plywood epoxy vs. strip-planking

Hi, I would like your input on the following.

I am in the planning stage for the construction of a 43-44 ft long range, medium-displacement, cruising sailboat. The boat is a multi-chine design; you can actually see her 38ft sistership on the following links
http://www.lordjimcroisieres.com/fr/index.php
http://www.chantiermer.com/presselordjim.htm
My 43 footer is essentially the same design but larger to allow a few modifications to the interior lay-out (and some more space for my 1m90, and the rest of the family).

A decision now has to be made about the construction materials and method, and I am still not decided on either plywood with glassfibre-epoxy (PW/GF/E) or strip-planking (SP). Several factors influence my decision, and this is where I would like your input.

A. STRENGTH of the hull.
By strong I simply mean that the construction can resist point impact with flotsam at high speed (floating container, rock, the occasional whale, ...), and come out of it with at most with minor damage. I take it that, build to the appropriate scantlings either method can provide a strong hull but ...
- would you for any reason prefer one method over the other and why?
- what about weigth?

B. susceptibility to ROT
Is any method more vulnerable, especially once holed or damaged, to woodrot?

C. integrated BALLAST
I would prefer to have the ballast (at least partly) be integrated to the hull, instead of bolted on. Can this be done with either PW/GF/E or SP?

D. REPAIRABILITY
Much cruising will be done within reasonable distance of major marinas, but some of it also at fairly isolated areas. Suppose there is a serious hit on a reef and the need to do some repairs oneself. Is there a preference for either method in terms of ease of repair?
:!: I am having the hull build by a professional yard and doing the interior fitting out myself. So any differences in degree of difficulty for the construction in itself are not an issue, only repair.

E. BUDGET
Any idea on how both methods compare in terms of construction budget.

Some additional information :
- the current design (the 38 ft) has been build in PW-GF-EP.
- the cruising program involves a 2-year tour of the Atlantic (Med-Caribbean (west and east)-US East coast-Canada)

I will appreciate any ideas you send my way.
Thanks, Philippe.

 

I would vote for the plywood. Strip planking would make sense on a rounded hull form.
For the ballast question, I am guessing you mean putting it in a fin or appendage of some sort as opposed to simply having inside ballast in the hull. I have heard of folks using lead shot poured in an epoxy slurry in a fin or appendage to give about the same specific gravity as iron and it has worked out well.
Well sealed plywood should give very good longevity and repairs would likely be easier than strip planking.

 

Thanks Gilbert.
Regarding the ballast issue. The architect indicated that since I am going from a 38ft to a 43ft while wanting the draft to remain shallow (1m85 max.), the ballast would have to consist of a bulb, added on to the keel, in order to have sufficient stiffness for this boat.
I was thinking however of not having all of the ballast added on (=bolted on), but to have also some of it integral to the keel, meaning on the inside of the keel (e.g. poured in). The idea is that, in the event that the bulb would come undone for some reason, there would still remain a certain margin of stability.
What I don't know is if a plywood-epoxy construction allows in any way for pouring in lead (or a slurry for instance) on the inside of the keel? Hence my question.

 

Go ply

Hello Phillippe,

I have built a few boats. Our 38ft cat is half strip plank and half ply. Both methods have their uses but if your boat is flat panelled with chines I would go ply.

Well sheathed with epoxy glass and well saturated with epoxy inside ply makes a great boat. You will have to cover some areas like the deck with a headliner to reduce condensation. On my later boats I use foam on the deck to save time and use ply elsewhere.

As to your worries about bonding the keel onto ply I am a little out of my depth as I have only built multihulls. However the use of unidirectional chainplates and good stitched fabrics allows you to reinforce stress points incredibly well. I would urge you to look into this technique and not to use mechanical fastenings only when it comes to high load reinforcing. With the right type of glass you can make the keel area bulletproof so the bulb won't worry you as much. Also composite chainplates are cheaper, lighter and never leak.

Having said that I should also remind you that bulb keels have been put on wooden boats for over 40 years. John Spencer's keels on his ply sneak boxes - Buccanneer, Ragtime, were similar to the keel on a Flying Fifteen developed by Uffa Fox over 50 years ago. Whilst not being exactly the same shape the loadings on the keel must have been similar and these boats haven't shown many concerns.

cheers

Phil Thompson

 

Thanks Phil

I take it by headliner you mean insulation ? If so, indeed this was planned, for condensation and heat protection (tropics).

The glassing technique for the keel gives me an idea I can take up with the architect.

