Your fixation of a skid plate’s
ability to absorb energy puzzles me.
Isn’t the purpose of a skid to… well… skid? Would that not imply its primary duty is abrasion resistance?
And do you put forth the premise that a Kevlar skid plate will resist abrasion as well as a Dynel or S-cloth skid plate?
Your fixation of a skid plate’s
The stems of most well made canoes are plenty strong and stiff and really don’t require additional impact protection. So for most people who choose to apply a “skid” plate, the purpose is to protect against abrasion. But to do so, the plate must be sufficiently impact resistant to avoid self-destruction which Kevlar felt unfortunately often is not. It does not have to break off in chunks to show it is doing its job.
Dynel is a good material for abrasion plates, keel strips, and rub rails because the fibers really do seem to be unusually abrasion resistant. It is not great for use on wooden boats which are to be clear finished because it retains a milky appearance when fully saturated with resin. The fibers also swell in the resin which can make for a thicker than desired laminate and a textured surface. This latter tendency can be reduced with vacuum bagging, pressure molding, or the use of mold release fabric (peel ply).
S 'glass is very strong and cures clear when fully saturated. It is easily cut and sanded and bonds very strongly to resins. A single layer of 6 oz/sq yd S 'glass can make a thin but strong plate with edges that can be very finely feathered and a surface that stands only a few mils proud of the adjacent hull presenting very little hydrodynamic drag. While the fibers may not be quite as abrasion resistant as Dynel, they are stronger. For a thicker plate, 2 layers of 4 or 6 oz/sq yd S 'glass can be used.
Aramids like Kevlar for all their good properties have some that are not so good. The relative lack of compression strength is a big one for abrasion plates and the outer layers of hulls. The aramid fibers also don’t bond to resins nearly as well as fiberglass and they are much quicker to separate from the resin matrix under stress, leaving intact fibers that have delaminated from the rest of the structure. The fibers are also very hydrophilic which can contribute to delamination problems. The fibers do not respond well to sanding so achieving a smooth plate with finely feathered edges is much more difficult.
Felts consist of many very short fibers oriented in all directions and compressed together. The fibers may be individually strong, but the whole material lacks structural integrity because it has no longer fibers running the length and breadth of the material. It has been used for skid plates simply because it is easy.
Unlike plain weave fabrics which tend to change shape, trading width for length and vice verse, felt maintains it cut shape pretty well. It does not fray along the edges like weaves do making it easier for folks who have no experience with fiber glassing to use. It lays down over curved shapes pretty well and one does not have to worry about cutting it out on the bias as there is no bias. This feature allows the shapes to be oriented in any direction desired making it more economical to use a single rectangle of material for multiple pairs of plates.
But all of those tiny, compressed fibers in the felt form innumerable interstices that wick up excess resin. Couple this with the fact that the skid plate kits that are commercially available usually come with an excess of resin. The resin is typically mixed up in one big batch and has a limited pot life. A boat owner without much prior experience with resin use slops it all on, not wanting to waste any. The result is a very resin rich lamination which makes the skid plate heavy, thick, and brittle.
For those who are interested in alternatives to Kevlar felt skid plates, Mike McCrea started this thread on another forum which offers some detailed DIY advice on how to work with woven cloth to construct an abrasion plate:
In the case of the OP’s boat, I suggested the possibility of removing the plates as that might be much easier than thinning them, smoothing them, and feathering the edges. The urethane adhesive that comes with most of these skid plate kits often doesn’t bond all that strongly to intact gel coat, especially if the surface prep was hasty or lacking, and the photos definitely suggest this application was a home job. Sometimes the bond can be broken more easily than one would guess allowing the whole plate to be peeled off with relative ease.
IF the hull is visually unacceptable, paint. Paint the felt before attacking it.
Felt on my Rendezvous is painted white on a maroon hull. I can see damage. Painting a vehicle or parts with white functions for seeing first rust.
Kevlar doesn’t sand well. Why would a Kevlar felt sand better than a Kevlar fabric ?
I wanted Blanc’s discussion of bow/stern strips. I’ll avoid distorting what he said but suggest we may be discussing two different materials properties and approaches to stem protection.
Felt absorbs impacts stress within felt. Adding more hull skin with a laminate strengthens a hull. The dividing line is discussed as does the extra skin absorb impact or strengthen the hull that is the hull with a fabric laminate continues absorb all impact where the felted hull receives less impact.
The two concepts are not splitting hairs but widely significant design approaches in all vehicles…including helmets and body armor.
and I’ve enjoyed your posts and Mike’s over at the tripping site.
I do my skid patches similarly to how Mike illustrated, I’ve always used S-cloth and epoxy resin. They’ve performed well but I plan to use Dynel on my Magic next spring when I add skid patches to it. I believe I can realize similar performance with less material build. And using peel ply will be a new technique for me, too.
I appreciate the time you’ve taken to post your tutorials. They are most useful for many of us.
Felt absorbs impact?
Maybe if you don’t soak it with resin first…
For something to absorb impact it either has to be elastic (malleable) or it has to be crushed. Resin-soaked and hardened kev felt is neither.
