Multihull Structure Thoughts

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

  1. oldmulti
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    oldmulti Senior Member

    The following is about a 37 year old design and build of a transportable 25 foot trimaran. Please read on, I assure you will learn many things EG just a side light about the FT 35 foot racing trimaran: “The mainhull LWL was only 28ft. Much knowledge was gained, especially that the rudder was trying to steer the bow of the float which was set 13ft to one side and 35ft ahead of it. This put a huge load on the rudder which broke on several occasions. Kelsall said that ‘more was learned from this design than any other, before or since’. In particular he stressed lessons about rotational momentum relating to weight, distances and centres of gravity.”

    Back to the “Estuary” tri. It is 25 x 17 foot that can be disassembled to 8 foot for trailing. The tri weighs 2800 lbs and displaces about 4000 lbs. The 33 foot fixed mast with stays has an integrated boom and jib boom unit that rotates around the fixed mast to produce a variant of the “Swing Rig”. The jib has a self tacking track on the forward jib boom. There is a reacher sail on a separate forestay furler. The mainhull length to beam is 8 to 1. The float length to beam is 13 to 1. The draft is between 2 foot and 5 foot depending on if the rudder and 1 or 2 of the mainhull offset daggerboards are down. The tri has a forward trimming daggerboard as well as an aft main daggerboard. The adjustable rudder is a design wonder as is the tiller

    The hulls are round bilge with steps in the mainhull to add room above the waterline. Now the hull design is a product of the time (35 years ago) and could be upgraded to a more modern shape. The designer builder of the 25 foot tri had spent 5 years building a 35 foot foam glass tri which he sailed around Britain and was influenced by its shape.

    The structure of the tri is mainly flat panels polyester foam unidirectional e-glass panels with round bottoms attached. The cross beams are foam glass 180 x 180 mm boxes with unidirectional flanges top and bottom. There is a strong forward fairing on the beams to add strength and handle the impact loads. His comments about beam design especially metal beams are very informative. The beams are “bolted” to the main hull and floats to allow disassemble. There are solid adjustable waterstays for additional support. The web site I will send you to has a very full description of how to build flat polyester glass panels. The boom and forward jib boom with an integrated rotating bearing around the mast is mainly solid glass with thicknesses of up to 12 mm of solid glass.

    The internal accommodation of a centre cockpit tri is good with 5.9 foot headroom around a very good galley. There are several real berths and a separate toilet area. The seating and table are small but useful. The entrance hatch is offset to one side from the practicable cockpit. The tri even has an inboard BMW D7 diesel with a folding propeller.

    The owner says the tris performance is good upwind but due to the “Swing Wing” rig it can out reach and run most 35 foot boats. He is very happy with the self tacking jib feature.

    You will need an hour to read his web site which gives further detail and his logic behind each decision. Plus there is good build technique information. Some of the sidelights about design and maintenance of other tris is very good. The web site is 1. GRP Estuary Cruiser. The Boat http://www.moonshinepublications.co.uk/index.php/grp-estuary-cruiser/1-grp-estuary-cruiser-the-boat

    Explore the navigation guides at top of the pages as they have many sub pages to read. A very interesting design.
     

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  2. oldmulti
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    oldmulti Senior Member

    Part 2 of the 25 foot Estuary cruiser.
     

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  3. bajansailor
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    bajansailor Marine Surveyor

    True, or very practical, but it sure as heck is very cute :)

    "I need a bay boat now, for shallow water work. Been considering Wood's 18' Skoota a bit, but those tunnel hulls (almost catamarans) are awfully tempting. I wonder if anyone has ever gone to the extremes in designing cat hulls with a lot of flare and thus reserve. The idea is taking out 6 people and not dropping too far to run skinny. But, a thread creep here"

    Please do start a new thread about this boat - I am sure it would become as epic as @DogCavalry Sea Sled build.
    And post some more photos of your Skoota please.
     
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  4. oldmulti
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    oldmulti Senior Member

    Fallguy. The Skoota 18 looks a reasonable design. Glad to see you have a waterfront property. I know a lot of people who have 25 foot plus "trailable" multihulls that do not trail them after the first few attempts. They may do it once a year to shift locations but once it is set up for water they find somewhere to leave the boat fully assembled on water. 30 plus foot masts are not fun to put up on a daily basis. Yes there are cats with a lot of flare in their hull like Wharram deep V models but they pick up a lot of wetted surface fast as they are "overloaded". Its better to have something designed to carry the load from the start that may have a wider hull but will be able to carry the overload better. Good luck on the new project if you start. I hope you can build the wharf soon so you can use your 32 foot cat.
     
