Hydrofoil Stabilised Dinghy specification covering all the main details so you can build one too – also to provide a framework for comparison and discussion of projects – to prevent reinventing the wheel each time.

This is part of a four part article

I hope you see the following as a commonsense specification. It is possible to make the foil a little bit more efficient – but we know this simple foil does work.

It is possible to make it much more complicated!

This is just about the worst possible hull for this experiment – but it still worked. Aim is not pure speed, but to reduce the need for hiking … and a little speed bonus for a very short boat!

Building hydrofoil stabilisation for a sailing dinghy this way is pretty easy. There should be more of it. Most have forgotten that the huge jump in the Moth Foilers was achieved in wood and glass – and all the carbon shenanigans came along much later.

The Advantage of working from a Common Base

No problem if you want to go another way using this basic information.

But the reason for using a common setup is the synergy it creates. Otherwise each developer continuously reinvents the wheel.

Stop … Start really defines the history of Foiling development. Including Monitor in the 1940s right foiling showed very little development indeed. Lots of projects – but very limited speeds and poor or no performance in light winds and upwind.

But in a few short years the Moths cracked all the serious problems and are among the few boats that will foil in light winds and in all directions.

What was the Moth advantage? Smart people self building foiling gear from basic materials like wood and fibreglass for easy low cost modification. Comparing and sharing their different approaches on the water each weekend.

Manufacturers developing “foiling boats” have had a much worse track record and many resort to cherry picking video footage and photos to give the impression of easy foiling. Hmmmm.

Information Sharing – as part of a hydrofoil construction group

The idea of this is it becomes a ready reference of information for anyone at all wanting to build a foil. There will be dataset that can be related to videos photos and other evidence. Also trends will become visible over time.

If you have several hulls using the same foil then provide each as a separate registration. Can be metric or Imperial:

Name, City, Country , Date of first sail

, Full foil profile – provide sketch or offsets if a non standard foil

Nose radius

Chord

Foil length according to the foil maximum length rule

foil finished weight

Core Material

Glass weights and layers used

Hull length, beam, weight and general type

Our registration would look like this

Michael Storer, Taal Lake, Philippines, November 2019

Full foil profile – NACA0010

Nose radius 3.17mm

Chord – 220mm

Foil length according to the foil maximum length rule 7 – 1220mm (4ft)

foil finished weight – not measured yet

Core Material – Solid Luan Laminated

Glass weights and layers used – Single layer 200gsm (6oz) glass (which might not be strong enough)

Hull length, beam, weight and general type – Hull 8ft x 3ft 9″x 65lbs (scow)

Motherhood statements/Intent of “Rules”

The idea of these “rules” is to limit the homebuilt foils specifically to a level of technology achievable for home boat builders. CNC and other methods can be used but only using the same set of materials. Professional builders can build as long as no methodologies are used that can be emulated with amateurs using minimal tools.

Hull Construction

Hull to be plywood. Glass can be used for protection or local joining/stiffening. Carbon fibre and other exotics not allowed anywhere. We used a single layer of 6oz (200gsm) glass. If you boat is heavier you may need more glass.

Retrofitting fibreglass hulls is fine, but must not be lighter than 8lbs per foot of length for boats under 10ft long. And not lighter than 10lbs per foot for boats over 10ft long

Monohulls

Monohulls only for stabilising foil.

Hull to be a monohull with no hollows in the hull surfaces intended to provide a multihull effect.

Sub Classes

A cheap class with a limited length range. A more expensive class for wider experimentation.

Class Lateral Foil A – 8 to 12ft long.

Class Lateral Foil Open – Any lengths outside the “A” range.

Hydrofoil Construction

Foil to be wood construction. No carbon, kevlar or other exotics, no foam construction. Can be hollow ply if you dare. The aim of this rule is twofold. To keep the cost down and to limit the technical development – and in particular to prevent the foils becoming too thin and sharp.

Vacuum bagging, resin infusion and other methods are disallowed.

Hydrofoil stabilisation sailboat Geometry

Non Tapered

Foil to be straight with no taper. Straight means leading edge, trailing edge and overall length are all straight. The tips may be rounded to a distance of 12mm from the extreme ends of the foil.

Foil Length

Foil, when pushed through flush with the hull on one side will have a maximum extension of 4ft (1220mm) on the other side from the part of the slot closest to the boat centreline.

Non rule – we set up our case with the foil at a 2 degree positive angle of attack to the static waterline – this is appropriate for assymmetrical foils.

Non rule – Just for information a symmetrical foil needs to be set up with a 4 degree angle of attack.

