Navigation - "30 degree rule"

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MikeB
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Navigation - "30 degree rule"

Post by MikeB »

Has anyone come across or used this? Something I came across recently - a simple rule of thumb to allow for tide on short crossings. For each knot of cross tide, allow a 30 degree offset to compensate.

Mike.

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Re: Navigation - "30 degree rule"

Post by TechnoEngineer »

Yes - it's a very useful rule of thumb. And works both in theory and practice (up to a couple of knots anyway).

I found it particularly useful for pulling into bays when being carried along by current past them.
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Re: Navigation - "30 degree rule"

Post by sleepybubble »

TechnoEngineer wrote:Yes - it's a very useful rule of thumb. And works both in theory and practice (up to a couple of knots anyway).

I found it particularly useful for pulling into bays when being carried along by current past them.
One presumes the rule does not work beyond 3 knots :)

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Re: Navigation - "30 degree rule"

Post by TechnoEngineer »

If your forward speed is 3 knots that is ;)
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Re: Navigation - "30 degree rule"

Post by PhilAyr »

Sounds good enough to me, but what would you define as a "short crossing" ?

Phil

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Re: Navigation - "30 degree rule"

Post by MikeB »

PhilAyr wrote:Sounds good enough to me, but what would you define as a "short crossing" ?

Phil
No idea - right back at ya - -

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Re: Navigation - "30 degree rule"

Post by PhilAyr »

MikeB wrote:
PhilAyr wrote:Sounds good enough to me, but what would you define as a "short crossing" ?

Phil
No idea - right back at ya - -
Okay point taken. Only one way to find out and that is suck it and see ! Next trip hopefully Ailsa Craig, or if I miss by 30 degrees N. Ireland !! ! ;-)

Phil

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Re: Navigation - "30 degree rule"

Post by ian johnston »

PhilAyr wrote:or if I miss by 30 degrees N. Ireland !! ! ;-)
Might not be a total disaster Phil; there's a few nice pubs I can think of on the Co Antrim coast! :o)

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Re: Navigation - "30 degree rule"

Post by mick m »

dos that work in the suthern hemisfear ?

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Re: Navigation - "30 degree rule"

Post by Jim »

mick m wrote:dos that work in the suthern hemisfear ?
No, you have to start at 60 and reduce by 30 degrees for each knot on account of the Coriolis effect....

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Re: Navigation - "30 degree rule"

Post by PhilAyr »

Jim wrote:
mick m wrote:dos that work in the suthern hemisfear ?
No, you have to start at 60 and reduce by 30 degrees for each knot on account of the Coriolis effect....
.... Nice one Jim. Now I am lost ! :-)
ian johnston wrote:
PhilAyr wrote:or if I miss by 30 degrees N. Ireland !! ! ;-)
Might not be a total disaster Phil; there's a few nice pubs I can think of on the Co Antrim coast! :o)

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Ian
...Now that would be a happy accident ! It's good to hear from you Ian. I hope we can fit in a few more trips before you put your uniform back on.

Kind Regards

Phil

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Re: Navigation - "30 degree rule"

Post by sleepybubble »

TechnoEngineer wrote:If your forward speed is 3 knots that is ;)

you know I was referring to the speed of the tide and the relative incremental increase in angle!

Actually this rule does have one further humorous application, if a tidal stream is at 6 knts then it suggest you just paddle off in the opposite direction, which is not completely inappropriate...

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Re: Navigation - "30 degree rule"

Post by Chris Bolton »

Actually this rule does have one further humorous application, if a tidal stream is at 6 knts then it suggest you just paddle off in the opposite direction, which is not completely inappropriate...
But it could cause problems if extended to a 12 knot tide.

Seriously, though, for a 3 knot paddling speed, unless my trigonometry is rubbish, it should be 20º per knot? That gives less than 5% error for tides up to 2 knots.

Chris

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Re: Navigation - "30 degree rule"

Post by MikeB »

Chris Bolton wrote: Seriously, though, for a 3 knot paddling speed, unless my trigonometry is rubbish, it should be 20º per knot? That gives less than 5% error for tides up to 2 knots.

Chris
I guess that's partly why I posed the question in the first place - I've come across it as 10º per knot based on a boat speed of 6 knots in a yachty context. A few rough and ready offset calculations give me 20º per knot as well and equates to the last time I worked it out properly and then applied it on the water. Mike.

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Re: Navigation - "30 degree rule"

Post by TechnoEngineer »

Yes the actual angles would be 19, 42, then 90 degrees. Increasing the steps to 30 degrees errs on the side of caution, and also avoids the potentially wrong idea of "60 degrees for 3 knots" that would be implied by a "20 degree rule".
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Re: Navigation - "30 degree rule"

Post by MikeB »

TechnoEngineer wrote:Yes the actual angles would be 19, 42, then 90 degrees. Increasing the steps to 30 degrees errs on the side of caution, and also avoids the potentially wrong idea of "60 degrees for 3 knots" that would be implied by a "20 degree rule".
Good point. Better to be up-tide than down-tide!

Especailly on those 3 knot tides where you've opted to offset at 90 degrees - - -

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Re: Navigation - "30 degree rule"

Post by RichJ »

Hi All,

We have found 20 degrees a consistently useful 'rule of thumb'.

