by
Cresswell Walker
This
article first appeared in Kit Planes in May, 1997.
The
Urge to Float
The
world is grey down to my knees - lake meeting sky eamlessly,
shadows of mountains behind, promising brighter days,
the air still. In my ears, the plop of rain on the tarp
and quack of a blue heron disturbed by my four year
old at the water's edge. Dabbling ducks on shore and
otters slipping. Logs captured in booms, as wet atop
as below, resting, waiting patiently like me for the
rain to end and the water adventure to continue. This
is the Shuswap. This is early September and we're here
on a family vacation with my Mini-Max, out of the trailer,
on the water, and ready to go.
In a region where water abounds, the temptation to move
from wheels to water was irresistable. A summer exploring
the land based haunts of Coastal B.C. ended abruptly
with an engine seizure gliding distance from YHE, Hope.
So after only one season on wheels and another winter
in the shop, my Mini Max project, "MukTuk", emerged
fitted with a new engine (and prop, but that's another
story) and a shiny new pair of very yellow webbed feet.
I had long been interested in building and flying floats.
With a lifetime of sailing, the marriage of air and
water in a single toy had to be tried. I also wanted
to build in wood. And why not? Wood 'floats'(sorry)
does it not? It's a lot nicer to work with than the
alternatives, and after all, people have been building
boats of wood for a very long time. And wood worked
so well in an airplane, I had to give it a try for floats.
How
do I Do It?
But of what design? All of the plans for wooden floats
that I managed to scare up seemed either too boxy or
unnecessarily complex to construct, and all were never
quite the correct displacement. So I set to work to
prepare my own plans.
From a cursory survey of existing float designs, I soon
decided that I wanted floats with fairly conventional
lines, yet with simple curves to facilitate construction
with marine plywood. I decided on the pentagonal section
similar to the Wipline floats. This provides for a 'v'
hull, tapered topsides and a flat top deck. I also decided
on longer, more slender floats. These promised better
"sea keeping" qualities, that is, less pitch critical,
more stable in a seaway and more easliy driven in displacement
mode. They would also be the most aerodynamic.
Scaling Up and Down
From photographs and various plans (including model
float plans), I scaled, then loaded the lines into AutoCad,
a PC based computer assisted drafting program. Next,
I used the program to "loft" and "fair" the lines, then
to draw bulkhead sections for each station. Finally,
I used the program to scale the float to a displacement
equal to 0.9 times the gross take-off weight, the generally
recognized minimum displacement required for safe operation.
In digital format, the float plans may be simply re-scaled
to any displacement. Then a push of a button, and the
plans were plotted, including full size bulkhead templates.
I toyed for a while with amphibious designs, but decided
the added complexity departed too far from the axiom,'do-one-thing-at-a-time-well'.
Building
Them
Construction from this design is simple boat building.
The bulkheads define float shape, watertight compartments
and provide compression strength. The stringers fair
the lines together and provide a gluing surface for
the plywood. The plywood skin gives longitudinal and
panel stiffness and, of course keep the water out. Simple,
conventional and affordable. Like a boat!
A table saw with a planing blade will rip out the stringers,
including the angled cross-section for the corner stringers.
For the float prototypes, I used yellow cedar because
it was there, but white pine, even clear fir would work,
and of course, sitka spruce if you can find and afford
it. Any reasonable 1/8" ply will fashion into frames
and skins, perhaps baltic birch or acume mahogany. I
used 4mm acume in the prototype and find them over-built.
The 3mm ply used in kayak construction would be ideal
for floats under 14 feet. Epoxy is recommended for all
'gluing'. With fillers, epoxy bonds any wood in a permanent
waterproof joint and is easily finished into shape.
The plywood skins are finished with an epoxy wood sealer
and painted with marine paint. For scuff resistance,
I placed a single strip of glass cloth in epoxy along
the centre bottom seam. The total weight for the somewhat
over-built prototypes with mounting hardware is 35kg's
(70lbs). I estimate this could be reduced by 10 lbs.
by using 3mm ply.
Alternatives
A word for all you aircraft building fanatics out there.
You could build these floats with some combination of
aircraft ply and fibreglass with polyester or epoxy
resins on the skins. You could build with the Edo rounded
top and concave bottoms. You could build molds for fibreglass
or epoxy/kevlar, or covered foam or build of aluminum.
But it would cost a fortune in time and materials. And
it would not make enough weight and performance difference
to justify the trouble, in my opinion. I think too many
potential float builders and float flyers are deterred
by the cost and over complexity and over design of floats.
Keep it simple and affordable and get it done. The prototypes
were built for less than $500 and they are in service
after a single winter!
Float Testing
At last the rain stops and some colour returns to the
world. Down to the foreshore, and I point "MukTuk" lakeward.
Clamber aboard, a couple of pulls on the starter and
the 503 Rotax, reasonably, starts. The air and the lake
are quite still now except for my intrusion, so I plan
a takeoff in the direction of my destination. Cockpit
and temperatures check, so full back stick, 4000 rpm
for one second to bring the nose up, then full throttle
for the run. Two, three, four seconds ease off back
pressure on the stick and she rises forward on the step.....five,
six, seven, eight, nine seconds and then the thrill
of acceleration as she comes unstuck and slides off
into the sky. No land takeoff can compare.
With
the floats, MukTuk's climb rate is reduced to about
two thirds of that of wheels, or to about 900 feet per
minute at sea level. Sink rates at idle is close to
800 feet per minute. 5500 rpm yields a cruise speed
of 60 mph indicated and 6300 rpm gives a maximum speed
of 75mph. Cruise consumes a good three and one quarter
gallons per hour, up from about two and one half gph
on wheels.
After liftoff from our family camping place at Herald
Provincial Park, I cruise southward at 500 feet over
Shuswap Lake past Canoe, Salmon Arm, then climb to 3500
feet over Notch Mountain and west overland to Blind
Bay and Chase. Instead of landing in the bay at Chase
as originally planned, I continue west along the Thompson
River toward Kamloops. On doing a fuel check at Montrose,
it is apparent to continue to Kamloops I'd have to fuel
up to return. I realize that I had forgotten to bring
along a liter of "Super Two Stroke". With no likely
source of this, or a satisfactory substitute two cycle
oil enroute, I turn and chug homeward.
At Salmon Arm I land for a break and sashay up to the
nice new 'sea plane' dock. On almost every beach and
dock there is someone who is just about to build their
own ultralight or has a story about this or that airplane
experience in their past. In any case, there is almost
always someone to catch a line and gam with and this
stop is no exception. Experienced airman, builder, or
aspiring novice, it is the people along the way that
make a day's pleasure.
After coffee, I lifted off from Salmon Arm but throttled
back at fifty feet to slow my accent to enjoy some of
the extraordinary bird life in this bay. Next, I cruised
the beach, but it is September after all. With floats,
the trick is to be high enough so as to not disturb
the wildlife or to make a nuisance of yourself, (and
to be legal of course) yet low enough to enjoy the sights
as only you can in a float plane.
Now nearly down to my reserve, it's hammer down and
roll on home accross the lake at top speed just for
the fun of it. I splash down just off the rock beach
and glide up to my temporary moorings opposite our campsite
in time for dinner. Easy as pie. Tomorrow I'm northbound
to Sicamous and Mara Lake. Wonder where I'll go the
day after?
If
you have ever had a hankering to fly off the water,
but were deterred by the cost and complexity of building
your own floats, think about MukTuk's new yellow feet!
