# Multihull Structure Thoughts

Discussion in 'Multihulls' started by oldmulti, May 27, 2019.

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### revintageSenior Member

Hi oldmulti,
Would you mind sharing the spreadsheet for calculating max wind from p.46? I have made an Excel spreadsheet for calculating beachcat design wind and want to compare. About F18/Tiger I get 16.7 knots, so the numbers seem to compare well.

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### ClarkeySenior Member

I can't thank oldmulti enough for running through those numbers - things are clearer now. I wonder where a Tiki 21 would sit relative to the other boats but don't want to hijack the thread and cause anyone more work.

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### oldmultiSenior Member

Clarkey. The Tiki 21 has a bruce number of 1.19 with a wind speed of 22.6 knots before reefing and a maximum of 7.4 knots average over 24 hours. Cooking Fat a Tiki 21 has circumnavigated. Please understand the numbers. A Hobie Tiger will be sailed at its limit of windspeed stability (the 16 knot figure). A Tiki 21, Kliss, Sardine Run etc will only sail at about 80% maximum of windspeed stability unless you are racing with support boats around.

Calculations about Bruce numbers and 24 hour averages make some very basic assumptions about a standard model of efficiency of hull shapes and rig effectiveness, which are not always the same in different designs.

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### oldmultiSenior Member

Revintage. Sorry about this but I will give you the mathematical logic for the calculations and you can cross check against the excel you have.

Obtain the stability of the boat (I am using foot lbs in calculations). Generally half the hull centre lines beam multiplied by the displacement including crew an realistic payload.

Find the height of the sail centre of effort for the “standard” rig. Generally the mainsail and 100% of the fore triangle.

Now divide the height of the sail centre of effort into the stability of the boat to get the lbs of pressure on a square foot of sail area.

Now divide the lbs of pressure on a square foot of sail by 0.004 to give X

Now get the square root of X and divide it by 1.15 to convert it to knots of wind speed.

EG Tiki 21 Stability 7956 foot lbs. divide by height of centre of effort of sail 14 foot = 2.71 lbs per square foot. Divide 2.71 lbs per square foot by 0.004 = 676.57. Get the square root of 676.57 = 26.013 Now divide by 1.15 to get 22.62 knots.

Sorry I am only partially metric and not fully up with newton meters etc.

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### revintageSenior Member

Will add my (metric) spreadsheet when I am back in town tomorrow. Might convert it to imperial too.

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### oldmultiSenior Member

The following is about a capsize of a trimaran. The first part is almost a direct quote. The full text is located at Surviving Flicka’s Capsize https://www.epoxyworks.com/index.php/surviving-flickas-capsize/

Flicka is a self designed and built 31 foot trimaran designed by Jan Gougeon, for the OSTAR and other racer cruiser functions. The tri is conservative in design with reasonable accommodation. The boat was doing its 500 mile qualifying run for the OSTAR in the Atlantic in the Gulf Stream area. The following is the start of the article.

“Jan Gougeon: The wind is shifting around to the Northeast so I figure if I can make good enough time I can get across the Gulf Stream before the Northeast wind. Absolutely, if it is blowing strong out of the NE you just don’t go across the Gulf Stream. It’s that bad.

So, Wednesday morning after I got my sun sights… going there was really a hassle but on the way back, man, I had the sun sights right. I was really gung-ho on the navigation thing. I was getting close to the Gulf Stream and it was getting pretty late and the wind was picking up strength. I decided I wouldn’t sail across and instead I would hove to.

I was hove to and playing around with letting the boat lay beam to the sea. That was absolutely treacherous. A trimaran, one thing it can’t do is lay beam to the sea in any condition. It absolutely can’t do that.

Jan: OK, here’s what the deal is when you lay a beam to the sea: as the wave comes it picks up the center hull and the outrigger. There is no weight on the outrigger but you are heeling 50 degrees instantly. That’s the flow of the trimaran–you can’t be beam-to-sea, you’ve got to be nose-to.

Right away I said, Huh, I can’t be beam-to cause I’m going to tip over instantly. So I put the main[sail] up and put two reefs in the main and took all the headsail off and the two reefs in the main. I tied the tiller over.

It was absolutely perfectly sitting there. And then I went down below and made something to eat and the boat almost tacked on me. So I said, boy, if it ever tacked on me, it would bear off on the other tack and go roaring off downwind and tip over on the other tack. I took the ties off the tiller and I let the tiller go stop-to-stop. It still stayed beautifully head-to-wind. It was sitting there like a duck on a pond. The motion was so comfortable and everything.

