practice hull

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lostintime
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practice hull

Post by lostintime » Thu Apr 12, 2018 3:33 am

Cabin fever set in and I built a 8' practice boat in my house, the wife was not as impressed with the mess as my daughter was with her new boat. I just used west system epoxy on some oak and birch ply I had laying around, but since marine grade is unavailable locally and shipping costs are enormous, would it be feasible/are there common alternatives that work if I were to roll a few layers of epoxy on a full size boat.

ps. It doesn't show well in the pics but has 3" rocker and floats well
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Re: practice hull

Post by lostintime » Thu Apr 12, 2018 3:42 am

The first pic was after a layer of fiberglass, inside frame is radiata pine laminated bows with oak stringers
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Lionel Connell
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Re: practice hull

Post by Lionel Connell » Thu Apr 12, 2018 4:55 am

Assuming that your boat is going to live on a trailer and not in the water.

If you use epoxy on both the inside and outside so as to completely encapsulate the timber you can use basically anything for a core in the sandwich. But, you really need to use glass cloth as a sheathing because otherwise you will eventually get stress cracks in the epoxy that will allow the water to pass. On a small boat say to 15-16ft one layer of 6oz cloth inside and out would be sufficient.Usually with at least three further coats of epoxy after you lay the glass. Perhaps 2 layers of 6oz cloth underneath if you think that you may be beaching the boat. For internal areas where you are unable to apply the glass cloth due to complexity it seems generally agreed that you need to apply at least 5-6 coats of epoxy to ensure a good water proof barrier. Remember that epoxy has has poor UV stability and needs to be over coated with UV varnish or paint.

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Re: practice hull

Post by Lopez Mike » Thu Apr 12, 2018 8:08 am

I need to start a post about my recent build. It might be very helpful for a new builder or even a very experienced one.

One of the things that saved all sorts of labor was to pre-coat all of the plywood before cutting out the shapes. It is a stitch and glue design. In my case it was a 24' hull so I first scarfed three sheets of 12mm plywood together end to end. Then rolled on a coat of epoxy. Then, before the amine blush came to the surface but after the epoxy was no longer tacky, I laid on the cloth and using a foam brush, applied enough epoxy to fill the weave. Then, again, while the cloth/epoxy combination was still at the 'leathery' stage, applied a last coat. When this had cured completely I washed the surface down with a water/vinegar mix and then sanded it all smooth on both sides.

Yes, there was some leftover pieces of plywood on which the epoxy and cloth was perhaps wasted but not that much. A boat uses up a lot of smaller bits.

This worked especially we'll when it came time to deal with the interior. No trying to coat complex shapes and hidden corners with cloth and resin.

I will start a new post with pictures. It will take a few days. I was able to have a floating 24' hull with exterior paint and a varnished deck in 150 hours of work.
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Re: practice hull

Post by fredrosse » Thu Apr 12, 2018 12:01 pm

Mike has mentioned a very important task: "One of the things that saved all sorts of labor was to pre-coat all of the plywood before cutting out the shapes. .....This worked especially we'll when it came time to deal with the interior. No trying to coat complex shapes and hidden corners with cloth and resin."

When I made my sidewheeler hull I neglected to do this, although when building started mcandrew1894 suggested "Coat all of your ply BEFORE you cut and fit it. It simplifies the job more than I can describe. 2 coats minimum. As far as interior, I would glass it too, at least below the chine in the bilge. My reason? You will be soaking it in water, oil and ash and then drop every sharp metallic object you ever bring on board on it, sharp side down! If it's glassed, it has some armor. "

I was putting down fiberglass armor inside my hull a year later, costing me about 5x the labor compared to doing it as suggested above, installing cloth inside the hull, between the frames was a big PITA, and the job is no where near as neat as it would have been if I had followed proper advice.

Probably the most important error I made in plywood boatbuilding, just passing it along as a REALLY important thing to do.
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Re: practice hull

Post by lostintime » Thu Apr 12, 2018 8:44 pm

That's a great idea I would not have thought of. Do you just put the fiberglass on the inside and glass the outside when it is shaped, or can you still bend it with the glass on both sides? I've seen stuff get really stiff with glass on both sides, but nothing I've made was near as long. Also do you sand the glue points, use peel ply, or have any trick surface prep steps before assembly?

