David Burch had a formula in his sea kayak navigation book for estimating duration of weak current (up to 0.5 kt). If I remember correctly, it read like this:
Duration of slow water in minutes =
60 / maximum speed in knots
For example, if max speed of the ebb current is 3 kts, then the current will run at about 0.5 kt for about 20 minutes before low tide.
Burch gave the formula as a rule of thumb. How well does it hold up compared with what you've paddled?
Haven't read, but one issue I see already is trying to compare based on low tide. At least in San Francisco Bay, the slack current when it changes from ebb to flood is about an hour after the actual low tide.
Slack tidal current at a given location needs to be assumed. If the current is ebbing at your current location, it is not slack low tide at your current location, regardless of whether or not it's low tide a mile away on the beach. These situations in and of themselves would have no bearing on the validity of this rule of thumb. You simply measure the current for 20 minutes either side of slack at your given location. The rule of thumb can't be applied without knowing the schedule and speed of the tidal flows in the location you're paddling.
I've never measured this for myself, so unfortunately I have nothing to contribute towards supporting or doubting this rule of thumb.
knot = nautical mile/hour.
mile & nautical mile = distance.
miles/hr & knots = speed.
knots/hr ~ miles/hr/hr
The twelfths rule? I shouldn’t be posting to this, because I really don’t know that much. I paddle tidal water a minority of the time, but our tides here are only a few feet. And, much of the time I am on a river where there is a downriver flow to begin with, and a good deal of the time I’m in shallow bays were the wind pushes the water around more than the tide changes it. So, I profess: I am no expert.
I’d like a few definitions.
Low tide: If there is a measuring stick in the water, low tide is the point where the water is at its lowest elevation, right?
Ebb flow: the movement of water leading up to low tide.
If I use those definitions, and I am on the Ocean, ebb flow has to reach it’s lowest point at the moment of low tide, does it not? On a river, I can see where water would continue to flow in the direction of the ocean after low tide, until the inflow of the rising tide is suffecient to overcome the normal, downriver, current. No answers here, but it might help me understand what you are talking about.
I always heard it explained as “amount of water that moves”. BoatsUS talks about range being the difference in elevation of the high and low tide. Can this rule also be applied to current? I guess not, or other posters would not be perplexed by the question.
The problem with the majority of people trying to apply rules and formalae is that they do not really know where those rules and equations come from, nor do they care to try simplest math to figure it out.
For example, the 60/Vmax rule makes assumption of 3 hour period between slack and max flow.
Derivation is following:
V=Vmax*sin(pi/2*t/T)
V - speed of current
Vmax - max flow
pi - 3.14, or 3 for kayakers
t - time
T - time between slack and max
For small values, x=sin(x)
So, we get V=Vmax*3/2*t/T
t=V*2*T/(Vmax*3)
if we set V to 0.5 kt, we get
t=T/(Vmax*3)=T/3 /Vmax
if T is in minutes, t is in minutes as well.
If we take T=3 Hours=180 min, t=180/3/Vmax=60/Vmax;
Anyways, lets take a look at the real world
Point 1- http://tidesandcurrents.noaa.gov/currents10/tab2pc2.html#114, Point Wilson, 1.4 miles northeast of
January 10, 1st reads:
0224 +7.0
0656 0
0915 -3.0
1206 0
1407 +1.8
1650 0
2037 -5.3
or
+7, 270min 0,140min -3, 170min 0,120 min 1.8, 160min 0,
230 min -5.3
0656 Slack
Before t=270/3/7=13min, David Burch 60/7=9min
After t=140/3/3=16min, DB -> 20min
Adds up to 29min, DB 29min
1206 Slack
Before 16min, DB 20 min
After t=120/3/1.8=22 min, DB = 33 min
Adds up 38min, DB 53min
If you look at the tide tables for various points in the NC sounds you see lots of variation. Then factor multiple inlets... proximity to inlets... rivers filling the sound... wind pushing water in or out of the sound. Too many variables for my little braims.
Not quite "ebb flow has to reach it’s lowest point at the moment of low tide, does it not?"
This is generally true on the open ocean, but there can be a significant lag between the time of high or low tide and the time of slack current when the shore geography gets involved.
There are two underlying things to remember:
“Water flows downhill” and the water to make the “bulge” a the high tide spot has to come from somewhere.
Tides are created by the gravitational pull of the moon pulling “up” on the water. This creates a bulge in the surface and water from other places has to flow towards the high spot to fill in underneath it.
The height of high tide is the balance of the gravitational force and the weight of the water.
Opposing that is “water flows down hill”. If there is a high spot here and a low spot there then water wants to fill in the low spot.
No, not the rule of 12ths The above formula is about estimating how much time you have of weak current flows (not still, just slow). Rule of 12ths is about height of water during the tide cycle.
Real World The real, practical value of a formula like that is your safety while negotiating an inlet. My advice would be to get there early and be prepared to stop and take a break until the current slowed down.
(nm)