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Old 28-06-2008, 07:34   #1
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Vessel Substructure to Support Rigging Loads

The Boat ‘Is’ the Structure

Who would have ever dreamed that a lake sailing, ultra light-weight, racing catamaran would encounter, and be capable of sustaining rigging loads comparable to those of an America’s Cup boat??

From a recent article in Seahorse magazine, “Alinghi, Birth of a Crazy Boat”, this paragraph emerges, “The boat ‘is’ the structure. These boats are now so complex, and the loads so high, that structural aspects take on a particular high importance: imagine that on a 1.2 ton boat you can reach 23.7 tons of mast compression…levels seen on an IACC design weighing over 24 tons! Also one cable in the substructure is sized at 56 tons…it is amazing that such light boats can produce such enormous loads.” (let alone absorb them, BE)
Alinghi Catamaran
….attached PDF

The point is ‘the boat is, or should be, designed as a structure’ to carry the rigging loads. In far too many cases with production vessels and custom vessels this is not the case. Most are concerned with a maximization of the interior layout, and bulkhead placements are supplemental to this interior layout configuration. Yes, care is taken with the placement of the mast and the need to support the compression loads of the mast, but seldom are the shrouds and stays tied to a 'designed substructure’ of the vessel…they are simply and traditionally attached to the outer hull skins and ends of the vessel’s shell.

This ‘flaw’ can be even more of a problem with a multihull vessel. The catamaran platform lacks the ‘backbone’ structure of the traditional keeled monohull, or central hull structure of the trimaran. Yet seldom is there any serious thought given to some sort of supplemental frame structure in these vessels. No wonder we see so much headstay sag. No wonder these boats can not carry a nice masthead genoa. Added to this equation, the shrouds are attached at a ‘shallow angle’ (half that of the forestay), so their aft-pulling capability is limited by both this shallow angle and the 3 point (tri-angled) configuration. Then at their point of attachment to hull sides (skins) there is likely no firm bulkhead backing, just skin. The shell of the hull is being asked to absorb the load, which it does in a ‘forgiving manner’ resulting in more forestay sag. It’s no wonder these vessels can’t carry a decent size headsail.

Now some would say this is not important, just utilize a fractional rig and smaller headsails. Rather interestingly this same Seahorse article discusses the preferred selection of multiple headsails and overlapping foresails for this optimized race boat. Naturally I found this observation rather interesting due to my preference for headsails, but I won’t rehash those arguments here, just reference a few subject threads I’ve contributed to these discussions. **see below

From another portion of the Seahorse article, “Alinhgi, Structural design:
The below-deck cable and beam truss structure is clearly visible in these two pictures, tying in the torsional loads that cause conventional cats to twist under sailing loads. The first boat sported a full carbon truss/tie rod structure on the centerline, looking somewhat like one side of a mast. This takes the place of a trimaran’s main hull, to carry the very high fore-and-aft loads generated by the forestay, mast and mainsheet.”

I have suggested such a fore-to-aft truss member be incorporated in the central wave splitting nacelle structure on my cruising designs:
A Longitudinal Beam & Headsails/Headstay Load
…excerpt..“Imagine a flat, on edge, mounted down the centerline on the underside of the bridge-deck. This flat plate will act as a rib to strengthen the fore-to-aft rigidity of the vessel. If a tow-bundle of hi-tech fiber was laid along the bottom edge of this flat plate, the rigidity could be even greater, akin to a bottom truss structure”…(or one side of a mast).

Back to the Sea Horse article,"On Alinghi the concept was taken an important step further. The aft portion of the central longitudinal truss member was divided into two separate members that ‘Y’ out to the stern beam. This completely solves the Achilles’ heel of catamarans, which is torsional deformation of the platform. Alinghi’s windward hull trims down by less than one-degree compared to the leeward hull at maximum load.
Finite element analysis and a specially written truss-analysis program were used to calculate the loads on every structural member and optimize their stretch behavior under all conceivable loading conditions as some of these loads reach 40 tons."

This torsional deformation subject matter is certainly an important one, not only in race boats, but cruising ones as well. It can prevent us from carrying our sailing rig in a most efficient manner, and it can rack a boat to a slow death.

The big Polish RACE catamaran "Warta Polpharma" added a cross X bracing of hi-modulus cable between the inner corners of its fore-beam and aft-beam in an attempt to stiffen itself in torsion and racking. Team Adventure was also contemplating the same arrangement (in fact I believe they did such an installation right after the finish of the RACE in preparation for a trans-Atlantic speed run). Of course these are non-bridgedeck cats.

Solid bridgedeck designs that we find with cruising cats are advantaged over the trampoline cats with respect to this cross-brace stiffening. But particular attention must be paid to the ‘flat-plate nature’ of most bridgedecks, and their construction composition, and their positive attachment to the main crossbeams in order that they are utilized most effectively.

