February, 2013

Centerboard Case Construction

The centerboard case is built out of two 6 mm thick plywood sides framed with 27 mm wide by 18 mm thick western red cedar. Exact frame parts can be plotted on the case side panels from the drawing details. The centerboard frame pieces can be cut using a bench top band saw or a very stable scroll saw and a lot of sanding. Prior to bonding the parts, the plywood sides should be glassed with fiberglass cloth and three coats of epoxy resin, per specification.

I  clamped one side of the centerboard case to a work bench, where I marked a 0,0  reference point and the X,Y points shown in the Hazardous Zero 9 Centerboard Case framing drawing. After "connecting the dots", I had perfect templates for the framing parts.

To the left is a section of the Hazardous Zero 9 Centerboard Case framing drawing I used to plot the full size patterns for the frame parts. Drawing Copyright Keith Callaghan.







For a temporary workbench, I used a 30" wide by 8' long sheet of plywood wrapped in plastic sheeting to prevent epoxy bonding to the work bench. 

The plywood sides are glassed with 200 gsm  or 6 oz/sq-ft fiberglass cloth and three coats of epoxy resin, per specification. The fiberglass cloth was cut with a 25 mm overlap all around. I used the West System 105 epoxy resin and their 206 slow hardener due to warmer than normal temperatures. The West System mini pumps work great and take away the hassle of measuring the right proportion of epoxy and hardener. All I had to do was pump the epoxy can once and the hardener can once to dispense the right mix. For the epoxy application method, I followed the instructions in the Gougeon Brothers book on Boat Construction. 

Here, one of the centerboard case sides is held down flat against the workbench after being glassed. I did the same with the other side, per Keith Callaghan's recommendation. I took the opportunity to epoxy the frame parts or other small parts as I went along

The other centerboard side undergoes the same treatment. I could've done both at once with a slightly larger workbench.

This picture shows the forward and aft frames bonded to one of the  sides of the case. Temporary 27 mm spacers were placed inside the case to help hold the spacing between the sides during final bonding. It is very important to ensure the pieces are flat against the workbench. I used multiple clamps to keep the centerboard case straight during bonding.

I used the West System 105 Epoxy Resin and the 206 Slow Hardener for glassing. For bonding the frames to the centerboard case sides, I mixed resin and hardener with the West System 406 colloidal silica to make the adhesive filler.

Once the case is closed during bonding, the 27 mm wide case is a very tight space to work in. I prepared these simple extensions for a brush, a flat end plastic spatula for removing excess epoxy and a round one for filleting. These tools worked well for me while finishing the inside of the case.




The centerboard case standing on the temporary work bench after bonding.

This picture shows the lamination of the top centerboard reinforcements. I found out later this step is done much later after the case has been inserted in the building frames.

It is true what they say about boat building. You can't have too many clamps! I like these clamps with rubber guards because they protect the plywood and they don't stick to epoxy. If you know someone who is building a boat, send them a set of clamps. You can't go wrong.

Lessons Learned

1. Install a bolt in the centerboard case holes to help with alignment of the centerboard case holes during bonding. Remember to insert a 27 mm sleeve or spacer inside the case. Hand-tighten the bolt.
2. The top centerboard reinforcements do not have to be in installed at this point. I will now have to either modify the building frames or slide the case in them as I mount them on the construction jig.

Scarfing The Planks

Scarf Joints For Mere Mortals

A scarf joint is a method of joining two members end to end in woodworking. The scarf joint is used when the material being joined is not available in the length required. It is an alternative to other joints such as the butt joint and the splice joint and is often favored over these in joinery because it yields a barely visible glue line.

If there was a boat building step I feared the most until recently, it must have been scarfing. I had never done it and I didn't know how to do it. While reading the scarfing section in the Gougeon Brothers' book on Boat Building, I discovered that the West Systems 875 Scarfer™ is available as an attachment for  circular saws. 

I ordered the 875 Scarfer jig only to discover that the outer foot plate of my old circular saw was too narrow to drill the needed holes to received the jig's outer plate and leaving a required 1/8" clearance between the outer plate and the saw blade. It became clear very quickly that this was not going to be the solution for me. Many individuals recommended the scarfer attachment as a viable option for making the required cuts. It also has a limitation to cut scarf joints in stock upto 9 mm or 3/8" thick.  I am sure it works great and, except for drilling a few holes in your circular saw foot plate, is the least expensive short-term solution. I wasn't too dissapointed in a way because I don't like buying single purpose tools. West Marine kindly accepted my return without any issues and that was that.

