BROOMSTICK
- A ONE-MAN HYDROFOIL TRIMARAN PROJECT
(PROGRESS
REPORT - OCTOBER, 2007)
Doug
Halsey
Fallbrook,
California
Broomstick
is the name of the hydrofoil trimaran that I designed and built and that I sail
on the inland lakes of Southern California (Lake Perris & Lake Elsinore). I
have always felt that hydrofoils were a magical sort of thing, and since my
wife & I have owned a number of dogs with names like Goblin, Wizard, &
Impy, the name Broomstick seems to fit into the family nicely.
Photo
1. Broomstick is a 15Õ long, 17Õ wide, 1-man trimaran that can be set up in a
variety of ways. This photo shows it sailing on foils, using its larger rig and
its smaller amas(pontoons).
Lake
Perris, June 4, 2007. (Photo by Terry Curtiss)
My
intention in building the boat was to produce something that would challenge my
engineering capabilities, be fun & exciting to sail, and be near the upper
limit of what would be possible for one man (of my age & size) to
transport, rig, and sail by himself. IÕm not aiming for any speed records or
competing for any prestigious trophies with this boat, but I hope to learn
enough to be able to dream about such things for a possible second boat.
Only
conventional materials have been used in this project. The main hull is planked
of foam, and covered inside & out with fiberglass/epoxy. The amas(pontoons)
are plywood. The crossbeam assembly is welded aluminum tubing with fabric
trampolines. Masts & booms are aluminum. The sails are Dacron. Some of the
parts are over 30 years old, from an earlier boat. The foils are all softwood,
covered with glass/epoxy. ThereÕs no need to spend a fortune at this stage.
Broomstick
is by no means a finished product; rather it is a continually-evolving
experimental platform. As such, the hulls, rigs, and foils are all highly
modular and easily interchangable. I have sailed it both with and without
foils, with 2 different sets of amas (pontoons), 2 different rigs, but so far,
with only the initial set of foils.
The
foils are arranged in the so-called "airplane configuration", with 2
deep-vee, high-dihedral surface-piercing foils just forward of the center of
gravity and a smaller horizontal foil at the bottom of the rudder. This
configuration was chosen for its simplicity, rigidity, and stability.
Over
the years, the question of whether to use surface-piercing foils or
fully-submerged foils has been a hotly debated topic, with each side claiming
numerous advantages. I think that the recent successes of l`Hydroptere
(surface-piercing) and the Moths (fully-submerged) shows that both types can be
equally successful.
I
believe that either type of foil could be used on Broomstick, but I chose the
surface-piercing option for the initial foils, mainly because they are much
simpler for the amateur to build. There are no flaps, no wands, no moving parts
on the main foils. Main foil incidence is set by bolting to brackets, and is
not normally changed on any given day. I have provided manual incidence control
for the aft foil (by simply adjusting the angle of the kick-up rudder), so that
I can try to sail with close to the optimum amount of foil in the water for the
given conditions, rather than just accepting the variation of foil area with
speed that surface-piercing foils automatically provide.
Vee
foils are used for the main foils, instead of the more common cantilevered
foils, because of the structural rigidity of the nearly triangular shape. This
allows them to be made of light wood covered by a single layer of 9-ounce
fiberglass, with no carbon fiber reinforcing, even though their chord is
smaller than it might be for cantilevered foils. Furthermore, for a given foil
depth, the span (horizontal projection) is twice that of cantilevered foils.
Initially, I was worried about extra induced drag that might arise for vee
foils with combined lift & side force, but numerical experiments in a
vortex-lattice lifting-surface code convinced me that vee foils are actually
quite efficient, especially when a short vertical fin is added at the bottom of
the vee.
The
large dihedral angle was chosen in order to discourage ventilation of the
foils, to improve performance in conditions with large side forces, and to
allow the boat to fly sufficiently high above the water with the desired foil
span. It also provides exceptional stability in pitch.
The
main foils were initially provided with anti-ventilation fences at 6Ó
increments along both inboard & outboard segments of each foil. I removed
them after observing flow separation originating from them on a number of
occasions. The boat sails much better without them.
