STABILISERS PART 1

Envoy is in Lefkas Marina for the northern hemisphere winter and we are home for the New Zealand summer returning to Lefkas during April.

This posting is based around an article I've written for Pacific MotorBoat magazine.

In pre-Envoy days boating around Auckland's Hauraki Gulf we owned several planing boats culminating in a 40 ft Oliver Royale GRP planing boat with twin Yanmar diesel shaft drives. We never then considered rolling to be an issue, however we realised very soon after buying our Nordhavn 46, Envoy, that full-displacement hulls are another matter, definitely requiring stabilisation to provide acceptable levels of comfort and safety when cruising moderate distances in beam seas higher than about one metre. I'm sure there are reasons for this that a boat designer could properly articulate. As much as I know is that in slight to moderate seas the flatter hull sections of a planing boat resist rolling to a greater extent than the rounded sections of a displacement boat. Also I have found that a fast boat on the plane seems to sit quite comfortably on beam waves without too much roll. On the negative side I understand that planing boats can roll dangerously in heavy beam seas and are more prone to capsize than ballasted displacement hulls like the Nordhavn. Additionally planing hulls pound when their flat sections hit waves, whereas the rounded sections of displacement hulls rise and fall more gracefully with greatly reduced noise and violent motion.

In our experience a planning boat like our previous 40 ft Oliver Royale doesn't roll under way to the extent that a displacement hull does

What follows isn't a technical treatise but comments based on cruising thousands of miles using Envoy's three stabilisation systems.

Stabilisers have been used for many years aboard commercial vessels but aboard pleasure craft only for about the last 40 years, considerably reducing rolling motion in beam seas, but with minimal effect on pitching caused by seas on the bow or stern.

Now some reader is bound to think “buy a catamaran and you won't have a problem” and I'd be the first to agree you've got a lesser problem, but we've met many folks who complain about their cat's motion both under way and at rest. Of course this is either better or worse depending on their design, as with monohulls.

Envoy at anchor with starboard stabiliser pole deployed with flopper-stopper to reduce roll from slight swell. The port pole can be seen in the vertical position

A web search shows many variations of stabilisers, but the mainstream systems fall into three basic types; passive, active and rotary.
We have no experience with rotary stabilisers, which are a more recent development utilising a flywheel contained in a vacuum chamber to provide a gyro effect, but they're reportedly highly effective in roll reduction both under way and at anchor with the advantages of having no protruding parts to cause speed-reducing drag or become fouled. They are able to be retrofitted either amidships or offset to one side, but have a significant investment cost. Presumably, if used for long periods such as overnight at anchor a generator would be needed to provide sufficient power – something we would not want to do.
Envoy has both passive and active stabilisers providing effective stabilisation under way and at anchor.

Envoy with both poles deployed with flopper stoppers to reduce roll at an exposed anchorage with ferry wakes

Next posting will discuss passive stabilisers in detail.