Design Idea

I agree with Petros. The HDML was seaworthy maybe but not sea-kindly according to Wiki -

"Although seaworthy, the boat had a considerable tendency to roll, especially when taking seas at anything other than directly ahead or astern. The cause was the round bilge midship section and a considerable reserve of stability, the effect of which was to impart a powerful righting moment if the boat was pushed over in a seaway. This, coupled with the round bilged hull and lack of bilge keels, would set up a rapid and violent rolling. One of the design criteria was that the boat had to be capable of turning inside the turning circle of a submerged submarine. To achieve this, HDMLs were fitted with two very large rudders and, to reduce resistance to turning, the keel ended 13 ft (4.0 m) before the stern. A side effect of this was that the boat lacked directional stability and was extremely difficult to hold on a straight course."

I bet the crews hated the damned things, but they certainly did get around.

- it is not clear what you want to do with this boat. It's a 70+ year old design that probably hasn't been used seriously for decades even by navies, was hardly ever used for its original purpose, was pressed into service in other theaters of war by a desperate nation battling for survival and never intended for pleasure.

 

The woodpecker looks a bit like the HDML. My basic idea was to take a good design and make it better. Lack of keel area is simple to sort. Obsolete to me means there is something better available to do the job.

 

Except that she's about half the weight with 2' less beam. With a total of 200 HP installed Woodpecker cruises at 12 knots and tops out at 13.5.

 

So Woodpecker is also a bit like a passagemaker lite but woodpecker is probably a lot more expensive and probably not quite so good at passagemaking. What I'm going to end up with is a passagemaker lite in steel also not as good as the real thing.
This for me is a project goal to help me learn boat design. I enjoy calculations but at the moment the only way for me to estimate a boat's weight is to find a similar design and guess. What do you think of this idea for the basic hull weight before fitting out.

calculate the wetted area using Taylor's formula then multiply by 4. 1/4 steel is 10lbs a square foot.

calculating the outside area does not help much because I don't know how many internal frames are required or what size and thickness they would need to be.

 

Well Mik guessing the weight accurately is why they pay us the big bucks.....It's a secret you see...

There are a dozen rules of thumb and every experienced designer has developed a method that works for him or her. This is why you need to serve an apprenticeship under someone actually practicing......to learn the "secrets" of the trade.....It takes years to figure this out and you get it wrong a few times and learn from that.

For steel vessels Tom Colvin uses the skin and bulkhead weight doubled for framing....but his are massively heavily framed hulls.

One way to look at comparisons is with a "cubic number"....Length * Beam * Depth (deck to fairbody). Divide the product by 100 and multiply by your "fudge factor" (less than one) and get the weight in long tons. Fudge factors range from .21 for light-weight high-speed yachts, to .55 for the heaviest construction. This is only for the first trial, as you refine the design you also refine the weight study.

 

Thank you Tad, the cubic number rule of thumb is exactly what I was trying to work out.

 

Have you viewed steve dashews boat 64 ,83, and now a 97foot is being built ?