Introduction: 2-Dog Backpack Boat: the Dogmaran
Have you ever tried to carry your kayak, your paddle, and your fishing gear a mile through the woods so that you could fish on the water with your two dogs? It doesn't really sound like fun, does it? I wanted it to be fun, so I made a Dogmaran.
The Dogmaran is an ultra-light boat for two dogs and their human. You can wear it like a backpack, trailer it behind a bike, or stuff it in the hatchback of your car. It's stable enough so that you can fish from a standing position, and it can be either rowed or propelled by kick fins (flippers), so your hands can stay free to fish or pet your dogs. When you're done for the day, the two hulls nest together for compact storage or transportation (car or bike!). Loaded with my flippers, life vest, fishing rod, and a couple of small tackle boxes, the total weight of this boat is around 35 pounds. I haven't weighed the oars, but they're lighter than my big tackle box.
Materials for Basic Boat:
- Two 4'x8' sheets of thin, cheap plywood. I used RevolutionPly 5mm because it was the cheapest stuff I could find.
- 1 Gallon Fiberglass Resin (and liquid hardener). I used Bondo from Lowes
- Fiberglass Cloth. I used an 8 square foot piece of Bondo brand from Lowes
- Nitrile Gloves -- I probably went through 3 pairs. I use Venom Brand, again from Lowes.
- Gorilla Glue -- not much is needed; if you already have a container, you'll probably have enough.
- Duct tape
- 10 feet of 1/2" Electrical Metal Tubing (EMT)
- 3/4" Plywood. I used scraps. Probably about 1 square foot in all.
- 8 Large Nails
- 4 Large deck screws
- Seat -- Make one (see below) or find one
- Sharpie or other permanent marker
- 1" wood screws (or drywall)
- 2" wood screws (or drywall)
- Drill and various bits, including a 3/4" spade bit
- Sander (a variety of types can work)
- Optional: Projector
- Saw Horses
- Angle grinder with cutting disc
Additional Materials and Tools for Seat:
- 1" webbing (although you could do this with rope if you want)
- 1/2" plywood
- 3 feet of appx. 2"x2" (e.g. from a 2x4 ripped lengthwise)
- Grommet tool
- Propane torch
- Drill and bits
Additional Materials and Tools for Shoulder Straps:
- Padded Backpack Straps (salvaged, or do a web search)
- 1" webbing (although you could do this with rope if you want)
- Three 1" Double Strap Adjusters
- One 1" Snap Hook
- One 1/4" eye bolt
- Grommet tool
- Propane torch
Additional Materials and Tools for Oars and Oar Locks:
- 10' of 1" Schedule 40 PVC
- Scrap 0.2" plywood (left over from boat)
- Fiberglass resin and hardener (left over from boat)
- 8" (or longer cable) ties
- Expanding foam (I used Great Stuff)
- 5/16" steel bar
- 1/8" x 1" flat steel
- 1 1/2" long 1/4" hex bolt
- Flush cutter
- Angle grinder with cutting wheel
Step 1: Parts and Setup
These are the part names that I plan to use in the later steps (hopefully I'll remember). The cup holder isn't very important, but I included a label because I don't think its purpose is obvious.
Step 2: Hull Design and Template Tracing -- Use Mine or Make Your Own
If you want to use my design, you can either project the attached PDF, or you can use the provided measurements to reproduce the design on your cardboard. If you are going to design your own, I describe my design process in "Method 3," below, and in the video provided above.
Preferred Method -- tracing with a projector: ***Note that the photos of my process don't show the updated templates that I have provided. The ones I provided here are better.
- Find a large, dark area.
- Set up a sheet of plywood in "landscape" orientation. Make sure that the face of the plywood is as vertical as possible.
- Height-wise, place the projector even with the middle of the plywood (so about 2' high if the plywood is on the ground.
- Project the black and white tracing template (attached) at the plywood.
- Adjust the distance of the projector until the black outline matches the outline of the plywood.
- Focus the projector and fiddle with the "keystone" control until all of the edges of the black line up as well as possible with the edges of the plywood.
- Trace the two shapes. When I did this, I projected another side panel, but it's better to just trace one, cut it out, and then use it as a template for the rest of your panels. It's easier that way, and you have a better chance of them being identical.
Method 2: Measure and Mark From Plan:
- Use the attached plan with measurements. The distances are in inches from an edge of the plywood.
- Make marks at each of the measured points. Then, as smoothy and artistically as you can, draw the edges so that they go through all of the points. Do this with pencil and re-draw until everything looks nice and smooth.
