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Steve's "Mobjack Bay" Skin-on-Frame Kayak Project - March, 2017

Dave Gentry's Mobjack Bay Kayak

Documenting the building of a Greenland-style Skin-on-Frame Kayak

In the spring of 2016 Sue and I built a skin-on-frame kayak for her use. It was based upon Brian Schulz's F1 design though it was built using frames and stringers vs. steam-bent ribs. You can check out the building of that boat here.

The Mobjack Bay was designed by Dave Gentry as a smaller Greenland-style kayak for lighter paddlers. The photo above is of Dave's prototype. It is definitely a low volume boat intended for day trips as opposed to camping or expeditions. There is not much storage space in this boat but I think it will suit the kind of paddling we do just fine. So, here we go!


Mobjack frame
The project began with cutting out the seven frames using full sized patterns provided with the plans. I was able to slightly modify the gunwales, keel and stringers I had prepared last year when building Sue's kayak. In the photo above you can see the basic frame layout. All those clamps at the bow are 'redirecting' the split gunwales to create the swoop of the shear of the bow.
Gunwale tips

Here is a closer look at that process. The gunwales were split (sawn) 54" back from their tips then bent upward to create the desired 'laminated' curve at the bow. The pine wood we're using is pretty stiff, even  at only 5/8" thickness, but this did work as the plans indicated it would with only about an inch or so of spring-back when the epoxy had cured.
 - March 5, 2017

Frame - March 9th
The gunwales and chines are permanently screwed and glued to the stems and frames. I am in the process of adding risers to fill the space between the tops of the gunwales and the top of the bow stem. This is according to the plans and should produce a nice even sweep at the bow. The boat project is on hold awaiting warmer weather. It takes a lot of firewood to bring the shop temperature up from low single digits that are forecast for the next week. The rivers are still pretty hard so that's OK.   - March 10, 2017
Gluing on bow risers
Today it finally warmed up enough (28 deg F.) to fire up the stove in the shop and epoxy the risers to the bow. Lots of clamps (21). Looking good.

As an aside, that red object under the saw horse is an inverted large, re-purposed plastic bucket that I use as a step-stool or seat. It is very handy since you can slide it into position with a gentle kick and it is super sturdy.
 - March 15, 2017

Risers & bow plate
After the epoxy holding the risers had cured I cut a recess for a small plywood bow plate. This piece is shaped to form a continuous curve with the tops of the risers and gives the bow its shape.
Hatch plate from side
I decided that it might be a good idea to have at least a little accessible storage space for things like a jacket, gloves, water bottle and lunch so I designed this hatch system to replace the aft deck beams. I added an additional cross piece and then screwed and glued the hatch plate (above) between the two frames behind the cockpit.

The covering fabric will go over the hatch plate and be sandwiched between the plate and the rim shown at left. The lid rests on a 5/8" lip inside the rim and will be held in place by a simple lip under the aft edge and by a shop-made latch at the front edge. The rotating latch will be low in profile so it won't be in the way of a wet re-entry. By the way, this is not a water-tight compartment; dry-bags will protect my stuff. - March 18, 2017
Hatch with rim & cover
   hatch detail drawing
  This assembly is made from 1/4" Baltic birch plywood and adds 28 oz. to the weight of the boat.

Mobjack floor (seat)
Today's project was to complete the floor (seat). I deviated somewhat from the plans here by choosing to fasten the floor slats into two panels and then fasten them to the frames with simple hidden fasteners. 
Floor retaining clip
Cut masic
Here's another view of the floor showing Tom Yost-style mounting backup plates. With the floors in I was able to get into the boat for a test fit. For a couple of reasons I decided to go with a larger, more modern style coaming. Most  important  was that I had a hard time getting into the boat. With the traditional, round coaming you enter the boat by sitting on the back deck and sliding forward until you plop down into the seat. The clearance on the masic was too tight for comfort. Also, I felt that I would have a hard time doing a wet exit through the smaller coaming.
Coaming form on boat
So, here is the coaming laminating form from Sue's kayak sitting in position - it fits perfectly! I left remnants of the masic to at least temporarily stabilize the coming.
- March 20, 2017
Frame with foam flotation
Today I cut and fit the flotation foam and generally made a mess of the shop. Cutting and trimming all those 1" thick pieces created lots of little, staticky, obnoxious  slivers of foam. An interesting sculptural process reminiscent of some projects in my senior year sculpture class - 50 years ago. - March 24, 2017

