Boat design and weight comparison

On a 25' planing hull, how does marine plywood sheathed with biaxial cloth and epoxy compare with a strip plank hull with similar sheathings, adjusting for the core thickness of course? My understanding is that the strip plank boat hull is one of the lightest and strongest designs. I am confident that the Western Red Cedar is far less expensive to purchase than Okoume 1088 marine plywood. Another consideration for me is the workability of the Marine plywood vs the strip plank approach. I am working solo on this project and struggle to invision myself man-handling a 25' hull panel while I try to apply the steel stitch wires, for stitch and glue design. If I build a supportive craddle to facilitate the construction I have negated alot of the advantages of this time of design. I would greatly appreciate some advice.

Thomas

 

Usually the sheathing schedules for strip plank hulls is quite different than a typical plywood version. If the strips were large enough in dimension, to accept hull loads, with only an abrasion resistant sheathing of fabric (like generally used on a ply hull) then the strip planked hull (of red western cedar) will be lighter. Generally stripped hulls have solid wood molded layer(s) and fabric on one or both sides, which act to stabilize the strips. This is a much different approach then used in plywood hulls, be that plank on frame or stitch and goo. The plywood panel bears the load entirely, with the thin sheathing to protect moisture ingress and other minor dings or scratches.

Strip planking offers a lot more options in hull shape then large panels of sheet goods can, which have to be conically developed. On the other hand a double planked plywood hull can be any shape a strip planked hull can be. The advantage is both skins go down at the same time, they cover more area in a single plank and the seams are self covering. There are many ways to skin a cat, literally, each having advantages and pit falls. Strip planking is very popular among amateur builders. The strips can be gotten out of larger stock easily, with a cove and bead they nest reasonably well while stacking them up, a round hull can be built, minimal framing is necessary, plus a few other things that the back yard builder usually finds handy.

To directly answer your questions, it would be very odd to have a 25' stitch and goo powerboat design with similar fabric sheathing schedule as a strip planked hull. The engineering principles are different. The strips have good longitudinal strength, but need help in cross panel load distribution (molding, sheathing and/or framing). Plywood has good longitudinal and pretty good cross panel load distribution, so doesn't need the help of additional veneers or heavier fabric skins.

 

Thank you for the info. It sounds like you would recommend marine plywood for a trailerable 25' boat, all things considered?
Thomas

 

Not necessarily, but the average back yard yacht builder surly prefers it. It can be employed in a number of different construction methods. Plywood hull shapes are limited, unless it is molded, in one of the few methods available. I like the stuff, pound for pound, it is very strong (stronger then steel) and a difficult material to replace with another, all things being the same.

Much depends on your experience, tools, skills and other interests and abilities. The selection of a design and of course it's available building processes, is a personal choice. This selection task can be one of the most time consuming things you may straddle the fence about, during the build.

Many also like strip construction, for it's ease of laying strips. In small boats the work goes quickly. I personally don't like the look of a bunch of little, varnished strips, except on canoes, but others haven't a problem with this.

Your choice will be a soul searching, for the method that best suits your ideas of what you can do, balanced with what you like. Compromise is the hall mark of yacht design and the selection of a design is directly reflecting the acceptance of these compromises, made by the designer. Good Luck . . .

 

Western Red Cedar and epoxy bonding

I read that Wester Red Cedar should not be considered a first choice for strip plank construction due to its potential problems associated with epoxy bonding. Does any one have any thoughts on this topic?

Thomas

 

Many woods can have issues with epoxy bonding. Most all can be handled including red western. Epoxy use is more procedural mind set and exsperence then anything else. If you follow instructions well, learn from mistakes and are a good problem solver, epoxy use is a cake walk. One of my major concerns with strip planking is the dimensions of the strips used.

 

PAR,

Could you please share with me some of your concerns with strip planking. I just ordered the Gougeon Brother's Book on Epoxy boat construction for additional reference.

Thomas

 

Strip planking is an easy to do construction method, that usually produces a boat that has a interior nearly free of framing structure, but can be on the heavy side, when compared to other methods. The ease that strips, particularly when coved, can be placed, plus no heavy, long planks to handle and the seamless, frameless interior are big selling tools, to the back yard builder.

All construction methods have good points and bad about them. Some are better suited to certain peoples skills, where others fit someone else. I don't have any bias for or against the method. I do favor other techniques, but that's me, your situation will require a different approach, again based on your desires, skills, tools etc. No one here, or anywhere else can tell you which is the right one for you.

Listing the advantages or draw backs to a specific method, can only be advantageous for developing your "wish list", generated during the design selection process. The winning method will employ the things you need the most.

This process can be a long, drawn out and painful one for many. Difficult decisions based on little experience and information makes it seem like a crap shoot at times. For what it's worth, all methods will produce a fine hull, if you are a craftsman, or a child only a mother can love if you loose interest, skip steps or take unwise short cuts. The world is full of these partly completed projects, many more then completed vessels.

The major mistakes I see the amateur make are over building, thinking it will be stronger (which usually makes it weaker in most cases), deviation from the lines, particularly in underwater areas and installing structures that at not in keeping with the type or style of craft being built. An example would be a standing headroom pilothouse on a 22' fishing boat. The designer didn't put one on it because he knew the boat wasn't big enough to carry this structure, but the builder thought other wise and slapped a big box on top of the hull. The extra weight, causes the boat to trim bow down and look terrible, so it doesn't get up on plane and looks like a floating mobile home.

With the amount of research you are applying to your effort, you'll likely not fall victim to the "child only a mother could love" syndrome, but every marina has one in it and there're easy to spot. Keep looking at designs, checking web sites for construction details and pick up some books (there are many). Check the Boat Design Book Store tab at the top of the page here and see if anything strikes your fancy. If you'd like to drop me an email (click on my name) I have round bilge power boat designs in your size range, using several different methods. Possibly one of them may be what you're after, or at least give you an idea what of you're after.

 

Far and away the best book I have found on this topic is "The Laminated Wood Boatbuilder" by Hub Miller. I bought my copy off Amazon.com a while ago. It is "A step-by-step guide for the backyard builder".

It covers everything from building the boat-shed, forming and pouring lead ballast, lofting and creating molds, making the strip planks, sheathing the hull, etc...

Buy it, read it a couple times, and you'll have a much better understanding of what method would be better for you.

Also, you may want to look at "The Art, Science, & Magic Cruising Boat Design" by Danny Greene, N.A. It has a chapter on boat-building methods which gives the pros and cons of the most popular methods out there. It mentioned the technique that I plan to use: building a strip-plank hull and then laying cold-molded veneers over it.
Unless you're designing the whole boat yourself, you should probably just read this chapter in the bookstore and make up your mind... I have found it a very useful book on many aspects of boat design- it's also easy to read.

 

Thank you for the information, it is appreciated!
Thomas

 

The best book i've read on strip plank is this one from Roger Simpson in Australia.called "Simpson on Boatbuilding"

http://www.boatcraft.com.au/simpson.html

Also look at Kiri as a replacment to W.R.C lighter and cheaper, in Oz anyway.

http://www.highpointtimber.com.au/

Dave