Instructions for Scratch Float Builders from Plans

Introduction

The plans and instructions assume a moderate understanding of wood working practices. The written instructions below will provide some direction on how to best cut and assemble the components.

Before you get too far along, a builder using these plans will need to determine where and how to reinforce the floats to carry the float attachment hardware. If you are using MukTuk's Float Attachment Kit, you will find instructions in the kit on where and how to reinforce the float for attachment. If you are designing your own attachment system, general guidance is provided on this topic in these instructions. This includes some suggestions on planning float attachment as far as the angle of float incidence is concerned, and float location with regard to center of gravity.

Reading the Plans

The plans included with the Plans are:

1. Plan of frames or bulkheads - full scale (1"=1")
2. Frame Station Locations attached as Appendix One
3. Un-scaled longitudinal section plan attached as Appendix Two

The first drawing, the plan of the frames or bulkheads, is included as a backup in case you need to recreate some of the materials included in the kit. The scale of this drawing is 'FULL' (1" to 1") to allow you to use the plans to create a full sized pattern and new frames if you require additional frames or to replace one of the frames supplied with the kit. The angles of each of the five corners of the float section are shown to assist in the cutting of the stringers if you need additional stringer material. The cut outs for each frame are also shown full size if you choose to cut out some of the bulkheads to make them lighter.

The frame locations are specified in Appendix One of these instructions, "Frame Station Locations".

The un-scaled longitudinal section plan is included in Appendix Two to give an overview of the completed float.

Strongback

You will likely want to build a "strongback" or "work beam" to assist you in laying up the frames. It is useful for laying out the float frames and stringers and to hold the frame and stringers straight and flat while the panels are being applied. Because the stringers come under tension when bent to the frames, the strong-back will hold the assembly in shape until the panels are attached. It will need to be the length of the floats and approximately 1/4" narrower than the float top deck. The slightly narrower top deck will enable the overlap of the side panels on the top stringers while the float is still on the strongback.

One way to construct the strongback would be to bond/nail a strip of 1/2" plywood to two parallel 2x4s that are just slightly longer than the float.

Materials

Stringers
A sitka spruce or comparable clear softwood is supplied in the kit. As the stringers function primarily as an attachment point for the plywood skin, strength is not critical.

Frames
The frames function primarily to create the float shape in construction and to carry some of the compression loads. The frames are to be constructed of 3 to 4mm (+/-1/8") plywood. Waterproof plywood is supplied with the kit.

Outside panels
The outside panels carry most of the float loads. They provide panel stiffness and resist point loading when mooring or beaching. They are exposed to the environment and of course keep the water out. They are the most important wood material involved.

The 3 mm plywood supplied in the kit will provide ample panel stiffness on floats 12 to14 feet long.

Adhesive
Adhesive is not included in the kit. Epoxy is the recommended material and is readily available at hardware and marine stores. It is strong, waterproof and easily finished. The "West" epoxy system works very well, particularly with the use of fillers for filling and fairing joints. Use a filling compound in the epoxy mix to ensure gap filling to create a watertight bond.

Finishes
These are not supplied with the kit as each builder will have their own preferences here. Epoxy resin and glass cloth strips on the outside of the hull is not necessary for strength but it can be applied perhaps along the keel join to absorb abrasion in beaching and handling on land. For the greatest durability, some builders epoxy saturated glass cloth to bottom surfaces, however, this adds considerable additional weight. You need to decide between resiliency and weight. A clear epoxy finish applied directly to the plywood is likely to be sufficient for most needs.

The wood must be painted or finished in some fashion to protect against water infiltration and rot. Many epoxy paints and even simple marine enamels are available in marine stores for finishing wooden hulls.

Construction

Preparing Frames
All the frames you need are included in the kit. However, if you need to cut additional frames, accuracy in cutting the frames will facilitate assembly and final appearance of the floats. Make them as precisely as your tools allow. A radial arm saw set at the appropriate angle will accurately cut both sides of each frame. For greatest accuracy, cut all frames from a single setting if possible Turn the material over for the second cut on each frame without changing the saw setting.

Note the following important points:

• Frames D and E are identical, except there should be no center cut-out hole in frame E in order to have at least two water-tight compartments in each float.
• Frame E is doubled at the step, by sandwiching frame E and frame E'. Frame E' is shorter than frame E and is attached on the rear or aft face of frame E. Frame E' defines the step. The bottom three stringers behind the step will butt into the back or aft face of frame E. This is partially illustrated on the drawing.
• The cut-out should be omitted from frame E and E'. This will result in two water tight compartments in each float. If more are required, be sure to omit the frame cutouts. Each watertight compartment will require an inspection port on the float top for inspection and bailing.

