on the design of ultra slim yachts

Interesting discussion and concept.

When I said "reduce sail on a boat" I shuold have made it obvious that I didn't mean reefing, but reducing the amount of sail in the design. I wrote it poorly.

I think thrust/resistance IS a good measure, but is it a linear relationship? Everything I see seems to indicate that increasing almost any dimension has a diminishing return and therefore that has to be factored into the ratios.

It's funny, we in OZ and the guys in the UK are so deeply ingrained to think of speed for overall length as being the true measure of performance. Like you, I like the European idea of using sail area. It tends to produce boats that are extremely sensitive but handle very well to my taste (like the International Canoe) and the old German sail area class dinghies were damn fine boats and much, much faster than anything the skiffs could create until the mid '50s or so.

I don't think you were shooting off at the mouth re wsa, I'd have thought off the top of my head that the plank on edge had less wsa, too. As I mentioned, I was looking at a compromise cutter not a skimming dish, very possibly the true dish would have had a higher wsa as they had wineglass type sections in some areas.

Cat rigs and unstayed masts are nice. Sometimes sail trim can be complicated, because of the lack of controls meaning that you haven't got direct control over shape in some areas, but that actually is interesting.

 

I agree, I don't think thrust resistance is linear. At least not in displacement mode. Friction will scale as the square, but wave resistance has got to scale as the cube or even higher. Then you have to start talking sail list to drag...

There is a limit to increasing one dimension over others. Some go for power = beam. The open 60's have an upwing SA/Disp ratio in the 40s, and SA/WSA ~ 8 or more. That is Power, so they are pushing the right dimension, and seem to be close to diminishing returns. I agree: you can take things too far in the end.

Length pays big time when disp/length goes down. On boats you can hike out on you can go to a beam to length ratio of 13 or so, then it is diminishing returns (all other things being equal) after that. Vis. this thread.

I'm with Bethwaite when it comes to sail power. I like the idea of the sailrig adjusting automatically to gusts. A wishbone boom would go a long way to getting that adjustment back.

 

I love wishbones. My brother's got one on his 38' cruising cat, and it's amazingly easy to use. It's only got a 4:1 tackle and you can just reach up lazily and pull in armfulls of sheet, on other similar boats you'd need to mess around with a winch. Great for lazyjacks, altogether superb.

The inter-relation of twist and outhaul on a wishbone can be a slight problem with some arrangements, when it comes to tweaking.

I'm not much into automatic gust response. My slalom sails (and the Mistral rig when downhauled to the max) are pretty automatic and I find it frankly a bit boring. I LIKE reacting to the gusts myself, it's part of sailing. It's a very personal thing though, and in a fast boat like a skiff it's different.

Sorry, when I was talking linear I was talking dimensions not forces. I've got the 'flu wjhich is why i'm not sailing and it's affecting my thinking.

 

I was thinking about automatic gust response on a big yacht; one that is not so easy to adjust without using the rudder. But I will keep your comments in mind. Getting just the rifht amount of automatic response will be a challenge though.

Get well

 

Years of experience...

It seems rowing have been building your "dreamboat" for years - relatively speaking that is. Can't something be learned from that? To illustrate proportions I grabbed this picture of the web: http://rowingvictoria.asn.au/files/BWSNBEYEIV%5CThe%20Melbourne%20Head%202004%20018.jpg

My question as I read the thread for the first time tonight was; why are you chasing the last 5% in hull design efficiency while leaving 40% on the table for the rig/sail and 20% on the table for ballast/keel?

It reminded of an article I saw years ago about the crab-claw sail and other difficult to implement shapes that are far superior to the marconi in efficiency. I was also thinking about the current "trend" among Panama class freighters to break up from fatique.

The fastest fish is said to be the Sailfish and it "hull" proportions are extremely clumsy by the standards discussed here. Of course it is using a different propulsion system, but it still has sails although not of the Marconi type.

With all due respect for the academic persuit of the perfect hull I would put my money on underwater appendages and "sail" propulsion with all the cool materials that are available today.

 

I forgot the sailfish link

http://www.amonline.net.au/fishes/faq/fastest.htm

 

Hi asathor,

Actually I am not chasing the last 5% - I have found that - vis the rowing scull. It's the 40% and the 20% that are the most difficult to get right. So I agree with you you have to concentrate on sails and keel to make it viable. That is where my money is as well.

I just thought to go for as deep a keel as can be engineered reliably, and for the cleanest rig possible. If with today's technology it is not possible - so be it. It still think it woth pursuing thought. Whether it is competitive against the wide boats is another matter - probably not.

