on the design of ultra slim yachts

To answer this, I will place myself virtually in the boat. The boat will be - what we call - "cruel" - harsh sudden movements. With her long and narrow hull she might go quite fast and therefore it might be very likely that instead of going over a wave she goes right through it. This depends entirely on the power she develops.
The curve of stability will be very steep, a high initial stability that goes up to an angle of say, 40 degrees. After 50, the end of the formstability is reached.

Any movement, due to lack of horizontal surface, will be like the release of a catapult sling that is stopped halfway in it's motion.

The upwind capacities of the slender hull theory are questionable: the lack of lateral surface shall make a secondary lateral set of foils necessary.

Further, the lack of buoyant surface will cause any wave to break over her - it will thus be a very wet boat all together, the continuous angle of heel will be so steep that live aboard is almost close to impossible.
In any design, even in aerospace, the human factor will remain the weakest element nontheless the most vital.

 

Ultra Slim Yachts

Some historical thoughts -

Van de Stadt's Black Soo 29ft x 6.6ft might be outside the ultra slim category but as an early ULDB design has demonstrated the advantages of long, thin and light. I believe at least one Black Soo is still racing competitively on the West Coast (USA).

Back in the 1960s Arthur Holt (East Coast UK)was building some super slim (perhaps not ultra) ULD racing yachts. Slithy Tove, 48ft x 6ft, entered twice in the Twin Handed Round Britain and Ireland (1970 and 1974). In 1970 she finished 2nd and 5th on the first two legs - demonstrating that under the right conditions the concept worked OK. Retired with damange in 1974.

I have recently sailed on a later Holt design Hanser - 36ft x 7ft ULD (two men could pick the hull up and turn it over when it came of the mold). Like Slithy Tove she has a relatively high aspect ratio fin keel. However Hanser's keel retracts to reduce draft down to a couple of feet. The linear accommodation works fine and the sailing performance is spectacular. Best of all she is easy to handle in all conditions - super light boats will always have a quick motion - but give me long thin boats over short fat ones for sailing off-shore anyday.

Cheers

 

This is the thread of Dionysis, however I cannot resist to reply to you Paul:

First of all an ULD and an ULDB are not per definition ultra slender hull designs; - this thesis of Dionysis is rather extreme not to say ultra-extreme.

Looking for "Slender Hull Theory of Sailing Craft" ( I did not found the item) I found a paper of Meade A. Gougeon "The Use of a Wood-Resin Composite for Marine Construction" and lots of other papers from Olin Stephens II regarding the 5.5 metre Antiopeia - going through all this I see that yachtdesign has indeed dramatically changed.

If we could see the slender hulls like the one Dionysis advocates, why should we bother about trimarans and catamarans?

I would suggest, in order to come to a serious approach give the L, B and D - the building material - the required freeboard - the required SA and the required V.

 

I would tend to agree xarax: there is a chance the boat could pitch and hobbyhorse, and your idea of a winged rudder makes a lot of sense. I just felt that with such a long waterplane, quite large overhangs, and a lot of flare, that this tendency would be minimised.

If the pitching is severe a winged rudder will have to be quite large to be effective.

 

D'Artois - LOA 84ft, LWL 68ft, BOA 6.5ft, BWL 5.5ft, SA 1,100ft^2, CANOE DRAFT 1.75ft, DRAFT 17ft, DISPLACEMENT 18,000lb, BALLAST BULB 9,000lb - selfstanding catrigged - 11knots upwind @ 30 degrees heel, 20knots offwind, strip plank/coldmolded

 

Deep torpedos vs wide, sloop rigged surfboards.

Hi dionysis:

They also need heavy keels to work. And often have large degrees of heel to get useful sail carrying ability. This is because, with a narrow hull, the Center of Bouyancy shifts very little to leeward as she heels over. So now almost all of your righting moment is coming from the Center of Gravity of her keel, which is shifting to windward, but is counter ballanced to a large degree by the Center of Gravity of the rig which is shifting to leeward. So now all you get to count is the cg of your keel minus the cg of your rig which can be a distressingly small number.

With a wider boat, the cg of everything but the keel shifts to leeward, just as with the narrow boat. But The cb (Center of Boyancy) also shifts to leeward. And usually at a much faster rate at normal sailing angles. This creates a spread between the boat's entire cg and cb, so you get to count the boats entire weight times this spread.

Indeed, narrow, deep, monos were tried in the old BOC (now called Around Alone) races. They worked quite well (had good ultimate stability too). That was until they were countered by much wider boats. With the same draft. These were much, much, much faster. (and much more stable upside down.)