- The panels have symmetry, so if geometry is your thing, you may want to make a template for part of each panel and then flip it to draw the rest. If you don't know what I'm talking about, just stick with steps 1 and 2.
Method 3: Design your Own. You may have different sized dogs, and you may not have dogs at all. Here is some insight into my design process, in case it helps...
- A decent amount of freeboard (distance between the water line and the top of the sides of the boat) when loaded with 350 pounds. My occupants weigh about 185 (me), 60 (Susie), and 38 (Murray). That's only 283 pounds, but I opted for a big fudge factor.
- Minimal drag (streamlined hulls).
- Flat bottom hulls (for dogs' comfort)
- Enough floor width and length for a dog. The designed width is about 16" at the top, tapering to 12" at the bottom. Having enough length wasn't an issue.
- Short enough for maneuverability in backpack mode but also as long as possible for stability and hull speed.
CAD Design Workflow:
I use CAD software called Rhinoceros, but I am guessing that most CAD programs have similar functions. Below is a quick summary my workflow. Here's a quick run-through of the information below, in the form of a video.
- I first used a control point curve tool to create the outline of one hull from a top view. I actually just created 1/4 of this outline, and then I used the mirror and join functions to create a symmetric polyline.
- Copy the curve orthogonally downward the height of hull (14" in my case). This curve represents the bottom of the hull.
- From a top view, scale down the bottom curve until it is about 12" wide, and until its length looks satisfactory.
- Loft the two curves to create sides.
- Cap planar holes and then analyze to determine the volume. My goal was to support 350 pounds, which would require approximately 10,000 cubic inches of volume below the water line. So one hull must have well over 5,000 cubic inches of volume. [Water's density is about 0.036 pounds per cubic inch, so 10,000 cubic inches is about 360 pounds of water. The weight of the water displaced is equal to the buoyant force that supports the boat.]
- Use the box and boolean split commands, along with a "guess and check" approach to find the location of the water line when a hull is displacing about 5,000 cubic inches of water. I included a diagram showing the water line for my dogamaran when the total water displacement is 9,974 in^3.
- If the water line looks uncomfortably high, rescale the hull to increase its volume. If the water line is ridiculously far down the hull, shrink the hull. The best dimension to scale here is probably height.
- When you're happy with the design, use the UnrollSrf command to create a flat view of the panels that you need to cut out. There will be two sides, a top, and a bottom. Delete the top panel.
- Create a 48" x 96" rectangle and arrange your pieces on them. You won't be able to get them all on one sheet. You could make a boat like this with one sheet, but you wouldn't be able to get two big dogs in it and still keep your butt dry.
Step 3: Cut the Panels
- Set the panels on some saw horses or some other support.
- Use a jigsaw to cut out one copy of a side panel and one copy of a floor panel.
- Trace the panels that you just cut and then cut out three more side panels and one more floor panel.
- Sand the edges of the panels -- unless your jigsaw makes nice, clean cuts. Mine doesn't.
Step 4: Tape and Glue the Panels
This type of construction usually goes by the name "stitch and glue." I have done this many different ways over the years. The way I did it this time is a little ugly, but I didn't care. I glued the seams with polyurethane glue (Gorilla Glue, in this case) because I don't like to work with fiberglass resin any more than I have to. At the same time, I wanted a strong bond and aesthetics aren't that important to me.
**Update: I have decided that next time I do this, I will use fiberglass putty (probably Bondo) instead of Gorilla Glue. The reasons I'm going to use fiberglass putty instead of Gorilla Glue are that 1) it sets faster, and 2) more importantly, it provides a thicker, less porous waterproof barrier at the seams. If my hull hits a rock, and the rock breaks through the outer fiberglass and plywood, the fiberglass putty is going to hold things together and keep me dry. In contrast, Gorilla Glue is puffy and porous, and I wouldn't count on it as a 2nd waterproof line of defense. Plus it's ugly and hard to clean (but I knew that in the beginning). At any rate, here's the method I used this time...
- Match up two side panels (Fig. A). Use strips of duct tape to tape their end edges together. [It looks nicer if you have the same plywood surface facing out.]
- Hold the side panels on a hard floor (concrete is good) and insert a bottom panel between the two side panels. As you push the bottom panel into place, this will pry apart the side panels. Stop when the bottom panel is an inch or two from the floor. (Fig. B)
- At each end, place a long strip of duct tape (maybe 1'), sticky side up, on the floor. Place the hull on top of the tape, and firmly attach the tape to the sides. Make sure you do this at both ends. (Figs. C & D)
- Now push the bottom panel the rest of the way to floor.
