Boat Shop Know-How The virtues of speed, or the lack thereof
Read more news

The virtues of speed, or the lack thereof

On May 12, my wife and I noticed, in a heavily clouded sky, a large, very black area to the north/northeast. We couldn’t see the cloud itself, only its effect.

The wind was a light south/southwest. By rotating to our right 30 degrees and watching the left-to-right cloud movement overhead, we knew conditions were deteriorating. (We were on land, but had we been at sea our rotation would’ve been 15 degrees to the right.)

And if we were on the water and knew our position, we would then have determined the nearest shelter to windward. With flat seas and no hazards along our course, it would’ve been a good time to exercise speed to get the heck out of potential danger.

Speed is a tool in the kits of all boaters. And speed is relative. Years ago, I sailed in boats that could only do about 4 knots, so speed to get out of harm’s way was problematic. We had to rely on an early response to situational awareness to avoid trouble. Today, thanks to advancements in naval architecture and technology, boats are faster. Speed is tricky; you have to apply the “Goldilocks Principle” and find the speed that’s just right for the prevailing conditions.


1. Some years ago, while delivering a boat to Oakland, Calif., in rough conditions, we crossed the crest of a wave that was not breaking at right angles, rather than letting the sea roll under us. It was a moment’s inattention. We fell off the back of that wave into a hole. Everyone on deck was in harnesses and tethered to strongly secured jacklines. No part of anyone’s body was in contact with the boat except for my hands with a death grip on the wheel. Once in safe harbor, we found that the fall of about 6 feet had loosened the bolts securing the boat’s fin keel and bent the rudder stock 6 degrees from vertical. Lesson learned: The faster you go, the more you must concentrate, because you have less time in which to react.

2. Hulls of modern construction are tremendously strong. They rarely fail. What does go wrong when moving at too high a speed for the conditions are “little things.” Crud in the bottom of the fuel tank stirs up and interrupts the flow of fuel. It doesn’t matter that your tanks were cleaned, that you use stabilizers, or even had your fuel “polished.” Murphy’s Law dictates that under bad conditions your fuel flow will cease. Linkages can come undone or break. Screws back off. Fittings and connectors loosen. Steering cables can jump from their sprockets. (When was the last time you checked the tension of your steering cables?) It won’t all happen at once, but one small occurrence at the wrong time can cascade into a potential disaster.


3. Not enough speed can also generate problems. Remember, rudders have to act against the flow of water. Speed equates with control. Moving too slowly usually exacts the penalty of sluggish response to steering commands. Powerboats with twin screws have small rudders because the force of the discharge current against them is so effective. You can steer a twin-screw boat just by coordinating your throttles. In fact, large rudders in a twin-engine boat would cause oversteering.

Slower-moving power- and sailboats have larger rudders to compensate for the weaker current flowing past the blade.


4. So what happens when you’re maneuvering in reverse? The weaker suction current flowing against the rudder makes the rudder less effective. At slow speeds, the torque of the screw tends to create a tendency to swing the stern to port, if you have a right-hand prop. The result is sluggish performance. One of the things I learned the hard way is that most

single-screw boats maneuver at too slow a speed when backing into a slip. You need control, ergo you need speed, but it has to conform to the Goldilocks Principle — in other words, it has to be just right. You can always position your boat in forward gear to take advantage of the torque-imposed port bias when in reverse. If she’s not swimming the way you want her to, apply correctional rudder with a burst of power forward.


5.
You can always shift into neutral and use your rudder with the sternway of the boat until steering becomes sluggish and then apply a little reverse power to regain steering control without having to struggle against engine torque. This applies to nosing into a slip as well; slip into neutral and use the rudder until the response at the wheel becomes spongy.


6. By the way, overconfidence in twin screws can lead to a problem. If you’re moving too fast for conditions and lose an engine, you’ll find it interesting trying to continue with a single power plant. Because of the outboard position of the props on a twin-screw boat, that one engine will introduce the effects of torque in forward gear.


7. When docking, remember not to maneuver at speeds that require you to slam into the opposite gear. Transmissions also can fail. Slow, but not too slow, and steady is a good rule. Don’t worry — a piling, the dock or another boat will stop you. You do have insurance, don’t you?


8. In conditions of reduced visibility, slower speeds are required, and slipping into neutral is a good way to handle that. When negotiating a tricky passage through rocks, reefs or shallows, slower is better than encountering something unpleasant.


9. A point for GPS aficionados to remember is that GPS positions at speeds below about 3 knots can be problematic, even with WAAS units.

 

This article originally appeared in the August 2009 issue.



BoatQuest