Sail Area in Regards to Cat Size

[scape]

why is sa/d ratio mostly just compared for as you say light-wind sailing?

[44'cruisingcat]

In stronger winds you would reduce sail area.

[Rapanui]

For most people, there is a minimum sailing boat speed at which they turn the motor on coupled with an expectation of how often they wish to motor. This minimum speed which for me is 4-5 kts is usually well below the natural 'hull' speed of a multihull and hence this speed is largely determined by the sail area, displacement and wetted surface, hull form, length/breadth etc is less important. The naval architects have combined this to give the metric SA/(Displacement)^2/3. This gives you a number which you can benchmark against other boats, a higher number should sail at your desired minimum speed in lower wind speeds. As has been pointed out, this also means that you will have to reduce sail area earlier in higher wind speeds for safety.
Other metrics such as the Performance Index (0.5*(Lwl*SA/D)^1/2) – This index gives an estimate of the relative boat speed to wind speed on a reach, i.e., a figure of .8 says a boat will do 8 knots in a 10 knot wind -quote, able you to benchmark the boat in normal sailing conditions and do take into account hull length.
You can take these metrics too far, to me their best use is to compare against known boat performance preferably from personal experience. In practice, there are too many unknowns to make this more than an approximation. For instance, how many people know the accurate displacement of their boats in cruising trim? Even the waterline length?

[Sand crab]

And there are about 20 other formulas that one could dissect which affect boat performance. It will drive you crazy. After awhile you get a better feel for a boat just by looking at it or by gaining knowlege like on the Forum here. You'll just know that's a faster boat or that's a slower boat.

[kaimusailing]

There is also sq root of sail area/cube root of displacement. Many modern cat rigs give a very large reaching and downwind sail area through use of a screetcher or gennaker on bowsprit and upwind with just main and jib. It's hard to maintain a taut forestay on a catamaran, so the jib is smaller than big overlapping genoas. In light air it is possible to bring apparent wind forward to close hauled even when sailing a reaching course, and another limit to sail area is the stability required to not capsize or pitchpole. Watch some of the AC45 videos and you'll see all these affects at their extreme.

[sandy daugherty]

Before we get too bogged down in numbers, lets consider the designer's goals and comfort levels. They may not be what the rosy prose in the brochure says.

Most designers are in the business for the long haul: make some money, put the kids through braces and college, have a nice boat to play with, live somewhere close to the water. The long haul means establishing a reputation in the target customer group. That may be charter operators, long distance cruisers, or racers. Unfortunately, there is no significant racing among multihulls that can't go home on a trailer. Monohulls have great Wednesday night class racing, one designs, and effective rating systems that build devoted participants almost everywhere there's water and wind.

Why this hasn't happened for cats would be a very fruitful discussion; the amazing success of the Farrier / Corsair tris proves beyond a doubt that it could happen.

Meanwhile, back at the big cat gang, maintaining a reputation means not having accidents. I believe that the fate of the wonderful Stiletto 30 was sealed by a single fatal accident, and one that really had little to do with the boat's design. It takes nerves of steel to produce a vessel that even hurricane force winds could blow over. For every other designer, its masts too short and beams too wide to be pulled over by a freight train.

So, the best plan for all but the gutsiest designer is to wax poetic about performance in the brochures, and hide the real numbers from the savvy shopper.

Test question: Do buyers really want to risk a calamity at sea for a bit more speed?

Apparently not.

[scape]

Originally Posted by sandy daugherty

Test question: Do buyers really want to risk a calamity at sea for a bit more speed?

Apparently not.

when it comes to an expensive cruising vessel with somewhat limited sailing experience, I would think not.



thanks for the input everyone! I can understand a bit about this now. i do wonder how different rigs (mostly sloop vs cat, or even junk or claw) would relate to these simple math functions and their efficiency and speed. it would seem link wind direction and wave chop would make for more important math than just sail area/displacement ratio's.

for instance a rotating cat bi-rig would, even at the same sailarea, likely perform better on a run than a similar sized sloop; correct?

perhaps this is why it is usually left to architects and engineers to figure out

i want to try and build a small boat (maybe a catamaran) which is probably going to be a mix of a few plans already available. i saw a cool retro video of a planing catamaran which had a cat rig of what looked like a windsurfer rig and it really got me thinking on some cool designs.

