Trailerable Multihulls

Discussion in 'Multihulls' started by JCD, Mar 4, 2008.

  1. Chris Ostlind

    Chris Ostlind Previous Member

    This Kevlar thing that everyone seems to have about abrasion resistance on the outside of the hull...

    I have repaired something on the order of 100 hulls over the last few years that have had Kevlar installed on the outside of the hull as some sort of protective measure against abrasion.

    While you do get some value in this direction with exterior Kevlar, you also make for one very ugly repair process for yourself or the yard dude who ends-up doing the work.

    Simply put: Kevlar is a very big nightmare to repair when it needs to be sanded and faired back into the existing laminate structure of hulls when doing repair work on the exterior, for virtually any reason. The very properties that make it highly abrasion resistant, also make it a big hassle to sand smoothly. What you will get when you apply your very cool, Fein 8" random orbit sander with 40 grit paper, is a giant fuzzy mess.

    Problems with Kevlar: It's hydrophilic, meaning it attracts water. This attracted water eventually wicks its way up the fibers where it finds a weak spot in the underlying laminates. If you have a wooden core, you now have a serious potential rot problem, largely unseen, until the whole thing makes its presence known through some breakdown of the rotten core section.

    If the core is not wood, then the Kevlar, itself, absorbs water and makes the boat much heavier and that will come to effect the performance of the craft.

    If you really need to have Kevlar as one of the laminates, then put it on the inside of the hull. Kevlar is extremely good in tension and extremely lousy in compression. Putting it on the inside of the hull makes it work in its best orientation when a collision occurs and it is pretty much not involved for day to day abrasion issues and maintenance, should you have that problem ongoing.

    My suggestion, if you are regularly exposing the boat to abrasion issues (regular beaching on tough material, etc) would be to add an extra layer , or two, of tougher, more easily repairable laminates than the standard E-Glass, such as S-Glass. This will allow you to monitor the high wear areas, make simple repairs with simple tools and avoid the expensive work that goes with reworking Kevlar.

    Please, give yourself, or the yard guy, a break and give some thought as to how the material is going to work best for you, should you want to incorporate Kevlar within your laminate schedule.
     
  2. masalai
    Joined: Oct 2007
    Posts: 6,824
    Likes: 119, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    Thank you Chris, so it may be good on "flack jackets" but NOT on the outside bum of a boat....
     
  3. Meanz Beanz
    Joined: Jun 2007
    Posts: 2,280
    Likes: 33, Points: 0, Legacy Rep: 585
    Location: Lower East ?

    Meanz Beanz Boom Doom Gloom Boom

    I understand that the Kevlar layer is often put one laminate in to aid repair.
     
  4. masalai
    Joined: Oct 2007
    Posts: 6,824
    Likes: 119, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    Heinz, Glass workers I know, wont touch it.... cheaper and easier to put a sheet of ply and glass over that with epoxy - - to clarify, endgrain balsa with glass either side (done in factory) - build the chine hull, then add a sheet of ply and then glass over that, then your choice of copper-arsenate 2 pot paint or whatever you can find (he he ) when wear through carelessness demands replacement then the job is relatively simple and straight forward....
     
  5. Meanz Beanz
    Joined: Jun 2007
    Posts: 2,280
    Likes: 33, Points: 0, Legacy Rep: 585
    Location: Lower East ?

    Meanz Beanz Boom Doom Gloom Boom

    Not a light solution. The boats that I have seen it used on are typically foam. What to do if you want light & stiff if not Kevlar. Extra laminates are a bit self defeating in the weight department, there are Kevlar substitutes like Twaron but I suspect they are no easier to get on with. Simply build a small full length hardwood keel to take ground on, not light... maybe foam or balsa and glass but sacrificial in nature. Weight has to be added, no doubt but not too much please. Don't like balsa core much... not sure I'd choose it to build.
     
  6. masalai
    Joined: Oct 2007
    Posts: 6,824
    Likes: 119, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    Ok they usually use a different bonding system - but reworking (reparing a kevlar outer) is disliked by all I have asked in the business......
     
  7. catsketcher
    Joined: Mar 2006
    Posts: 1,203
    Likes: 88, Points: 48, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    Kevlar is a bugger

    I have to concur with Chris about Kevlar - bugger to work with and pretty bad in compression. It blunts your scissors and all.

    Another good way to incerase abrasion is to use multiple light layers rather than a single heavy one - 2 300gms rather than a 600. As for me I say bugger the initial weight gain and go tough straight off. Repairs get heavy very quickly.

