Hydrofoil International Moth Build

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In 2006 I constructed a foiling international moth. The hull was constructed over a male mould using a carbon foam sandwich using vacuum bagging methods to consolidate the layup and removed excess resin. A set of hydrofoils were also constructed for this project. The hydrofoils were formed using a female mould and had an active control system to maintain a steady fly height.

Use the linkes to the left to navigate through the build sections.

For my 4th year project in 2004, at the University Of Glasgow, I worte a paper that considered an electronic control system for hydrofoils. This can be viewed here: http://www.boardrepair.co.uk/downloads/ConSysMoth.pdf

 

 

Hull Design

Where do you start when designing a new hull? I chose to look at the best current designs, the prowler mk1 and the Axeman 7 (this is in 2005).  I had already built a Axeman 7 hull a couple of years before, so the mould was sill lying around. Comparing the Prowler to the Axeman it was apparent that the shapes of the two were close, the prowler had rounded chines and a flat bottom compared to the sharper chines and rounded bottom of the Axeman.

The design approach adopted by many of the current builders is less is more. Foiling moths tend to spend most of the time in the air, so reducing windage and weight is perhaps more important than how the hull handles in the water. 

The Geko design follows this approach too. The following sections describe each aspect of the hull design:

Chines

To simplify construction the Axeman mould was modified so its bottom was flat cross ways, I also rounded the chines with a 50 mm radius. In retrospect adding the radius to the chines was unnecessary. The current thinking is hard chines are favourable because when the hull skims the water the hard chines shed water reducing drag. Another plus for hard chines is the build process is much easier!

Rocker
    
Rocker was slightly reduced compared to the axeman 7 with no rocker in the aft section of the hull. This was inspired from windsurf board design, where the fastest boards have no rocker in the rear of the board to promote fast planing. 
 

Free board

Since the hull doesn’t have to contend with waves the freeboard was reduced from 500mm at the bow to 330 mm to save weight. The decision to do this was made after the hull had been moulded. I had been toying with the idea while designing the hull, but no other moths had done this. At this time the first mistress appeared from full force boats with a much lower freeboard. I figured that if is was good enough for Mr May I too would adopt the smaller hull concept. Since i had to cut the hull shell down i was able to measure the material cut away - 1.2 Kg saved!
 

Offsets

The following table lists the offsets. Section 0 is the bow and section 6 the stern.

 

    Depths (from water line) Widths (from center line)
Section Distance from Bow chine & Center line gunwale Beam at flare Beam at Chine
           
0 0 -0.194 0.15 0.01 0.004
1 0.559 -0.191 0.15 0.072 0.067
2 1.118 -0.183 0.15 0.123 0.121
3 1.677 -0.167 0.15 0.149 0.147
4 2.237 -0.14 0.15 0.156 0.147
5 2.796 -0.104 0.15 0.138 0.108
6 3.355 -0.048 0.15 0.081 0.025

An excel spreadsheet showing the lines can be downloaded here

Bulkheads

Three bulkheads are installed. The front bulkhead is 0.9m from the bow. It provides support for the fordeck, crewdeck and racks.  It extends 0.2m above the gunwale.

The centre bulkhead is located at the rear of the daggerboard case. This allows it to transfer some of the foiling loads the the hull skin and stops the hull bending.

Transom to stop the water getting in!

Crew Deck

The crew deck has to carry out 4 functions. Provide a place to stand, provide attachment for the tramps, mainsheet attachment and rack support.

The attachment for the tramps and mainsheet is realised in the same manor. The crew deck extends 25mm beyond the hull skin. Through this carbon tube is glued allowing the tramp lacing and mainsheet to be attached. To ensure the carbon tube is bonded to the carbon skin correctly 5mm of foam core is removed around the hole using a bent nail and power drill.

Hull Construction

This section describes the build process for the hull. Use the navigation belowt to jump sections.

Hull Mold Construction


4.1 Mould


My mould started life as an Axeman 7 mould, however the bottom profile was altered considerable as describe earlier. In retrospect it would have been easier to start with a new mould as the side panels had warped over time and needed corrected. Live and learn!


The Geko hull has been designed so the mould can be easily made of sheet plywood bent over formers. Six formers, made of 12mm ply wood are spaced 559 mm apart. Remember to minus the thickness of the hull shell and the thickness of the mould skin from the offsets. The hull bottom and sides are flat. The formers are attached to the floor or another plank of wood to hold them firm. Ply wood or MDF, something around 10 mm, is bent around the formers. 