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  5. guzzis3
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    guzzis3 Senior Member

    I've said this before. I wonder if I should bother repeating.

    If you are buying or building a multihull the boat's payload is from the bit of the hull underwater. For given $ paying for longer wider hulls increases payload. Bridgedeck cabins don't. Unless you are limited by berthing costs registration etc a larger open deck cat is a better load carrier per $ than a bridgedeck cabin cat.

    But people still buy bridgedeck cats, which is fine when they are well designed, but some have too little bridgedeck clearance.

    And then there is the Horstman trimarans. Incredible accommodations for the size of boat. Decent payloads also. I would not be surprised if someone put a modern twist on them with a fractional rig and a few other changes. It would have to be an attractive package.
     
  6. redreuben
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    redreuben redreuben

    Guzzi, a Horstman with an attractive rig is a Cirro.
     
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  7. oldmulti
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    oldmulti Senior Member

    A short one about dreams versus reality. Dream a trailable multihull that can be set up in under 1 hour, sails well and has several berths, galley, dinette and full standing headroom. The reality is the farrier type trimarans need to be 30 foot plus to achieve the above. But the launch of a Farrier eg 31 foot means you are trying to pull up a 43 foot 150 lbs mast, this needs coordination and calm conditions.

    There are many foldable tris but those who have tried to have foldable trailable catamarans have had more difficulty and compromises. The best are Richard Woods Wizard, Sango and Skoota catamarans. They fold unfold as you launch them. The next best is the Maincat 22, Thomas Firth Jones Brine Shrimp and the Simpson Takeaway. All of these cats are below 26 foot.

    Other attempts at larger trailable cats end up with virtual rebuilds to get on or off a trailer. These are at best transportable. The only exception is the 36 foot Cat 2 Fold. The problem with all the large cats is the narrow hulls which man you can get some accommodation and headroom but you feal as though you are in a tube. Maybe the best compromise I have seen as an effective trailer sailor for inshore sailing is Kohler Maxi cat.

    But the thing that really peaked y intertest was the first trailable trimaran jpeg. That is a Crowther Twiggy 32 footer that when assembled has a 29 foot beam. The floats, crossbeams, mast would require a crane to move them around or a crowd of people. The eight 9 mm 1x 19 wire water stays alone would be heavy.

    Please be realistic about what you want. I knew a guy who had a F31 and he trailed it a bit but his ocean crossing dreams did not happen, his trailing dreams were painful especially on the road where fuel consumption and turning tight corners caused issues let alone mast raising. He eventually sold up and settled on a F25.

    Some jpegs give an idea.
     

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  8. guzzis3
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    guzzis3 Senior Member

    Laughing :D As far as I know they only came in one size ? but a foam cirro wold have to be an attractive proposition. There was one here in SEQ which broke. I thinkn it lost a float or something. Only one I've hear of so I assume misadventure, maintenance or whatever. RC is still in business I believe.

    I was actually thinking if you took a scarab 32 you could re-engineer that to lose folding and have bunks on the wings. THAT would be a very nice boat.

    oldmulti: It never ceases to amaze me that people put so much thought into spreading and connecting the hulls yet there is so little thought given to raising the mast.

    It's like the obsession with hull skins. No one thinks about all the time it takes to turn a basic hull into a functioning boat. It is well worth looking at Rob Denny's efforts in this area.

    Incidentally I am on a trailer sailer forum and they have some wonderful discussions on getting a mast vertical in a controlled safe and timely manner. The mast mate uses rubber bands to take the load and you just lift it controlling it as you go. There is also a self correcting rope bridle which keeps the mast centered and the jin pole also.
     
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  9. redreuben
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    redreuben redreuben

    Here in WA sailing is divided into two tribes, river and ocean, each have their attributes, but for river sailers to go to the ocean the mast must come down, the trimarans I crew on, sometimes go on the trailer, sometimes go down the river and the mast comes down afloat, either way its an operation, the mast on an 8.5 is a beast of a thing to wrestle down dodging the ferry wakes etc
    I agree with Guzzi3 lots of attention to folding not much attention to masts, might be a good topic for awhile.
    Might be interesting to cover tuning as well as most of it apart from the diamonds has to be reset. One boat I sailed on uses a strain gauge.

    edit; Loos Gauge. Professional Models - Loos & Company, Inc. - Cableware Division https://loosnaples.com/product/tension-gauges/professional-models/
     
  10. jamez
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    jamez Senior Member

    Speaking of mast handling the Cat2fold video shows the mast stepping procedure in some detail, as well as set up of decks etc.

