Underwater hull form for sailing catamarans

[Guest625101138]

So where are we???

I think the straight rocker has it. Basically the best sailing boat is one that sails fast and points high. Nothing new there - I learnt that from an experienced sailor and yacht designer named Joe Adams about 30 years ago. His yachts were considered radical then. He was one of the few designers that designed for performance rather than rule beating. Interestingly his designs are no longer radical but close to current practice.

For a "wickedly fast catamaran" - lets say 40ft in length and total displacement of 4 tonne (they can build a globe trotting 36.8m cat weighing 17t). Design speed is 20kts. Hull design condition is all weight carried on the lee hull and this hull in perfect trim at design speed.

It will have under-hull rudders and through hull dagger boards for efficiency. The bridge will be low windage. The bow will have ample reserve buoyancy and some lift to lower the risk of pitchpoling. By contrast the sterns will not have excessive buoyancy.

I will see what Godzilla produces and post it.

Rick W

[rob denney]

[quote=Pericles;179275]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.

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!

G'day,

I have been puzzling about this since it was posted. There is no doubt that what you say is correct. However, as it does not relate to my experience with actual boats, I have been wondering why.

Came up with the following:

1) The log has a much longer waterline, although it will still pitch in waves of critical height/length whereas the ball won't, so this is not much help.

2) Pitching is not just about hull shape. It is also about mass times distance from the centre of buoyancy. As soon as the ball has any of this, it becomes very prone to pitching, with only water friction to stop it.

3) All boats pitch. What we want is one that pitches least and stops pitching fastest. For this, the buoyant ends/high prismatic shape is clearly superior.

The Gunboat is indeed a quick machine, but it would be quicker, pitch less and float higher when pressed if it had no rocker, the same as the ORMA 60 floats. It would also be a bear to tack. Gunboat's hulls do not fit the stretched half ball analogy. If they did, it would have no rocker and very full ends, much like the Godzilla and Harry shapes.

regards,

Rob

[Richard Atkin]

The Wharram Tiki series are notorious for pitching. They have a lot of rocker. They are regarded as quite slow but very seaworthy. Perhaps the seaworthiness comes from the slow speed. I can see how these boats will follow the wave motion, instead of fighting it.

All the fastest beach cats don't have much rocker but are notorious for pitchpoling, but then they don't have much buoyancy or lift at the bows.

So.....it's not just theoretical. We have real life examples.

[Fanie]

Rocking comes from having narrow fore and aft hulls so somewhere near the centreline there is a pivot point. The waterline for a ball is round so there is no pitching in any direction unless you put something on it, it will roll from gravity.

The stick when sharpened would cleave the water, but the long parrallel sides would have a lot of drag due to the wetted surface area, and tacking or turning would be a bugger. Getting a proper form would be a trade-off from the long stick to the ball.

The morphing point would be where the stick is less rigit and the ball won't roll over. In my opinion some boats come pretty close - whether there is a perfect hull I very much doubt. Maybe some odd shape could become the standard some day, we're just not thinking in that direction yet.

[Pericles]

This description of the Gunboat 62 hulls is given at http://www.deltayachtsbrokerage.com/...World_gb62.pdf

The hulls are cylindrical in section below the water line for almost their entire length.-- Combined with a gentle uniform rocker along their length, this serves to dampen the pitching motion of the boat and with no vee forward forward, she is much easier to tack.

The stretching ball analogy. I should have said half ball. My failure to communicate. It should result in an underwater shape with gentle rocker rising towards the stern and stem. Never mind.

To everyone who has contributed, my grateful thanks and keep those comments and ideas coming. Exchanging information this way is about the rapid evolution of new designs, which benefits us all.

Guillermo, when we have completed the family move to Newhaven, East Sussex, we'll be about 30 kms from John Shuttleworth, so I shall be consulting him in due time.

