Interceptors effect on hull pressure

I didn't say interceptors were hype. Only the fact that many products in their marketing contain hype.

Agreed. But that is more of a financial decision than a design decision, per se. Dictated by the budget of the project. It also depends upon the raison d'etre of the interceptor for the vessel itself too. Motion control, trim correction, design correction etc etc. If the design of vessel satisfies the SOR without the appendage, why waste money installing one?

 

So it seems likely that the pressure cell induced by an interceptor will extend quite far forward (under certain conditions).
Would it not then be likely too that this pressure would spill sideways into the supply of a jet duct and any other low pressure area ie lnlets etc.

 

Define "quite far forward"..

And what magnitude is this pressure at the "quite far forward" location?

 

As soon as you "tap off" any flow from the semistationary fluid within the "interceptor pressure cell", there is no more surplus pressure to play with. The mere function of the interceptor is to collect the flow within the boundary layer, that has aquired a static pressure through the "braking action" of the friction (seen from a point travelling with the vessel). It must be understood that we want the WJ to ingest as much of the boudary layer fluid as possible, since this fluid contains "wake energy", that is recuperated in the pump. The jet efficiency is improved by the reduction in the nozzle velocity required for the required thrust.

Suppose that the width of the jet intake is small in comparison with the full transom beam. The difference with/without interceptor can be seen as a change in the radial inflow velocity composant into the intake opening. This flow is very close to a classic potential flow with a radial inflow into a sink, combined with a crossing parallell flow. The shape of the intake in the x -y plane is only physically correct in one single operating point, as I said before. A change in the radial/transverse pressure gradient will have the same impact on the potential flow as a change in operating point. It increases the secondary flow- for good or bad depends on the intake geometry and operating point. BUT a correct basic shape is always the best, since the interceptor introduces an increased drag.

An increase of the gradient will make the intake "feel" a change towards an increased velocity ratio (Vin / Vadvance). Depending on the actual OP (Vin /Vadv) and what OP the intake was designed for, you may see either an improvement or a reduction in terms of inlet efficiency. A corresponding change of inlet geometry would have the same impact.

On the subject of integrated interceptors in WJ-units: In backing situations, traditional trim tabs at both sides of the jet tend to block the reverse flow from the bucket. The best practical position for a trim device is thus directly underneath the pump. BUT in this position there is a very short "fetch distance" for the creation of a boundary layer, so an interceptor in this position is most effective with the looong, narrow type of inlets.

When installed for instance on catamarans, their most important function is in course-keeping. A WJ steering nozzle is stealing quite a considerable amount of thrust when deflected even a small amount. By the use of interceptors for steering at cruise speeds, where the nozzles are locked in straight ahead position, you have a gain in mileage.

 

By "two works out of Italy", do you mean papers presented at FAST 2009?

 

There are certain conditions and hull designs where that statement is not correct..as a series of slender catamaran tests conducted by the Berlin basin proved and as we have demonstrated numerous times at full scale.

There is a range of Froude number where a significant improvement in overall L/D is gained by purposely moving the LCG aft as necessary to balance a hull with a transom-mounted lifting device( tab, wedge or interceptor).

Quite a number of air-cushioned cats were built with transom lift devices just for that benefit and we've achieved good results with several catamarans too, although in the case of the latter we are often installing "oversized" trim tabs to accomplish dynamic motion control and they are larger than what would be optimum for the drag reduction alone.

The "nice" thing about interceptors is how little force they require to activate/deploy as compared to trim tabs.

 

No I was referring to these two papers:

De Luca, F. (2011). Experimental Study on Interceptor ’ s Effectiveness. Università degli studi di Napoli “Federico II.”

Brizzolara, S. (2003). HYDRODYNAMIC ANALYSIS OF INTERCEPTORS WITH CFD METHODS. FAST 2003 The 7th International Conference on Fast Sea Transportation (pp. 49-56). Ischia.

Both are on the internet and are pretty easy to trace with Google.

 

I agree with BMcF here. There is sound evidence that shows the benefits of adding an interceptor or stern wedge improves performance of even an optimised hull. It seems both devices create a more optimum pressure distribution along the hull. From my personal experience experimenting with this on catamarans, optimum performance is achieved with zero or slightly negative running trim angles. As usual there is a compromise: directional stability becomes a bit light.

 

I located some of the CFD work we had done for the intereceptors we provided for a very fast 74m cat..I believe that this graphic is from a 50-knot case.

I apologize for the "awkward" view; what you are looking at is the aft hull bottom and transom, from the centerline to one chine. The interceptor is full down but has been "hidden" so that the pressure distribution is visible throughout.

 

Sorry, I can't figure out where are the transom and the bottom. What is the flat panel protruding along the left edge? What is the curved white silhouette on the bottom plane?

 

The hull is "lying on its side in that view. That flat plate is the extension of the hull bottom, extending directly aft of the transom, that is an integral part of our interceptor design; we refer to it as the "seal plate".

The 3D pressure distribution is shown spanwise from the hull centerline to an outboard chine. You can see that the pressures can be quite high locally and just forward of the interceptor blade but diminish quickly as you move forward.

The "curved white silhouette" is the projected section area of the interceptor blade, fully deployed, that is removed/hidden from the view so that only the pressure field is visible..and all of it.

 

Ah ok, thanks. And how much of the hull length from the transom is visible in the picture?

 

roughly 3 meters..

 

Well, I was thinking in terms of percentage. Without knowing the other dimensions of the hull, the length of 3 meters doesn't tell much...
Do you remember what is the height/waterline_length (h/L) ratio of the interceptor in that CFD analysis? It's a sheer curiosity of mine, I would like to perform a very rough comparison with gurney flap, like the one in my post #19.

 

Sorry...I believe the waterline length was about 68 meters.

That interceptor had a fully deployed penetration depth of approximately 150mm (so that is what the illustrated pressure distribution shows) and the beam is the full transom beam of 4.9 meters.