What makes a boat sail faster in less wind? - SailNet Community
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post #1 of 11 Old 10-05-2007 Thread Starter
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What makes a boat sail faster in less wind?

Is it only the beam (width) of the boat primarily or other factors like weight and sails?
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post #2 of 11 Old 10-05-2007
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This reminds me of that rhetorical question: "Is it colder in the mountains than it is in the springtime?"

There are so many variables affecting boatspeed -- LWL, beam, wetted surface, draft, ballast, sail area, sail trim, wind speed, point of sail, etc etc -- it is almost impossible to answer your question definitively. All else equal, however, you MIGHT be able to say generally that the narrower boat ordinarily would have the overall advantage (as long as narrowness is not taken to extreme, in which case it might be a disadvantage). But if the boats have different beams, it is unlikely that all else would be equal, making the comparison difficult.

If you have specific boats that you are comparing it would be helpful to describe them. But if you are merely asking theoretically, then we can only reply theoretically, which may or may not be very useful to you.
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post #3 of 11 Old 10-05-2007
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This is a pretty open ended question with many answers.

Weight, sail power, reduced wetted surface, sail trim, boat handling/helsmanship and sea conditions all contribute to speed (or lack of it) in light air. Beam itself has little to do with it.
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post #4 of 11 Old 10-05-2007
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For a given boat, all that you can practically affect is her weight and her sail selection, although having a fair keel and a smooth bottom could also be considerations.
I think what you may be interested in is accelleration versus speed. Speed is limited by hull form, not much you're going to do about that one. Acceleration, or quickness, can be directly affected by the boat's laden displacement and size and condition of sails, paricularly in light air. Good, well cut sails will offer measurable improvement in performance, especially if you've been sailing with some older, worn out (read baggy) sails. They are simply more efficient airfoils. By the same token, everything that you carry on the boat requires energy to get it in motion. Simple physics tells us that if you remove 200 pounds of gear, or your brother-in-law, the boat will have less mass to get in motion and will be therefore quicker. In light air, where you might not expect to achieve hull speed, your top end speed may well increase as well. Acelleration is often overlooked, but is highly desirable. It will largely dictate how fast the boat the boat will get back up to speed after tacking or manoeuvering. It will also allow the boat to benefit from wind gusts that otherwise would have little effect other than an increase in heel.

The Portagee, a rapscalion with a speed obsession, frequently posts here and he goes through a process of annually discharging all unnecessary weight from his boat for the racing season. Come the cruising season he reloads those items that make wife and family comfortable, as well as his larger anchor and such he uses for cruising. I think he leaves the carpeting and the head in place year-round, but i could be mistaken.(g)

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post #5 of 11 Old 10-06-2007
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I've found that in light wind, the only thing that will make your boat go faster is A BIG ENGINE!

Just checking in.
Where ya'll keep'n the wimmin 'round here?
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post #6 of 11 Old 10-06-2007 Thread Starter
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I think I phrased my intended question wrong... I read discussions of some boat characteristics being desirable such as being able to sail in light wind as opposed to sitting dead in the water. I was under the impression certain boat characteristics enhance this performance and that certain boat makes and models sail well whereas other undesirable or not-as-fun sailing boats are slow. I thought the beam increases drag and I thought a narrow boat was a primary feature allowing ease of movement. Now I get the impression that just lightening up the boat and buying some better sails is all you need to move good in light wind. Or,... if it is the boat make and model itself that sails well can anyone list any examples of boats that sail well in light wind?
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post #7 of 11 Old 10-06-2007
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First, some boats are built like RV's, putting new sails on a big Morgan IO 41 will not make her sail faster than a Farr 40.

Disclaimer: the following is overly simplistic and there are far better/smarter people out there to explain this topic -- but I've got a little free time, so here goes...

kldzx9r, you are right that a large beam will cause drag but all surface area under the water will cause drag. A good "light wind" boat will have a small wetted surface. She'll need a lot of sail area, preferably high on the mast where the wind is stronger. And she'll also need good light wind sails that will fill in 2-5kts (heavy sails will droop and not hold shape in light wind).

