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By Graham Wheeler

 This build is to replace the older technology boats like the brushed motor LSH hydroplane with a more advanced hull, with the latest running gear and at a very modest

price. This is just a brief summary of the build. For more information about this boat, you are more than welcomed to come to a Club Day and speak to me or a Club Member about these boats.


The plan is taken from a Whiplash 21 - photocopied then it was reduced in size to 25.7inches (655mm) long.
After cutting out the paper parts and gluing them on to 2mm Birch ply they were cut out on a scroll saw and sanded to fit the paper - note that when you scale up or down plans you must make allowances for the thickness of your ply or the slots on the plan will be wrong.
Make up a flat board to build on (I used a piece of 30mm MDF)

The boat is flat from the transom to the sponsons, and the sponsons can overhang the end of the board.
Assemble the frames, making sure that the sponson built-in dihedral is on the left side. I use a spot of CA glue to temporary hold everything in place then later use epoxy glue to finally glue up.
Do the bottom skins first, use .8 or 1mm ply for these, and then the sponsons sides and bottom last. Sand all overhangs off, and then fit the deck also in 1mm ply.

Before adding the deck it pays to put in some flotation. I use the spray in foam but you can cut foam to fit if that’s what you prefer. The spray in foam makes a much firmer, stronger hull in my opinion but you do have to be a little careful with it.


These photos show the layout of the components, the battery needs to go as far forward as possible to get the C of G right.
I would have liked the motor a bit further forward but it makes getting the battery out very difficult. Motor mount is a bent aluminium plate epoxied directly on the floor.
Stuffing tube is 3/16 inch and goes into an Octura Offset Mini Strut System------- strut and rudder combo, no water pickup will be from a piece of brass tube bent into the prop wash. The motor controller is a Hobby King SS100amp, and at NZ$35 is a well priced unit.
The motor I am using is a Feigao 540 7S, 5,196 RPM/Volt, 5mm Shaft and are $70 NZ from Offshore Electrics.

Turn fin bracket ,just a simple aluminium angle bracket 2/ 3mm SS bolts, nuts are epoxyied in after fixing ,so as to be able to remove if needed 


Water pickup Bent from Brass tubing has now been fitted    


Making the cowl is a little tricky. The method I use is basically to carve a piece of blue or hard type polystyrene as a plug and then fibreglass it.
To explain that a little more; you need to place the poly on the deck of the boat and scribe the line of the deck first.
Then cut it with a band saw or coping saw to shape, - get it to fit nice and close all the way. Looking down on top cut out the top view profile, mark and cut the side view profile. Now the tricky bits - just carve, file and sand the shape that you like, finishing with about 150 grit sandpaper.
Place a large piece of glad-wrap over the whole boat (to stop the glue from sticking) and lay the cowl plug on the boat making sure that there is a nice close fit all around.
Cut 3 layers of 6oz glass cloth to fit easily over the cowl. Next use only EPOXY resin as Polyester will dissolve the plug. Apply the epoxy resin with a brush until fully wet out, then lay some more glad-wrap or thin plastic over the whole thing. Leave for at least 12 hrs for it to cure, then take all apart, trim the outside edge and sand smooth. That's it!


It pays to sand a small recess around the plug where it touches the hull, that gives you a thicker epoxy edge to sand and file to get the final fit.
I sometimes put a small sand bag on the top to weigh it down - gets a better fit that way.
Fill all pinholes with light weight spak filler and finish sanding.
Leave most of the polystyrene in the cowl for flotation.
You can also cut out a shape for a GPS to fit in and carve out the area to fit the motor.
The cowl hold down peg is fitted now as well.

  A sub hatch needs to be made as well which is the one that fits under the cowl and is taped down to make everything water tight. The cowl is only there to make it pretty and look like a real hydroplane!!!

Position of the receiver and servo battery also note water cooling tubes, one goes to a fitting on the out side of the hull, left side, so you can see it when going clockwise, as we race that way.


A couple of coats of yellow lacquer, just need some vinyl cut letters and the project is complete.


Finally finished, looking good with a few vinyl cut letters.

The hardware needed for this boat is,
Motor, Feigao 540S, 7s a 5196 Kv, or 8s is 4761Kv
These are $US49.99 from Offshore Electrics
The controller (ESC) is a Hobby King SS 100A from Hobby King at $US24.83. 

Prop shaft can be .150 or .130. I used .150 so a 5mm to150 collet will be needed.
The rudder on this was a Speed-Master mini hydro; the same for the strut, to fit a .150 flexi shaft
Battery is a 2S lipo, Flite-Max 5000,at $29.99.
Turn fin is home made from aluminium sheet, and a piece of angle aluminium for the bracket.