Newbie questions

Hi. I know nothing about boats but I'm interested, reading lots and trying to learn.

I have a couple of questions.

1) Capsize recovery: am I correct that in order to recover from a capsize, the ideal hull shape is a circular cross-section with weight concentrated at the bottom? A torpedo-shaped boat, as it were. If that is indeed the ideal hull shape for recovering from capsize, why is it not used outside submarines? Is capsizing not considered a major enough concern to shield against, or are current recovery measures considered sufficiently good?

2) Plastic materials: metal corrodes and wood rots, but plastic doesn't do either to an appreciable degree. Why isn't it used more as a boat building material? Is it structurally weak, even when very thick? Or are there other reasons to eschew plastic hulls? A soda bottle seems to float on water very well without significant degradation over time. Can this technology be scaled up to boat-size?

Please forgive me if the questions are stupid.

 

Ok here is my quick 2 cent reply.

1) Is it better to design a boat to resist capsize, than one that can easily recover from it? All things being equal I think you'll find a circular hull will capsize more readily than any other form. Get some books or do some internet searching for Initial stability and look at some different hull forms.

2) You mention wood and metal. Are you familiar with fiberglass. The resin that holds it stiff is a plastic.

Hope that helps a bit. This is a great forum with tons of knowledge. If you are even a little bit interested in designing you own boat, do yourself a favor and read a book or two on the subject. There are many out there and a quick search of the forum would no doubt lead you to some of them.

 

Why would you want a boat who's sole purpose is to recover from a capsize? Adding any other desirable characteristics will necessitate altering the design to come up with....current boat shapes. A torpedo shape would be hard to live with (or in) unless it was enormous, it's shape is well suited to underwater use but not surface use. It would need enormous ballast to provide reasonable stability (enough to get it to just below the max beam) and it would still be far too easy to roll (again for a hull big enough to be useable). The ends aren't conducive to smooth water flow (again...on the surface at the joint of two different fluids) so extra power would be needed to provide the same results as a more conventional hull shape.

Absolutely waterproof plastic types tend to be overly heavy for their structural strength (look at roto molded kayaks). compromises include polyester (which isn't waterproof, just resistant) and Epoxy (which is MORE water resistant but not proof) but both tend to make a decent hull with less weight. Also...most plastics that might be used are thermo plastics which need complicated (read expensive) molds and heat curing methods to manufacture usable shapes. They would also mostly resemble a clorox bottle.

 

Most production boats are GRP (Glass Reinforced Plastic) construction, so yes, we do use plastic, quite extensively.

It comes down to selecting materials for physical properties, cost, work effort, skills to handle, etc. In short, there are a great many variables to engineering any particular part or assembly of parts.

Yes, ballasted cylindrical shapes will recover quite well, but this is just one aspect of yacht design. This shape also yields very easily under load or can't "stand against it's press", meaning she'll want to flop over with wind pressure. Flattening out this shape a little "stiffens" this roll tendency considerably, especially if coupled with ballast.

To date I've only seen one fully plastic engine, though it did have ceramic cylinder sleeves, so metal would be an obvious choice here. I don't think plastic sparks plugs would be possible, but worth a shot maybe.

Plastic, especially those used in soda bottles, does degrade, rather rapidly in sun light. UV rays will de-polymerize this type of plastic very quickly, breaking down the molecular bonds that hold it together.

You may want to consider some engineering study and possibly yacht design, to satisfy you desires.