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Old 15-05-2022, 03:22   #1
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Do catamarans really have more wetted surface than monohulls??

When you consider the extra weight of the keel and the compromised hull shape does a monohull really have less wetted surface than a catamaran in comparable vessels?
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Old 15-05-2022, 03:39   #2
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Re: Do catamarans really have more wetted surface than monohulls??

The best hull shape for reducing wetted surface would be a half sphere. A spherical shape has the greatest volume for a given surface area. So a mono, with is much more spherical hull shape is lower in wetted surface than a similar sized mono, even thought the mono has more weight.

Also monos can shift the crew weight forward in light winds to lift the flat bum out of the water and reduce wetted surface area, but doing this on a cat doesn't do a lot.

I am not sure I would think that the keel and rudder on a modern mono would be a negative in light winds. As they are lifting foils they are essential for movement, especially upwind. Induced drag ,from developing lift, is not linear and it usually pays to have large foils working less hard, so large efficient foils are not a big worry in light airs. So monos work really nicely in light winds, because their foils are large and wetted surface is low.

BUt then when the wind comes up a bit more, a nice cat will have far lower wave drag and doesn't need to depower as quickly.

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Old 15-05-2022, 04:00   #3
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Re: Do catamarans really have more wetted surface than monohulls??

Catamarans, size for size, generally have more wetted surface area than monohulls. In a sailboat, frictional drag usually dominates at low speeds, so minimal wetted surface is important to light-air performance.

Eric Sponberg offered some illumination, on the subject at ➥ https://www.cruisersforum.com/forums...tml#post906344
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Old 15-05-2022, 05:03   #4
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Do catamarans really have more wetted surface than monohulls??

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Originally Posted by catsketcher View Post
The best hull shape for reducing wetted surface would be a half sphere. A spherical shape has the greatest volume for a given surface area. So a mono, with is much more spherical hull shape is lower in wetted surface than a similar sized mono, even thought the mono has more weight.


What if the mono, or your half sphere for that matter, weighs a million pounds? Still less wetted surface? You cannot make that analysis without considering weight.
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Old 15-05-2022, 05:42   #5
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Re: Do catamarans really have more wetted surface than monohulls??

While the engineering answer is just formulas, you have to determine what a "comparable vessel" is before you can apply them and there's the rub. There are so many combinations of beam and length and hull shape that it's pretty much impossible to get 2 people to agree that a given mono and cat are "comparable".
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Old 15-05-2022, 06:26   #6
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Re: Do catamarans really have more wetted surface than monohulls??

There are even more differences between cats than there are monohulls, so meaningful comparisons are really difficult. I think you are better off just researching performance characteristics.
  • How bad is the windage?
  • Does it have a stumpy little charter-safe rig wit a tiny jib?
  • Do they haul everything that could possibly fit and 4 heads?
  • Boards or stub keels (and some stub keels are pretty good)?
Unfortunately, production cruising cats seem to be evolving into charter cats that emphasize max cabin number and comfort at anchor. It's what the market buys. Ick.
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Old 15-05-2022, 07:14   #7
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Re: Do catamarans really have more wetted surface than monohulls??

For the same waterlinelength and displacement cats have more wetted surface than modern finkeeled monohulls
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Old 15-05-2022, 07:29   #8
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Re: Do catamarans really have more wetted surface than monohulls??

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For the same waterlinelength and displacement cats have more wetted surface than modern finkeeled monohulls
The problem is that a cat and modern fin keeled mono with the same waterline length and displacement will have significantly different square footage of living and usable deck area not to mention sailing performance, so most wouldn't consider them "comparable" even on just those two major factors. And once you do try to match just those two factors between two boats it rapidly becomes entirely subjective and we devolve into one of the classic CF unanswerable questions right up there with which anchor is best.
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Old 15-05-2022, 17:52   #9
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Re: Do catamarans really have more wetted surface than monohulls??

From some calculation I attempted, it seems that a 40', 10,000lb(156ft3)displacement catamaran with semi-circular hull cross section would have about 300ft2 wetted surface area.



Now let's say you have a 40' mono with a 15,000lb(234ft3) displacement, to account for about 33% keel weight, which is on the light end for a keel, I gather. Monohulls have more of a flat semi-oval cross section than a semi-circular one. Not sure how to effectively calculate the surface area of a monohull but I dare say that between the compromised shape(not really compromised because they rely on this hull shape and keel weight for stability) and the added weight, that a monohull would have a similar wetted surface area to the cat, or at least there would be a much smaller difference than many people seem to think.
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Old 16-05-2022, 08:42   #10
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Re: Do catamarans really have more wetted surface than monohulls??

Depends. a heavy displacement mono with a full keel, deep forefoot then can have more than a monohull. If mono is a modern shallow draft hull with fin keel then could be less.


Easy test is amount of bottom paint needed. I know I need more than a modern mono with same LOA, but less than a "Captain Ron" type hull.
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Old 18-05-2022, 15:11   #11
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Re: Do catamarans really have more wetted surface than monohulls??

This isn't a realistic discussion.......... Displacement makes a big difference. Take 2 30 foot sailboats, the Gemini 3000 catamaran and the Pearson 303, randomly chosen.


Gemini has a displacement of 6520
Pearson has a displacement of 10100


That's a really big difference......



If we are talking about bottom paint, I would suspect they are not much different.



If we are talking about drag, the long slender hulls will be considerable faster, as hull speed is related to the ratio of beam to length, not just waterline length.


Trying to make a comparison like this is like comparing apples to oranges as they say. These are small boats.




36' boats also random


Catalina MK II 36' displacement 13500
FP Mahe 36' displacement 11023




The weight differences will vary but the cats will nearly always be lighter.... except perhaps for some huge condo cat and some performance racing mono.


Within boats built for the same basic mission, cats are always going to be significantly lighter in weight.
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