Using Interceptors for Steering

There are a number of papers published on the subject but I don't have them readily at hand, unfortunately. One was the results from extensive testing of transom lift on various catamaran hulls that was done by model basin in Berlin, IIRC. That paper appeared in the mid 90s, perhaps at one of the Fast conferences.

The USN (NSWCCD) work on trim tabs and their effect on monohull efficiency was published by the principle investigaor, Gabor Karafiath, about 10 years ago.

 

Steering by interceptor was a failed concept. The goal in steering is to effect a turning moment about the instantaneous center of yaw. A drag device a short distance from that center slows the craft moem than steering it. A rudder can have a lift to drag ratio that is much higher than ten. So the rudder, much further from the center of yaw creates a much stronger yawing force with drag that is at least an order of magnitude less.
Also, Hickman's boat ripped past everyone else like the others were dragging anchors, but still often lost because the turning performance was abysmal.

 

Wrong! The creation of a turning moment requires a sideforce to be applied. What actually counts is the lift/drag ratio of the device used to create this force. At high speed, where most rudders are prone to ventilate, the interceptor will produce higher lift with less drag than a rudder. That is equally relevant for the steering force with waterjets; when the nozzle is turned for a steering effort, there are 3-dimensional secondary flows induced within the nozzle, which are actually "stealing" propulsive power.

The interceptor is using the energy in the boundary layer (that is already "stationary" relative to the moving hull). This fluid has been brought up to the speed of the hull; i.e. it has been energized. So using part of this (already paid for) energy to increase static pressure over a short hull side surface is beneficial for the total propulsion. But, like everything else marine, nothing is applicable everywhere. The problem I often see is that people don't understand that hydrodynamics can not be treated or understood intuitively; you have to go back to fundamental physical principles.

 

Exactly. Baeckmo nails it. Interceptors and trim tabs are lifting devices, not "drag devices". As with any lifting device there is, of course, a drag associated with lift that is produced. In some applications, the drag of the device is more than offset by reduction in total vessel drag. In other applications, a drag "penalty" is accepted as the cost paid for improved vessel motions in either the lateral or vertical planes...or even both.

 

Nope. Sorry. Neither of you understood what I wrote. Not gonna go all Ad Hoc on you. It's fine.

 

It was pretty clear...that you don't understand the nature and applied direction of a steering force produced by a side-mounted interceptor. So many successful interceptor-assisted systems operating successfully would tend to support the fact that the concept can work well if applied properly.

 

If the interceptors are on the hull bottom, ie not primarily steering devices, then yes, not very effective at generating a yaw moment, and yes, they have to use drag to do that and the moment arm is small. If they are vertical on transom sides, then they produce the same sort of force a rudder does and act with the same lever arm. Perhaps you didn't read the entire discussion. The folks responding to this thread are all very experienced. They design and build high-speed craft for a living.

 

My bad. I see that you are correct. That will teach me to jump in at the middle.

 

That's bit me more than a few times in the 35 years I've been in this game. ;-)

 

My mouth gets away from me sometimes. Once I went looking. Caught it a block away, messing with the neighbors daughter.