Mike Golding's secret weapon, part 2

We speak to designer Merfyn Owen about 'the Platex' inceptor device on Ecover III

Wednesday November 21st 2007, Author: James Boyd, Location: United Kingdom

Prior to the start of the Transat Jacques Vabre we wrote about one of the secret weapons fitted to Mike Golding's new Owen-Clarke designed Ecover III Open 60, namely what appeared to be an 'interceptor' as spotted by prying eyes when the boat was lifted out of the water in the Gosport.

Since then we have managed to corner designer Merfyn Owen, to find out about this innovation.

From the photos we saw of the boat during her inversion test in New Zealand, there appeared to be a slight wedge-shape to the underside of Ecover III's hull right at the transom. In fact this proved to be an optical illusion covering up something completely different, for as we saw in Le Havre prior to the start of the Transat Jacques Vabre, Ecover III is indeed fitted with an interceptor. Typically on power boats and ships, this device comprises a vertical plate, fitted to the transom, that drops down an inch or so (depending upon the size of the vessel) and is used to dampen pitching, but as Tom Weaver, newly moved over to the 'dark side' of the power boating world, informed us, is also in some cases used to improve lateral stability.

According to Owen, the interceptor fitted to Ecover III they have dubbed the 'Platex' (although in our opinion this sounds a little too much like a sex aid). "It has never been done on a sail boat before, so we came up with a different name for it. It is a little bit different because on a sailboat you have constant power and on a sail boat you haven’t."

Merfyn Owen

When it came to designing Ecover III the Platex was found to be a solution to one of the problems in inherent in designing a boat to work in a variety of conditions. "On these boats you need rocker," Owen expands. "Rocker reduces your wetted surface area in the light and in the South or when reaching in waves it allows the boat to pop up. It is great for the boat to pop up because you don’t have the bow sticking in and the pilots can move the boat around the ocean. When the bow is dug in, it is hard on the steering, you get water on the deck and you have to slow the boat down.

"Mike [Golding] was really fast in the South last time and part of the reason was because the boat had a bit of rocker. The downside of that is that in flat water and at speed you are giving away waterline length and you’d like to have the bow down, because that is quicker. So this Platex is a means of being able to do that - to be able to get the bow down, but to keep the boat having rocker so it is manoeuvrable in the South, while having slightly better wetted surface characteristics and less transom drag in light airs.

"Obviously putting a vertical plate down on the transom of your boat, perpendicular to the water line, doesn’t sound good - it is counter-intuitive, but with the bow coming down, you change the prismatic of the boat and all kinds of other hydrodynamic things go on, so if the trade off is positive, or even if it is even neutral, then you have a boat that will have the best of both worlds."

The Platex was an integral part of the design of Ecover III and in their R&D Owen-Clarke and Clay Oliver tank tested it at both 1/7th scale and ultimately at 1/3rd scale. However this only revealed limited information to the extent that Owen says they might not have fitted it had they not had the opportunity to test it at full scale thanks to their two boat test program with Dee Caffari's sistership, Aviva due for launch early next year.

Modern day offshore race boats are all about being able to change the configuration of one's vessel. We take for granted that we have different sized sails to adapt to wind strengths and points of sail, but boats like Open 60s, Minis and Volvo Open 70s have movable ballast be it in the form of canting keels or water ballast to alter their stability characteristics also to optimise their performance to sea and wind state. But there are downsides to both canting keels and water ballast - canting keel set-ups require additional foils to provide lift, no longer provided by the keel foil, while water ballast (originally used in a pre-canting keel world to offer lateral stability, and then with the introduction of canting keels, to alter fore and aft trim, and today a combination of lateral and fore and aft trim) required extra weight to be brought on board, sometimes increasing the displacement of the boat by up an extra 40%. Obviously there are times when this is an advantage, but other times when this extra weight simply slows the boat down. In addition to the reasons Merf Owen provides, this is potentially where the Platex could come into its own.

"It is a way of having a boat with two modes - bow down and bow up - without relying on water ballast, because ultimately when you look at the average wind speed in the Vendee Globe, the boats spend an awful lot of time without water ballast in."

The question remains - how to use it. The Platex develops lift at the back of the boat and therefore would be useful to drop the bow and to maximise waterline length upwind, but a fundamental requirement for a device like this to work is boat speed. Does the boat sail fast enough upwind for it to work? Owen thinks not. The main occasion it would be used seems to be in flatter water, where there is less requirement to lift the bow out, an occasion when the more powerful hull shapes with less rocker would be faster. While the Ecover team are playing their cards closely to their chest about the exact occasions they use it Owen does say that using the Platex downwind allows more aft water ballast to be loaded on for a given trim.

But there are a host of other advantages, many not obvious, many counter-intuitive. The Platex effects the way water comes off the transom. "You are changing the pressure and the boundary layer at the back of the boat, you are changing how the water comes off the back of the boat, so you are not getting the rooster tail effect and you are making a much cleaner break for the wave as it comes of the back of a boat - it is coming off a knife-edge, rather than a flat plate."

As ever in the fluids world, the effect is no only downstream, but the Platex also has effects upstream too, causing a pressure variation further forward up the hull. This may or may not have additional benefits. Previously we have seen projects attempting to alter dramatically the way water flows down a hull. Yves Parlier's record breaking 60ft catamaran Mediatis Region Aquitaine for example has a stepped hull like a sea plane (read more about this here) while Warren Luhrs' Open 60 from the 1980s, Hunter's Child, had a cunning fairing in the underside of her hull with holes in it to introduce bubbles beneath the hull on the basis that an aerated boundary layer (where the water flows slower than that around it, through being in contact with the hull) would have less drag.

Planing wedge on Gitana Eighty

Obviously direct comparisons can be made with the planing wedge arrangements fitted beneath the transoms of the Farr boat Paprec Virbac 2 and Gitana Eighty (read more about these here and here). These are very much more substantial structures but hope to achieve a very similar objective. While there remain a large number question marks over this in the sailing world, we can rest assured that this is no different in the motor boating and commercial shipping worlds, where many PhD thesis exist over the pros and cons of trim tabs (ie Virbac/Gitana-style planing wedges) versus interceptors ( Ecover's Platex).

For example on Ecover III the Platex is not fitted right on the transom (as it tends to be on motor boats and ships). This could affect the performance and there are many other factors - how wide should the Platex be, how much should it drop, is a square section the best, etc. "How thick do you make it? We’ve made it robust enough to be hit by a piece of wood so that it doesn’t break, but not so robust that it takes the transom off!" says Owen.

A major difference between the motor boat and sail boat worlds in terms of design, is that the latter heel. "On a motor boats, they don’t motor around heeled, and yet sailboats spend most of their time heeled - so is the system really going to work when the boat is heeled or is it going to work only when the boat is upright?" ponders Owen.

On Ecover the Platex is fitted a few centimetres in from the underside of transom and is a plate that drops down no more than 2-3cm across the entire breadth of the hull. It was located here rather than directly on the transom because here it could be attached to a bulkhead, says Owen and the whole deal is very light - "single figures in kilos". Thus should it prove to be a red herring, then the team haven't taken too much of hit.

Now AC90s should be reasonably quick boats too....

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