New Posts New Posts RSS Feed: Effects of tide on boat speed
  FAQ FAQ  Forum Search   Register Register  Login Login

Effects of tide on boat speed

 Post Reply Post Reply Page  123>
Author
Sussex Lad View Drop Down
Groupie
Groupie


Joined: 08 Jun 18
Online Status: Offline
Posts: 86
Post Options Post Options   Quote Sussex Lad Quote  Post ReplyReply Direct Link To This Post Topic: Effects of tide on boat speed
    Posted: 28 Oct 19 at 4:57pm
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.


Edited by Sussex Lad - 28 Oct 19 at 5:25pm
Back to Top
Rupert View Drop Down
Really should get out more
Really should get out more


Joined: 11 Aug 04
Location: Whitefriars sc
Online Status: Offline
Posts: 8377
Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post 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.
Firefly 2324, Lightning 130, Puffin 229, Minisail 3446
Back to Top
andymck View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 15 Dec 06
Location: Stamford
Online Status: Offline
Posts: 362
Post Options Post Options   Quote andymck Quote  Post ReplyReply Direct Link To This Post Posted: 31 Oct 19 at 10:38am
A very good explanation
So Itís 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


Andy Mck
Back to Top
GarethT View Drop Down
Really should get out more
Really should get out more
Avatar

Joined: 21 Apr 07
Online Status: Offline
Posts: 664
Post Options Post Options   Quote GarethT Quote  Post ReplyReply Direct Link To This Post 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.
Back to Top
Sussex Lad View Drop Down
Groupie
Groupie


Joined: 08 Jun 18
Online Status: Offline
Posts: 86
Post Options Post Options   Quote Sussex Lad Quote  Post ReplyReply Direct Link To This Post Posted: 31 Oct 19 at 4:40pm
Originally posted by andymck

A very good explanation
So Itís 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




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.


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.







Edited by Sussex Lad - 31 Oct 19 at 4:51pm
Back to Top
ohFFsake View Drop Down
Posting king
Posting king


Joined: 04 Sep 08
Location: United Kingdom
Online Status: Offline
Posts: 176
Post Options Post Options   Quote ohFFsake Quote  Post ReplyReply Direct Link To This Post 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.
Back to Top
eric_c View Drop Down
Groupie
Groupie


Joined: 21 Jan 18
Online Status: Offline
Posts: 55
Post Options Post Options   Quote eric_c Quote  Post ReplyReply Direct Link To This Post Posted: 18 Nov 19 at 10:32am
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.
Back to Top
Sussex Lad View Drop Down
Groupie
Groupie


Joined: 08 Jun 18
Online Status: Offline
Posts: 86
Post Options Post Options   Quote Sussex Lad Quote  Post ReplyReply Direct Link To This Post Posted: 18 Nov 19 at 4:34pm
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.


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


Originally posted by eric_c



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.
 

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.

 

Back to Top
tink View Drop Down
Far too distracted from work
Far too distracted from work


Joined: 23 Jan 16
Location: North East
Online Status: Offline
Posts: 421
Post Options Post Options   Quote tink Quote  Post ReplyReply Direct Link To This Post 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.
Tink
https://tinkboats.wordpress.com/

http://proasail.blogspot.com
Back to Top
ohFFsake View Drop Down
Posting king
Posting king


Joined: 04 Sep 08
Location: United Kingdom
Online Status: Offline
Posts: 176
Post Options Post Options   Quote ohFFsake Quote  Post ReplyReply Direct Link To This Post Posted: 18 Nov 19 at 4:59pm
Originally posted by tink

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.
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!
Back to Top
 Post Reply Post Reply Page  123>

Forum Jump Forum Permissions View Drop Down

Bulletin Board Software by Web Wiz Forums® version 9.665y
Copyright ©2001-2010 Web Wiz
Change your personal settings, or read our privacy policy