Reply
 
Thread Tools Search this Thread
 
Old 24 February 2008, 04:31   #11
RIBnet admin team
 
Poly's Avatar
 
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: 2 stroke YAM 20 HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 10,112
Quote:
Originally Posted by Cookee View Post
Hypothetical rubbish, there are so many other variables that it just provokes 100 different opinions - if a RIB and a yacht crossed the channel, the likelihood of either reaching their target is down to the quality of the navigation - BOTH would be affected in different ways by the wind and the tide!
Cookee - you are right that it is all hypothetical pondering - however I think the point you make is exactly the point that the article I read (I think it must be in a PBO I leant to someone as I can't find it) was trying to make --taking into account the effect of tide (and wind) is not purely the preserve of slow sailing yachtsmen.

Quote:
Originally Posted by BassBoy
On a point of pedantry!....... any yachty doing this length of journey would imediately realise that 18 hours of passage time would include about 3 tides of 6 hours each. If he/she were travelling north to south and tide was running east/west/east, (as in the English Chanel) then they would calculate the overall tidal effect east/west over that 18 hour period i.e. in this example the net result would be 6 hours east.
But that assumes you are leaving exactly when the tide turns. If you leave right in the middle of the tide then you get:

3 hours E
6 hours W
6 hours E
3 hours W

which will actually mean that in this hypothetical and theoretical situation the yacht can ignore tide when working out his course to steer (assuming he can make the whole trip in one straight line)
__________________
Poly is offline   Reply With Quote
Old 24 February 2008, 09:52   #12
Member
 
Doug Stormforce's Avatar
 
Country: UK - England
Town: Southampton
Boat name: Yoda & Obi Wan
Make: Valiant 750 & XS600
Length: 7m +
Engine: 150, 115 HP
MMSI: Various
Join Date: May 2004
Posts: 1,031
Quote:
Originally Posted by Polwart View Post
But that assumes you are leaving exactly when the tide turns. If you leave right in the middle of the tide then you get:

3 hours E
6 hours W
6 hours E
3 hours W

which will actually mean that in this hypothetical and theoretical situation the yacht can ignore tide when working out his course to steer (assuming he can make the whole trip in one straight line)
only if you assume the tide is constant across the Channel, which it is not. (i.e. tides of cherbourg peninsula considreably stronger than tides on S Coast England.
__________________

__________________
STORMFORCE COACHING
web - www.stormforce.biz
email - coaching@stormforce.biz
tel - 02380 231122
Doug Stormforce is offline   Reply With Quote
Old 24 February 2008, 13:22   #13
Member
 
Country: UK - England
Town: cookoo land
Boat name: tba
Make: ribcraft 595
Length: under 3m
Engine: Suzi 140
Join Date: Sep 2007
Posts: 316
Tidal vectors

OK, so it's come down to the nitty gritty of course to steer! We'v gone from the hypothetic to reality.

Get out your tidal stream atlas, look at each hour of tide and your estimated progress across the channel each hour. As Doug says, this will vary every hour. Make a note of each hours tides direction and speed. Plot each hour of that tide on your paper chart as a 'tidal vector' (don't forget your 3 chevrons if you want to be proper about it). Lay your compass course from the end of your tidal vectors to your destination. You are now much closer to an accurate course to steer. Don't forget your leeway.... most people use 5-10 degrees dependant on your point of sail or........ the 'apparent' wind direction on your RHIB for the duration of the passage. Don't forget to get the best/most updodate weather info you can for the 18 hour passage, and don't just rely on shipping or inshore forecasts, look at the synoptic/ surface pressure predictions and make up your own mind!
Don't forget 'Deviation' either, but then I guess there aren't many leisure ribsters that have a swung compass and a deviation card!


Alternatively, get a good book/DVD/training course and get your head round it that way!
__________________
BassBoy is offline   Reply With Quote
Old 24 February 2008, 13:22   #14
Member
 
Country: UK - England
Town: cookoo land
Boat name: tba
Make: ribcraft 595
Length: under 3m
Engine: Suzi 140
Join Date: Sep 2007
Posts: 316
Variation

Forgot to mention magnetic variation! Don't forget that either!!!!!!!!!!!!!!!!!!!!
__________________
BassBoy is offline   Reply With Quote
Old 24 February 2008, 16:51   #15
Member
 
SeaSkills's Avatar
 
Country: UK - Scotland
Town: Oban (mostly)
Make: Ribcraft, Humber,BWM
Length: 5m +
Engine: Outboards
Join Date: Jan 2008
Posts: 632
Send a message via Skype™ to SeaSkills
Quote:
Originally Posted by BassBoy View Post
Lay your compass course from the end of your tidal vectors to your destination.
Almost, but not quite right. You should lay off your course to a point along your ground track equivalent to the distance you would travel during the time you have used to calculate your tidal vectors.

