do they rotomold boats evenly or do they let the goo

setup a little heavier on the hull and ends and little lighter on the deck?

I guess the bow and stern points would naturally be thicker from head-on if the thickness of the goo was same everywhere at 90degs from mold surface.

 

Rotomolding is normally done on three axes to prevent that.

 

To some degree the plastic does pool in certain areas, but generally the mold is moved sufficiently to prevent much of it. Simply put, if you cut one up and took a set of mic's to it, you'd find some variances in thickness, but over all these differences would be a fairly small percentage of the total thickness average.

 

wouldn't it be better to mimic non-roto boat construction and make the hull a little thicker at certain places and the deck a little lighter?

Couldn't this be done just be tweaking the spinning a bit?

How about getting fancy by sticking some thin vanes inside the mold that would become stringers inside the hull as it rotos. Probably made of same stuff as the boat. Should be OK if they are "T" channel and the top of the T has a releasable adhesive strong enough to keep it in place for the first layers of rotoing, but weak enough so it wont tear the tough roto-hull apart on mold release.

All the "stringers" I've seen on roto-kayaks are grooves INSET into the hull, which isn't done on other displacement hulls so I assume its less than ideal.

One of the big criticisms of rotoboats is the strictly monocoque construction has disadvantages in weight, strength and shape holding.

 

The molds are rotated, as Gonzo mentioned along all three axises. The only reason for this construction is a very fast, cheap build. It's not particularly light, nor strong or stiff, just fast and cheap.

 

"It's not particularly light, nor strong or stiff, just fast and cheap."

True bit for things like a canoe it works fine.

The usual is to have 3 loaded boxes with an air valve, the first dump is the outer skin, the second a filler material and the last what will be the inner skin.

The canoes I saw being made in Maine used a truck rear axle with a monel mold to shape the hull.AS the mold was heated in a big open furnace.

On completion , after trimming and the wood seat and trim was installed there was still under 2 man hours of labor in the entire canoe.

 

Roto molding and injection molding are about the only types of tooling I have not built so I don't know from experience. But, the cost of building a roto mold and the machine to move it around looks very expensive. When you have expensive tooling you have to produce fast parts and be able to sell a gazillion to pay for the tooling, then the bigger you get the more massive the equipment. I watched a youtube video of them building, I think, a 20 or 22 foot boat and I would be willing to bet that the equipment had a 6 foot thick concrete base to keep from tearing the building down.
I can't see roto molding being in the future of boat building.

 

Only if you don't have to paddle it much. On roto molded canoes they can't get a fine entry and exit, they are more rounded and bulbous. For drifting down a river that makes no difference but for traveling by paddle, it adds up to a huge amount of resistance after a number of hours.

Their redeeming qualities, besides being cheap and easy to mass produce, are they are tough and can take much more punishment than a wood, fiberglass or aluminum canoe. They are highly abrasion resistant and bending that would break or permanently dent or crush regular boats is likely to have little effect on a roto molded thing.

 

Old Town does make woodies (at a hefty price) , but the sure sell a load of plastic world wide.

 

A while back I had an elegant Old Town kayak that was constructed of milk bottle material in some sort of foam sandwich . It was every bit as well designed and constructed as a similarly sized wood or glass boat of its size. So alright it was not one of the cheapies that you can buy at Wal Mart or Target stores. It was somewhat heavier than my self built kayak of ocumee and glass, but not the sluggard like the all roto molded ones.

Most of the kayaks that I see in Florida stores are sit on top (SOT) types which are probably more suited for fishing than the sit in types. I have yet to see a SOT that is as easy to paddle or as fast as a well designed and built wood, glass, or kevlar kayak. Most of them are dogs but they do serve the purpose because the buyers are fishermen not kayakers.

One thing the roto molded Tupperware yaks have going for them is that they are cheap, fairly durable, and easily replaceable.