PDA

View Full Version : Planking and it's effect on design



Excalibur
03-22-2007, 08:19 AM
I'm looking for a build project, and one of the boats I'm thinking about is an Elver 20. The designer specifies strip planking and says:

...This process of inter-nailing greatly reinforces the cross-grain strength and stiffness of the timber, and strip planked boats can often get by with a significant reduction in framing because of this fastening method...

So does mean that if I use carvel planking on a boat designed for strip planking that I have to add more framing? I understand that in the case of the Elver I should consult first with the designer (and I have), but on a more generic basis, is strip so much stronger than carvel that I need to worry about what the boat was first designed for? Is the same true of carvel vs lapstrake? Does changing the planking style incur great risk for the builder? To be more specific, I am asking this question in regards to sailboats in the 20 to 25 foot range that are designed/intended to be pocket cruisers and bay boats, not offshore or racing types.

Thorne
03-22-2007, 08:44 AM
Planking is an essential element of boat design, although some designs can swap back and forth without too many changes -- mostly ply hulls where the difference between lapstrake or flat hull sections is minimal.

No easy answer to this complex question. As I often say, find a plan for a boat you like, then build to the plan, man. Don't like the plan, find another -- but modifying major design elements is a game for the designer or other NA's in most cases, not amateur builders like you and me.

There is at least one Elver builder/owner on this forum, and of course many more you can contact online. From what little I know of the design, the Elver's "square-section strips top-nailed together" design is fairly unique, so I wouldn't judge other designs by Elver standards.

Excalibur
03-22-2007, 09:23 AM
Before I read all of the various threads on strip planking, I was ready to do just as the designer said. But after it got so many negative comments, I began to wonder. Rather than trying to outguess the designer, I think I'll happily take your advice and strip it (if I do the elver). I would do it dry tho (the designer says nails/no glue is an option). The Elver really appeals to me, and I can't wait to start. It should make a perfect (Chesapeake) Bay boat. Nice hat! If you are ever on the right coast and near St. Mary's City Maryland, bring the hat, you'll fit right in. :)

Thorne
03-22-2007, 10:58 AM
I've actually worn it to St. Mary's Cittie for 17th C military reenactment events -- it does fit right in!

;0 )

Here I am in your neighborhood wearing my "Keebler Elf Hat" while spying on the Royalist camp -- shortly before arrest.

http://www.luckhardt.com/grndmust01/images/spyinblue.jpg

I'd say when you get serious about building the Elver to be sure to contact the builders here. They will have some tips on what and what not to do, as the design is 30 years old and a lot has happened with epoxies and adhesives since then...

The only Elver tip I have is to epoxy and glass any fir plywood used on the boat -- but you'd probably use another type of marine grade ply and not have that problem.

Brian Palmer
03-22-2007, 11:17 AM
Excalibur,

I think you can adapt most carvel planked designs to another type of planking system, but I do not think you can generally adapt other boat designs based on strip or glued lapstrake to carvel design.

The reason is that lapstrake and strip plank systems lock the plank edges together so the planks cannot slide past each other, either fore and after or athwartships. But carvel needs the frames (and more of them) to hold the planks in alignment.

I have an Elver that I bought used. If you do not want to build a boat with strip planking, there are many other good designs out there in the same size range for different systems, such as glued lap and sheet ply construction.

-- Brian

Charles Burgess
03-22-2007, 11:42 AM
I'm looking for a build project, and one of the boats I'm thinking about is an Elver 20. The designer specifies strip planking and says:

...This process of inter-nailing greatly reinforces the cross-grain strength and stiffness of the timber, and strip planked boats can often get by with a significant reduction in framing because of this fastening method...

So does mean that if I use carvel planking on a boat designed for strip planking that I have to add more framing? I understand that in the case of the Elver I should consult first with the designer (and I have), but on a more generic basis, is strip so much stronger than carvel that I need to worry about what the boat was first designed for? Is the same true of carvel vs lapstrake? Does changing the planking style incur great risk for the builder? To be more specific, I am asking this question in regards to sailboats in the 20 to 25 foot range that are designed/intended to be pocket cruisers and bay boats, not offshore or racing types.

