Multihull Structure Thoughts

Constant Camber boats come in many sizes but the cats that are used for charter are generally in the 40 foot range. I will focus on the CC 40 used for charter. The cat is 40 x 22 foot weighs 11000lbs and displaces 19000 lbs. It carries 800 square foot of sail depending on local charter rules. The boats basic construction is 3 layers of 6 mm ply or WRC on a constant camber mould for the hulls. There is a strong gunnel timber and hull stringers at waterline level running the full length of the hulls. The keel lines were a slurry of epoxy and eg Micro balloons or QCells covered with a heavy glass layer internally and a glass keel layer externally. Some cats had fixed low aspect ratio keels others had daggerboards depending on the sailing area. The underwing was 18 mm ply with 3 x 6 mm on the forward underwing. The bulkheads were ply with timber reinforcement for the main mast and aft beams. The decks and cabin structures were 12 mm ply for the majority of the boat. The exterior of the cats is covered with e glass epoxy.

The charter cats were very strong and if well maintained will last for decades. For comparison the hulls of the 40 foot cruising tris were 3 layers of 4 mm ply. Even the 44 constant camber open bridge deck cats had 15 mm thick hulls and decks. As has been said Constant Camber boats are a relatively fast and cheap build approach especially when they don’t have much or basic internal accommodation. The jpegs are of a 40 foot charter cat, study print of a 40 foot constant camber tri and a 44 foot constant camber.

 

ETF 26 was conceived by the winner of the Jacques Vabre Race, Jean-Pierre Dick, and Guillaume Verdier, who designed the winning NZ boat for the 34th edition of the America's Cup.

ETF 26 is 26.6 x 14.1 foot weighs 785 lbs and the boat was designed to have a maximum crew weight of 580 lbs. The ETF 26 carries on a 44 foot carbon wing mast carrying a 315 square foot mainsail, a 120 square foot self tacking jib and a 285 square foot code zero. Add a 530 square foot spinnaker you are going to go very fast. The ETF26 proved to be able to adapt to different weather conditions: it can fly from 8 to 20 knots and reach 2.5 times the wind's speed.

The cat is full carbon with Carbon cross beams, Carbon / Epoxy S-foils and Carbon / Epoxy fixed “T” shaped Rudders. The Carbon Mast, Gooseneck and Carbon Boom with Outhaul. The mainly solid carbon skin hulls are for more robustness and repair-ability. The hulls mainly solid CF skins would be too thin to handle knocks if they were foam glass composites. The main foils rake can be moved from 0 to 2.5 degrees manually by a worm drive.

Notice the similarity to C class cats in size and shape eg jpeg. The C Class developments allowed this boat to evolve. The designers saw a slot in the market when they realized for most people the EG GC 32 was too big and cumbersome but the A class and C class cats were too racy for most owners who did not have the skills required to control them. So, after prompting from a 60 year old, they designed a slightly more robust C class type cat with twice the sail area that can perform well across a range of conditions. An independently test boat sailed in 7 knots of true hit 16 to 17 knots on its foil. Upwind with foils fully down the cat did 10 knots. The ETF 26 has hit 35 knots in flat water but can only hit 25 to 28 knots in waves. The ETF 26 has sailed through storm conditions. The mast and boom are sealed to help prevent a full capsize.

Light weight is the key to this boat ability to fly easily combined with its ability to ride close to the water about 1 and half foot means good handling characteristics helping make the ETF a safer boat to foil.

 

Now we talk about a creative tri that is trailable. The TNT 34 is 34 x 28.5 foot which can fold to either 12.4 foot or 9.5 foot for trailing. The weight is from 4000lbs for the shell to 4800 lbs sailing weight without crew and gear. The 49 foot mast carries a 618 square foot main, a 295 square foot self-tack jib, a 860 square foot genoa and a 1360 square foot spinnaker. Notice 2 things, the large beam and the ability to carry 915 square foot of sail upwind. This sail area is about 25% larger than most tri’s of this size.

What is the result? A Bruce number of 1.85 (this is high). A true wind speed (TWS) of 7-9 knots, wind angle 70 deg, boat speed 7-8 knots. TWS 5-6 knots, wind angle 70 deg, boat speed 6.5 knots. TWS 5-6 knots, wind angle 120 deg, boat speed 8-9 knots and TWS 7-9, wind angle 120 deg, boat speed 9-10 knots. That is a speed of 1.5 wind speed.