Mobjack Bay kayak frame on grass
Here's a view of the frame out 'on the grass'. The next steps are to oil the frame and then permanently glue in the foam. In the photo you an kind of see that I modified the forward deck beams. I cut them in a shallow 'S' curve, to give me a little more toe room and to have the forward ends blend into the sweep of the bow more smoothly. - March 25, 2017

Bending secondary forward deck beams
Dave Gentry describes this kayak as, "... specifically designed for lighter weight paddlers, and optimized for relaxed use. ... Recommended paddler size: 105 - 180 lbs." I weigh about 140 lbs so fall right in the middle of Dave's design weight.
 What I hadn't considered that light paddlers might also have smaller feet. My size 10's wouldn't fit under the front deck so I had to make some room for my toes. My solution was to 1) curve the deck beams (above), and 2) add secondary deck beams between the main beams and the gunwales. To make these things look OK I laminated a  long curved piece that starts at the frame behind the coaming, swoops around the coaming, over frames #3 and #2 and disappears below the deck fabric before getting to frame #1. The photos on the left show the three 1/4" thick  strips being laminated into this weird shaped piece. It looks like I'll have room for my feet at the expense of loosing a bit of the traditional look of the boat.   March 29, 2017

Frame finished from bow  Frame finished from side
The frame is finished and ready for skinning. Last minute details recently completed: Oiled entire frame with tung oil, final fitting of flotation foam, holes drilled through gunwales for carry loops and cross-lines.
April  5, 2017 - One month to get this far.
Frame finished 

Coaming on form
Clamping again! Here we (this takes the two of us to do well) have just wrapped the first of three laminations of thin oak strips around the coaming form. The strips were soaked (you can see the next strips in the trough beneath the boat, right photo) for several days. Then, with a bit of help from the heat gun, they nicely bend around the form. They will rest like that overnight.
Bottom, skinned
Today I also attached the polyester fabric to the bottom of the frame. The technique I've been using is to first stretch the fabric tightly over the frame, end to end, centering it on the keel, then staple 6-8" to the gunwale on one side. I duck under the boat and do the same on the other side, spacing the staples a couple of inches apart. All this, of course while pulling the fabric tightly over the frame. I then go back over the whole thing and pull & staple so that there are no wrinkles in the fabric - staples almost continuous along the gunwales. I'm using a really nice, $25 Surebonder pneumatic stapler and stainless steel staples. Looking more like a boat.  April  8, 2017

Hull skinned
Today I skinned the deck and bent the second lamination piece onto the coaming form. We're back to below freezing temperatures so I may wait a couple of days before continuing on the coaming. I'll work inside on the deck line sliders and maybe lay out the paddle on a nice 2x6 we picked up yesterday.  April  10, 2017

hull with first coat of paint on bottom
While waiting for the various steps (bending, gluing, curing) on the coaming laminations I decided to get the first coat of paint on the bottom of the kayak. This first involved varnishing the sides so masking tape would stick to the fabric. With the boat inverted, I masked along a line an inch above the chine and then applied a very thin coat of PL Preimium construction adhesive to the whole bottom of the boat. This stuff has to be worked into the weave of the fabric then almost entirely scraped off or it will bubble up and make a real mess. When that coat cured (2 days) I finally applied the first coat of  'Chestnut Brown' Rust-Oleum - a dark milk chocolate color. The plan is to paint the deck light tint of that color.  April  11 - 15, 2017

coaming and closeup showing holes
Coaming before being sewn to boat 
The coaming is finally done! It would have been a good idea to begin making the coaming earlier in the building process since there is quite a bit of down-time in the bending and especially in the laminating. It is difficult to add more than a couple of laminations (of seven) due to their tendency to slide around, up and down, when the thickened epoxy is applied.

I had lots of other stuff to do in the mean time, including re-making - slimming down, really, the longer oars I made for the dory last year. They are going from 72 oz. each to around 48 oz. including the sewn on leathers. The diversions just make this kayak's progress seem slow.

In the center photo you can just see the 1/8" holes through which I'll stitch the coaming to the deck fabric. The lower photo shows how much the fabric will be pulled up to make a nice water-tight fit.
 April  16 - 26, 2017