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post #21 of Old 05-19-2007
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Originally Posted by RichH
“A boatbuilders definition would be somewhat different.
A 'blue water' boat would have inherent/redundant strength at 5-6 times of the 'normal service design'. 'Coastal design' at about 3 times safety factor. 'Inshore design' at twice the inbuilt strength (safety factor).”
A boatbuilders definition of offshore capable is only what the marketing departments says it is.

Some very interesting thoughts in this thread but I think there are two thing wrong with the idea of categorizing a given boat offshore capable or not by a designed “safety factor”. First I think a range of 2 to 6 is much more then you see between the average boats in each group. Also the boats ability to live offshore is the result of a very complex interaction between parts and systems with each system having its own designed “safety factor”. But like the chain, the weakest link is the critical component so looking at the “safety factor” in the hull is not as important as looking at the “safety factor” in the rig or rudderpost because more failures occur there then in the hull. To take it a step further when I say a complex interaction between parts just look at the effect it has when you decide to step a mast on the keel instead of on the deck. That one change if you keep the same section affects the strength of the rig more then the added strength you designed in by using the more or less American industry standard factor of safety of 2.5 to 3 for rig design. And by the way that 2.5 to 3 is the same number used for design if the boat is a light day-sailor or a heavy cruiser. Other factors increase the margin of safety when you design a heavy offshore boat so the same factor of safety can be used for both the light and heavy boat. The exception to this would be if you used the European method of design represented by the “Nordic Boat Standard” where the factor of safety is 3.2 for the V1 shroud and 2.8 or 2.5 for the D1 depending on whether the boat has two lowers or one. But again, even in the NBS it doesn’t matter where the boat is being used because other factors determine the suitability of the boat for offshore service.

Let’s look at the difference between inshore and offshore boats. First let’s consider small boats which don’t have the same demands placed on them as in the larger boats. A good example would be the 22 foot Sea Sprite designed by Carl Alberg. I have owned two version of this boat with the first being a day sailor with an open cockpit and clearly intended to be a day sailor and no more. The second Sea Sprit I owned was built new by the manufacturer with input from the designer to be used on my first trans-Atlantic crossing. That boat had the same mast section and shroud size as the day sailor. In this case both boats had a “safety factor” in the rig that was the same but both boats were unmistakably intended for very different service. A small boat of suitable shape can be built just like any other boat of her class and be just as safe offshore. Of course you need to pick a suitable boat and I have some thoughts on that.

As boats get larger the loads increase compared to the size of the boat and offshore capable boats tend to be built stronger then inshore boats. But how much stronger is the question. Let’s look at my Tartan 34C for a moment. She is intended for near shore cruising and when she was built, CCA style racing. The upper shroud on my boat is 1/4" wire and it has a breaking strength of approximately 7,500 pounds.

Originally Posted by RichH
“Inotherwords a design-specific 'blue water' boat would be built at a minimum twice as strong as a 'coastal' design and 3 times as strong as an 'inshore' design.”
So does this mean that if my boat were being designed to go offshore she would have an upper capable of holding 7,500 X 2 or 15,000 pounds? That’s a wire with a diameter of 3/8” and I know that’s too large for a 34 foot boat. So I must be wrong and my boat is already designed for offshore work and we need to work backwards and find the wire suitable for coastal sailing or 7,500 / 2 or 3,750 pounds. I think that works out to be 3/16” wire and again that’s wrong because its way too small. If we just use this example I think it’s safe to say that RichH is using too large a range when he is comparing offshore to coastal boats and of course way, way too large a range when he adds inshore to the group because 7,500 / 3 is 2,500 or just a 5/32” wire and I don’t need to point out how small that is for a 34 foot boat. Work the number out for your own boat and see what you get.

How do you decide if a boat is suitable for offshore work when considering just the structure? Up until very recently boats were designed by using scantling tables. Those tables all used in one way or another the displacement of the boat for the design of the table. In other words a heavy displacement 30 foot 15,000 pound boat had the same size deck beam as a light displacement racing boat that was 45 feet long and also weighed 15,000 pounds. It’s easy to see which boat was stronger. Just like scantling rules I suggest that when comparing similar design boats the trick is to look at the displacement length ration, D/L = (Disp/2240)/(0.01*LWL)3. The higher number represents the stronger boat all other things being equal. Look at the Cal 40 and the Beneteau 393 as one example. Both are 39.3 feet in length and the Cal 40 has a displacement length ratio of 249 and the Beneteau is only 191. I think you need to select a style of boat that you are comfortable with and use the D/L ratio as a helpful indication of relative strength. Remember I said indication of relative strength and you still need to look at the details.

Originally Posted by Cruisingdad
Interesting definition... but what is the normal service design?? Or is that the million dollar question!! (smile).
That is a good question but I don’t think any practicing small craft designers are consulting with the insurance companies when it comes to design.

If you use a scantling table for hull design is there even a factor of safety to discuss? Not really because you don’t know the strain or the strength of the structure. In the last few years a lot of work was done and some numbers for load have been developed but even with modern FEA (finite element analyses) software designing by the numbers is still iffy as evidenced by the recent problems with keels dropping off one well know brand. The ISO method of hull design for local loads, popular in Europe because of legal requirements, is to assume a hydrostatic load from water a prescribed distance above the weather deck. But this is just for local loads and ignores global loads such as from the rig and bending moment from waves and is still just a crude estimate and only a step away from scantling tables. The ISO rule also ignores the hydrodynamic load. It is possible to calculate the required section modules of the hull the same way it’s done for ship design but the answer is not as useful as in ship design because the real load at sea is too hard to put a number to.

Principles of Yacht Design by Larsson and Eliasson has a great chapter about hull design by the numbers and if you have an interest and are mathematically inclined it’s worth a read. If you want to look at the strength of the hull in a more user friendly way then Dave Gerr’s book, Boat Strength is great and his scantling table is based on more then just the displacement of the boat. It uses a scantling number derived from length, beam, depth and displacement.
All the best,
Robert Gainer
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