As far as structural considerations are concerned, there simply is not the weight in the hull, keel and payload, to make fatigue an issue, if the hull is engineered well.

Those rowing sculls have a far greater length to beam ratio that my 13:1.

 

Proa or half try?

Hi dionysis.

It looks like I got a few things wrong. The Madge races and maybe the wave making of the two competing hull types. From the pictures I remember, the plank on edge was considerably deeper in draft than the skimming dish. But thats all just memory. And how reliable is that?

As for the proa. What I suggested may look like a proa but is not. A single out rigger differs in that she sometimes has the float on the windward side and sometimes has it on the leeward side. Thus the bow stays the bow and the stern stays the sern. She cannot carry nearly as much sail area as a tri (which is easily the all weather speed champ but carrying capacity chump) I guestimate it will safely carry a SD of no more than 15 and one probably closer to 12.

My original concept was for a cheap multi. 'Performance' be damned. Its main hull was 4 x 20ft, its outrigger would be 2 x 20ft, and its over all beam would be half its length + half the beam of each hull, or 13ft total. Its sail would be an eqalateral boomed lateen sail of about 120 to 150sft. Its loaded wt would be about 2200lbs and the outrig float would be about 600lbs. And it would have no wing deck. It would just have a wide single connecting beam with drag wires to add reenforcement to the hull alignment. Everything would be worked from below from a series of strategicly placed hatches.

I have a soft spot for multis when they were backyard affairs made out of ordinary matteriels by ordinary people, not corporate syndicates. I congradulate Ellen for her record, but I really don't identify with her much. She is an elite, and I a mere mortal.

Good luck with your project. I hope it is someday built. How about a 4 x 52ft version?

Bob

 

Me too. Your idea of switching sides wtth the outrigger is great and I still think about this option. Beats the pants out of having to carry 4 tons of lead at the bottom of the keel. But the stresses in the cross member would be very large - and you would have to go ultra hightech or big, heavy and complicated, and I don't like the idea of full length floats either. Perhaps they could be foil borne. Anyway, thanks for the encouragement.

 

Oh yea, 52 by 4 is an option, but then you have to bend over everywhere you go inside. I like standing headroom.

 

Rack and Pinion - With a Twist

I really like the idea of tying the Mast and Keel (ballast part) together even if the syncronized raising and lowering would be a joke unless they moved in opposites direction (yes, it was a first time).

Look at your watch :idea: Imagine the hands extended to see how easy it would be to balance the opposing forces if they originate in the same point.

And directional keel appendages and rudder(s) are free to be placed where you need them if you keep the ballast pod simple (low drag).

Now you have front and rear rudder , Moving ballast.

It's a Bullet Train, a Rocket, a hot dog without the calories?

Sorry my train of thought derailed, so I am going to go find some breakfast.

 

Catamaran Hulls

You guys sound like you know what you are talking about.Your thoughts on cylindrical hulls,with a cylyndrical bow made from 90 degreebend of cylinder of same diameter cut flush with the top side of the cylinder it is atatched to
It would create a bow that couldnt go under as the curve would constantly make it surface the more it sumberged..do ya think???

 

Is this thread still alive? Is anyone exploring the idea still? Made any progress on design?

I have been doing a lot of evaluation of slender hulls with the idea of building a fast coastal cruising yacht that is easily handled by one person. It would have accommodation for 2 and room for 4 for day sailing.

I want something that is trailerable but I do not mind if it requires a few hours to prepare.

I have been working on something in the range 15 to 20m WL and 1.2 to 1.5m BWL. This sort of boat would get 10kts with a tiny rig or even solar power as back up.

I am working on a design with a central accommodation section about 20ft long and the ends would be removable for trailering. Would also make it possible to build in a garage type space. A bit like the way they build large ships in pieces these days. This is not a lot different to collapsable cats and tris. Each hull section would be buoyant in its own right.

Rick W.

 

The central accommodation section can be beamier above the waterline, longer than 20ft, say30 ft, and curry all the essential functional parts of a sailboat (anchor and rudder included). The other two parts don t really need a standing headroom, as they could only house marine beds, ( but they need to be self righting too, if you don t want to have heavy and expensive collapse mechanisms and just be able to fix the three pieces into one by towing them one behind the other in the water).
It sounds like a nice interesting idea to me...

 

xarax
I appreciate you putting me onto this thread I will see if there are other replies.
Rick W.