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RL24 (Rob Legg)
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I originally posted this in 2014, http://www.sailnet.com/forums/gear-maintenance/173498-front-hatch-replacment.html, but I didn't want to necro.

We had temporarily put a marine vinyl covering over it that was velcroed onto the boat, classy I know. I have since decided to make something permanent for this upcoming season. On the last thread many were commenting on trying to make my own fiberglass hatch, however I am thinking it may be easier to use acrylic as it should bend easily.

Here is a pic from before the hatch was blown off, notice the bend that will have to be made.



Now, when I went to Menards I saw acrylic and I saw Lexan. Originally I was going to go Lexan as it was rated as being a lot stronger. However I came across this thread from sailnet in 2010, http://www.sailnet.com/forums/general-discussion-sailing-related/63781-lexan-vs-acrylic.html, and decided maybe acrylic was the way to go.

However after even further research I learned that acrylic expands and contracts with temperature differences. I was not going to make a frame to mount it in so its not going to have room to contract, I was going to put the hinge and latch directly onto the acrylic piece. Is this not going to work out?

The boat is an RL24 so its a trailer sailor and when not in use it will be under a tarp. We are in Wisconsin so the winter does get cold. Will I have to be worried about the acrylic breaking itself off the bolts?

If I am able to actually bolt everything right to the acrylic then the way I was planning on bending the sheet was possibly getting a foam piece and cutting it down to the same curve and size as the hatch and then using thermal reflective tape on it so it doesn't burn as I heat the acrylic over it and for it to the foam.
 

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Is that seriously 'fur' everywhere? Can't imagine a worst material to use for that.. but I know several builders did.

I had a small acrylic curved cover (similar shape, about half the size) made by a plastics outfit.. cost $200 or so and worked out fine, a piano hinge is directly attached (you can buy special 'plastic appropriate' drillbits).

I believe the way to DIY a curve like that is to make a male mold, place the acrylic in an oven until it will bend over the shape, but I suppose progressively and carefully heating with a heat gun might do - but with a greater risk of burning..
 

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The Laser 28's had a 3/8" thick acrylic or lexan forward hatch that had to curve perfectly to fit and seal properly. One of the class members built a male wooden mold with a plywood face over 2x_ framing, which allowed the plastic to be bent perfectly. I think that the plastic and mold were placed in an improvised oven until the plastic was pliable. I do not know how the plastic was forced into shape. The hinges and latches were bolted to the acrylic, but the bolt holes were drilled oversized and the oversized holes filled with soft caulk that cushioned the bolts from touching the plastic.
Jeff
 

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The Laser 28's had a 3/8" thick acrylic or lexan forward hatch that had to curve perfectly to fit and seal properly. One of the class members built a male wooden mold with a plywood face over 2x_ framing, which allowed the plastic to be bent perfectly. I think that the plastic and mold were placed in an improvised oven until the plastic was pliable. I do not know how the plastic was forced into shape. The hinges and latches were bolted to the acrylic, but the bolt holes were drilled oversized and the oversized holes filled with soft caulk that cushioned the bolts from touching the plastic.
Jeff
That would also allow for expansion/contraction without stressing the acrylic and cracking.
 

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RL24 (Rob Legg)
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Discussion Starter #5
Is that seriously 'fur' everywhere? Can't imagine a worst material to use for that.. but I know several builders did.

I had a small acrylic curved cover (similar shape, about half the size) made by a plastics outfit.. cost $200 or so and worked out fine, a piano hinge is directly attached (you can buy special 'plastic appropriate' drillbits).

I believe the way to DIY a curve like that is to make a male mold, place the acrylic in an oven until it will bend over the shape, but I suppose progressively and carefully heating with a heat gun might do - but with a greater risk of burning..
Yeah its carpet that lines the inside. At some point in the future I want to remove it and do foam backed vinyl or something.

Do you have the name of the company that made your hatch?

The Laser 28's had a 3/8" thick acrylic or lexan forward hatch that had to curve perfectly to fit and seal properly. One of the class members built a male wooden mold with a plywood face over 2x_ framing, which allowed the plastic to be bent perfectly. I think that the plastic and mold were placed in an improvised oven until the plastic was pliable. I do not know how the plastic was forced into shape. The hinges and latches were bolted to the acrylic, but the bolt holes were drilled oversized and the oversized holes filled with soft caulk that cushioned the bolts from touching the plastic.
Jeff
Yeah I was looking at doing 1/8'' or 1/4'' and was just going to do some kind of weather stripping to seal it. Any idea how over sized the bolt holes were?
 

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Yeah its carpet that lines the inside. At some point in the future I want to remove it and do foam backed vinyl or something.

Do you have the name of the company that made your hatch?
Just a local plastics outfit..
 

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RL24 (Rob Legg)
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Just a local plastics outfit..
Sounds good, Ill have to see what we have around here. Otherwise since the acrylic sheet is only about $30 and I can return the heat gun, if it doesn't work out its not an expensive mistake at-least.
 

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You can get pretty inexpensive heating blankets that are specific for bending acrylic. They're made of a material similar to quality air mattresses and have elements inside like a hot blanket for your bed.

I'm not sure if they work on thick material like 3/8" etc - they are generally used for making magazine racks, cup racks and that sort of thing.
 

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I like the heat blanket idea. It seems like you could build a simple mold out of door-skin plywood and 2x-'s, put the plastic on the mold, put the heat blanket on the plastic, put some sandbags on top of that and let it bake until the plastic sagged down onto the mold. Very cool idea.

McMaster- Carr shows something like that for around $100.00. But there may be other less expensive sources for a lower quality version.



