Tidal Wind Vectors - Worth Calculating and Considering Big Time

Tide can be thought of as being in a big bucket with the gods carrying you along. You have to either take it at some point or kedge up. However tide in doing this also moves your boat to produce a relative wind component.

Ok, you sail in only 2 knts max tide, which gives a 2 knts max tide-wind in the opposite direction, so what, stay out the tide until you dash to the windward mark?

Here I will prove to you that sailing incorrectly relative to tide will roundly thrash you in the results list, even with only 2 knots of tide. Remember most of us in the sailing fraternity are sailing between 4 and 6.6 knots up wind, so a two knot vector on apparent wind is very decisive, just as also a 2 knt move in the right direction in the giant bucket will be.

Further to that then there is an easier rule of thumb which will make you think of not just the tide's stegnth on those bouys and stalks out there up the course, but also it's direction across the course.

Example in Theory

We then have two components which either help you or exacerbate your issue. Firstly the tide moving you in the bucket either towards to away from your desired destination relative to your heading. Then in a sailing boat you have the following tide wind effects on a cardinal considering sailing close hauled to a mark in even tide distribution: 

12 o'clock tide to course- decreasing apparent wind strength on the beat - both STB and PRT tacks equally affected

3 O'clock - on the beat, Port is lifted and tide helps destination COG. Reverse for STB

6 O'clock- increased apparent wind strength, equal on both tacks and better COG on the beat

9 O'clock- STB is lifted and tide motion helps a better COG towards the mark

On the beat, the forces then resolve to mean that you want the tide under your bow, coming from the lee side- hence the Lee Bow Effect. You want to sail less time on the other tack where it is on top of your winward side.

Classically the three o'clock an 6 o'clock cross-course tide on a windward-leeward are where we start, before you can then consider Rodney Pattison's excellent diagram of Poole Bay in " Tactics; sailing to Win".

How Big is the Loss from Being on the Windward Bow on a Cross Course Tide ?

In the absence of tide if we sail at 6knts boat speed on the beat against 10 knts wind say, then we get an AWA shift from the true wind of about 10' say. So for a wind a 000' we have an AWA of 010' and a compass heading for a well pointing keel boat of 050' on port tack.

(We will leave lee-way out of this theory.)

With the opposite of lee bow, a 270' tide cross course at 9 o'clock 2 knts, then this is how bad it gets:

TWA     000
AWAa   010
tidWA   090
AWAb   020
HEAD   050
COG     072

Calculation (without leeway) : True Wind AWA 000 compass, boat speed knocks this more on the nose to 010 on port. Tidal vector diagram ( graphic to be added) then makes for another 10' shift for an apparent wind of 020'

000+ 10' + 10' = AWA 020

We sail then at a heading on the bow/centre line of the boat of + 40'

AWA + 40 = 060 heading magnetic

BUT the tide is also moving us to the right hand side of the course, and on my diagram this looks like more than 10' so take it as 12':

COG = 060 + 12' = 072

Seventy two degrees on the compass, meaning you are sailing 72' off the true wind in effect not making fast progress at all on VMG reduced by 22' which equates on my vector diagram to a loss of 1.5 knts VMG

This is 3ms so in a boat of say 10 meters, every 4 seconds or less you loose a whole boat legnth in VMG.

Now on sailing in the abscence of tide you would hop around the boat if you saw the opposite tack fleet were going faster given you wanted to stay on Port in this case and going 1.5knts

Now the one thing that is different here between port and starboard boats is that the STB boats have the tide on their lee bow:

They then get a lift - the tide wind vector plot remains the same, but you point the boat the opposite direction towards the new AWA vector with tide factored in.

A STB boat sailing at a nominal COG then of 320' on the compass, ends up doing a very lifted COG with tide wind and tide motion in account of 340. This means they get the opposite - lifted wind and a positive vector nearer to the mark due to the tide-giant-bucket resolving out to sailing a course at only 20' to the wind.

So you see the losses and gains are amplified by this : now at some point you HAVE to tack over to the unfavoured tack, so on a W-L course you eventually must do this to lay the mark . However you then spend much more of your time on the favoured tack and see how far it is taking you up.

In Practice

This situation of wind at 90' to tide is quite rare in many places in fact. In the UK our estuarial valleys tend to train the wind, and of course train the tide completely such that we tend to get wind against tide or with tide in our channels, river mouths and lochs.

Given a Vee shaped estuary with tide at 270 W and wind at 270 W, then we know to beat up in the shallower water at the sides were there is weaker current speed against us, and then come out under spinnaker in mid channel.

However now we know that we are losing a boat legnth VMG on the wrong tack EVERY FOUR SECONDS for this case in argument, then if there is a tidal bias of 45' degrees over the W-L course, which is a very likely scenario on the Clyde for example or around the inside of the Isle of Wight, then we may be loosing a boat length to the competition still every 8 SECONDS!  ( I will need to calculate this out)

Given we have a tide at 045 and a wind at 000, then I would need to do some diagrams to see if this remains a relative doubling of time per lost boat length or not.

Therefore, we need to just do a sanity test before we race by sailing up the course : look up not just tidal strength but direction of the breadth of the W-L  course and where the shallowest water is. You then want to put the lee side of your bow against the tide more than the windward side

You had better then have a very good reason for going over to the tide more on your windward bow, leaving this to well up the cone towards the windward mark when you are likely to be choosing slot for the layline; or alternatively if you know of a back eddy giving you leebow and positive COG or an extended area of shallow water where your eventual "wrong tack" will be penalised less


Further Thoughts and Reading 

So far we have looked at the theoretical model with tide steady and evenly distributed over the course in strength and direction. This is not a bad state of affairs for a dinghy event with perhaps only a maximum breadth from the laylines at half way up the course of 600-800m, laid out over an even seabed.

We then have to look at tidal sheer over shallow water as the depth decreases towards the shore, or over bars or sandbanks etc and bends in the tide round headlands, and finally back eddys or major diversions in bays. Also then you have current from rivers to think about which affects actually a lot of sailors because we like safe river estuaries to sail in!

On top of this we maybe need to sail on a biased start line, on a biased W_L course or at times when the tide is turning over distance raes.

Reading to consider are as mention Rodney Pattison's almost biblical testament with the Poole Bay example, but for the basic theory you could start with " Sail Smart" By Mark Chisnell , both of which are on Amazon and often in a good sailing chandlery near you.