I have been digging around in various web sites selling boats. It is
really amazing the claims made for speed over ground, fuel capacity
and range for some long range passage makers.
For instance, any full displacement hulls is going to have an upper
limit speed of:
11 knots if the water line length is 67 feet.
10 knots for a water line length of 56 feet.
9 knots for a length of 45'
8 knots for a water line length of 36'
And so on.
There are exceptions to every rule and the numbers above are
calculated from the square root of the water line length times 1.34.
Which is generally considered to be the practical upper limit.
So when you see a boat of 55 feet overall, with a water line length
closer to 49 feet, you can imagine the humor in claiming it will go
11 knots. The truth is more like 9.5 kn.The moment you see this kind
of exaggeration, it would be smart to take all the other claims with
a serious dose of skepticism.
As for the claims about fuel capacity. Unless the tanks have been
certified with an attached plate, specifying the exact capacity, you
would be well advised to be skeptical. In the case of really big
built in tanks, integral with the hull and where there is no
certificate plate, you can't rely on claims by the owner, the builder
or anyone except perhaps your own surveyor. And only then if you are
certain they are not influenced by any of them. As the surveyor will
have estimate the capacity by inspection and measurements. Of course
you can verify all this when you fill them, when empty. Of course
that is going to cost real money.
Failure to have these capacities reliably known, could get you into
big trouble on a long passage.
In case you might be tempted to think that this is all theoretical.
Think again. At one time or another I have had to deal with all of
these exaggerations. And by the way, bulbous bows don't have but a
tiny impact on overall speed in vessels of this size.
Mike
Capt. Mike Maurice
Tualatin(Portland), Oregon
I have been digging around in various web sites selling boats. It is
really amazing the claims made for speed over ground, fuel capacity
and range for some long range passage makers.
For instance, any full displacement hulls is going to have an upper
limit speed of:
11 knots if the water line length is 67 feet.
10 knots for a water line length of 56 feet.
9 knots for a length of 45'
8 knots for a water line length of 36'
And so on.
There are exceptions to every rule and the numbers above are
calculated from the square root of the water line length times 1.34.
Which is generally considered to be the practical upper limit.
So when you see a boat of 55 feet overall, with a water line length
closer to 49 feet, you can imagine the humor in claiming it will go
11 knots. The truth is more like 9.5 kn.The moment you see this kind
of exaggeration, it would be smart to take all the other claims with
a serious dose of skepticism.
As for the claims about fuel capacity. Unless the tanks have been
certified with an attached plate, specifying the exact capacity, you
would be well advised to be skeptical. In the case of really big
built in tanks, integral with the hull and where there is no
certificate plate, you can't rely on claims by the owner, the builder
or anyone except perhaps your own surveyor. And only then if you are
certain they are not influenced by any of them. As the surveyor will
have estimate the capacity by inspection and measurements. Of course
you can verify all this when you fill them, when empty. Of course
that is going to cost real money.
Failure to have these capacities reliably known, could get you into
big trouble on a long passage.
In case you might be tempted to think that this is all theoretical.
Think again. At one time or another I have had to deal with all of
these exaggerations. And by the way, bulbous bows don't have but a
tiny impact on overall speed in vessels of this size.
Mike
Capt. Mike Maurice
Tualatin(Portland), Oregon
