on the design of ultra slim yachts

Hmm - thanks Paul - as far as "Timed out again? " goes - that never happens to me - and I'm often logged on for hours....

 

Paul,
The above idea, without the April Fools' rack and pinion bit, is pretty much how the Mini650 class works. You may not exceed a max height from keel bottom to masthead, but the hull may be any where inbetween.
I always liked that system ;-)

 

Paul B:
I am serious. Modern cats, and old ones too, have two hulls. This thread is about a monohull. Therefore, it is not a shallow ultralight displacement boat. It has a deep keel with a massive bulb. The plank on edge boats match the criteria, at least for comparison to a similar type.

 

It is now 2003

Soon it will be 2004. Please do a little research into what's happening in the modern world before you post more nonsense. Stop reading books written before 1980 and go look at what is happening in the real world. Maybe subscribe to Seahorse and get a look at reality every month.

The closest big boats to the description of "half a catamaran" would be the current AC boats, at 80+ feet LOA and about 12 feet wide. They currently displace more than they should, to benefit from the rule, but they are still lighter (D/L) than most sportboats like the Melges 24.

The new Maxi boats like the Z86 class (Disney's Pyewacket) are similar, a bit beamier, but lighter still.

Both these boat typess have arc-like underbody sections to reduce wetted area. They are nothing like a Plank-on-Edge shape.

 

I heard that some time ago

I have never really delved into the Mini rules. I'm not a great fan of that sort of racing. When you hear about some of the better guys waking up with their prod broken from smashing into a ship I question the whole idea. It seems illegal to me.

I've said for years that anyone who enters these types of races should have to put up the insurance money or other funding to pay for any rescue mounted when they flip, dismast, sink, etc.

Crewed or shorthanded, mono or multi.

 

You need to look at the wetted aspect ratio, depth^2/total wetted area, for the available sail carrying ability. The wetted aspect ratio is the best indicator of hull L/D going to windward, and closewindedness is next to godliness for a fast craft on all points of sail.

With a big pod, the wetted aspect ratio is probably not going to be much improved over a catamaran of comparable stability. Remember, both top and bottom of a pod are wetted, while only the bottom of a hull is wetted. And you can reach down deep with a dagger board and not have any constraints on draft.

 

I did some preliminary numbers on the 100 ft by 8 ft design, and I arrived at a cantelever mast and a cat rig, no forsail, so no staying problems there.

Engineering wise, hull twist due to mast-keel moments would be large but I dare say well within todays materials to handle.

Wetted surface is only about 350 or so square ft, displacement about 8 tons, of which more than half is ballast, and sail area about 1200 ft squared.

To windward, without a canting keel, and about 15 ft deep, she will heel 40 degrees. No good since the boom end will be permanently in water, with a canting keel, no heel. This wuld allow for some wings for boom sheeting etc.

You would need to rethink some aspects of reefing.

Looks good so far if you ask me.

 

Big Boat?

I think your displacement sounds light, about what a current Open 60 weighs. Maybe you can do it, but it is such a long, thin structure I imagine you'll need to deal with a lot of bending and twisting loads. That will require structure, therefore weight, therefore sail area, therefore ballast, therfore structure...

The 1200 sq ft of sail sounds WAY low. I'm no fan of the stayless rig either.

Even with a canting keel it would heel (see Pyewacket's sea trial photos), but with so little sail maybe it wouldn't be too bad.

Are you going to model this? The hull concept (narrow 100 footer with wings) sounds a lot like something we saw back in 1988.

 

Definitely not a plank-on-edge shape.

The Big Boat, aft.

 

Yes PaulB, it will be modelled for sure. Your perceptive reply woke me up to my glaring error.

Hmm, I had forgotten about this particular bit of history. I was not thinking of such a monster though.

The above numbers apply to a 6.5 ft wide, 84 ft long design, hence the low sailarea. Waterline length about 72 ft. So 8 tons is about right.

So sorry for the confusion.

The quoted 100ft idea was to make for more accomodation. But your historical reference clinches it: 100ft is just too much.

Cannot see how to go without a simple and clean stayless rig. Anything more and we go down your structure/weight spiral for sure.

Bending strains are eased by not having a flat deck. Twisting strains need to be calculated though. Remember there are lots of cats this size, which are even thinner than this design, and are subject to much larger forces.

 

Let's see it soon

I imagine at 84 feet and 6.5 feet wide it is going to look different.

I agree there are some long skinny cat hulls out there, but they aren't housing huge rig and keel loads in the hull structures. The one that did (Team Phillips) didn't last long enough to hit the starting line of The Race. I think most of those bigger cats have lots of ring frames.

Your elimination of a headsail will reduce the longitudinal bending loads a lot. Using a cantilever mast will add a lot of other loads though, especially transversely through the deck.

The Big Boat at around 120 feet weighed something like 38 tonnes, right? It is certainly bigger than what you intend. For that weight it sure doesn't look like there is much boat in the water though.

What sort of rig dimensions are you considering? Something like an 80 foot "P" with a 30 foot boom will give you the 1200 square feet (2.67 Aspect ratio), but that sounds too short. You must be thinking higher aspect ratio main.

What about balance? Will the rig be way forward, or will the keel be way back? If the rig is forward then will the hull get real full up there, or is there enough boat out in frront that it has the required reserve bouyancy?.

Will you add a kite downwind?

At 6.5 feet Bmax will you have enough room for the canting mechanism for the keel to travel through 80 degrees or so?

So many questions on a project like this.

 

Logged out

Once again, the above post is mine.

I checked carefully, my post still said logged in user: Paul B, but the post shows up as guest. Seems to happen every time I post and take more than a couple of minutes to compose and send.

 

thanks PaulB,

I'd say reregister with a slightly different name and see what happens.

I will need to do the engineering first to make this even conceivably viable. Nevertheless:

Rig-wise, i would not go much over 80 ft, LCB is say 52.5% aft, the mast placed to suit. With such a clean rig, a little less efficiency aerodynamically can be offset.

With such a long boat you only need a little movable water ballast for and aft for trimming.

Canting keel under center cockpit. 4.5 tons is not that much to shif from side to side.

Assymetric kite - cannot see any problems - except for sail handling.

 

Re: Logged out

In your user cp, under 'edit options', check that options 2 (Automatically login when you return to the site) and 3 (Browse board with cookies) are both checked Yes.

 

Forget the wires and spreaders and the concept of a sloop rig. Go with a free-standing mast and you don't have to worry about spreaders. Use two masts or three masts. Keep the height low. Make the masts rotating for best efficiency on all points of sail.

Eric