A Fast Offshore Cruiser - 11. Foil design.

The foil arrangement will be a fin keel with a lead bulb and the rudder will be a single balanced spade. This arrangement has been chosen for efficiency reasons and that with modern materials and design they can be strong and safe enough to stand up to the rigours of cruising.

The foils will have their sections exclusively from the NACA series. The properties are very well known for these sections and therefore the performance of them is easy to predict. The dimensions and data are also widely available.

For the keel foil section there is a choice between the 4 and 6 digit NACA series’. The hydro-dynamics of each series can be summed up simply: -

4 digit: lower form drag at similar higher angles of attack (>» 4° ).

6 digit: lower form drag at similar lower angles of attack (<» 4° ).

From this it can be concluded that a 4-digit keel would not require to have as large a keel plan-form area as a 6-digit keel, as it could run at higher angles of attack, producing larger induced drags, with a reduction in viscous drag.

The choice has been made on a practical front of the boat being used for cruising. A larger keel should be stronger in impact situations, allows better distribution of loads into the hull and better arrangement of the keel bolts. So a foil of 6-digit sections, specifically 64 series, and a larger plan-form area will be used.

Bulb design.

The bulb has the job of lowering the centre of gravity of the vessel as low as possible, so the rig forces can be stood up to. There are various designs of bulbs that reduce cross tip flow or viscous drag. Having the mass of ballast lower, producing an increased righting moment, will outweighed these improvements by sailing the boat slightly flatter which will produce gains in resistance from other areas of the boat. To produce this an elephant foot type bulb has been used. This has increased WSA but the sharp corners will reduce cross tip flow and lower the C of G.

Starting at the top there is a keel stub. This will be made from laminated wood and is there to distribute the load from the keel onto the hull better, by increasing the contact area.

The foil, so long a price is not a constraint, will be constructed from stainless steel sheet. A general description of the foil is that it will have plating walls with closely spaced bulkheads inside to keep the shape. The leading edge will be rolled plate over a longitudinal plate, on the centre line, with permanent horizontal formers.

Assuming the keel is a cantilever with a point load at the tip will produce the foil scantlings.

As the foil is a complicated shape to calculate the section modulus (Z) it has been simplified to 2 flat, of the same t as the foil wall plates separated by half the maximum width of the keel root, e.g. 0.5*300=150mm. This will over build the foil as the majority of the material is further out form the centre line.

As can be seen from the calculations in Appendix A.8 the foil has been over built and will be left as such as this will compensate for any twisting or impact loading applied to the keel that are not been calculated here.

The bulb is cast lead. A shoe will be fitted to the sole to stop damage.

The keel bolts will start in the bulb with cross plates connecting the transversely opposite bolts to grip the lead. The bolts will pass through the foil and through the backbone of the hull to a large wooden backing pad. The bolts are not permanently fixed to the foil so this will be kept in compression when the boat starts heeling.

Arrangement.

The rudder is to have a single balanced spade arrangement. The other arrangements considered included twin rudders and various arrangements of skegs. The twin rudder arrangement, which has become so fashionable recently, thanks to the "Open" classes, was rejected because they only really come into there own on boats with wide sterns which roll the hull forward as they heel. Skegs were rejected on the grounds that they could be too strong in an impact situation. If the impact is great enough to bend/break a rudder blade what could it do to a skeg? The worst situation would be the skeg ripping out and causing serious damage. To protect the rudder the tip will be sacrificial.

Design.

The rudder stock will be perpendicular to the hull. This design will be easier to construct as the lower bearing is parallel with the hull, therefore not requiring a wedge between the hull and rudder root.

The alternative of a vertical stock would not produce a more hydro-dynamically effective rudder, as the rocker is quite shallow in this hull.

The rudder blade has a quadrilateral profile, rather than an elliptical profile. This profile was chosen to give the helm greater control in certain conditions. An elliptical rudder will stall across its entire span due to the constant down wash along the span. The quadrilateral rudder will be loaded up towards the tip therefore will stall there first, giving the helm notice of this by increased vibration.

The sections of the rudder will be of NACA 4 digit. This series are very good for rudders for the following reasons, 1. High stall angles due to large leading edge radius, rudders need to operate at high angles of attack, 2. High volume in the forward area, for the rudder stock, 3. At stall the CofP moves aft, giving the helmsman feel.

The rudder and stock like the rest of the structure will be built to comply with the ABS rule requirements.

In keeping with the rest of the boat the rudder blade will be made from wood. Vertical layers of epoxy laminated veneer or plywood, formed around the stock and frame, shaped to the desired sections will form the blade. Finished with layers of CSM and gelcoat will give resistance to wear and tear. This will create an interesting looking blade that will also be very resistant to bending. To produce the sacrificial tip the stock does not extend the complete span of the blade, stopping 20% up. So the tip will break off cleanly a small groove will run horizontally around the blade which will act as a stress raiser and starting the clean crack.

The stock will be of solid metal, rather than composites, because of cost and it would unnecessary to any different. The metal will be stainless steel due to it being in contact with the water. To stop the blade twisting over the stock 3 arms are slotted and welded through the stock, these arms are connected at their outer ends via a vertical strip will provide extra grip for the blade.

The calculations for the stock are in Appendix A.7