multihull scantlings

Discussion in 'Multihulls' started by rapscallion, Sep 23, 2007.

  1. catsketcher
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    catsketcher Senior Member

    There are other loads

    Hello all

    I agree with Rob to a large degree but differ from him in one respect. The reason that cruising boats may be heavier than Wild Thing (the Tennant tri I think Rob is referring to) is that the critical loads are often not sailing loads.

    My 38 ft cat has the normal strip cedar laminate of 600gm db outside and 400 gm uni inside and I have no problems with its stiffness or load taking ability. It could certainly be lighter for sailing loads but I wouldn't want it to be.

    The reason is unanticipated loads - the highest loads your multi will get is probably from dinghies, kayaks, run ins with marinas, logs and small rocks under the sand when you dry out. A racing boat will be well cared for and will not have to undergo such trials but a cruiser has to be strong and tough. Really tough.

    The problem with putting together a laminate schedule is that the engineer has to be given a proper concept of the highest load condition. According to my boat the only time the laminate has failed (by going beyond its elastic limit and deforming -its called getting a ding) has been when dinghies and the above have conspired to try to hurt my boat. Would I make the laminate lighter - no indeed according to the way my boat has performed the laminate is under engineered to cope with many impacts. A proper engineering of the laminate has to include factors that are hard to assess like the rough and tumble of a cruising life. Due to this the heavier laminate is a good choice.

    cheers

    Phil Thompson
     
  2. Bruce Woods
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    Bruce Woods Senior Member

    There are other loads

    Hi

    Well said catsketcher. My 35 ft cedar cruising cat also has 600db outside and after 10 years of abuse I certainly wouldn't want any lighter. The pruning back of safety factors through lighter laminates etc , that we are currently seeing in some "cruising" designs is a worrying trend. At the end of the day nothings fast about spending time in expensive boat yards performing repairs.

    A few hundred kilos saved in the laminate of a cruising boat pales into insignificance after loading her up with all the "cruising essentials".


    regards

    Bruce Woods
     
  3. rob denney
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    rob denney Senior Member

    G'day,

    6mm cedar/200/200 is 40% the weight of 15mm cedar/400/600. This is more than a couple of hundred kgs. It is considerably more than half the weight of your shell, which is a large proportion (20-25%?) of the finished boat weight. Maybe a ton in the two boats mentioned?

    A boat that is this much lighter will have considerably less inertia, therefore will be much easier to stop before you bump into things and will do less damage when you do.

    As for dinghies, kayaks and marinas, the impact zone for these is very small and could be locally beefed up if required, or fenders could be used. The same applies to rocks in the sand, although I personally would never sit a cat hull on sand or mud without using fenders/tyres/planks, regardless of hull thickness. The rock size that will damage a light boat with light laminate will be very similar to that which damages a heavy boat with heavy laminate.

    If you want proof of this, make some panels and do some tests. We did some impact tests of various laminates for survey boats when i was selling materials. The lighter laminates, thinner cores performed extremely well, much better than their relative weights would suggest. However, the govt department concerned had a minimum panel thickness (19mm), so that was that and this has been the defacto core thickness for cruising multis in Australia, (and for those designers who slavishly followed the Australian designers when doing their own strip plank boats), ever since.

    A few years ago, I built a 12m/40' harryproa prototype from 6mm bending ply (even lower properties than cedar) with 200 gsm glass each side. One of the ugliest and roughest built boats you have ever seen. It had a wing mast and lived in a boat yard for a while, sitting on 300mm high blocks. In a gale, it was blown off the blocks, landing on gravel and small rocks. The mast jumped out of the lower bearing and did a fair bit of damage to the sides of the hull, but did not fall down and there was a hole in one hull from a large rock, but otherwise no damage. I fixed it and eventually gave it away to a mate who thought he might be able to use some of it. He put it on a mooring which broke in a gale. The boat washed up on a West Australian beach just after low tide. The boat spent 3 hours being pounded beam on by waves which were "too big to launch a dinghy through", with no damage at all. The next day we tied some sheets of ply under the hulls as skids and towed it along the beach with a bulldozer, then put it back on the mooring.