I have had one hard direct impact on the kev felt skid on my Prospector, high up on the stem. It did not crush and it did not deform like rubber or foam. It did leave a mark much like a rock leaves when impacting my car’s windshield. I can tell you that the only place any impact was “absorbed” was on my body when I lunged forward in the boat as it stopped instantly. No - I’m sure the boat also flexed throughout it’s length, absorbing some of the impact. But the skid plate only chipped and slightly cracked - it absorbed nothing.
With that one exception on this one canoe, all other wear and tear on my stems has been abrasion. I see no advantage to these felt skids…none, other than ease of installation - and that’s debatable.
I’m with Steve
my anectdote is when we hit the rock full speed ahead the Kevlar skid plate got a hole punched right through it. Absorbed anything… not.
My thigh absorbed energy and so did the thwart that I broke when my leg hit it and broke it off.
OTOH - my Penobscot had been in what must have been a real doozy of an impact when it had no skid plate. Right on the blunt end of the vertical part of the nose, the stem was actually crushed in and permanently deformed in an area about two or three inches in diameter. Above the waterline, so of no consequence to me - but that blow was surely absorbed to some degree by the royalex hull at the point of impact.
yeah blame it
on the felt.
not to be …
‘For something to absorb impact it either has to be elastic (malleable) or it has to be crushed. Resin-soaked and hardened kev felt is neither’
steel is malleable and crushable…also HEAVY.
coefficient of restitution
resisting impact with materials largely depends on speed and weight limits.
Your communication skills…
really leave a lot to be desired. Much of the time I can’t tell what in creation you are actually saying. Confident I’m not the only one who feels this way.
Ignore this guy
Remember the time I tried to explain to him that cracks in gel coat due to a hull’s impact with a solid object were caused by the gel coat’s inability to flex as much as the underlying hull material? All the while he preached that cracks in the gel coat were proof that it had “absorbed energy” and thereby significantly reduced stress on the hull. I could explain the problem with that logic (again), but you get the idea.
how do you feel ?
The resin felt question:
If there is a less than resin filled felt but still cohesive before the impact…then hypothetically this product would offer more energy absorption than a filled felt.
Refer to crunchy candy bars
How that would be approached ? I dunno.
The OP’s hull has stem protection, my subject.
I agree others feel the same way. Probably most of Maine and some of New Hampshire.
Nonetheless ! the felts are on the stems.
cracks are evidence of energy absorption. The area absorbed energy and cracked. It’s eternal.
DK loves google
Google Datakoll. aka Gene something. And grab your beverage of choice. Its an eternal evening of reading how he has wrecked other forums.
He is doing a very good job on this one but I think and hope his time has run out.
But when it’s incidental and minimal, …
... you can ignore it.
Consider an eggshell. If you could coat your boat with eggshell, it would crack when the hull flexed, no matter how slightly it flexed. Yes, the eggshell would absorb energy when it cracked, but is it enough to matter? I say it's not. Gel coat is more flexible and a little stronger than that, but its flexural strength is still miniscule in comparison to any material from which a hull would be constructed. The stress it takes to make a paper-thin layer of gel coat break when flexed is so slight that you can (and should) ignore its contribution to strengthening the hull (or protecting it via energy absorption, as you prefer to say).
Using your logic, the paint on your car helps protect the metal when that metal is stressed. After all, the paint cracks if the metal bends more than little, so it must have absorbed energy, right? Well, you are right, but is it enough of a contribution to matter? Does your car benefit from the paint when you have a minor collision?
felt, gelcoat has the quality of absorbing energy then cracking or popping off and in that process protecting the laminate.
This is what happens. What you’re involved in is overstating a position in disagreement with a process that occurs and is foundational to your crit of it.
With gelcoat and felt stem protection, the facts are not negated on the VOLUME or DEGREE of the protection because the laws of physics state that the protection exists.
Try not to be dense?
Yeah - good idea. Speaking of dense, you might consider matters of degree. Put that steel on your stems and it won’t absorb any impact either - unless it’s density is such that it can be crushed (such as a “crush zone” provided by voids in the matrix ).
I can say something is malleable or not as a matter of degree (according to the force that can be reasonably expected to be appliied), and most people in the conversation will know what my point is…except for that one guy who has to divert from the point to avoid addressing the flaw in his ridiculous argument.
and that is a reason…
…to choose either over a stronger material?
That’s true, but you can’t see my point?
Using that same logic, I can truthfully state that when I jump off the ground, the whole earth moves in the opposite direction, and rebounds to its original position during the time that I fall back to the floor. There comes a point where theory, no matter how undeniable, is useless from any practical standpoint.
Steve gets it perfectly. Just because one material fails when bonded to a material with greater durability doesn't mean that failure is a desirable or intentional outcome. In this case, the design purpose of the gel coat is to take the wear of abrasion so that the structural material beneath doesn't. The fact that the gel coat cracks when the hull is flexed beyond a certain point is incidental, not a design feature, and not a benefit.
Excellent summary. n/p