The Router Scarfing Sled

I researched other options in the Gougeon Brothers' book on Boat Construction and on the Internet. I found references about a router scarfing sled in the book, at www.woodhaven.com and in a couple of videos in YouTube. This option seemed to be more logical since the router sled has more practical applications and has less limitations than the West System Scarfer.

This is the router scarfing sled I built from scrap laminated shelving material. Two angle guides for cutting ratios of 8:1 as recommended in the drawings and the construction notes. The top piece is a router track with a slot in the middle and guiding tracks underneath that glide over the angle guides. This jig provides me precise and repeatable cut.

Precise and consistent results can be achieved with the right router and knowing how to use it. I use a plunge router with a straight carbide bit as recommended by many boat builders. Mine has an adjustable depth screw that can easily be turned to gradually increase or decrease the depth of the cut.

To the right is a picture of my first scarf cut on a piece of scrap plywood. I performed several trial cuts to improve the quality of the cut. The recommendation is to make the cut from right to left so that the router bit cuts into the wood and no away from the wood as this can cause an undesired result shown in a picture later in this column.

Nothing like a sharp and smooth scarf cut.

Here, my first two scarf cuts held together with a clamp to ensure proper alignment of the plies of wood. This was a very clean scarf cut.









An example of what can happen if cutting the scarf with a router from left to right, away from the wood. Not a very good finish.

I made slight modifications to the jig to center the cutting area and square it off to the gliding tracks. A plywood clamp or strap is added to hold down the plywood on the workbench.

Further observation indicates a slight arching of the plank as a result of the clamping. The cut would not be even given the curvature, thus making a less desirable cut. This issue occurs mostly on the wider planks.

To solve the arching problem I insert wood wedges under the clamp. The plank is now flat against the workbench and I can initiate the scarf cut.

Plank 1a after the scarf cut. To achieve this result, I made 3 passes of the router removing about 2 mm at a time. The final pass can easily be fine tuned with the use of the depth screw in my router.

An angled view of the scarf cut on plank 1a. Once the router cut was done, the scarf cut was completed. There was no need for sanding.

One more look at the scarf cut on plank 1a. This is a section view. The clean cut and razor sharp edge produced by the router in this process is fantastic. I couldn't be happier.

Once I laid down the first pair of scarfed planks, I realized that as I had performed the scarf cuts in one direction for the aft planks, I would have to perform the scarf cuts for the matching forward planks on the opposite side. I proceeded to mark the locations where I would have to make the scarf cuts on all of the planks to avoid costly mistakes and waste of plywood.

It is a long process, but I am happy with the result and I have a repeatable process. The first cut took 45 minutes between setup, jig adjustments, etc. I was able to bring the scarfing process time for 1 plank down to 10 minutes. I explain the end-to-end process.

Preparation

1. Match the plank pieces
2. Lay out the 32 planks pieces over the full size drawings.
3. KEY STEP: Choose and mark the side where the scarf will be cut and where it won't be (over or under) on the forward and aft planks. Else, risk wasting a piece or, worse, bonding the planks incorrectly and wasting two pieces.

Scarf Cut (~10 minutes)

1. Pick up the plank and mark a parallel line 48mm from the edge to be scarfed and on the side to be scarfed
2. Lay the plank on the jig and align the edge of the plank to be scarfed exactly parallel to the reference line on the jig
3. Clamp down the plank to the jig. Wedge down under the clamp to further secure the plank edge over the jig (wider pieces mostly need this)
4. Adjust the router bit depth to start at about 3mm from the surface and initiate first pass cut
5. Adjust depth on plunge router and initiate second/last cut
6. Vacuum routing debris from the jig
7. Loosen up the clamp and carefully remove the plank
8. Inspect the cut
9. Lay down the plank over the full size drawings

For me this process was not about cutting 32 scarf joints. It was about being able to cut one scarf joint perfectly (or as close as possible to perfect) and being able to replicate that process 32 times. 

Bonding The Scarf Joints

After I finished cutting the scarf joints, I thought I was ready to start bonding the planks on the garage floor. Fortunately, I took a break to read the Gougeon Brothers' book section on bonding scarf joints. I quickly realized that I wasn't setup to go at it, at least the way I was thinking.

Aligning the planks precisely as they would over the full size drawings and clamping  the pieces during bonding to prevent slipping but without removing too much epoxy requires a surface to control these constraints during the process.

After seeing a picture of designer Keith Callaghan bonding one of the scarf joint of his HADRON dinghy Clearly, I wrote him an e-mail requesting some advise. Two keywords came back in his prompt reply. Workbench and clamping.

Clearly, the concrete floor was not going to work for me. This required a long enough workbench to accommodate at least the 14 ft. length. While I had the space, I didn't have the workbench. Or so I thought.

