Multihull Structure Thoughts

The Seatribe 870 is a sport catamaran that is transportable. The Seatribe is 28.6 x 17.4 foot with a weight of 2,200 lbs and a displacement of 3,350 lbs. The 33.6 foot fixed aluminium mast carries a 256 square foot mainsail and a 111 square foot self tacking jib. A larger 36.1 foot aluminium mast rig was used on one boat with a 280 square foot mainsail and a 118 square foot jib. A 450 square foot spinnaker available. The forestay is 8 mm SS 1x19 and the cap shrouds are 10 mm SS 1x19. The only winches are two Anderson 28 2 speed. The length to beam of the hulls is 10 to 1. The low aspect ratio keels draw 2.2 foot. The underwing clearance 1.9 foot. The hull beam at the gunnel is 4.2 foot. The outboard power is 20 HP.

The construction is basically foam glass. Hulls and decks are epoxy resin infused fibreglass Airex C 70 PVC foam sandwich composite. The underwing and galley modules are foam glass. Crossbeams are powder coated aluminium. Forward beam has a dolphin striker and bow roller. Mast support beam with dolphin striker. The tiller connections are aluminium. The cat can be disassembled and transported on a trailer. The component parts are not quite “trailable” category but are possible to be towed and assembled at a new location.

The accommodation is 2 double berths and a single/double. Each hull has a toilet space. The “galley” is at the front end of the open bridge deck cockpit. There is a cockpit roof cover option available.

The only “test” boat information I found says this cat is high performance and handles very well if kept light. It is described as an awesome little weekender that could be faster with a 5 foot bigger rig. A video on Youtube shows it reaching along very nicely.

A good fun design. The jpegs give the idea.

 

There are many ways to build a solid glass or foam glass multihull. They range from the old method of build a male frame mold with timber strips. You then screwed the foam to the mold, faired the foam, externally glassed the foam, faired the glass, take out the screws the do the glassing internally. Long, laborious and repeatedly fairing.

Next step was to do flat foam glass panels on a flat surface with vacuum bagging (and later resin infusion). Get your multiple panels, cut them to shape and tape the seams together. Again 1000’s of feet of taping which has to be filleted, surface sanded, glassed, sanded and faired. Do not underestimate the amount of taping of major components let alone the secondary components like secondary bulkheads, wing frames, furniture etc.

The next improvement was the creation of simple molds for hulls to hatches. This is a variation of flat panel molding. EG a cheap chipboard multi chine female hull mold. If well constructed and waxed the mold will last 2 hulls. You can glass an entire hull putting all strengthening materials etc as you hand lay, vacuum bag or resin infuse the hull. This will save about 800 square foot of secondary bonding and fairing for a 50 foot hull.

The next variation came from 2 directions. Rob Denny from Harryproa developed Intelligent Infusion which is a very simple complete half hull mold with decking and slots for bulkheads etc with resin infusion. One resin infusion run you have half a hull needing little fairing ready to insert bulkheads etc. This approach suits the Harryproa hull shape which have flat bottoms and are symmetrical. Web page:
Intelligent Infusion – Harryproa http://harryproa.com/?p=1845

Other builders such as Turning Point use cheap MDF, strip plank hull molds to create round bilge halve hull shapes. These hull molds if well constructed and waxed will last from 2 to 10 hulls. Turning Point build prepreg carbon fibre PVC foam hulls that are cooked for 18 hours at up to 180 degrees in these simple molds.

Please evaluate how much work is going to be involved in the various construction methods. Extremely complex shapes may need the old approach, but if the designer is good they will use a combination of flat panels and premolded parts to speed the build. Pulling EG a hull from a full female mold is the easiest if you can create a simple cheap mold. Finally, a man who professionally built a flat panel carbon fibre foam 42 foot racer trimaran said the boat would have been built faster and be significantly lighter had it been extracted from cheap built hull molds. The taping, fairing and fillers required added so much weight and work, the cheap molds would have been less weight and cost.

Some jpegs may help understand. Anything with II is Intelligent Infusion. TP is Turning Point.

 

There is also cnc cut polystyrene moulds like Greg Lynn’s trimaran “Girlfriend”
GREG LYNN YACHT – January 2013 http://glform.com/yacht/construction/january-2013/

 

It's also worth mentioning that for a cruising boat making hull skins by any method is a small part of the total build.

