THE Z H T A PROJECT
TECHNICAL PROPOSAL
January 20, 1988
OBJECTIVE: TO SAIL A RADIO CONTROLLED/COMPUTER CONTROLLED MODEL
OR DRONE SAILBOAT ACROSS THE ATLANTIC OCEAN. THE PROJECT IS TO
BE COMPLETED IN MINIMUM LEAD TIME AND WITH MAXIMUM PROBABILITY OF
SUCCESS. COST OF THE PROJECT IS TO BE KEPT MODERATE.
I. THE BOAT
General Considerations: The design selected must be strongly
built, readily available on the used boat market, and well
ballasted in proportion to its sail area. That is, the design
should not rely heavily on moveable crew weight for righting
moment. Hull, deck, rig, and appendages must be capable of
withstanding a knock-down or roll-over. On most available boats
this will probably require some reinforcing of the rudder, and
replacement of spars and standing rigging. Full keel/attached
rudder configuration has the advantage of being less susceptible
to fouling by sargasso weed or other floating objects, and less
likely to sustain rudder damage. Fin keel/spade rudder
configuration (possibly including a bulb keel) has a considerable
performance and control advantage, especially downwind.
Size: Several factors determine the minimum feasible size of the
vessel:
1) Must be large enough to carry off-the-shelf
navigation, computing, steering, and
communication equipment without serious
degradation of performance. The costs and
long lead times associated with
miniaturization are to be avoided.
2) Must have sufficient deck area for the
required solar panels for battery charging.
3) Rig must be able to support a masthead wind
instrument cluster.
4) Backstay must be a reasonable length for a
short-wave radio antennae.
5) It may be desirable for the vessel to carry
passengers during testing and de-bugging.
6) The model should be visible to other vessels,
so as to avoid risk of collision.
Several factors also determine the maximum feasible size:
1) Vessel must be small enough to rig and test
with minimum lead time.
2) Sails must be small enough to withstand
serious abuse in adverse weather conditions.
3) Rig must be small to withstand roll-over with
minimum probability of damage.
4) Reefing systems should be kept simple, and the
number of sailplan configurations required
during the voyage should be kept to a minimum.
5) The probability of being spotted by other
vessels should be minimized, so as to reduce
the risk of the vessel being "rescued" or
otherwise interfered with.
6) Vessel should be easy to transport and store.
7) Vessel must be acquired quickly and at
moderately low cost.
8) The danger, real or perceived, of causing
damage to another vessel must be minimized.
Preliminary evaluation of the above size parameters suggests a
vessel size of between 16 and 24 feet. Candidate stock designs
include:
Full keel designs: Cape Cod Bull's Eye (16 ft, 1350 lb.)
Cape Dory Typhoon (18 ft, 2000 lb.)
Pearson Ensign/Electra (22 ft, 3000 lb.)
Fin keel designs: Zypher (16 ft, 500 lb.)
International 110 (24 ft, 1000 lb.)
Wylie Wabbit (24 ft, 700 lb.)
II. STEERING
Steering will be controlled by an electrically-driven compass-
referenced autopilot. Wind vane type self-steering is ruled out
because of size and weight, vulnerability to wave damage,
difficulty in making automatic adjustments, susceptibility to
fouling the pendulum with floating objects, and the difficulty in
providing redundancy in the event of failure.
Several suitable off-the-shelf autopilot systems are available,
specifically the Autohelm 2000 or 3000. Although these units are
reasonably reliable, some redundancy is necessary. One possible
actuator configuration is to install two or three tillers on the
rudder stock (all below deck level). Each tiller will have its
own steering actuator. An electrically controlled clutch device
will couple only the active tiller to the rudder, and only the
actuator on that tiller will be powered. Similarly, two or three
control units will be available. On-board diagnostic software
will determine which tiller/actuator set and which control unit
should be switched on.
III. NAVIGATION
The model will carry one satellite navigation unit, plus one
additional satellite unit or one LORAN unit. Both of these
systems are available off-the-shelf for the recreational boating
market, complete with a standardized data output interface.
There will also be two remote-reading compasses, and two
knotmeter/log units. These devices will send data to the on-
board computer for maintaining a dead reckoning position, and/or
used in conjunction with the SATNAV or LORAN units to utilize
their built-in DR capabilities.
The on-board computer software will make course decisions based
on wind direction and speed measurements, the current vessel
position, and the pre-programmed or transmitted routing strategy.
IV. INSTRUMENTATION
In addition to the navigation instruments (knotmeter, log, and
compass), the model must also be capable of measuring wind speed
and direction. This could be a standard yacht masthead
instrument cluster, or possibly a heavier commercial anemometer
and remote direction indicator. Although the yacht type unit is
less expensive, lighter, and consumes much less power, these
devices have a poor reliability record and cannot be expected to
survive a roll-over.