Looking around in my garage, there is Merlin Rocket in there. The pieces are not together yet. But in my mind, I can see it. I could also see the space where a work bench could be built for bonding the planks but that would mean buying more materials for building a one time single purpose workbench. I would also have to move a lot of stuff around, like the building frames and the 2" x 8" x 14' boards for the construction jig... That's it! 

The construction jig used to support the building frames, mounted on saw horses, would make the perfect work bench for bonding the scarf joint.

First, I built the construction jig on the floor and around the saw horses and my tool tables to ensure I will be able to store them underneath the table when finished.

The finished temporary work bench with added boards. This workbench is suited for bonding  two full planks at a time. The space in the center is left open to solve the clamping challenges that Keith Callaghan warned me about. In that space I placed two sliding boards about 2" wider than the widest plank and 2' long. The purpose of the sliding boards at the scarf joint point is to move them sideways as needed to adjust for the curvature of the planks, have the scarf joint squared off over the sliding board and be able to place clamps precisely over the bonding area. The sliding boards are covered in plastic sheeting to prevent bonding with the planks.

In preparation for the bonding process, I place the full size drawings over the workbench and I lay the first two matching plank pieces over the drawing.

Once aligned over the drawing, I make several marks at 45° angles to help me later with the alignment over the workbench.

I can't remember the reference to this tip but it proved very valuable.

The method I used for bonding the scarf joints is the one recommended on page 109 of the Gougeon Brothers' book on boat construction. I used the West System 105 epoxy resin with the 205 Hardener to wet the scarfed area. 

After approximately 5 minutes, I added West System's 406 Colloidal Silica to the leftover batch of the above epoxy resin mix to create the bonding adhesive.

The adhesive mix is much thicker than what it appears on this picture.

I worked simultaneously on 2 sets of planks at a time. Once the bonding adhesive was applied, I aligned the matching plank pieces together using the 45° pencil marks discussed earlier. To the right, the scarf joints over the sliding workbench panels and under a plywood block wrapped in plastic sheeting while applying even clamping pressure from both sides of the planks.

About 24 hours later, I removed the clamps, inspected the bonded joints and the plank against the full size drawing. I couldn't be happier with the results. Not only clean joints that require little sanding but also perfectly aligned planks to the drawings.

While I stored the plank pieces safely against the wall prior to beginning the bonding process, I had to solve for their storage after bonding. A much longer space was needed. Once again, I looked around my garage. I found my ladders hanging from the ceiling so I decided to hang the planks from the ladders. I found a hanging system with Velcro straps at the local home center that worked perfectly for this purpose.

In March, I plan to round edges, epoxy coat the parts, and perhaps start building the building frames in preparation for planking the hull

Joining the MROA

This month I had the privilege to join the Merlin Rocket Owners Association with a full membership. Although there is not a Merlin Rocket class in the USA, I want to connect online with other Merlin Rocket sailors, get their advise and take full advantage of the library resources on the website. Thank you Keith Callaghan and Rob Holroyd for your advise and words of encouragement.

Many thanks to Colin Brockbank, Chris Phelan and Magnus Smith for their warm welcome and help during the registration process, the access to the library and the members only area. The content in the members area is extremely valuable to conduct my construction project.

Apparently the on boarding process reflects the speed of the Merlin Rocket and the individuals who sail them.

I received my 2013 Year Book and magazine shortly after paying my membership dues!

I now proudly display the Merlin Rocket decal on my vehicle so if you are one of the 700,000 Brits living in the USA and happen to see me while in Austin, Texas do the proper thing... HONK!








Where to sail in Austin, Texas

Lake Austin

Lake Austin is a water reservoir on the Colorado River in Austin, Texas in the United States. The reservoir was formed in 1939 by the construction of Tom Miller Dam by the Lower Colorado River Authority. Lake Austin is one of the seven Highland Lakes created by the Lower Colorado River Authority, and is used for flood control, electrical power generation, and recreation.

Lake Travis

Lake Travis is a reservoir on the Colorado River in central Texas in the United States. The reservoir was formed in 1942 by the construction of Mansfield Dam on the western edge of Austin, Texas by the Lower Colorado River Authority. Lake Travis has the largest storage capacity of the seven reservoirs known as the Highland Lakes, and stretches 65 miles (105 km) upriver from western Travis County in a highly serpentine course into southern Burnet County to Max Starcke Dam, southwest of the town of Marble Falls. The Pedernales River, a major tributary of the Colorado River, flows into the lake from the southwest in western Travis County. The lake is used for flood control, water supply, electrical power generation and recreation.