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Old 25-05-2018, 15:24   #1
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Catamaran Comparison (SA/D vs D/L)

I've recently become interested in catamaran specifications. In doing so, I've noticed it's not always easy to get the information you want/need for one reason or another. So, copying the methodology of Australian Multihull I've set out to collect the necessary data for calculating SA/D and D/L Ratios for some of my favorite cats. Incidentally, I calculated all the ratios myself, as there were errors in various published sources. Then, like the Australian publication, I put them in a chart attached, which I'm somewhat proud of since I think it really helps focus the data.

Curious what others think and would be interested to know if I should include some other boats. Note that I'm focusing on boats between 45 and 50 feet LWL for a live-aboard lifestyle with decent performance, but I'm obviously lured into evaluating others. That said, since a more LWL typically means better performance, it's a little unfair to compare a Privilege 585 with smaller boats, but it's still fun. Anyway, have at it!
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Old 25-05-2018, 15:44   #2
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Re: Catamaran Comparison (SA/D vs D/L)

I am on a ratings committee for multihulls in US gulf coast region. From experience I can tell you that manufacturer reporting of weight (used to calculate D) is very inconsistent. Even the weight gauges on cranes are rarely calibrated.
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Old 25-05-2018, 15:55   #3
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Re: Catamaran Comparison (SA/D vs D/L)

I believe it, but I'm not sure how else to go about comparing boats. I did look for various data confirmation as best as possible and worked with conservative numbers whenever in doubt, but there is certainly an inherent issue with whether the underlying data is reliable.
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Old 25-05-2018, 15:58   #4
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Re: Catamaran Comparison (SA/D vs D/L)

p.s. It's hard to believe that a Privilege 585 is only 44,092 lbs light displacement . . . but that's what they say!
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Old 25-05-2018, 18:31   #5
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Re: Catamaran Comparison (SA/D vs D/L)

MU
Quote:
Originally Posted by Escape_Velocity View Post
I've recently become interested in catamaran specifications. In doing so, I've noticed it's not always easy to get the information you want/need for one reason or another. So, copying the methodology of Australian Multihull I've set out to collect the necessary data for calculating SA/D and D/L Ratios for some of my favorite cats. Incidentally, I calculated all the ratios myself, as there were errors in various published sources. Then, like the Australian publication, I put them in a chart attached, which I'm somewhat proud of since I think it really helps focus the data.

Curious what others think and would be interested to know if I should include some other boats. Note that I'm focusing on boats between 45 and 50 feet LWL for a live-aboard lifestyle with decent performance, but I'm obviously lured into evaluating others. That said, since a more LWL typically means better performance, it's a little unfair to compare a Privilege 585 with smaller boats, but it's still fun. Anyway, have at it!
What are the units?

Edit: ran the numbers, our boat is off the chart, to the upper left...

SA/D is 28, D/L is 69.5.
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Old 25-05-2018, 19:04   #6
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Re: Catamaran Comparison (SA/D vs D/L)

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MU

What are the units?

Edit: ran the numbers, our boat is off the chart, to the upper left...

SA/D is 28, D/L is 69.5.
SA/D is standard units is ft2/ft3. Equation is:
SA/D = SA/(displacement/64.2)^(2/3),
where displacement is measured in lbs. I think "D" represents displacement in ft3 of water, not weight. The SA is sail area in ft2.

DLR should be long tons/ft. Equation is:
DLR=(displacement/2240)/(LWL/100)^3,
where displacement is again in lbs, and LWL in ft.

When we are looking at performance prediction, it is usually for long distance races whereby the upwind, downwind and reaching sail configurations come into play. So we have a weighted SA (arbitrarily 40% up, 30% down, 30% reach, which tends to fit our distance races somewhat).

Not sure what SA was used for the graph above. Ditto for D, equations, etc.
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Old 25-05-2018, 19:30   #7
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Re: Catamaran Comparison (SA/D vs D/L)

I too, am a bit confused by which units you have used. I have done this exercise for a number of boats for L/D using metric tonnes and metres. I came to two conclusions based on this data.

