Auto pilot

[robert sailor]

If you are going to cross oceans then a vane is a very nice thing to have however if most of your sailing is going to be coastal or Mex/Med/Caribbean then I would put my money into a good autopilot.

[TacomaSailor]

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

[hellosailor]

'- how good you are at balancing your sail plan'
+1!

Some less technical things to look for, if the model you are looking at says "for 32-40' boats" and you are in a 38? You're close to the max capacity that a perhaps overconfident marketing department is looking at. better to be near or at the BOTTOM of the suggested range, so you know the unit will have more than the minimum of power.

And I'd suggest also that "too much" autopilot just means it won't have to work as hard, so it should last much longer.

See if the maker, and the installing dealer, look nervous when you ask them blunt questions. Or if their answers are vague, when you email them.

[brianb00]

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?

First, that is very true. Larger seas and wind are going to mean larger loads on your rudder and tiller. It will also mean larger shock loads. Also the faster you go will lead to additional forces. Sail plan is key, as is sail adjustment/reefing. It is not the case that, without knowing something about the tiller load, you can size accurately for a boat. The AP manuf's use vessel weight as a guide, but it only is a guide. A keel hung rudder will have a very different load presented to the autopilot than a modern, well balanced fin keel and blade rudder. Go steer a a J105 or Class 40 and compare to a Westsail 32.

Suggest you speak to other owners of the same vessel or one of similar design below the water line and ask what their experience is. If they saw mechanical failures in the AP maybe up size.

You haven't mentioned what you are considering, but a boat of that size is likely going to need a Type 2 drive system and likely installed below deck. You can choose hydraulic or all electric drive and there are pro's and con's for both.

The best source of data I have found on AP issues are comments among racers in the various forums online. Single and double handers who ocean race put large strain on AP systems over many miles and are familiar with common failure modes. You might take a look at associated web sites/blogs.

Brian