Multihull Structure Thoughts

The Windrider 16 is a production trimaran originally built by Windrider in the USA. It was designed by Jim Brown and first built in 1995 and constructed in roto molded Polyethylene with detachable cross arms and had a carbon mast. The design was for a one person fun day sailor. Curtis Patzer from Canada decided it could make a nice camp cruiser. This is about the modifications he did to achieve an overnight to a week long cruiser.

The Windrider 16 is 16.58 x 12 foot can be disassembled by removing cross arms to about 6.5 foot. Weight is 250 lbs with a displacement of 750 lbs. The cat rig has a freestanding 24.7 foot carbon fibre mast above deck with a sail area of 82 square foot. The maximum draft is 1.3 foot over the keel and rudder.

The modification Curtis did was mainly controlled by the maximum payload of 500 lbs. First, he constructed a 9 mm ply platform supported by 25 x 75 mm framing on one side of the tri over the trampoline. This platform is lashed to the cross beams. On the opposite side is a narrower ply platform to accommodate a 100W solar panel and 1hp electric motor (a 1.2kWh LiFePO4 DIY battery sits in the vaka behind the seat). Next a small, light backpacking tent (Eureka Midori 1) with a side door and vestibule was obtained to provide shelter for the wider plywood sleeping panel. The final requirement was dry food, water and a 12 pack cooler stayed in the main hull under the spray skirt, and everything else went on the tramps in dry bags. Curtis did several trips of 2 to 5 days over the summer, covering about 200 miles, and had a great time.

His is a nice little camp cruiser for shelter water cruising. In the US Windrider 16 tris can be found second hand for under $3000 and the full fitout would cost less than $1000. A very cheap cruiser for one. Thee jpegs give the idea.

 

Oldmulti or any of the NA's tuning in ,
a theoretical question regarding 2 x 38' multihulls :

a) has 2 hulls, an open bridge deck & 12 - 1 beam/length hulls
beam 22'
b) has 3 hulls, 2 open decks & each of the 3 hulls is 15 - 1
beam 28'

Other than that they are identical, rig, sail, weight, et cetera, et cetera
If the twin hull version does 16knots under sail, what speed would the 3 hulled version be doing ?

 

Peterbike. This is a very general question. So, a general answer. The tri is likely to have a higher top speed in stronger wind due to more stability and if designed as a racer likely to have a higher average speed in lower winds due to less wetted surface. In moderate winds or just short of hull flying the cat may be competitive.

The real issue here the hull shapes and the wind wave and conditions they are designed for. To get a real answer you would need have some fairly accurate hull lines and have them assessed by a velocity prediction program.

 

Ah, apologies - I did not supply enough information.
Additional to above :
I am not thinking tri versus cat.
Think, a pod cat where the "pod" is the same size as the 2 outer hulls,
Or, a cat that got split down the middle & had another hull inserted central with a 15/1, b/l ratio & then while apart the o/a beam got stretched a bit.
Being a 'cruiser' hull lifting is not on the agenda - the windward hull may get close to leaving the water at most.
So, the drag/wetted surface area is increased 50%
Not concerned atm about it being a 'wrong' design - it is a thought exercise/learning moment.
I know design may/might be optimised to increase sailing abilities, but that is for another time/learning moment.
Thanks.

 

Explanation re thought/learning exercise : Eg.
Regarding round bilge vs. hard chine etc. many say "you must go round bilge because of drag"
But if we look at drag coefficients, we see that multichine is only slightly behind round, & that hard chine with the chines chamfered a bit - is only a little bit behind multi chine.
Result - chines do work with a slight drag penalty, but chines hulls generally being faster to build & fit out.
Some choose m/chine for convenience/speed of construction & accept the slight loss of efficiency.
Why wander down this path ?
There are racing cats that are getting long in the tooth design wise, they are useless to be recycled as cruisers because they have no internal space.
One way, to get around this is to add another (central) hull - the central becomes the living area & the 'floats' become 2 x stateroom.
No cockpits in any hull, side decks only.
No foils in centre hull - bare hull only.
It is said NO to 3 hulls because of drag, BUT what is the drag ? Is it acceptable for a cruising multi ?
There are compromises everywhere in a boat to get what you want within a certain size/budget.
So, back to the original question : "as above"

