Re: [bolger] Re: Plywood thickness for lapstrake hull

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• ... Me too. It has been 25+ years since I last saw that formula in a University classroom. IMO, moment of inertia formulas aren t really the most important
Message 1 of 14 , Mar 3, 2010
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On Wed, Mar 3, 2010 at 4:05 AM, Andrew <a.c.l.yen@...> wrote:

>. I = b(d^3)/12

Me too. It has been 25+ years since I last saw that formula in a
University classroom.

IMO, moment of inertia formulas aren't really the most important
factor with bent planking. Unlike static beams, planks are sprung
into shape and fastened at the seams. With lapstrake, you end up
getting the stiffening effect of a 3D shell effect/skin effect (or
whatever it is called!).

An extreme example of this 3D strength is a skin on frame kayak, which
fails the moment of inertia check miserably, but works as a boat just
fine. Other examples are all those paper lapstrake models which I
built up using thick paper, scissors and cellophane tape. The paper
'lapstrakes' fail a moment of inertia test individually, but gains
immense strength when fastened together.

The two controlling issues which I think are more important (in my
limited experience building just a small number of lapstrake full size
boats) are:

1) Looks. You need enough strake thickness to properly hold a fair
curve as it bends between the frames. A plank which is too thin gives
you an ugly wobble if it is asked to span too far between frames.

2) The "puncture" strength desired. Should you plan on having the
boat touching anything hard (a skin on frame kayak floats fine in the
water but can be trouble on a rocky beach).

3) Construction. If you go too thick, you will find it difficult to
bend and twist the planks to the needed shapes.
• Andrew, thanks for sharing the fruit of your studies. Do your calculations include stiffness of the glued portions of the overlapped strakes which are double
Message 2 of 14 , Mar 3, 2010
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Andrew, thanks for sharing the fruit of your studies. Do your calculations include stiffness of the glued portions of the overlapped strakes which are double the thickness of the individual strake?

--- In bolger@yahoogroups.com, "Andrew" <a.c.l.yen@...> wrote:
>
> ...Assuming a 120mm (4 3/4")wide plank with a 20mm (3/4")overlap:
>
> 12mm (1/2") lapstrake is 73% stiffer than 10mm (3/8") lapstrake
> 15mm (5/8") lapstrake is 240% stiffer than 10mm lapstrake...
> ...Lapstrake construction really does stiffen up a hull.
>

My Dad had a couple of Lymans when I was young. They were beautiful boats.

While I can appreciate a lapstrake hull being very strong, if it is made from plywood, the daunting task of protecting all that exposed plywood edge seems to me to be way to much to expect success. It will be very easy to lose the boat to rot/delamination.

Having read Ruel Parker's "New Cold Moulding" (approximate name) book, I would favor his method, or a variation of it in layers of thin plywood, even though he doesn't think it to be suitable to boats below about 25'. I would substitute thin planks made from thin plywood for the 3/4" common lumber stock he suggests starting with for his larger hulls of 40' or more.
• Hi Fred, ... ...snip... I don t have Gerr s book (yet), so I have to ask if this formula is intended to apply only to certain hull types or uses like, say,
Message 3 of 14 , Mar 3, 2010
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Hi Fred,

> My copy of Gerr's Elements of Boat Strength is at home, and I'm at my son's
> house, but I would suspect that 3/8" is pushing it. As I remember it,
> planking thickness is the Scantling Number raised to the 0.37 power, and
> Scantling Number is length times beam times midship depth divided by 1,000.
...snip...

I don't have Gerr's book (yet), so I have to ask if this formula is intended to apply only to certain hull types or uses like, say, ballasted keel boats intended for offshore voyaging. Certainly it seems that the displacement should play a factor. For example, I get values of approximately 0.54 for the Light Schooner and 0.61 for Birdwatcher, both of which have 1/4" side panels. I don't recall anyone complaining about the strength of the side panels on these boats, but of course neither of them are intended for offshore voyaging either. A more extreme example is a hypothetical 16' x 2.5' x 1' kayak. I get a value of 0.30 (7.62 mm), yet most are built with 4mm ply.

It seems that a second consideration must be planking species. If the formula is intended to apply to all species, then it must be biased toward lighter or weaker wood. Are two carvel planked boats the same "strength" if one is planked with red cedar and one with teak of the same thickness?

Nevertheless, I agree that 3/8" sounds light for a 25-footer of Chebacco's displacement. It also sounds like I need to get this book - if it's as thorough as "The Propeller Book" it should be quite useful.

Jon Kolb
• ... I ve worked though some simple examples both using Gerr s book, and using the beam formulas. You can find a discussion of the latter in Arthur Edmond s
Message 4 of 14 , Mar 3, 2010
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> I've ordered a copy of Gerr's Elements of Boat Strength.