I had to use a base measure in comparison so I used a Helia 44 for this as it is a boat I know well, with performance I know well, and then compared % differences for different boats. Schionnings, expecially the GForce scored nearly twice the performance, and other boats dropped from there. e.g. Chincogans 45% better, Crowthers 40% better, Graingers 25% better. As 44 says his boat would score off the charts. Of course this varies markedly with the design of the boat.

Two points were bleedingly obvious.

Firstly, the performance of the boat was inversely proportional to its load carrying capacity. I think Outremer have this in balance in that (at least from the 51 up) they have reasonable load capacity vs performance, cruising performance (defined below) being round 30% better than the Helia. However, on this forum many have criticised Outremer for their load capacity. Comfort specifications will vary from person to person.

Secondly, this analysis fails as the boats get longer as you have the effect of sailing performance increasing with wetted length and the weight of the boat typically goes up with the length of the boat. Thus, there are a number of 50-60 ft boats that I know sail quite well, that did not have impressive L/D ratios.

Finally, there is the issue of how you judge performance. If you want the hype of sailing a boat 20+ knots for a short time then that is fair enough. You need to go for a boat that is built for speed. However, this does not necessarily translate to long term cruising performance, when one considers all of the mitigating factors such as weather, sea state and crew comfort. I like to consider the measure as an average of daily averages. In other words when someone tells me their boat can do over 300km per day, then I will always ask them what their daily average over 10 days or 3000nm of sailing. Typically, for a cruising production boat such as the Helia 44 this will be 160nm. Even the fastest boat which is rated 100% over the Helia will only report about a 50% improvement on this daily average, but this will come at the cost of load carrying capacity and therefore comfort but of course this comfort definition will vary from person to person.

For me, as I said, by design or by experience, Outremer has the balance in that above 50' you will get 30% increase in average cruising performance with reasonable load capacity. Of course this comes at a price and many are still not happy with load capacity.

If you want a good cruising performance vs cost equation then you need to be looking at 55' plus, but of course then maintenance costs scale up with length and you are looking at greater carrying cost.

Lastly, I came to the conclusion that at the end of the day all of this analysis is pretty futile, as it typically induces analysis paralysis. There are production boats vis a vis Leopard, Lagoon and FP and nowadays with some notable exceptions they all perform similarly. There are boats with good performance vs comfort balances such as Outremer and Seawind, and there are bloody fast boats such as Schionning, Chincogan, some Graingers, and their European equivalents, but all of whom suffer load capacity problems as a tradeoff for performance. Leaving aside the hype you have have a good idea of how these boats perform both in short runs and long term cruising averages. Picking the differences in performance in different boats or brands in these groups is futile as it will depend very much on who is crewing the boat.

Just my 2c.
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Old 25-05-2018, 22:15   #8
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Re: Catamaran Comparison (SA/D vs D/L)

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Two points were bleedingly obvious.

Firstly, the performance of the boat was inversely proportional to its load carrying capacity.

Just my 2c.
Bleeding obvious maybe, but not accurate.

There are two ways to increase a boat's carrying capacity: make the hulls fatter, or make them longer.

A longer boat will have greater carrying capacity AND greater performance.
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Old 26-05-2018, 00:09   #9
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Re: Catamaran Comparison (SA/D vs D/L)

You must have missed the bit in the OP post where he said he was concentrating on boats 45-50' and the bit in my post where I said boats bigger than 55' will overcome the load capacity issue. I think that everyone is aware that a 90' catamaran will have both performance and load capacity, but that is entirely irrelevant to this discussion.

Also before someone makes the comment it is also a truism that these fast cats provide the advantage of sailing at low wind speeds when other boats may be motoring. Some, maybe most, would argue that this is the true advantage of cats with high D/L ratios rather than speed performance.
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Old 26-05-2018, 07:16   #10
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Re: Catamaran Comparison (SA/D vs D/L)

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Two points were bleedingly obvious.

Firstly, the performance of the boat was inversely proportional to its load carrying capacity.
Be careful here though, because IMHO the calculated "load carrying capacity" has even less rigor than the manufacturer's published displacement number. And I've never found that putting 3,000 lbs on a fast boat makes it slower than a boat that was already high D/L (i.e., slow) before adding the same amount of weight. And when I get home from cruising sabbatical, I can always remove that weight and have a fun boat to day sail or competitively race.

