Autopilot for a GBE?

[Island Time 1,246]

Those of you with the GBE type boats. What do you use for an autopilot (when cruising of course!) and what do you think of it?

I was asked this today by the new owner of Rental.....

[Clipper 346]

I had an ST1000, have now upgraded to ST2000. Original worked excellent when motoring and upwind.

Only used it to hoist or drop kites downwind.

I have mine set too far away from rudder pivot, so means slower response and less movement possible. But the boats track so straight, it is not much of an issue.

Hard to find a place to mount, I made removable mounts.

[Guest]

My Turissimo10 now called Crowded House had a autohelm st1000. Worked great, I can recall sailing home from Motuhei one day with it steering from the leeward cockpit, flying a hull with the two of us in the windward one on sheets.

[Tim C 23]

I have used a Autohelm on Pulse quite a bit singlehanded. When it works, it works well. I too have the pivot well beyond the recommended 400mm or so, It's just under 1m. So it tracks well, but doesn't tack well. Good for a single handed kite gybe though.

I went forward one day to put the anchor on the bow, got hit by a squirt, and peaked at 17 knots! And couldn't get back fast enough to do anything.

Getting the kite down is when they are most valuable short handed. And of course motoring a long way...

I'd love to do windvane steering one day, the modern carbon equivalent with trim tabs on the rudders. But I know how much time that would take...

[Island Time 1,246]

All good guys, I'm happy that a normal tiller pilot seems ok, once fitment is sorted. Thanks

 

Tim, as a wind vane steers to apparent wind, and that can vary wildly with boat speed, my understanding was that vanes don't work well on fast boats (Like performance multi's). Am I missing something?

 

The good electronic pilots steer to true wind when it's aft of about 90 deg, and to apparent when fwd of that, and it is my understanding that this was to avoid the problem. It was a function first asked for by race boats that will suddenly accelerate with a gust or a wave, and the resultant radical change in apparent wind dir, while true remained the same. Older pilots and vanes will try to follow the apparent wind....

[Black Panther 1,600]

Thinking out loud here. What if you introduced a feedback into the windvane that could adjust to boatspeed?

 

I;m thinking something like the "wand" on a foiling moth. As boat speed increases it would adjust the vane to an apparent wind further forward?

 

Probably not worth the trouble but an interesting intellectual exercise.

[Tim C 23]

All good guys, I'm happy that a normal tiller pilot seems ok, once fitment is sorted. Thanks

 

Tim, as a wind vane steers to apparent wind, and that can vary wildly with boat speed, my understanding was that vanes don't work well on fast boats (Like performance multi's). Am I missing something?

 

...

 

I've heard this theory and don't get the logic. Upwind or down, multihulls are sailed to the apparent wind, typically the wools on the sails. A wind vane, or electronic equivalent, would only do the same. Going downwind, it would bear away the boat in a gust, and come up with the lulls, just as a helmsman would. Possible on a beam reach getting it to average the required course may be difficult in gusty conditions. But then with a compass autohelm either the sails need to be trimmed, or the sails could well be stalled/over trimmed in a gust, which is not ideal either...

If the boat has too much sail up for the conditions then neither system is ideal. It's all about good seamanship.

[Island Time 1,246]

I think Tim, that it is logical, but I have no personal experience of AP control on really fast boats.

 

If you accelerate rapidly, the apparent wind goes forward, sometimes quite a bit. If the boat on AP follows the apparent wind, and tries to bear away to maintain the apparent angle, could it not pass through the true wind direction and cause a gybe?? Especially on boats that are at or beyond wind speed?

 

Even surfing in my boat will backwind the sails sometimes - I would not want AP wind steering in control in that situation - it would try to turn the boat right around!!

[Tim C 23]

I'm not sure it would. Remember on a performance boat the most aft you have the apparent wind is probably 90˚. So a big gust and a big change of course would be 20˚. To get to a gybe would mean the helm system hadn't reacted to the AW being aft of 90˚.

These large maxi Multi's in the Route de Rhum etc must have the dynamics sorted out, one way or the other!

[TimB 7]

I think the problem is this. Hard on the wind with autopilot under apparent wind angle control. if the boat goes down, knocked by a wave or whatever, a reasonably high performance multi, eg the GBE under discussion, will accelerate immediately and bring the apparent forward back where it was, so the pilot is now quite happy steering 5-10 degrees lower than the required course. The big French multis have solved this by using a brain (computer) to control the autopilot so it does what a good helmsman does and uses, boat speed, wind angle, course, both COG and BRG as inputs and tries to maintain a constant course to the wind with best VMG. A simple tiller pilot can't do this as it has only one input either course or wind angle.

 

tb

[Tim C 23]

I think human helmsman have a problem with this too! It is very easy to go low and fast, and be tacking through large angles. As easy to do this in the light as on windy days. Effectively one has to nominate a maximum boat speed, but that of course changes with every gust and lull. It's not easy maths...

[TimB 7]

Agree Tim, VMG is the key all the time. Applies down wind also, maybe even more so.

 

Tb