Revised 20-20B web page
Please note that this site is being reactivated
after about a year and
Advanced designs and detail fittings
Think of these designs as a simple way to get a rugged model
These designs have been built as prototypes and
Two of these designs are based on ideas presented by
Model 2004 November 2003
I consider this a logical evolution of the 20-20 idea of a cheap,
This version obviously reflects my experimentation with PVC
The model 2004 oar carrier is based on simple PVC fittings
Photos to be added here showing prototype construction details
Combining these water side
The 90-95 design with
Model 90-93 Jan Alkema invention---- RHM
The sketch below shows the basic idea and the numbers
Check the last of this page for a write up he did for
I refer to this as a RHM for rudder head mount.
that a simple model will reveal the action of
A side mounted vertical vane
This is a simple linkage and oar carrier that will allow
Model 90-94 USD vane system--- Another Jan Alkema
Model 90-94_Sht 1b
This is taking the RHM and USD ideas to combine in
innovative pendulum system for outboard rudders.
On boats with outboard rudders it can be troublesome to install a pendulum windvane system. The pendulum has to be free of the rudder and should not limit the rudder movements from full port to full starboard. That means that an extended support frame will be necessary. It will put extra weight on the transom. Another obvious drawback is that the wind vane system has to be mounted further aft of the boat, which makes it more vulnerable in harbors and crowded marina’s.
A lot of sailing boats with an outboard rudder and
wind vane system make use of a trim tab, directly mounted at the rudder. The
trim tab works as a servo system to generate enough force to turn the rudder.
It is unquestionable that a trim tab with the right
dimensions works, but on some points its performance is less than that of the
Trim tab or pendulum.
Let’s compare the characteristics of a trim tab and
a pendulum system and focus on the following aspects :
Support and vulnerability
Obstructions in the cockpit
The trim tab can be situated just at the trailing edge
of the rudder or at some distance of it. This
choice is mostly depending on the shape of the rudder. In figure 1. the trim tab
is directly mounted after the trailing edge. In figure 2. the trim tab is
mounted some distance behind the rudder.
In both cases the trim tab is working in the wake of
the rudder. The solution of figure 2. has the advantage of giving a bigger
torque, due to the increased distance between trim tab and rudder hinge.
The pendulum has normally a larger power arm as can be
seen in figure 3. So the pendulum with the same dimensions of the underwater
part as the trim tab, delivers more torque. This can be advantageous for heavy
or unbalanced rudders. Pendulums can operate these rudders without problems, but
trim tabs can be limited in their steering torque.
A trim tab develops a force which direction is
opposite to the rudder force. The trim tab is decreasing to some extend the
effect of the rudder. With normal dimensions the loss of rudder force is approx.
10 %. So the trim tab makes the
rudder less effective.
The pendulum force works in the same direction as the
rudder, so it assists the rudder and increases the total rudder action and hence
the steering efficiency.
Yaw damping is the ability to prevent or reduce oscillations in the course. Lack of this ability gives a zig - zag course, so it is an important characteristic of a course controller.
Yaw damping is not easy to explain, but the following
example may help to get an idea of it. In this example we are only considering
the influence on the rudder and we suppose that the vane is not turning during
the yawing motion of the boat.
When wave and/or sail forces are turning the boat (
yawing) , then there will be pressure on the rudder from the water flow.
When the rudder is fixed a force is developed on the
rudder which counteracts the yawing motion.
When the rudder is free, then it will line up with the
water flow and does not give any counter force to the yawing motion.
A rudder with a trim tab is not fixed or free but
controlled by the trim tab. When the boat yaws the water flow creates a force on
the rudder + trim tab and initially the rudder tends to line up with the water
flow and rotates a bit. But the trim
tab gets a greater rotation (in the
same direction) due to the linkage between trim tab and wind vane. The water
flow on the deflected trim tab creates a force
which prevents the rudder from lining up with the water flow. As a result of it
the rudder gives some counter force, which
damps the yawing motion.
This counter force is however smaller than a rudder
alone, that had been fixed.
A pendulum system, connected to the rudder can give
more yaw damping , and most when the pendulum is far aft of the rudder out of
the wake. When the boat yaws the rudder and the pendulum get a pressure from the
incoming water flow. The pendulum wants to swing out and the rudder wants to
line up with the water flow. But because the pendulum is much more powerful then
the rudder it swings out and turns the rudder opposite to the water flow
direction. So instead of limiting the rudder angle from giving in (this is what
the trim tab is doing), it increases
the rudder angle to create an increased counter force. So the pendulum system
gives a powerful and active yaw damping. The counter force of a rudder +
pendulum is bigger than a rudder alone that had been fixed.
This is true when the pendulum is out
the wake of the rudder. When the pendulum is close to the rudder then the active
yaw damping effect is less. But it
is always more than from the trim tab, because when the pendulum swings out, it
brings the blade far more out of the wake of the rudder compared with the trim
Support and vulnerability
An advantage of a trim tab system is that no heavy
support is necessary. The trim tab is directly mounted at the rudder.
The trim tab is well sheltered by the rudder which is
an advantage compared with a pendulum, which swings out and can pick up weed,
ropes and floating debris
As already mentioned an extended support frame is
inevitable for the normal pendulum system for an outboard rudder.
Obstructions in the cockpit.
The trim tab system has no steering lines and blocks
to the tiller or wheel. The trim tab directly controls the rudder blade.
For normal pendulum systems steering lines with
several guiding blocks are running through the cockpit to operate the helm or
the steering wheel. This is mostly a nuisance in the cockpit.
