Hatch Braces - Is this Overkill

[KCStudly]

Well, well, well, if we do the math, a 3/4 x 3 inch deep rib is actually 8x more resistant to bending than a 3/4 x 1-1/2 inch deep rib.

[eamarquardt]

Well Gus, technically we are both wrong.

The formula for a rectangular shape rib is: Moment = (width x height^3 (cubed)) / 12

So the answer is, "it depends". I may have exaggerated, but it is more than double.

Then a 1X1 would be 1X1X1X1 / 12 or 1/12th

A 1X2 would be 1X2X2X2 /12 or 8/12 th's or eight times as strong.

Given the proportions, the calculations for a 1.5 and 3 inch beams would be similar at 8 times stronger.

Cheers,

Gus

[eamarquardt]

Actually, I think laminating the rib was easier than cutting a bunch of pieces out of plywood and gluing them all together. In addition, the only wasted material was that which was lost from the saw kerfs. Minimal. Also I really like the side view of all the laminations.

Just my two cents.

Cheers,

Gus

Gus,
Your plywood lamination's look great. Did you run them through a thickness planer or were you able to keep everything level during glue-up? I had to run mine through my planer. I agree this method is MUCH easier, and cost effective over cutting from plywood. I will be using it for my hatch as I did for my perimeter profile.

I was able to keep things pretty even during the lay up. I did take a belt sander and sand it down a bit. I think running it through a planer might be a bit iffy due to the curve and narrowness and the fact it's not starting off flat. I'm thinking of building a jig to hold a router and before removing the ribs from the jig, route the top down flat from above and then flipping the rib and doing the other side. Then it would be perfect. The rib varied in height only about twenty thousandths of an inch. Next time I'll measure it as I clamp it and I'm sure I can get closer (although doing so is more just for the challenge as a variation of only .020" is plenty close).

Cheers,

Gus

[KCStudly]

I was able to run my galley wall edge lamination thru the planer; just had to walk them thru the curve as it fed. I was not even close to that tolerance with variations between slats.

I hadn't planned on this inaccuracy, so I had to scab a layer of 5 mm ply back on the sides of the parts to make up for it, but since they ended up buried under other construction it was not an aesthetic issue for me. Knowing what I know now, I would have made my slats wider and planned on planing to the final thickness.

Be aware that, just because they come out of the mold without spring back, does not mean that they are stable. The plywood slat version will probably do better, but the fir I used opened up nearly 3/4 inch with a change in season (made in spring, installed in summer). I would recommend sealing and or installing as soon as possible.

[eamarquardt]

I was able to run my galley wall edge lamination thru the planer; just had to walk them thru the curve as it fed. I was not even close to that tolerance with variations between slats.

I hadn't planned on this inaccuracy, so I had to scab a layer of 5 mm ply back on the sides of the parts to make up for it, but since they ended up buried under other construction it was not an aesthetic issue for me. Knowing what I know now, I would have made my slats wider and planned on planing to the final thickness.

Be aware that, just because they come out of the mold without spring back, does not mean that they are stable. The plywood slat version will probably do better, but the fir I used opened up nearly 3/4 inch with a change in season (made in spring, installed in summer). I would recommend sealing and or installing as soon as possible.

Good to know. Thanks.

Cheers,

Gus

[Larry C]

I was able to run my galley wall edge lamination thru the planer; just had to walk them thru the curve as it fed. I was not even close to that tolerance with variations between slats.

I hadn't planned on this inaccuracy, so I had to scab a layer of 5 mm ply back on the sides of the parts to make up for it, but since they ended up buried under other construction it was not an aesthetic issue for me. Knowing what I know now, I would have made my slats wider and planned on planing to the final thickness.

Be aware that, just because they come out of the mold without spring back, does not mean that they are stable. The plywood slat version will probably do better, but the fir I used opened up nearly 3/4 inch with a change in season (made in spring, installed in summer). I would recommend sealing and or installing as soon as possible.

