develop thrust from motion wind. (unless there is someone out there that can trim their sails and sail directly head to wind.)

ps you are correct that sailing is not about wind but is about wind shear between the air and water. If the water is moving relative to the geostationary land then it is the relative motion between air and sea that counts. There are some very good but now probably quite old books on yacht racing in tide that could be worth a read. Remember you cant sail in a hot air balloon.

[Ziggy]

Originally Posted by cwyckham

As Dockhead pointed out, apparently (pun intended), I was misusing the term true wind. I've never heard of ground wind. So yes, the wind over the ground was zero, but we were in a few knots of current, trying to cut across it. This gave us a few knots of true wind (apparent wind minus boat speed) on our beam, and we were able to use that to sail forward at about a knot(ish) which swung the apparent wind forward and increased it to a bit over three knots. We close reached across the current until we popped out the other side. Once we were out of the current, our apparent wind was just opposite our motion, our foresail backed and we stopped.

Chris

Chris,
Got it!

[oldcal46skipper]

Originally Posted by theway

Im just trying to gather some knowledge and experience from those with it. Motor sailing defined here as using the motor on a sailboat while also using the at least one sail - for faster speed or beter fuel economy, etc.

Im going up the ICW and like to get the genoa out whenever possible. Any tips would be nice to start a conversation about this. If there is already a thread please let me know.

thanks
austin

Hi Austin,
You have started a very interesting thread. I have read and enjoyed all posts. We drifted away from your original questions, but that is why I really enjoy this forum.

As an owner of IMHO the best motor sailer ever built, the Cal Cruising 46, for 29 years, 18 years living aboard and 9 years of wonderful cruising, I love the versatility of having a fast sailboat and 400 gallons of diesel fuel.

I do not intend to derail your thread, but answer your questions, as posted. We have only cruised the ICW from Jacksonville to the Chesapeake once, 2001 and only then due to some health issues from a Doc who suggested I go home, sit on my porch, give up cruising, etc., I suggested I did not need a second opinion, he could go .... However, in deference to my spousal unit, I agreed to go "inside up the ditch".

Our boat had MOTOR sailed around the world by the previous owner, Jack Jensen, the founder of Cal Boats, so she had some experience. Since your question was directed toward the ditch, I will not digress. Motor sailing in the ICW is problematical, traffic, wind direction, bridges, course, etc. we mostly motored, but then we are senior citizens and have sailed across the Atlantic, from Panama to Mobile, AL, Puerto Rico to Key West, and on, we did not bother to raise any sails in the ICW. Of course diesel fuel was about $1.00 USD per gallon in those days and we still had some Venezuelan fuel that we had paid $ 0.12 per gallon, (Pre-Chavez), in the tanks.

If we had the opportunity to cruise north again from Florida, we would go offshore and ride the Gulf Stream and avoid all of the Alpha Hotel powerboaters. When we returned south in 2001, we went outside and motorsailed the Gulf Stream counter current. Excellent! Just remember -
WX RULES. We are "Pleasure Craft".

Tom & Bobbie Vandiver, SATORI, Cal Cruising 46, Hull # 3, Home ported in Bayou Chico FL

[cwyckham]

Originally Posted by frant

Chris. Sorry you appear a bit confused. If the motor provides thrust (F) equal to one unit which leads to a speed of one unit (V) and you add thrust from the sails of one unit (F) then you will have a thrust of 2xF. Because V is proportional to the square root of thrust a thrust of 2F will give a speed of sq rt 2 or 1.41.

Very well described. Thank you for your clarification. Actually, drag isn't quite proportional to V squared, it's even worse than that. The form drag goes with the square, the skin friction drag by a little bit less than the square (laminar flow by V^3/2, but the more prevalent turbulent flow by V^9/5), but wave drag goes by the fourth power! Anyways, at slower speeds, well away from hull speed, V^2 is a fine approximation for illustrative purposes.

In the example that I gave, I said that I increased power enough to accelerate from 1 to 2 knots. I didn't say that I put in the revs that would have been the right amount to motor at 1 knot. So we're both correct, we're just talking about different things.

Originally Posted by frant

You might need to revise your vector physics. All forces and wind can be broken down to vector sums. The forward motion component of apparent wind is always from dead ahead. ...

Don't confuse the use of vectors with the principal of superposition. Only linear equations (such as F=ma) can be superposed (the effects of the individual components of the vector force considered separately and added together at the end). This does not apply to the wind and drag. You must treat the single wind vector as one, find the single force due to it, and only then decompose your forces into convenient components (such as thrust and heeling force, or whatever you like).

A simple example will illustrate: The wind is coming from the NE at 1.4 knots and striking a vertical pole. The shape and size of the pole is such that the drag force = V^2. What is the total force on the pole?

a) Use the total wind: force = drag = 1.4^2= 2
b) Consider the N and E portions of the wind separately: Decompose 1.4 knots from the NE into 1 from the N and 1 from the E. Drag is now 1^2 = 1 from the N and 1^2= 1 from the E. Combine the two forces to get a total drag of sqrt(1^2+1^2) = 1.4. By inappropriately considering two separate winds instead of the single wind vector, we've underestimated the force by 30%.