Have you an opinion of Ian Farrier's vertical strip foam method ? Always wondered how much time if any it saved..

 

Guzzis3. I have spoken to 3 guys who have used vertical strip foam. Yes its easier to shape the foam but as to if it saves time I am not to sure. It gives a truer shape but you still need to fair joints etc. then you have to fair the finished glass shell. Flat panel foam glass on a smooth surface is faster even if you have to tape them together. Farriers insistence on accuracy and peel ply helps, but I think vertical foam strips helps but is not a major time saver in a total build.

 

I'm building using male mold and vertical foam panels. I've never done any of the other methods. There is a lot of fairing to be done for sure. I'm on my second hull and the first took a lot of fairing, the second much less. For me attention to detail on the mold paid big dividends on the initial fairness.

 

A follow up to page 100 article (on this thread) on “Felix” the Turning Point 24 foot (now 26 foot) carbon fibre foam racing camp “cruiser” racing catamaran. Felix is 24 x 10 foot weighing 750 lbs and when ready for the water in racing trim with 3 crew and 300 lbs water ballast displaces about 1400 lbs. The carbon fibre rotating wing mast is 34 foot (ex Farrier 25 C) carrying 350 square foot mainsail area and a jib/genoa upwind. With the crew racks for the overall “beam” is 16 foot. This means a righting moment with 3 crew and water ballast is about 12,000 foot/lbs. The righting moment for a Farrier 25 C is about 19,000 foot/lbs.

Turning Point found the Farrier 25 C carbon fibre mast too heavy for Felix and decided to build a lighter mast section for racing. Now a BIG warning. The following mast design is for Felix and is a very near the limit on design. Turning Point are doing this mast for themselves, not a customer, so they are willing to take risks on the design and specifications. To give you the idea how different the old and new mast is, the new mast weighs with rigging 45 lbs, the Farrier F 25 C mast with rigging weighed 105 lbs for the same height of mast. You have been warned.

The new wing mast is a foam carbon fibre mast as per the specification’s jpeg. The mast was built in a mdf ply mold that did half the mast. The mast has carbon fibre with divinycell foam and resin infused epoxy. First half of the mast was molded then the second half was molded with a “flange” component incorporated. A central spine (web) is molded. Then all 3 components are joined together with a final wrap of glass outside. The mast is then post cured in an oven for 8 hours at 160 degrees. It helps that some of the Turning Point guys helped build America Cup foiling cat boats and foils so they know how to handle epoxy carbon and how to cook it. The second major assistance is they are within driving range of the Boeing seconds shop and can get some very cheap, very exotic material, very cheaply.

A very efficient and effective mast that saved 60 lbs of weight up high. Felix also had a stern stern inserted which added 2 foot to the waterline. This is a fast trailable camp cruiser that will scare many people silly, as it can sail faster than the wind. The jpegs give the idea.

 

If I remember , Turning Point was designed for the R2AK (race to Alaska) and was successfull !

 

The following cat is what happens when a man buys a second hand 34 foot Wharram remotely and finds when he finally sails the cat, it has significant rot issues. Solution, get a designer (Murray Isles Tasmania) to design a new cat and use the rig and equipment from the old cat. The new cat is 36 x 21.4 foot with an estimated weight 8000 lbs weight and a displacement of 12,500 lbs. If the original ketch rig was used the masts are 16 foot high round 110 mm timber or aluminium tubes with a 77 square foot jib, 151 square foot mizzen and a 181 square foot mainsail. The hulls are not a deep V but a rounded V allowing a bit more displacement and space at floor seat level. The length to beam is 7.9 to 1. The draft is 2.7 foot with a mini low aspect ratio keel to try and improve “performance”.

The guy also decided to change the construction of the hulls to strip plank cedar glass in epoxy. The bulkheads, decks and cabin are plywood (about 9 mm) timber. Basically following a Wharram type design but upgrading it as required to improve its “performance” and reducing the future maintenance. The crossbeam structure looks similar to the original Wharram Tangaroa design, solid timber about 290 x 145 mm “tied” to the hulls.

The accommodation is basic but effectively. One hull is an “owners hull” has a double berth, cabinets and a toilet. The other hull is the galley, seating (convertible to double berth) and a toilet.

The jpegs give the story and idea, I do not know if the cat was finished. If it was finished I would like to know if it sailed any better than a standard Wharram. The designer is good and has designed other multihulls so I would anticipate a reasonable cruiser.

