Interceptors effect on hull pressure

This report is from 1979 and much has happened since then.
There are lots of studies on interceptors which are much more extensive than this one.

Knowledge is always limited, knowledge can be limiting.

JS

 

Agree much has happened but not that much has been published in detail... If you can point me to any papers that give experimental data and good empiricla methods for calculaitng pressure distributions on hulls please do share with this forum.

For example midship interceptors that you are working on. Apart from Sverre Steen's work and two works out of Italy not much in the public domain since the system was invented back in 60s... Or can you point me to other significant works on midship interceptors?

 

You firstly need to account for the boundary layer...



Test have shown that Cp is dependent upon the coordinates of the x/h ratio.

 

The boundary layer is accounted for in that analysis, AH. As about the x/h ratio, I agree. The point was just to get an idea of the extent of the area interested by the increase of pressure due to the interceptor, for one chosen case.

I am looking at the figures 1 and 2 of the Russian pdf file shared by Sottorf, and am noting that the extent of the pressure disturbance appears to be up to 25-30% of the length from the transom. Pretty much what was qualitatively shown in the graphs in my post. But the values of the pressure coefficient are much higher in the experimental figures, even for a/L=0.01 case - which is the one I had used for the analysis. So much higher that I'd be curious to learn how did they define the pressure coefficient in that paper.

If it was defined as Cp = (P-Pinf)/(0.5 rho V^2), see my previous post for explanation of symbols, then it would mean that the experimental data indicate a much stronger influence of the interceptor on the upstream pressure field than my airfoil-analogy analysis has shown. And hence a possible stronger influence on a waterjet intake placed in that area of the hull.

Don't know, would like to see more papers and experimental graphs on that subject... Anyone?

Cheers


Edit:
I've found the reason for the big difference in numerical values between my foil-like analysis (post #13) and the experimental graph in the Russian paper by Sottorf (post #15). The experiment was carried at a fixed AoA of 6°. After I did the same with the gurney-flap airfoil from my previous post, the difference between the calculated and measured Cp has dropped to 15-20%. That's much more reasonable, considering the approximations of my airfoil-like model of the hull. See the relative graph:



Have to admit, it could also be a testimony of a good work done by Mark Drela on his X-Foil code.

 

That's the problem, there is little, quality, out there in research. I've a few papers on the subject, but they are not in total agreement with each other and often from a 2D analysis and not rigorous enough too.

Thus, i stand by my initial comment, until real qualitative and quantitative research can demonstrate otherwise.

I have not personally, on sea trials, noticed any such effect at all. And when you understand what an interceptor does, the results are not a "wow" moment either. Since we've demonstrated the same effects by simply moving the LCG and then repeating with an interceptor...same effect...and easy to spot from the LCG chase.

I suspect most general comments are based upon suppositions and too much inaccurate or incomplete theory. The effect of trim(LCG) changes on high speed vessels is well known and documented, but little attention appears to be drawn from these very obvious effects. Yet it seems the "wow" factor appears when an interceptor is used...as if it is new??

 

The "LCG chase" is a good argument to what you have been saying so far. As said before, let's hope to see some reliable research work on this issue pop out from somewhere.

Cheers

 

Performance Prediction of High-Speed Planing Craft with Interceptors Using a Variation of the Savitsky Method by Srikanth Syamsundar, Raju Datla, Stevens Institute of Technology. Presented at First Chesapeake Power Boat Symposium, March 2008

Tests were run at Davidson/Stevens with 20 deg deadrise prismatic hull free to heave and trim.
Speed coefficient Cv = 5, 10, 20, 30
Load coefficient Cdel = 1.0, 0.8
LCG location LCG/b = 2.25, 2, 1.75
Interceptor penetration i = 0, 0.25, 0.5 (I assume these are in percent.)

Appendix has results for each condition of velocity, drag, trim, heave, Ctm and drag.

Reduction in drag was found for speed bvelow Cv = 3 "drag increase almost exponentially at speeds beyond that"

 

This kind of makes me wonder why Kamewa are marketing there own interceptor integral to the jet system with a beam little wider than the duct. Anyone with a knowledgeable Rolls Royce contact?
Or is this just marketing and economics?

At high Fn numbers (mono hull) it seems common that the interceptor L/D will eventually exceed that of the hull. The further forward the force acts the more lift can be generated without over trimming. Reverse is true too, at low speed and with a smaller moment it could be a poor trim device.
This was shown as the case for catamaran ferries with lower Fn and the advice was to use tabs.

Whilst interceptors are not new the packages are now cheap, light, have ample reserve trim capacity and an active component of trim steering and roll available.
It is easy to get caught up when something is trending strongly even if it might be the wrong choice.

 

Correct. Otherwise many will opt to buy the "competitors" systems such as on offer here:
http://www.humphree.com/

This is so true for about 90% of the products on the market. And sadly, most people fall for the hype. And then to make matters worse, repeat the mantra from the supplier to others, as if it is a fact! You end up doing 'their' marketing and sales for them, for free!! You only have to read various pages on this and other forums as evidence of that!

 

Inteceptors effectiveness

I am aware that Humphree have all the knowledge but are not about to publicize it. As they are not a manufacturing company but a bunch a Hydrodynamic experts firstly.

One thing I never considered is if the hull pressure could extend far enough forward so as to have interaction with the forward produced hull pressure.

Anyway looks like a student found some time to make a thorough study on the topic (Nov 2011). Including constrained trim to validate lift and many others that should verify the interceptor to be not only a trim device. Wish it came 8years ago before I tried to figure it out myself.

http://www.din.unina.it/tesi dottorato/DELUCA_FABIO.pdf

Slightly off topic and with mixed results is wake disturbance.

 

Excellent find. It looks like a goldmine of good-quality info, will take me some time to read it all. Thank you very much!

 

It is important

to separate out the interceptor effect by itself and the impact due to the changed trim
that the interceptor lifting force is related to the penetration and the width
that the interceptor resistance is related to the penetration and the boundary layer
that the flow over the boat bottom is three dimensional and acts in the surface between air and water
that the pressure on the bottom is unequally distributed, both longitudinal and athwartships

that the majority of reports do not do this


JS

 

It is also important to recognize that the inteceptor alters the pressure distribution over the bottom and adds a bow down trim moment. This trim moment and resulting change in trim may be more significant in the overall effect on the boat than the change in lift by itself.

 

Yes, but that was the main theme of the previous discussion.

 

Itis not totally marketing hype. There are practical reasons for it even if it means a hydrodynamic compromise. Having an integrated trim control system means less installation hassles for the builder and one less supplier he has to get warranties from. Also on narrow beam vessels such as catamarans there is often no space to fit trim tabs or interceptors onto the transom. Having it integrated is then a nice solution.