I was able to feed the whole profile lamination's through the planer, and it worked good. My strips were a little wider than my final planned thickness so It worked out for me. I would recommend using wider strip to anyone considering this method, and plane to exact thickness.

KC... You make a good point about installing the lamination's soon after pulling from the mold. I too had a bit of springback, about 1/4" I used mostly Basswood which is very bendable. Also, I clamped both profile halves together so any potential springback would be the same on both sides. I let mine sit for almost a year before installing my bottom stringer. Keeping them clamped together helped keep them the same even though the profile is now a little different than the design.

When I do my hatch I will try some different methods to assure that their is no springback.

[len19070]

I've always had a different approach to building a hatch. Instead of building heavy to prevent "Spring Back", Use materials that Don't "Spring Back"

I use FRP and Aluminum.

I build 2 outside rails, 1" thick (by using 2 pieces of 1/2") And cross spars. Then I cover the outside with FRP, good side in extending 1 1/2 past the rails on both sides.

FRP does not "Spring Back" and is strong enough when bowed around the hatch to not flex...as well as being stable enough to Triangulate the hatch.

I also install a second sheet of FRP on the inside, good side in as a finish.

2 sheets is more than enough, then sheath the outside with Aluminum.

I seal the outside edges with this;






I know, I know. But doesn't water get into the sides?

No! I build the sides in 3" at the hatch, the FRP/Aluminum extends 1 1/2" past the galley opening and I put a second outside insert molding on the inside of the galley.

This insert molding is 3/8" tall with a vinyl insert that acts as the hatch's seal. Plus I extend the offset Hurricane hinge past the galley opening.

For the water to get in it has to run down, make a right or left hand turn and jump over a 3/8" tall molding that is under the FRP/Aluminum.

In addition the 2 outside corner insert moldings create a good Trough for the water to run down there by directing the water out and off the trailer.





Plus I extend the offset Hurricane hinge past the galley opening.


These shots are from 3 different trailers of mine, but the theory is still the same.

Happy Trails

Len

[oakinteriors1]

The two teardrops that I have built I skined them with 1/8" birch and have had no problems. I wonder why being how every one else either uses two sheets of 1/8" or one sheet of 1/4". My teardrop is 11 years old and has been in all kinds of weather and no major problems.

I have built 17' long kayaks out of 1/8'' Baltic birch...heck cedar strip boats out of 3/16'' x 3/4'' will take a beating for years...

[oakinteriors1]

I cut some 1/8 BB ply into strips and laminated a test rib.

I preformed the pieces into a tight circle to reduce springback when I glued them up. I clamped them to short pieces of angle iron screwed to plywood as a form. Preforming them was unnecessary. When I did the actual gluing up of the rib I used c-clamps as they worked better than the spring clamps.


Here is a picture of the finished laminate.


Here is a picture of the finished rib. There was no springback when I removed it from my jig and it is exceptionally stiff. The advantage of this technique is that a lot of the grain is parallel the entire length of the rib which makes it far stiffer than a rib cut out of plywood.


Actually, I think laminating the rib was easier than cutting a bunch of pieces out of plywood and gluing them all together. In addition, the only wasted material was that which was lost from the saw kerfs. Minimal. Also I really like the side view of all the laminations.

Just my two cents.

Cheers,

Gus

Same process as laminating Kayak Coaming Rings for skin boats..Only we have to use a heat gun on such tight radiuses when laminating strips of hardwood....

[Ravenwind]

Can you tell me what the 1/8 x 9/16 edge seal that Len used is called where it can be found?

[len19070]

Can you tell me what the 1/8 x 9/16 edge seal that Len used is called where it can be found?

its called "Black Edge Trim Lok" or "RV Seal Lock" It comes in many sizes and colors.

And I get it here;

http://www.ebay.com/itm/1-8-Black-Edge- ... d4&vxp=mtr

R&P Carriages is a very good outfit to deal with.

Happy Trails

Len