Multihull Structure Thoughts

Rigs aside, I quite like them. They have done a zillion miles safely and served their purpose in a very simple way.

 

Iridian. The rigs an acknowledgment to the original Crab Claw rigs used by pacific islanders 300 years ago. An original Crab Claw rig of the correct shape when tested against modern rigs provided more power reaching and running than modern rigs. "Marchaj's Sail Performance (1990/1996, McGraw-Hill International Marine) has Chapter 11 titled "The Sail Power of Various Rigs", which appears to be based on wind tunnel test by Marchaj "on the potential power of a number of rigs: Bermudian, latten, sprit, gunter, dipping lug and crab claw .... used to drive small fishing vessels". (p152) A considerable portion of the chapter is focused on crab claw sails, with claims of superiority over other rigs." The PDF is an article written by Marchai on how Crab Claw rigs work. The version in the Vaka's featured would not be as powerful but still has some of the advantages.

 

An obvious advantage of crabclaw rig is its higher area up where wind velocity is higher .

 

Also note that whenever the apparent wind angle is larger than 90°, aerodynamic drag gives a positive contribution to the sail's thrust (the force in the direction the boat is going), so it is perfectly reasonable to expect an obviously inefficient shape to be good for reaching (at sufficiently large angles) and running.

 

Pixel & Timber founder David Parrott designed a multi function multihull arrangement. His design can use common components to form a Proa, Trimaran or a Catamaran. The components are an 18 x 1.5 foot main hull, a 14.2 x 0.9 foot float hull, 13 foot cross beam tubes, rudders and centre boards. The basic rig is from a variety of windsurfers depending on the performance level required. The multihull option chosen is aimed at lake or bay sailing for 1 or 2 people.

The boats are: An 18 x 6.5 foot proa over the hulls or 9.5 total beam over the racks. An 18 x 13.25 foot trimaran with 14.2 foot long floats. A 18 x 13 foot catamaran. The common main hulls have a 12 to 1 length to beam. The floats are 15 to 1 length to beam. The sail area varies from 52 to 95 square foot depending on which windsurfer rig option is chosen. The associated windsurfer mast is attached to a beam or hull and supported by 2 carbon fibre struts attached to the beams. The weight varies according to the chosen model and options.

The hulls are rotomolded like PVC canoes or kayaks. The crossbeams (akas) are extruded plastic. The masts are often 2 piece carbon fibre. This tripod mast support design reinforces the mast and eliminates the need for the stub mast seen on many tacking outriggers. A custom clamp connects the struts to the mast, supporting it but still allowing rotation during tacks. A common rudder(s) and centreboard(s) are used in all models.

The intention of building the hulls in HDPE with rotomoulding is that the hulls can easily be recycled at the end of life. Fiberglass boats are very difficult to recycle at end of life.

This appears to be a good concept that allows you to have a variety of sailing options depending on your mood. Interesting concept. The jpegs give the idea.

 

@oldmulti thanks for the information, I will do some more reading on it. It seems like it could be paired with either an A-Frame Mast or a Over-The-Top setup to work for boats that do not shunt.

 

The Crab Claw sail is not a “poor” design, unless you are using that word in the frugal sense

In fact, it has many important attributes that make complete sense when you think about how it was originally used.

By placing the volume of the sail up high, a boat is able to better maintain power in big swell. Ever sail in big rolling swell? Hard to make progress when you lose wind every time you get to the bottom of the pit then sucked back before being thrusted forward again. This can be very dangerous in big surf, and traditionally these boats were designed to beach and then launch back out to sea through breaking swell. Getting caught inside is never any fun!

Another important attribute is in gusty weather the sail can be de-powered to regulate and spill the wind out the top by simply drawing the lower sprit up to the top. You don’t have to reef the sail in the traditional sense, as the sail will “close” to spill the wind. This self regulating feature allows for less babysitting and more margin of error in fast changing conditions.

The Crab Claw design also keeps the deck clear of a traditional boom and dropped sail, and the sail can be cut from a simple flat material. To reef the sail, you simply close the sail by drawing the lower sprit to the top mast sprit, and then wrap it around the mast sprit. This keep the sail off the deck, allowing plenty of room and clearance for cargo and the cabins the original voyaging boats typically had atop the decks. Remember these boats have lots of freeboard and flat decks, so everything sits up very high.

Also you’ll notice on the bigger Polynesian cats that they have multiple easier to manage smaller sails rather than one big sail, and that they typically have less overall sail area than a boat of similar size with a traditional rig, yet perform very well in comparison.

I think the proof is in the pudding as they say in that the boat OldMulti posted about is said to be capable of maintaining 7-8kn with minimal sail area for its size.

If you can’t already tell, I’m a fan. But then again, I’m lucky to get to see these traditional designs being used all the time and it’s amazing how well they perform around here. You could say they have a real “home field” advantage.

 

What SolGato said. Saved me a heap of typing...

