Study for Sail Area / Displacement Ratio of some cats

[catty]

Originally Posted by GordMay

catty:
Where did your Bill Gibbs quote come from?

Bill Gibbs’ “Afterburner” (54 Ft Cat’) finished 7TH in this year’s (61ST) Newport to Ensenada International Yacht Race, behind 1 Catamaran (“Loreal”) & 5 Monohulls (80 Ft “Magnitude”, 75 Ft “Akela”, 64 Ft “Medicine Man”, & 70 Ft “Oex”)

Sorry for delay, from The Multihulls Archives May 2008

[catty]

Originally Posted by Joli

Dave, I'll give it one last shot here. A couple reasons multis get sticky in very light conditions is because they have a quick motion and little weight to carry through the chop and lulls. It is tough to keep flow attached when the boat naturally has a quick motion, it is tough to keep boat speed with little momentum.

Steadying out the rig so flow can stay attached and keeping momentum through adding weight can in some instances be beneficial. I gave you an example of such a boat in post 33. Going to smaller sail when it is very light is another example.

Nothing in sailing is a zero sum gain. Every adjustment has an impact, some adjustments create surprising results.

Cheers,



Joli

Joli you are dead right, going from a nice breeze to a drifter , the left over slop will kill a multi quicker than anything, especially if its on the nose . Afterburner certainly doesn't suffer from insufficient sail area or from being overweight.

[2Hulls]

A perfect example. Multihulls in slop is the flip side of the very same issue.

Any boat is adversely affected by slop and lighter boats more so. Slop is just an opposing F in F=ma. Glad to see you understand this and how multis may be more adversely affected than heavy monos. Higher "m" means lower "a" for a given F.

Now apply this understanding to the original issue:

It is a cold hard fact that getting more mass moving takes more force and a boat with added mass cannot possibly be faster than it was before - all other things being equal.

Never, never, never.

Yep, a heavier boat will slow down less (from a lower top speed) than the same boat with lighter mass from any resistance. Then it takes more force to regain a lower top speed once the temporary resistance passes. The lower top speed is due to greater friction from pushing more water out of the way.

Get over it guys. This is reality. This is exactly the issue multihull critics like to keep repeating: load up that multihull with too much crap and the performance goes in the tank.

Guess what? The very same principle applies to monos.

Dave

[Hud3]

Dave,

F = ma doesn't account for the "quick motion" experienced by multis in chop, with light air. The motion shakes the wind right out of the sails and they stop dead.
What the heck was the subject of this thread, anyhow? I've lost track!

[2Hulls]

I understand that "quick motion" is a different, adverse result of chop - and multis don't have a corner on this market. Any lighter boat will get bounced around more than an otherwise equivalent heavier boat.

I submit that even when considering the adverse effects of chop, lighter versions of the same boat will do better overall. All of my discussion has been about effects of adding mass to the same boat - not about comparing different boats. Tossing in an argument about multis behaving badly in chop is a red herring to the "adding mass to perform better" assertion.

"OK crew, looks like it'll be a really light air day. We better flood the bilges and get some more crew to make sure our speed through those dead spots is as best as it can be!!!"

See?

Dave

[David M]

Originally Posted by 2Hulls

I understand that "quick motion" is a different, adverse result of chop - and multis don't have a corner on this market. Any lighter boat will get bounced around more than an otherwise equivalent heavier boat.

I submit that even when considering the adverse effects of chop, lighter versions of the same boat will do better overall. All of my discussion has been about effects of adding mass to the same boat - not about comparing different boats. Tossing in an argument about multis behaving badly in chop is a red herring to the "adding mass to perform better" assertion.

"OK crew, looks like it'll be a really light air day. We better flood the bilges and get some more crew to make sure our speed through those dead spots is as best as it can be!!!"

See?

Dave

.......Yes!

[Calculator]

Big Hulls - please say more about the K factor? Is this from Edmond Bruce's model studies of various L/B hulls?

Cal

[fastcat435]

Here you can find the formula,s for the texel rating and the spread sheet to calculate your own cat, it is still the best formula around for Catamarans.
Catamaran en Trimaran Club Nederland
The FastCat 455 has a texel rating of 98.64 for example and the
Catana 431 a rating of 127.71
the weight difference is 5500 kilo according to this spread sheet

Greetings

Gideon

[catty]

Originally Posted by BigCat

David is quite right, and Gideon is wrong if he means to imply that monos are slower than multis in light airs. A light multi has more wetted surface than a monohull, and it has even more when you add more weight to it. What David left out in his explanation for why multihulls are faster when the wind picks up is the "K" factor, which reflects the lower resistance of a much narrower hull. Cal is right about the effect of waterline length, but he is also leaving out the fact that the "K" factor gets higher if you stretch a hull to make it longer.

It is easier to design a narrow hull in a big boat, than in a small one, because the human body isn't scaling up as the boat gets bigger, and interiors are designed around the dimensions of the human body-if you are to have reasonable accommodations.

BigCat what sought of wetted surface are we seeing on say the 80' mono Magnitude and the 50' cat Afterburner. Well, wetted surface : SA ratio anyway.

Regards

Regards

[BigCat]

Hi, CSH - I don't have any specifics, but it is considered a truism - just google "catamaran monohull wetted surface"

[BigCat]

Hi, Catty

For example, on my website, you will find two boats, which have about the same waterline and displacement. One is a catamaran, and one is a monohull. The hulls of the multihull have about 30% more wetted surface than the monohull.

[Therapy]

Originally Posted by BigCat

Hi, Catty

For example, on my website, you will find two boats, which have about the same waterline and displacement. One is a catamaran, and one is a monohull. The hulls of the multihull have about 30% more wetted surface than the monohull.

I don't get it.
Do they weigh the same?
If so then the amount of displaced water is the same.
I guess it is the shape. I know a box is the lowest for the weight.
Strange...

[BigCat]

"I guess it is the shape. I know a box is the lowest for the weight." No, a circle is lowest for the weight. In three dimensions, a sphere.

If you google "surface to volume ratio," you will see why monohulls have less wetted surface than catamarans.

[Therapy]

Originally Posted by BigCat

"I guess it is the shape. I know a box is the lowest for the weight." No, a circle is lowest for the weight. In three dimensions, a sphere.

If you google "surface to volume ratio," you will see why monohulls have less wetted surface than catamarans.

I can't find what I saw the other day.

It was a link from here somewhere that showed several hull shapes.

I found this.

"2. Surface Area to Volume Ratio

A cube has the highest volume with the least surface area"

Much of this I don't understand well.

[waikikin]

Windage is another issue esp on bridgedeck cabin cats, you can easily represent the difference on paper by drawing a long thin rectangle(mono) against a much "fatter" one (cat) at various angles esp to windward where it really hurts. All the best from Jeff