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Scratch Building My 1/8th Scale Hydroplane

By Chris Breen





For many of us, 1/8th scale hydros are the most rewarding boats to model and race.  The boats look and run like real boats, the races short and often spectacular, and your hands will surely be shaking after a close finish.


When I began constructing a replacement for Llumar I decided to take a series of photos at various stages of construction and to write-up a brief outline of how I went about it.  This article is NOT intended to be a full set of instructions on how to build, but will perhaps suggest some solutions that will assist other builders to save a few dollars.


For those toying with the idea of moving into 1/8th scale here are some key aspects



Must be a model of a real hydroplane. – Note: does not HAVE to be an American Unlimited (although they almost all are). You could model a local boat if you really wanted to. There were also a couple of quite odd American Unlimited boats over the years and these could conceivably be modelled but most boats are “conventional” US Unlimited 3 point hydroplanes. 

Scale Unlimited boats are built to 1/8th scale, other boats need to be built to dimensions “approved” by the national body (eg the Scale Unlimited Hydroplane Association (SUHA).  The “approved” dimensions will be set so the finished model is very similar in size and power to an 1/8th scale US unlimited. Model dimensions are allowed to be plus or minus a bit compared to the precise scale dimension.



Glow only, 11 cc (0.67 ci) maximum capacity (almost all boats run an 11 cc motor, some boats run a 10 cc motor)


Paint Job

Models are expected to be finished to replicate the real boat. At major regatta events points will be awarded according to the standard you achieve.



Clockwise, usually 6 laps, 2 and ½ minute count-down start. 5 or 6 boats per heat


The Building

Llumar (version 1) was built from home-drawn plans based on photos only, and as a result was not exactly true to scale. Llumar version 2 has been built from plans prepared by the universally used source of plans for 1/8th scale boats, Newton Marine in the




 Having chosen and got your plans, now you need timber.  I bought a full size sheet of 1.5 mm and a half sheet of 3  mm marine ply from a local marine supplies shop on theNorth Shore.  For stick wood I found the local model shops were not very good so I purchased a small band saw from Bunnings ($175 or there abouts) and selected a nice piece of scotia or architrave or something and cut my own 6mm by 6mm and 6 mm by 3 mm stringers.

 For adhesive I used CA and my favourite epoxy resin, Epiglass HT9000.  This stuff is versatile and bloody strong!  You mix it 4 parts resin to 1 hardener and use it straight as laminating resin, mixed with glue powder for glue (thick medium or runny), and mixed with filler or micro balloons as a filler! Cures over-night unless you get the mix a little wrong then it may take two nights!  I found mixing very small quantities (1 teaspoon + ¼ teaspoon) a bit tricky to get the ratio right.  In larger quantities such as for a full on laminating session, no problems.

 The Hull

The first task is to photocopy the drawing sheets to produce a working set.  Note that on modern boats (at least) the sponsons are NOT identical, the outer sponson is wider, and the central “tub” is slightly offset onto the turn-fin side.  The plans for Llumar have the sponsons reversed correctly but the tub is of-set the wrong way.  This may be because model hydros often like a little extra weight on the outer sponson.  I needed to choose whether to mirror the real boat or follow the plans. I chose to mirror the real boat.

I then cut out each frame and former outline from the photocopy and pasted them onto the 3 mm ply with ordinary paper gum.  You want to use glue that easily washes or rubs off.  The frames are then all cut out using a bandsaw, scroll saw, grandad’s fret saw, young nubile slave or whatever you have to hand.




 The jig, used to support the framework during building must be straight and true. I made mine from 20 mm thick custom wood and something called “finapanel” I think.  I found the frames needed a bit of trimming and packing out to get everything to align. (Getting drawings perfect is very difficult, believe me, I know!)  Initial assembly was done using CA but CA is not fully water resistant and in any case I like to fillet all joints with epoxy glue.  Having assembled all the frame work and stringers I them embarked upon fairing the framework ready for skinning.  This gets a bit tedious. I have a tiny plane that is very useful for this task. It is only about 75 mm long by 30 wide and is probably my most useful modelling tool after a Stanley knife.

