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When Does Shallow Water Become a Problem?

A HydroComp Technical Report

Report 124

During sea trial analyses, users of NavCad and

FNH 0.0-0.4 0.6 0.8 1.0
PropExpert have encountered situations where the drag
and power were much greater than expected. In some of
these instances, the shallow water effects may be the Speed no loss 1% loss 4% loss 14% loss
cause of this added drag and power. This report is loss
written to help determine when shallow water becomes
a problem.
(NOTE: When FNH is greater than 1.0, we typically see
the speed loss quickly reducing to where we can even
OVERVIEW get a beneficial effect at high speeds, such as at planing
Boats and ships can be easily affected by shallow
water. Basically, we are witnessing a "Bernoulli" effect, We can turn this into a table of "critical depth" for
where the water velocity around the hull must be each FNH:
accelerated due to the restriction of shallow water. This
increase in velocity results in a higher drag comparable Minimum depth for
to this higher "effective" speed. In addition to this, the Speed
accelerating water can pull the vessel down (known as no loss 1% loss 4% loss 14% loss
sinkage or squat), causing the ship the act as if it had a
larger displacement. So when does shallow water cause 5 kts 14+ ft 6 ft --- ---
a measurable loss of speed?
10 kts 55+ ft 24 ft 14 ft 9 ft
15 kts 124+ ft 55 ft 31 ft 20 ft
We describe the parameters of shallow water
through the "depth Froude number", which is equal to V 20 kts 220+ ft 98 ft 55 ft 35 ft
/ (gH)0.5. The parameter V is the ship speed, g the
gravitational constant and H the water depth. In units of
knots and feet, FNH = 0.297 VKT / (HFT)0.5. In knots and This tells us that at "displacement" speeds, the
meters, FNH = 0.164 VKT / (HM)0.5. vessel will slow down as water depth gets shallower. It
One widely-used analysis of speed loss in shallow also shows that "shallow" water need not be all that
water is the Schlichting method. It can give us a general shallow, particularly at higher speeds. Remember -
sense of the potential speed loss. He found that there is these are typical values only and each ship will be
typically no measurable speed loss as long as FNH is less somewhat different.
than about 0.4. As FNH increases, however, the speed
loss begins to take effect:

A HydroComp Technical Report. Copyright © 2003 HydroComp, Inc. All Rights Reserved.


• A fishing boat normally can run at 10 knots in

deep water. If the water depth is 14 feet, then
we can expect a speed loss of about 4%. That
means that its speed is reduced by about one-
half knot.
• A supply boat is expected to run 15 knots in
deep water. However, during sea trials in 20
feet deep water it did not make this speed. The
14 % typical speed loss means that we would
expect a bit less than 13 knots on trial.
• A river work boat is trying to run at 10 knots in
9 ft of water. It will lose almost a knot and a

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