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Old 07-05-2008, 03:41   #31
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sail area/displacement?

Whilst this ratio may be useful as a starting point, before embarking on further analysis , it certainly isn't the only data worth looking at.

For instance if we take the Fusion 40 as an example, we have seen a tricked up version with extra sail area being flogged around a course by a cavalier 395 and another fusion 40 being flogged by a home built chine cat amongst others, at a Whitsunday regatta.

Why is this? could it be that the hull design has very high drag stern sections, as it appears performance was not the main driving force in their design, but the need to have a large boarding platform and enough buoyancy to support two large diesels aft of the accommodation on a maximum LOA of 40'?

Sure, the published displacement could be a little low, but they do appear to float on their lines with little apparent transom sink-age. Maybe a glance at the line drawings will also answer many questions. If the underwater sections look fast ,then they normally are.
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Old 07-05-2008, 06:12   #32
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Here are the specs of the Lightwave 45

Koen

Lightwave 45

SA : 116
LWL : 13.41
Dlight : 6.5
Dpayload : 9.5
SADR light : 34.08
SADR payload :26.42
DLR light : 74.94
DLR payload : 109.76
SWL : 778.9
SWL / D (%) light : 12
SWL / D (%) payload : 8.2
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Old 07-05-2008, 06:22   #33
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Originally Posted by Moby Dick View Post
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
When you consider meter boats, one trick in light air was to sink the boat (ie. open the bailers and flood the hull) This allowed the boats to carry through the lulls once momentum was built.

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Old 07-05-2008, 07:25   #34
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When you consider meter boats, one trick in light air was to sink the boat (ie. open the bailers and flood the hull) This allowed the boats to carry through the lulls once momentum was built.
Except that you have to curtsy to that pesky overseer of momentum, F=ma. He makes sure you don't get sumpin for nuttin.

Unless you took on that ballast after "momentum was built" you had the classic case of "pay me now, pay me later." Building the momentum with all that extra mass aboard to eventually spend it in lulls is very costly and is, at best, a zero sum game. Throw in the added friction of sitting lower in the water and creating bigger waves puts you in the red.

JMHO - and Newton's

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Old 07-05-2008, 08:41   #35
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"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. In this formula there certainly would be a noticeable difference between a Lagoon 42 and a Gunboat 66.


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.
My idea of a sail area-stability number was just another way of comparing boats to each other. I understand that it does not take all things into consideration, just like all the other ratios in this thread do not take everything into consideration. Sail area-stability is simply one more way of comparing the differences between various boats.

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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.
Wouldn't a heavy "barge" work against this formula as speed is half of the formula and weight kills speed? I think the key would be for naval architects to work with the other factors that create stability yet keep speed. Hey, it would be one more crazy number to take into consideration.
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Old 07-05-2008, 08:58   #36
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Except that you have to curtsy to that pesky overseer of momentum, F=ma. He makes sure you don't get sumpin for nuttin.

Unless you took on that ballast after "momentum was built" you had the classic case of "pay me now, pay me later." Building the momentum with all that extra mass aboard to eventually spend it in lulls is very costly and is, at best, a zero sum game. Throw in the added friction of sitting lower in the water and creating bigger waves puts you in the red.

JMHO - and Newton's

Dave
Just saying what worked. Can't believe the likes of Ted Hood would have done it had there not been a benefit.

Cheers,

Joli
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Old 07-05-2008, 09:08   #37
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Oh, it definitely will help a boat coast longer thru lulls and tacks. No question about that. But it's physically impossible to not have to buy that benefit with slower acceleration back to full speed and a lower top speed. Physically impossible. Period. No question. Take it to the bank. You cannot get sumpin for nuttin.

Tell Ted I've got a perpertual motion machine to sell him............

F=ma doesn't lie and he never compromises. There is no way that more mass can result in better overall sailing performance.

