mould warping

[rturbett]

I'm in the middle of "stripping in" a male mold with 3/4" strips, and the frame has developed a nice twist. What is the order that one should attach the stringers to keep this from happening?
Thanks,
Rob -my boat only turns to the left- Turbett

[yokebutt]

By locking down the strongback to the floor before putting up the stations. I've had pretty good luck with tabbing them to concrete floors with mat and polyester; decent in shear, but peels relatively easily when you lift straight up.

Yoke.

[rxcomposite]

You are in trouble. and you havent even laid a mold yet. the mold will probaly weigh a quater of a ton or more. Me, I nail the strongback to the concrete floor. I also make sure the strongback is really STRONG.

[rturbett]

Could you please define "strongback"? I have probably made a "weakback".

Also, I joined strips with a dowel to make them long enough. This has by luck saved me, becaus it has allowed me to flex the mold back to shape, and screw it in place. I will brush the inside with glue to hold it before proceeding.

[nero]

You are not bonding the strips as you lay them?

[rturbett]

Yes, each strip is being screwed to each station. I was trying to avoid having to unscrew 700 wood screws. Even though it was secured, there was just enough "flex" to let me force it fair. Adding the rest of the strips is holding it to shape. Almost done.

[rxcomposite]

Originally Posted by rturbett

Could you please define "strongback"? I have probably made a "weakback".

A "strong back" is a sturdy box like structure, or a series of diminishing dimensions of box like stucture intersecting the frames. It is usually cross braced to prevent warping and is usually solidly anchored to a concrete floor.
It looks like a bridge in structure.

For small boats, it is simplified into an inverted "T" bridge, with the "I" leg supporting the keel or profile, the _ supporting the frames.

The strongback supports the frames, the profile, the plug, and the female mold that will be laid over the pattern.

I usually lay the keel/profile first, then fit the station frames. I then fit the ribbands or long pieces of longtitudinals flush to the frames. When it resembles a birdcage, I check if the lines are faired.

I usually lay down the planking progressively starting at the keel, port and starboard at a time. Dont go whole hog and finish one side first as the kinetic energy stored in those bent planking is enough to twist your birdcage.

I use screws for the frames but use nails for the planking. leave the heads showing up until you have planked the whole boat. This gives you a last minute check if the whole thing is fair. If not it will be easy to pull the nails and make addittional shimming or timming on the frames.

When you are satisfied, drive the nails flush and set it.

[rturbett]

Yes, I made a weakback. Thanks for the info. I did put some strips on both sides, but then stayed with one side. That's when the problems started.
I used countersunk screws just so I would have the ability to go back and fix my mistakes.

[rxcomposite]

Seasoned professionals, as Nero has suggested, bond/glue the planking as they lay it down.

With you, since gluing as you set is not yet in your skill, you will have to coat the surface with resin. This will more or less make the strips stick together.

Do it after you have faired the planking by planing or using a power sander.

After sanding it, give it a coat of resin, let it go tacky then apply a layer of fglass cloth and resin. If you dont reinforce the wooden shell, the wood will "print through" on your mold. The wood tends to shrink because of the heat generated when the Fglass female mold starts to cure. All the care you made in fairing the mold will be wasted.

[nero]

"countersunk screws just.."

I have been using drywall screws with a fender washer. This stops the screw from sinking into the wood, and provides better pull to bring the strip up against the form. Most screws come out after the resin has set for a day. I leave a few to keep the planking up against the forms until I add temp. glue blocks from the other side. My first hull half started to lift off the form due to wood strip moisture content at 16 -18%. oops. Nothing to grave.

Let us know if your hull warps a bit. When you pull it from the forms.

Are you using the dry method of strip planking?

[wet feet]

Are you working on one side at a time?If you apply very many more strips to one side than the other,the uneven strains will not help.The advice in earlier posts about rigidity will still apply but you need to keep the strain balanced between sides.

[rturbett]

wet feet,
You are absolutely right. I started doing them evenly, then got greedy as I got tired of going back and forth. I have a plum line hanging at each station. I noticed the twist before I got to far ahead.

Do you glue the strips together, or just at the strip/station intersection?
Thanks,
Rob

[wet feet]

You don't give any clue as to whether the strips have hollow and round mating faces or if they are flat bevelled surfaces.If they are flat,my inclination would be to glue in order to restrict any individual strip movement under the pressure of fairing tools.I would not be too bothered about a perfect structural joint as long as the strips were to be held in place.The earlier advice about glass sheathing might need to be followed if you are contemplating vacuum bagging.If not and as you have told us it is a male mould,you could just seal the surface and look forward to only having to get a quality finish once you arrive at the outer surface of the hull.

[rturbett]

Wet feet,
I beveled one surface so that I could mate the strips without too much of a gap.
I plan on fairing, then filling the gaps with drywall mud. After sanding fair again I am probably going to glass it.
You are correct about individual strip movement. Just for fun, I started planing a section and could see that some of the strips would flex out of the way instead of getting cut by the plane. I will definitely glue strips together.
Rob