Ben
Yes - usually no allowance unless I state a factor which is arbitrary based on my experience with the pedal boats. Which is:
1. There is about a 5% increase in drag between dead calm and the smallest ripples. This may be related to how much laminar flow is maintained on the hull. The condition is detectable in the average speed I can make for a given heart rate.
2. Wind drag is far more significant than added wave drag going upwind. I have no way of really separating these factors. I have observed that the flared hull rides slightly higher going upwind or downwind into waves when their wavelength, combined with speed, is such that the hull rides level - this is a function of speed and wavelength but the hull rides level most of the time. At some point the waves are long enough that the hull follows the waves.
3. Any protrusion that clips the tops of waves kills speed. Any part of the hull or beams likely to clip waves needs to be faired to keep attached flow when passing through green water.
4. When running downwind into the back of waves speed drops dramatically if the bow buries rather than lift. This is mostly a function of the shape of the deck and shape of the bottom - the bow needs to lift rather than dive. I have found a 90 degree "V" on the deck in combination with the flat bottom and slight rocker works very well.
A hull with the best of features noted above will not experience a huge increase in drag due to waves - my estimate is that 10% would be reasonable. If you look at Delft Series data the increased drag due to ambient waves gets up to about 25% in the worst conditions where the hull gets rhythmic pitching - it does vary depending on the shape.
Windage becomes very significant upwind. Even on the sheltered lake where I exercise I often put more energy into overcoming wind drag than water drag. The annoying part is that a 30kph wind becomes 40+kph upwind and 20-kph down wind. So the assistance is a measly 25% of the resistance.
Rick
On 25/09/2011, at 10:25 PM, bjarthur123 wrote:
rick,
is it correct to think that you considered only hull drag in calm water when designing the hulls of your 250kg wing-sail proa, the one with a slightly longer ww hull? if so, how would things change to accommodate ambient waves?
ben
--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
> There are other real world factors that
> are more significant to a lw proa hull than hull drag in calm water.
> The most significant are appendage drag, windage and ambient waves.
Rick Willoughby
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