As much of the skinning as possible should be glued in place before releasing the model from the jig. Remember to seal the internal surfaces of ALL wood before you glue on a skin on that prevents you reaching any surface.  This takes a bit of fore-thought.  I used two or more coats of Epiglass “Everdure” for sealing.  I find fitting the deck is the trickiest bit, because of the compound curves.  You may find a few extra stringers and fitting the deck in several pieces is the easiest way.  You will need lots of clamps, drawing pins, cloths pegs.  Once skinning is finishes comes a lengthy period of filling gaps, pin holes etc, sanding and finally sealing with several coats of Everdure

 The Cowl. 

You can cheat and order a ready-made cowl from the US or you can be a real man and make your own.  Llumar’s cowl is 3 mm balsa strip planking glassed inside and out then filled, sanded, re-filled, re- sanded until I got sick of it.  To build the cowl I drew up frames at about 75 mm centres and spot glued the frames onto the top of the boat, and planked in place.  This helped get a good fit to the decked boat.  I then had a few hick-ups I will not go into with the result that my cowl is a good although not a perfect fit.





The best paint is, without doubt, two pot epoxy.  However I have been quite happy with polyurethane spray cans on my previous boats.  If using home handy-man spray cans be sure you get polyurethane based paint as ordinary paints (ie most of them) are not fuel resistant. I found a good range of polyurethane spray cans including a sandable undercoat/primer at The Warehouse (where everyone gets a bargain) for about $8 per colour. The biggest hassle with painting is doing the masking for each new colour.  Llumar has four colours, all common primary colours – one reason I chose this boat to model, so masking is a bit of a mission





 Fitting out, Decals, Set-up and Testing

 For many of us, 1/8th scale hydros are the most rewarding boats to model and race.  The boats look and run like real boats, the races are short and often spectacular, and your hands will surely be shaking after a close finish.

 When I began constructing a replacement for Llumar I decided to take a series of photos at various stages of construction and to write-up a brief outline of how I went about it.  This article is NOT intended to be a full set of instructions on how to build, but will perhaps suggest some solutions that will assist other builders to save a few dollars.

 Hint left out from part 1 (Hull construction)- Hydros usually need weight added to the fronts of the sponsons. It is therefore a good idea to construct some type of container for weights inside the sponson tips BEFORE you glue on the deck (I forgot).



You will need a rudder, a motor mount (and a motor !), turn fin and mount, a strut, a drive shaft and stuffing box, and radio boxes

 Radio Boxes.

There is nothing much unique about Hydro radio boxes so I will not say any more other than try to keep all the weight as far forward as possible. (As they say, "to add speed add lightness")



I have tried building my own rudders but the hassle is not really worth it - buy a good one, you will not regret it.   I purchased a Speedmaster brand rudder assembly. The next decision was whether to mount the rudder on the left or right side of the transom.  Again based on the real boat, I chose to mount on the opposite side to the turn fin.  (Note: quit a few 1/8th scales mount on the same side as the turn fin.  Opinion is very divided on which is better.)



 The Speedmaster blade is quite long and I may shorten it in due course.  Some opinion holds that the best place to end the blade is 1 to 1.5 propeller diameter below the prop shaft.  On the other hand I am told spin-outs are often caused by too short a rudder.

 The rudder servo in Llumar is a heavy duty Futaba sail servo and is mounted in its own box as far forward as I could arrange.  Try to keep everything as far forward as possible to reduce weight on the propeller.

 Motor Mount

This is one fitting I made myself (except for four rubber bushes from Octura).  The photo should be self-evident.  I chose to mount the motor on the floor of the tub to make things easy.  This may not have been a good idea as the floor is comparatively weak although I have strengthened the floor a fair amount.  Most mounts are supported on the walls of the centre tub but this makes it difficult to get at some of the fixings.  I made the decision to mount everything in the central tub so it is easy to get at.  Llumar therefore has no openings in the centre tub walls.




