Building A Foam/Ply Hydroplane

By Paul Williams.
Article posted 26th October 2007.

Outrigger hydroplanes are the purest form of racing boat. They have little or no pretension to scale, but instead are a totally functional object, that function being ultimate speed.

An outrigger, when reduced to its essentials, is simply three boxes held together with a couple of sticks. However, they seem to be a boat that few boat modellers are willing to attempt. Whether this is because they don't appeal from an aesthetic point of view, or because they are perceived to be too difficult to build, I can't say.

This brief article is not about how to design an outrigger, or how to build a specific boat. It's an overview of the techniques and materials I use to build my outriggers. Hopefully it will inspire you to give it a try!

The Kit

I start off with a minimal kit of parts cut from thin ply. I mark out one of the tub sides on a sheet of ply, then cut this out. Using this as a template, I double-side glue it to another piece of ply then cut out the second tub side. Repeat for the sponson templates.

A kit of parts to make a 12 cell, 700BB-powered outrigger.
I've cut lightening holes in the balsa doublers for the tub sides.

Lining Up

In order for the sponsons and the tub to line up correctly when assembled, the holes for the carbon sponson booms must be drilled accurately. This is a task for a pillar drill. If you want to do this by hand, then good luck!

Sponson templates and tub sides double-side taped together for drilling.

With the tub sides and sponson profiles double-side taped together along their respective datum lines, drill the holes using a sharpened piece of brass tube mounted in a pillar drill.

Drilling the sponson templates and tub sides to accept the sponson booms.

Making the tub

With the holes drilled, carefully peel apart the tub sides and sponson profiles. Sand the insides of the tub sides with medium-grade abrasive paper to remove both the plywood release agent and any glue residue from the double-side tape. Coat each balsa doubler in turn with medium-thick cyano superglue and glue to its respective tub side, wiping away any excess. When the cyano has cured, carefully sand flush and square. Return the tub sides to the pillar drill and punch out the balsa over the holes in the ply.

Using your engineer's square, carefully transfer and mark the location of each of the bulkheads on the balsa lining. Take each bulkhead in turn and cyano superglue to the inside of one of the tub sides, using the engineer's square to ensure each is glued perpendicular to the tub side. Add the 1/16th" ply transom in the same way. Repeat the process on the other tub side:

Glueing the bulkheads using an engineer's square, cyano and kicker.

All bulkheads (except for motor mount) now in.

Both sides of the tub joined together.

Take a length of 1/4" balsa triangle section and cut a selection of short lengths suitable for gluing into the corners of each bulkhead as a fillet piece. Glue these with thin or medium cyano and trim with the razor saw.

Balsa fillets for each bulkhead.

With the tub sides and bulkheads glued together, cut a block of pink foam to fit into the nose of the tub. It needs to be a good fit, not too tight, such that it forces the tub sides apart, nor too loose so the tub sides are drawn together, but a snug, sliding fit that does not distort the tub. Glue the block in place with either five minute epoxy, or foam-safe cyano. When the glue has cured, fire up your small hwc bow and trim the foam nose-piece flush.

Pink foam nose section, glued with 5 minute epoxy.

Pink foam nose section trimmed with hot wire, needs sanding.

Take your sanding block and sand the top and bottom of the tub to flush off the bulkhead edges, the balsa fillets and the foam undercut on the nose piece. Be careful not to snag the edges of the bulkheads when sanding.

NOTE: When sanding any part of the boat, be extremely careful to keep all edges as sharp as possible, unless noted otherwise.

Deck with hatches marked out and scored, and tub floor.

Cut a piece of 32nd/0.6mm ply to size for the tub floor, leaving it oversize by a few mm on the long edges and transom, and oversize by 5-6mm where it curves upwards over the foam nose-piece. It is vital that you place the tub on a truly flat surface when gluing the floor, as it is very easy to introduce a warp at this point. With the tub positioned over the floor, run thin cyano into the join followed with a quick squirt of kicker. Glue up to the the most forward bulkhead.

Gluing the floor with thin cyano.

Ready some strips of masking tape then mix up some epoxy (don't use foam-safe cyano here). Peel back the tub floor from the foam nose-piece and apply a coat of epoxy with a spreader made from a scrap of ply or balsa. Hold the tub floor to the foam with strips of masking tape. When fully cured, use your razor plan to trim the excess away, finishing with the sanding block. The overlap on the nose should be trimmed with the large engineer's square, leaving a small overlap that can be feathered by sanding.