    Where thin cores compare poorly to thick ones is in stiffness, but with the amount of furniture in cruising cats there is not a huge number of large, flat unsupported panels. Those there are can have stringers and/or ring frames installed. We did some fea on the lee hull (no furniture) of our larger harryproas. 12mm core needed a full length stringer and a couple of ring frames. Leave these out and a 25mm core was required. A no brainer as far as weight, cost and effort were concerned.

    If you want a slower, less seaworthy boat, or to make the materials suppliers (building, rigging, engines and sails will all be more expensive) smile, or save the designer the trouble of properly engineering your boat, then go ahead with the heavy laminate.

    Regards,

    Rob
     
  4. Bruce Woods
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    Bruce Woods Senior Member

    There are other loads

    Hi,

    Mr Denny, whilst some of your points are well considered and valid, I must disagree with others.

    The hull surface area of my particular cat is 80 sq meters. Using your figures of a 60% saving in weight between the "light" and "heavy " cedar laminates the saving after local reinforcing and stringers is still only a few hundred kilos as stated in my previous post. The remainder of the boat ( deck ,underwing etc) is constructed similar to most one off boats utilising ply , balsa core flat pannels etc.

    As you can appretiate there is little point in strip planking large flat or gently curved areas . Why rip up a flat sheet of material into strips and then glue it back together again flat?

    The impact from dingies through to winch handles, anchors ,dive gear, mooring buoys, floatsam, stilettos etc is a fact of life for cruising sailors. Running around with fenders is the standard response regardless of layup , to save the finish from damage . Local reinforcing of 200 g uni will end up spreading over most of the hull in reality. 600 db outer skin is a good practical thing for a cruising boat..

    Gees ,how many times does a cruising cat sailor accidentaly touch bottom ,crossing river bars ,nosing into new anchorages ,miscalculating the tide etc etc. Having spent a large part the past 25 years in the Kimberly region of Australia drying out between tides is a fact of life so the boat construction has to be right.

    Also planking a hull with 6mm strips would certainly pose a few more problems for the amateur, requiring much closer spaced building frames and loosing some of the natural self fairing tendancy of the strips. Fairing before glassing would certainly be a challange ( trying not to put your finger through it), all for saving of a few hundred kilos.

    Regards

    Bruce Woods
     
  5. rob denney
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    rob denney Senior Member

    G'day,

    While your laminate has withstood the abuse you describe my reasoning, based on the tests and experience I described is that the light laminate would have done as well and saved you considerable weight and cost, resulting in a faster, more seaworthy, cheaper boat.

    To answer your points:

    600/15mm/400 weighs 7.25 kgs persq m. 200/6/200 weighs 2.9 kgs per sq m. 80 sq m of this is 348 kgs, plus overlaps, plus the extra bog required to fair a laminate with 1mm high bumps at all the overlaps, plus any additonal resin from a non perfect hand lay up. Probably not far off 400 kgs. 5 adult males or 400 kgs of water. Your call whether this is insignificant or not. Rapscallion's tri has more surface area, fewer flat panels, so will be worse.

    Balsa core flat panels are even worse, as they are 600 gsm or 3mm ply each side, albeit at much better resin/fibre ratios.

    Why cut material into strips to make flat panels? 9mm hoop pine ply is about the same weight as 15mm cedar, gaboon is lighter, but much more expensive. Ply requires glass on at least one side, and 3 coats of epoxy on the other. 200/6mm/200 is considerably lighter.

    Race boats suffer as much, arguably more than cruising boats from your list of impact items (perhaps excluding stilletos;-), as their crews are rarely as considerate. They also take a lot bigger risks, at far higher speeds in shallow water and are much less maneuverable (smaller engines) in marinas.

    If your boat has centreboards and lift up rudders then, regardless of the laminate you take a (to me) very big risk drying out in areas where you cannot be sure the bottom has no rocks. If you have skeg rudders, only a small part of the v bow sees the sand, so the light laminate is plenty. If you have fixed keels, then the heavy laminate for the hull makes even less sense.

    6mm planking will need more building frames. With computer drafted and/or cut frames this is no big deal. Against this, they are easier to scarf, and there is far less resin to mix and apply, and almost no fairing after the glassing as there are no overlaps.

