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  #1  
Old 10-07-2009
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What makes a good light air boat?

I'm in the process of researching an upgrade - looking to find something in the 28-32 ft range. One of the characteristics I've identified as being important is the ability to sail well in light air as we have a lot of 5-10 knot days in the waters I primarily sail. So, my question is: what makes a good light air boat? I assume it is fundamentally physics and relates to things like: sail area; rig; hull design; keel design; weight....I'm probably leaving something out. I am sure it is a complex answer - if it's easier to point me towards a book, that would be great too. Also, examples of boats known to be good in light air are much appreciated.

Thanks
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Old 10-07-2009
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Couple things for starters:
1. Lots of sail, of course, but in a tall rig, to get sail as high as possible due to the wind gradient. The wind blows more strongly the farther you get from the friction of the water. On the Great Lakes it used to be common for boat builders to offer a "tall rig" version of a design, sometimes with pretty much the same overall sail area. In light air the gradient can mean almost no wind down at the boom but useful wind 30-50 feet up. I've had days on my 27 where it's a dead calm in the cockpit but my masthead wind sensor is showing 7 knots.
2. Low wetted surface to reduce parasitic drag. Light winds mean the boat isn't going to achieve hull speed, and at lower hull speeds, parasitic drag is proportionally more of a factor than wavemaking drag. The closer you get to hull speed, wavemaking drag soars in comparison.
Others can chip in.
cheers
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Old 10-07-2009
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Good comments by Diva above. The wetted surface area of the hull really comes into play in light air. Once winds build, there is normally more than enough hp available to overcome the hull drag, but in light air it makes a big difference.

Plenty of sail area too -- in an efficient rig design -- relative to the vessel's displacement. Sail Area-to-Displacement (SA/D) ratios are a good place to start when comparing similar sized boats. Typical, relatively conservative cruising designs will show SA/D ratios in the 15.5-16.5 realm. Once you start seeing numbers approaching 17 and above you are getting into a more performance oriented design, which theoretically should offer better light air performance.

But those aren't the only issues, and you'd be surprised how some seemingly stodgy designs can move along well in light air. The Sa/D ratios are based on 100% foretriangle measurement, so don't reflect overlapping genoas or staysails or any of the other tricks that allow a boat to pile on sail area in light air.

Also, there are things you can do to improve a run-of-the-mill design to help eek out light air performance: Use smooth racing bottom paint; keep the bottom clean of scum; install a folding/feathering propeller; get a spinnaker; purchase new working sails; don't pile the boat full of every convenience and sink it below its designed waterline; etc.

Having no idea what your budget is, or how you plan to sail the boat (family cruising, racing??), it's difficult to make suggestions. But in that size range I like the J-32, the Beneteau First 10R. They are a bit tight for family cruising, but definitely fall more on the performance (cruiser/racer, racer/cruiser) end of the spectrum.
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Old 10-07-2009
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We race and old C&C 35 which as 35' boats go is on the light side compared to a lot of current boats that added all kinds of creature comforts and the weight that goes with them

In are racing adventures we do really well in winds below 10 knots compared to some pretty modern boats like the first 36.7 and some J boats BUT as the wind speed gets higher say 15+ we will get beaten up pretty bad
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I mentioned my C&C 27. I have a Mk1 (1971), the rig is 33 ft. In the later versions C&C stretched it to 37 ft to improve light air performance. The 27 is a wonderful design, and the so-called Mk II which has a stretched stern is close to 28 ft loa. Even the mk 1 is a "big" 27, but still light and nimble.
Two C&C designs of the 1970s, the 26 and 29 (not the mk 2 of the 80s) are known for being capable in light air if a little tippy when the breeze comes up. A friend with a 29 was told by the broker to be prepared to reef the main when the wind got to 12 true.
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Great responses - thanks. To give a little more background on the boat's intended use: I'd use primarily for daysailing, but have three small kids and would like to take them for short cruises - say 1-3 nights (main reason for the upgrade). This would be primarily in and around penobscot bay in Maine, so lots of islands and would probably camp out at least one night. All of which is to say, I would trade cabin comfort for cockpit comfort (ie. prefer a larger cockpit and can live w/ spartan accomodations). I would also like something that is reasonably fast. I'm upgrading from an Ensign, which I love but find a little sluggish at times. While I would not consider going over 32', I would consider going under 28...say 26? Reason being that I have an interest in a baba 40 with my father - problem with that is: seems to really be his boat and I don't particularly like sailing it unless I'm going for a longer cruise (find cutter rig is a pain, too heavy and cockpit too small). Max budget is 30k - had figured I wouldn't spend more than 20k on the boat and leave 10k cushion for work.

Thanks again for the responses above.
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SA/D ratio would be where I would start. Check out this calc. it'll help you get started. Sail Calculator Pro v3.53 - 2000+ boats

My merit comes in just under 22 for SA/D. However I can't seem to find out how to calculate it. The numbers I'm getting don't make sense.
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Old 10-07-2009
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zz4gta,

SA/D = SA / (D/64)^2/3
The displacement needs to be in cubic feet, so:

Merit 25:
SA/D = 285 / (3000 / 64)^2/3
= 285 / (46.875)^0.6667
= 285 / 13.002
= 21.9
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Of course, if light air performance is really important, it might be a good idea to look at catamarans and trimarans. Multihulls do much better in light air, as they have far less wetted surface area.
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What makes a good light air boat?

A good light air sailor.

Have you ever been out on an a day when there is not even a ripple on the water? You look out over the bow and see a dozen sailboats stalled and the one or two boats that are moving. You think to yourself there must be some sort of breeze over there. Chances are there is not.

Light air sailing is work and patience. Sail trim is essential. Anyone can sail a boat when there is wind to waste. Pick a day when you don't think there is enough wind and go sailing. Get out there. Ease the out-haul on the main. Get some "bag" in the sail. Wait 2 min's to see if the boat responses. Ease the vang wait 2 min's to see if the boat responses. Do not make sail adjustments fast or in combination's. tweak a little and see what happens. Then tweak a little more. The more time you put into it, The more you will know how your boat responses. The BFS sailors will not spend the time to learn light air skills because it is not very exciting, but it is certainly a challenge.

The point I trying to made is light air sailing is a learned skill, the more you practice it the better you will become.
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