Hydrofoil Underside to be Flat apart from nose radius

Foil underside to be dead flat except for a nose radius of up to 6mm (Note – this is too large for most practical foils – do your research). The aim is not the highest tech but for something anyone can make with a bit of care.

Non Rule – ours is a 10% NACA section (NACA0010) with the co-ordinates set flat on one side and doubled on the other.

Non Rule – Our nose radius is 3.17mm. This is in the normal range for a foil of this thickness.

Non Rule – our foil is 22mm thick and a theoretical chord of 220mm. But trailing edge has been planed to a less fragile edge then glassed.

Trailing Edge Minimum Flat

Trailing edge is minimum 2.5mm wide.

Construction of Hydrofoil Case

Rectangular aperture

Aperture of the foil case is to be rectangular, of the same size as the case overall and not shaped to the foil profile.

Case is to be rectangular in form for its complete length

Non Rule – We haven’t noticed any sounds or ventilation so far.

Non Rule – Our foil case was made 25mm wide internally which allows enough space for the 22mm timber blank with added glass to have a sliding fit.

Angling of Foil and Case

Case is to be horizontal laterally to the design waterline of the boat and perpendicular to its centreline. This allows the case to be inclined to give the foil a preset angle of attack.

The case will not allow the foil to be raked fore and aft – clearance of case relative to foil to be no less than foil chord plus 5mm

The angle of attack of the Hydrofoil is not to be adjustable via the case or by spacers in the case.

No flaps or other devices to alter the configuration, lift or drag of the lifting hydrofoil

This is the hydrofoil case pushed through to one side while the hole adjustments are being made.

Centreboards and rudders

This is to stop boats from becoming too complicated. Regardless of the lifting hydrofoil they should be as standard as possible and able to sail without the lateral foil

No hydrodynamic lift from Centreboard or leeboard

Centreboard or leeboard must be set vertical in hull. (nomenclature – centreboard means a board in the centre of the boat – it can be swinging or dagger). Foil section to be symmetrical and aligned with centreline

No Hydrodynamic lift from rudderblade/s

No hydrodynamic lift from the steering foil of Rudders. They are to be vertical if single or can have a lateral angle of 15 degrees if dual. Blade/s to be symmetrical.

A horizontal T Foil can be fitted to the rudder but must be of a symmetrical section, no wider than 500mm or wider than 200mm.



Non Rule – we are trialling without the rudder hydrofoil as a major simplification. It works so far.

No changing configuration for one Regatta

Regattas and competitions – to be eligible for inclusion in overall results competitors must use one set of listed equipment through the regatta one hull, one hydrofoil, one sail, one centreboard one rudder.

Competitors may participate if they change gear from the above but their results will not be counted. They must sail in a sportsmanlike manner and not interfere unduly with boats who will be included in the results.

Loopholes

If a loophole is discovered the idea that the hydrofoil setups are to be easy for amateur builders and inexpensive because of building from cheap materials is to be used to allow or disallow a feature.

If you can see any weird loopholes please give me a yell!

Useful additional information on homebuilt hydrofoil construction – Non Rule

Location of lateral hydrofoil in the boat (Non Rule)

If the hydrofoil is too close to the centre of gravity of the boat then the boat can pitch nose down accidentally and the foil will pull the leeward side of the boat down – rapid capsize or break the foil (the displacement of the boat will be much larger than its weight. So it needs to be forward of the centre of gravity with the crew included.

This ended up being just behind the centrecase of this little boat. I would be cautious not to put the lateral hydrofoil case too far back. It is more or less at the midpoint of the boat as we know this type of hull forces the crew to be fairly far back. Many boats with bigger sails and short light hulls would need the lateral foil in front of the centrecase.

Hydrofoil Case Construction (Non Rule)

Our foil case was made of the same 5mm ply as the hull.

The spacers were 3mm thicker than the unglassed hydrofoil. This is adequate for one layer of glass. Spacers must be very straight or the box will not be straight and the foil will not slide.

We cut the holes in the side of the boat very slightly oversize and then used accurately sized ply pieces on the inside of the hull to adjust alignment.

The case was long enough to go out the hull sides. The wooden cleats or fillets to join the case to the skin of the boat have to have a 20mm bonding area to the hull and the foil case on the inside of the boat.

We also had an adjacent bulkhead to bond to the hydrofoil case.

We were concerned that if the foil was pushed through too far the end in the centrecase could lever the ply face off the case. We rounded the external corners of the made up case on the non bulkhead side and wrapped two layers of 50mm 2″ wide glass tape around the case at approximately 300mm (1ft) intervals