True Wind without speed transducer

[wingssail]

Originally Posted by rslifkin

I have always wondered why most sailors focus heavily on speed through water and ignore the GPS speed over ground readout. Coming from a powerboating background, in my mind, what really matters in the end is "how fast am I moving and in what direction?" So SoG and CoG from the GPS tell me that. It doesn't really matter if I'm moving a different direction than I'm pointing, or if I'm moving over ground slower or faster than through the water. That's cool to know occasionally, but is of no use for getting me to my destination.

Answer, Part 1.

Do you wonder why?

Sailboat racers led the way on this, beginning to use True Wind Speed, True Wind Angle, and Speed Through the Water in the 70's to 80's.

The reason is because to get the most out of your boat you need to understand how the sails and hull and keel and rudder are working to maximize your boat speed through the water. If you are fast through the water you will be fast over the ground. We have benchmarks (polar diagrams) and even more important, we are watching our boat speed relative to the true wind, true wind speed and the true wind angle, and the leeway, etc, minute by minute.

We must eliminate as many outside factors as possible, including the effect of current. When you look only at Sog and Cog, you have combined the effects of many things and it is hard to see what each factor is contributing. We need to separate out the components and optimize each one separately.

Current is the responsibility of the tactician and navigator.

Boat speed is the responsibility of the helmsman and trimmers.

So the helm and the trimmers concentrate on boat speed, and for that they need the instrumentation (or a very good seat of the pants feel).

The tactician and navigator watch the competition and where the boat is on the race course and how we are being affected by currents and land masses, etc, and for that they need GPS and current information.

What does any of the mean to a cruising sailor? Well, for most of us, nothing. We motor along merrily drinking a beer with little regard for whether or not we could be doing it faster. Who cares? we're cruising.

But for those who get a thrill out of knowing that they are sailing well, doing the best that they can given the boat and the conditions, we can utilize all of the information available, and strive to do, with just ourselves, what the racer does with a full crew.

Look, sailing is a sport, it is not just a passenger ride.

[wingssail]

Originally Posted by rslifkin

I have always wondered why most sailors focus heavily on speed through water and ignore the GPS speed over ground readout. Coming from a powerboating background, in my mind, what really matters in the end is "how fast am I moving and in what direction?" So SoG and CoG from the GPS tell me that. It doesn't really matter if I'm moving a different direction than I'm pointing, or if I'm moving over ground slower or faster than through the water. That's cool to know occasionally, but is of no use for getting me to my destination.

Answer, Part 2.

You are thinking that you only need to look at SoG and CoG. But if you are interested in a fast transit from one point to the other you can use some more information.

For example, lets say that you know that with a full fuel load, at 7000rpm, you should be able to go 44 mph. You look at your SoG and you see 41 mph.

But is that due to current or lack of boat speed? With good instrumentation you see that your speed through the water is not 44mph but 42mph. You ask why. Looking around you you see that the outdrives are not trimmed where you usually have them. So you make a change. The speed inches up to 43mph, but you are still a bit slow. Finally you decide that the fuel load was not taken on evenly and you trim tab down a little on one side and the boat speed comes up to 44mph

So now you are hitting 44mph and 43 Sog and you know the difference is 1 knot of current against you.

Most of your buddies wouldn't care. By now they are on their third brewski but you know you are doing really well and you are satisfied.

Or not. But if you care, you need those instruments.

[rslifkin]

Good point on the current vs speed thing. I tend to note expected wind and current before heading out, so I already have an idea of whether I'm expecting to be a little bit slow or fast.



As far as things like trim (sails or trim tabs on a powerboat), in my mind, it's always a matter of adjust to find maximum speed regardless of which speed you're looking at.