Gerr's Strip Plank Scantling Equations

Does anyone have experience using the Dave Gerr's 'Elements of Boat Strength' scantling number method for calculating strip-plank thickness? (Chapter 11) There seems to be a mistake in the calculation example given for 'Logger Bobber' in formula 11-3, determining the strip-plank thickness for an exterior only sheathed hull.

The equation given is 16.51 x Sn^0.3, the example, 16.51 X 2.97^0.3, which, if I'm not going crazy =22.89ish, whereas the example gives an answer of 24.7. This may seem to be a trivial difference for such a method of scantling determination if it is going to be rounded up anyway to say 25mm, but....

In formula 11-7 a strip-plank thickness is given for use with internal and external sheathing, given as 15.24 x Sn^0.34. The example answer I agree with. The problem is I would expect the thickness required to reduce if internal sheathing is added, which in his example, it does by 3mm if the answers are correct, but I don't believe the first one is.

The two equations given, 11-3 and 11-6 give a very small difference. Am I doing something wrong or is there a miss-print in the book? Which equation should I trust?

Also on page 157 (top right) it says, 'With internal frames, bulkheads and backbone structure, no internal hull sheathing is required or recommended Does anyone know a reason why it is not recommended?

While I'm at it, The ABS Guide for Off-shore yachts gives a formula for the required thickness of carvel planking.(7.5.1) It seems to give a small very small thickness, or alternatively a large frame spacing, (going by my very limited knowledge of wooden boat construction). Any comments/suggestions?

 

I would assume that it is not recommended because you will in essense be making a moisture-trap. Someone told me that when glassing balsa, back in the days, it so often turned into a water trap, rotting away the core (which the wood in a fully covered boat is), making it less strong, and in some cases directly dangerous.

Well, at least that is the reason I decided against it, and went for no cloth at all.

Take my "advice" with a grain of salt, though - I'm a newbie, but maybe that is the reason, and you might want to consider it, at least as a possible direction for your research.

 

If anyone was interested, it turns out it was a typo in the book. I managed to contact Dave Gerr and he confirmed this. The equation should be 16.51 x Sn^0.37

 

Well, I am

 

DanishBagger: I have read a similar argument against internal sheathing somewhere before, ie: that it creates a moisture trap. The ABS guide for Offshore Yachts specifically states that 'encapsulated' softwoods where used below the waterline must be considered non-structural core materials (4.7.4).
I have asked Dave Gerr what his resaon was so I'll let you know if he replies...

 

Cool, Tony, Looking forward to hear his response. It's great you contacted him, methinks.

Andre

 

Well, this is what he said...

Elements of Boat Strength has rules for modern wood-epoxy strip-plank both with and without internal sheathing. I personally prefer the systems without internal sheathing simply because there is so much less labor when not installing internal sheathing. (Remember, the entire interior is saturated/painted fully with 3-coats minimum epoxy.) The whole build process is simpler without internal sheathing. You can, however, build a somewhat lighter boat following the internal-sheathing methods--a notably lighter boat using the Lindsay-Lord method.

ABS is behind the times with regard to wood-epoxy construction. With properly designed and built wood-epoxy, there is no problem with water penetration between or under the skins.

Very helpful guy, I will have to go and buy my own copy now.

 

Also take a look at MacNaughton's scantling rules. They are based on Lord's work.

http://www.macnaughtongroup.com/scantlinsheathed.htm
http://www.macnaughtongroup.com/publishingown.htm

 

Yup, he certainly seems nice. Thanks for sharing.

I didn't know that it was a convenience-thing, with modern methods. When he writes that the interior is covered with epoxy, I went "duh, Andre! of course, why didn't that spring to mind!?" In reality, that is indeed covered as it should be.

Thanks again,

Andre

 

I found David Gerrs book for my application to be somewhat general. For a scantling #1.6 exterior weight of glass = 1500gm (50oz) There is no distinction for the use of uni. ie The require stiffening is at 90deg to the strip planking so why bother use woven glass with 50% of it parallel with the strips? I found Davids scantlings to be over conservative and not optimum for my purpose so I am using Tom's. Understandable, Davids are trying to cover a lot of ground.
There are flaws in both as the specific engineering is complex.

BTW---Great thread Tony.

 

I think Gerr list a lot of rules in his book but doesn't give enough details on why and how he ended uo with these rules...