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Re: beaching keel
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Hi Jens You probably know that polyamid (polyamide) and polyester are different materials. The long chains of molecules in polyester are linked together with esters whereas the long chains of molecules in polyamid are linked with amides. So the raw material has different chemical base and therefore different properties. For example, polyamids absorb more water than polyesters. When a plastic absorbs water, it swells. This is important when looking at the micromechanics of a composite. The swelling strains the matrix (i.e. epoxy) so that it cannot carry as much load. The matrix that is 'less strong' takes away from the strength of the finished composite. As you can imagine, it is very important to test materials after conditioning in the marine environment. Test data is available for most materials but finding data that represents performance in the marine environment is far more difficult. Before expensive testing I try to get an idea of potential performance. One example is to compare aerospace test data for finished laminate properties and 'hot-wet'? properties. If a product has little change between these two test conditions, it is a good candidate for further testing. However, this approximation is not perfectly accurate. For example, I have found products that are eventually damaged by salt water that tested exceptionally well in fresh water. You may not realize that fabrics for composite reinforcements generally have surface treatments to enhance wetting and adhesion to the matrix. Fabrics without the correct finish do not perform well. Here is an example: Many manufacturers of fabrics use silicone oils to lubricate their machines and even the yarns they weave with. Silicone is known to inhibit the cure of many epoxy systems. If one was to use such a fabric in a laminate, it may seem OK but perhaps would only give 1/3 of the strength of a fabric with the proper surface treatment. David From: "shakalboot@... [k-designs]" To: k-designs@... Sent: Monday, January 1, 2018 9:15 AM Subject: Re: [k-designs] Re: beaching keel
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Hello David, Your message is very interesting. I'm just about to build the hulls for my ECO 68 ( ) and also thought to reinforce the keels with kevlar. You write that polyester fibers are even more resistant to abrasion and thus more stable than Kevlar. ... I had to smile a little, because in my model airplanes I have glued landing skids under the hulls of polyamide fabric and epoxy resin for many years .... why not use it also in boat building ?? So my question: Can these reinforcements for the keels not also made of Polamid belt straps and epoxy resin? In Germany, there are by the meter in different widths ( ) -this is car seat belt material, this is 1,2mm thick and I think that can withstand any ground contact without any problems. best regards, Jens ---In k-designs@..., wrote : Hi Bernd I would have enjoyed seeing your beach landing.. Kevlar 49 (aramide fiber) is a wonderful material. It performs spectacularly for its weight when subject to impact tension loads. It is OK in? abrasion but does get fuzzy on the surface. Polyester fiber reinforced epoxy has poor impact resistance but outstanding abrasion resistance. I base this assessment on paddling custom whitewater kayaks in the early 1970s and years later working as a Materials and Process Engineer specializing in marine epoxy-composite structures. Custom whitewater kayaks are under designed by nature and abused by repeatedly slamming into rocks and sliding across gravel bars. They must flex to absorb impacts that would destroy rigid laminates 5-times stronger and then continue down river. A fellow named Chip Queitzsch accomplished engineering tests in the laboratory (1960's I think) to develop laminates that could survive the punishment of whitewater kayaking. He focused specifically on impact properties and abrasion properties, among others. Chip developed lab tested material properties for individual reinforcements and resins as well as combinations of reinforcements. His work showed that combinations of reinforcements could produce lighter, abrasion resistant and damage tolerant kayaks than previously imagined. The reason I bring up his work is that it is the first comparison of abrasion resistance properties of kevlar fiber reinforced composites and polyester fiber reinforced composites I know of. Later laboratory work by DuPont, and Union Carbide (manufacturers of Kevlar and Dynel) confirmed Chip's comparison of abrasion properties. Based on Chips work, polyester fiber reinforced epoxy is about the best composite that can be found for abrasion resistance. It is highly tolerant to abrasion and wears smooth. Thus, it is used in high wear areas rather than general hull structure. I don't think it competes with Kevlar. As I am sure you know, Kevlar is a staple of lightweight impact resistant structural composites including custom kayak hull structures. Later, when working as a Materials and Process Engineer, I recommended the use of polyester fiber reinforced epoxy to customers for high wear areas. It performed well for them. Also, it is relatively inexpensive and easy to laminate. David From: "Bernd@... [k-designs]" To: k-designs@... Sent: Saturday, December 30, 2017 8:50 AM Subject: Re: [k-designs] Re: beaching keel
? Hi Yes David the keel from my boats are overdesigned for the reason of beaching and some bumps on the way! Many sailors found us crazy what we did with our "PELICAN". Never a problem. Some scratches of course.? Keflar is otherwise the best protector. Only problem, the base has also to be strong. Even when whearing a bulletproof vest (Poliz and military) and you get hit, you get bruizes or have a broken rib. Craig your example from Hawaii is interesting. Keels on a multihull I find roundout stupit.? By the way, if the would use Keflar on the bows the would not have to rebuilt every year. Luiz was just writing that he has always to beach the boat and had never a problem. Damaging the bow or keel area was never reported from my small till the big catamarans (for my "big" is till 13,00m).? I mentioned this already elsewhere. Keflar is the most difficult material to work with, besides the costs. First you need special cissors to cut it (around US $ 100,-). You can not sand it ( try to sand a wol sweater). To apply Epoxy is very difficult, because you see no color difference is a part done or not..? A thrue story. We where sailing at the Baldenaisee near Essen/Germany with the jellow SC435.. I got hit by the main sheet and my glasses went in the water. I was so p...set that I turned the boat to land it. The landing place had a wooden front and there after gravel. I was at a good speed, just before the landing I moved back, so that the forward part of the hulls where out of the water. I was sliding with a good speed up the wood prodection and moved forwards. The boat was over the centerpoint and canted forward, I stepped of the boat without getting even wet feet. A guy was coming on in a rush. What no damage? I never could do this with my (Polyester) DART. I would have a hole in the boat. Yesss Sir, The good part of a wood/Epoxy construction with half the weight of the DART. I looked at him, "I do that always in this way"?;-) ?;-)) Bernd ? |
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