Semi-displacement boats don't generate lift?

Atkin drew fewer designs with reverse deadrise than he did with a "normal" V, as many box keels were sailing/motor boats.

 

I have a semi-planing hull. I'm awfully interested in whether anyone would be able to do cfd analysis on it after some modifications we made. The boat is performing well, but I wonder what happens if rocker is reduced by making the stern a bit deeper is all.

Been lurking the thread, have little to add..

 

If you read the articles on flaps they make it clear the intent is different from trim tabs. It's not to change the trim of the boat or ship, it's to change the pressure field along the bottom, creating a high pressure area under the after part (the stern, but forward of the transom). The rails of the Moxham and reverse chine strakes of the Vripack may do something similar by keeping flow under the bottom rather than allowing it to escape laterally.

I guess my question is, where is the state-of-the-art? How much can we now improve on, say, an Oldport Launch? Since I'm thinking electric power, assume any weight savings will be added back as batteries, for range.

 

An interceptor over the entire transom is the most effective option to increase the lift at the stern.
JS

 

But will that reduce drag in every instance at semi-displacement speed, which I would define as length Fr=0.4-0.9 (D/L 1.34 - 3.0), or is it compensation for a center of gravity that is aft of where it would optimally be for that speed range (or a sub-optimal hull shape)?

What would the designers of the Visby-class corvette say to that suggestion?

 

An interceptor is not a compensation for incorrect center of gravity. It creates a completely different pressure distribution over the aft bottom.
Since the Visby is powered by waterjet, it is not suitable to provide it with inteceptor in the way I suggest. The intake of the water jet is exactly where I intend to raise the pressure. Otherwise, Visby is an excellent example of a semi-planing vessel.

JS

 

Just a lurker, but the interceptor does not reduce any drag (either). It would always increase drag. Flaps as well. I was waiting for wiser men to answer, but might as well comment and let others remark against.

 

Show us your calculations. Don't guess, calculate and measure the result on an optimized boat. We have done it.
JS

 

Well, I was the one pushing back and asking for more discussion; so fancy you coming back and asking me.

In the simplest context, the drag coefficient, all others being equal for a flat plate same as the flow is about 0.001 versus a flat plate perpendicular to the flow with a coefficient of over 1.0. This is largely intuitive as well, so all I am really trying to do is get more discussion against Ditmore's question from 8:07am today in which he asks if an interceptor always reduces drag.. And wanting to understand how, all things equal, a drag equation which is the product of the constant can be less drag for a higher coefficient.

I am not the one who needs to explain what I don't understand!

Not to mention the fact I am wondering what an interceptor would do on my semi-planing power cat..so not trolling here as much as trying to learn..if I overspoke; it may have been on purpose

 

IMHO, you consider only the local drag induced by the flap or the interceptor which obviously induce local drag increase. But you must consider the positive effect of these
appendages on the hull trim or anything influencing the hull drag, which may result in a diminution of the total hydrodynamic drag of the hull , including appendage, versus the drag of a hull without appendage !

 

http://sassdesign.net/Interceptors in theory and practice.pdf
JS

 

It is not the drag per se, that is of interest, but always the lift/drag ratio! If there is a shape that generates the required lift (whatever that may be) with a higher L/D, then it will be superior, provided its other qualities (pitch stability for instance) are acceptable.

Note that at any speed for a surface vessel, there is a "lift" from buoyancy and a corresponding drag plus the lift and corresponding drag from the dynamic forces. Consequently, you have to understand the combination of two L/D ratios that follow different physics; the best combination (ie lowest total L/D) does not necessarily happen where both (L/D)displ and (L/D)dynamic operate at their respective maxima.

The interceptor is changing the pressure distribution of the lifting surface in general. If done correctly, the dynamic L/D is improved, in particular for a lifting surface operating on the boundary between fluids with great difference in density, like air/water or vapor/water (ie aerated or cavitating).

 

Thanks. So, I am technically correct, but missing on the big picture which is what I suspected. I saw the graph of the drag coefficient linked by HJS and unfortunately, not smart enough for a full interpretation, but Cd decreases...

So perhaps I'm even wrong in terms of total? drag.

So, my boat seems to perform a bit better with weight on the bow versus weight on the stern. There is a lot going on; I am sparing you all some of the details. I suspect my engines are a bit deep to start, weight aft adds more. Is it possible the boat can benefit by an interceptor (well, 2 as a catamaran)? In theory, how much lift could I get per inch of interceptor? Any ideas? 5 ton ship, call it 11000 pounds.

My transoms are 14 degrees which is 2 degrees past rigging spec for my engines. It was on purpose, as was nervous about bow lift. Is there any simple way I can test the idea of an interceptor? Say like 1" down off the transoms; fixed plate? It seems insane to me. The angle is 104 degrees then or actually cutting into the flow..

It would be easy enough to bolt 1/4" stainless plate on, but harder to do if I want to stay away from the cutting into the flow issue. I'd need to make a 14 degree mounting plate..

Thanks for the replies and I apologize for any hijack; trying to understand better.

 

That just makes the vessel trim more by the head. How much..depends how much buoyancy you add aft.
Also the ratio of the draft at the transom compared to max draft. By that I mean the transom depth relative to the DWL.

 

Need to be careful here...as that is just one 'spot' on the curve, so to speak.

The resistance of high speed displacement hulls, like yours, has a lazy U shape to it. The resistance is based upon a known/fixed LCG.
If that LCG (and hence LCB) changes, then it effects the shape and slope location of minima of that original curve.

So, simply put, it is what we call an LCS chase.
When tank testing the hull, you select a range of LCG locations +/- the design LCG, and see the change/effects on the resistance by moving the LCG fwd and aft.
Then and only then can you really establish if moving weights fwd is really beneficial.