DRUMHEAD MATERIAL

[adventureagent]

Drumhead, Cockpit Cover; whatever name you wish.

I've made cockpit covers for years, as needed. However, THIS boat's been beating me.

It is a small cockpit, measuring
31 1/2 by 17 1/2 inches.

The rim is about 1/2 inches, probably less at the back, underside. The rim is downturned amidships, but flat at bow and stern ends. Aft is a fairly short / narrow underside.

I have made a couple of covers out of not waterproofed cordura, which has worked for years on other boats.
I have made a couple of covers out of not waterproofed "taffeta" .

For elastic perimeter, I have used each application, to resolve: single shockcord, then doubled 1/4" shock cord and I have used 1/2 inch shock cord.

Each of these is a fine enough fit that installing on the cockpit of the boat up on my car rack is a bit "fussy", but they fit - and released easily.

In transit, I've had the covers sucked off the rim. Stationary, the sun shrinks the lighter and heavier material to a nice drum-like tension. It makes a nice "poong, poong" sound if you tap it. However, they of course loosen when the humidity gets high or if it's cloudy. They're moody and misbehave in these conditions.

Taught fabric, even with the vehicle parked, is enough to "pop" the covers. It's frustrating to stop enroute, to have strangers simply point to the cover on the asphalt, and to stop and find it dangling from a boat tie-down strap.

So I'm seeking advice as to what has worked for others who've had similar experience or know of some way to reduce or eliminate these issues. Is it materials (warp/woof), shockcord strength? I've tried pretty high tension, but it has to be affixed to the boat and with it vertical can be tedious. Maybe I need to use some tough, flat elastic?

I do have some old, flat stock, hard neoprene banding I used to make my whitewater skirts, but determining a circumference might be tricky. As well, I'm open for suggestions of how to "butt" join shockcord.

[Chris Bolton]

My wife and I make cockpit covers for our boats and they very rarely come off in transit. We use something like 4oz / 150g pu nylon. Lay a sheet of paper over the cockpit and mark the edge of the rim. Use this as a template to cut a flat panel. Take a 5cm wide strip of fabric as long as the circumference of the panel, fold in half along the length with the 'good' side out. Sew the doubled edge round the edge of the panel - I don't know the name of the stitch so see the diagram:

Leave a short length unstitched so that you can thread the shock cord. I normally use 5mm or 6mm, but on a plastic boat with a rounded rim I might use 8mm. To butt joint it I use whipping twine (Marlow no.2, I think it's waxed nylon, looks about 0.4 mm diameter) - overlap the ends a cm or so, wrap the twine round the overlap as tightly as you can, about 10 turns, then clove hitch to hold it, trim the ends close to the whipping, a bit diagonally, then take a new length of twine and wrap it round the whole joint so that the cut ends are bound in. The joint is about 50% bigger than the original diameter but that's never been a problem, I don't even notice where it is.

Usually we sew on a release loop like a spraydeck, and make it long enough that it reaches to where the strap goes round the boat, and thread it through the loop, so if it does come off you don't lose it - but I don't remember the last time I had one come off.

We use the same method for flat water spraydecks, just add a body tube to the panel - same type of seam. But for spraydecks, I unfold the the edges of the seams and apply impact adhesive, then squeeze them back together while it's still wet, so that it saturates the thread.

[ChrisJK]

Hi Chris
I have made a couple of hatch covers for transit and canal paddles using the material cut from those ton sand/aggregate bags and used grab adhesive to hold a channel for thin shock cord. i did also make a cockpit cover but I think I lost that somewhere. The hatch covers aren't that pretty but functional. I guess a better seam maker could do a better job.
I also have a protruding piece of cord to tie to the safety lines but only once had a flapper

[adventureagent]

Thanks, Chris (es). If=n the diagram, as I read it, it looks like a "straight stitch" but I think you're trying to suggest a more sophisticated stitch like maybe the one that looks like an endless V's and inverted V's. I've not done it that way, as it seems a much more refined method. However, I'm gonna try and find the 4 oz nykib and the template. I've used the release tab, long enough to fit under the boat tie-down. I've thought of the loop, and maybe I'll add that massive extension.

