The length of a boat
is one of the most important factors affecting the
boat's price. Yet, the represented length does not
always reflect the true size of the boat.
Weight -vs- the Weight Now
With the advent of
integrally molded bow pulpits and built in swim platforms,
there has been a lot of confusion over how a boat
length should be properly represented. A few, though
certainly a minority, of boat builders have included
the bow pulpit in the length over all (LOA).
Most others use what
is known as length on deck (LOD) that does
not include the length of the pulpit.
So which is the proper
length to use? Since the pulpit does not represent
useable space on the boat, the proper length to use
is the length exclusive of the pulpit, or LOD, though
that number is most often expressed as LOA.
Confusion can arise
because the pulpit is an integral part of the deck.
Therefore, to call it LOA is not inaccurate. Some
refer to length with and without pulpit, which eliminates
all confusion. If the builder leaves any doubt about
this, be sure to clarify it.
Not only are there
built-in bow pulpits today, but also built-in swim
platforms, or what may be called transom platforms
or extensions. Should these be included or excluded
in the length? Some of these are bolt-on molded sections,
while others are molded into and are integral with
the hull. The use of bolt-ons is rapidly passing,
though there are plenty of used boats with these things.
For the most part, I'd argue that bolt-ons do not
reflect useable hull length, and should not be figured
in as the hull length.
However, the integral
platforms do represent useable space on the boat,
as well as actually extending the length of the hull
proper. Therefore, this is rightly included in the
hull length representation.
these two boats, They are actually the same
LOA. The stern drive boat also has an
integral platform, but note that it does not
provide any additional internal hull space.
Therefore, we rate it as being 24" shorter
than the inboard boat above.
Many builders have
taken to applying model numbers like 2685 or 450 that
might seem to represent the boat length, but are actually
misleading. The Sea Ray 500 is actually a 45 foot
boat where the 500 seems to represent the length including
the bow pulpit.
There has long been
much misunderstanding about the lengths and weights
shown on Federally documented boats. How these numbers
are derived goes back to the World War II days when
many yachts were impressed into Coast Guarding services.
At that time, the Coast Guard wanted to know the useable
interior volume of the yachts. This was achieved through
a procedure called admeasurement.
Therefore, the "Registered
Length" shown on Federal documents represents
an average of the length on deck and the length waterline.
Basically, this number cuts the lengths of the overhangs
at bow and stern in half. Thus, if you have a boat
that is 50 feet on deck and 46 feet on waterline,
the registered length will be 48 feet. The same holds
true for the beam of the boat; it's an average between
the widest point and the beam at water line.
So what about the tonnage,
that doesn't ever seem to agree with the stated
builder weight? Well, the registered tonnage is derived
much the same way. In other words from a particular
formula that was designed to fit the Coast Guard's
needs. Hence, the registered tonnage shown on your
USCG documents bears little resemblance to the actual
weight of the boat.
These number should
be provided accurately by the builder, and usually
is. But keep in mind that that is with empty tanks
and no equipment. By the time you add fuel, water
and equipment, the draft is likely to change, though
usually not enough to cause concern. If you're marking
your hull to add a boot stripe, be sure to do that
with the boat fully loaded.
Weight -vs- Displacement
The weight of the
vessel as could be measured on a scale versus the
calculated displacement are not quite the same, though
the difference is not enough to concern boat owners.
Displacement is the volume of water that is displaced
by the hull of the vessel measured in pounds or tons.
This derives from a math calculation made by naval
architects during the design process. No one actually
ever physically measures the displacement of a hull
as that is far too costly to do. Displacement as a
representation of boat weight is no longer used by
pleasure craft builders, though it is always used
So where does the builder
weight come from? Commonly stated as "dry weight,"
it normally means the weight of the boat without any
fluids in tanks. Since most builders end up having
to ship boats, which requires obtaining an actual
measured weight, builders typically ship the boat
off to a truck weigh station where the weight is there
taken less the weight of the trailer. These weigh
stations, operated by federal or state governments,
are actually quite accurate to within 100 lbs. The
scales are tested and certified annually.
What about travel lift
scales, are they accurate? No, they are not sufficiently
accurate to rely upon. That's because the loads are
distributed over a number of lifting slings that can
result in considerable error. Nor is there any requirement
that the be calibrated and certified.
-vs- the Weight Now
Naturally, the builder's
dry weight and what your boat weighs now after fitting
out and filling all the tanks is not the same. You
can figure the difference with reasonable accuracy
by taking the tank capacities and multiplying the
weight of liquids per gallon.
Then you have to figure
weights added during fitting out. Typically, this
will be about 10% of the dry weight, assuming that
you are not making any major additions like a tender,
davit or other particularly heavy additions.
What About Water Absorption?
I've heard it said that boat hulls will absorb up
to 10% of their weight in water. This is a myth and
is not true. I doubt even a wood boat could absorb
that much, but certainly not a fiberglass boat. The
amount of water that a fiberglass hull is capable
of absorbing is insignificant. Even the amount of
water that could be absorbed by things like seat cushions
and deck cores is not going to be to the degree that
it will affect boat speed.
Most builders have
learned the hard way not to advertise boat speeds.
There are far too many factors involved for them to
do so reliably. There have been many lawsuits over
failures to achieve advertised speeds, which has
prompted most builders to remain silent on that subject.
If a builder does make a representation as to speed,
you'd best take that with a grain of salt and not
believe it until proven on a sea trial.