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Lotus

An other OC20 build !!

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Having made a few windshields, wrap the mold with felt, before tossing the acrylic over it and applying heat. Start in the middle and let gravity do the work. Keep the gun moving, working from the center out, in a spiraling pattern. Do not start and stop from the same edge. Always let the gun run off the edge and come back to the opposite side to apply more heat.

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On 9/22/2017 at 2:11 AM, PAR said:

Having made a few windshields, wrap the mold with felt,

 

Thanks for the advice PAR.  Will a cotton sheet do the same job as felt ? I thought felt is made out of nylon and it may melt with the heat used ! 

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Typically the heat (to get the plastic pliable) is 350 f, which felt will tolerate. Felt is now a combination of nylon, acrylic, rayon, polyester, etc., but this is called "craft" grade. Then there's the eco-friendly stuff which is all or most postconsumer stuff. You want the "blended" felt which is wool and rayon. get the 35/65 stuff, not the 20/80 stuff. I'm still fairly sure all the "felts" will tolerate the heat, as they use quite a bit, just to make it.

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Hi Lotus,

The flat sections on your transom look like they are the mounts for interceptor type trim tabs like Zipwakes. If so I would be very interested to hear about performance once you launch. On the Ocracoke 256 the ones we have just don't perform any usable function and are not currently being switched on. That is not to say that they can't be sorted at some point. I thought that deploying them would balance the boat side to side and fore and aft. Deploying one side makes the boat turn in a circle! Any feedback later on would be welcome.  

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Hi Capriosca,

Yes those flat section are intended for trim tabs , but was going to use the standard 9x18"(flap) Bennett trim tabs not that type you mentioned. Personally dont like that type much and think the flap ones are much more effective . Maybe I will incorporate an automatic trim controller ( bennett ) so that it will balance the boat automatically .

 

BTW yours is a great nice boat !! Well done !

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Lotus, also worth considering using a uv stable polycarbonate/lexan as opposed to an acrylic.

My current catamaran boat has lower windshield made of acrylic with upper shield made out of polycarbonate. 

After 10 years of service (offshore fishing) the polycarbonte has outperformed the acrylic in both long term clarity and durability (the acrylic now has a crack, and its thicker than the lexan!).

 

 

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Lenm polycarbonate seems much better than acrylic but unfortunately unavailable the thickness and tint in my country !.

 

Much more unfortunately is that it was a total disaster trying to form this wind shield !!    It came out all warped , even more when it cooled down . I tried to re heat it again on the mold itself but with no luck It will look fine wile heating but will warp again when it cools down. 

 

 I think the first mistake was the mold . I made the mold in a 'convex' shape ( to look nicer , like a car windscreen ) therefore the heated acrylic could not lay flat compleatly . I assumed that when the sheet will be heated it will take any shape ( sort of melting on the mold ).

 

The second mistake was that the sheet was too thick 5/16" (8mm) to be heated with a electric hot air gun . I even tried to heat it up with a diesel heater blower . 

I noticed that while heating one side it was still 'cold' on the other side , then while heating the other side the first side will begin to louse temperature immediately.

 

I have to do an other mold and start allover  again using 3/16" (5mm) or 1/4" (6mm)  instead of 5/16" acrylic sheet.

BTW the sheet to be formed is about 7ft x 2 ft in size !!

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Finally got it right !!!!!!! :)

 

All it needed is a perfect mold and a propane torch !!

 

BTW that wind shield is the same one that was all wrinkled :wacko:

 

 

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1 hour ago, smccormick said:

Nice, what changes did you have to make to the mould?

 

The first mold was curved both ways ( like a bubble ) so perspex could not lay flat when heated 

 

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Acrylics (plexi) are generally harder to bend than polycarbonates (Lexan) and both prefer to be "conically" or "cylindrically" bent, much like plywood or sheet metal. Polycarbonate can be cold bent and conforms to compound curves better. Acrylic also can accept a compound curve, though typically it's done in a male/female mold arrangement. I've had some success with compounds, but not very often has it been necessary. The way I figure it is, if I can get chipboard to bend around the mold, I can get plastic to take that too. It looks like your second attempt, is equally matched for the other portions and efforts you've preformed on this project. Congratulations.

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21 hours ago, PAR said:

The way I figure it is, if I can get chipboard to bend around the mold, I can get plastic to take that too.

 

Par, when you heat the acrylic sheet (  with heat gun or heat torch ) do yo heat one side only or both sides before you put the sheet on the mold ?

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I'll preheat to a degree, but generally I apply heat to the outside of the curve, which is in tension and needs to stretch. On small parts I use an oven, which does everything at once, but windshields I use a big torch like the one you have, start in the middle of the tightest curve and work in a spiral pattern away from it. This lets the weight of surrounding material help get the bend started and gravity can assist. I also use MAPP gas, not propane, because it's hotter. A laser temperature pointer is a handy thing, but as you get close, you'll see the plastic start to move and this is when you need to concentrate, on uniform movements and getting heat applied, in the direction you want. 

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Thanks for your help par !

I am asking because I'm not 100% happy with this windshield, there are still minor wrinkles . I will either heat it ones more or start allover again with a new sheet.

Par what sheet thickness do you prefer for this size of shield ? ( when flat it measured 6.5' x 2.5' )  

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1/4" would be my first choice and the weight of this stuff help with distortions, though it takes more heat. If you want a glass like appearance and can't invest in a huge oven, consider a male/female mold. A pain in the butt, but it does control things better. This said, I've made some pretty big ovens over the years and still have one I use for post curing. It was the other half's, but I got caught drying some oak in it one day and had to get her a new one. I pulled it apart and made a sheet metal box, wrapped with rock wool and some foam, sitting on a piece of 3/4" plywood. The two heating elements were spaced equally on the bottom and basicly the rest is the same as the stove was, (thermocouples, knobs, etc.). I have a few thermometers mounted in it and can hold temperatures quite well. This is a big deal for a single project, but the used appliance store can yield one for less than 100 bucks. Heat the oven to 350, insert the mold and plastic and keep an eye on it. It should start to "move" very quickly in an oven and if it doesn't, raise the temperature to 370.

 

Also you might want to consider some ratchet straps, across a thin piece of plywood, to hold it flat against the mold.

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On 10/3/2017 at 4:05 PM, PAR said:

1/4" would be my first choice and the weight of this stuff help with distortions

 

Thanks for your help Par this time its perfect  .Now I'm 100 % happy :D

I used a new 1/4 sheet ,clamped it to the mold and heated just the round corners , trimmed the extra material from the bottom side and it sits perfectly in her place . All left to do is just trim the upper side .

 

 

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I bet you noticed that the extra weight, of the thicker material came in handy when you got the bend started (it almost falls into place, once you hit the right temperature). Looks great, I don't see any distortions at all.

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