    If your cat is 25 years old as you imply, I am surprised it has such a heavy laminate. Whose design is it?

    regards,

    Rob


     
  6. Bruce Woods
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    Bruce Woods Senior Member

    Hi,

    Mr Denny,

    With a quick re read of my posts you will note the boat in question is only ten years old.The 25 years was referring to Kimberly sailing.

    I thought we were discussing the pitfalls of a designer specifing 200 gm outer laminate over cedar for a cruising cat hull. My comments and figures stand. Either the designer has little cruising experiece or the boat was intended for the race circuit.


    As someone once said it is not wise to wrestle a pig in mud as eventually you'll realize that the pig enjoys it.

    Regards

    Bruce Woods
     
  7. rob denney
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    rob denney Senior Member

    G'day,

    With a quick re read of my posts you will note the boat in question is only ten years old.The 25 years was referring to Kimberly sailing.

    That helps explain the heavy laminate, then.

    I thought we were discussing the pitfalls of a designer specifing 200 gm outer laminate over cedar for a cruising cat hull. My comments and figures stand. Either the designer has little cruising experiece or the boat was intended for the race circuit.

    The thread is about Rapscallion wanting a laminate for his boat. He got some advice suggesting he copy other boats. I put forward an alternative suggestion, backed up by examples and tests. I have had 30 years in the pleasure boating industry selling materials, designing and building boats, and clocked up over 50,000 miles of cruising and racing, all over the world. During that time, cedar strip cruising boat laminates have gotten inexorably heavier, for no good or obvious reason. My point is that Rapscallion will have a lighter boat that goes faster for less money and is equally able to stand up to abuse if he does not blindly copy what everyone else does.

    As someone once said it is not wise to wrestle a pig in mud as eventually you'll realize that the pig enjoys it.

    Or maybe it is as someone else once said, the fallback position of someone who is wrong, is to start flinging insults in an attempt to divert attention from themselves. Regardless, we have both had our say, Raps can decide for himself. Maybe he can let us know what he decides.

    My apologies for upsetting you, it was not my intention. I will stop replying to your posts.

    Regards
    rob
     
  8. ropf
    Joined: Aug 2008
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    ropf Junior Member

    Hello,
    i'm not going to design a boat, just try to understand *why* the designers do what they do. Therefore, I follow your discussion with interest.

    Bruce and Rob, both of you are concerned about impacts while no one speaks about rigging loads, hydrostatics... So the main assumption seems to be, if the boat is strong enough to survive the "usual accidents" so its strong enough for everything else. So far is the only question, who is the stronger opponent?

    Bruce claims this has to be the own boat, so the other (anchor, hammer, dinghy...) limits the impact energy. In Robs mind there is always a bigger danger (rock, kai, other ship...) so the impact energy is set by the weight of the own boat.

    Now it's time for the interesting part, let's express this in numbers. Bruce, the anchor is falling on deck, the dinghy is ramming the boat. Rob, your boat is falling on the rock. Or you are running against the kai.

    Because both of you are claiming, this is somthing between "normal" and "can always be happen", it should be a design goal. Please give numbers in therms of energy (weight, height/speed) both for the normal case (without damage) and the accident case (boat has to survive but maybe require repair).

    regards
    ropf

    Btw, if you are on collision to an big ship or big rock - maybe a anti ship missile is more effective then a stronger hull :) Sorry for my poor english.
     
  9. Asleep Helmsman
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    Asleep Helmsman Senior Member

    According to Jim at West Systems (Gougeon Brothers) they have been building 40 foot (12 meter) trimarans out of ½ inch (12.7 mm) cedar and fairing it down to 3/8 inch (9.5 mm) final core thickness.

    They cover this with one layer of 10 oz unidirectional carbon fiber set 90 degrees to the keel (and strips) on both the inside and outside of the hull.