 Although the first Geko design has rounded corners i would advise for much sharper chines. Round the chines on the mould with a very small radius, approximately 2mm, so the carbon fabric can wrap around easily. Minimal filler is then needed for the corner when the foam is applied.

In the photo of the mould above you can see the the flanges that are necessary to produce a boat with flares. The Geko design doesn’t have flares so this flange is not needed. One of the hardest parts of building the Axeman 7 mould was cutting the sheets of ply in the long curves required to build the flares. 

To stop the laminate sticking cover the mould using brown parcel tape and covering film for books. The covering film is available from Woolworths. I’d advise you also apply some wax release agent. CFS fiberglass supplies sells release wax amongst other useful stuff. To seal the vacuum bag apply sealant tape around the edge of the mould. This is thick really sticky stuff, a bit like chewing gum.

Hull shell

 

The shell is build from a conventional carbon foam sandwich construction. I chose to use 200 gms cloth on the outside and 160gms on the inside. This should save approximatly 250gms.  The inside layer is orientated at +/-45 degrees, while the outer skin is at 0/90 degrees. This allows the laminate to carry loads in all directions. The +/- is applied to the inside so joins are not visible in the cloth. Photo 3 shows the inner skin on the mold with the peal ply applied.

  

Next the foam is applied. 70Kg/m^3 Corecell foam from SP systems is used. Flat sheets are used for the bottom and sides and in this Mk1 Geko cross cut foam was used for the corners. The rounded corners required much fairing of the foam to produce a fair shape. Again, I would advise any would be builder to use sharp chines. Photos 4 & 5 show the foam being applied.

  
Four strips of foam glued together are used for the bow. If you wish you could include a block of wood where the forestay fittings attach. Photo 6 shows the bow construction.

  
Once the foam has been faired, the outer carbon skin is added and vacuum bagged. Once this has cured the shell is removed from the mold. There are several methods to help pry it of. First is to bang a wedge in under the transom. I eventually drilled a 5 mm hold in the middle of the mould from underneath and put a high pressure air line to it. This popped the shell of nicely!

Once its been removed replace the shell back onto the mold and construct an enclosure around the shell. I used the carboard that the foam came in. The shell now needs to be post cured for 12 hours to cure the epoxy further. Try to get the shell hot so you cant touch it.

At this stage i decided to cut the hull down as described in the hull design section. At the time i had access to a laser level that helped me get the gunwales perfectly level.

Laminating Technique

This is a quick section of laminating. There are some excellent resources available on the SP systems and National 12 website that discus laminating and vacuum bagging in detail. Below are some of my experience that hopefully you’ll find useful.

First precut and layout all the cloth and vacuum consumables before you mix any resin in the order that you will use them - i.e. vacuum bag on the bottom and fabric on the top. This allows you too work quickly later on when your working against the pot life of your epoxy.

First the inside layer of carbon is applied at +/-45 degrees. On sharp chines the cloth will go round better diagonally across the chine. Also straight joins are easier to fair in on the outer layer rather than diagonal joins.

Wet the mould out with slow curing resin and apply peel ply over the mould. Having peel ply on the inside performs two functions, first it provides a good surface to bond the bulkheads to. Furthermore I think that it helps distribute the resin through the cloth more evenly. This is my personal opinion, other builders only apply peel ply where bulkheads are present.

Try using a credit rather that a brush. I find it doesn’t disturb the cloth as much and large areas are easier to wet out quickly. The quantity of resin applied to the cloth is also more even. Aim to have the cloth damp rather that soaking wet. You should use the same weight of resin per unit area of cloth as the cloth weighs. So maybe 0.5 Kg for the inner skin. Buy a cheap set of scales from Lidl to measure the resin out.

Overlap the cloth by about an inch. Once the carbon cloth is on apply the peal ply. This needs to be saturated in epoxy to provide a good finish to bond the foam to. You may need to add more epoxy here but try not to over do it!

The whole boat needs to be vacuum bagged to consolidate the laminate and remove excess resin.

For the vacuum bagging a release fabric is first applied to the mould. This is a thin film with small holes spaced 1 cm over the entire surface. The purpose of this film is to stop the peel ply sticking the breather fabric, but sill allow the excess resin and air to pass from the peel ply. On top of this is the breather fabric that soaks up the excess resin and allows the vacuum to spread around the mould.

To seal the stack, a vacuum bag is placed over the top. The photo below is taken from an SP systems publication and shows the total vacuum bag stack.