Now this type of race (which is usually mostly down wind) is dominated by what look like large, wide, sloop rigged surfboards.

Bob

 

I suppose it is a matter of balance sharpii2. I would not be surprised that wider boats at more competitive - beam contributes more to usable stability than does depth of keel, no question. That does not mean that a long slim yacht isn't a viable sailing boat - just not a viable racer.

The long slim boat can carry only half the over 2,000 square feet of sail of an open 60 boat, yet it's speed is way over half the corresponding speed of the open60. That is efficiency.

 

Hi again

I agree entirely. In fact, most of my design concepts are narrower than usual. This is because I think the benefits of narrow beam may way outweigh the costs. I can imagine a thirty footer with a draft equal to its five foot beam (a square boat?). It would be able to punch through waves with relish. And it would be very unstable upside down. Not bad characteristics for a blue water boat. The trick would be getting the freeboard right. Too much and she will be unneccesarily tippy. Too little, and she would be a half tide rock. Once gotten right, I believe such a boat would be more seaworthy than its skipper rather than, in the case of the 'Open 60's', the other way around

It would be instructive and interesting to go narrower and narrower while keeping the same draft and displacement. Would there be a steady drop off in performance or would there be a notch somwhere?

In the late nineteenth century, the Brittish made a class of boats that were known as 'plank on edge' cutters. Some of them had as much as an 8 to1 length/beam ratio. They were usually deep sectioned and had similar displacement to boats of more moderate proportions of the same length. The draft could be double the beam or more and the freeboard was often inadequate. These boats could become submarines in rough conditons and were often counted as 'inland racers' for that reason.

At about the same time, on the other side of the Atlanic, Americans were making extremely wide shallow hulls that were known to their critics as 'skimming dishes'. They also often had inadequate freeboard and were known to capsize with little warning in squalls that were not much more than strong puffs.

One day, someone got the brilliant idea of racing these two types against one another. Two boats were selected that were about the same length and displacement and raced against one another. Each one's proponents predicted easy victory. Thus was born what was known as 'the Mage races' after, I believe, the narrow boat. The wide boat, I think, was called Schemer. As it turned out, the results surprized everybody.

On a triangular course, neither boat had a significant advantage.

Bob

 

hi bob,

I always strive to design a boat to be able to fend for itself in difficult conditions. One of the nice things about narrow boats, apart from the fact that they are eminently self-righting, is that their stability increases in direct proportion to the angle of heel. If in the extreme case their stability is predominantly due to ballast low down, then they just cannot be knocked down due to gusts. Lying beam-to in waves would be safe and confortable. The only danger would be pitchpoling while racing down a waveface.

Sure would - the narrower you got the deeper you have to go to keep the righting moment up.

The plank-on-edge boats, despite their bad press were fast boats, but they were wet, and dangerous. The marked depth of section of these boats was the outcome of having a full length keel. That was their failing.

You can design narrowness but without the depth of section of the plank-on-edge boats. This minimises the displacement - since you can hang the ballast down low on a foil, and hence make it far more effective.

Interesting about the "Madge" races eh! I did not know about them. The wetted surface of the pank-on-edge boats would, I dare say, have been a lot lower than their rivals, but their wave resistance would have been huge.

cheers

 

Old mades in different dresses.

Hi again.

I like talking to you because you're bold. In the above quote, I think you got it backwards. Actually the wetted surface area on the 'Plank-on Edge would be the greatest. That is because they are deeper than they are wide. You take the underwater surface area of the profile and you double it. Now. Take that same hull, lay it on its side, then step a mast on the other side. Now you have half the wetted area plus the keel which will be a lot less surface area than both sides the underwater structure supporting the keel of the plank on edge.

On the other hand, with the wider boat, all that beam has to be driven through any sea way. Hence mondo wave making.

I agree, somewhat, with another post. That you, by trying this will, but to a much lesser extent, be recreating some of the old problems of the plank on edge. The problem is the long keel fin. It will have considerable wetted area.
And it will also have bouyancy. To over come this, you will need a heavier keel bulb (to overcome the extra bouyancy) and more sail area (to overcome the added wetted area). In blowing conditions (where wave making, not wetted area, is king) and a smaller rig is needed for a given speed, this won't hurt you as much. But still, it will add up and inevitably take its toll.

Now. Imagine making the keel fin twice as long, and the bulb half as heavy and extending it not down but sideways above the waterline. The 'fin' now needs only to be dragged through the air. It now has no bouyancy subtracting from the bulb's effectiveness. In fact, its own weight is adding to the bulbs effectiveness. The only problem we have now is to devise a way to hold the bulb up when the wind pressure is insufficient. To do this we can take some of its weight allowance and build a float around it. In fact, we can make the float bouyant enough so it can hold the bulb up even with wind on the wrong side.