I have thought that bringing the base forward and using 2 rigid side poles to make a pyramid with the other "boom" free to move would make a simple and very efficient rig that would be REALLY easy to handle even single handed when sail area is very large. Really cheap to build also. Fraction of the cost of a marconi.

 

It's hard to sit here, riven by respect for each side. For one side, the Europeans were not ******. The crab claw sail has lots of important downfalls, as well. The high C of E is a problem. The claims for its speed are disproven by reality; the designers of the world's top foilers, multis, windsurfers etc are not stupid and they don't use crab claws. Nor were traditional western sailors, who travelled further than the Polynesians. The canoe in the vid cost the life of one of the world's legendary watermen, and navigating east to west by use of the stars and sun doesn't seem to be an art restricted to Polynesians.

I'm always cautious about what is "said". Who was it said by? Where they reliable? In what conditions was the performance obtained? Where's the proof? Respect should go both ways, and many traditional sailors, like the Buginese, adopted western rigs when they saw them. The only sailors I know from traditional Melanesian/Polynesian backgrounds adopted "western" rigs, and they are not ******.

I'm not saying that the CC sail etc don't work in some situations, but what is the comparison? How many of the guys who sail Hokuele (sp) use traditional Hawaiian road transport, communication, medicine, housing, etc?

I'm not meaning to diss traditional Pacific craft or crafts; it's just that sometimes I feel the praise for such things elevates them to a higher plane than the craft and crafts of other cultures, whereas they were probably of similar merit but different uses and situations. Sorry if I seem to be at cross purposes.

 

How do you know it's cheaper and easier? What are your bases for comparison? On what hulls, in what conditions, to what purposes, in what waters and by whom?

 

Borderline is a NZ 8.5m class racer cruiser catamaran that is 28 x 20.35 foot that displaces 2,200 lbs. The carbon fibre rotating mast is a TP 52 shaped wing section of 39 foot high carrying a 430 square foot full battened mainsail, a 205 square foot blade jib, a 320 square foot overlapping jib and a 1020 square foot gennaker. The length to beam is about 13 to 1. The hull draft is 250 mm and the asymmetric daggerboards draw 5.9 foot when down. The 15 HP outboard can drive the cat at 8 knots.

Borderline had 2 aluminium mast failures in its first year of sailing then the carbon fibre mast was installed. An 8.5m class mast section are EG made from SM Carbon, weighs 60 lbs, include hounds reinforcing. The Semi-Wing Rotating Mast section outside dimension is 141mm x 88mm with 6 mm walls.

The owner builder of Borderline is Andrew Potter who went to designer Andrew Eaton of Bakewell-White Yacht Design for the cat. Andrew is a brick layer and said it should not be hard to build a cat so 17 months later with the help of a few mates he launched Borderline. The “accommodation” is a single berth and some shelf space in the hulls with a portapotti in one hull. If Andrew wants to go cruising he adds a 320 lbs deck pod that contains a double berth, galley and limited seating. But this cat is about performance.

Borderline when tested in 12 to 18 knots could do 10 knots upwind tacking through 80 degrees. When reaching it can match or exceed wind speed. Its ‘natural’ top speed in appropriate conditions is 24 knots and Andrew said his closest brush with capsize on the Coastal Classic, the boat was doing the mid-20s, touching 28 knots. The overlapping jib would be flown up to 15 knots true wind speed and the blade up to 35 true wind speed. The mainsail is reefed at 25 knots. This is a fast cat that was Auckland bay champion in 8.5M division in 2012 and second in line honours over an 8.5M series in 2011.

Construction Cedar strip-planked bottom of hulls and bilge, topsides are 4mm ply with glass inside and out. There is carbon fibre reinforcing in high-load areas. Andrew built the beams first because they were the easiest shape. The beams are carbon fibre throughout, the compression strut, added later, is vacuum-bagged carbon Nomex, post-cured. The rudders and asymmetric boards are foam glass structures.

This cat has now been outclassed by some of the more aggressive 8.5M cats and tris (yes, this is conservative). The latest boats are 250 lbs lighter and carry more sail area. Borderline now has been converted to an electric cruiser cat. In its original form this boat was a very fast bay and coastal cruiser that could maintain very high speeds in racing. Well done Andrew. Sorry about the limited jpegs.

 

Sigh.

Because I've costed it out. Because a single sail with one control by 2 pairs of blocks, no battens and no rigging is cheap and simple.

And because LOTS of people have actually built sailboats with these rigs in all sorts of sizes and actually taken them sailing, cruising etc.

The reason racers use marconi rigs exclusively is because they are at their best with the apparent wind well forward. The boats are a package. Build it light and slim and you get low resistance. As the boat speeds uo the apparent wind moves well forward and the marconi comes into it's own.

In the real world of cruising loads, bouncy waves gusts etc a well proportioned crabclaw is a very nice thing.

You wouldn't take a lamborghini camping, you wouldn't take aland rover racing.

Having said that is is remarkable how conservative people can be. They will hang onto demonstrably inferior things long after they should just because they are reluctant to change.