The important part of this design is the floats to carry this sail area. The designer J Kostanski in 2009 designed the floats with a length to beam is 22:1 ratio and lots of buoyancy forward to prevent pitchpole. Floats have 200 % buoyancy plus optional float hydrofoils that provide an additional 40 to 50 % “buoyancy” (effective from 8 knots and up). So how do get a trailable boat with a 28.5 foot beam?

This is a new approach of pivot/telescopic folding system that has numerous advantages. A 2 foot aluminium sleeves pivots on the main hull gunnels that allow the cross arms to initially slide in the sleeves then pivot the float cross arms to fold the floats beside the hulls. A wire stay is attached to the cross arm when the float is at full width. The combined two movements of sliding and then pivoting of the beams allows folding in less than a minute by using winches. The system is subject to a patent.

The 2 foot sleeve is acting as solid long holder once bolted to the deck (similar bolting takes place on Farrier system). Once the beam is locked in the sleeve the float is fixed via cantilever to the main hull. To reduce forces a two stainless wires is provided for each float. The sleeve made of marine aluminium. Sliding is easy due to Teflon strips inside the sleeve and stainless steel strips on the beams. The beams require precise tooling to avoid jamming.

The construction is mainly foam glass with some carbon fibre especially in the cross beams. The dual rudders are optional and were done for an owner who wanted an outboard. The interior is comfortable and could be cruised for a reasonable period. There have been a few built but this is quite a project to trail mainly due to the rig, road regulations and the tow car requirements. The concept is good and could be used in a smaller tri that would be more appropriate as a trailer sailor.

 

Trimarans come in many forms but there was an interesting period where very wide bodied main hull tri dominated. It was originated by Arthur Piver’s plywood tris that had 6:1 length to beam ratio’s on their main hulls followed by other designers like Headley Nicol Vagabond series of trimarans. But designers then started to refine the model and build foam glass versions of these boats with round bilge hulls and better shaped floats. Robert Harris was the designer of Zipper a 38 x 24 foot tri that weighed 8100 lbs and carried 658 square foot of sail. The Zipper had a 5.7:1 main hull. Harris also designed the Swinger version which had swing cross arms but was basically the same boat.

But the most extreme of these types of trimarans came from Len Surtees. The Osprey a 38 x 26 foot tri that weighed 8000 lbs and carried 680 square foot of sail. If the boat looks familiar it is the boat Tristain Jones sailed through Europe. The Length to beam on the main hull was 5:1. Also around the same stage John Westell designed a range of swing arm tris including a 36 x 30 foot custom tri that had a wide main hull. There were several others including some production fiberglass production Piver designs.

Each of these boats made reasonable to very good cruisers. They could average 6 to 8 knots all day in 15 to 20 knot winds but their peak speed was limited unless they were surfing down a wave. This is a horses for courses situation. I have raced a 42 foot cat with 12:1 hulls against a wide bodied 6:1 42 foot Harris tri. Both boats had good accommodation and they were about the same displacement. In the same conditions the cat averages were 30 to 50% faster. Basically, choose what you want. Comfortable motion and sensible cruising performance especially in light winds which a wide bodied tri can provide or less accommodation and payload capacity but higher performance that a thinner bodied (8:1 plus) main hull tri can provide. PS Piver tris real problem was their float shape and rigs which is the reason they were not good all round performers. The jpegs show some samples.

 

What you have written does not address the advantages for cruisers in my post, but I will address it anyway.

Breaking a stick is a poor rig analogy as it does not consider the stays, their attachment points or the structure required to attach the stays to. No engineer would neglect this.
There are far more cantilevered bridges than suspension bridges (and far more unstayed trees than both), presumably because they are cheaper and easier to maintain, so suspension is neither 'cheaper or safer'. And no one worries about windage on bridges.

Put a little bit of bend in your stick (equivalent to improperly tuned or stretched rigging) and lean on the end and it will break very easily.

You may not consider drag reduction to be important, but anyone who sails upwind certainly does. See Shuttleworth's article on the subject.

"hundreds of wires" is several hundreds (2 end fittings, cotter pins, turnbuckles, chainplates, mast attachments, spreaders, seagull striker and fore beam) of individual components. On a mast, any one of them breaks and it all falls down. Fine if you check it regularly and replace it after x thousand miles, but worrying about it simply reduces the pleasure of cruising.
"Simplicity" is always best when you are not near a chandler, rigger or crane to lift the mast out.

 

I do not want to read into your post. So if there is a communication issue, lets clarify it. But...

'Most engjneers' is the logical fallacies of appeal to authority and majority. This retoric of yours avoids engaging the simple ideas presented. Why not engage the ideas instead? It might be a funner discussion.

