52 seamanship secrets
Now you can figure the amount of time needed to get to any point. Just
multiply the time to travel 1 mile by the number of miles you will travel to
reach your waypoint.
Note: these examples give you estimated times in transit based on your speed
through the water, and the underlying assumption is that your speed through
the water and your speed over ground are the same. This is not true in places
with strong current, and you will need to make a few adjustments. Adjustments
will also be required on vessels under sail and subject to changing speeds. We will
learn how to make these adjustments in Chapter 5.How to Make a Speed Graph in Th ree Easy Steps
A customized, ready-to-use speed/RPM table will tell you at a glance what
your speed through the water is at any RPM setting. Compare this speed
with the speed over ground reading
from your GPS receiver to quantify the
combined eff ect of wind and current
at any given time. You can also use the
table to estimate fuel requirements, time
en route to a destination, provisioning
needs, and fuel stop-off points.
POINTS TO REMEMBER
Q Make your speed runs in little or no wind and current. Th is keeps the
data much more accurate.
Q Find two prominent marks about a mile apart with good water all
around. You’ll need room to make a loop around each mark or near each
mark.
Q Select an area clear of boat traffi c. If you need to slow down, divert your
course, or stop for another boat, you’ll have to repeat the run at your
chosen RPM setting.
Q Load the boat for normal cruising. Top off fuel tanks and water tanks;
invite some friends.
- Make the graph. List speeds in 2-knot increments up the left side of
a piece of graph paper (see art page 54). Label across the bottom with
RPMs, beginning at zero and ending at the highest rated RPM for your
engine. Displacement power and sailing vessels might want to extend their
graphs only as high as the RPM at hull speed, since a displacement boat
can exceed its theoretical hull speed only by a small margin and only at
Supplies Needed
to Make a Speed Graph
X pencil
X graph paper
X watch or stopwatch