We now have a single outrigger! And as long as we don't get too greedy with the sail area, she will be reasonably safe. And, I might say, considerably cheaper than her deep draft conterpart. She'll go faster too.

I don't mean to say this to denigrate your concept, but only to point out that:

1.) Extreme measures rarely pay as advertised,
2.) They are almost never cheap, and
3.) Multihulls own the speed franchise. Imitaters beware.

The concept of high draft to beam ratios has its own vertues. But speed is not one of them. Safety and handling definately are. Making a supper narrow boat to compete with either multis or deep keeled skimming dishes for speed is, in my opinion, doomed to failure.

So here's my advice:

PROVE ME WRONG.

Bob

 

According to Russell Clark in Wooden Boat way back in '81, Madge met the New York "skimming dishes" and gave them a "square defeat". She also took on the Boston sloop Shadow (a "compromise sloop" halfway between the plank on edge and the skimming dish) and only lost due to an accident.

Re " The long slim boat can carry only half the over 2,000 square feet of sail of an open 60 boat, yet it's speed is way over half the corresponding speed of the open60. That is efficiency."

In a way it is, but if you're only half as fast as an Open 60 you'll be beaten across the line by old IOR half tonners which have much less than 1000 ft2 of sail.

I know what you mean about efficiency, but isn't it a bit like saying a Laser 4.7 is more efficient than a foiler Moth because the Radial goes about 75% of the speed with 59% of the rig?

 

Bob,

I may be wrong, but I don't think so - I presumed that the draft of the a plank-on-edge and it's wider competitors would be about the same, together wiith LWL - given this, then the girth measurement of the wider boats would be quite a bit larger = more wetted surface.

Fantastic eh! A Proa, or do it on both sides and you got a tri. As far as the proa is concerned - you have the problem of switching ends, so you have to reverse the sailplan, and the rudder/centerboards - very efficient, safer (unsinkable), cheaper - but oh how much more complicated. By the way this thin idea came out of a design for a proa. I still have a soft spot for them.

1.) There is nothing particularly extreme about this design. The hull is just one float of a high performance catamaran. The keel is just the same as a open60's keel.

With respect to monhulls:

When you put these two together, what you lose is stability, you gain by less wetted surface, and most importantly less parasitic drag. You would be surprised at the drag produced by wide hulls, standing rigging and flat decks etc. Wheather this hydro and aero drag would offset the loss in stability - I doubt it, but see points 2.) and 3.) below.

With respect to multihulls:

dito as with monos, ~ half the wetted surface, and ~ 0.25 the parasitic drag. Here though, the stability advantage is just enormous. I don't think I would be competitive, but I think more efficient.

2.) I bet to differ, easily 0.3333 times the expense of a multi and half the expense of a mono

Vis:

open 60 - 60 ft * 20 ft * 0.65 * 2 = hull and deck surface of open 60 = 1,560 ft ^2, then + swing keel + standing rigging + winches galore ++, kms of running rigging.

"extreme design" - 1,200 ft ^2 hull and deck surface (calculated), self standing mast ~ half price, drop keel ~ half price, one winch, very little running rigging, no standing rigging.

3.) Yep. But this design will give them a run for their money in some conditions.

Is this enough proof Bob?

 

Hi CT 249,

Yes. What's wrong with that? Efficiency is important, when you look at a boat overall.

An extreme example:

I think it was a couple of Observer TransAtlantic races where one competitor (Vendredi, Alain Colas from memory) was over 200 ft long, (could you imagine the machinery needed to control such a boat) going up against 50, 60 and 70 monos and multis. Lets say, it had won ( which it did not by the way,) by a slim margin over a seventy foot tri. Which boat is the more efficient?

I am trying to make the idea of efficiency as objective as I can. It has to do with for example:

1. thrust/resistance ratio
2. material expenditure/speed ratio

Then all the factors that distiguish a good from a bad designed boat.

I take your point though - the first(faster) boat to the finish gets the glory.

The ocean is wide enough to accept all comers.

I like the idea of designing boats that can do what others can do but in a simpler more elegant way. Designers left to their own devices, I expect would do the same. I suppose it's just a matter of taste.

cheers

 

Sorry, I explained it poorly. The point is that almost any time you reduce sail on a boat, you will improve its ratio of sail area to speed. Therefore, the simple ratio of SA/Speed is probably (well, almost certainly) not a true measure of efficiency (AFAIK).

