Motorsailing in dead calm

[John Vigor]

I seek the sage advice of Cape Dorians. In the latest issue of Small Craft Advisor, author Annie Westlund writes:

"In flat conditions, keeping the mainsail up helps with economy by about 5 percent when motoring, according to Tom Cunliffe."

Cunliffe is a well respected British sailor and writer.

A few days back I wrote a column in my blog disputing this. I maintain that if you are motoring in a dead calm at 5 knots, the apparent wind will be from dead ahead. Therefore the mainsail will merely flutter and not fill on either side. So there is no net benefit, and perhaps even a tiny detriment due to air friction on the sail.

But one of my readers insists, like Cunliffe, that there is a benefit.

Now, under these conditions, many of us will pin the mainsail down to one side or the other with the mainsail traveler -- just to keep the sail quiet. But this means it is backwinded, and creating a slight hindrance if anything.

I understand that pressure on a sail acts at right angles to the surface, and because of the curve of a mainsail trimmed for a beat there will be forces acting both forward and aft. But I can't see any net benefit from a mainsail fluttering in a calm.

Mostly, of course, conditions are not absolutely calm. There is usually a tiny movement of air from one side or the other, and this could certainly help. But the argument here concerns a theoretical dead calm and motoring at 5 knots.

Can there possibly be a benefit?

Cheers,

John V.

[Steve Laume]

The benefits might not be speed or efficiency through the water but I believe there are a few.

If it is very calm I move Raven's traveler far enough to windward (assuming there is a windward) that it stays full. If it is only the apparent wind created by motoring I figure I lose by friction and gain by lift. Not too much of a net loss.

By doing this I greatly stabilize the boat when it would otherwise be rolling around. It also increases my visibility to other boats and creates shade on a windless day. Most of all it I am in a position to take advantage of any puffs or an increase in wind if it should begin.

So we like to motor with the main up and it is rarely ever dead calm, Steve.

[winthrop fisher]

hi John V.

what differents does it make to you or not.

SCA is all about small boats under 25 feet long, its just like here on the CD board.

we are all sailors and we want wind...

come on, you can do better then that

just having fun with you... Winthrop

so how have you been

[Dean Abramson]

I too have trouble believing that there is any benefit re speed or fuel economy.

But it would help with rolling if there is any swell.

Dean

[Steve Laume]

Maybe by decreasing the rolling motion it actually improves forward speed, Steve.

[Warren Kaplan]

Our sails are powered by and large by apparent wind, are they not? A boat standing still in a dead calm is subject to no true wind and no apparent wind. But, a boat moving forward under power at 5 knots creates apparent wind. The question is whether that apparent wind comes at a sailable angle to the sail. If it does, then the engine created apparent wind might just add a minute amount of power to the sail. If the apparent wind angle is from dead ahead then it probably is of no use, but rather a hindrance. I suspect that the apparent wind angle under such circumstances is to far forward to power up the sail.

[Joe Myerson]

I tend to agree with Dean on this.

When motoring in a dead calm, I usually keep the main up, sometimes reefed and sheeted flat with the traveler locked amidships, because it seems to reduce rolling, making motoring more comfortable (or less uncomfortable).

Whether or not this improves fuel economy, I'm not sure, but it definitely makes the ride more comfortable.

--Joe

[drysuit2]

I tend to agree with Dean on this.

When motoring in a dead calm, I usually keep the main up, sometimes reefed and sheeted flat with the traveler locked amidships, because it seems to reduce rolling, making motoring more comfortable (or less uncomfortable).

Whether or not this improves fuel economy, I'm not sure, but it definitely makes the ride more comfortable.

--Joe

I'm in the "Rolling Club" too

[Dick Barthel]

John,

Joe Myerson and someone on your blog suggested that preventing roll may be enough to slightly increase speed. You haven't yet responded to Mr. Flanagan on your blog so I'm not sure of your opinion on the matter.

Assume motoring in a dead calm produces roll which in theory would slow you down as the keel works its way back and forth underwater producing turbulence. If you could minimize the roll and decrease the turbulence by keeping the main up wouldn't you possibly be increasing the boat speed but having nothing to do with apparent wind and lift?

Its seems plausible that rolling will decrease boat speed much the same as a locked prop might! Or increase it depending on your point of view. You devil you!

An interesting riddle from you as usual.

Dick

[Neil Gordon]

Those who let their props spin should argue that rolling will increase speed, as the hull and keel find the path of least resistance.

That said, unless the main is being flogged to death, I like leaving it up even if it's not providing lift, as it increases visibility about 10,000%. Nothing will slow you down like getting rammed.

[Neil Gordon]

By the way, not to coplicate the discussion...

But if you shift weight to one side and induce heel... you do two things... lengthen the waterline and allow the sail to fall into shape.

Is it possible to lock the prop while motoring and what effect would that have on boat speed?

[Maine Sail]

If you are dead into your own created wind there is zero "lift" on the sails.


Does a flag pole have more load on it with or without the flag in 5 knots of wind?

BTW I was on my boat this afternoon and busted out my digital strain gauge after reading this. My main sail, when hoisted, added about 20-25 pounds of load to my moored boat when pointing into the wind in about 8-10 knots. This is a net added drag..

[Carter Brey]

Is it possible to lock the prop while motoring and what effect would that have on boat speed?

Neil,

locking the prop while under power would increase boat speed quite a bit-- although the velocity would be centered around the axis of the propeller shaft and therefor angular and expressed in revolutions per minute. On a CD27, assuming a vertical distance of approximately forty feet from the prop shaft to the masthead, at 2,500 rpm and discounting the drag effect each time the mast and mainsail are dragged under water, that works out to approximately 1,136 miles per hour at the masthead, which of course would represent the distal extreme of the radius and therefor the point of maximum linear velocity.

Thank you for your attention,

Carter Brey
Sabre 28 MkII #532 "Delphine"
City Island, NY

[Dean Abramson]

I think that the amount of additional windage neutralizes or outweighs the created apparent wind. It's just my gut feeling; I am not a scientist, nor did I stay at the Holiday Inn Express last night.

Also something tells me that Warren's theory would only work on boats that can sail directly to windward. Still no science on my end, though. And my gut also says the sail, like John says, is "backwinded," but honestly I am not sure why. (Because the boom is windward of the centerline?)

I do like the idea of not taking the sail down when the wind dies (less work), and having to put it up again when the wind returns (less work). Unless it's wanted for anti-rolling, I usually drop it so as not to have the slatting.

But pulling it off one side with the traveler is a great idea! In 20 years of sailing, I did not think of that. I am no (rocket) scientist.

Dean

[Dean Abramson]

assuming a vertical distance of approximately forty feet from the prop shaft to the masthead, at 2,500 rpm and discounting the drag effect each time the mast and mainsail are dragged under water

However, this disrobes the crew, resulting in less drag.