Welding fiberglass.

[minaret]

You could make a dinghy out of plastic, but you would be restricted to a developable shape (ie no or minimal compound curves), similar to plywood boat building. And plastic welds suck, they are notorious for failure. It would probably sink.

Plastic boats like the Walker Bay, Hobies, and the like are roto molded. Having built some roto molds, I can tell you tooling is extremely expensive. The last mold I built cost over a million dollars, and that doesn't even begin to include the expensive stuff like the autoclave. It's only worth doing if you will be building and selling many thousands of parts.

[Delancey]

Originally Posted by s/v Beth

I have got a weird one. I have just recently became aware of welding for plastics. Various plastics, including HDPE, can be welded by a heat source and rod material. Well, our boats are really just plastic, and a lot of it. So- Can our boats be welded back together instead of glued with epoxy? I have done internet searches on this and found nothing about welding fiberglass. Has anyone tried this?

The distinction between our boats (FRP) and various types of welding extruded or sheet plastics such as HDPE lies in the fact the FRP "plastic" boat is a composite material consisting of fibers cast in a resin matrix whereas most plastic fabrication involves joining parts made of a material that is both homogenous and isometric.

Functionally, FRP panels can be welded through the use of filleted joints to achieve material strength connections. Like the composite materials being joined, this connection often benefits from overlapping layers of glass cloth making the connection itself a composite material. An example of this is the popular "stitch-and-glue" boatbuilding method.

It certainly would be possible to build a welded-HDPE 40' cruising boat, however due to the relatively low stiffness of materials like HDPE you would either need very thick material or a closely spaced lattice of frames and stringers to reduce the unsupported panel size down to an acceptable minimum.

[Delancey]

Originally Posted by s/v Beth

Most Polyesters (which fiberglass is commonly made of) are thermoplastic. I don't know if our Polyester yachts are (which is why I am asking the question)
Polyester - Wikipedia, the free encyclopedia
I did the basic homework on this one- I just need to know what type of polyester resin is in fiberglass.

Regarding resin types - the key there is the distinction between thermoforming and thermosetting. What the HDPE that is used in welding and your dad's leisure suit have in common is that they are both made from a type of plastic that can be shaped and reformed by heating and reheating. It's the thermoforming type.

What your dad's suit and your boat have in common is that the base polymer in both cases is a polyester resin base. The commonality ends there in that they are different in how the polymerization occurs. In the case of your boat a polyester-resin based resin is acted upon by a catalyst to produce a one-way reaction changing the resin from a liquid into a solid.

This one-way polymerization is typical of many resin systems used for composite layup, paint systems, electronic assembly, and foam products including polyester, vinyl ester, epoxy, acrylic, and urethanes. Sometimes the one-way reaction is a two-part system, sometimes a one-part that reacts with the air to produce polymerization like the varnish on your handrails.

Some resins bases can be found in both thermosetting and thermoforming types. Think about your acrylic port-light lenses, or your mom's acrylic Christmas sweater, or your boat's new paint job.

Generally, while polymerization of the thermosetting types is possible to do easily "in the field" the synthesis of the thermoforming types tends to happens on an industrial scale to produce the raw material in the form of small plastic beads. In the case of your plaid shirt, the beads are are melted down and extruded into a fibre and woven into a shirt.

[thinwater]

[QUOTE=s/v Beth;1473684.... I don't know if I have ever seen a HDPE boat with a hole in it.[/QUOTE]

Whitewater kayaks, all the time... but they are tough. But heavy and too flexible.

[nimblemotors]

Originally Posted by Delancey

It certainly would be possible to build a welded-HDPE 40' cruising boat, however due to the relatively low stiffness of materials like HDPE you would either need very thick material or a closely spaced lattice of frames and stringers to reduce the unsupported panel size down to an acceptable minimum.

My 40ft boat made of LDPE idea would use air pressure to create stiffness,
that is how inflatable boats work when made from floppy material.

[Delancey]

If you can get around the stiffness issue polyethylene plastics have a lot going for them in terms of tuffness or bash-ability as evidenced by their use in curbside recycling bins, kayaks, and all the plastic crap floating for the next thousands-of-years in our oceans.

Also on the upside, PE is very recyclable and in its various forms tends to have positive bouyancy.

Link below to an interesting approach to achieving flatness in an inflated structure.

A Massive Inflatable String Jungle Gym by Numen/For Use | Colossal

[s/v Beth]

So HDPE is just too floppy? How do Milk jugs do it then? I guess if they are without Milk then they fold into each other. I agree they are tough though. A while back I epoxied some thin plywood to HDPE- the outer part provided the abrasion resistence, the inner the stiffness.
I have some white plastic as a hard dodger on my boat now- it is stiff as plywood. Why couldn't you build a boat with this?

[Nicholson58]

Originally Posted by Andrew Troup

AFAIK the only plastics which can be welded are thermoplastics, not thermo-set plastics and definitely not catalysed resin-set, reinforced plastics.

However it's not my field, so hopefully someone else will clarify.

SPOT - ON. Totally correct.

