Martin PBM Mariner: A twin-engine WW II-era long-range patrol bomber

Model Airplane News, Nov 2003 by Sparks, Keith

When I started this project, I wanted to design something different and dependable that could fly off water. I had noticed that flying boats handle better than other floatplanes; rough "seas" and crosswinds wreak havoc on the little pontoon planes. Enter the Martin PBM Mariner. The full-size flying boat hunted submarines in the North Atlantic as well as in the Pacific, so I felt sure that a model of it could certainly handle the lakes in Texas!

This design has it ail when it comes to "different": graceful gull wings, an unusual tail group and a deep-vee hull design. As for dependability, I've flown this model more than 50 times and have had to use the rescue boat only once. Folks have asked me, "Whose kit is it?" and "Where did you get the plan?" When I told them it was my own, some accused me of hoarding a fantastic design. I wanted to prove them wrong, so here it is ... your next flying boat.

FUSELAGE

You will notice that I use the word "sand" a lot. Don't worry; 80 percent of it is done on blue foam, so it's fast and easy. Fuselage section half-patterns are on the plans; to make a whole section pattern, you'll need to fold card stock in half, and then glue the half pattern to the card with the fold as the centerline. Cut the pattern from the folded card, and both sides will be the same shape.

To begin fuselage construction, trace the patterns with a felt-tip pen on 2-inch-thick blue foam; transfer both centerlines so that you can properly align each section. Because the tail rises above the side-view centerline, on those pieces, you'll need to mark three lines to help with alignment; sections 28, 29 and 30 have dual centerlines. I used my scroll saw to cut the sections and then transferred lines to the sides of each section with a square block. Note: cut slightly outside the lines and make sure that the saw blade is square. Work on a sheet of foam rubber to protect the blue foam from hard objects on the work surface.

Bonding. Use four drops of epoxy to hold the sections together; keep the drops at least 2 inches from the outer edges. Bonding four sections at a time, start with section 16 and work your way to no. 1. Use weights for clamping pressure; as the epoxy cures, check the alignment marks frequently. Bond sections 17 through 27, and then sections 28 through 38. Finally, bond the three built-up section groups together, and secure them with masking tape (stretch the tape a little as you apply it so it will pull the pieces tightly together). I made a stand out of scrap foam to rest the fuselage on.

Shaping. To hold the sections together while you sand, put masking tape on the sides that you aren't sanding. If an epoxy bond breaks, just add a few more drops of epoxy and use the pen marks or sanding scratches for realignment.

Sand the flat spots using 60-grit sandpaper and work your way to the nose. Sand until the pen marks are nearly gone. Poke the pen into the foam about every fourth section so that you won't completely sand away the centerline references. When the original lines are gone, mark new ones before you continue to a new area. I made a sanding block of scrap foam to sand the concave surfaces of sections 17 through 29. The sandpaper must overhang the block to prevent its corners from gouging the foam. Cutting a radius at the corners of the paper helps, too.

While sanding the corners off the sections, I found it easier to use the sanding block as if it were a sled going down a flight of stairs. The exception was along the top of sections 22 to 29; I had to sand across the fuselage until the "steps" were gone.

I also used a cotton glove and a sheet of sandpaper folded in half with its sides glued together. I needed the glove for traction; otherwise, the paper would grip the foam and my hand would slide over the paper. At this point, the fuselage needed the canopy and ball turrets fitted. Test-fitting as I went along was the best way to go. You can omit these parts and paint them on later.

After going over the fuselage once more with 120-grit sandpaper, re-mark the reference lines and number the sections. Next, separate the sections at the following joints: 3-4, 6-7, 11-17, 28-29, 34-35 and 37-38. To separate the stubborn bonds, pull a fine wire through the gap between the sections to break the bonds. I used a hot wire to cut out the cores of the sections after I had made wire guides for the sections listed above. An alternative method would be to cut out the cores with a scroll saw. Try to keep the wall at least 3/4 inch thick and be sure to maintain this distance where the sections taper.

Bond the sections together again with a light coat of epoxy on the inner half of the sections (you don't want the glue to ooze onto the outside of the fuselage). Cut the lite-ply bulkheads 1/8 inch smaller than the fuselage sections on the outside edge. Fill the 1/8-inch void around the bulkheads with the softest balsa you can find, then sand these areas flush. I used the "sanding glove" with 180-grit sandpaper to remove any defects.