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The Masthead
Know It All Contest Solution
to the June 2012 Question which
considered whether existing thruster wiring can be used when retrofitting
a new, more powerful bow thruster.
The Know It All questions and correct answers are important design tips for students as well
as other marine professionals. We suggest that you file them away for future reference.
The Question Was:
The owner of the 37-foot motorsailer,
Pretty Penny
has asked you about installing a more powerful bow thruster. The pro-
posed new thruster and its tube will fit without a problem. The existing thruster wiring on
Pretty Penny
s 12-volt electric
system is good-quality, tinned-copper boat cable (BC5W2) 1 AWG, and is in good condition with excellent continuity. The
thruster’s power cables run from the forward end of the engine compartment, with the length of the run, from the batteries
to the thruster measured along the cables, of 16 feet 10 inches.
The old thruster was rated at 77 pounds of thrust, driven by a 2-hp DC motor drawing 205 amps. The new thruster is rated
at 132 pounds of thrust, with a 4-hp, 12-volt DC motor drawing 290 amps.
Can the existing power cables be used for the new thruster and why or why not?
The Winners Are:
Scott A. Harroun of the U.S Coast Guard; James Reddington, PE a senior forensic engineer and ABYC Marine Master Tech;
Douglas Martin; Paul Thunburg, a Westlawn student and USCG marine inspector; Charlie Johnson PE and ABYC Marine
Master Tech; Kim I. McCartney, marine surveyor; Peter Banks, Westlawn grad; and Leslie Allen, a Westlawn student all
submitted correct answers. This is a record-breaking eight correct answers received. Clearly, regular readers of
The Mast-
are inclined to be brainiacs. Under the contest rules, only the first three correct answers received can actually qualify
for the official and coveted title of Know It All, and our three Know It Alls for this question are thus: Scott Harroun, James
Reddington, and Douglas Martin. We wish to congratulate our three winners on being way too smart for their own good. In
honor of their excess of sagacity, perspicacity and unbridled brain power each should henceforth be addressed only as,
Mr. Know It All.” Naturally—in honor of their remarkable abilities—Westlawn caps, T-shirts, and Know It All certificates are
on their way to each of our three winners.
And the Solution Is:
The answer is no. The original power cables shouldn’t be used for
Pretty Penny
s new thruster.
The voltage drop for the original installation was higher than the ideal but appears nominally acceptable as:
AWG (American wire gauge) wire = 83,690 Cm (Circular Mills)
The distance to the thruster, is the total distance from the power source to the load and back again.
Voltage drop = 10.75 x 205 amps x 2 x 16.84 ft.
Cm = 0.88 volts
volts = 0.073 = 7.3% voltage drop
Ten percent voltage drop is considered the maximum allowable, so 7.3% was acceptable in the old installation. From ABYC
Standard E-11, Table IV; however, we see that the maximum allowable amperage for 1 AWG, outside of the engine com-
partment, is 165 amps. Though, the voltage drop is under 10% and
Pretty Penny
has been running her old thruster over
this cable, it doesn’t properly comply with ABYC standards. Indeed, ABYC Table IV indicates 2/0 wire is the minimum re-
quired on the old installation.
Using the existing 1 AWG cables for the new thruster would be worse:
Voltage drop = 10.75 x 290 amps x 2 x 16.84 ft.
Cm = 1.25 volts
volts = 0.104 = 10.4% voltage drop
This is over 10% and so not acceptable. Plus it exceeds the ABYC allowable ampacity of the wire of 165 amps.
We know the new thruster is drawing 290 amps. Referring to ABYC Standard E-11, Table IV, we see that he smallest size
that is rated for 290 amps is 4/0.
AWG = 211,600 Cm
Voltage drop = 10.75 x 290 amps x 2 x 16.84 ft.
Cm = 0.49 volts
volts = 0.04 = 4% voltage drop
This meets all requirements and is quite close to the ideal 3% voltage drop. This is excellent, but—in this case—heavy and
Sept. 2012 Page 15