Hull Rocker

We call bridge decks air compressors because of the effect they have, wave a piece of paper up and down and feel the resistance to vertical motion. This has a dampening effect on the ride over waves, air shock absorbers for water craft? It would seem like most of the resistance would be air/skin friction but there is compression from the wing and waves. Car comparisons might give an idea about smooth water effects, for waves it might be interesting to see what the offroad racers come up with.

 

Sorry I don't remember the details but I do remember that the theoretical figures we came up with were obviously no relation to the real world, so we moved on

At some point it becomes important though, like for hydroplanes and the Russian ground effect aircraft on the Caspian sea

Richard Woods

 

I thought various under-body geometries and tricks were banned on
Formula 1 cars because they worked well on dead flat roads, but if there was
some unevenness or waviness, the developed downforce was lost and the
vehicles were prone to flip end over end.

DCockey might correct me, but I would think that a bridge optimised for a
particular Froude number might be quite poor at other Fn for similar reasons.
If the drag due to the bridge is only small, I guess not much is lost or gained.

 

It's been quite a while since I closely followed Formula 1, but my understanding was the flat bottom rules were introduced to limit downforce.

Vehicles can "blow over" if the front of the vehicle rises off the ground enough so that the underbody has a large enough positive angle of attack that a enough pitching moment develops to cause the vehicle to continue pitching and eventually flip over. Essentially the same phenomena can occur with some high speed powerboats.

Mis-labled video of a Porsche race car blow over: http://www.youtube.com/watch?v=BgYyP8YKsas

Unlimited hydroplane blow over: http://www.youtube.com/watch?v=fcyEFTknFt8

I don't see any connection with multi-hull bridge aerodynamics and Froude number sensitivity. Race car and boat blow overs sometimes occur because the contour of the road or water surface starts the pitching motion.

 

An underbody ramp on a simplified bluff body shape such as shown in the attachment does not necessarially decrease trailing vorticity even when overall drag is decreased. Increasing ramp angle may increase the magnitude of the trailing vorticity but also cause a pressure increase on the body in the separation zone, with a net effect of decreasing drag.

There is a similar interaction on the upper portion of such a body. There will be some "hatch" angle, dependent on amount of side taper, length of the hatch, etc, which will have minimum drag. Closer to horizontal and the trailing vorticity may decrease but the base pressure will decrease causing an increase in net drag. A steeper angle than optimum will increase trailing vorticity but the drag will increase because the base pressure will not increase fast enough to offset the added drag due to the increasing trailing vorticity. There is a reason autos designed for low aero drag such as the Prius, Volt and Insight tend to have similar rear end shapes.

Boats, of course, operate on a free surface so variations in pressure are reflected in waves which can alter the relationship between trailing vorticity and pressure.

 

Has anyone checked to see if a horizontal net with separate beams has more air resistance? There is much less vertical resistance of course but 2 beams and an undulating surface of net edges must disturb the flow significantly with 2 vortices and the net. If all the exposing net cords were stacked vertically it would more of a wall than is commonly realized.

 

I only meant it in the sense that the lift and drag of a flat plate or airfoil at angle of attack will be different over a wavy surface to a perfectly flat surface,
just as the underbody of a car will behave differently on a corrugated road.

 

its mainly because a keelboat has a big lump of lead on it

Also a multihull has a lot of weight at the back, motors, beam, boom and mainsail and with the narrow waterline the only way to get hull volume is having more "rocker" aft

 

Just remeber the fundamental difference between an object moving past a waving surface, and an object moving with a wavy surface, say the Froude number dependent wave system generated by an object moving in still water.

 

Thanks Samnz,

I was talking to Brett Bakewell-White some time a go with regards to hull rocker on one of his opposition designers international one metre class (IOM) rc yachts that had rocker that carried on past the keel before kicking up to the stern and he said it probably was to hold the bow up when sailing down wind. I wondered whether the same thing applied to multihulls?

Bas

 

Reference post #8 and check the f-boat site.

 

Thanks Cav,

Sorry missed your reply

B