Keeping the boat (reasonably) stable
The Endeavour in Puffin Bay Cove after a rough run, paravanes still deployed. (Photo: Author)
This post continues the Glaciers of Kenai Fjords expedition, gathering photographic evidence of global warming in Alaska with researchers Bruce Molnia, Shawn Dilles, and Kim Angeli on board the Endeavour.
It was a long haul down Northwestern Fjord and then along the exposed southeastern side of Harris Peninsula. The wind and the seas had picked up, both from 90 degrees off our port side. The nearest land in that direction was the central Oregon coast about 1,200 miles away. The swells were not huge, but they were just the right amplitude and frequency to get the Endeavour rolling. When Patsy puts the coffee pot in the sink it’s time to deploy the paravanes.
Paravanes are the devices we use to stabilize the Endeavour and reduce the nausea-inducing side-to-side motion in heavy seas. All ocean-going power-driven vessels – especially those with efficient round hulls – which the Endeavour has – need some form of stabilization in some conditions. (Sailboats typically don’t because the wind against their sails and the water against their keel reduces rolling while driving the sailboat forward.) Paravanes don’t stop the roll – no stabilizer does that – but they dampen it, breaking up the oscillation. Imagine a truck with springs but no shock absorbers. Stabilizers are the shock absorbers.
A paravane rig consists of two long poles sticking out on either side of the boat at about a 45-degree angle, with weighted triangular paravanes – informally called “fish” – hanging by thin steel cables down into the water about ten feet under the surface. My fish are made out of plywood with lead ballast. They are only 600 square inches each, about the size of a king-size pizza, and weigh only 65 pounds. (I’ve used all-steel fish, but I feel they’re harder to handle and less effective for a given weight.) Underway, a slight downward angle of the fish creates pressure that, so far off the centerline of the boat, provides resistance every time the boat rolls, suppressing the roll. At speed, they make a noise like a Gregorian chant (on long passages I can make out words, I swear). When we don’t need them, we raise the poles to vertical and secure the fish on deck, saving drag and fuel.
Paravanes are common on salmon trollers and other fishing boats here in Alaska because they are effective, cheap, and easy to fix. I keep two spare fish and extra cable down in the engine room.
There are other ways to stabilize a boat.
Larger boats often have “active fin” stabilizers, which are fins that stick out of the hull under the water, one on each side, constantly being angled up or down by a motorized system to counteract each roll. When one side of the boat starts to go up, the fin on that side points down. Active fin systems work well when the boat is moving fast enough, but the gear is expensive, it takes up a lot of room below, none of it is any good unless the boat is moving, and the fins are always sticking out there – whether you need them or not – creating fuel-robbing drag and being vulnerable to damage. Also, active fin stabilizers are complex machines: if one breaks you’re probably not going to be able to fix it at sea, making for an uncomfortable ride home.
Some smaller high-tech boats have giant round gyroscopes bolted to the inside of the hull. They work spookily well, but they’re noisy, and heavy, and on bigger boats they have to be too large to be practical. On the Endeavour I would need two giant gyroscopes, each bigger than our engine. They would take up the entire companionway below and sound like Brobdingnagian beehives.
The simplest stabilizers are rolling chocks. These are long thin boards attached lengthwise down the hull on each side, about half the length of the boat, extending maybe a foot out from the hull. When the boat rolls, the extra keels bite into the water, slowing the roll. I once owned a 50-foot troller-style boat with rolling chocks and they seemed to work, up to a point. They’re fine for protected waters, if you don’t mind the extra drag.
The downside of paravanes is that someone (like me) has to go up onto the top deck in bouncy weather to deploy them. If I’m running the boat by myself, I throttle back to idle forward, use the trolling gear to slow our speed further – down to one knot or so, just enough for steerage – and point the bow into the swell, and engage the autopilot. Then I climb up the aft ladder to the top deck. If the autopilot has kept our bow into the waves, the rolling won’t be bad. Up near the smokestack, I release a pole on one side, lowering it from vertical by lines and blocks (pullies) to that 45-degree angle off the side, then I pick up the triangular fish (attached to the end of the pole with the cable) and drop it overboard. I watch for a moment, to make sure the fish is flying through the water as it should. Then I go over to the other side and go through the same steps over there. Back down on the lower deck I set short strut poles on each side, which keep the long poles in place in heavy seas.
Bringing the fish up is almost the reverse. We’ll be in calm water and I’ll bring the boat to a dead stop, not slow forward. No autopilot needed. Then I’ll go up top, put on gloves, pull a pole in upright with the lines, and then pull the heavy fish up by hand, running the cable over a roller. The boat really has to have no forward motion. Any water pressure makes it impossible to get the fish to the surface. Even so, this is hard work. I’ve decided that I’ll be officially old when I can no longer do it by myself. I’ll need to fabricate a motorized pinch-wheel device, something like crabbers use to pull their pots.
In practice, we rarely need the paravanes. It depends on the size of the seas and our heading. Within 45 degrees into or away from a swell (that’s half of the compass, in total) the boat won’t roll enough to bother. Even large beam swells can be fine, if they are spaced far enough apart or close enough together that they don’t reinforce the boat’s moment of roll. But when they’re abeam and the wrong frequency, they are essential. Like today.
Just before Ragged Island we bent our course slightly more west and entered McArthur Pass. In the swell shadow of the island the water became flat calm. We motored through the narrow, shallow passage, paravanes still down, and emerged on the other side in McCarty Fjord. We turned north, and three and a half miles under low clouds, we turned east into Moonlight Bay, and from Moonlight Bay we turned north into tiny, high-walled Midnight Cove. Another cirque. The sides were steep and high and the water was deep, well over 120 feet in the middle. We anchored at the far end in 60 feet of water, just off a waterfall plunging in with a roar from the forest above, and pulled up the paravanes.
It was only 1530 and there were five hours of daylight left, but the clouds were too heavy and low for more photos. We were in for the night.
— William Urschel