How thick should a hatch be?

[John Williams]

I've an Albin Express with a slightly leaky main hatch and I'm trying to improve the weatherproofing by making a "garage" (Google translation from Swedish websites) to stop water getting underneath it, and, as a by-product, to make it easier for crew to slide across the deck when tacking.

Picture below shows the current layout (not my boat)

I want to build a cover over the front half of the hatch rails, dropping to deck level just behind the ventilators. This picture shows the general design.

I'm seeking some advice on how thick it should be to support the weight of someone sliding across it - obviously it's unlikely to have a whole bodyweight on it, but some of my crew are pretty chunky I'd thought of making the cover fairly thin, then making a frame to fit across the top of the aluminium rails out of 10mm square carbon rods, spaced about 150mm apart. This would raise the top of the garage about 15mm over the top of the rails. My local supplier sells Coremat, so initial plan was a gelcoat, couple of layers of 600gsm CSM, layer of 2mm Coremat, then another couple of layers of 600gsm CSM, to give a thickness of about 5mm. I've costed this, and it's approx £25 of materials (excluding poly resin), but about another £60 for the carbon rods. I'm wondering if it will be easier/cheaper to exclude the carbon rods, but use more sandwich layers of Coremat & CSM, alternatively use carbon cloth rather than CSM if that will give more strength - essentially trading £80 of CSM/carbon rods for carbon cloth. I'd rather have it over-engineered with an extra sandwich layer or two across the span (about 680mm) than have a size 10 go through it, but really got no idea how thick it ought to be.

Any thoughts?

Thanks

John

 

[rwatson]

I would have thought that question shouldn't be "How Thick " the hatch should be, but "How Strong". Tensile, and shear stress, especially as you are spanning a distance, is the goal.

I had never heard of Coremat before. I found an explanatory video that provided an introduction, and a weight supporting demonstration. It looks very useful.

The impression I got from your description is that you are planning to use CSM on both sides. CSM is good in Compression, but inferior for the other stress orientations.
I would start looking as minimum, BiAxial Rovings of at least 400 gsm, on both sides of the Coremat. You can easily build a test section to try out.

The other thing that increases "strength" without excess material, is Shape. On a covering, incorporating Ridges of some sort can not only increase strength, but add valuable non-slip properties. Some subtle 5 mm forward facing V or U shaped "indents" would be typical. Strips of Coremat might be an effective way to build them. If you want a smooth effect, put the Ridges on the underside.

 

[John Williams]

Hi

Many thanks for the link to the video. I hadn't actually searched for Coremat on Youtube, so that was very informative. I'm not averse to using cloth as an alternative to CSM, just thought it might be easier to wet out and conform to the shape of the garage. I'd rather try to keep the top surface flat, hence my initial thoughts on supporting a thinner composite laminate on a carbon frame. Picture below shows a variation with strengthening ribs on the topside, however I think I prefer the flat surface.

The board I'm planning on using as the base of my mould has a subtle texture, which should provide a nice finish to the gelcoat. As regards trying a test piece, I'm guessing it would need to be the full span between the rails, otherwise I wouldn't get an accurate indication of any deflection in the middle when my 100kg bowman is sliding across it! Perhaps time to experiment, I can get 440gsm biaxial, so might make a piece with 2 layers on top, Coremat, 2 biaxial in the middle, Coremat, then 2 biaxial on the bottom. Place atop a couple of bricks the correct distance apart, and ask my bowman to jump on it!

 

[BlueBell]

Thick enough to keep your largest crew member from crashing through it, no?

 

[rwatson]

"I can get 440gsm biaxial, so might make a piece with 2 layers on top, Coremat, 2 biaxial in the middle, Coremat, then 2 biaxial on the bottom. Place atop a couple of bricks the correct distance apart,"

That sounds like the proper method, but sandwiching glass and Coremat, twice is not the best solution.
You would be better with two layers of Coremat in the centre, and two Biax layers on the bottom, and at least one layer on the top.
If that isn't enough, just keep increasing the outer layers.

 

[wet feet]

I'm going to go against the trend here and say that my estimation would be not too far from your original plan.I don't really see any justification for carbon rods,nor do I see quite how you might use them to good effect.I also don't believe you need to use anything more exotic than CSM as it isn't a wing on an Airbus,or anything that has undergone stress calculations to the nth degree.The absolute worst thing that could happen would be that a heavy crew member could drop a couple of inches onto a surface that is currently strong enough to take his/her weight.I would suggest 3mm coremat and if unobtainable,an extra 300gsm layer on either side of the coremat you can get hold of.You could test it by getting the second heaviest crew member to stand on it and if that works,send for the big guy!

 

[bajansailor]

The transverse stiffness of the top of the garage would be increased if you add some curvature or camber to it, rather than just having a flat panel.

If you incorporate a significant vertical flange across the aft end of the garage, this would add a lot of strength to it, and you could perhaps even mount some sailing instruments there if desired.
Are your wind speed / direction, log and depth instruments currently mounted in the cockpit on the aft bulkhead of the cabin?

 

[wet feet]

All good points.An instrument pod can be a very useful thing in that location,but it might limit the passage of the large fellow when tacking,in which case a smaller upstand similar to that on "Tikka" would still boost the rigidity somewhat.

 

[John Williams]

Thanks guys, as mentioned previously, all good points. We already have an instrument cluster on the mast, plus a rod kicker, but the crew manages to squeeze past it I'm intending on bringing the front of the cover down to meet the deck, and have the rear just enough distance above the hatch to allow clearance, but the hatch would provide support if the garage flexed a couple of mm. Agree with others this will improve stiffness. I once made the mistake of forgetting to reinstall the wooden strip on the front of the hatch after doing some work, and without the strip to stop excessive bend in the hatch, had to go shopping for £100 worth of polycarbonate to make a new hatch We have a slight leak under the hatch, water pools in the area where the first picture shows a filler cap of some sort, and makes it's way under the hatch. There's already a rubber sealing strip running between the rails, but I'm planning on building a dam of alu U channel and mastic to try to stop rearward water travel. Hopefully by putting the garage over the entire area in front of the hatch, down to deck level, no (or very little) water will enter this area, and the leak will be fixed.

 

[gonzo]

You should also have good drains at the forward corners. Many boats have tiny holes that get easily clogged and the water builds up until it leaks into the cabin.

 

[rangebowdrie]

The transverse stiffness of the top of the garage would be increased if you add some curvature or camber to it, rather than just having a flat panel.

This in spades.
A couple inches or so of camber will really decrease deflection under load whilst still allowing a fairly lightweight structure.

 

[John Williams]

I've rethought bringing the front of the hatch down to deck level, and am going to use a 20x40mm alu box section, screwed to the deck either between the hatch rails, or just in front of them, to fill the gap at the front, and support for the centre of the garage (the top of the rails is conveniently 40mm above the coachroof). This seems a simpler solution than to trying to get a sharpish bend in the hatch along the front edge, essentially the garage can be a flat lamination, with just a drop on each side where the garage can attach to the sloping sides of the rails. The top surface of the hatch is a couple of mm below the top of the rails, so I'm thinking of a strengthening strip on the rear underside of the garage so there's minimal clearance between the hatch and garage, but any flex should be negligible.

 

[wet feet]

It is,of course,your boat and you are free to do as you wish.Were I to have to slither through that gap I would find a moulding with rounded corners a much nicer solution.