Kindest regards,

Pericles

[Richard Atkin]

There is no ultimate hull shape. It depends on how you want to sail the boat.
If you build a cat that can do 60 knots for 2 seconds, but then turns upside down....then it is a very fast cat....and it isn't.
If you base it purely on passage making time, then you have to consider how much risk you want to take, and in what sort of weather. There are so many variables, that I think we will never agree on the "ultimate" design.

[Fanie]

I thought we agreed not to agree...

[Guest625101138]

The 4 tonne 12m GODZILLA hull ends up quite fat. It has a WL beam of 0.95m. Effectively semi-circular sections and of course no rocker given the design speed is about 2.5 times hull speed. The drag is not much less than that required to plane.

I have attached two sets of lineplans. One shows the cat in static trim and the other shows it in sailing trim.

I did a bit of fiddling above the waterline, bow and stern, along the lines discussed in earlier posts.

With a practical sized rig I determine it would need 30kts of wind to do 20kts.

Rick W.

[masalai]

Rick W. I am amazed and impressed. Your productivity is astounding. I am in awe. Back to my task of RTFM.........

[JCD]

Originally Posted by Rick Willoughby

The 4 tonne 12m GODZILLA hull ends up quite fat. It has a WL beam of 0.95m. Effectively semi-circular sections and of course no rocker given the design speed is about 2.5 times hull speed. The drag is not much less than that required to plane.

I have attached two sets of lineplans. One shows the cat in static trim and the other shows it in sailing trim.

I did a bit of fiddling above the waterline, bow and stern, along the lines discussed in earlier posts.

With a practical sized rig I determine it would need 30kts of wind to do 20kts.

Rick W.

Three words.

Amazing Rick.

J

[Richard Atkin]

Rick, nothing like a picture to express a point of view. Nice job.

Got a few questions again!

Why do your dagger boards point in the opposite direction to J's?

Are big daggers more important at higher speeds?

And by the way...your rudders still look gay. (that's not a question).

[Richard Atkin]

Rick have you considered getting hold of some VPP software for sail rigs....not just the hulls? Or does that not interest you enough?

[Freenacin]

Richard, the usual practise is to retract daggerboards as speed increases. You really only want them as big as possible when going to windward in light air.

Rick, the tapered daggerboards might make retracting them interesting. They seem to be quite low aspect ratio too. Wouldn't high aspect, constant chord boards be more efficient as well as being easier to build?

[Guest625101138]

Some good observations and question.

I mainly concentrated on the hulls based on the design condition stated.

Pushing a 4 tonne hull to 20kts takes reasonable grunt even if the hull is optimised for it.

The sail and keel were just exercises to determine if they could generate enough force. I tried to do 20kts in 20kt wind but the sail ends up ridiculously large. I simplified the foil analysis to using aspect ratio of 4 for both sails and keel. What I looked at was two winged sails located inboard of either hull as I can produce meaningful design forces. I have not gone into the interaction of two foils and these would be spaced enough so interaction would be low unless the wind is across the beam.

Anyhow the keel needs to be quite strong and I did not go into detail on its stresses just made it look like a section commensurate with the forces. It would need nice solid guides and would clear the deck when fully retracted so constant section is not really an issue - biggest loads are when it is fully extended.

I wanted to see how high it could sail. It should get up to around 35 degrees to true wind in 30kts. I doubt it would do this in open water because the sea state would be beyond its capability. Lifting a hull in calm water would be exciting enough for me.

One of the things I obseved as Godzilla was working was that the hull stayed a long time in "V" before it started to round out. Usually the round hull comes early on in the optimisation. I have noticed some of the latest big multihulls have "V" sections so these may drive better in a seaway with very little addition to calm water drag.

Rick W.

[Richard Atkin]

Rick
I thought your VPP software was limited to propellor power, but obviously you have progressed since then. I wasn't sure if you were using just raw maths, or if you were using an elaborate VPP with all the bells and whistles. It's all over my head.
Could you give me the name of a good VPP program so I could look at sticking a rig on my design? I don't care if it is hard to understand at first.