The Arctic Tern Project

Volume 5                           Issue No. 12                      December 2008

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Making Holes

Last month I said that this month I’d work on the sliding companionway hatch.  I changed my mind and decided instead to tackle mast placement and chainplates since I need to think about ordering my mast and standing rigging soon (cha-ching; big dollars).

Mast Placement

Ok.  So I needed to figure out where the mast is going to pass through the cabin top and then make a hole for it.  First thing was to get a center line on the cabin top (note that the joint in the cabin top lamination is off-center).  I did this by stretching a string line the length of my shed and lining it up at the bow and stern with a plumb-bob and then dropping a plumb line down from the string to the cabin top.  This was kind of fiddly and tricky and so ended up double checking with the laser.

 

The next thing was to measure back from the bow the required distance as specified in the plans and make an ‘X’.  This will be the center of the mast.  Luckily I had an off-cut of the mast profile that I’m going to use as this came in handy for making the hole.  I also lay-out on the cabin top the position of the doubler piece to strengthen the cabin top around the mast.

 

Then, using the mast profile off-cut, I made a template to use when cutting the hole for the mast.

 

 

The next step was to figure out the angle that the mast will pass through the cabin top.  I use a level to measure the slope of the cabin top.

 

 

Then I use my CAD program to figure out what the angle is and then add 1 degree for the mast rake.  This then gives me the angle at which the mast will pass through the cabin top.  The long, vertical angle in the drawing gives me the distance forward of the mast hole center that the mid chainplate will go since it is 20 inches lower than the cabin top (more on this below).

 

Then it snows.

 

Next I make up the cabin top doubler from three layers of 6mm ply.  Oh yeah, I forgot to mention that I drilled a small hole in the cabin top where I thought the mast center should be and dropped a plumb line down to the mast step.  Pretty much bang on so I enlarged the hole to ¼” and use it to help position the cabin-top doubler.

 

Initially I entertain the thought of laminating the doubler in-situ on the cabin top but reject this in favour of a form.  I use the old two-sticks method of scribing the cabin top camber on to a couple of pieces of 1” fir ply I have kicking around.

 

Then it snows some more.

 

The doubler gets laminated up in the shed (at -5C) and then taken into the house to cure.

 

Here’s a picture of my cat because she’s so cute.

Ok.  Once the doubler cures I mark off the cuts and have at it with the RAS.  I do the cut now while the double is still on the form because it makes it easy to get the cuts vertical.

 

 

I clean up the edge and mark a radius on the corners using a piece of pipe cut at an angle such that will stand vertical (this gives a slightly elliptical corner).

 

 

Then I test fit the doubler.  Looks good.

 

Then it snows some more.

 

Alright.  In this next shot you can see the wedges glued under the pattern that I’ll use to route the mast hole in the various pieces.  These wedges position the pattern at the correct angle so the hole ends up being angled correctly for the mast rake and cabin slope.

 

First I test out the pattern on a piece of scrap.  Seems to work ok.  I need to do a bit of fiddly repositioning of the bearing on the router bit in order to get sufficient depth of cut but I can make it work.

 

Next, I position and attach the doubler in place with screws and tack the pattern to it and then go at it with the router.

 

 

I cut all the way through the doubler and into the cabin top.  I then remove the doubler and attach the pattern to the cabin top and finish the cut all the way through (I only do it this way because the router won’t go through both the doubler and the cabin top because the bit is too short.

 

Then the doubler goes back on et, voila.

 

Next thing to make is the mast collar.  I initially tried to make this out of a chunk of solid African mahogany but it didn’t really work out so I decided to go with plywood instead.  I start off make the collar oversized to it’s easy to line up on the doubler.  There’s also a pieces that goes inside the cabin to trim the hole.

 

 

It also has a mast-shaped hole routed at the required angle.  I use a router bit with the bearing on the bottom (to follow the mast hole already cut in the doubler) to cut the hole in the collar.  I still need to use the pattern to get the hole at the correct angle.

 

The collar is made from two layers of 18mm and I cut one layer at a time with the router.

 

To cut the hole in the piece that goes inside the cabin I take the mast collar off the boat and use it to guide the router bit (still using the pattern to get the correct angle of cut).  I need to offset the pieces to compensate for the angle of the mast and the thickness of the cabin top.

 

 

Ok.  Then I do the final shaping on the doubler, collar and inside trim piece.  I use sandpaper stuck to the doubler to sand a curve in the bottom surface of the collar.

 

Chainplate Placement

The next thing to figure out is where the chainplates go.  This required some moderately heavy mental lifting and I’m exhausted from the effort.  First, I positioned and attached (temporarily; using only screws) the forestay fitting because I wanted something sturdy to attach a tape and/or string to in order to make sure the chainplates are positioned the same distance back on either side.

 

I used multiple methods for finding the position of the mid chainplate.  First, I put a batten athwartships at the center of the mast hole dropped a plumb line down at the sheer on either side.

 

This gave me a mark on the sheer directly below the center of the mast hole. 

 

Then I ran a non-stretchy string back from the forestay fitting to double check that this mark was the same distance from the bow on either side.

 

Then I ran a line out from the front of the cabin to the sheer and measured back from this as a triple-check.

 

 

Ok, all seemed as it should be.  Then I needed to move this mark forward about 3/8” to allow for the mast rake given that the chainplates are about 20” below the cabin top.

 

Ok, that gave me the position of the top of the mid chainplate.  I then drew up the chainplate angles on my CAD program based on the shroud lengths and angles from the plans (taking into consideration the 1 degree rake of the mast).

 

This drawing was laid out to give me distances from vertical which is easier to measure than angles.  The next step was to get a vertical line on the side of the boat.  How the heck was I going to do that?  I ended up using my laser but in order to do this I needed to make sure the laser was positioned exactly perpendicular to the center line of the boat.  To do this I dropped a plumb line on the far side of the boat down to the mid-chainplate mark on the sheer and then fiddled with the laser until it hit both the plumb line on the far side of the boat and the chainplate mark on the near side of the boat at the same time.  I needed to use binoculars to see the far side laser line.

 

Ok, so now I had a vertical line.  I use a square to get a line at right angles to this and 16” down from the sheer and from this I lay out the chainplate lines as given in the CAD drawing.

  

 

Since I can’t really shoot the laser on the other side of the boat because of space considerations I decide to make a pattern to transfer the chainplate lines to the other side of the boat.

 

First I try to use vapour barrier but it’s too stretchy to transfer the lines reliably.  Instead I end up using a sheet from some plans.

 

To transfer the lines to the other side of the boat I flip the paper over and position it along the sheer at the chainplate top marks and then mark the chainplate bottom marks from the sheet of paper.  It seems to work.

Next Time

Make and fit the chainplates.