I've worked though some simple examples both using Gerr's book, and using the "beam" formulas. You can find a discussion of the latter in Arthur Edmond's book Designing Power and Sail, but I make this observation with some reservation because the layout of the book has a zillion obscuring errors that make it hard to be sure you've got things right.

I also found that it was hard to get the engineering constants for plywood with confidence that the meant the same thing.

I think you will find that Bolger's construction is lighter that you get from most any rule. I think this is due to a couple different factors, including that Bolger had a highly refined intuition (guided by experience) about these things, and didn't think great stiffness was very important in very small boat. Also, rules tend to be accurate in some middle range, but off-kilter at the ends.
• On Wed, Mar 3, 2010 at 8:44 AM, adventures_in_astrophotography
Message 5 of 14 , Mar 3, 2010
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On Wed, Mar 3, 2010 at 8:44 AM, adventures_in_astrophotography <jon@...> wrote:

I don't have Gerr's book (yet), so I have to ask if this formula is intended to apply only to certain hull types or uses like, say, ballasted keel boats intended for offshore voyaging.

I think Gerr is conservative and designing for ocean use, so he wants a built-in margin of error. Bolger talks about this too, saying that a lot of scantlings on old boats were really heavy because they expected rot to cause a weakening over time. (It's like our famous I-35 collapsed bridge, which was designed for a safety factor of three, but over time they kept adding to the original structure.) Gerr's rules are based on plank on frame construction. He also has rules for framing member sizing and spacing, fiberglass covering, bulkhead number, etc.

Canoes, kayaks and the like are skin-stressed boats. Their loads are carried by the skin structure and because they're monocoque, they can get by with lighter members. But you give up puncture resistance.
Another interesting author is Basil Greenhill, the archaeologist who developed a taxonomy of boats. Any of his books are wonderful, especially where he describes skin stressed and plank on frame, their history and evolution.

fred s.

• ... Yes, Bolger mentions that you can build a Brick from half-inch plywood and it will last forever, barring rot, or build it from quarter-inch and you ll use
Message 6 of 14 , Mar 3, 2010
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On Wed, Mar 3, 2010 at 1:39 PM, Peter wrote:

I think you will find that Bolger's construction is lighter that you get from most any rule. I think this is due to a couple different factors, including that Bolger had a highly refined intuition (guided by experience) about these things, and didn't think great stiffness was very important in very small boat. Also, rules tend to be accurate in some middle range, but off-kilter at the ends.

Yes, Bolger mentions that you can build a Brick from  half-inch plywood and it will last forever, barring rot, or build it from quarter-inch and you'll use it more because it will be easier to handle but just figure on replacing it every so often. For proof, he tells the story of destroying a Tortoise by overdriving it.

Materials and type of construction is also very important. I just read about some tsunami driven shipping containers destroying houses in Chile. Shipping containers are steel monocoque structures weighing more than an order of magnitude less than a house but still able to destroy the house. Plank on frame is like house  construction, a conglomeration of bits and pieces held together mechanically at discrete locations. It will require heavy scantlings.

fred s.

• ... In my calculations I assumed that the stiffness of the hull will be related to the stiffness of the planks in bending. As we know, the round boat
Message 7 of 14 , Mar 7, 2010
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--- In bolger@yahoogroups.com, "daschultz2000" <daschultz8275@...> wrote:
>
>
> Andrew, thanks for sharing the fruit of your studies. Do your calculations include stiffness of the glued portions of the overlapped strakes which are double the thickness of the individual strake?
>
> --- In bolger@yahoogroups.com, "Andrew" <a.c.l.yen@> wrote:
> >
> > ...Assuming a 120mm (4 3/4")wide plank with a 20mm (3/4")overlap:
> >
> > 12mm (1/2") lapstrake is 73% stiffer than 10mm (3/8") lapstrake
> > 15mm (5/8") lapstrake is 240% stiffer than 10mm lapstrake...
> > ...Lapstrake construction really does stiffen up a hull.
> >
>
<snip>

In my calculations I assumed that the stiffness of the hull will be related to the stiffness of the planks in bending. As we know, the round boat shapes increase stiffness considerably over flat planks stiffness - but I still think that if you double the stiffness of the planks then you will - more or less - double the stiffness of the boat.

So that is why I calculated the stiffness of a flat 120mm wide plank with 20mm of it being double thickness (ie. the lap). What surprised me was the big difference between lapped planks versus non-overlapping planks. The huge difference between 12mm and 15mm was also interesting.

I realise that the stiffness of the planks will work against me in construction as will the extra weight so I was wondering if there were some potential savings/shortcuts to be had.

Andrew
• One data point: This weekend was the Snowe Row - over 100 pulling and paddled boats racing in Boston Harbor. A bunch were 26 pilot boats, planked up of 1/4
Message 8 of 14 , Mar 7, 2010
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One data point:

This weekend was the Snowe Row - over 100 pulling and paddled boats racing in Boston Harbor. A bunch were 26' pilot boats, planked up of 1/4" lapstrake, epoxied.