Richard

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Re: Navigation - "30 degree rule"

Post by Mike Mayberry »

On another note, where on earth are some you guys finding the degrees symbol on your keyboard? :)

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Re: Navigation - "30 degree rule"

Post by Robbie C »

for the Degrees sign, hold down Alt & type 0176 which makes °

Have a look here: http://www.levtechinc.com/pdf/specchrs.PDF

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Re: Navigation - "30 degree rule"

Post by Dave Thomas »

All of this seems to assume that you know the tidal stream fairly precisely! In most cases, the most you will know is that, for example, the maximum rate somewhere in the flow will be nominally 3 knots on a typical spring tide. Any estimate of what is happening in a particular small area at a particular time on a particular day in the spring-neap cycle might easily be out by as much as + or - 50%! So any rule, while giving an initial course to aim for prior to picking and observing transits, is going to need to be fairly heavily pessimised in poor visibility if not slipping down-tide is critical.
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Re: Navigation - "30 degree rule"

Post by mick m »

Dusent al this relate to a constant boat speed, if you stop for a brake, slow up to whate for others ect.... wont that throw your bering out to ?

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Re: Navigation - "30 degree rule"

Post by Chris Bolton »

I think it relates to average speed, Mick. If you paddle for an hour at 4 knots and stop for 20min, it's the same as paddling at 3knots.

Provided, of course, that the tidal stream flow is constant, both in time and distance - which it never is.

Chris

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Re: Navigation - "30 degree rule"

Post by downunder »

On an Apple computer for special characters go to

Edit > Special Characters > Click on the insertion point > Select the character you want and double click

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Re: Navigation - "30 degree rule"

Post by UlrikSchou »

Or hit alt Q... at least on this danish mac...
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Re: Navigation - "30 degree rule"

Post by Jon Wood »

On UK Mac try alt 0 to get º.
(It doesn't work on the numeric extension, though)

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Re: Navigation - "30 degree rule"

Post by mick m »

Chris Bolton wrote:I think it relates to average speed, Mick. If you paddle for an hour at 4 knots and stop for 20min, it's the same as paddling at 3knots.

Provided, of course, that the tidal stream flow is constant, both in time and distance - which it never is.

Chris
but if your in a tidal flow and stop for 20min , how fare will you drift , I thort the princapal of the exasise was to hit a destination point ? you wold need to re avaluat your heding to get back on corse ?
I realy think a lot of theas nav tools are designd for power /sail boats that can keep a constant spead

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Re: Navigation - "30 degree rule"

Post by mick m »

with a litel time on my hands I did a bit of reserch , and found this artical by Paul Cafan , for those who dont know the name , he was the first to secum navigate Australia 25 years ago, hers the link http://www.seacanoe.org/Crosplan.htm

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Re: Navigation - "30 degree rule"

Post by MikeB »

mick m wrote:
Chris Bolton wrote:I think it relates to average speed, Mick. If you paddle for an hour at 4 knots and stop for 20min, it's the same as paddling at 3knots.

Provided, of course, that the tidal stream flow is constant, both in time and distance - which it never is.

Chris
but if your in a tidal flow and stop for 20min , how fare will you drift , I thort the princapal of the exasise was to hit a destination point ? you wold need to re avaluat your heding to get back on corse ?
I realy think a lot of theas nav tools are designd for power /sail boats that can keep a constant spead
Any calculation to allow for tide assumes a constant speed - if it's a long crossing though, factoring in stops and working to an average speed will produce the desired result. The "30 degree rule" is really just a simplification - for short (whatever they may be deemed to be) crossings. I'd suppose a short crossing in this context might be 2 to hours max?

Mike.

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Re: Navigation - "30 degree rule"

Post by PhilAyr »

mick m wrote:with a litel time on my hands I did a bit of reserch , and found this artical by Paul Cafan , for those who dont know the name , he was the first to secum navigate Australia 25 years ago, hers the link http://www.seacanoe.org/Crosplan.htm
Thanks for that Mick. You have just confirmed what I have always thought. Cross tide paddling on a ferry glide can be hard work and a waste of time if the tide is greater than half your boat speed. My average boat speed (without a sail ) is about 2.7 knots, ( and that's on a good day ;-) ) so any cross-tide greater than 1.5 knots is wasted effort. Much better to leave further up stream and let the tide help push you along, or do the half and half method with slack water somewhere in the middle of your crossing. That way the opposing forces cancel each other out and all you need to do is paddle your plotted course. And don't forget you can always use technology and let the GPS do it for you !!

I suppose the "30 degree rule" is a bit like the "rule of thirds", and I certainly wouldn't want to rely on that for some areas off the West Coast of Scotland !

Phil

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Re: Navigation - "30 degree rule"

Post by parafinn »

Hi,
I agree with what Phil Ayr said,
"I suppose the "30 degree rule" is a bit like the "rule of thirds", and I certainly wouldn't want to rely on that for some areas off the West Coast of Scotland !"

In over 40 yrs of paddling I have never heard of this 30 deg rule.

Cannot see the practical application of such an arbitary rule when you have; paddler's fitness, technique, boat speed, wind, direction, wind speed, tidal patterns, eddies, wave patterns, hull shape, deck profile, boat trim, paddle shape, paddle feather, paddle trim, skegs, rudders and everything else to factor in.

Am I the only one that points, paddles and instinctively adjusts according to transits?

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