Use of PredictWind on passage – a question of seamanship. Written By David Hollier

To Go or Wait is usually the big question in any marina and it was certainly an issue for us on a recent passage from Whangarei in New Zealand to Minerva reef.

We use PredictWind to help us plan our passages.  It was late May, the weather in New Zealand was turning cold and we were keen to point our Leopard 46 catamaran (Geniet Lewe) northwards to escape the cold.  All the large maintenance tasks were complete and our crew for the passage (although a novice sailor) was keen. We tend to consult the 4 weather models offered by PredictWind to help us decide on an appropriate start date with the aim of sailing as much as possible and avoiding heavy weather.

In this case there was a low pressure system lurking around Fiji and the longer range forecasts were changing daily as to the potential path of this system.  This type of system can develop into fairly severe storms and we were not keen to encounter that mid passage. As the days went by and our planned departure date drew nearer, the grib files were all showing different predictions for this system.  When this happens, my weather radar goes on high alert as this means to me that the system is unpredictable and should be monitored carefully.

Our Customs pre-departure form was completed and submitted and our crew member was advised to be on standby.  The day prior to planned departure, the weather models were still at variance so we decided to delay and watch.  The following morning the forecast changed and the models came into alignment, with the passage plans being very similar. A call went out to our crew to join the boat as quickly as he could and we cleared customs that afternoon. There was a narrow weather window allowing us to get north of the system before it increased too much in strength The gust and CAPE charts offered as part of the PredictWind Offshore package also guide us here.  Often you will see an average wind strength of 25 Knots on the passage plan or gribs, but when you have a look at the gust chart this can paint quite a different picture. We try to avoid weather over 35 knots if we can but if it is going to this level or higher and is not thunderstorms, we may decide to weather it and have a strategy to run off if we need to, as long as we don’t need to be in it too long and the sea state is manageable.

The approach we use with the PredictWind passage plan is to adopt a rhumb line route unless the plan suggests there is a good reason to do something different.  In this case the plan initially was to follow the rhumb line and make decisions further north as we closed in on the low pressure system.

We set off from NZ with a light to moderate SE wind that gradually backed into the E and then the NE with a moderate sea state.  We expected as we approached the system to encounter some gusty squally conditions mainly from the NE. The worst we encountered was up to 38 knots.

Our daily routine is to download updated weather files every 12 hours and rerun the weather routing at each of these intervals.  As we approached the system, there was clearly going to be an optimum time and route through it. The weather routing models all suggested a route to the west of the rhumb line course that would take us nearer to the centre of the system.

We changed course and headed further west and this was around the time the squalls started so we were able to run off when stronger ones came through.  Fortunately it was daylight when we met the squally weather, so we could clearly see the squalls coming and reefed down in plenty of time.

Our progress matched the predicted passage plan so we encountered the system before it intensified as it moved south.  As we encountered stronger squalls we ran off to the west and ended up right in the middle of the system with very little wind for a while so we motored north for a few hours then as we came out the top of the system, a westerly filled in and we were able to resume our direct line course to Minerva.

The polars we use in PredictWind are those for a Leopard 47 and match our actual speeds quite well, but if the sea state is up or from ahead, I tend to reduce the polars by using the percentage adjustment factor, and using this method we are often surprised by just how accurate our predicted arrival time is.

We reached Minerva in four and a half days, with some motoring towards the end as the system moved south and the wind died.  There were two yachts in Minerva who had arrived late in the day before us and had encountered winds over 50 kts on their passage. One of their crew suffered a back injury after being thrown across the cabin and the other boat significant damage. They were not very complimentary about the well know meteorologist who had done their weather routing for them.  

We have used PredictWind over nearly 4 years and 30,000 nautical miles and firmly believe that it has increased the safety of our passages by ensuring we choose good weather windows for departure, and allows us to make good decisions based on up to date weather information and passage planning specifically relevant to our boat as we proceed through the passage.  This has proved itself many times and the comparison between our passage stories and those of other cruisers relying on other methods often results in stark contrasts between our passage stories and theirs, not to mention passage times.

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