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Old 07-05-2008, 09:44   #38
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Quote:
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This is my study on Sail Area / Displacement ratio of some cats which I think to buy..I can add more parameters or cats if you wish and please correct me if I have any mistake on this table.. I am waiting your comments
Hallo Mamo

Having this spread sheet is only usefull if the number are correct and for the FastCat they are not
Empty weight 5000 kilo
Sail area 118.5
waterline lenght 12.99


For the New Fastcat 555 the number are
Total length 58 ft 17.70 meters
Waterline length 56 ft 6 in 17.07 meters
Total width 31 ft 9.45
Draft 5 ft 5 in 1.52
Empty weight 9500 kilo 9.5 ton metric
Sail area 221 squire meters

Please correct the figures for the 455/435

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Old 07-05-2008, 09:56   #39
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It is my feeling we should come up with a different formula for payload , a larger cat or yacht will carry a higher payload compared with a smaller cat
For instance on the FastCat 455 1800 kilo payload will be fine as it will be for a St Francis 44 or a Privilege 435 or Catana 431 but A Catana 50 or a St Francis will take more stuff onboard while a Lagoon 380 or a Voyage 380 will take less weight on board
If we take 13 % of the lenght and transfer this into weight it will be more precize.
For the first mentioned catagory it will work out to 1800 kilo while for a lagoon 38 it will be 1500 kilo and for a Catana 50 or a st francis 50 it will be 2000 kilo,s
It will have an enourmous effect on smaller cats while the sailors on these cats are normally even more weight concious since they have to be .

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Old 07-05-2008, 12:39   #40
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Oh, it definitely will help a boat coast longer thru lulls and tacks. No question about that. But it's physically impossible to not have to buy that benefit with slower acceleration back to full speed and a lower top speed. Physically impossible. Period. No question. Take it to the bank. You cannot get sumpin for nuttin.

Tell Ted I've got a perpertual motion machine to sell him............

F=ma doesn't lie and he never compromises. There is no way that more mass can result in better overall sailing performance.

Dave
Dave, your not thinking this through. If you compare a (similar or sisters) light boat in light air to a heavy boat in light air the light boat will frequently be at a disadvantage. Your not just changing inertia of the boat in the lulls but you are also changing the moment of the rig and foils. When you hit waves what happens? I'll bet you shake the rig a bit? The heavier boat has less motion, it dampens out deleterious effects of surface irregularities better then a light boat. The heavy boat simply carries through much more easily then the light boat, flow stays attached, momentum continues, and average speed stays up.

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Old 07-05-2008, 12:44   #41
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In the real world, light winds are usually patchy, and weight can help you by giving you momentum that carries you from cat's paw to cat's paw.
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Old 07-05-2008, 12:44   #42
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Nice try, Joli. Again, you cannot get sumpin for nuttin. I suggest you think it through some more. If you're still not convinced, I'll offer to sell YOU the perpetual motion machine. Cheap.

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Old 07-05-2008, 13:31   #43
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Dave, believe what you will, here's a real world example. Not much different then what is done on light days with meter boats. I'm done here.

Beating to weather in light air, he induces heel with the water ballast to keep the boom to leeward, the mainsail filled, to increase the water line a bit, all of which adds up to an extra knot of speed.

boats.com - Boat Review/Test: Kanter 53

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Old 07-05-2008, 14:37   #44
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You guys go right ahead and keep believeing you can outsmart Newtonian physics. Some people believe crazier things than that.

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Old 07-05-2008, 16:25   #45
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Dave,

I too can envision instances in which a heavier boat will be faster in light air. As you point out it will take more force to accelerate a heavier boat but once up to speed I submit (at the risk of getting all Newtonian) that it will take more resistance to slow one down. This means that the boat will be able to coast through lulls and will be less affected by chop and windage. In a perfect world lighter is faster but in the real world of lulls and lumpy water a heavier boat may have the advantage. I'm sure you've sailed mono's as well as cats and have noticed that while multi's accelerate more quickly they also lose speed faster than a ballasted boat.

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