Score ply/strip = 2/0

Philippe

 

Your boat is definitely designed for sheet material, like plywood!
Here are my thoughts..

A: Can be designed to be the same, depending on the direction of the glass on each side of strips.

B: Depends on the type of wood you use. Shelman in Greece produces Okume plywood with fungicide in the glue

C: No difference, you can't pour lead on wood anyway. Have some lead pigs made for you and fasten them very well to floors, frames and stringers.

D: No difference

E: Ready made strips are extremely expensive compared even to high quality plywood. For example 100 Euro/m2 vs 40 Euro/m2. And that's before you add the extra time (gluing, fairing, sheeting), glass and epoxy. Strip planking can be cheaper if you make the strips yourself, otherwise is plywood much cheaper.

 

Raggi Thor
Thanks very much
Much information here.
Ply/Strip = 3/0. I guess this clearly sets a trend.

 

I agree with what has been said. If you want a chined boat it makes no sense to use strip planking. However, strip planking does have some advantages, including a much 'cleaner' interior without so much structure. But if you were attracted to wood strip construction, then get round bilge plans which take advantage of this building method. Van de Stadt has suitable plans on his site together with some good articles on strip building.

He also explains keel construction. Basically the whole keel unit will bolt on to your boat. With restricted draft this is likely to be a one piece lead casting of fin and bulb together.

There is a further option if you are keen on a chined boat. It is possible to build a 'one off' female mold out of cheap materials (MDF?) and then lay up your boat using any combination of glass / resin and foam. You then pull the mold off the boat and bin it, although some people have used it a second time. This gives you all the advantages of simple FRP construction without the need to build a plug or fair something built on a male mold.

It also allows you to easily build a keel bilge sump which adds enormously to the 'livability' of a vessel in heavy weather, and can be difficult to incorporate in either a strip planked or ply built boat.

 

To Crag Cay

OK, so I understand I am pretty much stuck with a bolted on keel, which catsketcher indicates shouldn't necessarily put me off.

What does bother me somewhat is your remark on the keel bilge sump. This is new info to me, and actually I was indeed hoping for. Any ideas on how this could be accomplished? You indicate it "can be difficult" but is it impossible?

Philippe

 

No it's not impossible, but would involve significantly more work and would have to be well engineered by the designer.

For both reasons they are normally omitted, but life in a boat without one (that is, most racing boats) can be unpleasant when a cup full of water sloshes all over the place and never accumulates in one spot long enough to be sponged out, let alone pumped.

I was impressed looking over the new J124 recently that they still go to the effort to incorproate one, despite having a high aspect deep fin keel.

 

Philippe, I received your email and haven't responded to this thread, primarily because the issues have mostly been covered.

Addressing your concerns of strength is difficult to answer, because there are many types of "strength" to debate. In general scantlings for both construction types (ply and strip) will result in similar stiffness, tension, compression, puncture, etc. requirements set down in the design brief, which is assembled at the beginning of the design process. Each method has advantages and disadvantages over the other, but can be accommodated with structure (other then the planking type) and engineering.

Both methods will rot, particularly if breaches in coatings (paint, CPES, sheathing, etc.) are not attended in reasonable order. This is true of all building methods, regardless of material used, including metal and 'glass. Part of good seamanship is maintaince. A 13 meter yacht is a substantial investment and most of use do well just to keep them in good order, let alone Bristol. The cheapest covered slip in my area will cost about $4,500 per year without ocean access (okay, there is ocean access, but 135 miles away, up a shallow river). Ocean or intercoastal access will easily triple those annual costs for a slip and it's just a place to park the boat, not maintaince. Rot can be fended off with quality materials and most importantly responsible upkeep.

"Integrated ballast" is an engineering issue and can easily be accommodated by a skilled designer. I'm in agreement that a percentage of all ballast be internal, if for no other reason then to trim her up as things change. I don't think it's necessary or wise (from a performance point of view) to have it all located inside, though a wholesome cruiser can easily carry all of her ballast inside, with the knowledge that she'd perform better with a majority of it moved outside and lower. The cruiser is a special animal and few designers now draw up what I consider a true cruiser. Most of what I see currently and in recent decades have been depowered racers or worse floating bath tubs that are ridiculously burdened, fat butted beasts, with stylish raked cabin fronts and radar arches.

Reparability is dependant on where you bump into a rock. Not the part of the boat affected, but the marina or yard that will be performing the work. Some will have little experience with a particular building method and will have difficulty in repairs. Of course it's crap shoot, depending on what part of the world you happen into distress, but both methods are generally accepted and practiced. I'd think ply would be easier to find qualified repair personal, but it a personal opinion on my part.