Turn Fin

Pay a lot of attention to the turn fin mounting.  Some time ago I remember doing some calculations to estimate the forces on the turn fin.  If I remember correctly the force can get up to something like 1.5 times the total weight of the boat in a controlled turn!! (who knows the forces in a bad spin-out).   Imagine picking your boat up by the fin and shaking it up and down vigorously. It needs to easily withstand such treatment.

 In Llumar the main turn fin mount is cut from heavy wall aluminium channel (on an angle to match the sponson rear transom).  The mounting is bolted through the sponson transom, through the double thickness interior frame, and treads into a thick piece of aluminium plate drilled and tapped for twin 4 mm stainless bolts.  At the rear of the mount a bracket bolts through into a similar ply pad and aluminium plate buried behind the non-trip framing and stringers. (Obviously you need to pre-plan all this before gluing the deck on).  Make every attempt to get the main mount surface parallel with the centre-line of the boat (which should also be parallel with the tunnel walls and the sponson ride pad edges)

 I have not yet procured a good turn fin so testing has only used the old fin off the old Llumar.  The fin itself must be stiff (1.5 times weigh of boat without flexure please?), but ideally should also be a thin as possible to reduce drag.  The turn fin also MUST be parallel with the boat's underwater lines.  Clamp a long straight edge to the fin and stand back and check - it is difficult to be confident without a long straight edge.

 Stuffing Box

I have always run 3/16 flex running in 1.5 mm wall thickness PTFE tube (it's expensive but VERY good in my opinion, $20/m from Ludowici Plastics in Henderson Place Penrose)  inside a snug fit K&S brass tube.  The brass tube is silicone glued into the hull to maintain a little flexibility.  I run a square drive at the motor and a ferrule at the strut. I just plain solder the ferrule to the 3/16 flex cable and rarely have had a failure of the solder.  One thing I always do however is to remove the cable after a day's running and fill the tube with ordinary engine oil before replacing the flex cable.  On some boats I remove the PTFE complete with flex.  The PTFE has a mall hole about mid-length into which I can inject oil at pressure and force out any water

Some opinion holds that the tube should have a gentle "S" curve to it.  This is supposed to reduce drag.  Llumar has such an "S" curve, but not deliberately.  The motor is slightly off-set , and the strut is central. After much gentle bending, straightening and re-bending the tube, everything finally lines up and the tube is "S" shaped !! Hint: K&S tubing is work-hardened when you buy it and very easily kinks.  If you heat red hot and quench in water it goes nice and soft. Re-heat and allow to cool slowly and it will regain some of its original strength. When bending your stuffing tube always insert your PTFE tube and your flex shaft to prevent kinking.


Again the Llumar strut is home-made (see photo). It is quite long to give good shaft support and is simply a heavy brass tube (same as stuffing tube) with a bush at the prop end to take the thrust.  The rest of the bearing is a length of the heavy wall PTFE used in the stuffing tube - works very well. The secret is to have it quite long.  This also makes it convenient to mount a rear ride plate. As thrust washers I assembly a stack of stainless washers and thin slices of the thickwall PTFE tube.   Again seems to work very well.

 Fuel Tanks

I run two 8 ounce tanks in Llumar this is enough but gives little margin for extended running.  Both are located in the central tub, so you can imagine there is not a lot of spare room (see photo).


  First Test Day

First water was Sunday February 22nd, quite a bit of wind but otherwise fine and sunny.  No canards (Flaps on the front cross-beam that help keep the nose down at speed), and no way of fixing the tail plane angle of attack.  Also the centre of gravity is still too far aft (recommendation is 110% of the sponson length (ie 10% of sponson length behind the sponson transom.  Turn fin leading edge should be on or slightly behind CoG).  All this meant I was not game to tune or throttle for max speed.  But otherwise very happy.  Tracking seemed very stable and predictable, not too much bounce in rough water turns.  Only at speed did the front get a bit flighty.






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