This is about the only way to bond to many thermoplastics like Nylon, Polyethylene. Glue won't stick to many of these but you can 'weld' to it.

[colemj]

Originally Posted by s/v Beth

So HDPE is just too floppy? How do Milk jugs do it then? I guess if they are without Milk then they fold into each other. I agree they are tough though. A while back I epoxied some thin plywood to HDPE- the outer part provided the abrasion resistence, the inner the stiffness.
I have some white plastic as a hard dodger on my boat now- it is stiff as plywood. Why couldn't you build a boat with this?

Milk jugs are polypropylene.

Mark

[minaret]

Originally Posted by s/v Beth

So HDPE is just too floppy? How do Milk jugs do it then? I guess if they are without Milk then they fold into each other. I agree they are tough though. A while back I epoxied some thin plywood to HDPE- the outer part provided the abrasion resistence, the inner the stiffness.
I have some white plastic as a hard dodger on my boat now- it is stiff as plywood. Why couldn't you build a boat with this?

Panel stiffness. Joints aside, if you make a plastic panel thick enough to be stiff enough not to oil can, it is incredibly heavy. Same reason most glass boats are cored. Same reason solid glass boats have so much chop matt in the layup. The necessary strength is achieved in half of the layup or less, but it would oil can severely if left that way. So they have to make it thick enough to provide the necessary stiffness. A plastic boat that is stiff enough not to oil can would weigh far too much. Walker Bay gets around this through faux lapstrake design, which only works in the small scale. Kayaks just go ahead and oil can. You will also note that even there, you can compare the same model of 'yak in fiberglass or plastic and the plastic models will average 20-30% heavier. Which is no big deal in a 'yak, 50 lbs instead of forty isn't necessarily a deal breaker. But in larger scale it just doesn't work.

[s/v Beth]

Does anyone know what the Portland Pudgy is made out of? (This is kinda fun having all you plastic experts around)

[nimblemotors]

Originally Posted by s/v Beth

So HDPE is just too floppy? How do Milk jugs do it then? I guess if they are without Milk then they fold into each other. I agree they are tough though. A while back I epoxied some thin plywood to HDPE- the outer part provided the abrasion resistence, the inner the stiffness.
I have some white plastic as a hard dodger on my boat now- it is stiff as plywood. Why couldn't you build a boat with this?

Epoxy will not stick to HDPE, almost nothing will, it is essentially very solid wax.
This can make it difficult to build things with it.

[GordMay]

Originally Posted by s/v Beth

Does anyone know what the Portland Pudgy is made out of? (This is kinda fun having all you plastic experts around)

The Portland Pudgy is rotation-molded polyethylene. The hull is double (hollow inside) and the area under the floor is filled with closed cell foam. It weighs about 128 Lbs.

[Delancey]

I can't say for certain about your hard top from the picture, but assuming it's made from Starboard, my guess that it's at least 3/4" thick and probably has much of its stiffness developed through shape-strength (camber, arch, whatever you want to call it) as well as from framing underneath and it's attachment to the cabin top.

I know I am going to mess this up but I will try to share an anecdote about one of the Gougeon Brothers I read somewhere a long time ago. Jan or Meode is at a conference somewhere to explain why wood is actually a good choice for a boatbuilding material when compared to other more exotic materials like aluminum or carbon.

After pointing out the fact that stiffness and the ability to resist deflection is what you are really looking for in a boat building material, he proceeds to pull several materials samples out of a briefcase. They all have the same weight and measure the same length and width and include samples of Western Red Cedar, aluminum, steel, and a piece of carbon fiber composite.

The dimension of the samples differ only in their thickness as a function of their density. Here's the part I know I am not going to get exactly right (but I can get it close enough to illustrate the point) let's say the steel sample was .060" thick, the aluminum sample was .180" thick, the carbon fiber sample was 1/4" thick, but the lowly WRC sample was 3/4" thick.

Can you guess which were the hardest and easiest samples to bend? If you guess the strongest and densest material was the easiest you are right!

Up to a certain point it doesn't matter what something is made of so much as how thick it is as far a determining how stiff or strong it is going to be. For example it is relatively easy to bend a wire coat hanger, not so much the wood closet rod the hanger is hanging on.

Nice thing about wood is that it really does grow on trees and if harvested responsibly is a renewable material. Of course to compete with modern materials in terms of durability you need to completely encapsulate wood in epoxy a la the Gougeon Brothers WEST System but I think it has a lot going for and should not be overlooked.

One other small bit about the PE plastics, the tend to suffer from something called "creep" which refers to the way in which the plastic molecules slide past each other and elongate under load. Poke your finger through a plastic bag to see what I mean.

[s/v Beth]

I spent the morning welding plastics and looking at stuff. I think that dodger may be ABS, I will have to get a sample and put it through my plastics id tree. The reason I say that is because it stuck great to thin plywood, the they became essentially one material, with one side wood and the other white. PE plastics are spost to make great water tanks, but the stresses on supporting the water are probably different than the stresses to keep the water out.
Anyway, going to make a plastic dingy, just haven't figured which plastic. It is not going to be fiberglass.