Carvel and lapstrake require more frames than strip planking. Some lapstrake designs can use less frames than a carvel planked boat of similar design.

Strip planking uses the same plank thickness as carvel planks, with the main difference is that strip plank is essentially square in cross section. Carvel planking will require a different (heavier) Scantling Number than strip planking...thus you only need to calculate the require scantlings and spacing for carvel planks.

Strip-planked hulls are significantly lighter than carvel hulls of the same design. The real need for consulting with the designer (naval architect) is that the carvel planked boat will be heavier than the strip planked version...and may have a significant impact on design performance (especially if you use a lot of epoxy in the construction) in several critical areas (depending upon the hull form): displacement, waterline, and draft.

I do not forsee a decrease in stability or seaworthyness of a design, originally intended for strip-planking, when carvel planked. Generally speaking, increasing displacement usually increases stability when the increased weight is soley due to hull materials. The exception to this generality is when the design relies mainly on form stability without balancing with displacement stability factors.

Brian Palmer
03-22-2007, 12:55 PM
I do not forsee a decrease in stability or seaworthyness of a design, originally intended for strip-planking, when carvel planked. Generally speaking, increasing displacement usually increases stability when the increased weight is soley due to hull materials. The exception to this generality is when the design relies mainly on form stability without balancing with displacement stability factors.

This is important since the Elver has no internal or external ballast, and, therefore, relies on form stability, if I understand correctly. It does not capsize when sailing because it has a wide shallow bottom and generous flair above the waterline. Most of the mass in the materials is above the waterline in this boat.

-- Brian

Thorne
03-22-2007, 02:40 PM
Here is the post by Steve Redmond (and a link to the thread) on why he designed the hull the way he did -
-------------------------------------

It's been a while since I've looked at the WB forum so forgive this late response --

Can't help answering the flat bottom comments, since the reason behind it may be of interest to design-oriented folks.

If you add rocker to an extremely light boat like Elver, or worse, rocker and round sections below the bilge, you must increase the fairbody depth to have her sit on her lines at the same displacement.

In simpler terms, everything you pull up must be compensated by another bulge that is pushed down to maintain the same volume. What this does is increase the body draft (by about double).

Then to get enough board-up lateral tracking stability beneath the body, you need to increase the depth of the skeg. You have now increased also the weight of materials used. You've also brought the bottom of the bow up so that it will likely slap at anchor.

You've also depleted the form stability, since the hull is now barrel shaped with the metacenter approaching the center of gravity. Probably you will need to reduce sail area and/or try to add enough ballast to compensate. But unfortunately ballast increases the displacement of the boat, necessitating a further change in the lines to adjust.

The new ballast unfortunately doesn't have much effect until heeled , and now makes the boat sinkable. That being a worry, you probably need to add enough ballast to make her self righting in the form of a more or less deep keel, and a self bailing cockpit. The displacement has now doubled or tripled, trailering is probably difficult with a conventional low trailer and standard automobile.

Costs, building complexity, and time to build have all now tripled. Sail area has doubled. Rigging complexity and masting requirements have increased as well.

In fact we now have a conventional small cruiser, one of thousands like her, which certainly can give pleasure, but not exactly what I had in mind. The reason I know all this is because I originally thought to design Elver round bilged, but was rapidly pushed by scribbled numbers and succesive alterations into flattening and flattening her, until it made no sense to imitate in her bottom anything other than an ironing board. That being a requirement, there was no reason to make her ugly just because the bottom was flat, so I did my best to avoid that, and stick with my original mental picture of her

Sad to say, I didn't design the flat bottom on purpose. I just couldn't escape it once the displacement was set.

Best Regards,

--Steve

http://www.woodenboatvb.com/vbulleti...ighlight=elver (http://www.woodenboatvb.com/vbulletin/upload/showthread.php?t=56683&highlight=elver)