Lopez Mike, a build thread would be great. That's a damn fine looking boat
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Re: practice hull

Post by fredrosse » Thu Apr 12, 2018 11:36 pm

With the epoxy on both sides of the sheet material, of course the material gets stiffer in bending, but with the typical moderate curvature found on sheet material boats this is probably a non-issue, but if it might be, make a test piece to verify how much bending you are comfortable with.

For a hull in the 15 - 25 foot range, the epoxy/cloth could be applied to the inside sheet surfaces, assembled to the hull shape, and then the outer hull coating of fiberglass cloth and epoxy can be applied after that. My 20 ft sidewheeler hull was epoxied to the frames while upside down, then the bottom (waterside) was glassed and epoxied. A few days later, turned +/-90 degrees to glass and epoxy each side, then turned right side up. Epoxy tends to run and make drip marks more easily than paint, so it helps to have surfaces near horizontal when filling the weave of the glass cloth.

I have applied new epoxy to old epoxy many times, and it sticks with excellent structural integrity even if the first layer is fully cured, and with no surface preparation (like sanding, or roughening surfaces) to get good adhesion. Just fully clean the old epoxy surface, and apply the new. I have always had this result with West System premium epoxy, as well as Raka epoxy, which costs far less.
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Re: practice hull

Post by Lopez Mike » Thu Apr 12, 2018 11:48 pm

For the sheets that became the bottoms and sides of my hull I only precoated the sides that were to become the insides and did the outsides later. But for all of the bulkheads I did both sides ahead of time. I'm not sure that I could have been able to tell whether the sheets were stiffer after coating. I'm sure they were but it wasn't a problem.

The only place on a single chine launch hull where curvature is always a problem is at the front on the bottom. There is quite a bit of compound curvature there. I dealt with it by running a large number of fairly deep cuts with my skill saw. Then after the bottom and side were wired together and the fillet of thickened epoxy was done inside, I filled the cuts with thickened epoxy and glassed over them. It's not possible to see them inside or outside.

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Yes, I need to get the post started on the build.
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Re: practice hull

Post by Lionel Connell » Fri Apr 13, 2018 6:29 am

Here is a further extension to glassing the plywood in advance trick that is used in making large model aircraft and light aircraft wings, this not only saves a lot of time but provides an amazing finish.

Take a large sheet of window glass and apply 8-10 coats of wax and buff.
Paint the glass with top coat paint using brush or roller, let dry. Several coats.
Paint the top coat with undercoat, let dry
Lay glass and epoxy over the under coat and lay the ply on top, vacuum bagging is best but not mandatory. Let cure.
Peel the ply off the glass and you have a paint finish that is equal in quality to a new car with ZERO sanding at any point.

Varnish can be applied in replacement to the paint and undercoat in the above process and you have a mirror finish wood grain surface with ZERO sanding.
This bright finish process really does require a vacuum bag to ensure that there are no air bubbles between the glass layup and the timber.

In both of the processes above it the important to paint a wet coat of epoxy onto the wood before laying the wood over the still wet epoxy/glass layup. It is best to apply the plywood to the epoxy layup by starting at one side and laying down the sheet with a rolling rolling action to avoid trapping air between the two.

Although this all sounds like it would be difficult it is not. Try a test on 2 pieces of ply 1ft square. Use this process on one and the standard glass layup and paint with sanding and filling on the other. By the time you are finished you will be totally amazed at how much easier it is.

For a painted finish where you will be cutting and joining pieces of the ply together most people would just start with undercoat in the layup and then paint the finished assembly after filling and sanding the joins.

For RC model aeroplane wings skins some builders will start by applying clear coat to the glass window, then graphics, then paint and so on. At the end they pull the ply off the glass and the result is mind blowing. With no painting skills at all they can achieve a perfectly flat glossy finish with ZERO sanding.

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Re: practice hull

Post by Mike Rometer » Fri Apr 13, 2018 8:11 am

That's a new one on me! Not heard of that before.
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