How much energy does sailing in chop take?

Another perfect sailing day, but still on the DL for another 4 weeks. G. mentioned wanting to filter out the chop input to the flap, so I added chop to the stability model to see the effect. The plot above is the work ( drag times distance ) for the foils in the case of a 3 ft wave and a 3ft wave with 0.5 ft chop. Vertical axis in ft-lbs (1 ft-lb = 1.35 Joules), horizontal in feet. The spike represent the extra work due to response to the chop by the flap.

It does seem like filtering out the high-frequency could be a good idea....all the extra work has to be slowing the boat down. But any filter will slow the response (e.g. to sharp waves) down so there will be some optimization.

There was one question about the work in the chop ( green line) case, so I plotted with a better scale. The wave and chop start at 200 ft. Before that, the work in the green/blue cases are the same. After the chop starts, the work by the flap responding to the chop is taking more energy out of the boat.

This model has some simplification so it is really not good for predicting absolute performance like the one Alan posted in Doug Culnane's website. I just ran this case to get an idea of the energy going into the response to chop, which ideally the control system would ignore.

Just need to figure out a way to implement filters without op-amps and batteries.

If one is good, are two better?

The poor performance of elevator control led me to try some other schemes. Leaving out the ones that did not work, the above graph compares the response of a flap only control scheme to a scheme that moves the elevator in the same direction as the flap, but at half the gain.

It is not clear that this is a better scheme but it does get all the control forces acting in the direction that the boat wants to move when the wand senses the trough.

The trough is an isolated half-sine of depth 3 ft and width 30 ft (1 m by 10 m).

Findlay/Turnock VPP from Uni. of Southampton

Just in case you have not seen it, this is a link to a paper from Findlay/Turnock on a velocity prediction program written in Basic/Excel. It does a good job explaining the inputs to a VPP and has a nice flow diagram for the logic of the program.

http://eprints.soton.ac.uk/52462/01/Findlay_Turnock_Foil_VPP.pdf

Accuracy of inputs

We had some off-line discussion about how to get a good moment of inertia for the dynamics model. Bifilar pendulum is a standard way. The Finn class uses the Lamboley swing method which can be found here ( www.lamboleyetudes.net). I decided to run the model with +/- 50% pitch inertia change from Alan's original estimate of 20 slug-ft-ft to see if it is sensitive to inertia changes.

Result is shown above for the case of elevator control. Although the high inertia case (30 slug-ft-ft) shows the expected slower response, the basic character of the elevator control ( height goes up before it goes down) does not change.

Still, if someone wants to measure inertia (Lamboley seems the easiest method) it would be interesting to know a more accurate value.