Thursday, July 22, 2010

Speedy Gonzalez

With the electric auto-pilot replacing the defective windvane, the
electric consumption on the boat has almost doubled to nearly 230
amp/hour per day, and with cloudy skies, we are having to run the
electric generator 2 and 1/2 hour per day. So I am hoping to get to
Rodriguez as quickly as we can so that we can have a replacement part
for the one that failed shipped to us. And I find myself watching the
speed to try and guess when we might arrive.
Well, talking speed on a boat (or an aircraft) can be sometime confusing.
First, there is the speed through the water. This one, everybody
understand, but not everybody knows that there is a speed limit (not
strictly enforced) for what is called displacement boats, i.e. boats
that don't plane with a hull going constantly through the water. In that
case, the maximum speed is decided by the length of the waterline, and
no matter how much sail you put on, you will not exceed it. Sometimes,
when you are surfing a big wave, you think you can, and indeed Papy
Jovial which has a hull speed of 7.73 knots once recorded 11.4 knots.
But then, after you have rushed down the wave, you have to climb up and
the average drops down to that famous hull speed. Anyway, with a speed
limit of 7.73 knots, we are quite happy when we can average more than 7
knots for the day, and this is what we did today with an average speed
through the water of 7.32 knots. Our average through the water since we
left the Cocos stands at 6.55 knots, which is wonderful.
But that speed through the water maybe very different from the speed you
achieve over the ground. This is because there are currents acting like
a conveyor belt, and it could work against you or for you. If you walked
on a moving walkway in an airport at 2.5 mph and the walkway is moving
at 1.5 mph, then the person walking next to you but outside of the
walkway will have to achieve 4.0 mph to stay with you. Since we left the
Cocos, we have actually covered over the ground 62 more nautical miles
than through the water. That is very pleasing. The bad news is that it
is over, and today Neptune, the god of the sea, only gave us 1 extra mile.
So now that you know your speed over the ground, you think that this is
it and you will be able to work out your time of arrival by dividing the
distance to your destination by your speed over the ground. Niet !
Here comes the VMG (Velocity made good), which takes into account the
fact that you are not going over the ground to your destination in a
straight line. Even if the wind allows you to take a direct course, you
will not go straight. The waves, the way the autopilot works, will make
you sail a sinusoidal course longer than the direct one. Today, in the
last 25 hours, we covered 184 nautical miles over the ground but we only
got 179 miles closer to our destination. And sometimes, you cannot set a
direct course, for example if the wind is directly in your face. You
then have to zig-zag (go tacking say the sailors) to receive the wind
alternatively on your starboard or port side with an angle big enough
that you sails can make you move forward. Let's say that the best you
can do is go 60 degrees from the wind (a convenient number since the
cosinus is 0.5), then when you go 6 miles over the ground, you only gain
3 miles towards your destination. The sailors used to say, when you go
tacking, it's twice the distance and three times the work, since you
have to handle the sails everytime you tack.
I apologize to the many sailors reading this blog for the simplification
and a language looking like intended for people who don't know much
about sailing. Well, quite a few of people keeping in touch with me
through the blog are not sailors and they might learn something they did
not know.
With all that, I have estimated, based on today's figures, that we will
arrive in Rodriguez on July 27 at 19:25 local time (GMT+4)

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