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1/8 Scale Electric Scale Hydro Build -  LLumar Window Film

After my previous boat was destroyed in a freeway smash in the States, ML Boatworks (www.ml-boatworks.com/) kindly supplied me with a replacement frame kit.  The following is an outline of the build sequence.

The ML kit comprises laser-cut plywood frames and longitudinal formers.    I found I wanted to add a couple of additional frames not included in the kit. I already had the Roger Newton 1/8 scale plans for the boat and strongly recommend that anyone contemplating building a scale boat from the ML Boatworks kits also procure a set of Newton drawings  (David Newton www.newtonmarine.com/) .  Note that this kit includes only the frames and longitudinal formers.  Stringers timber and skin sheeting ply must be purchased separately.  I use the best quality 1.5 mm plywood I can find for skinning.  In NZ there seem to be two grades-  a strong stiff cream coloured ply  (Hoop pine, I had some in stock but not enough) and a poorer (but still adequate) pinkish ply (mahogany??) I bought from “The Plyman” in Waitakere:

 frame on jig.jpg

Hull

The first task is to build a jig to assemble the framework around.  A key function of the jig is to ensure parts are assembled square and perpendicular. As much of the final skinning of the boat as possible should also be completed before freeing the model from the jig. 

I used 20 mm custom wood to build the jig. Spend some time inspecting the drawings and the laser frame components so that you correctly understand the form the jig needs to take before starting to cut any timber. In particular determine the exact tunnel width, whether you want the central tub offset to the left or the right, and where the break in the flat central hull floor occurs, ie where the flat underside section at the rear of the hull stops being flat and begins to rise as you move toward the bows.  The central tub will also have such a break in its floor BUT NOT AT THE SAME POSITION. Remember to allow for the longitudinal sponson former plywood thickness when cutting the width of the jig.

I began by dry assembling the frames around the jig, and then stapling the inner sponson longitudinal formers to the edges of the jig.  Carefully check everything is square and true BEFORE beginning to glue the joints using CA glue.  If necessary remove the staples and try again. I like strong boats (at the expense of some extra weight) so I often reinforce the major joint intersections with a 6 mm x 6mm timber fillet (didn’t help on the freeway...).  I recommend some particular reinforcing where the sponson transom frames intersect with the longitudinal formers as this is where boats often begin stress-cracking.  Sorry no close up photos.  If not reinforced by a timber fillet I always reinforce with a fillet of epoxy on every side of every joint.

frame on jig2.jpg

I reinforce around the transom (rudder mount), rear floor internal and external (strut mount), drive log hole through the floor, and motor mounting areas.

I did not buy stringers from the model shop as they cost a fortune and are now hard to get. I cut my own from a selected piece of moulding timber obtained from the local timber merchant, using a 10 inch bandsaw and a sanding block.  It’s surprising how much stringer timber is needed.  I also cut extra notches in some frames and installed some additional deck support stringers as I thought the deck looked a little unsupported in places and you don’t want someone flexing or breaking the deck when they rescue your boat.  Try to imagine where they will put their thumbs to pick up the boat and glue in a couple of stiffeners to support the deck.

Front Canard Spar

In the photos this looks to be carbon fibre, and it is. It is built around a 1.5 mm ply former, skinned top and bottom with 3 mm balsa sheet and then carved/sanded to aerofoil cross-section.  Finally a skin of carbon fibre cloth.  Carefully mark out where the spar penetrates the sponson walls and cut openings.  One trick I use to cleanly cut small openings is to heat a length of brass tubing of appropriate size red hot, and use this to burn through the ply surface – avoids damage often caused by a drill. Securely bond the spar ends to the interior of the sponson walls and tub walls.  These are high-stress joints so be thorough.

carbon spar.jpg

Tail-feather mounts

These again are built from a balsa core with a carbon fibre skin both sides.  Very light and unbelievably strong. (Visible in deck photo below.)

Skinning

Next comes the skinning.  I start with the sides and non-trips aft of the sponsons, followed by the sponson ride surfaces.  Block sand the surfaces to get a good flat surface to glue to. Glue on as much of the under surface sheeting as you can BEFORE releasing from the jig.  You can then glue in the main hull floor and tub floor etc.  I usually replace the hull onto the jig and weight it down while the glue cures at this stage.  Again I go around every joint and add a glue fillet (slow cure Epoxy).  I like the 1:4 mix epoxies such as Epiglass HT9000  ($$$$) or ADOS All-Purpose ($).

bottom sheeting underway.jpg

Once all the underside skinning is complete and all joints filleted with epoxy, the time has come to carefully seal the interior.  I apply two coats (or more) of Epiglass Everdure, plus maybe even some strips of glass cloth and epoxy at stress areas.  DO NOT SKIMP this step as it is the only way to keep the boat light as it ages.  You WILL get water inside the hulls, so you MUST adequately seal the timber to stop absorption.  In due course you must also seal the underside of the deck too – don’t forget... Don’t forget to provide drain holes either, or the water that WILL get in, will never get out....