Look at the old Herreshoff's with the "classic" shaped hull... when the wind is light she stands straight-up which means less surface area in the water which means she's easer to move in light wind. When the wind picks up she heel more which increases the waterline (a major factor in max hull speed) so she'll sail faster.

That said, a light wind boat probably won't be a good crusier and may not even be the best racer. Just as a Porche isn't a Hummer or a Winnebago.

I look forward to seeing the replies that say I'm full of $#!t.
:-)
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post #8 of 11 Old 10-06-2007
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Your biggest performance factors in light air are going to be LWL/Beam ratio, wetted surface area, hull smoothness, and sail area/displacement.

The only factors that can be "variable" are hull smoothness and sail area. Displacement is pretty much constant, a lighter boat will have less wetted surface. LWL/Beam is inherent to the design; a narrower boat is going to be preferred as it will generate less pressure drag (unless it is a ULDB design where the boat is VERY low displacement and designed to plane in downwind runs). ULDB boats are designed to have light displacement and a flat bottom so it can plane when "overpowered". The pressure drag is reduced because they don't displace much water, but wetted surface may not be minimized (a smooth hull has a bigger effect on these boats).

Sail area is of course variable; and the more sail you can hoist in light wind (like a gennaker or spinnaker) the closer you will be able to sail to the boat's maximum performance for that windspeed. Drag forces will prevent the boat from reaching the speed of the apparent wind; it depends on how much drag forces are generated due to the factors mentioned above.
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post #9 of 11 Old 10-06-2007
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Like so many things, I think it is the skipper. Keeping weight forward on the lee, playing the jib sheet by hand and selling his soulto the devil.

-Andy
Newport 17 - "Kohanna"
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post #10 of 11 Old 10-06-2007
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In light air anything that creates drag is the enemy and increasing drive is your friend.

On the drag side, in almost all ways, weight is the biggest single enemy in light air sailing. It means more hull in the water, which means more turbulence, more frontal area and more wetted surface. Boats with L/D's over 170 or so are operating at a real liability in light air. Long keels, skegs, large rudder areas, large amounts of deadrise all add to wetted surface, all add to drag. Boats with large diameter spars and lots of standing rigging, high freeboard, all kinds of gear on deck and in the rig have a lot of aerodynamic drag as well.

I somewhat disagree that waterline beam much of a factor. Cross sectional area really the more important factor. A boat can be narrow at the waterline, but still have an excessively high cross sectional area, and therefore a high drag. CCA boats are a good example of that.

Then there is the drive side. On the drive side, nothing succeeds like a large, efficient sail plan. With thier large drive for their drag, high aspect rigs really shine in the light stuff plus they get their rigs high enough off the water to help get into a slightly higher gradient wind range. Boats with standing sail pland SA/D's under 20-22 really suffer in light air.

Then there are the things that you can do to opimize the light air sailing ability of a particular boat. The biggest single thing that you can do is to get light weight sails (which will hold their shape in light air better than heavier sails which get pulled to flat due to gravity) and set them properly. Next comes keeping the bottom smooth, fair and clean. Sailing the boat, a small heel angle helps to keep sail shape and also since light winds often have a vertical component, a slight heel helps catch the falling breezes. On boats with comparatively hard chines and flat bottoms, heeling can also reduce wetted surface. On IOR, MORC, and Open Class typeform hulls, going bow down lifts the high wetted surface areas of the stern and so reduces wetted surface as well.

Depending on how light the winds are, you initially want to make your sail shapes fuller to generate more drive, and make sure the slot is open. As the winds lighten further, there is not enough force to push the wind around the sail so flatter sails actually work better in the really lightstuff. Again it is very critical to keep the slot open and air flowing. minimizing helm angle and the amount of movement of the helm is also important. Minimizing movement around the boat also helps since rocking and pitching interupts air flow over the sails and waterflow over the keel.

Jeff
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