chart and publication secrets 7
show heights at mean high water springs, abbreviated MHWS, which is the
long-term average of the local spring high tides. Both measures are conservative
for safe navigation provided that you use caution before attempting to pass
beneath a structure. For example, if you are counting on a 6-foot tide range
to squeeze your 60-foot mast under a bridge with a charted vertical clearance
of 55 feet, you are not exercising prudent, conservative navigation.
Th is leads us to ask, how do you know how much clearance you must have
to keep from accidentally and permanently shortening your mast, antenna,
or fl ying bridge?
Look on the chart next to bridge and power cable symbols for both vertical
and horizontal clearances. Determine the state of the tide (see more on tides in
Chapter 5). U.S. federal law requires bridges to show tide gauges on each entrance
side, telling mariners the vertical clearance at any moment. You must know the height
of the highest projection of your mast, antennas, cabintop, fl ying bridge, or tuna
tower. Use all of these factors to decide whether or not it is safe to pass beneath.
Sailing Vessels
Use your sailplan to determine mast height. This often shows the height from
the waterline to the top of the mast. If in doubt, contact the manufacturer or
class association for the exact specifications. If you carry instrumentation
atop your mast such as an apparent wind indicator or VHF whip antenna,
add a generous margin for safety.
SEA-CRET TIP
X If you are sailing a boat under 30 feet long and are unsure of its mast
height, use a rough estimate by multiplying the boat’s length × 1.5.
If the mast is extremely tall, add several feet for safety.Power Vessels
You’ll need two measurements for powerboats: one with all antennas and
instruments up, and one excluding any equipment that is easily lowered for
safe passage beneath bridges (such as many VHF whips). If you are unsure,
try these easy steps for accurate measurement:
- Attach a small block to the end of a boathook.
- Pass a line through the block. Attach a weight to the end of the line.
- Get up as high as possible and extend the device over the water.
- Feed the line out until the weight touches the water. Tie off the line.
- Pull the line aboard and measure from the end of the boathook to the
weight. Add any gear extensions to the measurement.