|
|
|
|
I agree with Dan, except for the "bow wave" thing
I am reluctant to jump in on this one since someone appears to have drug
Jax into it, but... being a techie type (and former physics major), I
can't resist. Hopefully, Jax (who often has correct ideas, but just as
often jumps in with either erroneous details or misinterpretations of
previous posts) won't beat up on me too bad.
So, are fuel additives available
that would effectively increase the hp/ounce of fuel burned even if
they greatly increase the overall fuel cost? Would octane boosters
help (or cetane boosters)?
The short answer is "no". Contrary to popular belief, the octane rating
of a fuel is not a measure of the energy content of a fuel. Rather, it
is a measure of how fast that fuel burns. As a result, increasing the
octane rating of a fuel does not make the fuel "more powerful". In
fact, contrary to what might be seem to be common sense, a higher octane
rated fuel actually burns slower than a lower rated fuel. This can
prevent premature ignition in high compression engines. Put another
way, it can prevent "knocking" in high performance engines.
Is it possible to temporarily increase the power
outpuit of a small engine by some means, perhaps a blower, a turbo or
even using NOS?
Some of the newer diesels have a turbo charger, but my personal
philosophy is to avoid them. This is based upon the notion that added
complexity will, on average, lead to more things to go wrong. Note that
others will disagree and advocate the use of smaller, turbo equipped
engines.
It should also be noted that turbo charging an engine does not produce
more energy (fuel mileage) from a gallon of fuel, it merely allows a
smaller displacement engine to behave as though it were a larger one and
consume more of the air/fuel mixture per gulp.
I've never heard of a NOx equipped cruising sailboat. While this could
increase the power of an engine, I see two problems with it. The first
is that I've never seen a fuel dock that could resupply you with NOx.
The 2nd is dealt with below in the discussion dealing with exceeding
hull speed.
I am primarily interested in times when it is a safety
issue, not for convenience so potential engine damage might be
acceptable.
Any ideas?
Simply put, displacement hulls don't plane. While you can get some
relatively flat bottom, fin keeled boats (like the classic Cal 40, any
of the racing sleds, or many of the newer racer/cruiser boats) to plane
while going down a wave (surfing), and in theory, you could put a
mega-horsepower engine into any boat, sustained planing of a
displacement hull under power is unattainable in practice.
There is this concept of "hull speed" that displacement hulls have to
contend with. This is often stated as 1.34 * sqrt( WL). While this is
good as a general rule of thumb for the maximum speed of a displacement
hull, most who actually own one can tell you that on a windless day and
with smooth water, they can routinely exceed this "theoretical maximum"
under power. I certainly can with my Tayana (a displacement hull if
there ever was one). The reason is that hull speed, while often touted
as a theoretical maximum, is nothing of the sort.
http://www.sailnet.com/images/conten...sc_figure2.gif
http://www.sailnet.com/images/conten...sc_figure3.gif
http://www.sailnet.com/images/conten...sc_figure4.gif
http://www.sailnet.com/images/conten...sc_figure5.gif
The above sequence of graphics (stolen from the excellent sailnet.com
web article at
http://www.sailnet.com/collections/a...leid=colgat006)
shows what happens as you approach and exceed hull speed.
Essentially, as you go faster and faster, the wavelength of the bow and
stern waves get longer and longer. As you pass the "hull speed", this
wavelength exceeds the waterline length of the boat and you wind up
sailing uphill with the boat trying to climb the bow wave. As you
continue to go faster and faster, the steepness of the hill gets greater
and greater and the amount of thrust required to do this gets similarly
greater and greater. While it's easy to exceed your hull speed by a
little, it's hard to do so by much, even if you double or triple the
amount of thrust the engine is outputting, it rapidly becomes a loosing
battle.
So the bottom line is that if your engine is powerful enough to push
your boat to hull speed against the wind and chop, a more powerful
engine won't buy you much.
I hope this little dissertation clarified the matter rather that
confusing it further. If you liked it, you should hear my 20 minute
treatise on the different kinds of sunglasses (tinted, reflective and
polarized) and the properties of each of the three basic kinds.
--
Dan Best - (707) 431-1662, Healdsburg, CA 95448
B-2/75 1977-1979
Tayana 37 #192, "Tricia Jean"
http://rangerbest.home.comcast.net/TriciaJean.JPG
|