# The "slow water rule"

-- Last Updated: Nov-20-10 6:21 PM EST --

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?

– Last Updated: Nov-18-10 9:07 PM EST –

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.

Varies here too
and looking at tonights tide and current tables the ebb current lasts a full two hours after the tide started to flood in.

Perhaps the work you cite was for ocean waters rather than estuarine ones.

In tidal races currents run about 5.5 mph at max velocity here.

at a given location

– Last Updated: Nov-19-10 7:10 AM EST –

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

All bets are off …
It’s very dependent on topography/bathography …

Mother Ocean always surprises…

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.

Boaters I know explained the amount of water moving during tidal exchange is roughly as follows (adapted from http://www.boatus.com/cruising/TomNeale/tip_08.asp)

Slack

Hour 1: One twelfth of range

Hour 2: Two twelfths of range

Hour 3: Three twelfths of range

Hour 4: Three twelfths of range

Hour 5: Two twelfths of range

Hour 6: One twelfth of range

Slack

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.

~~Chip

some math

– Last Updated: Nov-19-10 2:40 PM EST –

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
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

Point 2
http://tidesandcurrents.noaa.gov/currents10/tab2ac3.html#22
"The Race Point 0.4m SW"
0114 -4.2,
0510 slack
0757 +3.6,
1051 slack
1340 -5.0
1751 slack
2031 +3.5

Or -4.2, 235min 0, 170min +3.6, 180min 0, 170min -5.0, 250 min 0

Again,
0510 Slack
Before t=235/3/4.2=19min, DB -> 15min
After t=170/3/3.6=16min, DB -> 17min

1751 Slack
Before t=170/3/3.6= 16m, DB -> 17min
After t=170/3/5=11, DB-> 12 min

Here is what this mathematical masturbation tells me:

Numbers are off when the time between slack and max flow is not close to 3hours ( 180min) - mixed flows in the inlet being a prime example.

On average, the numbers are good enough to give a fairly accurate estimate.

If you desire to be more accurate, try this

period of current less than 0.5 kt in min will be
([rough hours between slack and max as 2, 3 or 4, fractions work] *20/Vmax)

– Last Updated: Nov-19-10 6:32 PM EST –

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.

It’s not a simple thing at all.

knots
Definition of knots, is nautical miles per hour. So knots per hour means nothing. Sorry pet peeve.

Bill H.

suiram is the real life of the party
kinda guy…

Stop it! My head hurts (nm)

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.

Thank you
Your summary is exactly the kind of feedback I was looking for.

OK, edited to fix that
Aside from that, do you have any experience with how accurate Burch’s rule of thumb is?

Well actually
Knots/hour is a technically valid unit of acceleration.

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.