 

Peterbike. What you appear to want is a cat or a tri more suitable to be a fast cruiser. Is the length to beam of hulls, the stability, chines etc are the major influencer or could they have an impact on a potential design option. I will try and add to the confusion to provide some information.

Length to beam is important when they are lower than 8 to 1. EG 6 to 1 has a negative impact. When the length to beam is greater than 8 to 1 hull form drag reduces marginally IF the weight of the boat is appropriate for the hull length to beam. If you have narrow hulls and an overweight boat the hull drag could increase. As Lock Crowther and other designers have found who had 12 to 1 catamaran hulls originally then designed 10 to 1 catamaran hulls with fuller ends the cats of a similar weight and rig would travel at the same or higher speeds.

Translation: the theoretical advantages of an EG 15 to 1 length to beam may not be gained if you have a heavy cat or EG as Woods found, if the hulls on a sailing cat were to close together the wave interaction of the hulls increases drag.

As for chines, agree chines if done well have a minimal impact on total drag and make building easier BUT resale value is down even if the hulls are done in very good materials. If hulls are done with flat panel sides and have a round bottom have a minimal impact on total build time. Woods says a good daggerboard will gain you more than the drag loses of a chine hull.

Next do not underestimate windage of hull structure and efficiency of a rig. Boats that have good rigs and low windage hull structures can perform well with average hulls. But the real truth here is told by EG Wharram’s 63 foot Gaia design. It has moderate hulls and a reasonable rig. The design can top 17 knots in open ocean. Good. The average speed across oceans is 7 to 9 knots. It can go faster but people sail slow for comfort. Yes, the latest carbon fibre 50 foot plus round bilge slim hull cruising cats can average 12 knots plus across an ocean in some (not total) comfort but even the owners of those cats slow the sailing speeds back to satisfy a spouse.

Look at the totality of what is required and optimise a design for that. The fastest hull shape with the most efficient rig on a low windage structure may end up being a useless cruiser. Several people are now finding a foiling cruiser with a wing mast rig etc may not have been the wisest choice.

Any other opinions?

 

This is about a small cheap tri built by Dean Hoskin from NSW Australia. Dean wanted a cheap simple tri to do day sailing on lake MacQuarrie. Dean had sailed before but wanted to create his own boat and choose a trimaran design. He said, it’s an experiment so make it cheap and easy to build and make it transportable by roof rack on a car.

The tris dimensions are not published but it is 8 foot long which is the length of a sheet of 8 x 4 foot plywood. The mainhull is a single chine dory shape. The floats are 8 foot long with an asymmetric shape for lateral resistance. The mast step for the aluminium pipe mast. The mast looks about 52 mm diameter. The sail is a home made plastic sail with some battens. The sheet for the mainsail leads though a cabinet door handle aft as a “turning block”.

So, the materials used in the build. To quote Dean “It’s made from the cheapest 3mm plywood I could get.” The cross beams look like 2 layers of 19 x 75 mm bolted to the floats and main hull. The deck mast structure looks well built and the free standing mast base is a timber block glued to the hull bottom. The rudder is controlled by ropes to foot pedals allowing the hands free to control the sails. A simple paint job with house paint was done.

The tri sailed well for a small boat but it had its limits. Small waves and a max of 20 knot winds. On one trip there was a large crack heard and an immediate return to shore happened. The tri had a limited life after that and the cheap ply rotted.

Tomorrow we will discuss Deans next trimaran, again cheap but 10 foot long with improved sailing capability. The jpegs of the 8 foot plywood version is shown.

