Gerr's Strip Plank Scantling Equations

[Tony Fish]

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?

[DanishBagger]

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.

[Tony Fish]

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

[DanishBagger]

Well, I am

[Tony Fish]

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...

[DanishBagger]

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

Andre

[Tony Fish]

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.

[Raggi_Thor]

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

[DanishBagger]

Originally Posted by Tony Fish

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.

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

[Roly]

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.

[Raggi_Thor]

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...