    Joe
     
  10. rob denney
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    rob denney Senior Member

    G'day,

    I cannot supply numbers, and consider there are too many variables for a simple number to make much sense. What you describe is dropping objects on the boat vs dropping boats onto objects. Both ways the thicker core and laminate will sustain less damage. In both cases, the objects are so much stronger than the boat that there will not be much difference in the distance dropped to cause damage. Drop an anchor point first from waist height, you will damage anything other than steel. Hit a rock at 6 knots, both will be holed. Is it worth the weight, cost and effort involved in the heavier laminate to be able to hit a rock at 3 knots and survive, rather than 2.5? These are guessed numbers, but I think they're in the ball park.

    I would also ask those who reckon their hull and laminate is as light as it can be for impact resistance whether they have ever tested lighter, thinner laminates. The resistance to damage is not at all linear and, if there is a bulkhead, frame or stringer at the point of impact, there is even less difference.

    I have recently paid a small fortune to have a 20m/66' harryproa lee hull engineered from first principles, including finite element analysis. The engineer is not involved in selling materials. The hull came out at 15mm cedar, with 400 double bias glass each side, with a couple of stringers and some carbon to take the rig and beam loads. This is actually a lighter laminate than the 15m harrys (which are half the weight of any similar sized cat). The reason being the hull is bigger, so there is more material to take the loads.

    There are better ways to prevent local impact damage than overbuilding the entire boat. To name a few that feature on harrys: A dinghy ramp so dinghys don't bash the topsides, anchors that stow on beams fore and aft so they don't get dropped on deck. Zero rocker hulls, kick up rudders and no daggerboards or fixed keels so the bow hits things, not the middle of the boat. The bows (all 4 of them) are lightly glassed high density polyurethane, allowing impacts at much higher speeds than cedar glass, and also limiting the damage to an easy repair.

    For what it is worth: Rapscallion, who started this thread has decided to go the lightweight way. He is building a 12m/40' trailerable harry with 10mm core (plastic honeycomb) and 200 and 300 gsm basalt cloth each side, also engineered at considerable cost. This boat will arguably see more impact loads being trailered around the country than a marina based boat.

    Asleep: The boat Jim is referring to is Adrenalin, a radical racing F40 trimaran with a huge rig. Not a cruising boat, but it does show how little laminate is required to withstand massive sailing loads and indicates how far out of whack cruising boats have become.

    regards,

    Rob
     
  11. Asleep Helmsman
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    Asleep Helmsman Senior Member

    Howdy,

    There is one consideration in hull thickness irrelevant of any loads, expected or otherwise, and that’s floatation during high stress conditions like foundering.

    Foundering; according to dictionary.com is to fill up with water and sink. Now the filling up with water part is bad enough, but that whole sinking thing is just a little too much for me.

    So your choices are a thicker, lighter than water, core or strategically place expanded poly-something-or-another floatation.

    Why not both?

    Create your original hull shape from cedar, for stiffness in the core, than epoxy strips of medium density poly-something-or-another.

    These thoughts just came to me out of nowhere, so precede with caution before you start thinking we have solved once and forever the hull thickness problem.

    Where you see “Why not both?” that’s the exact moment of truth, for the dual core thought, it’s maybe not original, but I did come by it honestly, probably because Rob mention polyurethane in the bow of a wooden stripped-planked boat.

    Now that brings me to the next question. Rob, earlier you alluded to other methods of construction, why not elaborate here. That way I won’t lose my way trying to navigate this site.

    I am asleep after all.


    Take Care,
    Joe
     
  12. rob denney
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    rob denney Senior Member

    G'day,

    Should have been polystyrene, not polyurethane. Sorry. Buoyancy is good, air in sealed chambers and bonded on foam will both do the job, if they are done properly.

    What do you want to know about other methods of construction? Better that You ask questions which I will try to answer rather than me writing stuff which may not be relevant.

    regards,
    Rob
     
  13. catsketcher
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    catsketcher Senior Member

    I am not trying to upset you Rob!

    Gday everyone,

    Rob I have stopped writing this until I mused over it because you have done the testing and I haven't BUT and I had to put in the but, I wonder about laminates and fatigue. Maybe this is one of the reasons that we have cunky laminates.