 

After the disintegration of the plywood 8 foot tri Dean Hoskin decided to build an improved second version of his trimaran. Again, a single person simple to build, easy to sail and transport trimaran.

The new version is 10 x 8 foot with a 2 part windsurfer mast and a Laser mainsail. The main hull is longer but the major change was narrower symmetrical flats with low aspect ratio keel. The weight was similar to the previous 8 foot plywood tri as the 10 foot version is built in foam and has permanent crossarms.

The build again is cheap using what looks like 50 mm Styrofoam which is glued and bogged into position. I do not know but suspect a light e-glass (200 gsm?) covers the external surface which could be finished with epoxy. The low aspect keels on the floats are plywood and bogged on with some glass reinforcing. The mast step for the free standing mast is a PVC tube. The cross arms look like a flat timber plank. The rudder came from the 8 foot plywood tri.

The tri sailed better than the 8 foot version handling chop better and being faster. Still an inshore boat.

The jpegs give the idea.

 

This is a response to an enquire on another Blog which I am dual posting here. The Piana 30 performance cruising catamaran was built by Naval Force 3 shipyard in France. It is capable of Atlantic crossings with a good crew and moderate conditions.

The Piana 30 is 29.5 x 16 foot with a weight of 4480 lbs. The rig is a sloop of unknown area. The draft is 2.65 foot over the low aspect ratio keels and rudders. The power is 2 x Honda 10 HP outboards.

The accommodation has a double berth aft in each hull. In the port hull is a galley and in the starboard hull is part of the toilet area and toilet. Forward in both hulls is a single berth. The bridge deck saloon has a large dinette capable of 6 people. The headroom in the main saloon is 5.75 foot The cockpit is comfortable. There is storage in the forward wingdeck lockers.

The build is mainly plywood and timber. I do not know the structure but imagine EG 6 mm for hull sides, 9 mm for hull bottoms and decks and 12 mm for underwing and some bulkheads. Internal furniture acts as structural stiffeners throughout. A well thought out structure.

From the limited comments I have seen about sailing performance the owners were happy on all points of sail.

The jpegs give the idea.

 

Hi. This is going to be in 3 parts as Jim Gallant the builder design of a series of trimarans developed some fun tris for sailing around his local area. The first tri is very simple and will not require much detail.

The tri is 20 x 17 foot using a Solcat18 rig. The main hull is a Solcat 18 hull with 5.2 Nacra Cat hulls as floats. The daggerboard from the Solcat 18 is used in the main hull.

The main hull and floats had some strengthening with simple bulkheads under the cross beams. The cross beams are cut from a mast section from a mono hull. There was some additional fiberglass work around the beams but minimal other modifications. The only significant change was the support wires for the rig.

The tri sailed well after a little tuning and work out where sheets should lead etc. The tri initially proved to be a handful until all the secondary lead positions and handling bugs were worked out.

Now Jim was inspired as to what speed he could achieve and then saw the final jpeg with Eric Tabaly and said I could do one of these.

The jpeg shows the tri followed by Eric Tabaly tri. There is a video of Solcat Nacra tri sailing along very well.

 

I will continue Jim Gallant tris tomorrow but something else attracted my attention. A 6 day 22 hour run across Atlantic done by a partial foiling boat. Excellent. The fact that it is the fastest monohull “cruising” boat to achieve this time is amazing. The fact it is considered faster than EG the 100 foot racing Commanche monohull on all points of sail is incredible. The boat has no problem doing 25 knots for hours at a time and they don’t go above 30 knots as the boat is reaching its structural limits. The boat is “Raven” which we spoke about in the development stage earlier in this thread. “Raven” is a 111 foot mono with AC 75 monohull foils attached.

