Clipper's Vera Cruise Build

Clipper's Vera Cruise Build

Friday, December 26, 2014

Center Skin Panel Backing Plates

Before installing the center skin panel, I need to add backing plates at each butt joint. This allows me to tie the skin panel into the structure on all sides of the panel. The approach is one of two different ways of dealing with the need to add multiple plywood panels on a boat and joining the panels together.

One approach is to scarf join the panels together end to end and then attach them to the boat. This can yield a very nice exterior finish and if done correctly, can be as strong as the rest of the panel. In areas of extreme curves or bending, this is probably the only way to join the panels unless the panel is applied in narrower pieces.

The scarfing approach has a few drawbacks for the builder who is primarily doing the work by themselves. First and foremost, the additional size of the joined panels makes it difficult to handle. On my boat, three panels would have to be joined together making the entire piece, 24 feet in length before trimming to size. Needless to say, this would be very difficult (but not impossible) to manage by yourself. Trimming the panel to size requires numerous sessions of installing and uninstalling ( as well as trimming and sanding) the panel before it is ready for installation. I do know of a builder who used this approach and was successful with it.

The second drawback from my perspective is that the larger panel requires a considerably longer time to install, with a corresponding increase in epoxy that needs application, and screws that need to be driven in place. I find my stamina to be challenged if it takes me longer than 2 hours to accomplish a single task. A 24 foot panel installation would undoubtedly take 4 or 5 hours (possibly longer) to get glued into place, all the screws driven down, and all the excess epoxy cleaned up. This estimate is based upon the length of time it took me to install a single panel and is an aggressive pace. Because of the time needed, trying to keep workable epoxy available during the installation would be very challenging.Again, not impossible, but at my age, I don't need the stress.

The second approach is to use butt joints. This allows panels to be installed one at a time and then joined together when needed. The joining together could, in theory, be done separately from the panel installation. A butt joint requires a backing plate to be installed in the inner surface of the joint. This backing plate needs to extend a sufficient distance onto each skin panel (6 inches each side in my case) and needs to be attached to both skin panels.

The biggest drawback to the butt joint approach is that the the backing plate is sometimes difficult to conform to a curve and therefore, the butt joint should preferably be in an area that is relatively flat. I had to deal with this (more in a few moments).

I have heard of a third approach that is similar to a butt joint in which the two panels are joined with fiberglass cloth on both sides, but I was not comfortable with this approach from a strength perspective. Admittedly, I am not that familiar with this technique and there may be more to it than I am aware of.

Because of the drawbacks to the scarfing method, I elected to go with butt joints on my skin panels. I have discussed this in several of my postings previously. The remainder of this posting will cover the installation of the center panel butt joint backing plates.

The skins already installed at the front and rear of the boat have a slight  curve in the up and down direction. In addition, the front panel also has a slight twist to it in relation to the chine and sheer. The backing plates are from the same 9mm plywood used for the skins. They are 12 inches wide to allow 6 inches of mating surface on each side of the joint. The length is cut to allow the panel to fit between the sheer and the chine.



Because these plates are relatively short, getting them to conform to the previously mentioned curves is difficult due to the stiffness of the plywood. I decided to try and pre-bend them a bit using steam. I was a bit concerned about the steam heat causing the panels to delaminate, but the book I have on building plywood boats talks about using boiling water to get panels to bend when fitting them to the boat. So I figured it would be okay to try and if I didn't like the results, I could always try something else.

So I broke out the steaming technique I used previously on the chines and sheers (using plastic bagging and a steam generator). The panel was wrapped in the plastic sheeting and then connected to the steam generator.





Notice that I have ratchet straps around the panel. More on this in a moment. These panels would not require much heat since they are fairly thin. After steaming them for about twenty minutes, they became pliable enough that I could press down in the center and then start ratcheting the straps to get a slight curve. Not much of a curve is needed.



These were allowed to dry out and they retained enough of the curve to make them fit to the skins much better. There was no evidence of delamination so I deemed the experiment a success.

The plates were placed into position on the skins and cut to length. My plan was to epoxy them into position and used short silicon bronze screws to hold it together while the epoxy cured. The screws would be driven from the inside of the boat and WOULD NOT extend through the outside skin.



Doing the epoxy glue up was fairly straightforward for the aft panel plate, but the forward plate needed a twist in order to conform correctly to the skins. I had to use some clamps and a board to "persuade" this plate to stay in position. I found that the screws had insufficient bite to pull the panels down to the skin so I had to apply pressure at every location where I drove a screw. This was not easy and I intend on acquiring some deep throat "C" clamps for the next set of plates.





You can see in a couple of the pictures how I had to place a board to push against the backside of the twisted plate. And clamps are installed at opposite corners of the mating area. I was able to get everything down correctly, but as mentioned, I will be using more clamps next time.

Finally, a shot showing both backing plates in position. You can clearly see how the plates will allow the center skin to attach at the joints in addition to their other connections on the sheer and chine. In a couple of days, after the epoxy has cured sufficiently, I will be attaching the center skin panel. Then I can move on to the starboard side.



