A windy day in Mobjack Bay

We have been anchored in various spots in Mobjack Bay for the last ten days or so. Quiet, peaceful and serene. That is, until the other day. We had a line of thunderstorms ride over us, bringing gusty winds touching 50 knots a couple of times. We were oriented fairly well as the winds were mostly out of the southwest.

The winds were strong enough to kick up some waterspouts as the gusts were funneled down Back Creek just west of us. With the winds came some pretty heavy rain, so between the rain and the waterspouts, visibility was poor.

Prior to the arrival of the storms I had been watching their advance on my radar app. While there was a NOAA warning for high winds and hail, I didn’t pay a lot of attention. (NOAA is always saying that..). I was looking at the intensity of the cells and it kinda looked like the strongest cells would pass both north and south of us so I wasn’t too worried as we made our way to the pilot house to watch the show.

As the skies darkened I took one last look at the radar update and saw the gap between the two cells disappear and nothing but red filled the screen in front of our location symbol on my iPad. At that point the wind arrived and I was beginning to think I had underestimated the intensity of this one. Probably should have started the engines as well. Nothing behind us to drag into, however.

When the wind arrived the gusts went from 20 knots to 50+ knots quickly and the direction changed 20-30 degrees with each gust. AT 85,000 lbs, we don’t usually get pushed around or heel much. But a couple of the gusts caught us broadside, pushing us over about 20 degrees and sending us off in a new direction.

The sensation when we reached the end of our chain was more a subtle deceleration than an abrupt snatch and I would ascribe this to the fact that we were anchored in deep mud and I would guess the last ten feet of chain was in the mud. I go into all this description because as trawler cruisers we prepare our boats to handle this sort of weather event but rarely see it. What follows is anecdotal, one person’s experience in one specific event, but possibly some general rules of thumb can be drawn from the lessons learned.

First off then, a bit about our anchorage. We chose the spot to provide some protection from the south. There were some tall trees on land but most of the terrain around here is low and the river beds slope gradually to the shore so one can’t anchor too close to land. In the photo below you can see how far out the docks run in order to find navigable water.

After the storm abated I went out in the dinghy and checked depths. Turns out we could have moved about 200′ closer to shore. When we arrived here a few days prior, the wind was out of the north so limited what we could do with what was then the lee shore. Once I knew a storm was coming we could have moved a lot closer to land and the protection of the trees.

Our tackle consists of a 55kg Rocna Vulcan anchor with 7/16″ HT chain. We use a 1.25″ nylon snubber that feeds from a deck hawse down to a bow eye at the waterline and then to the chain, connecting with a soft shackle. The effective length of the snubber is about 13′.

At this location we saw 10′ at high tide and had 70′ of chain out, measured from the bow eye, so 7:1. When we pulled anchor the next day I estimated that the anchor was sitting 3′-5′ below the depth read by the transducer. Based on our track before and after the event, I would estimate we dragged about 10′.

All that description reads like a pretty typical set up and anchoring routine. One question some might have is about the size of the snubber line, at 1.25″ way oversize for the job and unlikely to stretch at all. In the past I used lesser diameter line in order to provide stretch and reduce the violence of the snubbing event. I tried to estimate the loads, then subtract the strength degradation due to knots, wear and tear, the change in direction at the bow eye, etc. Then I tried to estimate what the “just right” amount of stretch should be, 10% would be helpful, 20% might weaken the line..? And what wind strength should I use in the equation?

In the end, I gave up on the goldilocks snubber, installed the mother of all snubbers and forgave stretchiness as a feature. In this one specific weather event, with this bottom material and this scope, it turned out I didn’t need any stretch. The boat rounded up at the end of each swing relatively smoothly. I can attribute that observation to either the weight of the chain or the mud it had to pull through, or both.

We anchor a lot in the Bahamas, usually in good sand. A typical set often looks like the photo below.

We see 30 knots in the Bahamas often. A couple of times I have gone down to observe the action of the anchor and chain in these conditions. What I find is that with 70′ of chain out, in 10′ of water, the last 20′ will stay on the seabed through most of the swings of the boat. At the end of the swing the direction of pull changes about 20 degrees and there will be a modest tug at the chain to anchor shackle. So at 30 knots, the chain maintains at least some catenary even at the end of a swing and the “snub” is gentle. The feeling of motion while on the boat is minor at 30 knots, more noticeable at 35-40 knots and at 50 knots the sensation is strong enough to suggest I need to hold onto something.

A note about all this for any sailors out there reading this. When at anchor in high winds on our sailboat we would actually sail from one swing to the next, accelerating right up until we came to the end of the chain. The arrival was abrupt, along with the change in direction. We also covered a lot more territory than we do with this trawler. On Privateer we decelerate as we approach the end of the chain and then fall off in the other direction. This action is easier on the chain and snubber than it was on the sailboat, so sailors might take all this with a grain of salt and stick to their stretchy snubbers.