Building Floats


Over the years I always have enjoyed Float Flying.  It always amased me how so many guys struggle with this. Worse yet, when you offer advise, you get the "I have been doing this for 20 years, you can't tell me anything new".

Well the fact is, there is a real science to setting up your plane for floats, and a real science to the float designs.


How to determine the size floats required?
Measure from the back side of the prop to the rudder hinge line.
70% of this length for normal (Lake) waters, 75% for rougher waters

What position should the floats be mounted?
A good float design will have a step. On a 40 size model, the step should be 1/2" to 3/4" in front of the CG. This normally puts the bow of the floats about 2" in front of the prop.

how far apart should the floats be spread (stance) ?
normally, 20-25% of the wing span, narrower will be unstable in rougher waters.

What incidence should be used?
for flat bottom wings, 0 degree
Semi Symetrical 1/2 degree
Fully Symetrical 1 degree
This means that if the top of the floats are level, the wing will have a 0-1 degree positive incidence


Water Handling?
on the bigger planes I like to use water rudders, on 1 or both floats. Linkage can be used to the planes rudder, or I like a seperate servo mounted in the floats (needs water proofing) On a small foamie, I would be inclined to drop a long wire to the rudder from the planes rudder, so that the water rudder is totally submerged when standing still. Old credit cards work very well for a rudder. In my case, I stole my wifes credit card and cut the rudder shape with a pair of scissors hehehe.

Earnst makes very nice water rudders for 40 and 60 size planes. A little expensive but they are a good design.


What is a good Float Design?

Well there are many opinions. My observations is that a flat bottomed float is difficult to control. They tend to skid in a turn. On a landing you can find the plane going sideways until the edge of the float digs in and then your upside down with a water soaked wing.

SIG makes a build up float around 42" that is a good design. It is a V hull from tip to stern with a good sized step. These floats were a favorite for the Great Planes Super Cubs. The only problem was that the floats tended to develop suction and they were difficult to get them up on the step. But when they were setup as described earlier,  they did a pretty good job.

Guys were adding chines to the insides to keep the water spray away from the props. Water spray will bog down a 2 stroke. 4 Strokes seemed to have more torque and did better with the spray.

I started adding triangle stock to the edges of the floats on the front V hull, effectively making it a Tri Hull. WOW, what an improvement. It had a hydriolic effect and makes the float jump up on top of the water instead of plowing thru the water. No more suction. No more Spray. Instantly up on the step and developing takeoff speed, meantime the stubborn oldtimers were running in circles trying to make a wave to jump off of to break the suction.


I don't remember the manufacturer, but I did have a set of 56" floats that were a foam core.  2 pieces of plywood 2"x2"x1/4" are inlaid on the top for the attachment points, a hole is drilled in the center of each deep into the foam, then wooden dowels are epoxied thru the plywood into the foam. The dowels almost come out the bottom of the foam. Next the sides and top are lamenated with 1/16" balsa and the bottoms are sheeted with 1/64 plywood. This is the same design as the SIG floats described earlier, a stepped V Hull. Again I added the Triangle stock to make the Tri Hull.


A word of caution, Electricity and water don't mix so well. I had my electric Kadet on floats and the controller went up in smoke because it got wet. Someone suggested bagging the controller to keep it dry, but this is high power, 100 Amp controller and it needs cooling, so bagginhg it was not an option. I highly recommend Nitro 4 stroke or Gassers. Smaller electrics would be a lot less expensive if it fried.


The float design I currently use is similar to the SIG built up floats. I say similar because I no longer have the plans for the SIG floats, so I developed a design using AutoCad. The nice part about using Autocad is I can scale the plans to whatever size float I need. Of course the little notches for the keel and stringer need to be adjusted each time, but slowly I am developing a nice library of different sizes.


I have just finished a foam plane made from a 3 view drawing, it's the Cessna 140 with 48" wingspan. I decided to make a pair of floats for it.


Step 1 - measure the plane from the rear of the prop to the rudder line, 75% is approx 24 inches.


Step 2 - go to AutoCad and scale a set of floats for 24", adjust the keel and stringer notches for .125" x .250".


Step 3 - print the plans at 1:1   I was able to get all of the bulkheads on an 8.5x11 page and I printed 2 pages. The top of the float drawing was scaled to fit the page, all of the pertenant locations are dimensioned, so I just drew the design on the balsa wood. I used .125" thk balsa for the bulkheads and top plate, except for bulkhead #9, which is .250" thk balsa with a .125" thk plywood bulkhead lamenated on the inside surface.I used a spray adhesive and glued the paper templates to the balsa for the bulkheads.



Step 4 - Cut out the Bulkheads and the top plates.


Step 5 - attach a second landing gear at the rear of the cabin. The gear is facing backwards.



