Originally Posted by mattnone
Ok here is the next question. I have been looking at these videos on YouTube about the autopilots, most that I have seen say that you need to up size if you want to use it when you are in heavy waves and wind
. How true is that and how would you calculate for the right size for a 38ft that's what I'm looking to buy?
Here is what I would guess is way more than you wanted to know:
Our boat is a Caliber 40 cutter
that weights 24,000 pounds in long distance cruising trim. The first 50 Calibers of this type were built and sold as a 38 and then a swim step/reversed transom was added and it became a 40. Caliber made no hull
, rig, interior
changes when going from 38 to 40.
The two important Autopilot
numbers, IMHO, are "HARD OVER TIME" and THRUST for the motor/drive mechanism that actually moves the rudder
The Hard Over Time tells you how quickly your rudder
can be moved. A comparable measure is “stroke movement per second.”
The Thrust tells you how much force can be applied to move the rudder.
The third important number is STROKE LENGTH – how far can the actuating shaft connected to the rudder quadrant move?
The fourth important measure of autopilot performance is the amount of yaw the course computer allows before it starts counter steering
We used an Autohelm
ST6000 course computer with a Type I00 (type 1) Linear Drive
for a lot of ocean and coastal sailing. The numbers, as shown in the install manual, are:
The Type 100 Linear Drive
(12V and 3 amps) can move the actuating shaft 1” (one inch) per second. The maximum stroke length is 12” so the hard over time is 12 seconds. On my boat the rudder will move 32 degrees from side to side or 64 degrees in a full stroke so the drive can move the rudder about 5 or 6 degrees per second.
The Type 100 thrust is specified as 650 pound feet but the manual also specifies the maximum torque that can be applied to the rudder shaft is 540 pound feet.
We have used the autopilot in breaking seas of over 10’ and 35 knots gusting to over 50 and never felt there was a lack of power or speed of response. When the autopilot is set to full gain it quickly and accurately steers the boat… UNTIL sailing downwind beyond 120 degrees apparent.
That is when the allowed yaw (course deviation) becomes important.
We found that when sailing beyond 140 apparent in more than 4’ seas or more than 20 knots apparent wind
- the ST6000 course computer allowed the stern of the boat to be pushed too much off course when hit by a quartering sea or when a big gust came from above (lesser apparent wind angle) than the prevailing wind.
The bow of the boat would then yaw up to 10 degrees and we would quickly sail towards a broach or roundup. The autopilot would counter steer, but I learned from a lot of manual steering
experiments, that IF the bow moved more than 5 degrees off course that more than 15 degrees rudder would be immediately needed to bring the bow back on course. The autopilot was too slow to react and too slow to feed in more and more rudder. The Type 100 linear drive could move the rudder 6 degrees per second but the bow would yaw more quickly than that.
Once past 10 degrees course deviation max rudder was needed to force the bow down. I am a big strong and very experienced racing
helmsman and when I allowed the bow to get too far off the wind – even my 200 pounds and 6’ height was not enough to quickly steer back on course with our 3-foot wheel
This is a design flaw in our boat but illustrates that standard autopilot specifications do not tell the whole story. Upwind and reaching were perfectly manageable in almost all conditions but deep down wind in gusting winds or biggish seas was a serious problem for the autopilot.
I installed an AutoHelm
GYRO-RATE TRANSDUCER and that magically solved
the problem. When that device is installed the ST6000 course computer will start aggressively counter steering as soon as the bow moves 2 degrees off course (when the gain is set to full). The ST6000/Type 100 combination then was able to handle a full spinnaker
at 160 degrees apparent in 20 gusting to 35 knots and 5-foot seas with no problems and complete confidence on my part.
The Gryro-Rate transducer did not change the power of speed of the Linear Drive but it did allow the rudder to be moved much sooner and prevented the bow from picking up rotational energy as the stern was shoved sideways.
My brothers Tartan 42 cutter
(21,000 pounds) had an AH4000 wheel
pilot installed. It could steer nicely up to 15-knots apparent in most conditions but anything beyond that and the clutch
would release. But, the boat was such a treat to steer/sail (it was, after all, designed as an SORC ocean race
boat), that not having the autopilot work was a good thing most of the time. Except when pounding north along the Baja
coast where every inch of the 750 miles was upwind into more than 15 knots.
So your answer is not so simple. It really depends on
- boat design
- how you sail
- how good you are at balancing your sail plan
- how much of your sailing life the autopilot must provide service