Semi-displacement boats don't generate lift?

fallguy · Sep 10, 2023

Dolfiman said: ↑

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.
Click to expand...

Ad Hoc said: ↑

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.
Click to expand...

interesting, thanks

BMcF · Sep 11, 2023

Ad Hoc said: ↑

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.
Click to expand...

We purposely design surface effect ships with the LCG aft of that required for the optimum running trim and then "correct it" with transom trim tabs or intercepters. That design detail results in a reduction of the total resistance by a very significant amount..in excess of 5% in calmer conditions at speeds in the 40-50 knot range (and higher).

BlueBell · Sep 11, 2023

( Haven't read the thread. )

Don't forget about "squat" ( anti-lift ).

HJS · Sep 11, 2023

BlueBell said: ↑

Don't forget about "squat" ( anti-lift ).
Click to expand...

Elaborate on what you mean.
JS

BlueBell · Sep 11, 2023

HJS said: ↑

Elaborate on what you mean.
JS
Click to expand...

The drop of pressure below the ship's bottom causes a vertical sinking of the ship's hull in the water, at the same time and depending on the vessel's block coefficient Cb she will trim forward or aft or will sink deeper into the water on even keel. The sum of all vertical sinking and trim is called “SQUAT”. A down force, the opposite of lift.

Log in or Sign up

Semi-displacement boats don't generate lift?

fallguy Senior Member

BMcF Senior Member

BlueBell . . . _ _ _ . . . _ _ _

HJS Member

BlueBell . . . _ _ _ . . . _ _ _

Stern taper on semi-displacement hulls

Flexsys and SRW(Semi Rigid Sails)and.....Money!

semi submersed kayak

Semi-Submersable hull dynamics

Development of a reliable performance prediction tool for semi-planing hull forms

Calculation of Semisubmersible RAOs using Morison Theory

Flat stern bottom extention on displacement power boat?

Waterline length includes the rudder? Displacement speed.

Barge efficiency-displacement speed

Prop/rudder optimization for longe range single engine displacement vessel 30k lb displacement