- Add more duct tape to securely close all of the seams. Use strips that are about 8" or longer, and place them perpendicular to the seams. Get the seams tight. You may need to remove tape that looks loose and re-tighten or replace it.
- Turn the hull upside-down. I wanted my bottom panels to be "inside" the side panels. To do this, I had to use a screwdriver in a few places to pry a side panel over a bit so that I could "stuff" the bottom panel back into place. (Fig. G)
- When everything is satisfactory, cover all the seams in strip of duct tape running parallel to the seams. I did an experiment to see if masking tape was as good as duct tape, but it wasn't. Don't use masking tape; it won't hold well, and it won't come off easily later on. (Fig. H)
- Turn the hull back over.
- Wipe the seams inside the hull with a wet cloth (to help the Gorilla glue).(Fig. I
- Apply a bead of Gorilla glue to all of the seams. Wear some nitrile gloves and spread the Gorilla glue at least 1/2" on each side of the seam. If you wipe your gloves, leave them outside-out carefully, you can use them again when you fiberglass the hulls. (Figs. J,K)
- Wait. Preferably overnight (Fig. L). If I had used fiberglass putty, I would only have needed to wait about half an hour.
Step 5: Remove Tape and Prepare for Fiberglassing
- Remove the tape.
- Sand any glue that expanded to the outside of the hulls. (Fig A)
- Sand anything else that you want to be smooth.
- If you find any big holes, repair them. (Figs B-E). I had one issue to fix...
- My problem: On the hull where I used masking tape, some of the tape released, and the two side panels opened up to make a big crack.
- Solution: I cut a piece of plywood and simply hot glued it to fill in the space. The fiberglass went right over the hot glue; no problem.
Step 6: Fiberglassing
I'm not an expert, and I hate working with fiberglass. But the stuff is so useful that I can't seem to stay away from it. If you want to learn from an expert, there are many experts out there. My methods were unorthodox and probably a little ridiculous. I wanted to see if I could avoid using throw-away brushes, and I managed to do that, but I did go through three pairs of nitrile gloves. My method is described below.
Part 1 (Figures A & C) -- Setting up and establishing my batch volume and hardener mixing ratio:
- Move everything outside, on sawhorses. Fiberglass resin is incredibly stinky and bad for you. I had a little breeze, so I didn't set up a fan. In retrospect, I probably should have set up a fan to blow away even more of the fumes.
- Find a disposable surface to set all of your stuff on. I used one of my plywood offcuts (Fig A), but I wish I had used cardboard.
- I cut the bottom off of a 2-liter bottle and used a measuring cup and a Sharpie to make a mark at a level that would give me approximately 200mL of resin. According to my calculations, 1ml of resin needed 0.01294 mL of liquid hardener, so 200mL of resin required about 2.6mL of hardener. At first I used too much, and my resin hardened to jelly before I used it all, but I eventually settled on this ratio. I seem to do most of my fiberglass projects on cold days, so I was surprised by how quickly the resin set up. On this day I probably had around 5-15 minutes of working time, the variation due to my haphazard measurement.
- I did all of my work by mixing a series of separate 200mL batches of resin, stirred with a stick of folded cardboard.
Part 2 -- Encapsulating The wood:
- The goal here was to cover everything so that my boat will be watertight, and so that the wood won't rot.
- Whenever possible, I oriented the hull on my sawhorses so that the surface that I was working on was horizontal. Then I poured a strip of resin on the surface and squeegeed it as far as I could with a square of cardboard (Figure D).
- After I finished one surface, I rotated the hull on my sawhorses without regard for having the uncured part touching the sawhorse. It wasn't an issue.
- In the end, I embraced finger painting. I dipped two or three fingers into the resin (Figure E) and then painted it on. I only ended up going through 3 pairs of nitrile gloves in all, and I only had one glove tear. Using my fingers was especially helpful in spreading resin along the ugly Gorilla Glue seam. My cardboard squeegee was no match for the irregular surface.
Part 3 -- Adding fiberglass cloth to the outside of the seams:
- The purpose of the fiberglass cloth is to add strength to my seams, and to provide another waterproof barrier.
- I pre-cut my cloth into strips about 3" wide and 2' long (Fig B). Cutting fiberglass cloth when your hands are already covered in fiberglass can be really annoying. Little strands from the frayed edges get ripped loose. I try to minimize cutting while I'm messy, but doing some of it is unavoidable (for me, anyway).