To allow for the possible loss of wind measurement capability,
the model will also measure its heel angle. This information,
combined with course, speed, sail trim settings and reef
configuration, will allow the on-board software to estimate wind
speed and direction with reasonable accuracy. A short wind test
maneuver, performed hourly, might be used to improve the accuracy
of this wind estimate if the masthead cluster is lost or
malfunctions.
V. COMMUNICATION AND TELEMETRY
The primary communication system will be via short-wave radio,
pre-tuned to transmit and receive "packets" of digital
information at selected times. Even at a relatively slow data
transmission rate and with sveral repetitions, these
transmissions will only take a few seconds for each contact
between the model and the land-based communications station. A
range of frequencies can be used to maximize the probability of
making a successful contact under varying propagation conditions.
It will be desirable to use one ham station on the east coast of
North America, and one station in England, linked by telephone
to the central control and information center.
Satellite bulletin boards may also be extremely useful, but
antennae aiming will not be possible from the vessel.
Contacts will be attempted on the order of twice daily.
Transmissions from the model will include:
1) Present position from DR.
2) Last SATNAV/LORAN fix.
3) Present vessel speed and heading.
4) Present wind speed and direction.
5) Heel angle, sail trim settings, and reef configuration.
6) Battery condition and charge/discharge current.
7) Time history of items 3,4,5,6 since last contact.
8) Results of diagnostic routines.
Transmissions to the model will include:
1) Routing strategy modification, based on weather information
available on shore.
2) Modification of time and/or frequency of next contact
attempt, if necessary for propagation conditions or security.
3) Initiate switch to arrival mode, initiate real-time control
with specified parameters.
4) Modify arrival waypoint if necessary.
In addition to long-range communications, a VHF radio
(approximately 25 mile range) may be installed as an
interference-avoidance aid. This radio would be activated once
the model is at sea, and continuously monitor channels 13 and 16.
If a call from another vessel is detected, the radio would return
a taped message explaining the nature of the voyage, along with a
request to report the model's position.
VI. SAIL HANDLING
Sail trim: In order to minimize the possibility of tangles and
fouled rigging, running rigging must be kept as simple as
possible with 1:1 sheets and control lines where possible.
Linear actuators, similar to those used for the autopilot tiller
control, will be used to adjust sheets. Note that the duty cycle
of an actuator used in this way is extremely intermittent, and
their reliability should be far greater than that of the
actuators used for steering. The mainsail will be controlled by
a single-part sheet and one or two actuators. The jib will be
most likely be a conventional non-self tacking working jib, with
an actuator controlling each sheet independently. This
arangement will probably offer the least possibility of a
tangle, maintain good trim through the widest range of wind
speeds and angles, and work well with roller furling. A single-
sheet arrangement with a boom or tacking track might still be an
attractive alternative, however.
Reefing: The jib will be roller-furled around a rigid luff-
support spar. Although luff-support reefing/furling systems are
subject to reliability problems on larger vessels, the approach
here will be to use a grossly oversize system. There will be
only two positions for the jib: furled and unfurled. The furling
spool will be driven by a continuous chain drive from a small
electric motor.
There will be one fairly deep area reef in the mainsail. A
linear actuator will slack the halyard, tighten the reef clew,
and tighten the reef tack simultaneously. No lacing lines will
be used.
In order to insure smooth working of the mainsail luff, steel
lugs will be sued at the inboard end of each batten. A
lubrication system will introduce lubricant into the track. In
moderate weather, the reef will be exercised about once a day to
prevent salt build-up or freezing of moving parts.
All actuators and servo-motors for trim and reefing will be
located either inside the main boom, or in a special shallow
compartment located just below the main deck, sealed from the
rest of the model's interior. Because a small amount of water
will enter this compartment through the ports where control lines
penetrate the deck, here must be provision for draining or
pumping this water overboard.
VII. POWER
Electrical power will be provided by several large lead-acid deep
discharge type storage batteries, maintained by solar panels on
deck and possibly also on the model's starboard side.
Approximately 100-150 watts of peak rated charging power will be
required for a summer crossing, which entails approximately 20-30
square feet of solar panel area (using off-the-shelf marine solar
charging panels). For a winter crossing, it is estimated that
approximately three to four times the peak rated charging
capability will be needed, unless other power-economizing
measured are taken. A low power mode will be available to the
model, with reduced steering input, less frequent sail trim, and
possibly delayed or abbreviated radio contact.
VIII. ON-BOARD COMPUTER
The on-board computer will probably be an off-the-shelf MS-DOS
laptop computer, selected for reliability in adverse or more
computers will be installed for redundancy, although only one
will be environmental conditions. Two operating at any one time
under normal conditions.
IX. DEPARTURE/ARRIVAL
Departure and arrival will require real-time steering control
from an escort vessel. This will be accomplished by sending
course instructions to the autopilot by means of conventional
designated radio control frequencies. If the model reaches the
designated arrival waypoint before the escort vessel is on
station, then the model will heave to and send an intermittent
homing signal on VHF frequencies.