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Old 27-04-2008, 00:28   #16
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mamo,
For purely selfish reasons, would you be interested in adding all the FastCats, Catana's and Gunboats? ..perhaps some Privileges as well?

Thanks for doing this!

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Old 27-04-2008, 00:40   #17
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Interesting to contrast a Harryproa such as the Visionarry sports
Visionarry Sport Specifications:
Leeward hull length: 15 m / 50'
Windward hull length: 10 m / 32'8
Beam: 8.3 m / 27'
Empty weight: 2.2 tonnes / 4,928 lbs
Payload approx: 1.5 tonnes / 3,360 lbs*
Sail area: 72sq m / 774 sq ft
Draft: Rudders Up 400 m / 1'5"
Draft: Rudders Down 1.5 m / 5’
Righting Moment: 15 tonne metres
Berths: 2 x queen size and 1 x single in the windward hull; 2 x pipe cots in the leeward hull

SA/Dexp2/3 ratio of 30.24 when loaded up ( using feet and pounds)
The cruiser adds about .8 tonne or so
Robert
PS the SA /D is not such a valid comparison but you can do the maths. It is nearly 20 in metric, and about 15 for the cruiser when loaded with 1.5 tonnes
Robert
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Old 27-04-2008, 09:17   #18
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This is my new table comparing catamarans, I need your comments and help for missing numbers

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Old 27-04-2008, 13:59   #19
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mamo,
It keeps getting better. This spreadsheet is a keeper worth saving.

David
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Old 28-04-2008, 05:18   #20
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Hi Mamo,

Talking about fast cruising catamaran (especially in light winds) , Lightwave's are perhaps worth looking at. For their new 45 feeter , they claim sails area of 116,5 sqm for light displacement of 6,5 tons. That seems to me quite remarkable..
You may check their web site

The Ultimate Catamarans by Lightwave Yachts

Anybody with first hand info on these cats ??

Cheers

Yeloya
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Old 28-04-2008, 07:26   #21
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Again , contrast a Visionarry with a metric ratio of 46.1 lightship and 32.6 with 1.5 tonnes on board

Dn't understand the displacement to waterline equation, but 15m and 3.7 tonnes would surely be reasonable.
Robert
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Old 29-04-2008, 12:23   #22
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Would you mind putting SF 44? Specs are

LOA 13.450
DWL 12.100
BEAM 7.200
DRAUGHT 0.900
DISPLACEMENT 7000 Kg (Light Ship Condition)
ENGINE 2 x Yanmar Saildrives 20 Kw Each
BRIDGEDECK CLEARANCE 0.600
MAST HEIGHT OFF WATER 18.710
MAINSAIL 65 SQM (Incl. Roach)
ROLLER FURLING GENOA 50 SQM
PAYLOAD 2500 KG (Nominal
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Old 30-04-2008, 06:39   #23
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Well, that makes life for boat designers much easier. Just stick a taller mast on, increase the sail area, and you are done. Fastest cat, top of the list etc. I wonder why they don't all just do that....?

I wonder if hull design, windage, stability and other factors play a part? Particularly as a sail with a true wind on the beam becomes a beat as the apparent wind moves forward.... . And for downwind sailing a specialist sail (spinaker/cruising sail) is often used anyway.
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Old 06-05-2008, 05:22   #24
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And Maren, I'd say that terhohalme's English is much, much better than our Finnish-way better than mine, anyway.
Actually, I didn't know where he was from. I tend to assume any mistakes people make are either due to writing on the fly or using a device like a blackberry.

I remarked because it struck me as funny.
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Old 06-05-2008, 10:14   #25
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What would be interesting is a sail area to stability ratio. This would describe the boats most likely to capsize. The area under the stability curve would describe stability. Adding on more sail area in order to go faster would not look like such a good thing with this figure....you would need more stability to accompany it.

Now take this figure and multiply it by boat speed at a given wind angle and wind speed, and you would have a good idea of boat speed-stability. Or in other words, which boats are fastest and yet still give you a lower probability of capsizing? This would be really nice thing to know. The higher the number, the better.