    My little folder has 600gm outside Kiri and a layer of Dynel sheathing as well. Then it has lots of filler. I have pretty rocky beaches on the lake so it gets well used on the bum.

    The 7 metre one I built with two layers of 600gm DB on the bottom - way over the top hydrostatically but trailer sailers get really beaten up by trailers and rollers. I like the idea of a really tough laminate on a well used trailer sailer as I can't help but give mine heaps when a cross wind or chop hits when retrieving.

    I also have a wooden keel covered with uni and DB. The little cat has no keel and so no rollers - real pain. Just popped the 7's hulls in the water a week ago and it slid into the water amazingly. So my take is - save weight somewhere else - put in on a tough laminate where the trailer and the beach will bang it and make sure the bottom can take rollers - probably a small keel.

    Final mention - a trailer sailer will probably need a high compression core in the keel area too - 80kg foam is bad for rollers - go cedar or high density foam or suffer having to use padded cradles. I complain about padded cradles on my little 19 footer - a big cat will be much worse if the ramp is shallow.

    cheers

    Phil Thompson
     
  8. JCD
    Joined: Jul 2006
    Posts: 359
    Likes: 3, Points: 0, Legacy Rep: 36
    Location: Coney

    JCD Follow the Bubbles!

    Hello Meanz Beanz, and everyone else,

    Sorry for the confusion above. That was actually a question that I had posed to Richard Woods because I needed to get clarity on whether he meant balance as you have denoted 50/50 in/out or if he meant the weights being more equal to each other on my recommended callout. I thank you for the answer you have provided.

    I agree with your point regarding the "balanced" structure being stronger as a whole component. I would like to share my thinking on why I designed as I did. My first confession is that I really did not try to be "weight discreet" when it came to areas below the waterline and althought I was conscientious of it, my primary focus was strength, strength and strength and screw the added weight if that was the result of the goal.

    For me, the strength can be averaged (balanced) equally in the interior or exterior as it is the simplest and fastest way to go, or the minimum strength required on the inside can be calculated and the added "extra" (unbalanced) strength can be placed where it would best serve its purpose, on the exterior. This adds more time, cost and labor which I don't consider substantial for the end result.

    The required strength must exist regardless of whether the "balanced" or "unbalanced" lay up is chosen by the builder/skipper, and it does, but in the case of the "unbalanced" lay up, the "average" residual strength in the interior is placed on the exterior where it best serves its purpose. This is not really anything new or not generally accepted as true, instead, builders have moved away from these in the interest of reducing time and labor costs. The builder/skipper for the TR27B will get the required strength anyway, but I wanted to provide a better option that they can choose or reject at the expense of little more time and varied glass cloth, for almost the same weight with greater strength.

    For me, the extra 1000.00US for different cloth weights and the extra 100 hours for the stronger lay up will be the way to go and if built by a pro that charges for the pound regardless of complexity, it just can't be beat.

    Regarding the Kevlar discussion in prior posts, my sentiments are the same and it appears that everyone is on the same page. My understanding is that it is difficult to work with and a real PITA. It is a dry sponge that finds water in a dessert and if there is even the slightest crease or kink in the cloth it will not bond with 500 atmospheres of vacuum. For me it is considered a disposable cloth that once it is broken, it must be discarded and should not be repaired unless you’re willing to subject yourself to a very unpleasant experience. The only sensible thing I have heard about it is that it should be used on the interior of a component and it should always be between other skins because of bonding concerns. In the case of the TR27B, it would be foolish to use it because its puncture properties should be in the exterior to protect the core/hull from penetration. A slick surface can be best achieved with Dynel if that is the property sought.

    It is reasonable to design for what could happen, (grounding, coral, floatsam etc), but to what point? What about the naval madman that fires off a torpedo or the unlucky whale with screwed up sonar that has to come up for air? Then there is this whole romantic view on beaching your bows on erotic beaches where the surge will quickly turn the beautiful sand into a belt sander for the hulls. Wouldn’t keeping her off her bottom and walking in 3 inches of water to the beach be a better idea?

    Perhaps it may be better served if we viewed this in regards to skin thickness opposed to weight. The following can be used to find skin thickness based on cloth weight and I have included the same for the metric proficient.

    Thickness = (Wt of Dry Glass (oz/yd^2) -0.3)/610
    Thickness = (Wt of Dry Glass (g/m^2) -9.7)/ 813

    The callout in bold is obviously the simplest. My recommendations are the callouts not in bold, which allow better orientation, strength and finish for almost the same weight obviously with increased labor. The lesser weight should always be laid up as the outermost skin whether interior or exterior because the smaller weave allows for a better finish and higher glass/resin ratio. Refer to the scantlings I provided and copied below.