The newest boat in the race ("engineered" and built by my brother, Don - I was on the shakedown cruise Friday evening) was a 6-oared 32' coxed gig (?) planked up of 1/4" ply and epoxy, with NO ribs or transverse framing - none - just 1x10 thwarts.

A bit light? At 300 lbs, maybe, but with a purpose. The race is a "LeMans" start, with crews running down the beach to the boats, which are crowded bow-on-the-sand; boats must be boarded, backed out, and turned around before getting down to the racing. In an attempt to gain an edge, the crew of seven youngsters from New York (only one of whom had ever seen her before) picked her up - all 32' - carried her up the beach, turned her around, launched her bow-first, and boarded over the stern. All this before the rest of the fleet had managed to turn around and start pulling.

My point? In 20 years of building, rowing, and sailing such very light boats, hull failure has not been an issue. They are far lighter than any scantling rule would allow, but have held up just fine, and refinement/lightening continues.

Use common sense, yes. Bolger's scantlings will make a solid reliable boat, with some safety factor for inexperience and construction errors. But if you know what you are doing, you can sometimes build much lighter.

--- Andrew wrote:

> Hi,
>
> I read somewhere (probably on www.chebacco.com) that someone has built a lapstrake Chebacco in 3/8" plywood instead of the designed 1/2". The builder had obtained Phil's blessing for this alteration.
>
> This would lighten the hull and would probably be easier to plank.
> <snip>
• Hi Andrew It depends on how you build it and how you treat it. My friend and I are currently rebuilding a Young 780 20+year old ply 26 trailer sailer of
Message 9 of 14 , Mar 7, 2010
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Hi Andrew
It depends on how you build it and how you treat it.

My friend and I are currently rebuilding a "Young 780" 20+year old ply 26' trailer sailer of perhaps a ton displacent. Its had several owners and lots of use. The hard chine hull is mostly 10mm ply and sound, with "egg crate" framing on the bottom, but not a lot on the sides. What is not so good is where the bottom panels twist up at the front they were laminated 2x5mm and full of voids leading to rot induced failure. Also below the sole were water ballast tanks, later filled with expanding foam, also resulting in some rot.
The choice of 10mm for the basic structure was a success, design mistakes, construction errors, and retro expanding foam led to failures.
I have a Diablo poorly built with construction ply 10 yrs ago which has had but little use, and the paint is holding it together.
I built a Gloucester Gull with 10mm bottom, 6mm sides, epoxy stitch&glue style 10yrs ago. Lives outside with latex house paint finish and it will last forever.
I built a CLC kayak with 3mm epoxy and house paint, lives outside and should also last forever.
Ply needs "appropriate" framing, good sealing, and ventilation. So built and looked after, a Chebacco of 6mm topsides and heavier bottom would in my opinion be a safe if not a sensible thing to do.
Wayne Poulsen
Fremantle West Australia

--- In bolger@yahoogroups.com, "Andrew" <a.c.l.yen@...> wrote:
>
> Hi,
>
> I read somewhere (probably on www.chebacco.com) that someone has built a lapstrake Chebacco in 3/8" plywood instead of the designed 1/2". The builder had obtained Phil's blessing for this alteration.
>
> This would lighten the hull and would probably be easier to plank.
>
> This got me to thinking about a Chebacco planked in 3/8" but with a double thickness bottom. The net weight of the hull would be about the same but the weight would be lower. I wonder if the Chebacco 25 would be strong enough with 3/8" planks and 3/4" bottom.
>
> I would appreciate your thoughts/wisdom.
>
> Andrew
>
• ... Stiffness is related to the square of the depth of a beam, so doubling planking thickness would quadruple stiffness. In calculating the scantling number,
Message 10 of 14 , Mar 8, 2010
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On Sun, Mar 7, 2010 at 4:37 AM, Andrew wrote:

--- In bolger@yahoogroups.com, "daschultz2000" <daschultz8275@...> wrote:

In my calculations I assumed that the stiffness of the hull will be related to the stiffness of the planks in bending. As we know, the round boat shapes increase stiffness considerably over flat planks stiffness - but I still think that if you double the stiffness of the planks then you will - more or less - double the stiffness of the boat.

Stiffness is related to the square of the depth of a beam, so doubling planking thickness would quadruple stiffness. In calculating the scantling number, the product is directly related to the product of length, width and depth. In the case of the rowing gig in relation to the 25 foot Chebaco, the length is 7 feet longer, but width and depth are  much less, so its scantling number is lower. It also doesn't have to carry the loads imposed by the mast and sail. Clinker built gigs are skin stressed, monocoque boats that don't need framing except to add some marginal stiffness and shape retention under stress. A carvel built gig would need substantial framing.

fred s.

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