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Effects of tide on boat speed |
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Sussex Lad
Far too distracted from work Joined: 08 Jun 18 Online Status: Offline Posts: 359 |
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Topic: Effects of tide on boat speed Posted: 28 Oct 19 at 4:57pm |
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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.
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.
Edited by Sussex Lad - 28 Oct 19 at 5:25pm |
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Rupert
Really should get out more Joined: 11 Aug 04 Location: Whitefriars sc Online Status: Offline Posts: 8956 |
Post Options Quote Reply Posted: 28 Oct 19 at 5:59pm | |
Makes sense to me.
Might explain why the Topper handicap has increased so much since I was racing them 15 or so years ago. Bit of a bandit on a lake, now, but probably still hard on the sea. An excellent example. |
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Firefly 2324, Puffin 229, Minisail 3446 Mirror 70686
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andymck
Far too distracted from work Joined: 15 Dec 06 Location: Stamford Online Status: Offline Posts: 397 |
Post Options Quote Reply Posted: 31 Oct 19 at 10:38am | |
A very good explanation
So It’s all about % change. And that an advantage going with the tide does not make up for the loss going against the tide if that is 50% of your speed, as you will spend much longer in the adverse tide. With a 20% difference, you are winning as there is less of an effect and you will spend a proportionally shorter time on the adverse leg and get more goes at the good leg. The only way to reduce the effect would be to have a shorter course for the slower boat, multiple marks. Or golf handicaps. A |
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Andy Mck
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GarethT
Really should get out more Joined: 21 Apr 07 Online Status: Offline Posts: 714 |
Post Options Quote Reply Posted: 31 Oct 19 at 1:24pm | |
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.
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Sussex Lad
Far too distracted from work Joined: 08 Jun 18 Online Status: Offline Posts: 359 |
Post Options Quote Reply Posted: 31 Oct 19 at 4:40pm | |
Yes, the slower the boat, the longer it's pushing the tide and the losses are not made up going the other way. In very light winds a slow boat aint going to make it whereas something a bit quicker will. The ultimate punishment for choosing a slow boat or being a junior. Sailors who have been at it for years in tide are vaguely aware that slow boats in tide are at a disadvantage but IMO most are not aware how big that disadvantage is.
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. Edited by Sussex Lad - 31 Oct 19 at 4:51pm |
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ohFFsake
Far too distracted from work Joined: 04 Sep 08 Location: United Kingdom Online Status: Offline Posts: 219 |
Post Options Quote Reply Posted: 18 Nov 19 at 8:54am | |
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.
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eric_c
Far too distracted from work Joined: 21 Jan 18 Online Status: Offline Posts: 382 |
Post Options Quote Reply Posted: 18 Nov 19 at 10:32am | |
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.
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Sussex Lad
Far too distracted from work Joined: 08 Jun 18 Online Status: Offline Posts: 359 |
Post Options Quote Reply Posted: 18 Nov 19 at 4:34pm | |
Estuary sailing? Simple there and back course with the tide turning at the "there" mark?.......yes, it happens at some clubs no doubt and the slow boats will get a bonus although I guess most of the race is sailed in slack tide? Thumbs up to them anyway.......I have no idea how many clubs This would effect. If different PY's could be calculated Coastal and Estuary would probably need to be segregated
Yes loads of variables including the varying flow rate on any given day (rule of twelfths) and yes, it does make every week different. Personally I much prefer coastal sailing for that very reason. |
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tink
Really should get out more Joined: 23 Jan 16 Location: North Hants Online Status: Offline Posts: 788 |
Post Options Quote Reply Posted: 18 Nov 19 at 4:54pm | |
Every twelve weeks must be the same though surely and it is predictable. The chaotic nature of a feature lined inland lake is truly never the same.
Just being mischievous. Each has its merits. |
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Tink
https://tinkboats.com http://proasail.blogspot.com |
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ohFFsake
Far too distracted from work Joined: 04 Sep 08 Location: United Kingdom Online Status: Offline Posts: 219 |
Post Options Quote Reply Posted: 18 Nov 19 at 4:59pm | |
Haha - was just about to say the same!
Weirdly, as a lake sailor I find I can deal with tidal rivers ok, as the effects of the tide can often be reasonably predictable and / or "read" with reference to the banks. The varying moving carpet of a bay defeats me! |
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