I like the idea of whipping two cord strands together. Lacking practice at this too, instead I used several tiny zip ties. However, these have the little square plastic that houses the actual "lock" for the tie. This added a crappy dimension and was quite a task to feed through the hem I'd made. Not fun and it did help me to privately exercise my English as a second language.

My method of doing the drumhead is/has been to simply lay the nylon over the cockpit rim, good side down.
I then stretch a shock cord around the rim.
Next I draw the edge of the rim by simply dragging a marker (of any type that is visible) completely 'round.
Then I take the material that is the main body of cloth and pull it up to the rim. This includes "gathers" (bunches?).
Again I mark this all around with the marker. This is what would be called "the good side" of the fabric.
I remove all from the boat.
I cut 'round the whole, following the marked "good side". I simply link the markings as some unmarked places exist, as I cut.
I bring the edge of the "good side" to the original line drawn on the cockpit rim.
I lay any material folds in one direction so the sewing machine will slide over it and the rest with relative ease.
Using cordura makes for large bumps of material, and in this case, I may cut some "v"s to make it lay flatter.
I pin these folds as I go.
I sew the marked circumferance to "near" the stern end of the outline. This is where the shock cord inserts and ties.
Usually I don't sew this part shut, and I've completed the circumference once.
I do one more row of sewing and with that I put in the "ripcord" of cloth.

I hope I've been clear on that, but it's at least a "rough idea" of what I do.
This method is pretty simple and works easily to any boat rim, it seems. (except THIS one)

So, I'll put this aside for a bit, then get at it. As paddling here in Ontario is generally winding down (today I was the only paddler and only two powerboats were out the whole day on this big, spring-fed lake).

Again, thanks for the replies, each.

[Chris Bolton]

the diagram, as I read it, it looks like a "straight stitch" but I think you're trying to suggest a more sophisticated stitch

No, just a straight stitch. The diagram was to show the good sides together and the hem folded down, rather than the straight stitch through an overlap.

[adventureagent]

So ... the RUNOUT of this TOPIC (or the drumming-out):

I used my standard method again, of one sheet and no 5 cm strip for the shockcord.as is the practice of Chris Bolton and wife. I'm best with minimal bits and pieces and fittings. Simple mind, simple plan. Pandemic had our local stores closed, so I resorted to the 'footprint' of my solo tent. This material was already recycled for that purpose from a much larger, discarded tent.

What was making my earlier drumheads come off was 1) sunlight 2) heavy Cordura. It seems that I could make an infinite number of drumheads / cockpit covers out of the heavier material and none would stand dryness. It didn't have to be really, really hot out, just not 'moist'. This would make the Cordura shrink, if only ever so slowly. I tried heavy shockcord and light shockcord. As my 'new' boat's cockpit rim is quite narrow, the shrinkage consistently pulled the skirt off the rim. Sunlight generated heat just made this happen faster.

My last iteration, just a tent-floor grade of polyurethane coated material, is not so 'cast iron' strong as to be pulled off by just dryness / heat. I used a shockcord of the diameter we would use for decklines (forgive me for not micrometering it). I have this and a bunch of other bits stored in re-sealable plastic lunch bages. It's much easier to rummage through and handle amid the tonnes of stuff I've kept.

This has been on the boat constantly, other than when I've been paddling, for over a month. It 'gives' enough that when dry it's taught. When it's hot, the thing truly sounds like a Tom drum when you tap it. A nice, deep 'boom'. So that's my source for future drumheads, I guess. The Cordura worked well with the wider rim on my XP 18, I'll have to find other uses for the remainder of the 'bolt' of Cordura I've had for several years.