- At this point, I really embraced finger painting. Unless the seam was already "wet" with resin, I dipped two gloved fingers into my resin and panted the resin over the seam. Then I laid a strip of cloth on the seam (Figure F) and used my fingers to smooth out the cloth (Figure G). I'm not going to give more specific suggestions, because I'm not sure I stuck to any one method. I just found a way to smooth out the cloth.
- After I had done some smoothing of a strip of cloth, I made sure that it was thoroughly "wet" with resin. You can tell when it is. Again, I dipped two gloved fingers in my 2-liter bottle resin bucket and painted it on.
- It's okay to leave some cloth hanging beyond the ends of the hulls. It can be trimmed off later.
Part 4 -- Sanding
- I put on my mask so that I wouldn't be breathing fiberglass dust. I don't have a picture of the power sanding, but you can see me sanding lightly (without a mask) in Figure H.
- My goal was mostly to remove sharp material. At this point, a lot of people would be thinking about preparing for a second coat, but for my purposes, this was enough. I sanded off bumps and frayed edges of cloth.
Part 5 -- Leave it outside.
Leave as much of your stuff outside as you can, for as long as you can. You may have stopped noticing the fiberglass smell at this point, but it's there. Let your stuff air out for a while if you can.
Step 7: Make the Brackets and Add the Struts
There are a lot of ways to do this. I will share my method here, but these measurements only apply if you use the same hull design that I provided.
- Cut two 50.5" lengths of 1/2" EMT (galvanized steel tubing).
- Cut the brackets -- 8 rectangles of 3/4" plywood, each with dimensions 4" x 3".
- Draw a line and an X mark, as shown in Figures A and B.
- Measure 21" from each tip of each hull and make a sharpie mark at the top of the gunwale.
- Use 1" screws to attach the wood on the inside of the gunwale, with your drawn line even with the top of the gunwale and the X 21" from the tip of the hull.
- Use a 3/4" spade bit to drill through the X mark in a direction pointing toward the X mark on the opposite plywood rectangle. Do this for all 8 rectangles. [Figure C]
- Remove two brackets from one side of each hull. These should be the ones that face the seat. Use a saw to cut openings in the rectangles as shown in Figure D.
- Replace those brackets on the boat's hull.
- Use leftover plywood from making the hull to cut out 4 rectangles 1 1/2" x 3". Screw these onto the non-slotted rectangles as shown in Figure E.
- Get 8 large nails to use as strut retainer pins (to hold the struts in the brackets). Drill holes, slightly larger in diameter than the nails, through the slotted brackets and the metal tubing that runs through them, as shown in Figure F.
- Secure the metal tubing by inserting the retainer pin nails through the slotted brackets and tubing.
- Sand the brackets to remove sharp edges and cut off any nails that passed through the hull. [Figs G and H]
Step 8: Make a Seat -- or Find One
When I was doing my initial testing, I used a plastic chair seat that I think came from IKEA over 20 years ago. But I wanted something that was foldable, so I finally made my own seat. If you've made the boat, a chair is pretty simple. I think the figures speak for themselves. But here are a few notes.
- Figure A shows the dimensions of my seat, but here is no reason you have to use the same dimensions that I did.
- To strengthen the seat, I added a piece of 2"x4", ripped lengthwise (so about 1.5"x1.75") to the front and back edge.
- Instead of being fancy and doing some sewing, I passed the webbing through drilled holes and tied knots in at the end of the webbing to keep it from pulling through the seat.
- **My most important tip here is that (as you can see in Figure B), at the front of the seat, the webbing passes out through the bottom of the seat but at the back it passes out through the top. This gives the front of the seat a nice upward tilt.
- The seat has two kinds of straps. One type is typically found on camp chairs (like these Crazy Creek chairs), and the purpose is to adjust the angle of the seat back. The other straps support the seat between the two hulls. Figure C shows how one of these seat straps hooks onto the strap post. In the bracket. The strap post is just a sturdy deck screw.
- If you look closely at the seat supporting strap, in Figure C, you can see that it has simple holes that I melted with a hot nail, and it has grommeted holes. The simple holes tore, so I added gromets. I recommend using grommets. You can add a series of them in each strap to adjust the height of the seat -- higher for rowing, lower for using kick fins. If you don't know how to install a grommet, there are plenty of videos out there.