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Secondly, this analysis fails as the boats get longer as you have the effect of sailing performance increasing with wetted length and the weight of the boat typically goes up with the length of the boat.
Not exactly sure what you are trying to say here, but generally as LWL increases the sailing performance also increases. Not sure where the failure is in the analysis? Yes, we can always find some exception, like a manufacturers 33 ft model has minimal interior furniture and outboards, and the 38 ft has diesels, gen, lots of interior wood furniture, etc. But the weight associated with adding five feet of hull to each side is usually pretty light in the scheme of things.
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Old 26-05-2018, 09:04   #11
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Re: Catamaran Comparison (SA/D vs D/L)

Any thought to adding Catana to your chart? From my perspective, Catana has always been the 'production cat with performance'. Narrower hulls, daggerboards, the new ones are supposedly 'light-weight' due to extensive use of carbon fiber.

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Old 26-05-2018, 09:34   #12
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Re: Catamaran Comparison (SA/D vs D/L)

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MU

What are the units?

Edit: ran the numbers, our boat is off the chart, to the upper left...

SA/D is 28, D/L is 69.5.
That's pretty damn impressive, 44! What make/model of boat do you have??
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Old 26-05-2018, 09:35   #13
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Re: Catamaran Comparison (SA/D vs D/L)

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Not sure what SA was used for the graph above. Ditto for D, equations, etc.
I used the same SA for each -- the total sft of the main and genoa.
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Old 26-05-2018, 09:53   #14
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Re: Catamaran Comparison (SA/D vs D/L)

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Bleeding obvious maybe, but not accurate.

There are two ways to increase a boat's carrying capacity: make the hulls fatter, or make them longer.

A longer boat will have greater carrying capacity AND greater performance.
You can crib a little with cross sectional underwater profit. changing the box of prismatic coefficients. Other then increased WL performance probably suffers.
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Old 26-05-2018, 09:56   #15
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Re: Catamaran Comparison (SA/D vs D/L)

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Firstly, the performance of the boat was inversely proportional to its load carrying capacity.
In the range we're talking about, there's no argument you're generally correct, although see the comment about the Helia 44 and Saba 50 below.

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I think Outremer have this in balance . . . Comfort specifications will vary from person to person.
Agreed, and therein lies the rub.

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[T]his analysis fails as the boats get longer as you have the effect of sailing performance increasing with wetted length and the weight of the boat typically goes up with the length of the boat.
Right, so that's why I'm focusing on ~44-51 feet only.

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[T]here is the issue of how you judge performance. If you want the hype of sailing a boat 20+ knots for a short time then that is fair enough. You need to go for a boat that is built for speed. However, this does not necessarily translate to long term cruising performance, when one considers all of the mitigating factors such as weather, sea state and crew comfort. I like to consider the measure as an average of daily averages. In other words when someone tells me their boat can do over 300km per day, then I will always ask them what their daily average over 10 days or 3000nm of sailing. Typically, for a cruising production boat such as the Helia 44 this will be 160nm. Even the fastest boat which is rated 100% over the Helia will only report about a 50% improvement on this daily average, but this will come at the cost of load carrying capacity and therefore comfort but of course this comfort definition will vary from person to person.
This is an excellent point to keep in mind. You seem to like the Helia 44 a lot. I do too, it's clearly a top performer for a production boat with great load carrying capacity, though I am also drawn to the Saba 50 (which ironically has a slightly lower load carrying capacity and lower SA/D vs D/L ratios).

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For me, as I said, by design or by experience, Outremer has the balance in that above 50' you will get 30% increase in average cruising performance with reasonable load capacity. Of course this comes at a price and many are still not happy with load capacity.
What is the load capacity of the Outremer 51? Have you been on one? I need to give them a second look as I've only been on a 45. I was not impressed with the finishing of Outremer 45, however.

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I came to the conclusion that at the end of the day all of this analysis is pretty futile, as it typically induces analysis paralysis.
Not for me, but I think your ultimate point is that in this size range it's all "about the same." I'm not sure that's true, but I respect your view.
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