Fast Cruising Catamarans - How Fast ?

[Hud3]

Hello, Christy and Joe. I remember your boat and your lovely family from the Caribbean 1500 a few years ago. Welcome to the Forum!

[switchcat]

We remember you of course! I see you are back in the Caribbean - we have been cruising the Med the last couple of years. Currently we are in Siracusa Sicily - we are heading back to the Caribbean this winter - maybe we can have a Killer Bee at Sunshines with you.....

Joe and Christy
Welcome to Zia Later!
5MileWIFI.com

[BigCat]

Originally Posted by mikereed100

This thread had got me thinking about cruising cats and light air performance. It is my understanding that at lower speeds friction (wetted surface) is the major cause of drag. As one approaches hull speed, wavemaking becomes the dominant speed-limiting factor. Given that much cruising is done in light airs would it not be more advantageous to have a hull shape that mimimized wetted surface at the expense of top end speed? What I am getting at is that for light air performance perhaps a wider hull with a flatter bottom might be a better performer than a slender hull with a more rouded or V'ed profile. The wider, flatter hulls would have an additional advantage in that a cruising payload would depress the hulls less, resulting in a slower increase in wetted surface and theoretically less decrease in light air performance as the boat is loaded. Bridgedeck clearance would also suffer less. Makes me wonder if a stripped down Bahia (remove generator, ac, extra cabins, heads etc.) might not outperform an Outremer in light air. Looking forward to your thoughts.

Mike

The lowest wetted surface is found in boats that have sections that are sections of a circle, not flat ones. Slamming against waves when going to windward will stop a boat that isn't sharp enough forward, however. The shape that is optimum for speed is typically 'v' forward, round in the middle, and flattened aft. I dislike slapping counters, however, and carry the 'v' right aft to prevent slapping, but round the turn of the bilge with a large radius to minimize wetted surface. Most cruising isn't done in 'light airs' but rather in moderate winds-averaging about 12 or 13 knots.

[Hud3]

Originally Posted by switchcat

We remember you of course! I see you are back in the Caribbean - we have been cruising the Med the last couple of years. Currently we are in Siracusa Sicily - we are heading back to the Caribbean this winter - maybe we can have a Killer Bee at Sunshines with you.....

Joe and Christy
Welcome to Zia Later!
5MileWIFI.com

That'd be great! Killer Bees are on me!

[mikereed100]

Originally Posted by BigCat

The lowest wetted surface is found in boats that have sections that are sections of a circle, not flat ones. Slamming against waves when going to windward will stop a boat that isn't sharp enough forward, however. The shape that is optimum for speed is typically 'v' forward, round in the middle, and flattened aft. I dislike slapping counters, however, and carry the 'v' right aft to prevent slapping, but round the turn of the bilge with a large radius to minimize wetted surface. Most cruising isn't done in 'light airs' but rather in moderate winds-averaging about 12 or 13 knots.

Agreed on all counts. I only mention a flatter section hull for it's improved load carrying capacity and by "light air" I mean anything below about 15 knots. In other words, a full main and large headsail are being flown.

But my question remains. In these conditions is a narrow, easily depressed hull at a disadvantage compared to a wider hull with less surface drag given the same weight?

Mike

[yeloya]

Being for quite a while in this forum as a relatively unexperienced sailor/cruiser, I just can't believe that most people didn't realised yet that everything that we talk are all about compromises and trade offs..

I believe every designer knows very well how to make a boat that is faster in light winds, that points higher, very seaworthy, etc.. The trade off is less space, the use of very expensive/high technology material, less load carrying capacities, etc.
For everytghing you get , there is a price to pay and some trade offs. I wanted a fulll island bed, the price I am paying is the slamming under my pillow and a bit more drag. If you like extra head space, you will have to accept more windage. If you prefer larger sail area for better performance in light airs, you will have to be more familiar with putting - removing reefs and be a bit more worried when the wind raise suddenly.
Everyone has different tastes and preferences. That's why we have 100 different types of cats around. The important is to understand what you want and what you have to give up to have it. This is sometimes extra money, sometimes a dagger board that you'll have to deal with, sometimes less space.

Back to the topic, the question "how fast yr boat is meaningless" unless we talk about what are tradeoffs. A 300.000 Euro more to be paid for the same size of boat to be 1 or 2 knots faster ? Norrower galleys and by how much ? Will everybody on board need to be qualified sailor to achieve this ? etc.etc..