    Hull Bottom
    Total Dry Laminate Weight
    Inside 12.5/6oz/yd (424/203gsm)
    Outside 20/10oz/yd (679/339gsm)


    Recommended Fabric and Schedule (1.109 #’s sq ft) (.046ksm)
    Inside
    Cloth 7.5oz/yd (255gsm)
    Cloth 10oz/yd (339gsm)
    Core
    Spun polypropylene yarn 4oz/yd (136gsm)
    Cloth 12.5oz/yd (424gsm)
    Cloth 10oz/yd (339gsm)
    Dynel 4.2oz/yd (143gsm)
    Outside

    Results:
    Bold skin thickness for the hull bottom = .029” on the inside and .048” on the outside.
    = 0.76mm on the inside and 1.24mm on the outside.

    Recommended skin thickness for the hull bottom = .028” on the inside and .049” on the outside.
    = .71mm on the inside and 1.27mm on the outside.

    First observation is that the thicknesses are not that small. Second is that although the difference between the inside and outside for the bold scantlings and the recommended scantlings is small, the recommended is by far superior because it has more strength (beef) on the outside where it is required and the different cloths such as Polypropylene and Dynel produce a fairer and stronger lay up due to the orientation and weave of the cloth for very little increase in weight. Yes, I know that it is a lot of glass but, remember that category B is no joke either.

    I also called out a sheer rub strip and keel and stem shoe or rub strip, whichever way you choose to identify with it.

    Sheer Rub Strip
    Total Dry Laminate Weight 10oz.yd (255gsm)
    Width 7” (178mm)

    Keel Shoe/Stem
    Total Dry Laminate Weight 12oz.yd (406gsm)
    Width 7” (178mm)

    As you can see, the sheer strip is 7” wide (177mm) and the skin is .015” (0.30mm) thick. The stem/keel strip is the same width and the skin is .02” (.48mm) thick. It isn’t excessive, but that is a lot of sacrificial thickness to add to the rest of the skins beneath them. Yes, I suspect a skipper exercising poor judgment could go through that thickness in an hour, but I don’t think most skippers abuse an investment of such value.

    Could I play with it a little more? Sure. Are the metric weights ideal to call out based on existing stock, probably not because I am calling out in imperial and only provide metric as a courtesy, but that isn't to say that it couldn't be done and then readjusted by any builder, as long as they don't go too far above the required weight, and definitely not far below, if at all.

    Thanks
    J:cool:
     
  9. catsketcher
    Joined: Mar 2006
    Posts: 1,203
    Likes: 88, Points: 48, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    Polyprop

    Hello JCD

    What is the polypropelene yarn next to the core? - I would steer clear of it if I were you. I think Dynel is polyprop and I wouldn't put glass over Dynel.

    As to Dynel - I used it on my small boat and didn't on the next one. It sucks up an awful amount of resin - very thirsty. It is not especially tenacious and according to my bottom of dinghy test doesn't stick that well when crazed. Good for sliding over rocks though but you will have faired your boat's bum so there will be some filler there.

    Maybe you should increase the density of the filler ... Dunno but that is what is the first stage under the paint - anything tough will be hard to fair - maybe aluminum fleck in the clear resin over the filler. Anyone done this or similar?

    cheers

    Phil Thompson
     
    1 person likes this.
  10. catsketcher
    Joined: Mar 2006
    Posts: 1,203
    Likes: 88, Points: 48, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    Ps

    JCD

    I wouldn't do the two layers inside. No real point as you can get very nice 600gm DB and biaxs. Laminating multi layers of glass has to be worth it or you will curse yourself during building. As for the outside laminate I would only double below waterline - use one layer above.

    Glassing over a fully cured glass layer is not especially easy - the knobbly surface needs prepping. If you sand too much you will take away strength of the initial laminate. Doing two or more laminates at the same time is tricky.

    Secondary bonding considerations, ease of building and cost would have me recommending a single laminate with another only below the waterline (outside) for trailers and rocky beaches to have a slug at.

    Also I would say your keel strip is way underdone. If going on rollers you will have huge pressure on the laminate above each roller. One layer of 440uni or DB will not be enough unless the rollers are very well adjusted and core is very high density. Check your rollers and hull shape to see if you will have a concentrated keel load.

    cheers

    Phil Thompson
     
  11. Chris Ostlind

    Chris Ostlind Previous Member

    Hi Phil,

    I've done a few bottoms of boats with graphite saturated epoxy. These boats were intended for tough, abrasive beach and water conditions and the material holds up extremely well for the purpose.