A Macgregor 26 is mebbe 30% or so of the speed of a 60' ORMA trimaran, yet it has less than 30% of the sail of an ORMA 60. But surely you'll agree that the Mac is less efficient than the tri.

Your boat would also probably be less efficient by your speed for dollar criteria than something like my 1968 timber half tonner, because you'll always win that contest by going simpler and smaller won't you? It's just the physics of yacht sailing; the speed of big boats and big sails does not increase with their cost and size.

You could also say that if you have two boats designed to win race, the boat that costs $100,000 and wins a race is more efficient than one that cost $1000 and finishes second, 1 second behind. The expensive boat fulfilled its design criteria (winning) the other one didn't. While I prefer boat #2, aren't there many different ways of looking at efficiency?

The boats you are thinking of are (1972) Vendredi Trieze (Jean Yves Terlain skipper, about 120' long, three headsails and no mains, Carter design. Second place behind Alain Colas' 70' ish tri Manureva (Tabarly's old boat that had retired in '68).

In '76 the monster was Colas' Club Med, 236' long four-masted schooner. She was second behind Tabarly in his 73' IOR ketch.

Re planks on edge wsa. I'm looking at the plans of Madge as I type and I've measured the midships skin girth very roughly. Madge has the same skin girth as the later "compromise" boats like Burgess' Nymph to within maybe 7%; I'm just measuring with a bit of paper. Nymph was deeper than the earlier skimming dishes but still 2' shallower than Madge (which was 8' deep and 7'9" wide!). My measurement are rough and I don't know if I have plans of an equivalent skimming dish but there maybe isn't a huge difference in wsa in those old boats.

PS do you reckon a seaworthy unstayed mast is really half the price of a normal rig? I suppose you're looking at a much smaller section. Is a drop keel half price? They are normally more expensive AFAIK.

 

CT 249,

While reefing will improve SA/Speed, this is relative to the same boat under different conditions and not relative to another boat, and has to do with the reduction in heeling arm.

So I still think that thrust/resistance is at least one objective measure of efficiency that is worth considering. Perhaps not the only one, and in the final analysis maybe only a component of a more complete measure.

As far as the Mac is concerned. While the Mac may sail at half the speed of an ORMA tri on it's less than half the SA, it is the second half of the speed difference that is the most telling in terms of efficincy at least for the tri; since the tri needs to overcome a substantial increase in resistance as it speeds up, which it does with ease because of let's say for argument's sake, it's lower displacement to length ratio, and lower parasitic drag and longer LWL.

So I agree, things are not so straightforward at first thought. Different boats are at their most efficient at different speeds. I am just exploring whether these "design efficiencies" are comparable.

If we compare say two boats, each at their best design efficiency, which is more efficient? Perhaps you are right. But what about the little america's cup cats, compared to the ORMA tri's?

Fair point. But for argument's sake. The long thin mono weighs more than twice that of a half tonner. That has to be taken into account.

Yes there are, but that does not discount these different "qualities" of efficiency as valuable measures. After all, we do this in our heads all the time when we compare boats, and also what we mean when we talk about optimising a design. Let's become a lot more explicite about it, that's all.

Yes it was Club Med that I was thinking about. Thanks for that. Club Med's SA/Disp, and SA/WSA was way low, to allow Alain Colas to manouvre it safely. This obviously limited the efficiency of the boat among a host of other limitations.

Wetted Surface Area: There goes me shooting my mouth off about it being "quite a bit larger". As I said (given draft and planform are the same) the girth measurement of a plank-on-edge boat should be smaller than a wider more conventional hull. It still makes sense to me, but I stand corrected.

Re keels: I was comparing ballpark prices with open 60s, which have canting keels. Yep, may not be cheaper than bolted on keels say, but certainly cheaper than canting keels.

Re: unstayed masts. I think they are beautiful. You can taper them in section to your heart's content, you can taper the wall thickness, they are better aerodynamically, structurally more efficient etc. etc. No standing rigging, no chainplates, no backstay adjusters etc. etc.

Pricewise - to buy - they are expensive - to homemake - I think they can be cheaper. But that all depends on the boat design itself.

An unstayed mast in a boat with a large righting moment leads to larger section inertias. You may be able to get away with a larger but thinner section but there is a safety limit. If you can minimise your righting moment by making your monohull as efficient as possible, and taking into account the reduced parasitic drag, and the added efficiency of the cleaner sails etc. say, then you can minimise section inertia = layers of carbon and glass, and build a mast comparabe and arguably cheaper to build and cheaper in the long run.