 

Hedley Nicol. And Vagabonds are about 8 to 1 on the waterline, Cavaliers, Wanderers and Islanders are a bit wider.

 

The idea of breaking a stick shows that a column in compression requires a far greater force to break than if it is in simple beam or cantilever mode. Columns are strong. Even stronger when supported by stays to keep them in column. Putting slight pre-bend in a mast is a simple way to get the mast more rigid because you know the "eccentricity" of the mast and can design accordingly. Push it forward in the middle and then lock it there - really well shown by the skiff type rig - huge load on a small pre-bent and locked section.

And yes simple beam bridges are most common, for small bridges over roads and railways, just like cantilever masts are more common on Lasers, OKs, Finns and many singlehander dinghies. When loads are low the cantilever or simple beam is a great thing. But when the loads get large, say in a long bridge with a high load, most bridges resolve compression and tension into different members. The stability of my 38 footer is 16 times the stability of my 19 footer so the engineering may need a different approach. One that can handle this exponential increase in load. Engineers have to determine where this cross-over point is where a cantilever is more work than a stayed truss.

But I do think that if unstayed masts were the best option for larger boats then would they not be used in open classes like IMOCAs or a deed of gift AC boat or in record setting sailing boats? They don't seem to be. A clean slate and the best in the business go with the stayed rig. Or the charter boats made just for ease of use.

This is my point about most engineers. I like to be reasonably humble in the face of other people's knowledge and it seems that almost all sparmakers and yacht designers design masts with stays, sometimes only three, like the AC cats last time, sometimes lots more, like the Hobart monos or 18ft skiffs. Learned people like AC mast engineers like using stays and lots of other engineers who work for mast companies or yacht design firms seem to as well. The plethora of stayed masts is either because it is a system that works well, or because people are blind to the charms of going freestanding. I don't think it is a logical fallacy to argue that in a widely diverse field such as yacht design, that favourable traits are copied and are more common in successive generations. Ease of use is paramount these days as we see a decrease in the need for speed in many cruisers. I would argue that a stayed rig still fits the bill for most users.

I don't argue that unstayed mast are silly for everyone. I should temper my words to reflect that. Just like my wishbone rig is fab for me and my sailing but rarely seen on other cats, it could be a great option for someone with a certain set of parameters. I would like to see more interesting boats out there in a sea of white generic similarity and certainly proas and tris and interesting cats and monos should be respected for the traits they trial, so that the successful ones can be used by the next generation of sailors. Happy free mast building all, I however, will sit this one out unless the boat is quite small.

 

The Shilo was designed in the early eighties by Malcolm Tennant as a compact cruiser with spacious accommodation for a family and good performance for the rough conditions of New Zealand waters. Shilo is 30 x 18.3 foot and weighs 6800 lbs and carries 2700 lbs payload. The sail area of the rig is 520 square foot on a “cutter rig”. Actually the rig is a fractional rig that carries a masthead genoa. The jpegs of the boat have an interesting variation of a roller furling jib option if you look at the bow arrangement.

This boat is the half way house between an older style English full wingdeck cat and a more modern partial wingdeck cat that has a good rig and decent boards. The hulls length to beam are 9.2:1.

The boat was built with composite strip planking with glass and epoxy structure with ply timber crossbeams. This design could easily be converted to a foam glass solution with ply bulkheads or full foam glass. Shilo has a simple interior, that is practical that does not have full headroom in the bridge cabin. The Shilo is a sporty, no nonsense cruising catamaran for coastal and limited offshore waters.

 

Essence 6 is 22.15 x 14.76 foot weighs 992 lbs with a 29.5 foot high aluminium mast carrying 194 square foot main and 65 square foot jib. The Essence is a designed by Tomas Jonas Janda. This is a very attractive day sailing, limited cruising catamaran that can have a fractional sloop rig or biplane rig. The Essence 6 has excellent sailing capability and many have been produced. It’s basically foam glass round bilge hulls with large aluminium cross beams. The boat is trailable with a bit of assembly. Jpegs below.

Now Tomas Jonas Janda has designed the Independant IC36. It is an open bridge deck trailable catamaran of 36 x 20.30 foot weighing 4850 lbs carrying a 49 foot mast with a 452 square foot main 194 square foot jib and a 495 square foot Code 0. There are 3 mast options an aluminium section that weighs 208 lbs. A carbon tube of 153 lbs or a carbon fibre wing mast that weighs 130 lbs that also can increase performance by 8 to 12%. The Independent IC36 is claimed to be capable of sailing 1.4 times faster than the wind due to low hydrodynamic resistance below the waterline, with a narrow hull profile and low all-up weight. The rotating mast is moved further aft and high displacement bows offer high top-end speed and avoid bow pitching. Carbon compression beam extends forwards to carbon asymmetric bowsprit.

