Effects of tide on boat speed

David, I hope you don't mind me lifting a quote from the other thread It seemed like a good point at which to start a new topic. 

Originally posted by davidyacht Sussex Lad ... please can you outline what aspect of sea sailing skews the PY system?  I accept that tidal gates or changing tides can skew the results, but surely racing on the open sea in a constant tide is akin to the conveyor that forms the basis of the Lee bow discussion? I sail on a tidal estuary and I really find it hard to fault the PY system.

With the tide in some directions the negative effects on the slower boats is fairly obvious and most folk will notice.

With tide on the beam on a single tack leg boats clearly have to head into the tide slightly to overcome the conveyor belt effect and arrive at the mark in a pleasing fashion........The slower the boat the more they have to head into the tide, they have to point the boat in a different direction. They are effectively sailing in a different direction which is effectively longer in distance and time. If tide is a spring (had an 8.1 metre last month, no sailing though) and the wind is light the difference is astounding.

As I said, this example is quite clear and straightforward. However if we look at legs that are parallel with the tide and do some simple math it reveals a bit more. 

A simple example. Sailing there and back (on a beam reach say) between 2 marks one nautical mile apart. Two nautical miles in all.

Slow boat travelling at 4 knots

Fast boat travelling at 8 knots

There and Back with no tide.

Slow boat takes 30 mins overall.....15 there, 15 back

Fast boat takes 15 mins overall.....7.5 there, 7.5 back

straightforward and obvious, no loss for either

There (against) and back (with) 2 knots of tide 

slow boat takes 40 mins overall.....30 there, 10 back

Fast boat takes 16 mins overall.....10 there, 6 back

Tide Penalty for slow boat is 10 mins

Tide Penalty for fast boat is 1 min

I hope my maths is correct, not a strong point of mine,  no doubt I'll be corrected if wrong.

 Clearly there are other factors involved in the real world that may improve or worsen the situation, waves, wave direction. In my example there would also be slightly different apparent wind on each leg but IMO the example does give a very clear picture of the problems of tide when using a set of numbers that are heavily biased towards non-tidal racing. 

As others have said, boats are quite often chosen to suit the qualities of a particular location but there is still quite a large variation in boat speed in most handicap fleets, particularly at handicap opens.

Juniors have little choice on what classes they choose. Historically our topper sailors have faired very poorly despite being better sailors than some adults......not very encouraging for them.

Originally posted by GarethT At our club we race around high water, so are always against a foul tide on the way to finish, thus slower boats will be sailing against a stronger tide than faster boats.

Ouch.


For handicapping purposes It appears  tide seems like an insoluble problem.  Neap, spring, slack, different flows for different locations.

The one thing tide has in it's favour is that it's predictable and flow can be averaged out over one cycle of 28 days (or 27.???). If tidal PY calcs were attempted this wouldn't  be useful for single handicap races but might prove helpful for a club series?


The previous example I gave was originally part of some work I did for a club magazine a few years ago on a now dead PC, still have the hard drive though. At the time I did apply a tidal correction formula to the standard PY list and ran it alongside the normal results.....It never really gained any traction.

Originally posted by ohFFsake Also worth considering that if the general effect of the tide on the race course is favourable, it will tend to benefit slower boats more than faster boats.

Wind against tide will benefit some boats which are not particularly quick up wind.

If you are reasonably good at getting your boat upwind against chop, it will suit you.

Then going down the extended run, boats which surf better than others will benefit.

In the limit, a fast boat can make a lot of ground against the tide downwind!

The chop from wind-over-tide amplifies the effect of skill differences too.

An asy which allows you to take a big detour out of the tide can be a big help....

Tide certainly helps avoid every week being the same.