Motorboat Steering

Posted By: Mark N, Nimble Arctic, Jax FL
Date: 3/19/07 7:29p.m.

Report: outside forum guidelines

In Response To: Vagabond under motor-power (pic) (Rod M. WWP-19 "Spring", Nimble Vagabond "Loligo" S. Gulf Isl)

Rod, I'll start off by admitting that I have never been on a Vagabond so my comments will be based on my experience on many motorboats 10-1100'. I asked how much motorboat experience that you had because in my experience, most sailors don’t realize that motorboats don’t steer in the same manner that sailboats do. As you know from your years of sailing, a sailboat steers as a result of the force created when the rudder is deflected into the water flowing past it. This makes a sailboat naturally resist directional perturbations as long as it is moving through the water. On the other hand, an outboard powered motorboat uses vectored thrust for steerage vice a rudder. This means that the amount of steerage available is more dependant on throttle setting than on boat speed. This also means that the outboard motorboat is less resistant to directional perturbations because the lower unit does not make a very good rudder. For these reasons, outboard motorboats tend to be very hard to drive when at low throttle settings. (As a side note, I/O motorboats handle just like outboards. Inboard motorboats and sailboats with props mounted directly in front of the rudder lie somewhere between a sailboat and an outboard motorboat depending upon how big their rudders are.) Now for your problem. First off, I really doubt that your Vagabond has any fundamental design problems that make it that hard to drive. If that were the case, I would expect the Nomad to have the same problem, and having read their board on and off for several years, I have seen no such complaints. (You may want to post your questions there.) Also, I have confidence that Ted Brewer and the late Jerry Koch would not produce a boat design with such a flaw. I also do not believe that changing the trim of the boat and/or engine will have much impact on handling. With the flat bottom, I find that my Nimble is pretty resistant to weight and I expect that your boat is the same. It will take a lot of weight to appreciably change the boat’s trim and given the flat bottom, it probably won’t matter much. I do find that it is easy to load my Nimble too much by the stern and this hurts boat speed. Similarly, I don’t think that trimming the motor up or down will do much except move the bow up or down. I would suggest trimming the motor so that your boat is flat at your most common cruise speed and with your normal weight loading. If your problem just occurred at lower speeds with low power settings, I would think that this is just normal motorboat wallowing. Given that you still have the problem at 3000 RPM, I suspect that you have a mechanical problem with your steering system. The steering should not have any “slop” and the force required to turn the wheel should be uniform full throw. If you have any play at all, I would check all of the cable attachments if it is a mechanical system and bleed the system if it is hydraulic. One last point is that you may want to play with the gain on your autopilot in order to get it to drive better. Given that how your boat steers varies with your throttle setting, I recommend that you set the gain to give you the best performance at your most common cruise setting. After playing with mine for a while, I finally found a compromise setting which works pretty well under most conditions. Even so, this setting is too slow to hold a course when sailing slower than ~2kts or when motoring at idle in winds. It is too fast when motoring at full throttle and causes the boat to oversteer. Hope that this helps.