Foils, Sails, Power, Drag
The sailing world is going foiling mad. Time alone will tell whether it’s a good thing or not, but what we can say is that it’s fun and fast, and very different in feeling. What is the same for the sailor is that we need to understand a far greater range of sail settings in order to optimise the performances of these boats.
We don’t know whether its a good thing or not that everything is evolving on the basis that the foils go THROUGH the water, rather than skim over it, because Vestas Sailrocket who shattered the world speed record operated on hydrofoils which did NOT lift tboat clear of the water in the same manner as the foils we see commonly used on Moth, Windsurfer and kite, and even the Nacra and AC boats. The foils Sailrocket used appear to be skimming (planing???) Small platforms that understand the principle that planing on bubbles across the surface of the water is better in terms of performance than a wing cutting through the water and kicking up turbulence. Remember the amount of wake (turbulence) you can see is all slowing the boat down. The AC foiling boats, both this generation and last, displace a massive firehose of water, which when minimised will see a huge increase in speed from the current 30 wish knots (this generation) up to 50 knots last generation 50s. If you are interested in the brief story see https://howtospendit.ft.com/vehicles/29573-the-foil-force and google Sailrocket technology.
The development of foiling has highlighted some aspects of sail design which have been a constant source of challenge over the years to designers, and also to sailors who have to put up with the less than perfect power of rigs to adapt to the massive range of speeds and situations that foiling boats are sailed in.
Planning means that the boat skims over the water rather than go through the water – in theory reducing the resistance of the water on the hull by a huge proportion of displacement mode. Equally, when a foiling boat gets airborne the resistance of the water is reduced by even more, and there is no need to have as much power to keep foiling. That is, until you fall off the plane, or dip off the foil, or hit a wave or other obstruction (plastic bag, fish etc) or slow for whatever reason.
Consider this: on a conventional planing boat, or windsurfer, the power needed to plane far exceeds the power needed to keep on the plane. It’s the same with cats – the power needed to fly a hull (and then increase speed dramatically) is far less than the power to keep it going. Certainly if you have windsurfing experience you know that you have to go a long way off the wind to start planing, and then come back up once you are planing. Going broader to the wind is one way of producing more power in the sail. If you pump you can again get planing, but check out a windsurfing starting line and look at the stars who can power their way onto the plane, pumping upwind, and compare them to the others who are nearly reaching to get enough power before planing. Sailboats are the same – the good sailors plane earlier than the average – like windsurfing but not as noticeable because ultimately the speed difference between boats that are not planing / not foiling and those that are dictates the gap (“delta”) that will develop whilst the non planers get going. This often dictates the outcome of races in planning classes.
Now we establish that the optimum shape of the rig to produce enough power to get foiling or planing is very different and a lot fuller than that needed to optimise speed once planing or foiling. Essentially the fuller the sail, the more drag it produces. When we sail at 5 knots the drag produced by the sail is negligent, because the air is light. When we sail at 50 knots we can feel the air becoming “heavier” – as our face is pulled around slightly by the force of it hitting us, and we can therefore surmise that clean, non turbulent airflow over the sail becomes critical. Whilst air drag is still not as significant as water drag (water being heavier than air), the reduction of water drag through foiling/planning/flying a hull means that proportionately air drag becomes more significant and important for us to understand if we are to explore the full potential of the speed that we can now achieve.
So how can we look at potential, and how can we look at power and rig settings to achieve something useful from a very “geeky” topic, and convert that theoretical difference in to speed?
For sure this challenge applies at the upper end of achievable speeds with skiffs, windsurfers and even older boats like the 505 and FD. On flat water those boats can achieve speeds where top end speed is dictated by sail drag. Therefore logic says they need power to get going, and sleekness to keep accelerating once the water resistance has decreased. As sailors how do we contribute to the solution of what essentially is a designer’s challenge?
If we bend the mast with rig tension to flatten the rig, we will certainly contribute to top speed. Then we have a challenge getting enough power to get onto the plane/foil if we flatten it too much, because power is produced by shape. We can have the fastest setup in the fleet, but it’s totally useless if we take ten seconds longer than everyone else to get going (planning/foiling). We therefore arrive at a point where we realise on a stayed boat the hardware (shrouds, lowers and cap) settings are a compromise between power to get going early and flatness to optimise top speed.
Windsurfers have developed adjustable outhauls and cunninghams (downhauls) to get as close as they can to optimum settings – of course on an unstayed rig that means that mast stiffness will dictate sail shape as you apply downhaul and outhaul. They are pretty advanced with design and ability to alter shape profoundly. If we look at skiff rigs, we have a lot of adjustments but not too much cunningham/outhaul and even kicker adjustment to make a huge difference to the draft of the sail.
This blog has attempted to highlight that we live in exciting times, but that the huge advances in boat design are still nowhere near optimised in terms of rig development for the modern hulls and foils. It also highlights how much we must keep up to speed with changing technology in order to race successfully and coach better. At Toplevel Sailing we do just that, and are happy to discuss technology as well as conventional sailing boats.