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Sunday, February 26, 2017

STABILISERS PART 3

Envoy is in Lefkas Marina for the northern hemisphere winter while we're home for the New Zealand summer returning to Greece in April.

Our last Post discussed passive stabilisers.

This shot shows Laurie holding a bird, connected to a winch for lowering into the water. You can see the holes on top of the bird for adjusting the stabilising effect

Active stabilisers consist of a fin mounted below the waterline extending out each side of the hull using mostly hydraulics but in some systems electric motors to move them like aircraft ailerons to counteract rolling motion.

Envoy out of the water with stabilising fin visible

Envoy has hydraulically driven fins with the hydraulic pump powered by vee-belts from the main engine crankshaft and a 12 volt DC pump providing a constant flow of sea water to cool the hydraulic fluid. An electronic motion detector signals the hydraulics to move the fins to suit differing sea conditions with adjustment available via two thumb wheels in a pilot house control box.

The main advantages of this active system are that it's more effective than passive stabilisers providing 80-90 per cent roll reduction (compared to about 70 per cent), it deploys quickly with no physical effort required, it's easily adjustable using the electronic control system and causes negligible speed reduction.

However on the negative side the initial and ongoing cost of active stabilisers is expensive and during ten years we've spent several thousands of dollars replacing seals, sea water cooling pumps, vee-belts, filters, a hydraulic pump with its load adaptor, a servo valve and electronic components. Another issue is that they are highly specialised equipment and generally require trained and authorised engineers to provide service.
We're just about to make another significant investment to remove the fins and replace the through-hull seals, something which is required about every five years. This is a job the majority of owners including ourselves wouldn't attempt do themselves and special equipment is needed to remove the fins.

Set of through-hull seals

We're also going to make a modification so that when the system is not in use the fins hydraulically lock in the centre position. Currently the fins move at rest (which is noisy and potentially damaging) in all but the most sheltered of anchorages, so when necessary we secure them in the centred position through a fiddly process of using manual locking bolts. The actions of locking them and then unlocking them takes about 15 minutes.

An active stabilisation system also requires a significant investment in spare parts and we carry a spare sea water pump, hydraulic motor, load adaptor, set of vee belts and oil filter. The fins can potentially be fouled by flotsam or lines although this has only happened to us once in a marina. Although some active stabilisation systems can be used at anchor with a generator running to provide power ours cannot, in any case we wouldn't want to have an engine running overnight.

Partially hidden by the starboard side of the wheel is the stabilisers' control system

Using both systems we've occasionally found it necessary to alter course by about 30 degrees to “tack” and take the waves more on Envoy's bow when encountering closely-spaced, steep waves over about two and a half metres breaking directly on our beam.
A full displacement hull like Envoy definitely requires some form of stabilisation and the vast majority if not all Nordhavns have a system fitted. We're pleased to have both active and passive systems aboard Envoy but if restricted to one would undoubtedly select the passive system for its reliability, economy and ability to use at anchor.

Next Post will take a light-hearted look a the use of the title “Captain”.

Saturday, February 11, 2017

STABILISERS PART 2

Envoy is in Lefkas Marina for the northern hemisphere winter and we are home for the New Zealand summer, heading back in April.
Upcoming Posts - Stabilisers Part 3, Cruising in Auckland's Hauraki Gulf aboard 48ft motor yacht Moritz, Using the title Captain, Envoy's 2017 cruising plans.

Our last Posting introduced the subject of stabilisers, now we'll talk more about passive stabilisers.

A passive stabilisation system consist of two poles each about seven metres long mounted near amidships on the vessel's beam, which are stored vertically and when in use lowered to be deployed out from the vessel's side using a downhaul, topping lift and fore and aft guy-wires to keep them in position. They are mostly made from aluminium or steel though less commonly using laminated timber.

A large metal plate weighing about 25kg and shaped like a delta-winged aircraft, known as a “paravane”, “bird” (the term we use) or “fish” suspends from the end of the pole using a combination of chain and nylon line, the latter providing some spring to reduce shock loads. The front of the bird is weighted to make it “fly” through the water in a nose-down position approximately five metres below the surface and the top of the bird has several chain attachment points to adjust the bird's angle through the water. The further aft you attach the chain the more the bird tends to dive deeper and the more aggressive becomes the bird's stabilising action at the expense of increased drag.

This image shows the port pole and bird deployed with the downhaul, topping lift and fore and aft guy-wires

Due to the birds' weight and large awkward shape some form of winch is used to deploy and retrieve them (we use a light block and tackle) and with familiarity this process takes about 15 minutes.
When not in use we store the birds well out of the way on the boat deck, but if intending to use them we move them down to the cockpit so they're ready for immediate use.

Centre-right is a bird stored on Envoy's upper deck

The poles provide a distinctive fishing-boat-like appearance that won't suit everybody but certainly creates interest among other cruisers.

Stabiliser poles provide a distinctive appearance - love it or hate it! Port deployed, starboard raised

On some boats the poles are stored in a near horizontal position along the superstructure and this probably makes for a tidier appearance.

This system's big advantages are effectiveness - reducing roll by about 70 per cent, low initial and ongoing cost and reliability since there are no mechanical or moving parts - in the ten years we've owned Envoy no part of the system has needed replacement or maintenance. Just now we're having the birds epoxy-painted over the galavanising which is rusting slightly.

It's disadvantages are that it slows your boat down by about 10 per cent, there is some risk of the birds fouling flotsam or lines in the water (although ours never have) and the system cannot be rigged and de-rigged in shallow water or confined spaces.

Another huge benefit of this system is that by suspending a different type of metal plate the system provides highly effective stabilisation at anchor. Envoy's plates, known as “flopper-stoppers”, each consist of two flat plates of stainless steel plate joined together along one side with a hinge. When the boat rolls downward the hinge allows the plates to close together and drop rapidly and when the boat rolls upwards the hinge allows the plates to open and resist upward pressure. This system is extremely effective enabling us to anchor in places where roll or wakes would normally make it too uncomfortable. Flopper stoppers are comparatively light and easily deployed and retrieved by hand.
They can be used in depths over four metres.

Laurie holds a flopper-stopper

Here the port flopper-stopper has been deployed alongside a jetty to reduce the effects of a side-swell during a gale in Cephalonia


Next posting will look at ACTIVE stabilisers and our view on what is the "best" option.