True/App wind speed when log jammed

W

WayneS

Well-Known Member
Joined
21 Jan 2002
Messages
1,036
Location
Hampshire
Visit site
A recent passage with a fouled log highlighted the fact that our Raymarine "ST60 Wind" uses log speed to work out true wind speed rather than GPS boat speed.

Is this normal cos it seems a somewhat flawed principal to me.

I had a quick browse through the manuals and could see no way of switching it to use GPS speed data.


Any suggestions?

W
 
It has to know the boat speed as it only ever "sees" apparent wind. The manufacturer uses his own instruments to do the calculation, you may not have a GPS for all he knows....
 
GPS wouldn't give the same principle anyhow.... sailing at 5kts (boat speed) against 2kts tide, so SOG only 3kts. Apparent wind 20kts (sailing upwind), so true wind using the log 15kts, but would be 17kts using GPS speed.

The instrument needs to know how fast the boat is moving to calculate true wind speed... obviously it can only read apparent wind, the rest is open to calculation/interpretation.

I think you'll find that this is the only way to do it.... it may have problems, but can you suggest any better way??
 
Agree 100% that it needs a speed input but I would have though that it would be "better" to use a GPS speed if one was present.

I would have though that there should be a way of selecting which speed reading to use for the calculation.

I think I owe Raymarine a call to see what their take is.

Wayne
 
[ QUOTE ]
The instrument needs to know how fast the boat is moving to calculate true wind speed... obviously it can only read apparent wind, the rest is open to calculation/interpretation.

I think you'll find that this is the only way to do it.... it may have problems, but can you suggest any better way??

[/ QUOTE ]
It depends what you mean by "true wind speed".

Using the log information, the instruments may exaggerate a constant wind speed of F4 to become a F5 with the tide in one direction, or underestimate it as a F3 with the tide in the other direction. That may give an impression of the fact that the sailor 'feels' a stronger wind when the tide is carrying him into it, but is not the generally recognised way of describing wind force on the Beaufort scale. So strictly speaking, GPS info would give the "truer" wind speed, in the sense of the actual speed of the wind relative to the Earth's surface, which is what Met Office forecasts etc. are based on.
 
I have always understood that true wind speed is that which would be measured by an anemometer on a pole attached to the ground (ignoring issues of wind speed variation with altitude), rather than that which would be measured by an anemometer on a pole secured to a minimum wind resistance raft drifting on the tide.

If this is the case then using COG and SOG (from the GPS) to derive true wind from apparent is clearly more accurate than using speed through the water (from the log). This may not be the way it is normally done, but that does not make the normal method correct.
 
[ QUOTE ]
I have always understood that true wind speed is that which would be measured by an anemometer on a pole attached to the ground

[/ QUOTE ]
Completely agree with that Dave.

I think I'm starting to get myself in a pickle... I've sat down and worked it through properly, and can now see what you guys are saying... my example was flawed in the fact that the apparent wind speed would have been different if there wasn't the tide to contend with.

Sorry chaps for my mistake.
 
Discussed this with a top coach recently.

There are three types of wind, apparent wind, sailing wind and true wind.
The apparent wind is the wind blowing over the deck, the sailing wind is the wind that the boat would see if it was just drifting with the tide, and the true wind is what an anchored boat would see.
So yes, whilst a GPS derived speed would give an accurate true wind, it's not important. The sailing wind is what's important, both in direction for planning a beat to windward, or deciding if you will need to gybe when running, and in strength.
To see what I mean, take it to extremis. Imagine you have a 10 knot tide under you, and a 10 knot true breeze against you, and you're sailing to windward at 5 knots through the water. The apparent wind would be approximately 25 knots. If you used your GPS speed to calculate the true wind speed it would say "I'm doing 15 knots and the wind is 25 knots, therefore the true wind is 10 knots"
Is this of any importance to you? Not really, it's interesting for observing the effect that the tide is having on the wind, but it's a pretty meaningless number, you certainly couldn't set the amount of sail you normally do in 10 knots true.

But with water speed being used for the calculation the computer says "I have 25 knots of wind and I'm doing 5 knots, so the wind speed is 20 knots" Now that is number you could use to base your sail choice on, especially if you're actually running down wind and about to turn upwind. If you're true wind is reading 10 knots you might reasonably assume that you wouldn't need a reef, and be unpleasantly supprised when you suddenly have 25 knots apparent over the deck and your dinner on the floor.

Ok, so you're unlikely to have 10 knots of tide very often, but it still makes a difference.
 
[ QUOTE ]
Unless you are a motorboat then you do want true wind?

[/ QUOTE ]

Why?

It's the sailing wind that creates the wave conditions, as this actually is the wind flow over the surface of the water.
 
[ QUOTE ]
There are three types of wind, apparent wind, sailing wind and true wind.

[/ QUOTE ]Had an interesting(?) example of "sailing wind" a few seasons back sailing into Langstone Harbour on a flood spring tide with a very gentle southerly wind. As soon as we were in the channel the current matched the wind speed - several knots but dead calm and no steerage!
 
Exactly! And a true wind reading that's addamently insiting that there is still 5 knots of breeze is slightly irrelavent.

Thinking about it a bit more though, I guess the true wind would be useful (and I speak from a racing point of view here) info when you're contemplating what will happen to the breeze as the tide slackens in shallow water, or if the tide changes. But given the choice I'd take the sailing wind any day.
 
A common mistake made by race organisers is to set a start line square to the true wind that they feel on their anchored committee boat. If there is a strong tide along the start line then it won't be square to the true wind as felt by the racing boats and one end will be favoured over the other.
 
exactly! And this was the exact context that we were talking about this in the first place!

Interestingly, the effect is different when going down wind, as the tide then becomes larger in comparison to the apparent wind, so a true beat to the sailing wind will result in a one sided run back to the start line.
 
The raymarine E80 shows three types of wind speed (when GPS and log connected and wind sensors working)

Apparent
True
Ground

True is based on water speed of boat (from log)
Ground is based on ground speed (from gps i.e. sog)
 
So how do you work out the figures if you are in the Alderney Race on ebb tide, sailing NE with a NW wind. The tide can be up to 10 knots, but lets say 8 for now.

You are sailing at log speed 5 knots, with GPS SOG at 3 knots astern. (Not that I would leave the comfort of Guernsey until the tide turned, if going to Cherbourg).

What would give you the correct apparent and true wind? Log speed or GPS SOG?
 
Well beam winds are a little harder to do in your head, but the principal's the same.
The point is, you have the ground wind which you could measure using GPS speed and direction, coupled with a compas input.
However, this is really only of academic interest as you won't at any time be sailing in this wind. Especially with beam winds the direction will be massively different.
With log input you have the true wind that you will be sailing in. So this is a number and direction that you could use to plan your next sail change.
 
[ QUOTE ]
However, this is really only of academic interest as you won't at any time be sailing in this wind.

[/ QUOTE ]

I stood on the SE coast of Alderney and watched a sailing boat heading for Cap de la Hague from Guernsey, sailing astern.

Instead of a beam wind, explain a close hauled situation - as the above sailing boat had - until the tide turned.
 
I think it can be useful to have an idea of the 'ground wind' from the point of view of following what the weather is doing, and also what you might expect when the tide turns, or you round a headland etc.

Both in terms of change of windspeed and shift of wind direction (all other things being equal).
 
You must log in or register to reply here.

Share this page

Top