The elements of boat strength

@TANSL , that also happens.
Bottom line @rjwintl , don't assume someone here thinks you're dense because they are overexplaining. Worst thing that could happen on an educational forum is that they take you at your word, and stop explaining.

 

I thought this forum was gonna be helpful instead of critical … I play by the “rules” and research the hell out of anything I do to be SAFE !!! I built my wooden boat WITHOUT any Nautical degree and got it titled and registered by the Virginia Dept of Wildlife Services boat division. I saved Thousands of dollars doing it myself and it passed their design parameters. Building houses (over 2000), here and overseas, I used my carpentry knowledge to apply to boat building. I’m a Tesla type to many of this forums Edison type!!! Edison told Tesla that the transfer of electrical power without a cord was impossible, well , Edison was wrong! I believe in innovation! rjw

 

That chip on your shoulder is going to give you a spinal injury. Relax, ask concrete questions. Accept the answers as good value for the money. If you have knowledge to share, share. Like I said, no one thinks you're a dummy.

 

That “chip on my shoulder “ that you perceive only surfaces when a "concrete" (reasonable) question gets belittled or talked-down to by the so-called experts! Here’s a bobtail for ya , "ya don’t need a gel-coat on a 12 foot wooden fishing boat to be safe" ! … rjw

 

With your background I don't understand your attitude at all. I especially don't understand what you have against Gerr. The guy wrote a book to help would be designers. The book is not the law, nobody forces anyone to use it. It's popular because if you follow the content you can produce a sound structure for a displacement monohull of conventional proportions without having much prior training.

 

Registering & Titling Your Watercraft https://dwr.virginia.gov/boating/registration/procedure/

What did the Virginia DWR review? I did not know any US states reviewed the technical aspects of homebuilt boats. I thought they were only interested in verifying that the cost was accurately reported by checking receipts, bill of materials, etc.

 

Dave Gerr paper from 2010 about the origins and derivation of the "Scantling Number" system and rules described in Elements of Boat Strength https://www.gerrmarine.com/ELEMENTS_OF_BOAT_STRENGTH/BoatStrengthIBEX.pdf

An excerpt from Gerr's paper:

How the Sn Scantling Rule Was Created

Where does one start to create a scantling rule? With real boats. We’ve already seen how difficult it is to know what the real loads are on a boat. What we can know with good certainty is the specifications of boats that have been built and have given good service over many years— dimensions, displacement, speed, type of service, material, and scantlings actually built to. These data can be analyzed (regressed) to obtain the desired rules. The data and the results are constantly checked and rechecked against other real-world boats, against basic class rule results and against first principles.

With any such process, the results are not going to and should not match any other specific rules’, or methods’ results, but the scantlings from the resulting new rule should produce consistently reasonable structures, that meet reliable criteria for strength.

The real boat data available to work with came from:

- My own designs actually built and in service (and from the MacLear & Harris office and from Cape Dory Yachts) -
- Boats from other designers for which I had data
- Boat data from the three-volume set of Fishing Boats of the World
- Additional wooden boat data from USCG NVIC 16-60, Scantlings for Wooden Passenger Vessels
​

This totaled hundreds of different vessels as a reference database.

Results were checked against:

- Engineering by first principles
- ABS
- Lloyds
​

Again the goal was not specifically to meet any of the class rules, particularly given the unknowns in loads, but to be close on the majority of the structure. One of the interesting findings was how strong many real-world boat structures actually are, even without meeting the requirements of class rules. Stiffness and strength in real boats, often are considerably greater than such rules might indicate.

One of the interesting findings was how strong many real-world boat structures actually are, even without meeting the requirements of class rules. Stiffness and strength in real boats, often are considerably greater than such rules might indicate.​

 

Yes, the VA DWR checks the “seaworthy-ness” of a home built boat on the auspices of Safety … in addition to submitting receipts for your boat materials , and pictures of all sides and interior construction of your boat they want it sent digitally over their website (I submitted it both digitally and photo copies in the mail)! They also require you to pass the USCG /NASBLA ABC3 Boater Safety Course ( 8 hr seminar , then a comprehensive test, no open book , not as easy as you might think either) … LOL !!! … nothing like 21st century regulations … rjw

 

Dave Gerr's Scantling Number (SN) is based on the product of the Length, Beam and Depth of the hull, with some rules on how to determine the values to use. Displacement is not used.

From his paper (link above) on why displacement was not used:

"Originally, I was going to use displacement in tons as the basic reference number. This works well, but it has two drawbacks. One is that there is a slight difference between U.S. and metric tons. More important, you often don’t know what a boat will weigh or what it does weigh. The designer or builder would then have to guesstimate displacement. I didn’t want to start calculations with a guess. This would require preliminary calculations based on an estimated displacement and subsequent recalcs to match the final designed or as-loaded displacement."​

I am confused by how a design can progress very far without an estimate of displacement. How is an initial set of lines derived?

 

Seriously man, seaworthy-ness from photos? You mean like "boat appears to be in one piece", "the paint is shiny" and other such important things. There is no applicable standard for structure or stability in privately used boats in the US, not even for commercially buildt ones.

 

You're saying this, like it is a bad thing, or something extremely difficult to do?

 

You misunderstand the difference between critiquing/questioning...as any one would do and faces and can answer objectively, to that of taking any comment as a personal criticism, which is not engineering.
There are no emotions in engineering.. just facts.

 

The book is not about the whole design spiral, only about structure. If you have the dimensions his formulas produce scantlings wich you can then use to calculate structure weight. This weight you then use in the final displacement calculations. This is exactly why Gerr's method can not be used for more extreme boats (ultralight, very heavy, etc.), the scantlings are not very well correlated to the actual boat, he just assumes "typical" shapes and displacements. The results are usually enough to produce a strong/stiff boat and allow final shape adjustments without recalculating the scantlings.

 

That was Gerr's objective as described in the paper. What I find confusing or perplexing is the implication that it should be possible to design a boat without a weight estimate. How is the hull shape designed without a weight estimate?

How sensitive are minimum scantlings to displacement/weight in reality?

 

The clue is here:

L x B x D is not far off a simple:
length x beam x draft x Cb = displacement.
I don't have the book, thus can only infer...but it is suggestive to me that in his fudge of coefficients, he has taken the equivalent of a Cb into account to arrive at a "nominal" displacement based upon the 3 main parameters.
Which one could do - roughly - anyway, if one is in possession of said parameters.