Cheers

Yeloya

[David M]

Originally Posted by BigCat

The lowest wetted surface is found in boats that have sections that are sections of a circle, not flat ones. Slamming against waves when going to windward will stop a boat that isn't sharp enough forward, however. The shape that is optimum for speed is typically 'v' forward, round in the middle, and flattened aft. I dislike slapping counters, however, and carry the 'v' right aft to prevent slapping, but round the turn of the bilge with a large radius to minimize wetted surface. Most cruising isn't done in 'light airs' but rather in moderate winds-averaging about 12 or 13 knots.

Actually, a parabola if you want to compare the ratio of the surface area under the parabola (volume) versus the length of the parabola from the x-axis back to the x-axis (wetted surface). Compared to a half-circle, the parabola has a more favorable wetted surface to volume ratio.

[Talbot]

I find this a bit of a strange question. For a catamaran, speed = weight. The lightest fastest cats are also the ones least capable of taking additional weight for cruising.

If you are long term cruising, your main concern is (or should be) avoiding damage to your gear.

Thus all this talk of comparison is fundamentally flawed. It is impossible to compare like for like (cruising weight for one person is very heay for another, and lightly loaded for a third), plus sea conditions will also make a big difference.

Finally, the biggest difference - one where the sails and rigging are set up correctly and the boat is being sailed to her optimum.

[David M]

As is being hinted at, Its all a big compromise. There are no truly innovative technologies except in perhaps the area of strength to weight ratios. The physics of the interface between the atmosphere and the water will always be the same. Every time one thing is improved, it is almost always the case that something else has to be made worse. Get the boat that works best for your purposes. There is no ideal boat. The boat has to fit the individuals purposes. No one boat will ever suit everyone....its not even close.

Naval architects will boast about how their boat is the best. But you have to ask in return...best for me?

So what happens is people debate what is the best design when it would probably be a lot more productive to debate what is the best design for you.

[Adaero]

Originally Posted by David M

As is being hinted at, Its all a big compromise. There are no truly innovative technologies except in perhaps the area of strength to weight ratios. The physics of the interface between the atmosphere and the water will always be the same. Every time one thing is improved, it is almost always the case that something else has to be made worse. Get the boat that works best for your purposes. There is no ideal boat. The boat has to fit the individuals purposes. No one boat will ever suit everyone....its not even close.

Naval architects will boast about how their boat is the best. But you have to ask in return...best for me?

So what happens is people debate what is the best design when it would probably be a lot more productive to debate what is the best design for you.

Posts such as this quoted above is the reason I continue to read and sometimes (not as often as some) try and contribute to this forum. The above should make sense to every sailor who reads it and will hopefully sink in.

As I said in a previous post the original question is totally irrelevant to 99% of the people who read these boards. If anyone here wants to take their boat to the absolute limit, maybe try and lift a hull or similar then thats up to them. To me this is just ridiculous as cruising is getting somewhere safely in a reasonable amount of time, it's not a "dick measuring contest" or "my boat is faster than yours"! As said above what is right for me might not be right for everyone else.

The guys with the Switch Cats will be happy to read that in this months Multihull Mag there is an article stating that a Swiss guy who has a Switch 51 has bought the company and will be releasing a totally new Switch 55 next year.

Adaero

[BigCat]

Well, the easiest way to make a boat fast is to give it longer hulls. Let's say you want two double berths and two heads, a modest dinette, a reasonable galley, and a reasonable nav station. You don't feel the need to have a sofa, or a bar. Well, you could do this in a 40 foot cat, but it would be a bit stubby. So design a 40 foot cat, lengthen the hulls, increase the beam , but don't increase the accommodations. You now have a fast boat. The more you stretch the boat, the lighter it becomes in proportion and the narrower the hulls become in proportion. That's all there is to it. You will get more bang for your buck this way than you will get with carbon fiber, epoxy, mylar sails, or any gadget you can buy.

Mike, I'm not sure where you get the idea that flat sections are more burdensome than curved or v sections. I don't agree that they are. Flat sections may be conducive to planing, and also, unfortunately, to pounding. If you load them up you get a section approximating a square, which is not an optimum shape in any way. No hull shape in common use today in catamarans is anywhere near too deep and narrow to have more draft than is optimum, even when heavily loaded.

And, the least surface to volume ratio possible is in a circle in two dimensions, and a sphere in 3 dimensions-however, it is only in winds of under, say 5 or 6 knots that this is a big issue.