    Typically, this has been applied to the bottoms of fly fishing dories and other boats that would see a lot of rocks and heavy gravel at launch and retrieval. The last boat to get this treatment was a coastal cruising canoe/trimaran of my own design that is going to be used for a lot of unique adventures on unknown shores.

    The new owner reports a hull that is resistant to rocky shores and heavy duty grit of a typical wild beach. There is very light scuffing on the easily repairable graphite bottom. Should the owner encounter a heavily gouged bottom, it's an easy repair with more graphite and epoxy with a modest sand and buff for fairness.

    The graphite in the mix is a self-lubricating compound that wears away, marginally, with each encounter with a harder surface, much like the ends of a typical graphite pencil. The lubricity provides a slippery surface for the boat as it glides over the harder rocks and gravel.

    I posted a couple of photos below to show the effect of a graphite bottom on the boat mentioned.

    I don't see any reason why an aluminum saturated mix would not provide similar service.

    Chris O
     

    Attached Files:

  12. Meanz Beanz
    Joined: Jun 2007
    Posts: 2,280
    Likes: 33, Points: 0, Legacy Rep: 585
    Location: Lower East ?

    Meanz Beanz Boom Doom Gloom Boom

    The aluminum idea sounds interesting but I would be concerned about copper based anti foul, I would assume there would be an issue. The graphite sounds good Chris, I have no knowledge of how this behaves, are there any reaction issues with other metals or is it pretty inert stuff?

    Phil, the extra fine layers below the water sounds reasonable, I did wonder about Dynel as its suppose to be good abrasion resistance but I figured it added little in the way of strength and I know from doing decks with it a fair amount of epoxy disappears into it. Surprised to hear you used it, I take it you'd not do it again?
     
  13. catsketcher
    Joined: Mar 2006
    Posts: 1,203
    Likes: 88, Points: 48, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    Dynel

    Hello Meanz

    I used Dynel because I wanted something slippery. After also putting it on the bottom of my dinghy I am less enamoured with it. Also you do have a pretty thick layer of filler on most custom boats - 2-4mm so this has to be gouged before it gets to the Dynel. After almost two years of pretty lintermittent but rough use the little folder still has to show the Dynel anywhere. Ther are some deep gouge marks in her filler but the Dynel hasn't had to earn its keep yet. It probably is a bit silly to have something on the bottom that makes no contribution to coping with trailering or sailing loads.

    The thing that worries me about Dynel is when you put in under the load bearing laminate. Dynel has such different physical characteristics from glass that it may not want to distribute the load to the glass laminate properly. It is very stretchy (Low E) and I do not know how it goes as part of a laminate. It has lower peel resistance (in my findings) than E glass.

    My call is to chuck the Dynel, especially under any glass, go graphite in resin or similar after fairing.

    cheers

    Phil
     
  14. Meanz Beanz
    Joined: Jun 2007
    Posts: 2,280
    Likes: 33, Points: 0, Legacy Rep: 585
    Location: Lower East ?

    Meanz Beanz Boom Doom Gloom Boom

    Yeah, I figured it was best left for decks and old timer hulls.
     

  15. tspeer
    Joined: Feb 2002
    Posts: 2,212
    Likes: 178, Points: 63, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    What you're pointing out is a classic problem with handicap racing. The point of handicap racing is not to have a fast boat. The point is to have a boat that is not as slow as its rating says it is.

    There's a similar issue with mixed one-design's that sail in a handicap race. You're really not sailing against the other sailors in that day's race. You are racing against sailors in your own class from times past. Some classes attract top talent, and some classes do not. The rating for a handicap class includes not only the speed potential of the boat, but also the talent of the people that have traditionally sailed that class, if the handicap is based on statistical performance (such as the Portsmouth Rating System). As you point out, if you have a fast rating, then you have to really be on your toes to sail up to the rating. That's typical of a high-performance class that attracts good sailors.

    So you can interpret your class' handicap this way, "If I were racing in a one-design fleet and sailed the same way I sailed today, where would I come out in that fleet?" If you sailed well enough to have won in a one-design fleet of your design, then you will be sailing well enough with respect to your handicap to win the day's race on corrected time.

    For example there was a Formula 40 catamaran that used to race in our area, and the standing joke was that he had to finish before the race started in order to save his time!
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.