 

I know very little about this catamaran done by Buckley Yacht Design in 2015. The CC 27 Magnificat is 27.3 x 16 foot with a weight of 6,000 lbs and displacement of 8,300 lbs. The mast height and sail area are unknown. The length to beam on the hulls is approximately 7.5 to 1. The keels could be low aspect ratio or daggerboards. The draft is 1.5 foot over the rudders I assume. This cat’s design concept is close to Derek Kelsall’s KC-27.

This cat was designed to have a lot of space below decks as indicated by the hulls wide beam. Each hull contains a 4.5 foot wide double berth. The headroom in the hulls will be 6 foot plus with about 5 foot in the main cabin. The galley could be in the hull or main cabin. The main cabin contains the dinette. The toilet is in a hull.

The construction is strip plank cedar, glass, epoxy and plywood. The Xray views indicate frames every 3 foot with floor boards and bunk tops providing longitudinal stiffening. Spacious, full bridge deck catamarans have a lot of structure to build and can take as long as a 30 foot, shorter bridge deck cats to build.

The first of these cats is being built in Britian and the dimensions of the cat indicate it can go through the French Canal system which would be fun. I suspect this design is ocean capable but it most certainly would be a very good coastal cruiser with OK performance with the right rig.

The jpegs give an idea but if your interested contact Buckley Yacht Design directly. The final 2 jpegs are Kelsall KC-27.

 

Threads like this led to some interesting discussions. I ran across one discussion between a long time builder of catamarans and a naval architect designer. A new aluminum cat design was being discussed. The cat design is 38 x 20 foot that displaces 16,500 lbs. Rig details unknown.

The guy who designed the cat did some FEA (Finite Element Analysis) on the design and then concluded from his reading that 20 frames and a 6 mm aluminum skin would be needed and was this OK. The naval architect said what are the statement of requirements that you are working to, as the structure may be to strong or weak depending apon the task you intend. The FEA guy was a bit unsure. The naval architect encouraged him to define his needs and review his FEA as if you don’t feed in worthwhile inputs you can get rubbish outputs. All civil so far.

For some background a 38 foot aluminum cat can have a 3 mm aluminum skin with a frame every 600 mm and stringer every 300 mm to the other extreme with a 38 footer having a 10 mm thick aluminum skin with a bulkhead every 6 foot and virtually no stringers. Pay your money take your choice. It depends on your intended purpose of the boat and building skill. The 3 mm hull is likely to be lighter but less capable of handling knocks the 10 mm hull would not notice.

Now the fun begins. A long term cat boat builder in effect said from his experience that he had more knowledge than naval architects / designers about what works in power cat designs and did his own design work based on his experience. Naval architect said if the design part is done correctly, you will get a better result. Boat builder said he has had to correct defects in several designs and as a result does not trust a designer to get it correct.

Both parties are correct for different reasons. The 100 year Skenes Elements of Boat Design is consider a text on how to design, but the basics of the book are built of years of practical boat building experience then some maths was applied. Modern naval architecture is built of years of maths, testing, understanding of materials and miles of sailing experience. A good naval architect designer who has sailing experience will design you a very appropriate structure to suit your needs that will be cheap to build with minimal guess work. A boatbuilder may get you there with an few deviations along the way, but be sure the vessel will not leak.

In short a naval architect is likely to get you to an optimised design result faster. A boat builder will get you a practical but may be less optimised result faster. Both have there place. But, please understand guess design is OK to about 30 foot after that please understand something about topics like panel stiffness, strength, buoyancy distribution and stress or you could get into real trouble.

The jpegs are the cat that started the discussion.

 

Re scantlings on ally cats, the cat in my avatar photo is 15 m long and a tad under 5 m wide. Her hull bottom plating is 6 mm, while the side and deck plating (including the vertical inboard sides of the hulls - they are assymetric) is 4 mm.
The frames are 100 mm x 6 mm flat bar, spaced 1000 mm, and the longitudinals are 50 mm x 6 mm flat bar, with one per section (eg one between the keel and the first chine - she has 2 chines).
And she has been in operation for the past 21 years with no structure problems reported.
The head honcho of the company that built her went bananas when he saw that she has 6 mm bottom plating - he thought it should have been 4 mm, and in retrospect now I am sure that 4 mm would have been fine.
She does not go offshore, and she cruises sedately along the coast at about 8 - 10 knots with passengers on board - she did achieve 26 knots though with 10 people on board on trials after new 150 hp O/B motors were fitted some years ago (she originally had a pair of 70 hp motors, then a pair of 115 hp motors, before moving up to the 150's).

