Semi-displacement boats don't generate lift?

I'm less interested, in the context of interceptors here, in moving weight vs understanding if I get stern lift from interceptors. But I understand, as you say, there are a few other things to consider.

 

The US Navy and Coast Guard clearly disagree with you concerning flaps, Fallguy. See the links pasted by CGBarry at Stern Flap Design https://www.boatdesign.net/threads/stern-flap-design.52417/. What I'd love would be if Cusanelli & Karafiath or others would get in this thread and tell us about how their work on flaps has affected new vessel design.

Seems like the Saab Visby class has been a great success and the US Littoral class a total failure. Score one for Sweden, Jurgen.

I think you're right to be talking catamarans, Fallguy. At semi-displacement speeds, slim hull multihulls rule!

I disagree with baeckmo that L/D ratio is what matters in this context.. If lift increases, that might reduce drag, but in the end drag alone is the meaningful measure.

I've seen that paper before, Jurgen, and am impressed with your idea of mounting an interceptor forward of the transom, under the hull, making it a deployable step. Seems like this helps make a boat good at both semi-displacement and planing speeds by allowing the CG to be farther forward of the transom than would be optimum for planing alone, yes?

 

This week there was a first test with a new concept. It is a boat with the dimensions 8.3 x 1.9 loaded to about one ton. On the very first run, 21 knots were achieved with a 30 horsepower engine with a standard propeller.

It is important to remember that the aftship interceptor should not be equated with a normal step. It's a completely different technique that aims to create lift with minimal drag. All the components must be matched against each other so that the minimum total drag in relation to the total weight is achieved.

Could not upload the movie, the file is too big.

 

That's very cool! Is the CG farther forward than typical planing boats? What are the learnings concerning semi-displacement speeds, perhaps 7-15 knots for this boat?

 

Today the boat was tested down to 8 knots with a 20 mm interceptor. The result is much better than expected. The CG is about where it usually is for planing boats.
JS

 

Localized drag of any shape, based on the coefficients is always higher for any shape not in the plane of the flow. So, the flap in a down position has more drag over its area than it would in the plane of the flow.

If anyone wishes to explain to me that tbis is wrong, I'm all ears.

Of course, this may or in cases does fall apart in the context of an entire hull. But I have not seen it quantified and if JS did, I'm not smart enough to understand if it has an equation, per se. Of course, a great amount of study is not simple to interpret in 30 minutes of reading.

 

Are you referring to vcg? LCG is fixed, correct?

 

I had the same feelings about this when discussed on another forum many moons ago. I finally accepted that the drag induced by the interceptor, was less than the gain in surface drag aft of it when deployed. Its counter intuitive, until you understand whats going on aft of the interceptor.

 

More important is understanding what is going on in front of the interceptor. Aft of the interceptor, the bottom is dry in my concept.
JS

 

Sure, Im not sure many realise that. Anyone who has flown older gliders with block wood air brakes will recognise your interceptor concept, but with opposite effect, when deployed air speed falls and sink rate increases. Its why had problems initially with the concept, but air is not water and a boat is a buoyant object.

 

I'm only referring to the local drag of the flap or interceptor and have been the entire time.

We've concluded that I'm correct, but that over the entire body or hull it can vary.

But I've yet to see the math.

The drag formula I'm familiar with is

D = 1/2 • fluid density • velocity squared • area • drag coefficient

If the drag coefficient increases, all else equal, so does drag.

In order for total drag to decrease, when velocity increases, fd same and area same, the drag coefficient must change.

But the above formula does not account for pressure...

So, if the pressure decreases through hull lift; the formula is not good enough.

Or, the formula, rewritten for velocity alone as the dependent variable, is missing the quotient for pressure and lift.

Unless fluid density is theoretically lower if the hull not pushing down, so maybe fd is sort of a bad bit of the formula?

discussion?

 

Certainly there is drag induced into the mix, much the same way aft hung trim flaps do, but they are also beneficial depending on flap angle and speed. The formulas and equations are beyond my level of everyday competence, so I am not one to be adding much to the discussion .

I was interested to hear the interceptor has been effective as low as 8 knots.

 

My boat makes quite a spray off the engines at high speeds; I think from engine too low. So the attraction for me is lift.

https://youtube.com/shorts/bVDeH7g10BA?feature=shared

 

Your drag formulation is the usual one in one fluid medium, but a boat evolves in a 2 fluids medium with a so called free surface and gravity effect. So in short, for the drag of a boat as a whole, you have 3 + 1 components :
. The frictional drag (in relation with the wetted surface of the hull)
. The residuary drag (also called the wave drag, which is responsible for the creation of the gravity waves, it is the most complicated component to estimate, no direct (physical) formulation available).
. The areodynamic drag (of all the things above the sea surface)
. If any, the transom drag (if still not clear of water)

For a power catamaran, I did an application to compute the drag and the relation power installed versus speed prediction on calm water, may be you can test it if you can collect the requested input data :
SA-VPP power catamaran https://www.boatdesign.net/threads/sa-vpp-power-catamaran.65725/#post-921104

As regard the interceptor, I think the key point is its effect on the boundary layer upstream of it, moreover if the flow is already detached (being the root of the wake). Then the interceptor changes completely the pressure field upstream of it, contributing to the extra lift. And this extra lift can then reduce significantly the residuary drag component.

 

Not withstanding the comments about the negative pressure field it creates, these appendages do reduce the resistance, but only under certain conditions.
If you have a hull where, for whatever reason, the LCG is not in the ideal location for that hull, using trim tabs/interceptors can "artificially" move the LCG.
When plotted on an LCG chase of resistance curve, the appendages enable the hull to run at a more optimum condition.

Since when doing an LCG chase in a tank test, there is a minima and either side the resistance increases.
Most designs - by that i mean the LCG in the design condition - are not always on that minima, either owing to: space, layout, weight growth, or even just poor design etc.
But these appendages "move" the resistance along that curve closer to the minima thereby reducing the overall resistance.