Adjust Tabs or Autopilot?

[JLR]

After owning my 56 MY for the past 13 seasons, I really should know the answer to this question but I do not. Hence, the inquiry. I recently fixed a problem with my Bennett Auto Tab Controller (ATC). That's the little gizmo that keeps the tabs in the proper place to keep the attitude of the boat at the "set point". After the repair, I went to reset the set point and I did so at cruising speed (about 16 knots) choosing the set point that maximized my speed on my gps. All good so far. Everything runs great using the ATC when I am not going down wind (ie following seas). In anything following (even as little as a one foot following sea), with my autopilot on and the ATC on, she tends to bow steer too much, creating a zig zag course on the pilot. Now she has never handled really well in a following sea but with the set point chosen for speed, the bow steer (downwind only) doesn't make the pilot happy. I have two choices and I was hoping for some help. I can raise the bow a bit and lose some speed or I presume I can adjust the pilot to reduce its sensitivity. I guess my question is this--do I re set the set point to loose speed or do I try to get my autopilot (a Robertson/Simrad unit) to slow down enough to not overly compensate for the bow steer. Thanks for the help.

[sgharford]

Great Post! Not the same boat (43DC cruising at 17 knots), but the exact same problem. She's a pig in broad or following sea with the trim tabs maxed out for speed. Taking a knot to knot and a half hit by trimming bow up solves 75% of over steer problem for me. If anything like mine, I can't see how an autopilot could react quick enough to compensate for a boat that is operating in a dangerous stance at that speed. I figure the motor yachts are top heavy and tend to roll a few more degrees than say a Convertible, continuously messing with the way the rudders and props interact with the hull’s wetted surface throughout the (exaggerated) roll. Stating what you already know on plane: moving center of effort back and getting that rear keel to dig a little deeper definitely makes her handling more respectable. If nothing else, seems only fair/safe to let the Autopilot operate in that same realm too. Maybe in the end you'll even save time and fuel by running a straighter course?

[JLR]

I also forgot to add another confounding factor--the stabilizers but those I leave alone. I guess I never had this problem (the autopilot oversteer problem) until I set the set point recently with the bow down attitude. It just seems odd to me that even a very small following see would create problems for the pilot so, in the end, I guess I will just raise the bow a bit until a compromise is reached between the pilot and the tabs.

[GJH]

Can you tell us more about the ATC? Frankly, I can't recall hearing of them and am pretty sure I don't have one?

Plus every time I see you posting a 16 knot cruise, I am flummoxed, because I have never seen anything near that; I think the fastest we have got her to go is 14 knots at rated WOT with a totally fresh bottom and gear, in dead calm perhaps with a little current helping us along. I have 650HP 8v92Ti's turning the OEM 30x29 four blade props through 2-1 reduction Allisons. I have to conclude I don't know how to get the boat on plane and use the tabs correctly. It hasn't been a big deal, because we are hull speed cruisers, but it bugs me. The boat weighs in at 74,000 pounds, average, which is to spec.

In bad quartering seas, the autopilot gives up. In following seas, it does OK once we get synched up to the waves speed-wise. These 56's are great boats but have a big flat butt for their length.

[JLR]

George - here is the link for the ATC (actually a newer version than the one I have). It is cheap and easy install and great, especially for boats that cruise a lot while changing speeds a lot. http://www.bennetttrimtabs.com/autotabcontrol.php?cID=4

Also, we have the same boat, the same engines and are turning the same gear with the same props (perhaps different cupping). I turn up to 2325-2350 or so and I cruise at about 16 knots at about 2125-2130 rpms. I never run with more than 1/8 tank of fresh water in the rear water tank and never use the forward water tank.

Tabs are almost all of the way down and the stabilizers are on rather than centered unless I am docking. I carry no dink or other heavy, optional stuff except perhaps for some built in extra seating on the bridge. We have not loaded her up with stuff as we use it for cruising and rarely stay aboard more than 7 days at a time.

Also, I am told but cannot confirm that Roger Penske was a prior owner of my boat so perhaps he did something to the engines but since I have majored both, I believe both are pretty much OEM with the exception of the injectors I use which are Interstate McBee injectors.

Finally, every winter I try to go through the boat and discard anything that I have not used for the past few years believing that lightening the load pays off down the road.

No other secrets here other than having the bottom and gear cleaned quite often and the paint and props redone every third year.