Dave Gerr Boat Strength Scantling

I am reading Dave Gerr book ''Boat Strength''. I was wondering were do the scantling come from. His own experience or a standard?

Also ISO 12217 do they talk about scantling or is there another ISO standard that talks about scantling?

It is for the design and construction of a 30 foot sailboat.

Thank you

 

I've got the same book. They would be based on his own experience as well as on that of others he has learned from. For example, he passes along Lord's scantling rules for plywood without much comment. I've read NAs say around here that they will yield somewhat conservative values but since they are meant to include people building their own boats I would say that conservative is better than flimsy.

From what I've read most scantling rules should be the same, regulators (usually) like recreational boating but recreational rescuing ... not so much.

 

If you want to get lighter hulls, it is preferable that you use some of the current standards. Specifically for small boats, including recreational ones, the ISO 12215 standards have been created. ISO 12217 deals with stability, rather than scantling. It is doubtful that some of the scantlings obtained with Dave Gerr's rules will be admitted today, especially in composite materials.

 

Does anyone know of any comparisons of Gerr's scantlings with those in ISO 12215?

 

You can make the comparisons you want but, in construction with composite materials, always keep in mind that a thickness, even if it is very large, may not be enough to achieve adequate resistance. It is mandatory, in accordance with current standards, to perform an analysis of the stresses supported by each layer which, in the procedures used by Dave Gerr, whichever they are, is not contemplated. So, I'm not sure what comparisons could be made.

 

No comparison is possible because for Gerr's 'rules' there are no operational conditions. Under ISO or any other engineering-type rules, we are designing for operational envelope, thus acceleration, speed vs sea state. This will define lightness of the craft, and the conditions when the craft will actually break down.

With Gerr's 'rules' we don't really know what was in his mind. One can get same weights, but how about operational limitations?
In my opinion, knowledge of how to make formulas in Excel is not enough to compile structural rules.

 

8 years ago I investigated the safety factor of the keel attachement and compared Gerr's design to the ABS-rule, valid at that time.
Gerr's static safety factor was almost twice as high.
http://www.remmlinger.com/droptest.pdf

 

That would make sense. Generic scantling systems probably tread a fine line between being useful and making you a snack-in-the-water to circling lawyers. Governing bodies or NGOs seem like they would have the advantage of being harder to sue.

EDIT: wow, that sounds pretty cynical....

 

Hi,

We used Dave Gerr's SN for our build after check them against ABS (steel hull). They are a bit beefier, but as we have a heavy displacement hull, it wasn't a massive consideration.

I thing it might be worthwhile to mention the material you are thinking about.

Cheers,
Mark

 

https://www.gerrmarine.com/ELEMENTS_OF_BOAT_STRENGTH/BoatStrengthIBEX.pdf

This link provides some supplementary info to the book 10 years after it was published

 

This paper is more interesting than the book itself If Gerr had included it as chapter into his book, would be a great value as principles and assumptions are reviewed.

 

Thank you all for your reply. Very interesting information.

If I use the Dave Gerr scantling method and then I compare with the requirements of ISO 12215, I should be on a good track for a 30 foot sailboat.

Do you know if the ISO 12217 for stabilty will work for a catboat type desings, shallow-draft, wide beam 2:1 LOA?

Someone ask for the material. I am pretty sure that I will use plywood, cold molded and/or strip plaking covered in epoxy with fibeglass cloth.

 

I do not recommend it. Use ISO 12215, if possible, the differences may be appreciable.
For motor boats larger than 6 m (LOA) in length you can use ISO 12217-1. For motor boats smaller than 6 m in length you have to use ISO 12217-3. ISO 12217-2 is for sailing boats.

 

Cheers for this.
A most interesting read...!!!!

 

If you plan on using sheet material (plywood), why not just go aluminium ?