    I feel that the light laminates on the early Crowther cats like Pumpkin Eater - (10 oz over 12 mm foam) and similar ones on Nicol tris did little to get cruiser to follow these examples. These boats were foam which should mean an increase in laminate compared to cedar or Kiri. Remember that D Flawless blew apart with her epoxy laminate when a conservative cedar boat like Songlines (a beautiful 14m cat by Neville Lloyd) just filled up a hull when both had a centreboard case blown out by a collision. Songlines sailed home but D flawless was found in bits. Why the two different failure modes? Did Songlines have much greater rip stopping in her chunky laminate? This rip stopping is what Nigel Irens did in B and Q and Sodebo by using high shear cores and pulling both laminates together every few metres.

    I had occasion to literally run into Pumpin Eater with my Twiggy - not my fault I insist - and the hull just deformed in a really plastic way. Most unlike what I would want. Devils Three was falling apart near my Twiggy in the 90s. Considering Laser sailors - I was one of these too - know that stiff Lasers get soft after a few seasons of hard racing and even skiffs and Cherubs which are cored composite start off nice and stiff but end up heavy, soft and full of dings is not the question of how heavy the boat is to be related to how long you want it to be stiff for?

    I think this will be one of the problems about getting a laminate much lighter than usual. The long term fatigue testing has been done on boats which are using older style resins and cores. Even then getting the designers to make public the laminates and having the builders follow the plans means that we won't really know what laminates have served well over long periods of time in cruising use.

    Cheers

    Phil Thompson
     

  14. rob denney
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    rob denney Senior Member

    G'day,

    Takes more than a boat discussion to upset me. ;-)

    Fatigue is a fascinating subject. The Gougeon's built a test rig and tested a bunch of different materials and laminates, and wood and epoxy came out at the top, apart from carbon/epoxy and wet wood. Not really a surprise when you think about the cyclic loading at high stresses (relative to ultimate strength) trees are subjected to.

    330 gsm/10 ounce glass over 12mm/half inch foam is not a lot different in one off strength/stiffness and impact tests to 200 gsm/6 ounce uni across 10mm cedar. In terms of fatigue and long term multiple small impact tests (dinghies, stilletos, anchors), the cedar would be way ahead.

    D Flawless was designed as a bay racer, and was subjected to cyclones, ocean crossings and a very tough race program by a fearless skipper (Gavin). It hit a whale in mid ocean and was eventually blown ashore. I know nothing about Songlines, but I bet it was not as far out of it's comfort zone as D Flawless was. How fast was Songlines going, how far down were it's centreboards, what did it hit, what was the weather, what was the laminate round her cases are all questions to be answered before you can make any comparison of the two.

    I don't know what Pumpkin Eater and Devil's Three were built from (suspect polyester as Shaun Arbor built them, I think?), but Lasers and production Cherubs are both built from polyester, which in thin films has notoriously poor fatigue properties. A cat I do know about is the epoxy laminated XL2 (very similar to D Flawless), which has had a hard 30 years, and the hull and decks are in very good nick. There are also all Derek Kelsall's early boats (10mm foam/300 gsm glass/polyester) which have stood the test of time extremely well. It is much harder to build a quality foam boat than a strip planked one.

    Maybe a heavier laminate will last longer (far too many variables to make this an all encompassing statement), but it is like the "how fast you need to go to damage the boat on a rock" question. A very small difference in speed for a big increase of the laminate, cost, work and weight. If you want it to last forever and bump into things at speed, build it from 6mm steel or 8mm alloy.

    Re your last paragraph on knowledge of boats, materials and laminates: The first large strip planked boat was the Tennant tri. 200 gsm glass over 6mm cedar, using WEST epoxy. All these materials are still available. The boat is still going strong as are hundreds of other lightly built (by today's standard) boats from the same era. You cannot get much better real life test results than that.

    If you build lighter than what was usual for these boats, you may have a problem. But building lighter than what is now considered normal, you will just have a better, cheaper boat.

    See my earlier post on why I think laminates and cores have increased, and why for these reasons, copying what everybody else does is going to cost you money, time and payload. Not that the designers or materials sales people are complaining about that.

    It is one of life's mysteries that people spend a fortune on sails to extract a small speed gain for a short time but won't spend any money having an engineer remove hundreds of kgs from the boat for a big gain over the life of the boat, probably paid for by the saving in materials.

    regards,

    Rob.

     
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