 

CT 249 asked what basis are claims made for performance of sail types. Simple answer is if someone has done a relatively unbias test in the real world I will accept that information. If you have done an equal or better test I will review and accept that information. Marchaj's Sail Performance (1990/1996, McGraw-Hill International Marine) is one of the very few people that has done some serious analysis of rig types. Another person is at Proa File | Testing the crab claw sail https://proafile.com/multihull-boats/article/testing-the-crab-claw-sail
Attached are 2 further PDF's on wind tunnel tests of older style of sails. Marchaj is a very knowledgeable guy on sail aerodynamics and has lead many other to do both expensive professional and home research to either verify or dispute his stuff. I understand CT 249, this has been an issue for you since about 2004 (first entry I could find), but please understand there is some scientific support for the claims.

Now about modern rigs. Yes, modern fat head high aspect ratio soft and wing sails have better lift drag than older style pin head Bermudian and other types of sail. Evolution has helped enormously since 1990's. This is the reason reefable wing sails will take over in the future, but 200 years ago when the height of sail cloth was cotton or flax and any spar was timber the crab claw was remarkable cheap and efficient. Modern wing rig information can be found at Wing sail technology https://www.scoop.it/topic/wing-sail-technology/?&tag=%23SoftWingSail

Also please read up on vortex control. This is the difference between winning and losing in a lot of Formula One motor racing and helps modern aircraft with upturned wing tips fly further. If you can get a vortex in the right position and controlled in the right way, on the tip of a sail or wing, you can improve the lift drag across the entire sail or wing surface. A funny thing about crab claw sails, they have at certain angles of attack and angle of yards, excellent vortex's coming of the top tip of there sails. Ancient experience being understood with modern science.

 

@guzzis3 how would you tack a crab claw on a non proa?

A frame or over the top of the mast?

 

The 72' and the 50' vakas were both paid for by the wonderful Dieter Paulman. The designs appear to have been chosen primarily as a cultural statement, a function they have performed well. Neither of the two I know of (Marshalls and Fiji) sail well enough to provide a regular cargo service. I sailed on the Marshalls 50 with the weird crab claw sail, which is unlike the crabclaws the locals use. They told me they only use it for publicity shots. At sea, they use a conventional main. They have tried solar/battery electric motors and coconut oil instead of diesel, but reverted to diesel. The oar is indeed huge. And a handful for 3 people in big seas. As well as the exceptional cultural awareness the boats have provided around the Pacific, they have a superb training program which produces excellent sailors.

Marchaj's comparisons were with unvanged, highly twisted marconi rigs, so the superiority of crab claws is expected reaching and running. The proa file guy only tested variations on crab claws, and only one of them (the Marshallese one) worked, so his results are dubious.

Crab claw sails are great. We put one on the Mini cargo proa ( video at http://harryproa.com/?p=3155 MARCH #3) and it worked well, in conjunction with a lee board. It is not possible to build a better sail with flat materials, and more distributed loads (less reinforcing) although the hot racing canoes have a funny little bend in the top boom and tailored sail cloth at the junction with the bottom one. The bend is usually a grown knee scarphed and lashed to the straight part with fishing line. To understand how much bend and shape and where it goes "it is necessary to be at one with the sail".
They are slow to reef as they need to be lowered, the reef tucked in and re raised. Lifting or lowering the boom to depower in breeze causes them to flog hard due to the boom weight, so it is easier to just drop them in the water and wait for the squall to pass. They work well but fall down if caught aback on a stayed to windward canoe mast.
In big seas, light wind, I suspect they would flop about and be useless due to the extra weight aloft and lack of battens.
We put one on an A frame mast in the Marshalls, it was easier to tack/gybe it than to shunt (although the Marshallese make shunting look easy), but it could not be eased far enough for downwind sailing and the extra weight aloft was noticable.
The Marshallese ones (model 4 in the Piazza paper) don't have more sail up high than a marconi rig.
The vortex theory used to be the reason crab claws were "lifting" sails, ie they purported to lift the bow. This was certainly not true on the local boat I sailed. See the video at Mini Cargo Ferry Prototype – Harryproa http://harryproa.com/?p=3155 FEBRUARY 2020 #1 We pitchpoled it with comparative ease. Righting it, not so much.

We gave serious thought to using a pair of crab claws on the cargo proa to avoid the cost, complexity, maintenance and skill levels required for a 'modern' rig. In the end, we went with a telescoping wing rig which is easier to build by unskilled locals, uses the same materials and tools as the boat plus a small sewing machine, is cheaper, safer to use and has near zero maintenance. It should also go upwind better, which is critical for servicing remote Pacific communities. And it was great fun to develop.

5,000 stainless screws for a 28'ter (Skoota 28)? Each one is a potential/probable rot starter unless they are screwed into epoxied holes or removed and the hole filled with epoxy resin. At 20 seconds to insert each screw, that is 27 hours of screwing. ;-), a bit less to remove them (unless the slots are filled with epoxy) and about 5 times as long to fill the holes with resin. The Mini Cargo proa has zero metal fastenings.