If you want to go faster, which would you choose: a new headsail or a fully faired and professionally finished set of foils?

If you currently have a silky smooth keel, which is symmetrical in section and blemish free, along with a rudder that’s as smooth and as slippery as a dorsal fin, then the headsail could well be the most effective option. But a walk around a typical marina at this time of year suggests that foils are often the most neglected part of the boat.

What lies below the waterline may be out of sight and out of mind, but the keel and rudder are crucial lifting surfaces and are every bit as important as the sail plan. You wouldn’t trust an aircraft with a pristine wing on one side and a battered and abused one on the other and a boat should be no different.

Underwater neglect doesn’t stop with the foils. From rough surface finishes to yawning gaps around saildrive legs and skin fittings that sit prouder than a cluster of giant limpets, there are plenty of areas that cause underwater drag.

Tidying up below the waterline

Tidying up what lies beneath the waterline can make a big difference to a boat’s pointing ability and straight-line speed.

If you take that off-season stroll around the boatyard, you will soon see what you’re up against. Look at that hot ship with its marble smooth surfaces and as many hard angles as you’ll find on a dolphin – is it any wonder that they often have the first windward mark rounding to themselves?

If you can assemble your crew for a few working weekends to tackle the underside, all will gain in the collective effort of making the boat more competitive. In reality, this is often difficult to achieve, but getting the job done professionally may not be as expensive as you might think. There are plenty of builders who can do it for you in a fraction of the time, to a greater level of accuracy and at roughly the price of a new headsail.

I talked to two UK South Coast racing boat experts, Hamble-based Richard Faulkner and Cowes-based David Herritage. Based on their suggestions, here are five key underwater areas that could change the performance of your boat.

1. Section symmetry

Checking that sections are perfectly symmetrical is second nature to those running grand-prix boats and high end racing one-designs such as TP52s and King 40s. But many production cruiser-racers can have asymmetrical keels and/or rudders that will produce uneven performances on each tack. Making sure the foil section is as the designer intended is a good starting point.

There are two basic options here: 1) Ask for section templates from the designer or, 2) simply test to see whether your foil is different on each side. Then pick the better section shape and re-profile the other side to match.

“It’s not that difficult a job, but it is time-consuming so people tend to employ an expert,” says Richard Faulkner. “Broadly speaking the process involves stripping all the old paint off and getting back to the original epoxy coating. From there the correct sections are built up using a micro-balloon filler, if required, and a high build epoxy paint.

“Given the potential for improvement it always amazes me that more people don’t do this, but I guess modern electronics and other go-faster gear is sexier.”

2. Perfect foil

“For boats that drysail we would use an epoxy paint system. Awlgrip’s 545 is popular with racing sailors, as is Durepox, both of which achieve a good finish,” says Faulkner. “Of the two we find that the Durepox tends to resist the yellow discolouring with age better.

“On foils we would rub back initially with 120-240 grade before moving to 400 grade and then finishing with 1,000 grade. For boats that will stay in the water, we use Nautix A4 antifouling as it is a good hard coating.”

One of the tricks of the trade, especially when wet sanding a white-painted finish, is to use a guide coat which can be sprayed on before the rub down. This allows you to see any low spots.

3. Weed cutters/skin fittings

The gap between the top of the rudder and the underside of the hull is not just an obvious trap for weed, but an area that is extremely hard to get at from aboard the boat.

Fitting a weed cutter is a simple and cheap way of minimising the chances of this area being fouled.

A short stainless blade with a sharp leading edge is fitted just ahead of the leading edge of the rudder. Some boats fit weed cutters ahead of the saildrive leg as well.

Ideally all skin fittings should be flush with the hull. Skin fittings that stand proud should be replaced with flush fittings, but if this isn’t possible then fittings should at least be faired in with filler to reduce drag.

4. Hull finish

For drysailed boats with no antifouling, the top end racers will rub down to 800-1000 grade although there are differences in opinion as to whether this level is really necessary. For antifouled surfaces, 400 grade is generally the practical limit.

Teflon-based antifouling tends to be favoured over the soft self-polishing types, as it is harder. In addition to Nautix A4, Tri-Lux and Interspeed, both from International, are also popular choices.

“McLube’s HullKote is a popular surface finish for underwater,” says Heritage.

5. Saildrive leg/prop blades

The popularity and simplicity of saildrive units has been driven largely by the ease with which engines can be installed. For raceboats they are also lighter than a conventional shaftdrive. But the gap between the strut and the hole in the hull through which it protrudes can create unnecessary drag.

“A simple composite fairing can be fitted to reduce the gap,” says Faulkner.

Prop blades should be kept smooth and clean with a simple ten-minute rub down using 180 grade. Anodes can also be tidied up in the same way.

Ballpark costs

Costs will vary depending on the size of the boat and its condition, but for a typical production racer-cruiser of 38-40ft approximate prices would be:

• Keel and rudder coated and fully faired £4,000
• Advanced foil work – including alloy section templates and keel weight checks £7,500
• Saildrive fairing £500
• Weed cutter £100
• Skin fitting fairings £350