The apparent wind is the only single wind that your sails see, and the fact that it is larger than the true wind (if you're going upwind) helps you sail faster. Of course, nothing is for free, it also slows you down going downwind.

Originally Posted by frant

ps you are correct that sailing is not about wind but is about wind shear between the air and water. If the water is moving relative to the geostationary land then it is the relative motion between air and sea that counts. There are some very good but now probably quite old books on yacht racing in tide that could be worth a read. Remember you cant sail in a hot air balloon.

Well said.

[frant]

Chris, We are getting very close to an agreement. We do agree that apparent wind will increase the pressure acting on a sail (bernoulli ). This increased pressure does lead to increased forces acting on the boat. Problem is that these increased forces are acting in the wrong direction (drag and heeling)with NO increase in forward thrust. You simply cannot sail direct to windward.

[cwyckham]

Originally Posted by frant

Chris, We are getting very close to an agreement. We do agree that apparent wind will increase the pressure acting on a sail (bernoulli ). This increased pressure does lead to increased forces acting on the boat. Problem is that these increased forces are acting in the wrong direction (drag and heeling)with NO increase in forward thrust. You simply cannot sail direct to windward.

You're not sailing direct to windward. Assuming the true wind is forward of the beam, the effect of your forward movement is simply to rotate the angle of the apparent wind forward a few degrees and increase it's velocity. Unless you're already at hull speed, you can use this increased velocity to increase both the lift and drag/heeling forces on your sail, accelerating your boat. This isn't a dreaded infinite loop, because, as you correctly pointed out, drag on the hull increases very quickly, so you'll find an equilibrium point pretty quickly at some speed above that you would have if the wind speed over your deck was equal to the true wind speed.

Of course, if you started close hauled, then when the wind swings forward due to your boat speed, you'll be pinching and you'll have to fall off a bit. Your velocity made good will generally still go up a bit, though, due to your higher speed.

Again, the key point here is that the boat only feels one single, combined wind, not two winds. You cannot consider the true wind and boat speed induced winds as two separate effects! It just does not work that way.

[frant]

Originally Posted by cwyckham

You're not sailing direct to windward. Assuming the true wind is forward of the beam, the effect of your forward movement is simply to rotate the angle of the apparent wind forward a few degrees and increase it's velocity. Unless you're already at hull speed, you can use this increased velocity to increase both the lift and drag/heeling forces on your sail, accelerating your boat. This isn't a dreaded infinite loop, because, as you correctly pointed out, drag on the hull increases very quickly, so you'll find an equilibrium point pretty quickly at some speed above that you would have if the wind speed over your deck was equal to the true wind speed.

Of course, if you started close hauled, then when the wind swings forward due to your boat speed, you'll be pinching and you'll have to fall off a bit. Your velocity made good will generally still go up a bit, though, due to your higher speed.

Again, the key point here is that the boat only feels one single, combined wind, not two winds. You cannot consider the true wind and boat speed induced winds as two separate effects! It just does not work that way.

I think we will have to agree to disagree on this one. But trust me that you cannot gain any forward thrust from the motion wind. Any increase in pressure caused by a greater wind speed flowing over the sail will not result in a net increase in thrust in the forward direction. (by virtue of the apparent wind moving forward and the sail being trimmed closer). If you ever get the opportunity to sail on a speed windsurfing board you will see what I mean. The boom does not have to be trimmed to allow for variation in point of sail. You simply trim close hauled and the speed rises and falls depending on heading. Fastest point of sail is approx 135 deg and at that point the apparent wind actually falls. when coming in from 90 deg to the wind to close hauled speed falls considerably but apparent wind increases in strength considerably. If it were possible to gain propulsion from the forward motion component of apparent wind then close hauled should be fastest point not slowest.

[frant]

Actually will revise my statement that you cannot gain any forward thrust from the motion wind. It will come down to the angle of attack and the lift to drag ratio of the airfoils and rig. The AC catamarans can probably benefit. For a cruising vessel the lift to drag ratio of the rig will mean that the drag rises faster than the increase in lift which has the same net effect. ie in practical terms you cannot gain any forward thrust from motion wind.
What this means for motor sailing is that it is easier to trim and keep flow attached and the sails working. When yacht racing in light airs the extreme difficulty is accelerating the boat (F=mA) and keeping flow attched to sails at low windspeed. Once up to speed it is noticeably easier to maintain sail trim (telltales flow) in the higher apparent wind and therefore to maintain boatspeed. This is probably where the concept of sailing on apparent wind developed.

[capn_billl]

It sounds as if the engine is compensating for the fall off as you approach maximum speed. In which case it is not affecting peak velocity, but Velocity made good. As you are spending more time at optimum velocity.