Raven is 111 x 24 foot with a displacement of 55 tons (123,000 lbs) with 20,500 lbs of bulbed keel is hung on a fixed 16.4 foot long fin. The 2 x 23 foot long foils also have ballast. There is 22,400 lbs water ballast if required. The foils lift about 50% of the boats weight which reduces the wetted surface and reduces the heel to about 10 degrees when powered up. When the rig is fully powered up the potential top speed is near 35 knots but the crew try to sail at a maximum of 30 knots due to structural limits.

The build is carbon fibre foam and EG Titanium, light weight interior fitouts, chamfered deck edges etc. There was even special light weight paints and build techniques (in full moulds) that reduced the paint weight by 400 kgs. The design by Boltin and build of “Raven” took Baltic yachts 2.5 years to do. Next all that was required was 17,000 miles of sailing to get the computer software to trim sails to match the foil flap angles to get the optimum stability for speed and sea conditions. Whilst racing they still have to have a full time foil trimmer and sail trimmer 24/7. PS When racing the engineer/hydraulics person is working hard as well. Back to the build, start with a full size timber mockup which is used to make full female moulds. Now Baltic Yachts build skill comes to the fore and they built a boat that weighs half the weight of other 110 footers they have built. This boat has real accommodation on board. Think about a 200,000 person hour plus from conception to racing across the Atlantic. Yes, we are talking 20 plus million and the owner is willing to replace the rig when the crew think an upgrade is required.

Performance is high performance multihull category and Raven has not been fully tuned yet. There will be more of these boats coming soon and will be a real threat to multihulls if the idea works in small boats. The AC 40’s are already working well.

The jpegs give the idea.

 

I think it would be interesting to see Raven line up against Comanche, or even a current generation IMOCA, but her performance in conditions that were close to ideal for her wasn't awe inspiring. To put it in "high performance multihull" territory is very generous. She was beaten over the line by two 20 year old design trimarans (MOD 70) that are over 40' shorter. The trimarans were about 2 days ahead - or about 30% faster.

None of the Ultim Trimarans were racing, but they're an order of magnitude faster again than the MODs - and still 11' shorter than Raven.

Granted, the tris are bare bones racers, and Raven has amenities, but I'll wait to see the performance of the Ferrari Hypersail before I start comparing offshore mono and multihull racers.

 

Jim Gallants trimaran research for a mini Hydroptere foiler was done after he had built a canoe trimaran. He saw the Eric Tabarly small foiling tri which he developed “Paul Richard” a semi foiler 54 foot long for ocean racing. The first jpeg is a study of Tabarly small tri followed by a sketch of Paul Richard (which was built in aluminium). From these inspirations and Hydroptere (jpeg 3 and 4 in an upgraded version know as DCNS) he stated to build his Solcat 18 foiler.

Jim used the main hull of the Solcat 18 then built 2 floats and put foils on them. Sounds simple but as you will see there is a lot of work and thought behind each component. From the information found so far the tri is 18 x 20 foot with a weight of about 350 to 400 lbs. The rig has a 190 square foot mainsail and a 88 square foot jib. Each foil weighs about 35 to 40 lbs. The foils are 7.5 foot long, 1.9 foot wide at the widest point, and 0.6 foot wide at the tips. The section of the foils is NACA 0012 profile with the thickest section of the foil being a third of the way back from the front. It is a simple effective shape but better more complex foil shapes are available but harder to build.

The tri could foil it in about 10-11 knots of wind single handed and 12 knots with a buddy. Due to where Jim sails a foiling reach may only last a minute before having to jibe. So, when the wind is less than 10 knots, it’ll fall off a foil. Top speed is not really known as the tri has never had enough space to reach a top speed.

Thee build of the tri is the Solcat 18 main hull with some reinforcements, aluminium cut down mast cross beams the floats, foils and aluminium connection points. The floats are 3 mm plywood skins with foam carbon fibre bulkheads. The float shape is based on the foils lifting the float clear of the water and floats only having enough buoyancy to support the tri in light winds. The foil build was of a sandwich construction of the following on both the top and bottom surfaces, working from the outside in:

• 2 layers of E-glass on the outside
• Next layer in, biaxial carbon fiber for twist resistance
• 2 layers of uni-directional carbon fiber
• 4 mil Okume plywood panel on the bottom, 3 mil Okume plywood panel on the top
• 1 layer uni-directional carbon fiber inside the plywood layers
• Hardwood stringers and divynicell panels to fill the interior cavities.

