UK-Cherub Class

Get Your Heart Racing

Building a Single Skin Rotating Gantry

The gantry on Pocket Rocket needed modification to allow for the use of a T-foil, the shape of the existing one made this difficult, I was unsure about its strength and stiffness as it was made from foam struts with a curve along their length. So I decided to build a new gantry.

The methods I used to build the gantry are outlined below, they may or may not work for you, involve the use of dangerous chemicals and failure while sailing renders your boat uncontrollable, if you intend copying any of the techniques used please ensure that you understand all the risks involved.

The Shape

The Gantry is to be hinged on the back of the boat near to the corners of the transom for strength, it can be thought of as being made from two isosceles triangles with the tips at the transom and the short edges meeting where the rudder pin is to go. To give it strength in compression the edges of the triangles were to be folded over to make flanges which would also serve as a location for the rudder pin. As most of the strength and stiffness of the gantry is in the triangular shape and the flanges, the stiffness of the flat panels between the flanges is not critical to the overall stiffness of the gantry so single skin construction can be used, which should save some weight compared to using foam sandwich.

An important part of the design of the gantry is the hinged attachment to the back of the boat, this must be strong and stiff enough to take not only the steering, drag and lifting loads but also to prevent the spreading of the gantry under these loads.

Building the Mould

The mould was built from various off cuts of MDF and it was shaped so that the moulding would slip off at the end and so that the bottom of the gantry was at 90 degrees to the rudder pin so that the bottom of the gantry and the rudder stock were roughly parallel, this avoids creating a cam that acts to self center the rudder when the T-foil is lifting. The sections of MDF were glued and screwed together and the corners of the mould were rounded off with a router and sand paper to avoid any sharp corners, making lamination easier and giving the finished gantry a better look. The mould was then saturated with polyester resin and covered in a light glass cloth, this would help to seal the mould and allow a vacuum bag to be used when laminating the final piece. Polyester resin was used as it is cheap and would cure quickly, for a single use mould like this using Epoxy would be overkill. The mould was faired with car body filler, and then painted, and sanded through the grades to 400 grit wet and dry. Car body filler was again used as it allowed a quick fill, sand cycle and was cheap. With the mould complete it was coated in several layers of realise wax ready for use.

Building the Gantry

The shape of the gantry would provide much of the strength and most of the load would be through the edges and through the flanges so the laminate spec was devised to use unidirectional tape in the flanges with woven cloth to provide some toughness to the flanges and stop them from cracking along their length.

To spread the load and to create the bearing surface for the hinge I made up two glass tubes on the same diameter rod as was to be used for the hinge, these were then held in alignment with a long rod and attached to the mould before laminating the remainder. If I were to build the gantry again I would probably do this slightly differently as these pieces were awkward to hold in place, and added an extra two laminating steps to the construction.

From the inside of the mould the laminate spec is:

  • 200gsm +- 45 tight woven 50mm cloth tape along the centreline and extending up to the flanges
  • 300gsm Uni along full length flange
  • 300gsm +- 45 woven carbon over the whole gantry
  • 300gsm Uni short sections along flange, around hinge
  • 300gsm Uni along full length of flange
  • 200gsm 2×2 twill 6 small tapered layers around the rudder pin bearing point on the flanges
  • 200gsm 0-90 2×2 twill over the whole gantry

This was then covered with peel ply, bleed film, breather cloth and finally the vac bag, the vacuum pulled and allowed to cure. For more details on the vacuum bagging process please read the Vacuum Bagging article.

Once cured the Gantry was removed from the mould, some extra reinforcement was added around the hinges on the internal part of the gantry. Finally it was given a clean to get rid of traces of realise wax, lightly sanded and given a few coats of varnish to protect it from the UV, and mounted on the boat.

The new gantry including the hinge pins weighs 680g plus approx 100g added to the boat for the hinge points and the hard point, this can be compared with the removed parts of the old gantry weighing 1250g. Some of this weight reduction will be lost in the purchase system for the T-foil but it will still be lighter than adding a T-foil adjustment system to the existing gantry.


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Lessons Learned

  • Use Bigger mould Flanges – the flanges on the mould did not extend far enough past where I wanted to laminate, making it difficult to ensure that no fibres, peel-ply, release film, bleeder cloth or epoxy went over the tacky tape creating leaks.
  • Hinge simplification – I would attach the hinge bushes internally after laminating the gantry to reduce the number of steps in the lamination, the downside of this is that the alignment of the mould would be more critical.
  • Use more release agent - the laminate stuck to the mould in a couple of spots, making it difficult to release and damaging the mould.

Further Reading

tech/2682-gantry.txt · Last modified: 2013/06/25 15:55 (external edit)