Investigating the 'Platex'

Out on Mike Golding's Ecover we see the effects of his Open 60's interceptor while designer Merf Owen tells us more

Friday September 19th 2008, Author: James Boyd, Location: United Kingdom
Two of the most interesting innovations in the Vendee Globe (and the Volvo Ocean Race) have been the use of go-faster devices from the power boat world. In the Volvo Ocean Race race the Russian boat with spray rails running along her topsides from the bow aft, while in the IMOCA Open 60 class the Farr boats have been fitted with similar but less prominent 'reaching strakes' and two - Gitana Eighty and Virbac-Paprec 2 have planing wedges, drop down tabs beneath their transom. Both the Owen Clarke designed Open 60s Ecover and Dee Caffari's Aviva are fitted with an alternative to planing wedges, known 'interceptors' (that Mike Golding refers to as the 'Playtex'), while Jonny Malbon's Artemis Ocean Racing has a similar device.

Designer Merf Owen has explained the interceptor concept to us in the past. To recap it is a vertical plate located beneath the hull just a few centimetres in from the transom that can be dropped down by 10-20mm or raised flush with the underside of the hull. On Artemis Ocean Racing it is fitted externally at the bottom of the transom.

Mike Golding has now had the opportunity to test it in anger- so when does he use it? "I use it pretty much everywhere. With a bit more testing I think you could use it partially, but we haven’t managed to test that. The good news is that it seems to be working. It is not a massive gain, but the beauty of it is that it gives you a boat with less rocker that can have more rocker. When we are hooing downwind in big seas you can take the Platex up, the bow pops up and away she goes."

While the Farr boats have planing wedges, Golding describes the Playtex as creating a planing wedge only with water. This has the effect of pushing the bow down. "It does work and it works greater at higher speed, but the reality is that at high speed the bow is digging in too much and the boat is being steered by the bow, so when you are driving the boat the helm gets very heavy. The cut off point is about 17 knots of boat speed. If you were hand steering you could push it to 20 but it would be hard work. It is faster with it down."

However while the Playtex has a noticable effect on the fore and aft trim, Golding says that it doesn't provide a noticable difference in speed, which is odd. Hooning downwind, the Playtex is lifted up and at speed the bow will pop up by as much as one foot and while this makes for a less bone shaking ride, it doesn't make the boat faster. "It is not a turbo. I don’t think you’d see a return on it around a short course. But It is a long term gain. There are little things you can do with it - some are intuitive but some are less clear."

When we sailed Ecover there was not enough breeze and boat speed to see its effect on the bow, but it was most noticable how having the Playtex down cleared up the stern wave. See the video of this here

Below designer Merfyn Owen provides a little more insight into the device:

The Playtex is an adaptation of a trim device called an interceptor first devised by the Russian Navy for high speed patrol craft and now in more general use on high speed catamaran ferries. So far as we know OCD are the first to have adapted this technology and fitted it to a race boat.

The Playtex contributes to a reduction in drag in a number of ways. Its effect manifests itself in bow down trim when deployed and bow up trim when not deployed. A yacht fitted with a Playtex device is able to achieve variable trim without the addition of ballast although the Playtex can be used in conjunction with ballast and equivalent drag benefits are still derived.

With variable trim modes a yacht is able to benefit as follows:
1) In light airs the hull’s rocker provides a yacht designed with an interceptor a smaller waterplane footprint and so produces lower wetted surface area and less drag.
2) In moderate airs when deployed the Playtex drives the bow down, lifts the stern with a corresponding reduction in hull resistance.
3) In heavier airs and in a seaway when surfing at high speed the interceptor can be raised whereby the yacht’s rocker induces a flow/pressure distribution to allow the bow to pop up. This reduces drag from the rudders and load on the pilots thus allowing the boat to be driven hard with a bow up (non-burying) attitude.

Yachts designed with insufficient rocker suffer, perform less well in light airs and have a tendency to bury their bows at speed and in a seaway, leading to control issues and a need to ‘de-power’ early. The Playtex system works both upright and heeled, with and without ballast and over a significant range of boat speeds and at all wind directions. Reductions in hull drag are significant, values typically being slightly above or below 10%. Gains in seconds per mile would range between 2% and 3%. The values given are conservative averages, drag reduction in some cases is as high as 16%.

In one third scale tank testing the effect of the interceptors can clearly be seen. In the following two photographs the pictures were taken with a camera mounted in the same position on the testing carriage. You can use a ruler to measure bow immersion, compare the angle of the sheer line or visibly by eye note how much more rudder is visible at the back of the boat.

The Playtex consists of a vertical carbon plate running between two graphite coated carbon bearing plates. It is split each side of the centerline and raised by a system of levers controlled by two separate morse cables. On Aviva and Ecover 3 the plates are buried within the thickness of the transom so as initially to make the system less obvious to the casual observer.

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