General
dimensions are as follows:
Overall Length = 15' (Main Hull)
Length of Amas (Pontoons) = 11'
(Longer Set)
= 7' (Shorter Set)
Overall Beam = 17' (Between Ends
of the Crossbeams)
= 13'8" (Between Vertices of the Vee Foils)
Total Sail Area = 108 Square Feet
(Smaller Rig)
= 140 Square Feet (Larger Rig)
Total Weight = 200 lbs. (Without
Crew)
= 350 lbs. (With Crew)
Main Foils:
Dihedral = 60
degrees
Chord = 6"
Span = 22.5"
(Horizontal Projection at Bottom of Hull)
Depth = 19.5" (Below Bottom
of Hull)
Section shape =
NACA 0012 (Approximately)
Fin at Bottom of Main Foils:
Chord = 6"
Span = 6"
Aft Foil:
Chord = 6"
Span = 24"
The
boat was first launched in June, 2003 and sailed as a conventional trimaran for
the rest of that year, using the longer amas, a daggerboard, and the smaller rig.
It performed reasonably well in this mode, for a boat of this size, but was
somewhat hampered by design compromises made with the foils in mind (small ama
volume, low crossbeam clearance, etc.).
By
early in the next year, I had completed the foils. Broomstick flew successfully
on its third outing (May 10,2004), reaching an estimated speed of about 18
knots and fulfilling a dream that I had had for many years. Shortly after that,
however, a serious illness in the family was diagnosed, and I was not able to
find time to sail for almost 1 & 1/2 years. During that time, though, I was
able to build the smaller set of amas and put together the larger rig.
At
this point in time (October, 2007), I have more time to devote to the project,
but have still only sailed with the foils a total of about 20 times. Several of
these days had too little wind, and there were problems with the main-foil
attachments and the aft-foil controls on several of the other days. I am
gradually overcoming these problems & the boat's performance is improving
steadily.
This
year, I have been using a Velocitek S10 (GPS speedometer) to log performance
data (speed, heading, etc.) and, on several outings, I have been accompanied by
a friend & professional photographer (Terry Curtiss), who has taken
literally hundreds of photos each day. Synchronizing the clocks on the camera
and the Velocitek S10 helps me determine not just how fast the boat was going
at any instant, but also how the boat was set up, how high & at what pitch
& heel angles it was flying, and much other useful information. Top speed
measured so far on a reach is 20.6 knots, with a best 10-second average of 20.2
knots. I am more uncertain about performance to windward at this stage, but I
believe the boat has exceeded the true wind speed at a heading of about 60
degrees from the true wind.
The
performance described above was achieved in winds estimated to be about 15
knots, using the larger rig. I expect (and hope) to achieve much higher speeds
in stronger winds, probably using the smaller rig.
(I
would like to at least exceed 1 / 2 the current World Speed Record.)
Any
major improvements in reaching speed and windward ability (in the windier
conditions that I am most interested in) probably will require significant
reductions in aerodynamic drag and increases in righting moment. My Velocity
Prediction Code (VPP) indicates that reductions in drag of the foils would be
of less importance, at this point.
Possible
aerodynamic improvements might include:
1
- Reducing parasite drag (streamlined crossbeams, internal halyards, more
attention to detail, etc.)
2
- Eliminating the amas. This may be feasible on reaching legs (in speed trials,
for example), but I wouldnÕt want to try it on windward legs.
3
- Adopting a more modern conventional rig (square-top sail, carbon-fiber mast,
newer sail materials, etc.)
4
- Adopting a more radical rig, possibly a large-chord wing mast. Full-chord
wing sails are ruled out by my desires to focus on the foils, to keep expenses
down, and to be able to rig & unrig it myself.
Righting
moment is a key factor in the design of any high-speed sailboat. Broomstick
obtains its righting moment in a conventional way: 1 - Crew weight is
positioned far out to windward.
2
- Heeling increases the area (& hence lift) of the leeward foil and
decreases the area of the windward foil.