     
  11. Russell Brown
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    Russell Brown Senior Member

    Are you talking about a Cirrostratus tri? If so, the "Cirro" is a beautiful bird in comparison to a horseman design.
     
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  12. oldmulti
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    oldmulti Senior Member

    Mast lowering and raising on a trailable boat (mono and multi are similar but there are variations) is a multi dimension problem. Lets start with some history. Thames barges, 150 years ago, were EG 85 foot long displacing 100 tons and carrying 70 foot long solid timber masts. The barges went up and down rivers with bridges over them. The barges had to raise and lower there masts to go under the bridges. While sailing, the skipper and a single (often a boy) crew could sail to the bridge lower the mast use the boats momentum to push under the bridge then raise the mast to sail on. How. The 1000 lbs mast/sail has a counter balance weight below a tabernacle pivot point.

    With a suitable amount of ballast in the mast, raising and lowering is almost effortless. If you wish a 5 year old could operate a simply installed geared flagpole raiser at the pivot. This is a large (2 or 3 ft) diameter rack gear with a geared pinion and crank to take ant effort out of the drill. Now this system works if you can afford a 1000 lbs plus counterbalance weight below the tabernacle point. In short, the only modern version of this I have seen is in Phil Boldger creative monohull designs.

    Next is the trailer sailor crowd. Smaller boats with light masts depend apon a few wires and human push power to raise the rig. An EG 50 lbs, 25 foot mast is relatively simple on a low wind day. After that you need a structured plan to control the mast raising process. Now, you will be amazed at what can be done. The jpegs of the Fastnet winning Rolly Tasker 77 foot Siska monohull offshore racer with a 90 foot mast sailed on the Swan river and often went offshore racing. As Redrueben said there is a bridge between the river and sea. Siska could lower its mast as it approached the bridge, go under the bridge under power and raise the mast in 5 minutes. Notice Siska rig. The mast has its caps and lowers in a single plane so the entire rig could pivot for up and down with its boom and winches. The mast depended on runner and lower forestays for fore and aft control below the forestay.

    There is a 45 foot monohull ketch that uses a jockey pole on the fore mast is in the third jpeg. I suspect this boat is “Yankee” that sailed through the French canals and was feature in National Geographic magazine. It would be a process but it was possible to do by the boats crew. When the mast was down shifting the masts around would require a lot of friends as the main mast would weigh at least 400 lbs.

    Now we need to discuss a feature required by larger boats that raise and lower masts. The deck either has to be relatively flat for and aft or the mast has to pivot on a tabernacle which allows the mast to lay relatively flat for raising or lowering. Next the mast base needs a structure that can have eg a bolt that can allow the mast to pivot on.

    As PAR a yacht designer says “I've seen and used counter weight arrangements, but unless you live on the canals of Europe, they are much too heavy on anything but a 20' pocket yacht. The easiest arrangements I seen, used and designed hinge the mast aft and use a tackle on the headstay or at the base of the mast. If necessary, a whisker pole is used, if the mast has to come way down. An A frame option can offer more control. The tabernacle design is key and the higher the purchase on the mast the less advantage you need to hoist and lower. I have a 25' mast I raise by tackle on a 4 foot tall tabernacle, when I launch it. On the 5 to 1 tackle I use to hoist (or lower) it's initial line pull is over 100 pounds, so a hand cracked trailer winch is used. If the pivot point wasn't 4 foot off the deck, say only one foot, I'd need a nearly 500 pound pull on the same tackle. These systems are pretty easy to work out, but the engineering needs to be worked out carefully, as the loads can be huge. The tabernacle mount and strength/stiffness need special attention as do the hard points for the tackle purchases.”

    This is part one of a 2 part effort, more on multi hull masts tomorrow. Jpegs give some idea.
     

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  13. oldmulti
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    oldmulti Senior Member

    Part 2 of mast raising and lowering. Multihulls have their own mast raising and lowering issues. If you have a non rotating mast on a multihull that is at full width when you raise the mast it is no harder than a monohull. But may multihulls have rotating mast fractional rigs. This adds extra complexity, as often the cap shrouds are not in line with the mast. The diamonds are swept back making the mast a difficult shape to move around and the mast base needs often is set on a ball shaped mast step to allow rotation. Also, in the case of folding multihulls that unfold on the water the mast needs to be raised on land and held in position until the EG tri is unfolded on the water and the caps attached to the floats. The problems grow larger if you have larger chord wing masts as their shape is not conducive to simple mast base attachment.