Cut strips of 1/4" balsa triangle section and glue into the bottom of the tub where the sides meet the floor. I like to use the mitre block to make the corners mitre together. This is not necessary for strength, I do it purely for the sake of neatness. Repeat this process where the deck will meet the sides, leaving these fillets slightly proud of the tub and sanding back flush when the glue has dried.

Adding balsa fillets to strengthen where the floor and sides meet.

Cut another piece of ply, oversized per the tub floor, for the deck. Repeat the gluing process of cyano up to the bulkhead behind the foam, finishing with epoxy. Trim and sand the excess as before. Cyano glue a piece of scrap 32nd/06mm ply over the transom, trim and sand flush with the sides.

Some designs of outrigger I've seen don't have a deck as such. All my designs always have decks with hatch openings as adding a deck vastly increases the torsional rigidity of the boat and gives extra area for taping the hatches which helps keep the inside of the boat dry.

The positions for the hatch openings can be determined in advance and marked then scored on the deck before the deck is glued to the boat, or you can glue the deck on and then identify by feel through the ply deck where the bulkheads are and mark out hatches to suit.

Decide the extent of your hatch openings and mark using square and rule. As a guide, leave 10mm either side of a bulkhead and 15mm off the tub sides to give plenty of tape-down area for the hatches. Use a hole stencil to mark radii in the corners of each hatch opening, then cut out using your craft knife. Roughly cut across each corner, leaving enough material behind to sand a neat radius. To do this, take a piece of abrasive paper and wrap around a piece of 8-10mm dowel. Finish by sanding the edges of each opening to remove splinters.

The almost finished tub, with the deck added and the hatches cut out.

Return the almost finished tub to the pillar drill. Using your brass tube bit, core through the foam nose-piece. Set the tub aside for the moment and turn to...

The Front Sponsons

Cut two blanks from a sheet of pink foam. Sand the wire-cut faces to remove the "stringers", then mark a datum line parallel from one face on each blank.

Coat one face of each sponson profile in turn and glue to its respective blank. When cured, core out the sponson boom holes on the pillar drill. Cut two lengths of brass tube long enough to thread through both blanks, thus holding them together for the next operation, which is to hwc around the sponson profile. Don't allow the hot wire to run off at the ends of each run; instead, as the wire nears the end of a cut, pull it up and away sharply into the waste. The ridges left can be sanded flush afterward.

Sponson templates and foam blocks.

Carefully sand the two blanks together to remove the foam undercut, then separate. Glue, using foam-safe cyano, the bottom sheet, then the top sheet, feathering the nose of each as necessary. Cut a temporary cutter guide from 1/16th ply to give the desired anti-trip angles on each sponson. When you have hwc the anti-trips, sand and sheet over, then cut the chamfer on the outside edge of sponson. Sand and sheet over the bevel. Finally, sand the transom and cyano glue a ply capping piece in place, sanding the excess flush with the sides.

Templates glued to blocks, keyed with brass tubes.

Initial profile hwc to shape, trimmed to accept balsa leading edge.

Balsa leading edges sanded flush.

Ply sheeting to sponson tops.

Ply sheeting to planing surfaces.

Anti-trip angles cut and sanded.

Completely sheeted, just requires brass tubes to be epoxied in.

Rear sponsons; same technique as the fronts, just smaller.

Tub, front and rear sponsons.

The finished hull, with carbon booms, brass tube mounts and bodyclips retaining sponsons.
368 grams total weight at this point, target weight: 1750 grams ready-to-race.

Front on.

Rear sponsons cyano glued to tub.

The last job, a "belt and braces" measure if you like, is to dribble a line of thin cyano along each seam. When this is dry, sand flush.

Finishing

To take the boat to a point where it can be run, there is a list of additional tasks to be completed. The first task is to install the motor mount and drive system. Then, install the rudder and steering servo. See the article on radio control systems for some tips. If you're going to race the boat you will need to install an isolation loop.

When you have finished all the tasks that require gluing to the hull, you can go ahead and seal the boat using your preferred sealing product. I always used to use epoxy laminating resin thinned with isopropyl alchohol. While this does a good job of sealing the wood, it needs a fair bit of work when cured to get an acceptable surface for painting.

For this boat, I decided to try "G4 Damp Seal" which I purchased (with a some G4 thinner) from CFS Fibreglass Supplies.

Update

Here are some pictures of the finished boat, complete with wire drive, motor etc.

© Copyright Paul Williams and www.fastelectrics.net, 2010.

This article may not be reproduced wholly or in part without the written permission of the author and www.fastelectrics.net. If you would like to use this article or the accompanying pictures/diagrams please email articles@fastelectrics.net.

Last modified: 08th July 2010 @ 09:06