Questions on plug building...

[Pavook]

Hi,
I am looking at building a plug for a 37 foot sailboat. I have researched the topic here and elsewhere but I still have some questions. Perhaps some of you can help. If anyone knows a book or guide that has a step by step procedure described I would appreciate the reference.

We have looked into cnc milling, but its too expensive. Seems like it might take longer to do a traditional plug, but would be cheaper (although perhaps slightly less “true” to the CAD shape). And either way, you still have to hand fair the plug. We will have the frames CNC cut and set on a steel base.

To make this simple, here are my specific questions:

- what type of material can be used to skin the frames? Is strip planking the best? Can battens with a Okume door skin covering work (its just a simple sailboat hull, but are the compound curves too much, will the door skins conform to all of the curves, particularly at bow?)

- if strip planking, what is better, using wood, or structural foam? What is the best thickness / width of the planks?

- if, once planked, the plug will be covered with a layer of fiberglass, can there be small gaps in the planking?

- once the frames are cut and set onto the steel base, can anyone give me a rough estimate of the amount of time it should take (with reasonably talented labor) to plank, glass, fair, sand and buff the mold? (roughly 600 sq feet of surface)

Thanks for your help.

[gonzo]

You can putty the gaps in the planking. It should take a crew about 4 to 6 weeks to fair a plug. Then it will take about a week to ten days to do the mold.

[Pavook]

thanks gonzo. crew of how many, 3? and i guess strip planking is the way to go not 1/8th ply?

[Raggi_Thor]

You can have the frames cut with a cnc router or a laser. Taht will save you some time and you know the hull is fair (or as fair as the cad model :-) .

[gonzo]

I find that three people work fine for strip planking. Two put them on and one take care of the glue and cutting. For fairing and sanding you can always put more people on the job.

[Raggi_Thor]

I don't have any experience from plug building, so this is just my thoughts...
If you cut the frames to make a female mould I think it's better to strip plank than to use plywood. Maybe for some areas you can use battens and plywood while you use strips for the areas with most curvature. Have you seen the pictures from building the B18?
http://www.mboats.no/B18/B18_building_pictures.htm
I had planned to skin the round bottom with two layers of 4mm Okume plywood.
The curvature made it difficult, so the builder chose to use strips of pine wood.
Building the hull for this little boat took two men two days including sanding, tehn add a day or two for painting and more sanding(?).
I don't know if you can scale this up to a 36 feet mould, you may want a better finnish for the mould.

[rxcomposite]

Pavook,

We had a long discussion here on plug building. Even sent some thumbnails. Pros and cons of male, female, birdcage, foam core.

Please see my post, Jfblouin, or Danielsan. These will lead you to a lot of good tips by other seasoned boatbuilders.

regards,

rx

[Pavook]

Thanks all.

[yokebutt]

Pavook,

We normally just set up two legs for each station and use a laser to mark a convenient waterline on each one, that way you don't need a flat base to get eveything in one plane.

For quick and (relatively) easy low-tech one-offs, it isn't too difficult to make wooden stations with longitudinal battens a few inches apart, (depending on curvature) and then strapping strips of core-foam from sheer to centerline. On the last boat we made with this method,(31 foot sailboat) we ripped the foam into 2 foot strips, bent one on at about the middle of the boat, and screwed it on to the battens from the inside. For the subsequent strips forward (and aft) we held a foam-strip in place next to the previous strip and scribed the edge of it that went against the previous one, cut it and installed it. and so on all the way to the ends. Once the plug was covered in foam we laminated the outside, flipped it over, ripped out all the wood, and laminated the inside. With this method the material forming the solid 3D surface becomes part of the boat instead of going to the landfill, and you don't have that many joints between the foam-sheets to pucker and misbehave.

Yoke.