Many cats are seen to have basically a flat bridgedeck structure with some attached fore-to-aft stiffening beam/ribs on the bottom side. These attached beams/ribs do act to cut down the unsupported panel size and give a stiffer walking deck to the saloon, but they only act in a small manner to prevent the bridgedeck from bending fore-to-aft. And they don’t contribute to the athwartships or diagonal bending problems at all. If we give a little camber shape and/or add a corrugated channel shape to the flat panel deck we improve things a little bit further.

Athwartships bending is a most serious concern as our rig’s shrouds are always acting to bend our vessel up in half around the mastbase pushing down. In open-deck cats the stiff crossbeams along with their dolphin strikers and gull strikers resist these bending loads. In our cruising cats it’s the main bulkheads we rely on to do this job. But so often we see vessels with less than desirable bulkhead arrangements. I believe it is very important that their be a minimum of two major bulkheads, one fore, one aft, and that akin to the open-deck boats these two bulkheads must be continuous across the whole beam of the vessel. And these bulkheads need some ‘beef’ rather than just be made of some ‘flat sheet’ of ¾ or 1 inch plywood or the sort. What’s wrong with a good stiff 2 - 5 inch thick panel of hi-tech sandwich cored material. Now you have a ‘bulk’head! Make sure you have a good bond between this super bulkhead and your bridgedeck and you are on your way to a stiff boat. Throw in a couple of ‘diagonals’ of hi-tech yarn (possible buried within the other structures, skins, and panels of the boat) to triangulate things.

Now you’ve got a stiffer boat and some good substructure to mount your sailing rig to.

In the particular case of my rather unusual mast-aft design, I tie the shrouds directly onto the same ‘super’ bulkhead that my mast is mounted on. And one of my backstays (the mizzen forestay) is tied directly to the same super bulkhead right at the base of the mast. Its upward force is countered by the downward force of the mast. The other two backstays are tied to another bulkhead at the stern, while the forestays are tied to the ‘flat plate’ rib structure down the centerline of the boat.

Sail Aerodynamics
Mastheaded Vs Fractional rig
Mast-Aft Sailing Rig
Cutter Rig to Windward
Attached FilesSeahorse_Aug2000.pdf (927.4 KB, 137 views)

...tried attaching the Seahorse article in PDF form but it was larger file size than permitted on this form, so you will have to go here
Boat Design Forums - View Single Post - Sail Loading on Rig, Rig Loading on Vessel

Brian Eiland
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Old 28-06-2008, 14:02   #2
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That was something I liked about Bob Oram's designs. The composite chainplates are built into dedicated bulkheads, not just glassed onto the hull skins. Also the bulkheads themselves are tilted forwards so they are in a straight line with the shrouds, so there is no "bending" force on the bulkheads, just pure tension. The same applies to the forebeam bulkheads.

Another interesting approach is an Australian built boat called "Cut Snake", which has a mast beam which is "V" shaped when viewed from above, with the mast at the apex of the "V", and the composite chainplates at it's ends, so the mast compression and chainplate loads are all contained in one structure.

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Old 28-06-2008, 22:03   #3
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V shaped structure like this?

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Old 30-06-2008, 17:14   #4
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That's the idea
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Old 30-06-2008, 17:36   #5
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Thank you Mr. Eiland. This was my principal objection to a mast aft configuration. The engineering challenge to craft a vessel that can carry these enormous loads will be terribly expensive, and will require even more exotic materials and processes to construct. Will we ever know how much was spent designing this Alinghi?
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Old 01-07-2008, 00:54   #6
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Brian, you seem to think huge loads is a good and desirable thing. You also seem to think a boat that needs excess engineering to eliminate the torque is a good thing.

I would suggest that instead of getting more and more complex, resulting in higher and higher loads ands ever more ingenious ways to absorb those loads, it makes more sense, particularly for a cruising boat, to reverse the spiral and make the boats simpler and the loads lower.

Easiest way to do this is a proa (as shown by a cruising 15m/50' harryproa weighing half as much as most cruising 12m/40' cats), but even on a cat it is not rocket science to put the rigs in the hulls rather than suspended between the hulls and to use rigs that don't impart collossal loads into the entire structure, and if they do, make them as localised as possible and resist them with tension which is easy, rather than compression and bending which are complex, heavy and difficult.

Would Alinghi be as fast if it was a proa with an unstayed rig? No, given the state of art of proas compared to cats. But a conservative estimate of the cost to design and build Alinghi would not be far short of a million bucks, plus a couple of prototypes, and if this was spent on a proa, then I am pretty certain it would be lighter, longer, more stable, carry more sail, have less windage and be faster


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Old 01-07-2008, 04:45   #7
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That is all fine and dandy but the market place will not support hi tech solutions to build a rigid boat. Boats flex, and people live with that, it is simple economics. If Alinghi cost $20 million and breaks half way through the series who cares? The average joe is more concerned that his Lagoon 41 can be bought for a reasonable sum and will not break period. (ie strucural failure). We jack the rig to 30k and my wife leaves when I do it. She does not like to watch the furniture moving.

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