But hey, let's not split hairs. That degree of accuracy gives you a course to steer that you could only steer with an auto-pilot anyway

__________________
SEASKILLS TRAINING
Web; www.seaskills.co.uk
Email; info@seaskills.co.uk
Tel; 07525 012 013
SeaSkills is offline   Reply With Quote
Old 25 February 2008, 01:12   #16
Member
 
Country: UK - England
Town: cookoo land
Boat name: tba
Make: ribcraft 595
Length: under 3m
Engine: Suzi 140
Join Date: Sep 2007
Posts: 316
zzzzzzzzzzzzzzzzz

SS

Yup.

BB

Like you, I'm nodding off with this thread but hope it's been useful nontheless.
__________________
BassBoy is offline   Reply With Quote
Old 25 February 2008, 07:01   #17
Member
 
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,610
Quote:
Originally Posted by BassBoy View Post
9D

On a point of pedantry!....... any yachty doing this length of journey
Errr
Pablo's original post just said "crossing the channel". If we're going to get pedantic, crossing the channel Plymouth to Roscoff is a slightly longer journey than Dover - Calais.

The original post also said (and I quote) "following a set compass direction".

My other post was basically pointing out that anyone who blindly follows a compass heading on a long (or short, depending on the weather) leg is not going to find themselves where they thought they would, unless they did a lot of maths first.

And having done the maths for one boat, it's going to be totally different for another - probably even if you compare two "random" 6m ribs!
__________________
9D280 is offline   Reply With Quote
Old 25 February 2008, 09:12   #18
Member
 
Country: France
Town: Nantes
Boat name: A l'Attaque
Make: Sea-Way
Length: 6m +
Engine: Not here yet
Join Date: Aug 2007
Posts: 455
I very much enjoyed reading all your messages, thank you.

I now have another question which will sort the seamen from the seaboys (or the inconscient from the safety aware). You've just come past the Needles on your way to Alderney and your GPS packs up, what do you do?

Do you :
A) Get your pocket GPS ou and put in your new way point
B) You've got everything on board to plan your route and you continue with your compas
C) Your GPS was only a backup and you where doing everything with a compass anyway
D) You turn around and go home quick before you lose sight of the coast

At my present state of experience and equipement I would probably choose option D but i'm interested in your replies.
__________________
Pablo is offline   Reply With Quote
Old 25 February 2008, 11:27   #19
Member
 
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,962
Quote:
Originally Posted by BassBoy View Post
....Get out your tidal stream atlas, look at each hour of tide and your estimated progress across the channel each hour. As Doug says, this will vary every hour. Make a note of each hours tides direction and speed. Plot each hour of that tide on your paper chart as a 'tidal vector' (don't forget your 3 chevrons if you want to be proper about it). Lay your compass course from the end of your tidal vectors to your destination. You are now much closer to an accurate course to steer. Don't forget your leeway.... most people use 5-10 degrees dependant on your point of sail or........ the 'apparent' wind direction on your RHIB for the duration of the passage. Don't forget to get the best/most updodate weather info you can for the 18 hour passage, and don't just rely on shipping or inshore forecasts, look at the synoptic/ surface pressure predictions and make up your own mind!
Don't forget 'Deviation' either, but then I guess there aren't many leisure ribsters that have a swung compass and a deviation card!


Alternatively, get a good book/DVD/training course and get your head round it that way!
Feck that. Isn't this why you have a rib?
Quote:
OK, so it's come down to the nitty gritty of course to steer! We'v gone from the hypothetic to reality.
That's better. Which way's the tide goin...west? That'll be a few miles off then. Which way's the wind blowin'...westerly F3? That'll be a few miles the other way. Just drive straight across. Given that it's only gonna take about 2minutes to correct a mile and I betcha a fiver you can't drive a straight line when the winds blowing, why bother with the hassle.
__________________
JW.
jwalker is offline   Reply With Quote
Old 25 February 2008, 12:25   #20
Member
 
Country: UK - England
Town: cookoo land
Boat name: tba
Make: ribcraft 595
Length: under 3m
Engine: Suzi 140
Join Date: Sep 2007
Posts: 316
Seaboy!

Pablo

All things being equal, I would press on using 'dead reckoning' but might make the decision to put into Cherbourg as it's a safer approach (commercial shipping asside). A few miles out going to Alderney (Bray Harbour)can mean the difference between the Swinge or the Alderney race!

IF however the viz was very bad, I'd head back to Yarmouth!
__________________

__________________
BassBoy is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 14:16.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.