Possible
increases in righting moment might include:
1
- Using a trapeze to move the crew weight farther to windward. This will
require me to improve my sailing technique, since I already feel like I have
too many lines & too few hands.
2
- Adding a second crew. This may be beneficial in stronger winds (though
cramped), but would seriously limit the foiling possibilities should the wind
drop.
3
- Allowing the crossbeam assembly to twist, so that the windward foil can
generate some negative lift. I believe that I can arrange things so that this
happens automatically, when needed, without any active input while sailing.
4
- Setting the windward foil at a negative incidence, by manual means, to produce
negative lift. This could be done most easily in speed trials, where the boat
only has to sail well on one tack.
Of
course, increased righting moment means higher loads on the structure. Until I
get more experience sailing on the foils and more confidence in the structure,
I am reluctant to use any of these
methods to increase righting moment. I am already breaking too many things!
I
would never claim that hydrofoils are the perfect kind of boats for everyone
(there are way too many practical problems), or that Broomstick is the perfect
hydrofoil sailboat (several others are currently performing much better). I hope to be able to report on
significant increases in performance in the near future, but for me, with my
own interests, capabilities, and limitations, this has already been the perfect
project. 
Photo 2. Broomstick
on its trailer, ready for its first sail. Any boat this wide must be dismantled
for towing & storage. Each component is light enough to be carried easily
by one person. Fallbrook, June 24, 2003. (Photo by Doug Halsey)
Photo 3. The boat is
assembled & rigged on the beach or at the waterÕs edge. It must be anchored
in deeper water to deploy the foils. The visible scars on the foil surfaces are
the result of removing the anti-ventilation fences. Lake Elsinore, August 24,
2006. (Photo by Terry Curtiss)
Photo 4. At
sub-foiling speeds (semi-displacement mode), the foils partially lift the main
hull and provide enough righting moment to keep the amas out of the water. The
boat can be very fast in this mode. There is no sudden change when the main
hull totally clears the water. Lake Perris, June 4, 2007. (Photo by Terry
Curtiss)
Photo 5. Sailing
with the smaller rig. The wishboom rig imparts lower bending forces to the main
hull and makes it easier to get under the sail when tacking & jibing, but
jib luff-tension is not adequate. A newer version of this rig, to be completed
soon, will have the mast extended and will use a conventional boom. Lake Elsinore, August 1, 2006.(Photo by
Terry Curtiss)
Photo 6. Broomstick
at close to 20 knots (in spite of dangling lines & poor jib
trim). Lake Perris, July 12, 2007. (Photo by Terry Curtiss)
Photo 7. It is
obviously possible to capsize Broomstick.
This happened the first time out with the larger rig & the smaller
amas. The winds were the strongest IÕve sailed in yet (estimated at 25 knots).
I was just luffing & trying to untangle some lines, when a gust hit &
over I went. At low speeds, the foils do not provide much righting moment, so
the boat is especially vulnerable. Fortunately, it is very easy to right, even
without outside help. The small amas displace only a fraction of the boatÕs
weight, so it floats with the main hull in the water, rather than 8Õ up, and
the mast lies parallel to the surface, with no tendency for the boat to turn
turtle. Windage of the ama & trampoline act as a weathervane, causing the
boat to rotate into the wind within a couple of minutes, at which point, my
weight is sufficient to right the boat. Lake Perris, November 15, 2005. (Photo
by Terry Curtiss)
Photo 8. No serious
crashes have occurred while the boat was foiling. This minor crash was caused
by a slipped or bent main foil attachment. The foil brackets & crossbeam tubes
are not very streamlined, so they stop the boat very quickly when they hit the
water (even at very low speeds). Lake Perris, June 4, 2007. (Photo by Terry
Curtiss)
Photo 9. The
incidence of the aft foil has been controlled by an up/down control wire for
the rudder blade. This is what happens when the wire breaks & the rudder
kicks up all the way. Later, a similar incident cracked the rudder & tore
off the aft foil. A redesigned rudder & aft-foil control system is nearing
completion at this time. Lake Perris, June 20, 2007. (Photo by Terry Curtiss)