[jzk]

Originally Posted by David M

As is being hinted at, Its all a big compromise. There are no truly innovative technologies except in perhaps the area of strength to weight ratios.

Technology will always improve.

[ldrhawke]

Originally Posted by BigCat

Well, the easiest way to make a boat fast is to give it longer hulls. ...............................You will get more bang for your buck this way than you will get with carbon fiber, epoxy, mylar sails, or any gadget you can buy.
...................

Right on!

Designer comments on ,"Longer is Better"

Obviously light material in construction of any cat helps, but at a certain point it make no economic sense. When mainly trying to keep a cat light, by using exotic new materials, that are costly to purchase and install; it quickly gets the point that it makes far more economic and performance sense to simply stretch the boat a few feet instead.

Striving mainly for lightness, by continually making structural sections thinner and lighter can be dangerous. At some point, even though they may have adequate measured strength to ultimate failure, there is the unanswered question of material fatigue from cycling and age.

There is also the comfort factor which isn't easy to measure or place a value on. A cruising catamaran can become too light. It then becomes akin to riding in a large Hobie cat, just look at the large ocean cat race boats as they bounce across the waves. A heavier cat is more stable and does not bounce off the waves like a cork, making the ride less physically and mentally fatiguing. Plus, shorter boats are less stable and hobby horse much more easily.

Also, often wave sound levels inside a lighter constructed boat go up. Plus, thinner poorer insulted hulls can sweat and cause mildew/odors to develop inside the boat. Thin cabin roof materials also transfers the suns heat more easily to make the cabin less comfortable. I've seen pictures of used cats that the owners have covered the complete cabin ceiling with tin foil trying to keep the suns heat out.

Isn't this the Cruisers Forum? I'm all for keeping the weight off and the performance in a catamaran, but sometimes I think this site sounds more like the Racers Forum?

The humorous part is to see the cruiser that spends $100,000's more to get a short lightly constructed cat and then loads it down with a Gen set, AC, Washer/drier, etc; only to end up with a mediocre performing catamaran in the end.

[yeloya]

Every time one thing is improved, it is almost always the case that something else has to be made worse. Get the boat that works best for your purposes. There is no ideal boat. The boat has to fit the individuals purposes. No one boat will ever suit everyone....its not even close.

Naval architects will boast about how their boat is the best. But you have to ask in return...best for me?

Thx David, that's exactly what I was trying to say..

We do agree that speed is not the major concern, otherwise all motor yatchs are there..

However, being able to sail in lighter winds (not necesarily faster) does still matter, provided that you accept the tradeoffs..

Cheers

Yeloya

[fastcat435]

Originally Posted by David M

As is being hinted at, Its all a big compromise. There are no truly innovative technologies except in perhaps the area of strength to weight ratios. The physics of the interface between the atmosphere and the water will always be the same. Every time one thing is improved, it is almost always the case that something else has to be made worse. Get the boat that works best for your purposes. There is no ideal boat. The boat has to fit the individuals purposes. No one boat will ever suit everyone....its not even close.

Naval architects will boast about how their boat is the best. But you have to ask in return...best for me?

So what happens is people debate what is the best design when it would probably be a lot more productive to debate what is the best design for you.

You are right David

there is no Ideal boat , all boats ( cats ) are compromises and with some you can live and with some you cannot.
If speed is your thing than the weight has got to go down and taking it down by making the hull or anything structural is a recipe for disaster so the only ways to save on weight are
Using stronger materials like S glass, carbon fibre, Basalt fibre , epoxy, foam or lightweight balsa , making laminates instead of solids, And using prereg or resin infusion to minimize on the resin used.
Where most can be saved besides the above is on items in and on the boat like substituting glass with Lexan
A porcelain toilet bowl with a plastic or carbon version.
Using dyneema instead of polyester lines
PBO side stays , carbon rig
Lightweight batteries, Led Lighting and so on and on.
This does not weaken the structure but actually increases strength since a lighter boat has less stress.
It also provides the often needed payload to make a boat more luxurious without suffering to much in performance.
The idea is to create an enormous payload like 2200 lbs per 10 ft of length and that way we can end with the almost perfect cruising catamaran. ( Perfect does not exist)

Every time one thing is improved, it is almost always the case that something else has to be made worse.
And in case of getting weight out the cost is the worse part of it all.
It is very cost expensive to save on weight
It will always be a compromise although many sailors believe the have found the perfect boat.

Greetings
Gideon