Lee bowing

Tacking with the tidal flow from leeward. It pushes you to windward, as you'd expect, and the change in apparent wind lets you point higher.
 
Put simply imaging trying to cross the tidal flow sailing directly into the wind. If on one tack the tide takes you further away from where you want to be yet on the other tack the tide takes you back towards where you want to be. Time taken to cross the tide is roughly the same but you end up having crossed nearer to your destination.

The other effect is that dependant on the relative speeds of the tide and wind the combined effect moves the apparent wind effectively giving you lift if the tide is pushing the boat on its lee side.

I had trouble understanding this at first until in one race I wrongly thought it would be "quicker" crossing with the tide rather than angling the bows "against "the tide. How all the other boats proved me wrong by getting so far ahead of us!
 
THERE IS NO LEE-BOW EFFECT - Dave Perry

One of the most fascinating and timeless controversies in our sport is over what effect current has on
how we sail and race our boats. Beginning in early 1979, Peter Isler and I filled hours of time debating
the effects of current, and it wasn't until mid-1980 that he finally parted my clouds and shook me loose
from years of misconceptions and incorrect assumptions. Here then is my understanding of the effects
of current, substantiated by several of my more mathematically-clever friends.

Assuming that we're sailing in constant current direction and strength, No! As we've determined, the
direction and strength of the current created wind is the same no matter at what angle the boat is aiming
or at what speed it is moving. The presumption of the lee-bow effect is that if you are sailing directly into
the current you can pinch slightly, putting the current on your leeward bow, and the current will push you
up to weather. This is obviously false because the only direction the current can move you is in the
direction it is going (the stick on the river).

The presumption of those who believe that in current a boat will have a different apparent wind direction
and strength on opposite tacks, is that on one tack the boat will be slowed more by the current than on
the other. The extreme example is when port tack takes you right into the current, and starboard tack
takes you across it. The illusion is that on port tack it would seem that the boat is still going forward
toward the wind, but that on starboard the boat is being swept away from the wind by the current.
Therefore, the apparent winds must be different on the two tacks.

The fallacy here, though, is that the judgment of going toward the wind and being swept away are made
in reference to fixed objects such as the mark, an anchored boat, or land. In reality, both boats are being
affected equally by the current and the wind "sees" both boats in the same way. In other words, if you
were following the race in a motorboat and were in the ocean where you couldn't see any land for
reference, the boats would look identical on either tack, and in fact you would have no clue that there
even was current unless you knew from charts or perhaps from the surface condition of the water. Put
another way, if you're sailing on a boat with apparent wind strength and direction instruments, they'll
read the same on both tacks because the boat is affected in the same way by the current on either tacks
(the stick in the river again).

-- Excerpt from Winning in One-Designs by Dave Perry,
http://www.ussailing.org/member/library/wiodcurrent.htm

 
Excellent stuff Dylan
Back in the mists of time there was an article in "Yachts and Yachting" called The Lee Bow Myth . I didn't believe it at first, having been indoctrinated by the accumulated wisdom of the Sailing Club. However after some thought and computer modelling I agree , it is a total myth.
A yacht in a current stream is like being on moving carpet and two yachts of exactly equal performance take exactly the same time to beat upwind from A to B because over the whole piece they experience the same conditions. Its all relative you see.
Well done Dylan , but I bet there will still be loads of posts to the contrary.
 
Technically it isn't correct to say that there is no lee bow effect. The question was posed in the context of a channel crossing and if you time your crossing with the tide on your windward bow and I wait until it's on the leeward bow then I know who'll make the fastest crossing. That is the lee bow effect, you go faster and point higher on one tack than you do on the other. If you're talking about beating dead to windward, i.e. using both tacks equally, then that's a different kettle of fish.
The article is very much in the context of dinghy racing, where everyone goes around a course at the same time as each other, so that the assumption of a constant current direction and speed can apply.
 
I think the problem is that there are two different effects both of which are called "lee bowing". The one we are talking about only applies when you are crossing the tide and you expect the tide to turn during the crossing.

Anyone who starts talking about current in place of tide is talking about something completely different.

It is easy to show mathematically that lee bowing the tide works
 
Michael Reeve-Fowkes, The Yachtsman's Manual of tides.

P47, he believes in it and illustrates it graphically, which is good enough for me.

I have certainly found it to be a useful tactic on the few channel crossings I've done. (about 60 at the last count, a number of which were races in which I didn't come last!).

I don't think that scanning and pasting is legal, but if any one PMs me I'm sure something could be arranged.
 
I agree with Evadne. The boat feels a component of the wind caused by the tide moving the boat through the air, so you are effectively lifted when you lee bow.

It is very worthwhile planning to use the lee bow effect on any coastal or offshore passage that is cross-tide.

Of course a dinghy in a race you will be tacking more often than once per turn of the tide, so you'd be better off taking advatage of small windshifts, tidal back eddies, etc. etc. to work to windward.
 
Exactly so. In the context of crossing the Channel (as opposed to round a race course) and depending on the distance between start and finish, the boat speed and so on, it can make a huge difference. In our 'milk-run' case for example from Poole to Cherbourg where we passage plan on 9hrs for the 60mls from entrance to entrance, that gives a possible 6hrs of tide on one beam versus 3hrs on the other beam, or at a different time 4.5hrs of each or permutations between. If we want to go southbound across in a SW wind, planning to cross with 6hrs of west going and 3hrs of east going will give a course to steer that could be as much as 15degs more off the wind than the same trip taken with 3hrs of west versus 6hrs of east tide. That equates to a much better wind angle and higher boatspeed. more comfort and a shorter trip. To make the point even more apparent, some cruising boats could not point well enough to lay the required course in the worst of the tide options and would add insult to injury by needing to tack, an even slower trip.
 
You are all correct of course about a Channel crossing, but there is another type of occasion when lee-bowing can be significant, and that is when close-hauled and e.g. trying to clear a bend in a river. It is possible for an experienced sailor to pinch up a bit and gain more ground from the tide/current while another sailor allows himself to be set towards the bank and forced to tack.
 
Lets try again.

In the context of a channel crossing. S Coast to Cherbourg is 60mls. If wind is on the nose assuming 90 deg tacking angle you will sail at 45 deg to the real direction you want to go which is roughly 85mls. As Robin says with the tide most of the time in one direction say 6hrs at an average of 4kts thats 24 mls the tide can counteract the direction of tack so you sail into the tide for the longer period (part of the lee bow effect!)

In the context of the shift in apparent wind its best to exagerate the tide for analysis. Lets assume wind speed on the nose of 10kts and cross tide of 10kts. A stationary boat in the water will feel the apparent wind coming from 45 deg and by pointing straight across the tide it will be close hauled going directly to the point it wants to be. Now in reality the tide is never as strong as the wind but if the boat points into the tide it gains the benefit of whatever apparent wind shift the tide contribute to.

If anyone disagrees they can pick up the bar tab for the early arrivals at the scuttlebutt cherbourg run!! /forums/images/graemlins/grin.gif
 
You must log in or register to reply here.

Other threads that may be of interest

Sea Change
Rusty trailer- which zinc spray to use?
2
Replies
25
Views
876
ducked
D
A
Finding crew
Replies
2
Views
263
Phil_boat
P
P
which moisture meter?
Replies
19
Views
438
gjgm
G
P
Essential pre purchase or even pre survey checks
2
Replies
28
Views
912
Refueler
Refueler

Share this page

Top