All of a sudden the sun came out so I ran down below, grabbed my sextant, got up there and took a sight. I was working out the sun site down below and all of a sudden a wave a little bigger than the rest came along and broke just as it got under the boat. I mean, it flipped me over so fast that there was no… the centrifugal force was so great that it would be like the Loop-O-Plane [carnival ride]. You’re standing on the bottom of the boat looking down at the water but you are not falling down yet, you know what I mean? In other words, the boat isn’t using its outrigger at all.

The proa* is the way to go. Absolutely, it is the way to go. I’ve got this neat boat all dreamed up already. I’ll show you when I get home. It’s self-rescuing, self-righting, won’t sink, and you can get out of it. It’s got all the answers to everything in this multihull thing.

Anyway, the trimaran–no one has figured out the answer until now. No one has said why they tip over; I absolutely know why they tip over. I can make a little model and a diagram and show you instantly why they’re absolutely treacherous if you leave them beam-to-sea. Absolutely treacherous.”

The conditions were the wind was blowing 24 knots. The seas were crashing right over the boat. Remember, this is the gulfstream where wind and wave conditions are influenced by currents etc creating difficult conditions at times.

The message here is this is a skilled man doing a single handed sail to qualify for a race. His boat did not capsize by wind pressure but by wave action. The full article is very informative and gives clear messages about the need for drogues etc. and careful weather selection.

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### trip the light fandangoSenior Member

Jan seems to be describing flipping a pancake..

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### peterbikeJunior Member

Old Multi, does anyone know what Jan said about proa's ?
thanks.

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### oldmultiSenior Member

peterbike. This is the only mention I have heard about a Proa from Jan. At the end of the Epoxyworks article there was this quote "It’s unclear whether Jan is referencing the AZULAO (trimaran) that had previously capsized, or Clifton’s later Dick Newick-designed AZULAO II (42 foot proa)." You will find details of both these boats on this thread. Does anyone else know if he drew a design or built a model etc?

TTLF. In some of the seas I have sailed in the middle of Bass Straight I can fully understand Jan's description. We were reaching in daylight and could steer our way through the waves but the 8 meter seas, with breaking wave tops, that on 2 occasions landed on top of the cat I was sailing on. The seas were worrying. Had I been sailing at night or on a 33 foot tri I would have gone with the seas and gotten to cover as fast as possible or deployed a drogue.

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### oldmultiSenior Member

The Vizable 105 is a cat designed by a designer architect. The cat is 34.4 x 20.5 foot with a 5600 lbs weight and 7100 lbs displacement. The hulls length to beam is 14.5:1 with a prismatic coefficient of 0.645. The total sail area is 753 square foot cutter rig on a 38 foot mast. The mast can be a spreaderless fixed section spar or a rotating spreaderless wingmast for added performance. The cat has a high aspect ratio kick-up rudders and a single kick-up centerboard, for leeway prevention.

A cutter rig provides an effective sail plan for short handed sailing and comfortable cruising. The foresail can be tacked from port to starboard by a swinging bowsprit leaving the stay sail in clean air. The sail area is distributed in three smaller sails, reducing the loads in the hardware and sheets. A smaller mainsail is much easier to trim, hoist and reef. All sail controls are routed to the central helm stations for safe operation of the rig. The mast can be raised and lowered without a crane and transported on the deck for storage.

With an open bridgedeck configuration, the catamaran can be dis-assembled and either shipped in a container or transported by road on a bespoke trailer.

The hulls at the gunnels are 4 foot wide. The numbers indicate the cat will be fast with an efficient rig. The cat should be fast enough to be raced competitively and capable of accommodating a crew of 7. The hulls provide standing headroom in the entry, galley, head and nav station area. The aft bunks are 3.3 foot wide and a single forward tapered bunk on STB provides another comfortable sleeping cabin. Folding table and seats provide a dinning area in the galley area.

VIZIBLE105 was designed for homebuilding but could be a production cat. A modular design allows homebuilders to plan the construction in stages, while being able to sail the boat as early as possible. Also, the owner/builder can choose to have the platform fixed (all beams glued in) or to be able to dis-assemble the catamaran. The stages are: complete only hulls and beams, trampoline between the beams - ready to sail, next you can add a hard bridge deck sole and finally do a complete hard top (enclosed saloon).