Step 6 - Place the model over the top pieces. Use a plumb bob from the CG location to the top of the floats. There is a location where bulkheads #5 and #6 are lamenated together, the location is shown with 3 parallel lines. These lines show the location where all the bulkheads will be glued. Set the step ( the center line of the 3 parallel lines) about 1/2 inch in front of the CG. Mark the place on the top sheets where the landing gear will attach. Here we need to lamenate small pieces of .125 plywood for strength. You will notice that Bulkhead #4 is in the way, so we will put the plywood and notch #4 for clearance. The rear mount is clear of the bulheads.



Step 8 - Cut 4 plywood pieces .875" x 2" and epoxy them onto the top plates at the specified locations. Note, the top pieces are slightly oversized for sanding, so it is important that these plywood pieces are snug up to the stringer and do not extend past the bulkhead edge. Now CA Bulkhead #4 in place.

Now is a good time to install the attachment hardware. Use epoxy or CA under the "T" nuts.

Note: the plywood spreader, this is to guarantee that both side are the same. The plywood is only for setup.



Step 7 - using thin CA, tack glue the stringer in a couple of places and then glue each of the bulkheads in place (Except Bulkhead #4). Next glue the stringer permanently. Next cut strips for the keels, .125" x .250 length as required. Glue the rear keel in place. Place the front keel in the slots at bulkheads #5 and #4, thn mark where #4 is. Remove and soak the keel in water from #4 forward. Now starting at #5 glue the keel at each bulkhead carefully bending the keel as you go. It will find it's own curve.



Step 9 - Now is a good time to trim the stringer and keels. The keels should be sanded to match the shap of the bulkheads, making a sharp V. Do not trim the top yet.


Step 10 - add sides, .062: (1.5mm) balsa. The side should sit on top of the top plate (everything upside down now) and firmly up against the bulkheads. The sides should have 1 nice straight edge, don't worry about trying to shape the other edge now. Once the side is glued in place, you will have a gap between bulkheads #1 & #2. Cut a small piece of balsa to fill these gaps or if the gap is very small, use epoxy to fill the gap. Wetting the sides from Bulhead #2 to #1 helps with this complex curve.



Step 11 - add bottoms, .062: (1.5mm) balsa. These need to be traced from the sides, there is a slight curve, make sure the pieces extend past the keel. I used some 5 minute epoxy on the bulkhead and keel surfaces and thin CA on the side to bottom seam.


Here we need to start trimming away excess material, carefully remove excess with your modeling knife and then block sand to a nice flush and sharp edges. DO NOT ROUND THE CORNERS.


The 1st bottom piece gets trimmed and countoured to meet the existing keel and bulkhead line.

Finish the bottom sheets, trim and block sand all surface.


Next we will add .250" thk (6mm) balsa pieces to the bow. We will add layers until the final length of the float slightly exceeds 24". Probably 3 pieces, but we want to do them 1 at a time, it will make it a lot easier to shape the bow. Each layer is epoxied in place and then shaped. Remember, no rounded edges, keep the contour going by block sanding each side.


After lamenating and shaping 3 pieces, measure 24" form the stern and mark the bow. I like a round bow, so I used a 1.8125 dia plastic cup (that's what I had) and drew an arc. Then shaped the bow to my marks.


At this point, I have built a conventional V hull float, very similar to the 42" SIG float. Next I will add the triangle stock to the forward hull, block sand the sides, and feather it at the bow. Making the forward hull a "Tri Hull" greatly improves the performance of the float.


The triangle stock is difficult to install. What I have done here is to epoxy the flat area just to where the curvature starts. Next I will steam the wood and slowly pull it to the shape needed.



Even with the use of steam, I still had breaks. So CA them back together and after they are shaped it makes no difference.

Recently, I experimented and found this to be a better way. Using a maximum of 5mm thick balsa plank, Cut the 1st pieces (4) to the desired width, and then carve a 45 degree angle on 1 side. This is equivalent to the width of triangle stock. Glue this piece to the float, it will bend easily. Then block sand to the countour of the float. Then keep laminating and sanding each piece to the desired height. The end result is the same as triangle stock, without the breakage.
The floats need to be covered so they will be water tight. Larger floats I would seriously consider fiber glassing, but on these small floats I am concerned about weight. I used silver Micro Lite covering.

The floats are attached to the plane and 2 spreader bars, fore and aft are installed. The spreader bars will keep the floats from spreading apart and also help to keep the floats parallel. The only better arrangement would be an "X" shape, which I use on larger floats using Plastic coated wire (think fishing tackle). Here I used 1/4" KS Streamline Tubing with a length of balsa epoxied inside.


The floats are adjusted so that the main wing has 1 degree incidince relative to the top surface of the floats. Adjustments are be made by shimming the landing gear.


I installed an Earnst .40 to .60 size water rudder and an old micro servo which is Y-Corded to the rudder channel. This rudder is also retractable if you add another servo to retract it. The rudder is spring loaded (rubber band) , so if it hits something submerged in the water, it will kick up and then return to the normal position once clear of the obstacle.


Well next is to test fly it. I'll try to take many pictures of the water flight.


Thanks for reading.