Combining the advantages.
My boat, a Westerly Konsort, has an outboard rudder.
When I started to design a wind vane system I wanted to have the best of both
worlds. So the advantages of a pendulum system should preferably be combined
with the advantages of a trim tab system. Would that be possible?
After many sketches and a lot of thinking I came up
with a system which I later named the Rudder Head Mounted (RHM) pendulum
or oar system. See the sketch in figure 4.
The pendulum has the horizontal hinge mounted on the
Essential are the two restraint lines from the transom
side to the oar carrier or pendulum tube. When these lines are loose, the
pendulum can swing to each side, but it can not turn the rudder. The system is
disconnected. When these restraint lines are tight, then they form a fixed point
on the tube which will be a pivot point. (point
P) When the pendulum swings out then the rudder is forced to turn.
Figure 5 shows how the swing out movement of the
pendulum gives a rudder movement, when the restraint lines are tight.
For stretching or adjusting the restraint lines I use
clam cleats on the aft cockpit sole. To release the lines, simply pull them out
of the cleats.
This Rudder Head Mounted (RHM) pendulum combines all
the advantages of both the pendulum and trim tab :
It is a true pendulum with the
power of normal pendulums.
It increases the rudder action.
· Yaw damping is better than with a trim tab
No heavy support frame is
· There are no steering lines in the cockpit
One minus point remains. The pendulum is not so well sheltered after the rudder as the trim tab.
I made the prototype of the oar carrier and the oar from plywood. I used SS hinges which are normally used for the rudders of small dinghy’s. The first sailing tests were carried out in 1981 and it worked from the start. Figure 6 shows the prototype of the oar carrier and oar, which fitted very well with the shape of the rudder.
This RHM pendulum system is combined with a separate wind vane which is mounted on the push pit. Via thin stainless steel cables the rotation of the vane is transmitted to the little tiller on the oar.
Figure 4 shows how the cables are running from the wind vane to the small tiller arm on the oar.
I used the prototype of the RHM pendulum for some 5 years. After that I have rebuilt the system, using stainless steel tubes. Ball bearings are used for the oar rotation and in all blocks to get as low friction as possible. I also made the blade retractable. It is still in use, after 20 years, with only small modifications up to now. See figure 7 and 8.
Position of the cleats for the restraint lines.
The position of the cleats should be close to the
sides, using the full width of the transom. The restraint lines have to remain
reasonable tight without slack or over tension for rudder angles of
+/- 20 degrees, which is the max. range for rudder corrections during
On my boat it appeared that the cleats should be
positioned higher than the connection point
P on the oar carrier or
pendulum tube. On my system the vertical distance between P and the cleats is
about 0.25 m.
In general it may need some trial and error to find the best positions of the cleats.
Well mannered behavior.
On every pendulum system the oar needs feedback, otherwise the system will over steer. When the oar is initially turned by the vane, it will swing out, but during that swing the oar is rotated back, to arrive at a certain swing angle and so at a certain rudder angle. The information of the swing angle is fed back in the turning of the oar through the linkage between vane and oar.
The RHM pendulum system needs more feedback than a
normal pendulum, because the pendulum rotates together with the rudder, so that
rotation must also be compensated as an extra.
The geometry of the cross beam and wires and blocks
however ensures that there is more then enough feedback in the system, to get a
well mannered behavior of the pendulum without any sign of over steering.
The system gives a good yaw damping in downwind and
broad reaching courses on a lively boat like the Westerly Konsort. Note the
straight track on figure 9 during a running course in force 5.
On windward courses the system works also very well.
Note the oar at work, most of the time out of the wake
of the rudder in figure 10.
The wind vane part.
Up until now the wind vane part has not been highlighted. The wind vane part has been developed separately from the pendulum.
Before starting with the RHM pendulum I used a big wind vane, directly coupled to the rudder. I had taken that wind vane from my previous boat. It had an adjustable vane axis tilt angle. An article on that separate wind vane has been published in PBO nr. 170 Febr. 1981.
After building the RHM pendulum I combined it with the existing wind vane. The combination worked very well, although the vane was a bit oversized for operating the small oar.
Some 6 years ago I designed a new type of wind vane, the Up Side Down (USD) wind vane, which could easily be connected to the existing RHM pendulum via the thin stainless steel wires.
See figure 11.
The interesting feature of the USD vane is that the vane action increases when the boat heels. It automatically adapts its action in the right way to the sailing circumstances.
When heeled with some weather helm, the USD vane gives more action for a better accuracy. On a downwind course with no heel the USD vane action is less, which enhances course stability.
The USD wind vane has been described in the article “ Which wind vanes work best “ (PBO nr. 414 , June 2001) .
How to operate the RHM / USD system.
At the start the vane is locked is a vertical position
and the restraint lines are slack, the blade is put into the water. It appears
that the oar is following the rudder without swinging out, also when the boat is
steered manually. With slack restraint lines no forces are exerted on the
When the restraint lines are tightened
and the vane is set on the desired wind course and released, then the
system is taking over and will steer the boat.
Normally I put the tiller in the upright position to get a free and
To disconnect the system I first lock the vane in a
vertical position and then pull the restraint lines out of the cleats.
The described construction principle of the Rudder Head Mounted pendulum has been used for 25 years now. I made many sailing trips with it to Denmark , UK and France. Up till now I have not experienced any shortcomings in the system.
In my opinion it is a feasible and satisfying solution for boats with outboard rudders.
RHM and USD are working together perfectly, making sailing trips even more enjoyable.