Planning for the Challenge
Meticulous planning is essential with any challenge of this nature, and as we have no engine, it will be vital to make sure that we get it absolutely right. There are a host of factors that we need to take into account when planning the route:- Route - avoiding shallows / rocks and other dangers
- Wind direction
- Wind speed
- Tide Times
- Tidal Flows
The Process
To start out, we plotted a simple route using software provided by the supplier of the GPS system (Garmin). On the map, we have plotted 24 waypoints along the route which takes us out of the thames, along the North coast of Kent, past Margate, Dover, Dungeness, Beachy Head and Finally Brighton.
By plotting the route, we ended up with the total distance (140nm) and the distance / bearing between each waypoint (see Fig 1)
Fig 1 - Plotting initial route
Entering this information into a spreadsheet allows us to then do some more calculations!
Entering this information into a spreadsheet allows us to then do some more calculations!
Estimating our VMG (Velocity Made Good)
Velocity made good is an important factor in determining any sailing route. Sailing boats can't travel directly into the wind, and Asymmetric boats like the tiger also don't like sailing directly downwind, so we need to factor into any calculation the effect that this is going to have on our speed between the waypoints. For example, if we need to sail directly into the wind, we will need to bear off to 45 degrees from the wind, and then zig-zag back again to make ground. Although the boat will maintain good speed doing this, the extra distance involved means that our actual speed between the waypoints is reduced dramatically.
Whilst out practicing, we have been measuring out speed relative to the wind with gps devices. When you download this information into software such as GPSActionReplay, it will give you a good idea of the speed that you can attain relative to the wind:
Fig 2 - Plotting Speed and VMG from practice sessions
You will see from the above graph that we really don't want to be sailing into, or directly against the wind (as we turn away from the wind, our speed increases)
The VMG data from the GPS helps us to build a model for what our VMG will be relative to the wind direction. Now here is where we have to make a bit of a punt, because the gps doesn't measure the VMG that accurately head to wind... so, we've built a model to estimate the %age effect over or under the average speed of the boat the angle we are sailing to the wind will produce. In other words, if our course is directly into the wind, our velocity made good we have estimated will be around 30% of the average boat speed. If we are on a broad reach, we should be sailing a good 20% faster than the average boat speed - this gives us this, revised graph:
Fig 3 - Point of Sail effect on VMG
We can look on the charts to see what the tide will be doing where and when, but first, we will need to know when we are likely to get to where we are trying to predict the tide for... let's move on a step for that first...
Do some tests with 8 sample wind directions and you can produce a diagram of time taken by wind direction (below)
Using our practice data to create a VMG model for the our boat
The VMG data from the GPS helps us to build a model for what our VMG will be relative to the wind direction. Now here is where we have to make a bit of a punt, because the gps doesn't measure the VMG that accurately head to wind... so, we've built a model to estimate the %age effect over or under the average speed of the boat the angle we are sailing to the wind will produce. In other words, if our course is directly into the wind, our velocity made good we have estimated will be around 30% of the average boat speed. If we are on a broad reach, we should be sailing a good 20% faster than the average boat speed - this gives us this, revised graph:
Fig 3 - Point of Sail effect on VMG
Next step - Tide?
All along the course we have set ourselves will be tides that will seriously affect our boat speed - we will want to start on the Thames at high tide so that the tide will help us to sail out of the bendy river - it would be nearly impossible to sail upstream on the Thames, so this part is essential. However, if we leave the Thames on the stroke of high tide, what will the tide be doing at Margate, where shallows and currents are also very strong, then at Dover - some of the most tidal waters in the world??We can look on the charts to see what the tide will be doing where and when, but first, we will need to know when we are likely to get to where we are trying to predict the tide for... let's move on a step for that first...
Timings
OK, we have our waypoints, the course bearings, the distances and a model for our VMG for each point of sail. Now we need to enter wind direction into the equation... With wind direction, we can predict the point of sail that will be required for each course, and with our handy model above, we can predict what effect that point of sail will have on our VMG. Enter an average speed guestimate based on our practice sessions and bingo, we can work out how long it will take us to get from waypoint to waypoint (forgetting tide for the moment) and ultimately what effect the wind direction will have on the time that we arrive in Brighton.Do some tests with 8 sample wind directions and you can produce a diagram of time taken by wind direction (below)
Fig 4 - Estimated total time taken (in minutes) by wind direction
Looking at the above, and remembering that I've not factored tides in for the moment, it would look like a WNW to NE or S or SSE is what we are looking for - a Northerly being the most favourable (as suspected) If we get a northerly wind and our boat speed average stays at around 10 knots, we should be drinking a beer in Brighton just before 1900 in the evening, however, even with the same boat speed, with a Westerly wind, we will be arriving into Brighton when the clubs are kicking out at 0330 in the morning, quite a difference!
That's how far I've got at the minute - if there is anyone reading this that has any comments about it at all, please feel free to email me on matti@sunsail.com - it's the first time I've done this, so if anyone has any advice, it would be gratefully accepted....
Matt
Tidal Factor
I've not got round to this yet (watch this space), but the next plan will be to work out the tidal flows at selected waypoints along the route for the day that we will be aiming for (01st September at the moment) and assuming that we leave on the dot of high tide, which happens to be 0515 in the morning at Woolwich Ferry. Once I have all of the flows and directions, I will factor this into each leg of the journey, adjusting the bearing (which affected Point of Sail) and the speed accordingly. A quick look at the tidal flows around the South East coast has shown that as long as we get to Margate before the tide turns, we should be able to go with the tide all the way around... of course we will need to be very sure that this is the case and that at no point we will be heading up wind against a strong tide.That's how far I've got at the minute - if there is anyone reading this that has any comments about it at all, please feel free to email me on matti@sunsail.com - it's the first time I've done this, so if anyone has any advice, it would be gratefully accepted....
Matt
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