Tuesday, August 20, 2013

Drawing out the next major Zip part

At first glance, the Zip has some rather odd frame designations.... Specifically, frame number "5-1/2".

It seems odd until you understand that the designations are logically based on boat design drawings. If you look at a cross-section drawing of the boat, there are several vertical lines that divide the hull into sub-sections. Each one of these vertical lines is called a "station." The numbering starts at zero (the transom), and ends at "F.P." (which stands for "Forward Perpendicular") at the foremost tip of the boat. In the case of the Zip, there are 8 stations including "F.P.", dividing the hull into 7 sub-sections of about 24-1/2" each. Frame # "5-1/2" is the frame placed between stations 5 and 6. It is located near the base of the stem.

Frame number 5-1/2 is a very important part. As the instructions indicate, the horizontal line formed by the floor member of frame # 5-1/2 is the plane from which the rest of the boat is set up. The instructions specifically state to take extra care to ensure that this part is accurately measured and constructed.

I just finished drawing this part, and I thought it would make a good blog post to illustrate how the Glen-L plans are copied to the wood.

First of all, you fold the carbon paper in half, so that the lines you trace through it are transferred to both the wood AND the back of the actual plans. Then you place the plans on the wood, and flatten them down securely. Push pins help for this.

Secondly, trace the plans for the part you want to build. As I mentioned in an earlier post, the centerline is THE crucial reference point for the boat. ALWAYS carefully draw the centerline. Make sure you extend your traced centerline beyond the needed dimensions of the part, for this reason:

Third, draw "sight holes" that are centered on the extended part of the traced centerline. I like to trace a small coin for this purpose. Use an exacto knife to cut your sight holes through the paper. You'll need these holes to align the plans after you flip them over.

Here I've traced out the floor timber, and have cut sight holes on the centerline.
Next, remove your push pins, plans, and carbon paper from the wood. The half-width of your part should be clearly visible.

Half-width of the floor timber drawn onto 3/4" Douglas Fir marine plywood.
Now, un-fold the carbon paper, and place it normally onto the wood where the second half of your part is to be drawn. Flip the plans over, and you'll see the carbon-copy you made on the back of the plans. Carefully and accurately align the sight holes on the extended centerline that is traced onto the wood. Again flatten the paper down securely. Now you're ready to trace the second half.

Plans flipped & aligned. Ready to draw the second half of the floor timber.
Trace the carbon-copied lines onto the wood, just like you did the first side. When you're done, the full part will be drawn onto the wood.

The full floor timber drawn onto the wood, waiting to be cut out.

Monday, August 19, 2013

Still fairing

The long, slow and laborious process of fairing continues. I was happy that over the weekend, I did make port and starboard contact at the bow. Although it's hard to tell if it's just the angle of the photo, it looks like I need to balance the symmetry of the joining sheer lines. I also have a small gap there on the front starboard side that I need to fill with epoxy.

The great news is, even though fairing is taking a long time, it is getting closer to being done.

Monday, August 12, 2013

Zip stem construction

Today's blog post is about assembling the stem for my Glen-L Zip. The Zip's stem, as with many Glen-L designs, is made from two identically shaped pieces of 3/4" marine-grade plywood, laminated together with marine epoxy and fastened with silicon bronze screws.

This is now the third stem I've built; the other two being for a Squirt and a Utility. Learning from previous experience, I did a few things differently this time.

First, I spent much more time and care in shaping the pieces beforehand. I feel very comfortable this time around with the shape being correct.

Second, I used less epoxy. The other two times, I globbed a bunch of thickened epoxy on, just to make certain there was enough. In reality, that just creates more of a mess, because the majority of it squeezes out.

Third, I kept a putty knife ready to scrape away the epoxy that squeezed out from between the two layers of plywood. This makes for much less sanding afterwards.

Also, having learned a tip from a fellow boatbuilder, I used an awl to quickly align the pre-drilled holes before driving the screws back in. This proved to be a great time-saver, since it can be a little tricky to mate the holes back together with epoxy spread over the wood, (particularly while wearing latex gloves, goggles, and a respirator).

For the epoxy, I used Glen-L Poxy Shield, thickened slightly with #2 silica. Poxy-shield is mixed in a 5:1 ratio, using 5 parts resin to 1 part hardener. It can be hard to find very small mixing cups in order to measure small batches of epoxy, so I've used my own method. I take a regular off-the-shelf mixing cup, and measure the distance between the pre-printed volume marks. So far, all the cups I've used had a distance of 8mm between the 4oz and 8oz markings. So, I add marks every 2mm between them to show the 5oz, 6oz and 7oz lines. That way, I can pour in 5oz of resin, then add hardener until the mixture reaches the 6oz line. There are, no doubt, more accurate ways to go about this. However, this way is quick, simple, cheap, and effective.

Perhaps not the most scientific method, but it works.
After mixing and thickening the epoxy, I spread it out on the mating surfaces of each stem piece, making sure the whole area was covered. Then, I put the pieces together & aligned a couple of holes with the awl. I dipped the silicon bronze screws in a little epoxy, then drove them down into the holes. Then, I clamped the pieces together solidly, driving the screws in further as needed. Afterwards, I scraped away the epoxy that had squeezed out from between the layers before the epoxy became unworkable. Then I left it alone to let the epoxy cure overnight.

Everything ready to go: Stem parts, Poxy-Shield resin and hardener, #2 silica, mixing cups, mixing stick, chip brush, awl, screwdriver, extra screws (just in case), clamps, and putty knife.

Glued, screwed, clamped and scraped with the epoxy curing.

On a final note, I also worked on the Utility over the weekend... still fairing. It seems like a never-ending process, but it's getting there.