Underwater hull form for sailing catamarans

[Fanie]

Nicely commented, Pericles. If I may add, the mass of the boat will also oppose rocking as will the sail. Any boat will rock in a specific length of wave as the floating area tries following the waterline angle.

On Youtube there is a video of a surface hull trimaran, very light and very fast. I was impressed that it schemes the surface only, probably have a daggerboard and rudder for steering... sorry cannot remember the name. A shallow hull under speed will tend to lift up from the water and lean towards plaining while a V-ish hull that sits deep has to continuous displace water to move. On the other hand a shallow draft is more windy. So it depends what your app is. If it's racing you'd go for the shallower surface draft, a leisure craft would gain by sitting deeper in the water. Compromizes.

[Pericles]

Fanie,

Thanks for the compliment.

Lightness is a desirable feature for a catamaran, even a cruising catamaran used for leisure. Length and beam need to be maximised for speed and stability. The FARR site is worth a search.

http://www.f-boat.com/pages/faq.html#anchor150609

Designing one's boat from scratch using free software is great for pictures for the walls, but as PAR makes very plain, the skin is not the structure.

http://boatdesign.net/forums/showthr...143#post179143

Who was it who said “If I have seen farther than Descartes, it is because I have stood on the shoulders of giants”?

I was put in my place very soundly many years ago. My office supervisor said to me "Ambition without ability is a disaster on the horizon". I took it to heart.

All the best,

Pericles

[Richard Atkin]

Pericles...comparing a ball to a telegraph pole? You've got to be kidding me. Attach a mast and sail between 2 balls and watch what happens. Attach a mast and sail between 2 telegraph poles and watch what happens. Measure the pitching by watching the top of the mast. Remember we are not including ballast in this equation. The idea is to have resistance at the ends. A ball has no ends.

[Richard Atkin]

Okay, so maybe I jumped in a bit too soon! Ummmm.....okay so that leaves us wondering how much of the pitching is created by the up and down motion of the waves, and how much of the pitching is created by a mixture of momentum and accelerations and sail forces etc.
If long slim hulls with no rocker have a wave piercing effect, then they will be less influenced by the up and down motion of the waves.

[Richard Atkin]

Okay...but then how much wave piercing is acceptable until it becomes unsafe?

hmmmmm.....

sorry Pericles....I humbley accept that it's not as simple as I thought. I guess it just depends where you want to make the compromises.

Have I just made a fool of myself?

[Richard Atkin]

Pericles, quote "I was put in my place very soundly many years ago."

That's a very british attitude. You guys need to loosen up a bit with all the big boss, little apprentice stuff. Ofcourse all technology is progressed by standing on the shoulders of previous 'people'....not necessarily 'giants'.

Young people discover new things. Old people discover new things. The technology is all around us, so we are all in the same boat.

[Fanie]

Here is a vid of a cat speeding, one can observe the under water hull shape while sailing. I have also stumbled across other pictures since reading the thread and they seem to be fairly similar.

http://www.youtube.com/watch?v=O5wPt3xz1K4

comparing a ball to a telegraph pole?

I thought the two extremes explained it fairly well. The ball would go up and down while the pole would pitch some. Cut the ball in halve and put a weight under it for righting moment and you have a monohull

So if you have two balls amd a pole it seems you have a multi-hull. If this is correct we are multi-hulls... which could be why some of us work so well and we all do a bit of pitching here and there

Looking at some racing vids earlier on, some of these cats push their leeward hull so far into the water they pitch-pole over foreward. Why don't they just add some fins at an angle on the hull fronts that would force the hull up out of the water if it gets submerged to beyond a certain point ? I think this could make for a nice safety feature on any cat hull.

[Fanie]

Wrong link sorry...

Here...

http://www.youtube.com/watch?v=lsTXG...eature=related

[Freenacin]

Originally Posted by farjoe

Is faster tacking really due to more rocker?

I have a small hobie like cat with lots of rocker but no dagger and it tacks like a pig.

On the other hand I have another slightly bigger cat with a lot less rocker but a sizeable dagger and it very rarely misses a tack.

There a dozens of factor which effect how fast a boat will tack - rocker is just one of them. Perhaps I should have said "all other things being equal"

Periclese, your telegraph pole vs ball example is a bit strange. How would you measure pitch in a shape that has no ends? However a sphere has virtually zero resistance to roll - in any direction, where a cylinder does have resistance to pitching.

Read Chris White's "The Cruising Multihull" - he discusses rocker and it's effect on pitch and yaw.

[Richard Atkin]

Fanie, quote "I thought the two extremes explained it fairly well. The ball would go up and down while the pole would pitch some. Cut the ball in halve and put a weight under it for righting moment and you have a monohull"

Put the ball halves into a cat configuration, and remove the "weight under it for righting moment". Do you see a stable boat? The waves might not make it pitch (in the way you describe)....but think about all the other forces. That was my point.

We need to remember that cats are not 2 monohulls stuck together. The rules for rocker must be different.

[Richard Atkin]

I'm allowed to say all this stuff even though I have had no experience. (I was promoted to senior member a couple of days ago).
Check out the words below my name and soak it up.

[Pericles]

Freenacin,

The ball in water is a thought experiment. You'll have to try it for real, I guess! Place ball in bath of water and create a series of waves. The ball will bob up and down but it will not pitch fore and aft. It may roll in any direction as you would expect, but pitching is also about being rolled back in the opposite direction by same wave passing under the ball. Ergo, there is no pitch to measure. Look at the underhulls of Gunboat 62.

http://www.deltayachtsbrokerage.com/...unboat-en.html

http://gunboat.info/home.swf

Fanie, I am delighted you know what I am trying to explain.

Richard, stretch the two halves of the ball and you end up with Gunboat series hulls. High speed and minimum pitching. Great designers visualise.
Choose your giants' shoulders very, very, carefully!

Best wishes,

Pericles

[Guillermo]

Perry,
I think you'll find this interesting:
http://www.john-shuttleworth.com/Articles/NESTalk.html

Cheers.

[Freenacin]

Originally Posted by Pericles

Freenacin,

The ball in water is a thought experiment. You'll have to try it for real, I guess! Place ball in bath of water and create a series of waves. The ball will bob up and down but it will not pitch fore and aft. It may roll in any direction as you would expect, but pitching is also about being rolled back in the opposite direction by same wave passing under the ball. Ergo, there is no pitch to measure. Look at the underhulls of Gunboat 62.

http://www.deltayachtsbrokerage.com/...unboat-en.html

http://gunboat.info/home.swf

Fanie, I am delighted you know what I am trying to explain.

Richard, stretch the two halves of the ball and you end up with Gunboat series hulls. High speed and minimum pitching. Great designers visualise.
Choose your giants' shoulders very, very, carefully!

Best wishes,

Pericles

I don't know where you got it into your head that Gunboats have a lot of rocker. In fact they don't. The fact that they can float in 2 feet of water (according to their website) demonstrates that clearly. 2 feet over 62 feet is clearly very little rocker. Which is why they don't suffer from hobbyhorsing.

Good article Guillermo, although it does contradict what some people have "discovered" while playing with their balls in the bathtub.

[Fanie]

Fanie, I am delighted you know what I am trying to explain.

You have to be a fisherman to have noticed the difference between round floats and the pencil like ones I always knew fishing was good