So that is it for now. Hopefully, by the beginning of the new year, I will have the port center skin attached and the starboard center panel well along. Take care and enjoy the remainder of your holidays.

Sunday, December 21, 2014

More Side Panel Skinning

Well, I had hoped to get more work done since the last posting two weeks ago. Things being what they are, I was only able to complete the second front panel and start on the port center panel.

The port front panel presented a few more difficulties than the starboard panel. This had mostly to do with the lack of space on that side of the boat. I also had some difficulty getting the panel into position because of the curvature required. The front of the panel kept getting hung up on the back wall of the garage.

However, eventually I did get it into position and glued down. The only real difference between this panel and the starboard side is that this panel lays over the end of the starboard panel in the nose. That means that the starboard side panel had to be sanded down flush with the stem first.





Another area I had more difficulty with on this panel was getting the forward most screws in. Like the starboard panel, I had to use air powered angle tools (drill and wrench). But for some reason, the wrench would run out of power before completion of driving the screw so I was forced to get in there and complete that by hand. Needless to say, that was difficult in that I had to drive it down while keeping the driving bit firmly planted in the screw (so that the head wouldn't strip).  Afterwards, my arms felt like they had been in a boxing match.

When fitting the panel, I noticed that it did not lay quite flat on the stem. The amount of curvature on the panel in this area is a bit more than is easily handled by the panel. You can see in the next photo the small gap in the center.



After I removed the panel I worked the fairing on the stem a little and this improved the  fit, but I was still forced to use some levering action by placing a board against the wall and pulling down on it while driving the screws in. Of course, I couldn't do this by myself and had to enlist the aid of my son.

As mentioned, I eventually succeeded in getting the panel glued into correct position. It gives the boat a new character when both front panels are on.




At this point, the only remaining work on the side panels was to fill in the center section. These were going to require six feet of plywood as well as two backing plates per side. This is the work that I didn't get much done on.

I did get a couple of preliminary fit pictures. The first one illustrates the butt join on the forward end of the center panel. Eventually, there will be a 12 inch wide backing plate (also made from 9mm plywood) glued and screwed to the inside of this joint (as well as the other joint on the aft end). The backing plate needs to extend six inches on each side of the joint (hence the 12 inch width). It will also extend the full length of the join between the sheer and the chine. (approximately 32 inches).


One other aspect of this backing plate is that it needs to conform to the curve of the skin panel. The curve is not excessive but the shortness of the backing plate means that it will be difficult to force into a curve. My concern here is that that stiffness will tend to flatten out the curvature of the skin at the joint. A flat spot would be aesthetically undesirable and might affect the performance of the boat.

I am going to experiment with pre-curving the backing plates before gluing them into position. I have yet to perform this pre-curving and I do not have any photos of them yet.

In the the final photo, you can see that the center panel is trimmed to length and in position but still needs trimming on the chine edge. What I particularly like about this photo, however, is that it shows off the lines of the boat quite nicely.



So as you can see, I don't have a lot to discuss this time around. I will have some more free time after Christmas as well the New Year weekend so I will be trying to get the center sections completed. After they are done, skinning will be on hold for awhile while I complete work on the bottom of the boat.

Several tasks needs to be accomplished on the bottom before I can start adding the skin panels. First, I need to install the battens into the notches in the frames. These notches will need to have limbers added before the battens are installed. The limbers are an area cut out on the outboard side of each notch that will allow water to drain to the aft end of the boat.

In addition to the battens, I need to do a bit more fairing work on the frame bottom edges. And I also need to pre-drill the two drain plug holes in the transom.

All in all, quite a bit of work to accomplish, before the skins can be added. I am still hoping to complete all of this (including the skinning)  by the end of February but with the onset of winter and colder weather, I am not sure this will be possible. We'll see.

So, that is it for now. I will continue to work the boat and in a couple of weeks, I should be able to show more progress. Until then, take care.

Sunday, December 7, 2014

Skinning Continued

After last week, I was pondering how I was going to apply the remaining skin panels to the boat. My thinking was that I needed to have some idea where the center panel (next one after the aft panel) needed to end so that the front panel could be covered with a single sheet of plywood and at the same time, leave the joint between the center and front panel flat enough to use a butt joint.

Readers may recall, that I had originally intended on making a single 24 foot long piece of plywood using scarf joints and then attempting to apply that panel in one shot. Knowing what I know now, I am glad I did not attempt to do that. More on that in a few moments.

The scarf joint approach was originally considered because I was afraid that the butt joint wouldn't conform to the curves of the boat all that well. However, the scarf joint approach presented several difficult problems to overcome. First, the length of three panels would have exceeded the length of my garage by 3 feet. I was unwilling to store the panels outdoors.

I though perhaps I could make a heavy paper template of the panels on the boat structure and then rough cut the 24 foot panel down to a more manageable size. But information I read seemed to indicate that this approach was risky because of the different flexibility characteristics of heavy paper when compared to plywood.

The second major challenge was how I was going to handle that large of a piece of plywood when fitting, and even more so when gluing the panel to the boat. I am working alone on this project and do not have a reliable source of extra hands so this was a major consideration.