Cutting Stringers
All the stringer material you should need is included in the kit. However, if you need additional stringer material it is easily ripped to size on a table saw. Use the angle cutting capability of the saw to cut the outside angles on the center and corner stringers. A loose friction fit with the frame notches will make assembly easier, but this fit is not critical.

Be sure stringers are about one inch longer than required to span the required frames. This will facilitate clamping during construction.

Initial Assembly

Be as accurate as you can during assembly. This will result in a strong and good looking float, with smooth, fair lines and no leaking! It will also save you considerable sanding.

• Begin the assembly by measuring and marking each frame location on the strongback in reference to frame A using the frame locations specified in the separate sheet called "Table Frame Station Locations".
• Temporarily fasten the three top deck stringers to the strongback top between frame stations B through I. (The float is built upside-down on the strongback.) Identify the stringers by comparing the cross section profile with the cross sections on the full scale plan of the bulkheads. Do not fasten the stringers forward of B. The stringers must be strongly but temporarily fastened to prevent movement of the finished frame when bending the ends in to meet the front and rear template. Just tack down the outside stringers with small (1" to 1 º") finishing nails that can easily be pried from the strongback after the side and bottom panels are installed. Screws are ideal for the center stringer. The fasteners in the centre top stringer should be screwed in from the inside face of the strongback so that they may be removed after the float has the side and bottom panels in place.
• Check that the three stringers of the top extend slightly beyond each end of the float ( i.e. beyond station A and I.)
• Epoxy frame E' to the back or aft face of frame E.
• Except for frame A, epoxy each frame in place checking with a square to ensure each frame is perpendicular to the strongback and top stringers and square with the float centre line.
• Epoxy the bottom corner stringer and centre stringers to frames D, E, F, G, H, and I. Use wire or clamps as necessary to hold the stringers to frames while epoxy cures. Use fillets (excess epoxy rounded into the joint) as necessary to attach the stringers to the narrow frames.
• While leaving the top stringers firmly anchored to the strongback at frame B, temporarily shim the stringers ‡ inch above the strongback at frame A location. This is how the slight droop of the float bow is created.
• Epoxy frame A to the bottom stringers squaring it to the strong back.
• Epoxy the bottom corner stringers and bottom centre stringer to frames A, B and C. This will require some bending of the stringers and wire or clamps to bind the stringers to the frames while the epoxy cures.
• Epoxy the balance of the stringers in place.
• To provide additional panel gluing area, install back up blocks of short lengths of left over stinger material inside the faces of frames A, E, E', and I These should be fitted between the stringers and stand just proud of the frame edge enough to allow them to be sanded flush.
• You will need to be sure that water can drain to the lowest point in the float, beneath an inspection port. You do not want any compartments that cannot be inspected to hold water unknown to you!. Cut "limber holes" (drain holes) in the frames along the centre stringer so any water entering the floats will drain to a place beneath the inspection ports.
• Cut the stringers flush with the ends at frame A and I.
• Take a deep breath and put on your muscle shirt! Using a sanding block large enough to span at least two stringers, sand all the stringers and backup blocks using 40 to 80 grit paper to prepare them for mating with the panels. Always use a block, and sand by running the block along two parallel stringers so as to not to round the corners of the stringers. This sometimes requires considerable sanding. Sand with the block until you are confident the plywood panel will sit tightly on all stringer surfaces, frames and backup blocks. (The top faces of the top stringers can be sanded later when the frame is removed from the strongback.) Sand until the face of each stringer is flush with the frames, adjacent stringers and the back up blocks. This is also a time to smooth and fair any bulges that show up in the stingers that will determine the lines of the finished float. This step is critical for watertight bonding of the panels to the stringers and for the overall quality of the finished float.

  Attachment Points
  Before attaching panels, add reinforcement at attachment points as necessary. Usually, doubling the stringers and bonding an inside plate of plywood at the attachment points is sufficient.

  Install fasteners and backup blocks for water rudders, cleats and other hardware where such will be located in places inaccessible to inspection ports.

  Outside Plywood Panels
  The kit includes all plywood panels scarfed to full length.