The hull shape adds stiffness with the gunnels acting as a longitudinal stringer adding rigidity. The only shapes needing moulding are the hull outer shells and the underside of the central pod, everything else can be built on a flat table, infused or vacuum bagged. The large beams provide structural strength and handy storage space. For construction simplicity, the hull windows are externally mounted, avoiding complicated construction details and reducing construction time. The hulls shells are monocoque, no complicated moulding smooth throughout for easier finishing. An interesting design done by a general designer not specifically a boat designer. I hope one has been built so we can all evaluate its performance.

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11. Joined: May 2019
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### oldmultiSenior Member

Hi. Any person who may be interested in a second hand Crowther Spindrift 37 cat please message me. The cat is located in the Cranborne area of Victoria, Australia. The Spindrift 37 is a full boat with rig, sails, motors and reasonable fitout etc. BUT!!! It has in the starboard hull wing deck joint area a problem. Each plywood bulkhead has fractures that will need a rebuild. There is some rot in a few small area's. One of the 2 engines would need replacing. I have nothing to do with the boat. The price advertised is \$18,000 Australian. It is laying in a boat yard where you could do the work or have someone do the repairs.

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### bajansailorMarine Surveyor

I am on the other side of the world, hence am not a potential punter, however I would love to see some photos of her if you have any?

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### oldmultiSenior Member

bajansailor. I am not associated with the boat for sale. All I can do is direct you to the seller. It is a slightly modified Spindrift 37 cat. Some jpegs below of similar Spindrift 37 cats.

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### oldmultiSenior Member

A little story about a “production cat” that started development in 2007 and by 2012 was still being developed. The Lightspeed 32 is 32 x 18 foot transportable tube catamaran. It has a cloud of sail on a very light cat. Details are a bit vague as there appears to be only one boat built and production boats were priced at \$225,000. In a world where Reynolds 33 sold for \$120,000 the Lightspeed 32 was always going to have a problem.

Why the interest in the boat? One quote “At last year's Newport to Ensenada race in light conditions, the Lightspeed 32 was kicking Afterburner's (52 ft beachcat) butt before both dropped out after midnight”

The Lightspeed 32 needs to be a very fast cat to outsail Afterburner. So, what is the cat about? The prototype was designed by Marc van Peteghem and Vincent Lauriot Prevost and is a carbon-nomex structure with carbon fibre beams and mast. The design is clearly performance oriented, but it appears to be designed to be a little more forgiving that some other boats. It could never be called a cruiser, but it’s not a full out racer either. I see it as a compromise between amateur racer and more casual daysailer. The website reports top speeds of a little under 25kts, but 20kts seems pretty common. The Lightspeed 32 has limited accommodation in the forward part of the hulls.

As a sailor said “As an experienced LightSpeed-32 sailor, I will tell you that the boat feels just about right in terms of weight balance. In really light air, under 5, the crew has to stay in front of the main beam, but the rest of the time you just sit on the comfy benches. I've had the upper knuckle of the bow about three feet under, and it sure didn't feel too stern heavy at that point. Here are a few shots I took last summer of the boat in the SC harbor. The mast raising system is amazing, it's a giant aluminium A-frame with a system of blocks and winches. The trailer extends to allow the hulls to be bolted to the crossbars.”

To quote one tester “I was lucky enough to ride along for the most recent test, and witnessed the LS32 perform in a variety of conditions, from light and spotty to full-on breezy-all in the course of one and a half races on Saturday morning. Aside from the thrill of sailing faster than the wind speed the boat manoeuvred very well.”

The reason for the half race was “minor stress fracture sustained to its starboard hull during the Newport Unlimited Regatta on Saturday. So when, halfway up the second beat of the second race, as we were flying a hull on starboard tack, a high-stress section of the hull began to crack.” The builder said “"Each time we sail, it's a process of finding the weakest link," Stookey explained. "Once we find the weakest link, it exposes the next weakest link-that's what happens, fundamentally."

Translation of the above. You have a cat that was designed to be very advanced for its time trying to use Stilleto build technology. The designers did a good job in producing a fast boat that could be sailed by average sailors in many conditions. The downside is a carbon nomex, carbon fibre build is not cheap and the boats price was probably reflective of the investment but not of the market where there were several cheaper, lower tech but nearly as fast boats available. The final issue is it took 5 years to develop the boat from initial build to near full development. During this time the GC 32, Toro 34 etc were starting to be developed and marketed with stronger marketing organisations. A little warning, be very sure of your market niche if you plan to “productionise you idea”.

Jpegs tell the story.