To review the equation for lift, lets solve the equation, plug in some sampe numbers and see what happens. That will settle the question of apparent wind and any velocity increase.

L = p * V^2 * (foil area) * (coefficient of lift) p = air density pretty much a constant at sea level. The coefficient of lift is dependant on angle of attack, I think we can safely assume it will be at max if fully trimmed. foil area, also a constant unless reefed. so that leaves velocity over airfoil as the only variable. Since Velocity is squared with wind speed, and drag is also squared with boat speed, I would expect that boat speed would linearily increase with wind speed.

Anyone who sails closer than a beam reach is sailing "to the wind". Using ice craft as an example where drag is a constant speeds of 115mph with a wind speed of 30mph implies that at least some synergistic effects DO occur from apparent wind. My experience with a laser confirms this as with carefull sail adjustment I can reach planing speeds in winds of 10mph or less, (the loss of speed caused by turning TOO close drops boat velocity back to wind speed and it takes considerable time and effort to regain speed). The increase of lift from the squared of the wind velocity more than counteracts the loss on the polar diagram of decreased lift efficiency as you sail closer to the wind.

[zeehag]

when my anemometer says wind (dont matte3r what KINDA wind,,LOL) is less than 5 kts, i use perkins to power the boat with sails --as many as needed-- up. is 2200 rpms and very lovely thing to do -- keeps my heavy brick in motion...
in pacific ocean, 1+1=>8, as southerly drift makes up for the rest and then some. even my boat goes faster here than it will anywhere else ..LOL.....

the only cone in my boat is with ice cream inside it and yummmmmmmmmy....

[frant]

Originally Posted by capn_billl

It sounds as if the engine is compensating for the fall off as you approach maximum speed. In which case it is not affecting peak velocity, but Velocity made good. As you are spending more time at optimum velocity.

To review the equation for lift, lets solve the equation, plug in some sampe numbers and see what happens. That will settle the question of apparent wind and any velocity increase.

L = p * V^2 * (foil area) * (coefficient of lift) p = air density pretty much a constant at sea level. The coefficient of lift is dependant on angle of attack, I think we can safely assume it will be at max if fully trimmed. foil area, also a constant unless reefed. so that leaves velocity over airfoil as the only variable. Since Velocity is squared with wind speed, and drag is also squared with boat speed, I would expect that boat speed would linearily increase with wind speed.

Anyone who sails closer than a beam reach is sailing "to the wind". Using ice craft as an example where drag is a constant speeds of 115mph with a wind speed of 30mph implies that at least some synergistic effects DO occur from apparent wind. My experience with a laser confirms this as with carefull sail adjustment I can reach planing speeds in winds of 10mph or less, (the loss of speed caused by turning TOO close drops boat velocity back to wind speed and it takes considerable time and effort to regain speed). The increase of lift from the squared of the wind velocity more than counteracts the loss on the polar diagram of decreased lift efficiency as you sail closer to the wind.

Your equation for lift is basically correct ie you will get an increase in lift from the airfoil proportional to the square of windspeed. Problem is that drag will also increase in proportion to square of windspeed. Then not only must one consider the angle of attack of the airfoil to the wind direction (lift/drag ratio will be highest when the c/line of the airfoil is parallel to airfoil). Lift is perpendicular to c/line, drag parallel. The other complication with motion wind is that it moves the apparent wind forward. This means that the airfoil centreline moves closer to parallel to the direction of travel (required thrust direction). Consequentially the greater lift produced is directed closer to perpendicular to direction of travel and the greater drag is directed closer to the opposite direction of travel. Plug in the numbers with a relatively poor lift to drag ratio of a cruising sailplan and the high angle of attack required to set these sails and you will find that net forward thrust is actually lower when moving forward than when stationary. The effect is amplified if the higher the motion wind. Therefore when motorsailing the net forward thrust provided by the sailplan is less than the net forward thrust provided by the sail plan at the lower sailing only speed.
I have spent a lot of time laser sailing also. Simply the force (thrust) required to accelerate the boat from standstill up onto the plane is much greater than the thrust required to maintain the boat on the plane. You will of course be aware that a bit of body kinetics will enable you to get over the hump and allow the boat to enter the lower drag planeing mode. I sailed a laser regatta on the Gippsland Lakes a while ago and spent the rest of the week cruising towing a laser behind. At reasonable high towing speed it was possible to adjust the towline length so that the laser was surfing down the sternwave with no tension on the towline whatsoever. Adjust the towline so that the laser was dragging up the back of the sternwave and your arm could be ripped off.

[tager]

DO SAIL. DON'T MOTOR!

[capn_billl]

I don't have the time or the hard data right now, but maybe someone can post a couple of vector diagrams with real numbers or calculated lift with boat at rest in 10mph real wind, and a second with boat at 10mph and apparent wind 15mph at 45degs forward.