My next step is to fit ALL the hardware into boat (ie hull minus deck).  Experiment with where you are going to locate batteries, fuel tanks, motors, struts, ESCs, switches , plumbing, turn fins, etc etc.  The objective is to get everything in AND the CoG in the correct place (about 50 mm aft of the sponson transoms).  On electric boats with flex drives we are typically locating the motor about 350 mm forward of the main transom.  Some of the Seattle boats used solid straight shafts which means motors must go much further forward.

decked.jpg

Now the deck can be commenced.  I began by building the aerofoil bull-nose section either side of the central tub.  I cut four formers and glued these in the correct  position on the four longitudinal formers (on the ML Boatworks longitudinal formers, the shape and position is marked with a dotted outline).  I then “plank” between the shapes using narrow strips of 1.5 mm ply.  Don’t forget to seal interior surfaces as you go as you may not be able to get at them later on.  I found this method built a very strong and light aerofoil shape but one which needed a fair bit of external filling and sanding to get fair. 

After block-sanding the framework and  double checking you have installed all the necessary concealed equipment, tubes, plumbing, you can start the deck proper.  I fit it in four pieces with a butt joint directly above the inner wall of the sponson.  This will ease achieving the compound curvature on the deck.  I tape paper over the area,  trace the edges with a broad pencil and cut out the deck section using the paper pattern.  I use the paper pattern method for most skin sections as they are often an odd curved shape.  You may need to trial fit and trim a couple of times so cut a little oversize.  SEAL THE UNDERSIDE then glue down.  I use copious tape, thumb pins and weights with the hull again sitting on the jig. You may wish to insert some foam blocks in the interior spaces before gluing down the deck.

interior layout.jpg

For LLumar I house all the equipment in the central tub in a single space with two hatches.  I used two hatches so I could keep a continuous frame across the boat at the sponson transom position. Refer photos.  The third small hatch at the tub bow allows access to an operational canard flap control horn and is separate from the main space

Now comes the tedious bit – trimming oversized skin sections, sanding, filling, sealing, filling, sanding, swearing, sealing etc etc until you get a perfect surface or just get too tired. Remember to keep all underwater edges square and sharp.  This reduces drag and assists the boat to get up on plane.

Cowl

I purchased a cowl from www.rcboatcompany.com but it arrived a bit late and did not match the shape of my hull well, so I elected to mould my own. Old one is For Sale if anyone wants it....

 

cowl plug.jpg

Based on the Newton Marine drawings and photos, I built up a strip-balsa cowl directly on the hull. Balsa formers at about 75 mm centres and lots of narrow balsa strips & CA glue – All from about 4 mm balsa.  Once complete, copious polyfilla  type bog and sanding until you are happy with the shape and smoothness of the surface.  Next several (4 or 5) coats of Everdure to form a hard smooth surface finish on the “plug”.  Next a coat of PVA fibreglass mould release  (NZ Fibreglass Ltd in Mt Wellington) and a thick layer of fibreglass to make a female mould.  I stiffened the mould further by buying some plaster-of-paris bandages  (Takapuna Art Supplies) and coating the mould (while still on the plug) in a two part plaster cast.   The glass mould is a single unit but the plaster support is in two pieces. I can now make replacement cowls that perfectly fit my hull.

cowl moulded and hull painted.jpg

Tail Feathers

These are glass mouldings from www.RCboatCompany.com.  Assembly is pretty straight forward.  Don’t forget some internal strengthening as desired.

Painting

As Llumar is an electric boat I did not need to worry too much about paint type.  I just used ordinary Dulux  Spaypak Quick Dry Enamel as the LLumar colours are pretty conventional.  Once decals are applied a final clear coat of the same product finishes the job (no good for methanol fuel but seems Ok for petrol). Spend as much time on finishing, undercoating, sanding re-undercoating etc as you can stand.  I get frustrated and my boats are built to race, so I don’t go overboard getting a perfect finish (and it can all get wiped out by a drunk Mexican in any case...)

Decals

These come from Mike McKnight  www.thunderboatgraphics.com/ in the US. His site includes instruction for application.

Turn fin mount

I cannot emphasise how solid the turn fin and mount must be.  You must be able to pick the boat up by the turn fin and should be able to even shake it up and down WITHOUT ANY FLEXURE on this component.  I strengthen the mount area for Africa.  A rear brace through the non-trip and into the central tub wall may be good insurance.

Servos, ESC & Plugs

It is important that the steering servo is strong enough.  I use a Hobbyking HK15298B and use a 6 volt receiver pack.  This servo can then deliver 20 Kg-cm of torque and has been more than adequate.  Cheap too, and standard size.... ESC needs to be a High Voltage unit rated at about 200 Amp min, as we run 8S or 10S as a basic battery set-up (Note 8S uses a larger prop and  pulls more amps than 10S).  I also run 8 mm plugs on the battery side and 6 mm plugs on the motor.

Props

Props are a matter of trial & error but as a starting point I have had success with a X646 3 bladder on 10s and a X455 on 8S.

Drive Train

On Llumar I run a 3/16inch flex cable in a thickwalled Teflon liner.  At the motor end I have a home-made square-drive collet, so I have a floating cable at the motor end. Many boats run ¼inch cables for strength, but I cannot remember when I last broke a cable...maybe the floating collet is the key.

completed2.jpg

 

So there you have it LLumar version III......

Chris Breen

 

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