Jeff
 

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Unless your existing hatch is pretty thin you'll have a problem doing that - the outside surface of your existing hatch will become the inside surface of the plexi so there will be a difference in the "radius" of the curve.
 

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RL24 (Rob Legg)
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Discussion Starter #14
Unless your existing hatch is pretty thin you'll have a problem doing that - the outside surface of your existing hatch will become the inside surface of the plexi so there will be a difference in the "radius" of the curve.
There is no hatch left to use as a mold it was blown off and sunk. What I meant was using the opening itself and having something covering the open hole and using that as a "mold." Idk, all I know is it's going to be interesting making this work.
 

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That would also allow for expansion/contraction without stressing the acrylic and cracking.
You definitely want to avoid point loading of the acrylic at bolt holes and enlarging the holes is a start. Suggest you distribute the clamping load and use an adhesive caulk like GE Silpruf. Whatever caulk you use, be sure it is compatible with acrylic.
 

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When I looked up plastics a while back I kept finding "eplastics" with some good prices. Turned out they are local to me so I went down and picked up supplies from them. They had some cool videos of them manufacturing intricate things I got to watch while waiting for them to cut some starboard for me.

If you were able to make a template of the outside of the frame of where you want your new hatch to fit up against I am sure they could form a piece of acrylic to fit. I don't know what it will cost you but their capabilities looked impressive and their plastic prices were cheap compared to where I was buying it from before.

Here is their promo video.
 

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I'd bet my company heats and forms acrylic sheets, 24" to 104" square 200 times a day. Works out for the guys on the floor during the winter but come summer, even in Maine . . . it's like an oven out there!

That said, acrylic is thermo-forming like everybody is saying. Polycarbonate can be cold formed or brake formed also, but with a large radius like this it would probably be near impossible to do that properly. Since it's acrylic, heat is your only option, but the problem is heating **** only the area you want to bend **** (tried to emphasize without yelling). To do this, the first thing I would do is make a frame with the proper radius and angle. Take your time and make it nice. The hatch will be there for a good long while and the frame will be in your shed for almost as long. Make it several inches wider and longer than your acrylic requires and be sure to cover it with thin plywood, especially the curve. Slitting the back 3/4 of the way thru on your table saw at 1/4" O.C. will make this happen. Fill the screw holes and sand it. Now, sandwich the lower section of acrylic between the mold that you've created and another piece of 3/4" plywood and clamp well. The top edge should be 3/8" from the start of the bend. Loosely clamp plywood to the top with spring clamps that can be removed quickly. This is your heat shield. When rolled over the mold, the edge should be about 3/8" from the top transition point. So, to sum up, you have a frame, cold acrylic clamped to it on the bottom with a piece of plywood, another piece of plywood on the top and about 3" exposed between them. Looks like a stiff band-aid. Now use a heat gun and SLOWLY heat the part that you can see thru. heat both sides, back and forth heating all the exposed area evenly. When it starts to go, heat it another 10 seconds but don't let it twist. Now, gently lay the top into place, tug a little to get it to follow the mold, and clamp it into place. Let it cool for 1/2 hour or so. 5 minutes would probably do but let it find it's new shape slowly. Once cool, finish cut to size, sand the edges smooth, scrape the edges smoother with a razor, maybe flame polish them. Adhere in place using 3M VHB tape. Mask and back-bed with Dow 795, be sure to tool the bead.
 

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RL24 (Rob Legg)
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Discussion Starter #18
I'd bet my company heats and forms acrylic sheets, 24" to 104" square 200 times a day. Works out for the guys on the floor during the winter but come summer, even in Maine . . . it's like an oven out there!

That said, acrylic is thermo-forming like everybody is saying. Polycarbonate can be cold formed or brake formed also, but with a large radius like this it would probably be near impossible to do that properly. Since it's acrylic, heat is your only option, but the problem is heating **** only the area you want to bend **** (tried to emphasize without yelling). To do this, the first thing I would do is make a frame with the proper radius and angle. Take your time and make it nice. The hatch will be there for a good long while and the frame will be in your shed for almost as long. Make it several inches wider and longer than your acrylic requires and be sure to cover it with thin plywood, especially the curve. Slitting the back 3/4 of the way thru on your table saw at 1/4" O.C. will make this happen. Fill the screw holes and sand it. Now, sandwich the lower section of acrylic between the mold that you've created and another piece of 3/4" plywood and clamp well. The top edge should be 3/8" from the start of the bend. Loosely clamp plywood to the top with spring clamps that can be removed quickly. This is your heat shield. When rolled over the mold, the edge should be about 3/8" from the top transition point. So, to sum up, you have a frame, cold acrylic clamped to it on the bottom with a piece of plywood, another piece of plywood on the top and about 3" exposed between them. Looks like a stiff band-aid. Now use a heat gun and SLOWLY heat the part that you can see thru. heat both sides, back and forth heating all the exposed area evenly. When it starts to go, heat it another 10 seconds but don't let it twist. Now, gently lay the top into place, tug a little to get it to follow the mold, and clamp it into place. Let it cool for 1/2 hour or so. 5 minutes would probably do but let it find it's new shape slowly. Once cool, finish cut to size, sand the edges smooth, scrape the edges smoother with a razor, maybe flame polish them. Adhere in place using 3M VHB tape. Mask and back-bed with Dow 795, be sure to tool the bead.
Thanks for that, especially with the picture.

Yeah we are definitely going to have to go the plexiglass route, I had a quote from a local fiberglass company done. $676. I was like well the boat was $900 so I'm going to have to pass and test my hand at plex.
 
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