Basic boat design

Hi
I am considering building a steel cruiser (approx.40ft) from a ''purchased'' design and I am keen to understand the sea worthiness of relevant designs. I understand the basic principles of displacement, semi displacement and planing. As I propose to navigate at least around the med. if not further, I know that a planing hull is out. Displacement would do the job but would it be too slow? semi displacement would be fast enough I think but how seaworthy can this type of hull be?
I have seen some large (one would think) ocean going vessels with seating or cockpit areas that could be swamped with large waves or large areas of glass leading to living quarters which could be smashed. Is it obvious that these should be avoided to maintain maximum seaworthiness or am I missing something?

 

Marchaj complains about that kind of thing in "Seaworthiness: The Forgotten Factor". Fast boats seem to be popular these days, regardless of suitability for longer passages. I'm not really interested in motorboats personally, but I like the idea of a displacement trawler with relatively low power and generous fuel tanks. The farther you go in that direction, the less exciting your boat will seem to a novice, but on the other hand, the more options you will have to explore distant areas.

 

Modern planing hulforms (especially Bernard Olesinsky forms) actually tend to be pretty good for sea-keeping. But still not as good as displacement forms.

You're right about glass panels and their ability to smash. Remember, though, that it is a distributed load, not a point load and a window is a plate structure so the actual failure pressure (normal to the pane) is reasonably high. What has been known is for windows to smash due to the incorect transmittion of slamming loads. That happened on a magazine's test some months back. (one of the ybw mags, search www.ybw.com for window smashing or similar).

Maximum boat speed for a displacement craft of that length would be about 8 or 8.5 kts. It really depends what you want to do with it (speeds, conditions etc.). I'd go and talk to either the sales-people (read around the subject too, so you know what's actually crap and what's someone's pet peeve) or preferably talk to the designer (or take a designer to see the boat and get an opinion). Lastly, test sail it and make sure you like the motions.

Personally, I'd have a 40 ft yacht, any day of the week, but there you go...

Tim B.

 

A fast planing monohull receives it's stability by way of it's speed. When there are high seas, choppy as well as the Med can be, the fast boats are actually useless, with the exception of the wavepiercing designs, of course.
Another forgotten factor is the "seaworthyness" of the crew. This factor is always negelected, many found that out to their surprise; the med can be a terrible sea in a heavy Levante, Tramontana or Mistral, to mention a few.
On the other hand, with a real fast boat, you may run for any storm, or don't go out at all.
With a displacement boat, you may ride it out if you are a seaman enough.
Contrary to the north-Atlantic, the med is spoiled with harbours, even at long distance cruising you will always be in time to look for refuge.