One of boating’s most uncomfortable experiences is to be in a windless anchorage with a swell surging in from some meteorological disturbance below the horizon. Given some wind, a boat will tend to lie head into it. In calm conditions most boats prefer to align themselves beam-on to any wave action, rolling hideously as they do so.
Fortunately, a remedy is fairly easy to contrive. You need a flopper stopper, a somewhat exaggerated description for what should more correctly be termed ‘roll reducers’ or, more pompously, a ‘roll attenuation devices’.

There are various proprietary products on the market but here’s one you can fashion yourself. It’s not an original idea by any means but it works well as I can testify, having made a few myself. Quite apart from its effectiveness, they the considerable virtues of mechanical simplicity and a conveniently stowable shape. It comprises a triangular plate (see below) weighted in one corner and fitted with a triple bridle.

The principles behind its operation are illustrated below. Here’s how it works.

• When the boat rolls towards the flopper stopper, the weighted corner drops and the plate dives.
• Then, as the boat begins to roll upright again, the three-legged harness snaps the plate horizontal again and the increased drag dampens the roll. Couldn’t be simpler!

The construction details are obvious. Although I have heard of folk using plywood, adding an extra weight to counter the buoyancy, but ideally the plate should be non-buoyant. I used a sheet of glassfibre (about 8mm thick) but equally practicable would be aluminium or steel.

The flopper stopper should be sized to match the boat – a bit of a guess really. The plate shown here had sides 700mm in length before the corners were rounded off (to help protect the topsides). It worked very effectively on our previous 30 footer, but I think it could have been just a tad larger on 40ft Shindig.
 
Any kind of weight will do but it needs to be fairly heavy if the plate is to dive quickly. The one shown here on the top corner is a 5lb dive belt weight. And no, the splices are not unravelling -- the ends were left long and not heat-sealed to make handling easier

If possible the plate should be suspended below any surface wave action. The deeper the better. It helps to lead the attachment line back to the cockpit so depth adjustments can be optimised.

If using a spinnaker pole, the topping lift should be rigged to the pole’s end, not the usual mid-length point. This is because the vertical snatch loads can be considerable.