Stern flaps and interceptors

A question if I may...how do you select a value for the longitudinal length to be used in the formula that you provided in your notes (Lf) ?

 

Afraid that Lf is arbitrarily selected, based on the necessary flap area. I'd say that you use the transom beam for width, roughly 5 degrees of aoa, then Lf will fall out from the area needed to produce the necessary lift.

In racing circles, you find long, narrow flaps. Due to the low aspect ratio, the induced losses are high and the resulting lift/drag ratio low. But I guess at very high speeds, it is also a question of trimming "sideways", and in that context those narrow and long flaps may be producing less surprises than a full-span flap, but that is just my speculation. Anyone who knows (I mean knows, not speculates)?

 

We (my company) employ a lot of trim tabs and interceptors as part, in some cases, all, of the 'effectors' in our vessel stablization systems. For that application, trim tabs are typically much longer than what is most hydrodynamically efficient - e.g. as much beam as possible and minimized length. Longer trim tabs add to control forces (viscous and accelerated added mass reaction terms that are not present in a 'static' trim tab scenario) and obtaining the most control force is what we are after.

For prediction of the interceptor lift, we rely entirely on a limited set of actual test data acquired quite a number of years ago and do a 'lot' of scaling; the validity of which has never been fully proven out. That's not to say the intereceptors do not work; they do work and very well. It's just that we don't have a neat analytical design 'tool' for our interceptors like we do for our trim tabs and trim tab design process.

Nearly 10 years ago, we paid a consultant to run a 'high end' CFD analysis of a proposed interceptor application and received back lots of pretty-colored plots of pressure distributions that seemed to make sense. The cost of the analysis was prohibitive at the time and unfortunately we never got the chance to instrument and validate the predictions..something that was supposed to occur but didn't.