Once the stack is assembled, a vacuum is applied to compress the laminate together and remove excess resin. A special fitting is available for connecting the vacuum hose to the bag, however I insert the tube under the edge of the bag and seal with vacuum tape. The end of the vacuum hose needs to be wrapped in breather so that the bag cant seal the tube. The suction need to be appled while the resin cures. I like to leave a blob of resin on the top of the bag so I can judge the cure state.

Bulkheads

A usual construction of 3 bulkheads is used. The centre bulkhead is against the rear edge of the dagger board case to help structurally. The dagger case is mounted 100 mm forward of the current position used (2005), however the dagger board is vertical so the front hydrofoil is located in the same position as the prowler.

 
The bulkheads are pre-laminated and vacuum bagged on a prepared flat surface and shown in the above photo.

   
To fit the bulkheads large radius fillets and 50mm carbon tape is used. The above photos show them being ftted. 

Foredeck

The King plank is the usual fair of a 50 mm deep foam core beam with 400 gms cloth either side.

Carbon tape is used to secure the king plank at each end.

A pre-laminated sheet of carbon is wrapped over the king plank. It overlaps the hull by around 20mm and is faired in.

  

Crew Deck

The deck is a single sheet of foam that overhangs the gunwales by about an inch to allow the tramps to be attached. 
First laminate the underside of the deck and add extra carbon where the daggerboard will go, also add carbon at the transom. I put three layers extra on at the centerboard with each layer getting smaller to spread the stress. Just one at the transom will be fine - this helps take the load of the racks and gantry.

 Once you've laminated the underside of the deck let the resin gel then stick the deck down to the hull with a strong mix -eg 45% microballons, 45%  microfibers & 10% silica peanut butter consistency.  I left the fabric overhanging the front edge by an inch, then bent it up the front bulkhead when the deck was glued down. This ensured the inner layer of carbon is bonded to the bulkhead.
 
   
You will need to push the deck down so it goes into a curve. Use a long piece of wood with lots of weighs and cramps. 
 
 
Once this is cured laminate the top of the deck and wrap the fabric round the gunwale and down the topsides for a inch. This will need to be vacuum bagged because the carbon wont sit nicely round the edge of the deck by itself.
 
  
To attach the tramps I drilled holes, removed some of the foam core, using a bent nail on a power drill, and inserted small pieces of carbon tube. I think a better method would to glue a glass or carbon rod along the edge of the gunwale, then drill holes through the deck just in from the edge to allow tramp lacing's to pass around the rod.
 
   
Wooden blocks are made to support the racks, I actually fitted these at a later stage. The outer carbon skin and foam core is removed, then the block bonded to the inner skin and a fillet added so there is a good bond with the outer skin.
 
The daggerboard case is approximately 13cm from back to front. This is added after the deck fitting. The daggerboard case is made around the final foil first, then cut the hole in the deck and hull to take it.
 
The mainsheet is about 10cm behind the centreboard case. It is attached in the same way as the tramps. There are two small holes through the gunwale about an inch apart, the strop attaches through these.

Foil Design

The foil design was carried out in a similar way to the hull design. The current ideas were studied and small improvements were made.

5.1 Section Choice.

The foils sections are Tom Spears H105 sections. The horizontal foils are cambered 4% (need to check this) and the vertical foil has no camber. This section was chosen over NACA 64 012 series sections because it has been optimised for use as a hydrofoil. Pressure peaks have been smoothed out so the foil is less likely to ventilate. A second bonus is the drag is less for the same thickness of foil. This allows stronger structures of the same drag to be made.

Foil Building

This section is going to be about building the Geko hydrofoils. Check back soon for more information and some photos. I've included a couple of photos of the control rod and hinge detail.

  

Foil Mould

The hydrofoils are manufactured in female moulds. This is for a couple of reasons:

  • Producing multiple sets is less work - ie front and rear foils
  • The foil section is more controlable

The moulds were constructd by routing out a rough foil section from a setion of plywood, then using a template to apply filler to form a good surface.

I assembled the mould in a n shape so the top of the hydrofoil and half the vertical foil is moulded in one piece and the mould for the vertical mould is shared between each half. While this means multiple foils cann't be molded at the same time less time is spent making the moulds. The photos below shows a close up of the mould with half a foil section in it and the complete mould with a vacuum bag round it.

  

 

Foil Moulding

Moulding the foils is fairly simple. I used 2 layers of 200gms square weve carbon on the outside to provide hoop strength to the foil, then 3 layers of 400 gms unidirectional carbon fibre for the main structual strength. The unidirectional layers are 80mm wide.

The rear 25mm of the foil forms the flap. I had a change of design in respect to the hinge design.