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In article ,
(Parallax) wrote:

snip Cruising
sailboats are not terribly concerned about the overall cost of their
fuel because they will always use a small amount.
Most sailboats have a smaller engine for their displacement than a
powerboat because their primary power is supplied by free wind.
However, fuel use (hp/liter or hp/ounce) and overall power sometimes
are important for a sailboat. A long distance cruiser can only carry
a small amount of fuel and if he has a long period of no or contrary
wind, he might want to be able to motor for a long time with little
regard for the cost of the small amount of fuel. He might consider it
rreasonable to pay a premium for a fuel that was formulated to produce
more more power for ounce of fuel burned even if that fuel was twice
as expensive as conventional fuel. 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)?

Not significantly. More significant is a large, slow prop... more later.
Also, I believe diesel has a slightly higher btu/pound rating than gas.

A related issue is that most sailboats have a fairly small engine
intended only to get the boat up to hull speed in calm water. Putting
in a larger engine would be considered wasteful because then the boat
would have to always carry around that heavier engine even when it
wasnt needed. 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?

Yes, but KISS would preclude that, though turbos wouldn't spin hard at
most cruising speeds. I suspect NOS wouldn't work, for the same reason I
was told never to use ether to help start: premature ignition.

This might give the sailboat the ability to better
motor off a lee shore in strong winds, motor against a strong running
tide, etc. I am primarily interested in times when it is a safety
issue, not for convenience so potential engine damage might be
acceptable.
Any ideas?

We have a Yanmar 2GM20F on our 7,000# boat. At full power, in calm
water, we can motor at 7.2 knots, our hull speed is 6.65. When I wanted
to get home a few minutes earlier a couple of weeks ago (I could see the
front bearing down), I firewalled it and did 6.5-7.5 knots for those two
hours, got into the slip, got the first (most important) spring line on,
and got plastered by a monsoon. If I had to, we could pretty much
maintain hull speed through anything -- though I usually slow down.

We used 2+ gallons for that trip. More usually, we would use 0.6 gallons
at 5.5 knots. If I were cruising, it'd be closer to 0.4 at 5 knots.
(yeah, I'm retentive enough to have checked all these numbers: we've
averaged 0.30 gph for 1200 hours.)

In other words, consumption is directly related to speed once a real
load is on the engine (5 knots doesn't improve consumption), and
increases significantly as you approach and exceed hull speed.

The diesel it replaced would have used about twice the fuel for the same
speeds because it swung a little 12x8" prop at about engine speed. The
Yanmar swings a 16x10" prop at about 1/3rd the engine revs. The
difference: drag and bite.

Our 15 gallon tank gives us about 40 hours' range at high cruise, up to
60 hours if I'm not in a rush -- about 300 nm. Add another tank or a
couple of jerry cans and I'd be happy with an honest 6-700 nm range,
particularly since Xan moves on a breath of wind.

Get good sails, particularly light-air ones. That'll work nicely.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/