Re wing engines or twins
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Marty wrote:
Re "come home system"
I would now not have a separate smaller prop. If I were to have another
single
engine boat, the come home system would turn the main shaft and prop.
Probably
run by hydraulics off the gen set.
exodus52@aol.com
Marty's come home system consisted of an Isuzu QD-60 (60hp, 4 cylinder)
engine; Velvet Drive 5000 "V" drive transmission and a Gori 18x15 two blade
folding prop, the largest they made at the time. This arrangement, because
of the excessive slip of most of these small prop wing engine
installations, gave a FLAT WATER cruise speed (2200 RPM) of only five
knots. A sister ship in Hong Kong performs the same.
You can imagine that the thrust required to give five knots in flat water
conditions would be less than useless in the 50-60 knot winds and 20+ foot
seas Marty was experiencing.
I THINK IT IS SAVE TO SAY THAT SMALL POWER, SMALL PROP, OFFSET WING
ENGINES, AND MOST BOW THRUSTERS, ARE FAIR WEATHER FRIENDS AT BEST.
I'm not sure that hydraulically linking the 25 to 30 severely governed
horse power of most gensets to the main shaft would be any better in the
conditions Marty experienced.
Marty's choice for Exodus II of twin engines and props will give total
propulsion, shaft, prop and rudder redundancy at the cost of some
efficiently and prop protection. "Log knockers" or twin fins and skeg
rudders can give back some prop protection and feathering props can give
back some efficiently at long range cruise speeds by allowing single engine
operation.
Twin engine / props also allow a shallower draft and easier trucking which
fit into Marty's future cruise plans.
Paul's "Doulos" concept is ultimately redundant but, like all engine only
propulsion systems, still does not solve one of our most frequent
problems - too much air in the fuel tanks.
If every design choice was not a tradeoff or compromise in some way TWL
would be very boring.
Bill & Stella Kimley
Seahorse Marine
Tel&Fax 86-756-331-3403
zhyachts@pub.zhuhai.gd.cn
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It has been said here before: boat design is a process of making tradeoffs.
It is equally important to be aware of what is being traded off right from
the start. A wing engine is a typical tradeoff. The wing engine provides
an alternate method of getting home but there is a tradeoff here that was
clearly illustrated in the recent posting describing the sad loss of a fine
vessel. "Get home" here does not mean get home under extreme conditions.
Any cruiser going offshore often enough will encounter the conditions
described. If this piece of bad luck combines with loss of the main engine
(and unfortunately, extreme conditions frequently does just that), the get
home engine (and very likely, a second full power engine) is not the
seamanlike solution. I feel that it is imperative to be prepared for such
a situation by being ready to deploy a drogue or sea anchor. This device
will prevent the broach feared by the skipper of the ill fated vessel. It
will stabilize the vessel and allow time for finding the source of incoming
water, finding the cause of the overheating and provide the most important
ingredient in an emergency at sea: opportunity for the adrenalen to drain
away and for calm consideration of the situation. Just think of this
emergency in terms of a sailing vessel. Suppose the same thing happened
and the main "engine", the mast, had become inoperative. In most sailboats
the engine is very much like that "get home" engine, it will not be
sufficient to buck seas and 50-60 knot winds. The sailor must be ready to
set head or tail to the wind and seas and wait it out. The drogue is the
solution of choice for many sailors.
Personally, I would not take a course broadside to such wind and seas
to head for the dubious shelter of the shore even if I had an engine
capable of doing this. Closing with the shore, even a nice soft, sandy
shore in extreme conditions is my idea of a bad nightmare! Again, the
drogue is an option under extreme conditions even with an operative and
powerful engine.
One more point: just this year I had my engine (on a motorsailer)
overheat and quit in a prolonged squall at 2 AM in the gulfstream. We were
motoring because the wind had been light from the wrong direction.
Fortunately, with reduced sail we were able to heave to while we sorted out
the overheating. The cause of the overheating was the extremely rough
conditions combined with the heeling caused by the violent wind. We had
sucked air into the cooling system, this caused a barely adequate gasket on
the inlet strainer to admit more air into the system. The point here is
that two engines would not necessarilly have helped us. In all likelyhood,
both engines would have overheated.
I would suggest that everyone who might be caught offshore be prepared
to survive without any outside help. This definitely means having a way to
hold the boat at a safe angle to the worst seas in the most violent winds.
It also means having at hand the information and tools and spares to take
care of losing the use of one or both engines due to overheating or clogged
filters. Another aspect of survival at sea is the preparation needed to
estimate the volume of water observed in the bilge, estimate its rate of
entry, trace its source, reduce entry and pump out without power. Until
these issues have been addressed, please, do not go out there! Another
item to keep in mind is the grim fact that many people trying to escape
from small boats in heavy seas to the apparent safety of ships die in that
most difficult and dangerous transition.
Perhaps it would be useful on this net to have a discussion of
parachute vs drogue; series vs single drogue; bow vs stern deployment.
Frank Lawlor
(HOBO, a wing engine vessel abuilding)