Interceptors effect on hull pressure

Here are two more references for your collection.
Both are from FAST 2009, Athens, Greece, October 2009.

PERFORMANCE OF A PLANING CATAMARAN WITH MID-MOUNTED INTERCEPTORS
Sverre Steen, Sverre Anders Alterskjær, Andreas Velgaard, Ingebjørn Aasheim

A SYSTEMATIC CFD ANALYSIS OF FLAPS/INTERCEPTORS HYDRODYNAMIC PERFORMANCE
D. Villa and S.Brizzolara

 

Thank you BMcf for sharing that profile. I have been curious about seeing such a comparison of your hooked type interceptor vs a flat plate for a while.
It seems that the pressure profile from the tip of the interceptor fwd is much the same for for a flat plate. Interesting though to see how little pressure spill there is at the chine, for somehow I expected more.

 

I would be very interested to see a long slender hull form (or even low L/B hull forms), with a decent LCG chase (ie more than just 3 points), and then establishing the least EHP at the most optimal location of the LCG. And then with everything else the same (except now at the new optimum LCG location), run the model to confirm the LCG chase and then with an interceptor at the said optimum location to see what difference, if any, is found. I've not seen any data that demonstrates this.

Would be interested to see if any is published. Especially with a range of B/T ratios too for the same L/D ratio.

 

Exactly. And in most cases a "fence" is installed at the chine to block the cross-flow and loss of static pressure locally there. Every little bit helps.

 

The very systematic results were published quite some years ago..one of the early Fast conferences if I'm not mistaken. Dr. Muller-Graf maybe? I'll have to do a lot of digging but I'll find it and pot a copy. But beyond that..for nearly two decades now, we've been very successfull using that information and duplicating those results..sometimes to very good effect. On one catamaran ferry project I recall, shifting the CG aft and adding transom lift (tabs in that case) bought them over 2 knots of additional speed. A very current project involving a moderately high speed monohull patrol craft gained about 1.0-1.3 knots on the top end by adding trim tabs.

And of course there are hulls that benefit not at all in any drag reduction or increased efficiency from lift at the transom..although I don't ever recall a case where adding our transom tabs or interceptors for ride control were shown to slow a vessel any....so we're good with that, since ride control and not drag reduction is what we're all about.

 

I think that may have been a boat I did down under in the early 90s

Sounds far too familiar

 

I'm pretty sure that more than one case was uncovered; the one I was specifically referring to was a Norwegian cat..



You can't get that hull drag prediction done for me if yr on here posting, now can you.

 

You can put LCG anywhere as long as you have sufficient long wet surface in front to create the required boundary layer thickness.

What do you mean by 3 points?

JS

 

Gesss...no pleasing some ppl

See below:



In my previous post (#8) of the same figure, ( shown here: http://www.boatdesign.net/forums/at...terceptors-effect-hull-pressure-lcg-chase.jpg ) to save money many tests for an LCG chase are done with 3 runs. Therefore the curve is a crude extrapolation of the 3 points (shown in red) to give a smooth extended curve of LCG v resistance. The more points you use, i.e. more than 3, the better the results.

 

The interceptor, including the "pressurized cell" in front of it (~40 > 50 times interceptor protrusion), should be regarded a "hydrodynamic lifting device", analog to a traditional hydrofoil.

If the L/D value of a "hld" is higher than that of the basic hull, it will give an overall reduction of drag when taking a portion of the total lift, even with this hull at its optimum trim. Now, from f.i. "Bannikova et al", the L/D of the interceptor is considerably higher than that of planing monohedrons; typical values being 12 to 16 versus 5 to 7 for a planing hull. In this case the interceptor has its place as an "integrated planing hydrofoil", to be compared with the hydrofoil assisted hull. The L/D of ordinary trim tabs are far lower than that of the interceptor, while a fully submerged foil will have roughly 12<L/D<20.

Whether the "hdl" (interceptor, foil or trim tab) lift is applied at the transom, combined with a corresponding change of LCG, or it is arranged closer to the existing LCG is more a matter of convenience (and possibility to arrange adequate ventilation); what counts is that it is carrying a share of the total lift at a higher L/D ratio than the bare hull. The optimum trim of the hull may change with this lift assistance, of course.

In real life, the interceptor is cheaper, less prone to damage, and easier to control than a submerged, external hydrofoil, which makes it useful in spite of its lower L/D. The possibility to use the interceptor to influence wave-making drag or transom separation drag at displacement speeds has yet to be studied, but some preliminary data point this way.

 

This is what I suspect too, but would prefer to see some data on this and its real effects and limits.

It is not so easy to model, since at scale, the interceptor is terribly small, and thus the tolerances on manufacture and installation must be very high indeed, for any research to be conclusive and/or supported by actual sea trials.

 

Here is some data on a "displacement"- boat:


and here is an other:
http://www.boatdesign.net/forums/boat-design/ecodream-optimized-motor-boat-43342.html

js

 

An other example:

At my suggestion this boat was equipped with a 7,5 mm interceptor at the stern. The speed increased thereby from 14.0 knots to 16.0 knots. That is a speed increase of over 14%.

JS

 

Not quite right (in red). It's the Marginal L/D of the hull, not the overall value, that is important for comparison, and for a craft operating at it's design loads, this should be quite a bit higher than it's average L/D.

Figure the added lift of the interceptor. Compare the noninterceptor hull drag at design load to the drag of the same craft running lighter plus the interceptor drag.