Planing Catamaran Itzacat Speed Sailing Hydroplane - YouTube

[Rapanui]

The whole issue of performance versus safety is interesting. There is no doubt that a a cat that performs well in light weather is capable of being capsized or pitchpoled. I have had a Sailcraft Iroquois for 10 years of which many have been capsized, and a FP Maldives for a similar length of time of which several have been capsized. Both did over 7000 miles without mishap but the potential was there. Even my FP Athena could be capsized or pitchpoled. I am in no doubt that the major manufacturers in chasing the charter market have chosen the road of safety over performance. Having had first hand experience of people chartering where the skippers are qualified but some have no intrinsic feel for seamanship I can understand people like Lagoon and others who have followed the safety route but I personally do not want such a boat with the poor light wind performance that results. Its also worth pointing out that all the manufacturers have done is moved the threshold of danger upwards, they have not eliminated it.
My personal philosophy is to choose a boat with reasonable light weather performance and to rely on seamanship, common sense and early sail reduction to keep safe. In most cruising areas even the English Channel, light winds predominate. Poor light wind performance will significantly increase the percentage of time you have to motor, something I hate. The performance metrics allow me to compare potential candidates against boats I know. They are not the whole answer but are useful in making comparitive judgements

[44'cruisingcat]

Originally Posted by Sand crab

That's really good. Most are around 22. I looked at your gallery and profile but couldn't find any specs on your cat. Care to share?

Sail area is 90m2, displacement 6 tonnes.

Initialy I converted to feet and pounds in my head. Though maybe I was out a bit, so I used a converter and re-did, got 27.71 .

So I wasn't far off anyway.

[44'cruisingcat]

Originally Posted by sandy daugherty

Before we get too bogged down in numbers, lets consider the designer's goals and comfort levels. They may not be what the rosy prose in the brochure says.

Most designers are in the business for the long haul: make some money, put the kids through braces and college, have a nice boat to play with, live somewhere close to the water. The long haul means establishing a reputation in the target customer group. That may be charter operators, long distance cruisers, or racers. Unfortunately, there is no significant racing among multihulls that can't go home on a trailer. Monohulls have great Wednesday night class racing, one designs, and effective rating systems that build devoted participants almost everywhere there's water and wind.

Why this hasn't happened for cats would be a very fruitful discussion; the amazing success of the Farrier / Corsair tris proves beyond a doubt that it could happen.

Meanwhile, back at the big cat gang, maintaining a reputation means not having accidents. I believe that the fate of the wonderful Stiletto 30 was sealed by a single fatal accident, and one that really had little to do with the boat's design. It takes nerves of steel to produce a vessel that even hurricane force winds could blow over. For every other designer, its masts too short and beams too wide to be pulled over by a freight train.

So, the best plan for all but the gutsiest designer is to wax poetic about performance in the brochures, and hide the real numbers from the savvy shopper.

Test question: Do buyers really want to risk a calamity at sea for a bit more speed?

Apparently not.

I disagree. Designers like Morelli & Melvin design cats that PERFORM, and have a very good name. Chris White, most Australian designers, Schionnings, Oram, Craig Schionning, Shaun Arber... all design boats to sail, and all seem to have been able to make a reasonable living over a lot of years.

The conservatively designed boats are those intended for the charter market. Where they will be sailed by people with no idea whatsoever about reefing. And where they are usually required by the charter companies to run engines for hours a day, so motoring is no real hardship.

It always amazes me, watching the charter boats sailing across Whitsunday passage, with 30 knots wind against 3-4 knots of tide, fairly shallow water, most of them on a beam reach with every stitch of sail up, sheeted in as tight as they'll go...

Those boats really are pretty idiot proof.

[Pat Ross]

Originally Posted by Intentional Drifter

Hi, Scape, and welcome to CF!

You might want to check out Calvin Markwood's Multihull Dynamics, Inc. - Home

Here is a good link for formulas Multihull Dynamics, Inc. - News Article


Here is another good article on how Cal Markwood approaches his work Multihull Dynamics, Inc. - News Article

There are many other articles available in the Technical Section Multihull Dynamics, Inc. - News

Enjoy, what your doing now is how we all got started for the most part

Pat

[44'cruisingcat]

Originally Posted by Sand crab

It goes both ways. This one uses feet and pounds, or tons or cubic feet. Convert to metric and it will give you the same number.
SA / D Ratio
Remember it's only a ratio of sail area to displacement. You can view it or look at it as either metric or standard and it's still the same number.

At 27.71, it says my boat is a high-performance racer. In multihull terms it certainly isn't. A reasonably quick cruising boat, but not a racing boat.

[Sand crab]

That SA/D link is for monos. I've never seen one that applies to multis. It thinks every multi is high performance. Yours is better than most. What exactly is your cat?

[Pat Ross]

Originally Posted by 44'cruisingcat

At 27.71, it says my boat is a high-performance racer. In multihull terms it certainly isn't. A reasonably quick cruising boat, but not a racing boat.

Here is a graph showing Chris White's definition of multihulls based on rigging and waterline length by Bruce Number. He breaks it down to 4 categories

[44'cruisingcat]

Originally Posted by Sand crab

That SA/D link is for monos. I've never seen one that applies to multis. It thinks every multi is high performance. Yours is better than most. What exactly is your cat?

It's an Oram 44C, but slightly modified. Bit taller, wider, heavier, and a little more sail area.