    So, let’s talk about what is possible. The first jpeg is of “Pados”, a Creative 42 design that has a rotating wing mast that can be raised and lowered by one person, although the crowds this attracts always provide additional assistance! Many Farrier tris raise and lower their masts regularly. The process varies depending on the size but a mast base that has a attached plate for the EG ball to insert in the rotating mast base allow the mast base to be under control. Then either a boom attached to the mast or a whisker pole allows the mast to be raised. In some cases temporary wires either side of the mast controls sideways movements. In other cases a temporary support pole is set up to provide a lifting point for the mast. If your rig has such luxuries as rod forestays that cannot be bent using a boom lift is the best. The jpegs sow some of the variations.

    The jpeg diagram of the mast raising of a Seacart 26 involves an addon temporary A frame to provide a mast lifting point to allow lift the mast. This is mainly an on land mast raise prior to launch.

    On water mast raising and lowering is possible if you set it up correctly. The jpegs of the Seawind 24 catamaran shows what is required. This Seawind has a rotating mast of about 150 lbs weight that can be lowered and raised by one person on the water to get under a bridge. The mast step foot of the mast has a permanent structure connected by a bolt. There are adjustable ropes on the mast caps to provide side movement control.

    As with all these systems the important thing is the design and build of the mast step mast base combination. This needs to be well thought out and strong. Second you need some form of lateral control of the mast as it rises or lowers. Wing masts can generate a lot of power even in light winds which needs to be controlled. Bolts or shackles that attach rigging need to have “captive” pins or nuts or you will have a raised mast with you running around trying to find a lost nut at the wrong time. As Redrueben said you also need to able to tension the rig to correct tensions for efficient and safe sailing. Finally please check all your lifting rope connections and shackles prior to a lift or lower to ensure you have control as it happens.

    Be careful folks 150 lbs dropping on someone head is not fun. The jpeg give an idea.
     

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  14. guzzis3
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    guzzis3 Senior Member

    The wizard/Sango system might be worth a mention. Admittedly their masts don't rotate.

    The mast attaches to a U shaped slot at the front of the cuddy cabin with a simple bolt through the section. This holds the mast in line as it's raised. You can even have the mast left vertical without the shrouds in place, but obviously if you tried to sail like that you'd get breakages. With the mast vertical and well supported you can attach shrouds at your leisure.

    Lifting tackle is your choice.

    Simple system gets the job done. Despite a non rotating rig these boats have a good racing record. The bungee raising system I mentioned above would work well coupled with the control this system ensures. It would make for very quick mast raising.
     

  15. oldmulti
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    oldmulti Senior Member

    This is a Kurt Hughes earlier trimaran design for simple fast coastal cruising and day sailing. The tri is trailable but requires assembly if you use the simple aluminium cross beams. At this size the aluminium beams are light and relatively easy to move around. The Hughes 26 trimaran is 26.7 x 22.3 foot with a weight of 1350 lbs and a displacement of 2215 lbs. The 35 foot mast (rotating mast is an option) carries a 295 square foot mainsail, 150 square foot blade jib, 300 square foot screecher and a 540 square foot spinnaker. The draft varies between 1.5 foot to 6.5 foot over the daggerboard. The tri has a kickup rudder on the main hull. The main hull length to beam is 10.3 to 1 and the floats are 15 to 1. The mast is raised and lowered by a separate “A” frame. The outboard can be a 5 HP.

    The accommodation is simple with 3 single berths (1 wing berth can “convert” to a double, a small galley and limited seating. There is only 4.5 foot headroom. This tri is about sailing first, cruising second.

    Construction is the rapid CM plywood/epoxy. The main hull and floats are with plywood panels made on cylinder moulds. There would be 2 layers of 3 mm ply. The pre curved plywood panels are then tortured into the required hull shapes. Stringers and bulkheads are then inserted into the tortured hull shapes to finalise the hull. The 6 mm plywood decks, deck frames and stringers are added. The exterior is covered by an e-glass cloth of eg 200 gsm in epoxy. The simple option is to add aluminium tube cross beams with waterstays. These cross beams are a fast build and relatively cheap to do.

    The performance of this tri is good to very good. Kurt Knows how to produce a fast trimaran and many of his clients enjoy racing so words like carbon main, Kevlar jibs etc feature in second hand versions.

    There were several versions of the Hughes 26 resulting in a carbon fibre cross beam version. The beams are light enough to be lifted off by one person. Setup should be about an hour. This version has much bigger amas than the same size folding trimaran, farther out there, and with a higher speed shape. It has no seastays.

    An interesting fast trimaran that can be built by a home builder. Excellent tri Mr Hughes.
     

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