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### oldmultiSenior Member

With regret, I am writing this item. I have sailed on 3 versions of this design and found them good cats but after looking at one recently and doing some investigation we need to talk about the structural problems that design is having after a few years of use. The design is the Spindrift 37, Spindrift 40 series by Lock Crowther. They were designed in the 70’s with foam glass or balsa glass hulls. The remainder of the cat is plywood. Underwing, deck, bulkheads, crossbeams and cabins etc are all plywood. Dozens of these cats have crossed oceans and 2 to my knowledge have circumnavigated.

What is the structural problem? Fractures in the hull bulkheads crossbeams joints and the splitting of the underwing from hull gunnels.

So, let’s analysis what is going on. In the first case the cat was home built and to plan. The cat was in the Indian ocean and got hit by a series of waves in the forward underwing which cause a fracture at the underwing hull gunnel joint after that the forward wing frame cross beam started to separate from the forward hull bulkhead. Next the main cross beam started to separate from the hull bulkhead. The movement caused a fracture along the underwing hull joint aft of the main cross beam. I spoke to the owner builder after the family was rescued by a passing ship and the boat was abandoned.

The second case was a heavily built (3000 lbs overweight) Spindrift 37. Going to windward in a short sharp head sea a 1.5 x 1.5 foot hole was punched in the 12 mm plywood forward underwing which let water into the lee hull. The forward underwing started to fracture at the hull underwing joint due to additional weight of water in the lee hull. This boat was near a bay and ran down wind to the bay pumped the water out and got home to be repaired. The repair required a strengthening of the internal structure with additional stringers and much filleting. The wing deck hull joint was also reinforced. If this had of happened mid ocean it would have been a dangerous situation.

The next boat is James Evenson and Kim Jensen Spindrift 37 “Zingaro” which was on a final approach to Honolulu from a 16,000 miles South Seas adventure when things went wrong. Zingaro was on starboard tack, with only a ‘postage stamp’ of jib out, dragging drogues, and still making 6-7 knots over the bottom. At 2:30 a.m., the boat was picked up by one wave and slammed down by another, followed by a wrenching motion and “a tearing sound like ripping cured fiberglass off a piece of plywood,” says James.

“The rear main beam was broken, and subsequently all of the bulkheads and stringers on the starboard side,” says James. “A lot of the hull-deck joint also failed.” All that was holding the hull to the rest of the boat was part of the deck. The jpegs show part of the damage. The story is at Zingaro's Bad Break Is Bad Break - Latitude38 https://www.latitude38.com/lectronic/zingaros-bad-break-bad-break/

The final case is in a boatyard after sailing across the “Bight” from Perth to Melbourne Australia (3000 miles of open, often rough, water with minimal ports). The Spindrift 37 has fractures in the bulkheads at the hull underwing joints. I do not know when or how these fractures occurred but they are similar to others described or seen.

In NZ a Spindrift 45 (same construction concept as the 37/40) called Broken Arrow had a significant hull separation from its underwing and main crossbeam due to a failure in the main beam hull joint. I cannot remember more detail but I have the article somewhere.

Except for Zingaro and Broken Arrow, I can assure you there was no rot in any of boats in the areas affected by these failures. I cannot make a statement about the quality of the materials or build quality of each of these boats.

The 2 main issues though are: The hull underwing joint needs to be first class. These boats were designed in a time when people thought several layers CSM over the hull underwing joint would be strong enough. If people used CSM and epoxy this made a poor joint as standard CSM have “binders” that epoxy does not like. It needed a minimum of several layers of biax e glass with epoxy. Also, it depended on a 12 mm ply underwing being attached to a 35 mm gunnel strip with glass on both sides. There needs to be more beef in that joint with a fillet or chamfer of some size being placed in the hull underwing joint. Several 37/40’s and 45’s have had the modification of adding a chamfer and to my knowledge none have failed.

The second issue is the hull bulkhead cross beam joints. In these designs they were just glue joints of separate components. If the glue joint or plywood faces are not of good quality, these can work and separate over time. If the boat is overloaded these can also potentially weaken the joint at that point. Rudy Choy designs for example did some similar joints but backed it up with 2 foot diameter plywood discs placed at 45 degrees to the face of the cross arm bulkhead ply and glued to the plywood faces of the crossbeams and bulkheads.

Most of the failures were on relatively low mile boats (I do not know about Zingaro) so I do not think it’s a fatigue problem. Please check these areas if you propose to buy one of these designs. BUT it is not just these designs but any cat that does not have fully integrated full crossbeam bulkhead units with continuous top and bottom flanges from the beam down into the hulls. Also the underwing hull joint is critical and needs careful inspection.

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