So which boats would go like hell and are inherently safer?
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Old 06-05-2008, 15:20   #26
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What would be interesting is a sail area to stability ratio. This would describe the boats most likely to capsize. The area under the stability curve would describe stability. Adding on more sail area in order to go faster would not look like such a good thing with this figure....you would need more stability to accompany it.

Now take this figure and multiply it by boat speed at a given wind angle and wind speed, and you would have a good idea of boat speed-stability. Or in other words, which boats are fastest and yet still give you a lower probability of capsizing? This would be really nice thing to know. The higher the number, the better.

So which boats would go like hell and are inherently safer?

This is starting to sound like an arms race. Higher mast takes you to top of list, so now more beam for more stability takes you back to the top of the list then... oh no we've just catwheeled.
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Old 06-05-2008, 15:47   #27
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Red face Don't forget resistance- which is a big subject

"Now take this figure and multiply it by boat speed at a given wind angle and wind speed, and you would have a good idea of boat speed-stability. Or in other words, which boats are fastest and yet still give you a lower probability of capsizing? This would be really nice thing to know. The higher the number, the better."

There is yet another relevant set of statistics, and that is resistance. A really wide heavy barge with a huge rig would meet your criteria, but it wouldn't be fast because of resistance. Weight, waterline length, and the length to beam ratio at the waterline for a single hull (the K factor) are some major components of resistance in multihulls.

When sailing to windward, windage can be a big factor in fast multis, as well. There are more factors, such as wetted surface, which figures in light airs performance, and prismatic coeffecient, which can only be optimum for a particular speed.

This is why the lightest multis with the longest, skinniest hulls are the fastest, if given a big beam and big rigs. However, being fastest isn't everything in cruising boats. Accommodations, comfort, sea-kindliness, seaworthiness, and load carrying ability are also important design goals-not to mention a reasonable price (for most of us, except for those who buy boats in the Gunboat class.)

It is the strength / weight / longevity/ dollars comparison that persuades me to go with vinylester / balsa. I am convinced that using your dollars to buy more length is the cheapest and most effective way to increase speed, and I think you will find that almost all yacht designers agree on this. The K factor interacts with waterline length, because it is a multiplier of the square root of the waterline length. The K factor is the degree to which you can adjust the maximum displacement speed, usually given at 1.35 x the square root of the waterline, by using a larger number than 1.35 in inverse proportion to the waterline beam length ratio of a hull. If you take a given hull and make it longer, you increase not only the waterline length, but also the K factor, so you get a beneficial synergey here.

Also, you can't just make multis beamier to make them faster, because, many believe, if you take this past a certain point, you increase the likelihood of pitchpoling by tripping over the lee bow.
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Old 06-05-2008, 23:30   #28
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The only problem of increasing that waterline is the cost of finding a place to park it. If cruising to out of the way places this is not a problem. Robert
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Old 07-05-2008, 00:10   #29
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Yes, waterline definitely costs more for dock space. Keep it on a buoy, which is not a possibility everywhere, of course. I have been many places where it is possible, however, and my own future plans include such a place. Pretty much any place where it is good to have a boat will have decent natural harbors. If there aren't any good natural harbors in the area, your big cruising boat is basically a daysailor, which you may hope to take on a voyage elsewhere some day.
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Old 07-05-2008, 02:19   #30
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SA/D is only relevant for performance in LIGHT winds. Think about it. A force is only needed to accelerate an object, or to overcome friction to keep it at a constant velocity. So, to keep a boat running at a given speed, you only need to overcome water resistance with the force exerted by the sails. The slimmer/low resistance design has lower resistance for a given speed. Whatever its weight.

The force from the sails also has to accelerate the boat to that speed as well, of course, but generally the force is fairly constant, apart from the gusts. So, a heavier boat may take a little longer to get to speed, but only a few seconds. A heavy boat will not speed up in gusts as fast as a light boat, but it will not slow down as fast either, because of inertia.

In light winds, you only really have the gusts, so high SA/D boats may go faster.

So a heavier boat that has the stability to have bigger sails, but that has low water resistance, could be faster than a ligher boat with higher SA/D
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