The bridge deck clearance 2.65 foot. For trailing it will require an SUV capable of towing 7800 lbs (3.5 ton).

The hulls and decks are Vacuum Epoxy sandwich construction. Airex core, glass, carbon, Kevlar, Dyneema® reinforcements in epoxy resin, but 30% more stiffness and 2 times more resistant than polyester and vinylester.
Dyneema® fibre at bow, aft and keel reinforcements from Kevlar. Hull and deck are bonded with epoxy glass in one solid hull. Structural bulkheads laminated to reinforce overall hull construction. No gel-coat to save weight. Whole boat is painted in standard RAL 9003 (any colour is possible). The main beams are made of epoxy carbon sandwich.

This boat is still under development but based on Tomas past designs it should be a good fast design.

 

This is the first time I've seen dyneema glassed in and used as a re enforcement, if I understand it correctly, a great idea,.. something re vintage was considering. Avoiding gel coat to save weight is interesting if weight is becoming an issue, thanks again Oldmulti.

 

The subject is unstayed masts for cruising boats, not bridges, 18' skiffs, AC boats or IMOCAS. Maybe drop the red herrings and reply to the reasons why unstayed masts are better for cruisers.

Wrong. See (or address) my previous post about the structure required to support the stays. Which, incidentally, is composed of 5 cantilevered unstayed beams (hulls, cross beams) on most stayed rig cruising multis.

How many wires does this require, how often do they replace the rigging, check the tensions, sail dead down wind, reef in a squall, or hoist/lower the main on the water? ie How suitable is a skiff rig for a cruising boat?

Maybe, but it is several orders of magnitude less than the Golden Gate bridge.

The argument about whether unstayed rigs could handle the loads ended with Team Phillips, 20 years ago. This had more righting moment than any race cat or tri ever built, and fewer crew than the rest of the fleet. The hulls broke, the rig was excellent. Unstayed rig yachts like Matese Falcon, Yacht A and others further prove the point.

A relevant comment! Anyone got any deas why the charter boats don't use unstyaed rigs?

Go ahead and argue it, then. Tell us why it is so good for cruisers, starting with the reasons I gave showing it isn't.

You do, apart from Lasers and Finns.

Yes, you should, but you haven't.
,

Writing negative posts about unstayed rigs and then trying to defend your views with red herring posts is hardly "sitting this one out".

 

^^
Rob et al
Instead of cluttering this fine thread ,with thread drift ,feel free to continue the mast discussion here...
Rob Dennys' unstayed masts https://www.boatdesign.net/threads/rob-dennys-unstayed-masts.63398/

 

Robert Harris did several versions of his 32/33 trimaran. The initial tri was a wide bodied main hull cruiser with no wings that was 33 x 20 foot weighing about 6500 lbs with 460 square foot of sail (I am working of memory as I cannot find the drawing). Then he in 1975 created a racer version of the same boat with less displacement and more sail which he called Barrard Flasher but basically the same layout. The Flasher was 33.9 x 21 weighing 4600 lbs and had a 488 square foot rig with main and jib. Hull and decks are constructed of Airex foam fiberglass sandwich construction. Blue jpegs. Following this he then created a third version called the Harris 33 in 1980, which had small wing deck extensions that added room to the main cabin. This version was 33 x 22 foot displaced 7500 lbs carrying a 39 foot mast and 500 square foot of main and jib. The prototype was double diagonal timber covered with e glass with aluminium tapered cross beams. This boat was also foam fiberglass with Airex foam in production.

The Barrard Flasher had a centre board in the main hull and a 29 foot waterline, the Harris 33 version had a centre board in each float and a 30 foot waterline. The Harris 33 version was capable of sailing at 15 knots plus under the right conditions. As time went on Harris improved his designs from a good base. I will tomorrow show a 1968 paper Harris wrote which shows his clarity of thinking about multihull design at an early stage. He had quite a portfolio of tris, cats and monohulls.

 

The following document was written in 1968 and gives an idea of the thinking at the time about multihull design. Most of the thoughts are appropriate even today. The only significant difference is bow design, buoyancy distribution and structural concepts which have advanced greatly due to material advances. A bit of history. The full paper is over 2 thread entry posts on this page and the next page 53.

Tomorrow light weigh structures on boats about 25 foot.