• With the float frame still in place on the strongback, attach both side panels. As with all panels, rough fit the panels oversize, and trim only after expoxied in place. Use staples or finishing nails to hold panels in place while epoxy cures
• Trim and sand panel edges flush (again with the large sanding block always running on two parallel stringers)
• Remove any fasteners holding the top stringers to the strongback that you will not be able to access with the bottom panels in place.
• Epoxy each of the four bottom panels in place. Bevel and match the center edge of each pair of the bottom panels before epoxying in place
• Trim and sand each bottom panel.

FINAL Assembly

• Remove float from strongback
• Trim and sand side panels flush with top
• Apply a wood sealer to all inside surfaces to protect wood from rot. At least two coats of a clear sealant available from a marine supplier will give the best protection with the least weight. Take care not to apply sealer to top stringer glue surfaces.
• Install any attachment hardware you will not be able to access with the top panel in place.
• Epoxy top panel in place and trim and sand to final fit
• Block sand the outside end faces of A and I and epoxy a doubling face panel of left over 3mm plywood on the transom at I.
• Rough shape and expoxy the wood block to the bow at A, and finish sand to shape in place.
• Epoxy a back up plate on the aft step face. This is important for strength and water-proofing in this high stress area.
• Cut holes for inspection ports. An inspection port should be located directly above the lowest point of each watertight compartment and wherever it is necessary to facilitate fastening hardware. The round, screw-in plastic or nylon marine inspection plates are recommended for use in the floats. They should be large enough to allow the builder's elbow and upper arm to enter the float to facilitate installation of attachment points.
• Fill all staple holes and other voids with epoxy filler mix to seal floats completely.
• Finish sand entire float surface.
• If desired, apply one or more layers of 2" cloth strips in epoxy down the center keel. This should be adequate protection for floats in normal service, but additional cloth can be added where desired at some weight penalty.
• Sand and fair epoxy/glass to suit.
• Apply finishing material to floats. Two part marine epoxy paints are ideal for this purpose.
• Install cleats, rudders, float attachments and other hardware using polysulfide or comparable marine caulking compound.

Water Rudder(s)

Water rudders are not required for water takeoff and landings, but are necessary for control when taxiing and docking in any but the lowest wind conditions. If operating in moderate wind and water conditions, a single water rudder may be adequate, otherwise one rudder should be installed on each float. Even if you choose to build initially without water rudders, it is suggested you install back up blocks in the stern of the floats should you subsequently decide to install rudders. MukTuk has a water Rudder Kit and plans available. Alternatively, aluminum water rudders designed for ocean kayaks lend themselves well to float rudder installations.

Aircraft Ventral Fins

All float installations reduce stability of an aircraft in yaw by adding substantially to vertical surface area forward of the vertical axis of rotation. The builder should consult with the aircraft manufacturer or designer to determine ventral fin requirements for their aircraft.

Installation

Two critical factors determine float installation: 1) step location in relation to center of gravity and 2) angle of incidence of float to wing chord. The builder should consult with the aircraft manufacturer or designer in this regard.

In the limited experience of the author, the following information is offered.

Step Location
Direction on float step location in relation to c of g varies in the reference literature. Some sources indicate the float step should be located directly under the c of g, others suggest the float step should be some moderate distance behind the c of g. Two methods for locating the step in relation to the c of g have been found. One is to locate the step behind the aircraft c of g a distance equal to 1/3 times the width of the float at the step (e.g. With an 18" wide float, the step would be located 6" behind the c of g). The second is locate the step 5 degrees behind the c of g (e.g. The step should be located on a line 5 degrees aft of a vertical line extending down from the c of g point on the wing).

The closer the step to the c of g the less stable the float/aircraft, though the nose of the aircraft can be drawn higher giving better prop clearance on tractor aircraft. If the c of g is too far forward, the aircraft tends to plow nose down when power is initially applied and resist planing.

Angle of incidence and stance
The angle of incidence is the angle of attack of the wing in relation to the float's longitudinal axis. In the authors limited reading on the subject, this varies from 3 degrees to approximately 12 degrees. The greater angle of incidence results in greater parasitic drag from the floats but faster takeoffs. The aircraft manufacturer may be able to provide some direction on this subject as well. Floats should be 4 ‡ to 6 feet apart depending on wing span.

General
Other elements of float attachment such as determining attachment points on your aircraft need to be determined by the builder as dictated by the requirements of the aircraft. Again, consult your aircraft manufacturer in this regard.