Yachty aerodynamics question

[Pedro]

Here is a fun question for those interested in sailing and aerodynamics: does the size of an anemometer's cups affect the rotational speed? And does the radius of the arm supporting the cups affect speed likewise? All given same loading and wind speed.


A320 sailor

[ChrisVJ]

The size of the cups, if adequate, is not important. The advancing cup would also be bigger/smaller.

The radius would change the revolutions, obviously, but the instrument is geared to show wind speed for the built radius.

[SoundBarrier]

yeah what Chris said!

Here's another one....

Why do they have those insane bullbus weights at the bottom of the keel. Why not make it a moveable wing so as to generate more control?

[werbil]

SB,

They already do.

Most new maxis have the lead on a keel that can be swung side to side by hydraulics. However, racing rules require a certain amount of stability with the bulb hard over - otherwise an unintential gybe or round up could be nasty. The biggest problem with this system is building it strong but light enough - one failed spectacularly on the Sydney to Hobart a couple of years - actuating arms broke, keel swang violently from side to side until it departed the yacht.

Most yachts with this system do have a controlable foil close to the keel in addition to the rudder.

Like aircraft, weight is critical in racing yachts. A certain amount of ballast is always required in monohulls, and the longer the moment arm you can attach it to the more righting moment you achieve. More righting moment allows you to carry more sail area, and to point higher into the wind.

[singpilot]

My Hunter 433 had a winged keel (fixed) with a large 'bulb'. The difference in draft between the normal keel and the keel I had was just short of 2 feet. The boat was a 'Caribbean Spec' boat, so the shoaler draft was a plus.

[Miraz]

SB - mainly for stability, the 60' boat that I normally sail is certified to self-right from 168 deg of roll when carrying a full main and head sail...takes a lot of righting moment to drag that much sail through the water!

I'm not a fan of swinging keels, we use water ballast - we can add 8 ton of weight to the right bits of the boat when we need it, and get rid of it when we don't.

[TyroPicard]

ChrisVJ

Why not answer the question?
The advancing cup would also be bigger/smaller.
What???? Hilarious.

[LurkerBelow]

Back to the original question ...

looking at a spare parts catalogue for weather instruments
in the distance past, I recall seeing that they offered a rotor
with larger cups to boost low speed capability. This is obvious
when you think about it - bigger cup -> larger force -> more
torque -> better ability to overcome stiction.
If you move the cups closer to the axle, the faster the rotor
will rotate for a given wind speed. However, you then reduce the
torque. There would also be interference effects come into play, reducing
the performance.
Cup anemometers have problems in gusty weather where the rotor
inertia causes overspeed indications at times

[rubik101]

Talking of cups, would similar, half-cup, protrusions on the walls of aircraft tyres not rotate them up to a few hundred rpm prior to touchdown, so reducing the shock loading and tread wear on touchdown? I guess the tyre manufacturers would complain at the reduced wear rate.

[singpilot]

I believe there was a program back in the 60's that did exactly this, it did, in fact spin the tires up, but the wear patterns did not change/decrease enough to warrant the cost of the mod.

The touchdown spoolup wear is inconsequencial compared to braking wear and heavy taxi and turning wear.

Keeping your partners' feet off of the brakes at touchdown is a biggie too. You'd be surprised at how many pilots have no idea what they are doing at touchdown with their feet. Brake applied pressure on the EICAS would prove my theory about certain suspected offenders.