Cost (budget) is a fair question, but you must consider this simple fact; the costs associated with the planking of a yacht (43 - 44 foot) will account for about 10% of the total budget of the yacht, possibly less. Strip is more labor intensive, but you'll save on framing. Plywood requires less labor to plank, but you'll have framing to pay for. In the end it about balances out, with a slight edge to plywood (maybe a couple of percent toward the total build cost).

Planking material, as a general rule in wooden craft, is the finest wood on the boat, for understandable reasons, it keeps the wet out of your socks. Resale and insurance value will be higher on a strip built yacht, though given a choice, I'd prefer molded construction, which will rate higher still.

In the end, the call is ultimately in your lap. A molded hull has very few shape limitations and is the most durable and light weight of the non-composite wooden methods. Strip has some shape limitations, but not many and good value/reparability. Plywood (unless double planked or molded) is quite limited in the shapes it can be twisted into. Plywood also doesn't have the reputation some of the other methods enjoy (mostly unjustly deserved) so value will be lower, but reparability will be better in most places you plan to travel.

Honestly, if I were you, I'd talk it over with the designer and the yard building her. They'll be the best guides to providing you answers to your questions. Most designers love to explain their creations and method choices to owners and clients. Try not to force a yard that specializes in molded construction to build a strip hull. You'll not get one as good as a yard that specializes in strip. It's sort of like going to a fine seafood restaurant and ordering steak. You may love steak and they may do a fine job of it, but likely not as well as a steak house, if you get my drift.

Good Luck and keep me in tune with your progress.

 

Yes, thats a good point, so easy to forget.

 

Thank you all very much.
While I am reading this, your remarks generate new questions and thoughts. I am going to think this over and come back to you.
Greetings
Phil

 

update

Hi,
your information has spurred some action.

First I had a talk with the manager of a yard nearby about cold molded construction. They did all the repair work on our S&S 41, which is cold molded, after we had a boat backing in our starboard side last year. And they did a great job (if interested insome pics check my posts on "long range passagemaker" - don't know how to make the link here directly)
Actually the ease with which the hull was damaged is the reason why I hadn't taken cold molded ito consideration and have been looking into plywood and striplanking. The other is that the current Montaubin design is plywood.
He confirmed that cold molded would be possible for this Montaubin design and that he would build it more solid than our S&S which is fairly light it seems. Add to that a glassfibre/epoxy covering and the result should be a strong, puncture resistant hull. And, as PAR indicated, better resale value and it would seem better for insurance coverage.

Then I mailed the designer and asked him if he would be able to redesign for could molded (I asked him the same question for aluminium before, but this he could not do, not being familiar with it), and what possible impact, if any, this might have on the design?
He hasn't responded yet but send me a sketch drawing of a 43 ft version (see attachment).

Now I have new questions :
- it would seem to me from the drawing that there is not much in terms of deep bilges, let alone a deep keel bilge sump as Crag Cay indicated. Could this be accomodated for of would this mean radically changing the design?
- this boat woudl apparantly weigh 7,8 tons (lège=no charge). Now, our S&S is 2 feet shorter and weighs in at 10 tons. How can a boat which is longer, and would be build to increased scantlings, end up weighing so much less?

PS : I'll post this question also on a separate thread, but do you have any idea from this sketch how this boat would behave? It looks like a very adult version of a dinghy (planign hull); is this a good candidate for a passagemaker? Can you tell?

Cheers
Philippe

 

My experience it limited to plywood stitch and glue but it would be a good fit for your design. The largest hull we have done in that method was a trawler just under 46' x 13' with a displace well over 30,000 lbs http://www.devlinboat.com/constructionsockeye45.htm and the largest sailboat we have done was 28'6" http://www.devlinboat.com/dchogmog.htm . Stitch and glue makes for a very uncluttered interior because there are no ribs or frames, the bulkheads work as those. Maintenance is low and repair is easy on a plywood epoxy hull. Use proper plywood and a breach in the glass skin won't do any harm to speak of if you dry the area out and repair during the annual haul outs for zinc replacement and bottom paint. Integrated ballast is no problem as you can pour lead into the keel and or bilge. The keel is made off the boat and after roll over it is glassed on with keel bolts and then filled with lead ingots and lead shot to fill the smaller spaces around the ingots. The shot is mixed with epoxy and poured in as the ingot are laid in. It makes for a very solid and stable keel. Can't comment on other types of construction because I have never built using them.
---Joel---