I considered a hybrid approach where I would install the aft panels separately. The thinking here was that the aft panel butt joint is fairly flat so a scarf joint was not necessary here. I moved forward with that approach and installed both aft panels. That's where I was at the end of the last posting.

Installing these aft panels made me aware of the amount of work and time required just to install the 8 foot piece of plywood. I was was battling cure times and my personal endurance just getting these installed. I began to realize that installing a much larger piece of plywood was going to be very difficult for me and run the risk of having the epoxy cure to an unworkable state before I finished installing the panel.

I decided to re-examine the possibility of using a second butt joint near the front of the boat. I first needed to get some idea of where an 8 foot panel would end if attached to the front of the boat. Since this area is curved, it was not easily measured. Only by attaching a similar sized piece of wood could I be sure where the second butt joint would be.

I had two full 8 foot long pieces of plywood that were left over from the aft panels. They were narrower but that wasn't a problem. So I clamped them into position on the bow.


 This gave me a rough idea where a full panel would end. Then I clamped a scrap piece of plywood over that approximate area clamping to the sheer and the chine and over frame 4. What I was looking for here was the amount of curvature of the panel in a vertical direction. It didn't appear to be too pronounced and I began to have confidence that a second butt joint would work in this area as well.

I examined pictures of other examples of this design built by other builders and I saw that many of them also had butt joints in this same approximate location. At this point I was still thinking that I would install the center panel first and then install the forward panel last. But as I though about it more, I realized that installing the front panel first would be easier and then I would only have to fill in the center with the relatively flat center section. I considered the idea for awhile and could not think of any real reason not to do it this way, so that is the approach I ultimately decided to use.

The first order of business was to get a rough trimmed panel. I laid a full sheet of plywood in position and clamped it into place. Then I marked off very rough outlines where I needed to cut the panel.


After cutting these rough outlines, the panel was much easier to clamp back into position. From here I could refine the amount of material to trim off. As material is removed, it becomes progressively easier to re-install the panel because it is lighter and conforms to the curves better.





You'll notice in the first of the four photos that there is a vertical line marked on the panel. This would be the cut line for the but joint. Also notice how well the panel conforms to the curve when viewed from the inside. This is a testament to the design of the boat. I did notice that my chine fairing in this area was still a bit flat. This is in the area of the center clamp in the third picture.

I had to take a couple of days and build up the chine in that area before proceeding. Once that was completed, I re-installed the panel again and the fit was much better. Before proceeding further, I took the panel and checked it against the other side of the boat. This served two purposes. First a sanity check to insure that both sides of the boat are the same (they are). Secondly, I was able to use this first panel as a template to rough cut the second front panel from another sheet of plywood.

I took that second panel and test fitted it to the boat on the port side and saw that there would be no problems in getting it to install. A plus was that the chine on that side did not need any additional work. So when I get to that side later this week, I can proceed immediately to the fitting and installation.

Back to the starboard front panel. Using the same sequence of steps I outlined in the last article, I proceeded to get the panel installed. As a recap, this basically meant tracing the structure outlines on the interior of the panel, using these tracings as guides to layout the screw holes, and final trimming the chine edge of the panel. This edge needed final trimming before installation because the bottom skins will eventually butt up to this edge and it would have been nearly impossible to clean this up after the panel was installed. The tracing of the chine on the panel gave me the reference points I needed to determine the cut line for this final trimming.



You'll notice that the curvature of the panel at the aft end (the butt joint ) is relatively flat and there should be no problem joining the center panel at this location.

One other area of difficulty I had to contend with was at the very front of the panel. Because of the closeness of the boat to the garage back wall, there is insufficient room to get in there to pre-drill the screw holes into the structure and to install the screws during assembly using my battery powered drill. It is also a difficult area to clamp because the stem and breasthook get in the way.

I solved these problems by using an air powered angle drill to drill the holes and an air powered wrench and socket fitted with the screw tip to drive the screws. You can see these in the last of the previous photos.

Here are two photos showing the area in question. You can also see in the second photo, the overlap of the panel on the stem. This will need to be sanded back to the stem. When the other front panel is installed, it will overlap this first panel in this area.



Because of difficulties I had with removing the temporary steel screws and plywood washers on the aft panels, I decided to remove these screws earlier in the process on this panel. Remember that these temporary screws are there simply to hold the panel in place while the epoxy cures. They are eventually replaced with permanent silicon bronze screws. I waited several hours until the epoxy was hardened, but not fully cured. The one by one, I removed the temporary screws and installed the permanent screws. I left the clamps in place to avoid putting any strain on the not fully cured epoxy.



These screws will eventually be filled over with putty and sanded smooth before I apply fiberglass cloth to the hull in a few months.

So the weather is a bit cooler today and I have other pressing things to take care of but I am hoping to to get the other front panel on by next weekend. Then I can begin to tackle the center panels. Those will involve the additional work of making and installing backing plates in the butt joint areas. I will explain all of that when I get to it.

Until next time.

Take care.