SAFETY CONSIDERATIONS
Horsepower Ratings
How did we get these rules for power on
outboard boats? Is there a better way?
Most of the horsepower (or kilowatts in some
countries) ratings for monohull outboard boats, used today, were developed
back in the 1950's by the BIA and the Yacht Safety Bureau. They ran
tests on various boats and came up with the formulas for different types
of boats. Primarily the limits on power are due to two factors, one is
simply handling of the boat. Putting too much power on the back of a boat
can make it handle erratically, spin out, and difficult to steer. The
other is weight. Putting a much larger engine on the transom can
make the boat sit stern down and make it easy for a following wave to
roll over the transom and swamp the boat.
Also different types of hull shapes handle power in different ways.
Flat bottom boats with hard chines are much easier to skid sideways in a
turn, or to catch a chine and trip the boat, possibly flipping it, than a
boat with a vee or rounded bottom. Boats with a shorter transom
height sit lower and are easier to swamp than a boat with a full height
transom (20 inch). Engines come in standard shaft lengths, generally 15
inch, 20 inch and 25 inch, i.e. short shaft, standard, and long shaft.
But, over the years both boats and engines have
changed considerably, so shouldn't the rules have changed? Well,
probably, but with everyone required to use the same formulas, at least
they present a level playing field. Everyone has to use the same rules.
Tests conducted in 2003 indicated that many boats may actually be
overpowered using these formulas. More testing needs to be done.
Is there a better way? There may be. Way
back when, ABYC developed a test course standard for boats that don't have
to meet the US Federal regulations. This course is basically the
same one that was originally used to obtain the formulas and measures a
boats ability to maneuver through a course with turns, without becoming
unstable. The Coast Guard used this test course when it developed a
separate standard for boats 13 feet or less, with one or two passengers, a
20 inch transom height and less than 40 horsepower. These were popular in
the 80's, and called thrill craft, before the advent of Personal
Watercraft.
In the 1980's the Coast Guard and Mercury Marine
did a lot of testing on different types and sizes of boats instrumented
with accelerometers. These instruments measured the acceleration sideways
in a turn. The basic theory was, the more power, the greater the
sideways, or lateral accelerations. Unfortunately the data revealed that
there was no correlation, between power and lateral accelerations in a
turn. There was more of a correlation between acceleration and the boat
hull type.
The International Standards Organization (ISO) has
developed it's own test standard, a collision avoidance test, or barrier
test. An imaginary barrier is set up on the water and the boat with
rated power, run at full throttle, has to avoid "hitting" (actually
crossing over) the barrier by turning at a specific distance which is
calculated based on the length of the boat. The test is repeated with
larger engines until it can't avoid the barrier. The highest power with
which it could complete the turn is the rated power.
On the more technical and scientific side, some
naval architects and engineers have done research into resistance on
planing hulls and developed formulas based on weight, wetted surface at
speed, resistance, angle of the vee and other factors. These
formulas are used frequently to determine power for larger inboard powered
planing hull boats but aren't really applicable to small (under 20 feet)
outboard powered boats.
So where does that leave us? The Coast Guard
has been doing some testing comparing the ISO standard to the US standard
to see how a boat rates under each standard. Whether this will develop
into a new standard is open to speculation.
Start-In-Gear Protection: Another important safety
consideration is to prevent engines from being started while the boats
drive is in gear. This matters because people get tossed out of
the boat when the boat suddenly starts and jumps forward.
Statistics showed a significant amount of accidents in which
someone had gone aft to work on an outboard engine, the engine
started up in gear and the person was thrown over the transom of the
boat. So the USCG adopted a regulation requiring outboard motors
with greater than 115 lb. of thrust (about 2 HP or about 1.5 KW) to have
a device that prevents the engine from being started when in gear.
However, ABYC has a
standard, P-14 Mechanical Propulsion Control Systems, which requires
start-in gear protection on inboard and inboard/sterndrive boats as
well. So most power boats have start-in-gear protection. Not to be
confused with Kill Switches.
Kill-Switches: Emergency engine stop switches
that stop the engine if the device is tripped by the operator being
knocked down or thrown out of the boat. Usually this is a simple
lanyard attached to the key, or a switch. When the lanyard pulls
the key out or trips the switch the engine stops. There are more
sophisticated devices on the market. This is now a USCG
requirement. ABYC does have a standard for it, A-33
Emergency Engine/Propulsion Cutoff Devices, and most boats produced
today have them, especially Personal Watercraft. The industry
standard for PWCs requires them to have a Kill Switch.
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Copyright 2007 All Rights Reserved Revised 04/02/2020
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