Not an easy build. As you can see from the jpegs the float to foil aluminium attachments involved a lot of work although the components look easy the alignment of the components would have been hard. 1 degree of difference in an angle of attack on the foils could be a real problem. The T foil rudder was another piece of good initial engineering and build. Jim did some excellent work the foils etc.

The 18 foot tri was sailed for a while then Jim said I can improve this and we move on to the next version. The jpegs give an idea of the 18 footer (and some of Tabarly and Hydroptere) work.

 

The final instalment of Jim Gallants home built and designed foiling trimaran. After the Solcat 18 version Jim was still not satisfied with the overall performance. The 18 footer could fully fly but was a little hard to handle and there was insufficient buoyancy in the main hull when not flying so Jim got a second hand Boston Whaler Supercat 20 catamaran hull for $150 an started from their.

The overall 20 foot version was 20 x 22 foot with a weight of about 500 lbs. The sails were larger and made by a professional sail maker to suit the tri. The foils were the same basic dimensions but were made stronger.

Now we get to the improvements. The crossbeam is a 20 foot 115 x 3.2 mm wall aluminium tube which was stronger than the old mast crossbeam from the 18 footer. This was then attached to the 20 foot main hull. The foils were remanufactured and strengthen. The steering system was redeveloped initially with a steering wheel which proved to be confusing to use on each tack and to fast in response so another push pull system was developed connecting to a tiller arm which worked much better.

Next came an improvement that worked but proved to be structurally weak. The floats on the 18 footer were solidly fixed to the cross arms and foils meaning when there was movement in the floats it would alter the foil angle of attack. Jim on the 20 footer let the floats rotate on the crossbeam but had the foils held solidly on the crossbeams by an aluminium brackets.

Now I will quote Jim Gallant: “Yes it was incredibly fun to fly over the water, but the boat kept breaking, though normally only when 2 people were sailing it at the same time. There are terrible forces at work when foiling a 500 pound boat and 400 pounds worth of people at near 20mph speeds. Aluminum cracks.”

Translation. The 20 foot foiler was a much better trimaran foiler but even though it was stronger and more buoyant and capable of 20 knots, the forces on the tri were large and parts cracked or broke.

Jims next trimaran was to be a non foiling trimaran. Jim then took up kite board foiling for fun.

An interesting development path. The jpegs give the idea.

 

Hien Ngo found his windsurfer Kerma Sailbord was a little unstable in light winds so he tried to improve the situation to allow him more sailing time. He thought a trimaran version would help. He had an old Bruce Jones board that could be cut up and used as stabilisers. The resulting trimaran worked well enough that Hien could do more sailing instead of swimming.

The over concept is 10.6 x 5.2 foot with the main hull board 2.2 foot wide. The weight is unknown. The sail area is a 45 square foot windsurfer rig. The rig is underpowered but works well. He plans to go for a larger rig later. There is a dagger board in the main hull.

The construction is a Kerma Sailboard (10.6 foot) main hull complete with 4 bolt connection points drilled in the main deck to hold the cross arms with bolts. The floats are cut from a shorter Bruce Jones sail board. Short connecting aluminium cross arm pieces are then pushed into the foam of the cut out pieces of the Bruce Jones board. The cross arms are cut down pieces of a wind surfer mast that slide over the short tubes inserted into the Bruce Jones floats. The windsurfer cross arms are then bolted to the main Kerma board. Result a simple tri.

The board and sail combination is much more stable in any wind speed and performs reasonably well.

The jpegs give the idea. A fun approach to sailing.