Monday, September 10, 2007

Plank bogging

I've been working on getting this port hull-half-half ready for laminating. Here's the keel plywood set into place and screwed to the battens:

Next up was using the dremel to cut a V-groove between each of the planks, ala Henny. This should give you an idea of what that looks like:

See those horizontal join lines near the bottom of a few of the planks in the picture above? This particular foam is A550 offcuts from Noah's and it came from the factory with those join lines. While fairing the foam yesterday some of those join lines started to crack, especially where they were under tension from a mold curve. Arggh -- just what I needed. Anyway, something for the rest of you to watch out for. I fixed them by dremeling a small slot over the join, bogging it, then screwing the two halves back together with screws and small pieces of wood.

After dremeling the grooves between the planks yesterday, I went ahead and started bogging them as well. Unfortunately, the dweeb that's in charge of this project (that would be me) managed to run out of epoxy before the job was done, and it was too late to make a run to Seattle to get more. Thankfully my loving and patient wife agreed to be an HOV lane dummy this morning, so we could be at Fiberlay as soon as they opened (8 am) without having to sit forever in traffic.

I've been thinking hard about this practice of using bog between the planks, and I'm wishing I had explored the polyurethane approach more. The groove and bog approach has the following drawbacks:

  • It takes a lot of bog, especially if you're using V-grooves like I am, which adds a lot of weight (bog is heavy). Henny grooved his planks in order to guarantee a good vacuum seal, but I'm doing it mainly because it's easy.
  • It's difficult to guarantee that you've filled the groove 100%.
  • If the groove is open to the back (e.g., if your planks are not perfectly fitted to one another) then you end up with squeeze-out, aka wasted bog.
  • If you are filling gaps\grooves which are open in the back, then the bog has a tendency to settle as it cures. (I've learned that thixotropic means "resists gravity", not "immune to gravity".) Which means that after several hours, your carefully screeded bog lines are now bumpy, low valleys between the planks. This happened to me, and I ended up going back a second time to apply a thin skim coat over the bog lines to bring them back up to level with the planks.
  • If you over-bog and leave it too high, sanding it back down is difficult (as always) without damaging the foam on either side.

Polyurethane glue would seem to fix several of these issues: it's lighter than bog (I think), its easier to trim off excess squeeze-out, it's cheaper than bog, and its foaming tendency would help to fill gaps. I think I will give it a more serious try, on the starboard hull half. I didn't do it on this one, because I had this image in my mind of dancing around with a plank-in-hand, with wet glue applied, and not being able to get it screwed down next to its mate in time. The groove+bog approach is easier in that respect.

If you are still dedicated to using bog but don't want to apply it as you're planking, I would suggest close-fitting your planks a small distance apart (maybe one or two popsticle-stick thicknesses), and not doing the dremel treatment - that way you only have a tiny area to fill with bog, to minimize the weight. Not that I'm an expert in any of this stuff.

Anyway, last job for tonight was the skim coat on the bog lines. Here's how it looks currently:


After some final touch-up sanding in the morning, I think I am ready for lamination of this hull-half-half. Ouch - I can feel my back aching already.


Menno said...

Hello Jay,

I used polyurethane glue for planking my floats, and I can tell you that's not perfect either. It is a lot of work to get a sufficient thick layer of the glue on the 1 cm thick panel edge without spilling and dripping too much. Mounting a strip dripping with glue is not fun at all. The glue method will probably be lighter, but glue can weigh a lot as well. I was surprised at the ammount (weight) of glue I had to use on my floats. Further: with the glue method you'll probably still use bog (although not much) to fix all the places the strips are not perfectly level. Lastly: on the main hull it seems impossible - at least on my hull - to avoid quite broad seams between the strips in the lower part of the hull which the glue won't bridge for certain. So you'll probably have to use the 'bog method' on that part anyway.

Long story short: I used the dremel-bog method on my first main hull-half and was very happy with it. Much easier working - no mess while planking - and not too much worries about seams between strips. Also in the end much faster than glueing (even if I have to do another run with putty to correct the 'sagging' in some seams).

I think the disadvantage of mounting two strips a bit apart without making a v-groove is that it's much harder to fill the whole seam for sure.

I guess you'll be glassing the inside of the hull shortly? I hope you will not have any more troubles with bubbles this time.


Jay said...


Thanks for your detailed comments. You made me feel better, that I was still using bog for my plank joins. Looks like there are tradeoffs to either approach.

The seams between planks are usually due to the compound curvature of the mold (especially lower in the hull) pushing the planks apart at various places. The only way to fix this is to scribe each plank so that is closely fits to its adjacent one. This can be finicky work but helps to minimize the bog needed. I'll try to take some pictures of the scribing process, or at least how I do it, next time.

You are right that with an open seam between planks, you sometimes don't fill the entire seam up. The only solution for this -- and it's what I did on my floats -- is to fill those seams in from the outside of the hull (it didn't take long, and I was doing the screw-holes at the same time). Obviously the bog will normally have reached total cure by the time you get around to this, so it may require scuffing up the existing bog to get a decent bond.

Let's indeed hope that I don't have any bubble trouble this time around. It should help that I'm not glassing over a completely enclosed float in a rising temperature. :-)


Anonymous said...

Amazing progress every time I look at your site. Wow.
Just a quick couple of comments.
1) Other Farrier builders in the Toronto area I've been told took that approach of leaving a gap between planks. The dremel approach seems smart so as to contain the putty. If I were mixing epoxy I think I would do the same.
2) I have used a polyurethane construction adhesive that is more viscous than the glue. Holds onto the edge really well. The polyurethane adhesive has a fairly decent working time. I had lots of time to smear glue along the edge of the foam put it in position hold it in place with one arm and screw the first few fasteners with my other arm. I found that I tended to squeeze the foam planks perhaps too tightly together. My joins on the floats tended to be universally low on the inside and high on the outside. Lots of excess glue squeezed out below that I just cut off with a knife when the float half was removed. This excess glue tended to cause a smear where there was a batten but the cured glue remains sandable without appearing as a hard spot.

I'm watching you guys working on the main hull with great interest. Thanks for sharing all your work.


Jay said...


Thanks for the kind words. I hope the pictures and commentary are helping folks build better boats than I am.

Using the dremel, if you get a true V-groove yes this will help contain the putty. However I am trying to be more weight-sensitive, and I think I'd rather fill in an open-back thin slot and waste some putty, and save on the weight. Builder's choice of course.

On the floats, I too was able to just hold each plank in position with one arm, while screwing it from the backside with the other. This is impossible though with the main hull - the planks are just too long, and you can't get a good "reach-around" most of the time. Once I realized that, it was the tipping point that caused me to stick with the epoxy.