 

Buy an existing boat and convert it to what you want is always an interesting option. But how much of a conversion is always the problem. I know of a guy who brought a 32 foot sailing cat and spent 3 years, nearly full time, converting it to a powerboat. The boat worked but he had spent so much money and time he had to sell it almost immediately. It took over 2 years to sell as it was not a “known” design and the $ received did not equal the $ spent. Next was a guy who purchased an old 27 foot Stiletto catamaran, extended to 30 foot, put a bridgedeck cabin on it and went sailing with its original rig gear etc. for $6,000. He did all the work himself and hopefully sailed it along coasts as there was no designer involved. Stilleto 27’s have strong hulls and crossbeams but were never intended to have full bridgedeck cabins etc on them, as the rig and rudders were designed for a lightweight cat. The following tale also brings up other issues.

Start with a 34 x 22 foot weighing 5800 lbs open bridgedeck lightweight coastal racing catamaran called “Intoxication” then convert it into a 42 x 22 foot weighing 9400 lbs offshore bridgedeck cabin cruising catamaran. This minor modification only took 3 years of professional boat builders time and involved cutting the boat in half, inserting 8 foot in the middle, strengthening the entire structure, obtaining a “new” second hand 68 foot carbon mast which was cut down to 53 foot to fit the lengthen cat and upgrading sails and gear to match the new requirements.

The jpegs below give some idea of the modifications required. The cost of this unknown, but I could easily imagine you could have built a new 42 foot cat for similar money and brought a second hand 42 footer for less. The really troubling part of this story is the need to “strengthen” the structure. It is not just about an additional layer of glass on the hull. It is the extra reinforcement along the keel lines, upgrade of rudder systems, chainplates, engines etc. But the killer would have been the upgrade of the crossbeam structures to handle the larger weight and increased rigging forces. If not done well you just have wasted a lot of money and put yourself in danger.

Putting an extra few feet out the bow or stern is not a major problem but significantly modifying the length by cutting a boat in half and adding 60% to its weight is a major design and build undertaking. Please evaluate the real cost of doing such an task as you may find it to be a very expensive way to get what you want.

 

I've seen a Gemini catamaran converted to outboards with rig removed. The owner sailed and as age caught up made the conversion. He was happy with the result.

 

Scuff. Agreed there are many successful conversions of multihulls to change them from their initial form. Some are minimal changes to major conversions. When you start cutting boats in half, adding weight and having to rebuild many components to strengthen them the value for money equation starts to look questionable. The only person I know who makes a value for money difference by cutting a power boat in half inserting and inserting EG an extra 20% of length then relaunching it is a builder in Florida. He can actually increase the speed of 60 to 90 foot mono power boats with the same engines because he understands pressure distributions along the hull. Some boats are just overpowered and too fat for their initial designed length. Back to more humble stuff.

The Cat 24 is a simple catamaran for day or coastal sailing. The cat is 25 x 14.9 foot with a weight of 2,360 lbs and a displacement of 4,200 lbs. The fractionally rigged 32 foot mast carries 210 square foot mainsail and a 90 square foot jib. The dory bottomed hull length to beam is 11.5 to 1. The draft over the spade rudders is 2.4 foot. The hull width at the gunnels is about 3.75 foot. There is no mention of the keel options but this design could have low aspect ratio keels or daggerboards.

The accommodation is claimed to be 6 persons for day sailing with limited accommodation. Probably 3 berths loo and small cooking area. Headroom in the hulls is 5 foot over about 4.5 foot of hull length.

The structure of the cat is plywood with timber stringers and ply bulkheads (6 and 9 mm). The main and aft beams are plywood boxes reinforced with timber. There is a need to laminate some plywood panels EG the forward beam underwing but majority is flat panels. There is a solid floor in the cockpit. This design could be adapted to foam glass panels although the designer mainly speaks about plywood.

This cat should perform reasonably well when lightly loaded. The cat is capable of carrying a good payload and has for example a 250 litre water tank (weight of water 560 lbs). So as a mini coastal cruiser you could carry a reasonable amount of stuff for a few weeks.

A simple design that should perform well. The jpegs give an idea. The designer comes from Bestboatdesign.com