Step 9: Prepare Some Shoulder Straps
If you're planning to carry this boat for a long distance, it's nice to have some high-quality shoulder straps. I used ropes at first, and they work, but I wouldn't want to go far with rope straps. My closest fishing spot is about 1.5 miles away, so I wanted to have comfortable straps.
Figure A shows my simple straps. I took straps from an old backpack, and then I tied them together with paracord. I also added some more webbing to the strap that goes behind me (top in the figure). The two front straps each have a grommet for hooking the strap to the strap post.
The red locations show locations for strap adjusters. The yellow shows a snap hook. The blue circle shows how I loop the straps under the struts and then slip the grommet over the strap post (a.k.a. deck screw).
Step 10: Make Oars and Oar Locks
The oars have PVC pipe handles and plywood blades. The blades are encapsulated with fiberglass resin, and the handles are filled with expanding foam. I happened to have some extra 1" PVC sitting around, but if I ever make more oars I will probably use wood. The PVC oars work fine, but PVC isn't easily recyclable, especially if you fill them with foam like I did. I added the foam because I thought it would add some integrity and rigidity to the oars. It did that, but it also kept water out of them, so they don't sink and fill with water when they're lying idle in the water. That's probably the biggest advantage of adding the foam. Here were my steps...
- Trace a blade design on plywood left over from building the boat hulls. Make sure the grain is running parallel to the length of the blade.
- Decide how far the PVC will overlap the blade.
- Using a bandsaw, carefully cut a slot (Figure A) in the PVC. This notch should be wide enough to accept the plywood blade and long enough to allow for the overlap that you want. I thought about making a jig to get a perfectly straight cut, but I dismissed it as an unnecessary waste of time.
- Test fit the blade in the PVC and adjust if necessary.
- Use a permanent marker to mark the PVC slots where you plan to add cable ties. (Figure B shows my spacings)
- Cut V-shaped notches in the marks' locations. I used a flush cutter for this (Figure C). Flush cutters are great for cutting PVC (and lots of other things) safely and precisely.
- Insert the blade and use a pencil to mark the locations of the notches. (Figure D)
- Drill 1/4" holes in the marks' locations.
- Encapsulate the blade with fiberglass resin. (Figure F)
- When the fiberglass has set, ream out any holes that got filled-in with fiberglass. Try not to ream all the way to the wood; you don't want water getting to the wood.
- Insert the blade, fasten it on with cable ties, and clip the cable ties.
- Find a drill bit that's just big enough to fit the tube from the expanding foam can. Then drill a series of holes in the PVC pipe.
- Fill the pipe with expanding foam (Figure G). I started from the center of the pipe and inserted foam a little at a time, working toward the ends. I didn't want to start from the ends and then have air gaps trapped in the pipe, but drilling so many holes probably would have prevented that anyway.
- Wait for the foam to cure and then break it off. (Figures H and I)
- Make some oar locks that look like mine, but not so ugly. If you can do this, you probably don't need directions. The important parts of my oar locks are 1/8" steel welded to 5/16" steel rod. [It's a long story, but the reason mine look so terrible is that I had the idea that the oar locks themselves needed to be able to rotate on two axes. So my 5/16" steel rod was welded to a nut around some 1/2" threaded rod. After I realized my mistake, I just welded the nut in place. It's ugly, but it works.] Figure J shows my abomination of an oar lock.
- Drill holes for your oar locks. Don't give them any slop. You want a tight fit. With the length of your levers, friction won't be a problem.
- I attached my oarlocks to my oars with 1/4" hex bolts and lock washers.
Step 11: Rod and Cup Holders
The rod holders and cup holders were easy. However, I did make one miscalculation with the rod holder. I need to reinstall it with more forward tilt. At its current angle, my it holds a rod so vertically that the lure hangs too close to the rod. With a little bit of movement, the lure is liable to swing over and latch on to the rod. Very annoying, but easy to fix.
As you can see, I made the rod holders and cup holder out of PVC segments. To attach the PVC to the struts, I drilled 1/4" holes in the struts and used 1/4" carriage bolt with a washer and a regular nut. The carriage bolt head digs into the PVC and doesn't rotate.
Step 12: Bike Trailer
I just added this. We're going camping near a "rail trail" that passes several lakes and ponds, so I put a trailer together using some wheels and a connector from our old Chariot jogging stroller. I think I can fit both dogs and some gear in the boat. The rest I can carry in my panniers. Of course, the dogs will probably want to get out and run some, too. I'll try to get some pictures from the trip.
First Prize in the