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Old 20-10-2009, 06:10   #1
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Cruiser Displacement and Sail Area

When looking at 2 different boats of same length; what is the sailing performance difference for "normal" conditions (not real light air like 5 knots) between the boats with different displacement, but same displacement/sail area? Say for the 2 boats below:

Boat 1
displacement - 30,000
DLR - 320
displacement/sail area - 17.5

Boat 2
displacement - 25,000
DLR - 250
displacement/sail area - 17.5

Both have the same power ratio for their displacement so would seem to me to have same sailing performance in general. Given the heavier boat is going to be more comfortable and I think may be faster at higher wind because it may standup better at the higher wind before having to reef.

If you have any real ratios etc from your experience to go along with your answer that would be nice.

Thanks
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Old 20-10-2009, 07:42   #2
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There’s some good general discussion and links on the subject at:

Sailboat Design Ratios
Sailboat Design Ratios

and

Sailboat Ratios
Sailboat Ratios
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Old 20-10-2009, 08:42   #3
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Looking at the ratios is the first step. You should also look at the beam and hull form. I'd guess from the numbers that the heavier boat has easier bilges (less pounding).
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Old 20-10-2009, 09:09   #4
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Boat 1 is a really heavy cruiser. Really heavy. Knowing nothing more about it than the two specs you've given, I'd have to guess that it's going to be a dog to sail. Boat #2 is going to be stout and stiff, along the lines of an Island Packet. It will probably have difficulty keeping up with modern production boats of equal waterline, but it should at least move along in medium air.

As a point of comparison, I sail a boat of 30,000 lbs displacement with a D/L of 190 and a SA/D of 18, which is going to seem like a rocket ship compared to the boats you're looking at. However, I'll still be sailing when boats like the ones you've described are motoring. The counterpoint is that I'll have to reef a lot sooner. To me that's a fair tradeoff for not having to listen to the engine on light-air days.
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Old 20-10-2009, 09:22   #5
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If these are production boats, then a good way to compare performance is to look up the PHRF racing handicap on them. These numbers are seconds per mile difference over a typical race course. They will give you something closer to real performance than looking at the ratios.

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Old 20-10-2009, 15:24   #6
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So is everyone saying that the heavier boat is going to be slower, even though it has more sail and both boats have the same displacement/sail area ratios? Isn't this just like saying a heavier car with more horsepower is as fast as a smaller car with a smaller engine? I like the idea of the heavier boat for comfort (because I know my wife likes that), but I still like to sail faster. In the past I've kind of mainly looked at the displacement, but started thinking that the sail area ratio was probably the thing to look at. Just so everyone knows my current Cal-39 has a DLR=245 with a disp/SA=17.7 so I know how that works out sailing. I'm disregarding hull/keel design for know (but both the boats AI listed are modern cutaway keel designs)
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Old 20-10-2009, 15:30   #7
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Quote:
Originally Posted by Don Lucas View Post
So is everyone saying that the heavier boat is going to be slower, even though it has more sail and both boats have the same displacement/sail area ratios? ...
In light airs it probably will be. The amount pf wetted surface is so much higher. In most cases it will require more sail to be used to get the same speed. Carrying a lot sail can be hard on the crew. If you look at the specs on 'cruisers' you'll see a pretty good range of boats where some just aren't designed to carry enough sail to match their displacement.

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Old 20-10-2009, 15:41   #8
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I've read in boat design books that the ratio that you really want to look at is SA / wetted surface rather than SA/D. Displacement works reasonably well for comparing normal similar boats and is usually the only number of the two that you can get your hands on.

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Old 20-10-2009, 15:42   #9
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But Paul in my example both boats are carrying enough sail for the same power ratios. I think it is accepted that regardless of the power ratio the heavy boat is going suffer some at low winds, which is why I said to forget that aspect. Most of the heavier boats use a cutter or ketch rig to reduce the work load in handling.
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Old 20-10-2009, 15:47   #10
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Don,
In your limited case you'd expect both boats to be reasonable close in speed on reaches. Close hauled or downwind, then other design attributes might come into play. On a reach, waterline wins.

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Old 20-10-2009, 22:09   #11
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Hmm, I guess Oh Joy displacing 15,000 and having 800 ft of sail means she'll fly? I guess that's why the PO made it 500nm in 48 hrs from Venezuela to Puerto Rico in 25 knots of breeze.
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Old 21-10-2009, 00:29   #12
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Neither boat is light. Differences are small enough that shape probably matters more. Paul's point on SA/D is on the money if you have older or weaker crew members, or you can twist it around to say that for the same muscle power you can run a larger faster boat that gets a bit of comfort from extra length.

Personally I agree shape matters more than small DLR differences, and in my experience comfort is as much about tactics and how you drive the boat.
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Old 21-10-2009, 01:53   #13
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With length fixed aren't we looking at just a plain heavier boat or do we need to know the beam?

If the boats are the same beam the heavier boat will sit deeper in the water - wetted area.

If the boats are different beam the boat may not sit deeper in the water but wetted area is still increased.

IMHO - Bigger sail area negates this only when the wind is enough to take advantage of it. In lighter conditions and especially gusting conditions the lighter boat has an advantage in acceleration. Each time the boat decelerates and has to accelerate you are fighting the mass of the boat.

Lighter displacement, bigger sails with a deeper keel and lower CG.

If you want to know what goes faster look at race boats.
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Old 21-10-2009, 08:18   #14
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Quote:
Originally Posted by Ex-Calif View Post
If you want to know what rates better (even though it may not go faster) look at race boats.
Fixed it for you.

I think there is too much fixation on "light air performance" and "windward performance". Look, unless you're racing around the buoys an extra couple of degrees of pointing ability won't offset being undercanvassed/unbalanced on a reach or other issues that would arise with lighter displacement in different weather conditions.
If you want to race, buy to the rule- but not if you want to go long distance.

Our boat will go in the mid 8's in about 10-12 kts of breeze
Disp: 30000 lbs+
Beam 12'
Draft 8'
LWL (unheeled) 34'-long overhangs.

Shape counts more than just the raw numbers but given sufficient SA, the ratio counts more than the raw displacement. Remember, wetted area drag is significant only at the slowest speeds - and you need mass to allow you to ghost between puffs (that's more shape dependent than anuthing else IMO)
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Old 21-10-2009, 09:14   #15
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I think that I want a heavy boat that is going to sail well for it's size. I know my wife doesn't like crashing along and getting overpowered, which is another posting of "what do women what". So I need to be careful that it is going to be a boat she likes sailing on and this part of next boat is going to be fully on me for knowing. I normally don't ask about any boat model as it tends to become too many "mystery options" so I chase down owners etc instead. But the numbers I quoted are from (not exactly these boats numbers as the question was met to be "in general"):

42' Cheoy Lee Goldern Wave
42' Tayana Vancover
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