Cold-molding over planking

I'm about to embark on restoration of a timber Twister (28' yacht, Kim Holman design). I have 3 options re hull restoration. 1. Simply restore planking - it seems to be in reasonable condition, 2. Repair and glass it, or 3. Cold-mold over the old planking. Any thoughts on this would be welcome. I'm wondering too how to terminate the cold-molding (or glass) at the ballast so that I don't simply create a moisture inviting open ended sandwich (if you catch my drift!). Any thoughts welcome. Cold-molding is my preference at this point due to the increased strength for an old boat (1967) so if anyone knows of a good book on how to cold-mold over old planking, I'd also be grateful for that information! Thanks! Rick Location - Port Stephens, NSW, Australia

 

Hi,
my personal preference is to restore the existing planking if it is in reasonably good condition. If the hull is structurally in need of reinforcement then cold molding one or two layers on the existing hull is a good option. The cold molded layers should be rebated in to the existing wood keel, between the existing plank rebate and the ballast. I would not recommend fiberglass coating a planked hull. If you want to install a waterproof coating then my suggestion is to rake out the seams and spline them with softwood splines (western red cedar is good), and then coat the hull with Dynel or Xynole polyester set in epoxy resin. This coating reduces the absorbtion of moisture which prevents the splining from overstressing the plank to frame joints. The coating should cover the hull and the ballast keel to prevent the ingress of water.
I hope that these comments are of some help.
All the best with the project.
David

 

Hi David.Your comment "I would not recommend fiberglass coating a planked hull".Please expound. I would like to hear your point of vew on this issue. A glass sheath if don properly can solve manny issues but detract from the designe and value of a planked boat. My rebuild has a 4mm Meranti sheath that terminated at the keel with epoxy between the keel and the sheath .Not ideal.Iwas advised to glass the hull up to the water line. Sounded a bit dum just moving the leaky bit higer up.

Regards
Butch

 

Hi Butch,
I do not like fiberglass sheathing on a planked hull because the fibrglass is hard and brittle when in composition with the resin. This means that unless it is a very thick laminate it will crack at the plank joints and this defeats the purpose of the sheathing. I recommend a Xynole polyester or Dynel because it is somewhat elastic and will move with the wood to some extent. I recently did a boat this way and it sank at its mooring in a major storm and was underwater for five days. Even though the planking became saturated and cupped, the coating remained adhered to the hull and when the planking dried out again it returned to its original shape with no discernable cracking or separation.
One thing to bear in mind also is the type of epoxy used. The quicker setting epoxies set up very hard but brittle. The slower setting epoxies set somewhat softer and more resilient and will work with the movement of the wood to some extent. My personal preference is for a Shell Epon 3 with a Versamid 140 hardener. This is a very thick epoxy and hardener and the mix is one to one. It takes about four hours to kick off. When I worked at the Wooden Boat Center in Marina Del Rey in the mid 80's we did some experiments comparing this mix with a West Epoxy. We did sample mixes in coke cans with a putty knife embedded in the mix. One blow of a hammer shattered the West mix but the Shell mix we could beat on all day and it just deformed and absorbed the blows. We had to cut the epoxy off the knife in the end. Admittedly the shell mix was not as strong as the West in the ultimate strength tests but it was stronger than any wood we used it on so was not the weak link.
If you have already sheathed the hull with a 4mm plywood overlay then you have probably eliminated the movement associated with a planked hull and a glass coating will probably be OK. I would still recommend using the slower setting epoxies however. The keel joint is another matter. I would suggest that you glass or dynel over the ballast keel as well as the hull, but at the keel make the laminate much thicker at the keel joint possibly four layers tapered back in to both the hull and keel. In this way you will add strength to resist the movement at the keel. This is not infallible but at least gives you a chance. Make shure that the keel bolts have been tightened down before glassing, as the reduced moisture content in the wood will loosen them and allow more movement at the keel joint.
I hope that explains my thinking and experience a bit more.
All the best with the project, I hate to see the old woodies scrapped for want of a bit of work.
David.

 

Thanks for your input David much appreciated

 

Thanks from me too David - that's really helpful. Another quick question though, if you don't mind. The rebate into the keel would need to be the depth of the cold-mold strips, of course but how would this be faired in so that the ends of the strips don't spring out? I mean, would you rout out the bottom of the rebate to get the depth and then plane out above? I guess this would involve fairing back into the bottom plank too? Regards, Rick

 

Hi RFNK,
the rebate has to be the depth of the overlay at the ballast keel side and tapered back in to the hull surface in a smooth curve to meet the curve of the lower hull. There is often a slight knuckle at the existing plank to keel joint and this can be used to mask the new transition. It all depends on the original design. Usually the new overlay will create a slight crease line even if the original design did not have one. Usually the best way is a saw cut, to the depth of the overlay with a skil saw ,at the base, and chisel out the tapered wedge, then use a rebate or small block plane to smooth out the transition. If you are careful you can also use a mini grinder with a sanding pad for the rough work.
I hope that this helps.
All the best with the project.
David