Doug
With rudder ventilating it really does not matter much what is preventing leeway. The hull and the rudder are both working to prevent leeway unless the rig is very unbalanced. The hull and the rudder are planing surfaces producing the lift to reduce leeway. The highest L/D for the rudder is about 9 and will occur around 3 degrees to true course. The hull will have lower L/D up to about 6 degrees. So if leeway is not obvious then you are probably achieving less than 6 degrees. If this is the situation there is no point trying to reduce leeway further by making the ventilating rudder bigger.
I need to point out that these figures will vary a little depending on size of boat, speed and other parameters but are good enough for discussion.
I would not expect that you will improve anything by changing the section shape of the rudders. They could be flat plates with pointy ends and work just as well. Maybe something to test.
The case where bigger blade might help is if the rig is not well balanced and you need a lot of steering input to hold course. If the blade is working more than a couple of degrees off straight then its AoA is the steering input plus the hull leeway. At 6 degrees true AoA on the rudder (say 4 on the hull plus 2 on the helm) the rudder L/D will be around 7. So you are below the best and more blade area would improve result for that point of sailing.
Over a course there is a lot of variation with the loads. To make meaningful comparisons you need reliable polars from an accurate model or you spend days match racing against a reference boat to find the optimum. For example increasing the blade area will slow you down off the wind unless you raise it.
Rick
On 16/05/2011, at 2:59 PM, Doug Haines wrote:
From: Rick Willoughby <rickwill@bigpond.net.au>
Subject: Re: [harryproa] Re: Design your proa HUll exercise
To: harryproa@yahoogroups.com.au
Date: Monday, 16 May, 2011, 14:48ToddThe V hull will produce lift but nowhere near as efficiently as a cambered centreboard. I also doubt that it will be more efficient than a flat bottom hull. Using your numbers you have increased the wetted surface more than 30% just to increase the lateral surface area. There is a huge cost in terms of extra hull drag. The lift coefficient for a deep V section could easily be half that of a flat panel. So 18sq.ft V may not be any better than 7sq.ft flat.I have some reasonable models for flat panel planing and Savisky for planing V hulls but it stops at 35 degrees deadrise for vertical lift. I know anything steeper than 45 degrees deadrise will dig in rather than slip sideways. So if the deadrise is greater than 45 degrees it should be good at preventing leeway but still not as good as a slab side.RickOn 16/05/2011, at 2:11 PM, tsstproa wrote:Hull length for length. Hull displacement for displacement.
Say on a 20 foot hull with a 400lb displacement.
V hull with a 13'' draft and Square bottom hull with a 6'' draft.
V hull 38sqft wetted surface area, square bottom hull 28sqft wetted surface area.
V hull with 18 sqft lateral area and Square bottom hull 7 sqft lateral area.
V hull skidding out and not producing lift I doubt it!!!
V hull with more than double the area how do you figure it would and not produce lift?
Also at high angles of heel do you see the difference in under water plane shape? The wider the square bottom hull becomes the more defined is becomes.
Placement of rudders on beam is more about convenience for the models. The rudder placement on the model in this clip is to close to the hull. If you look at the end of the clip. The same model with the reverse flow sail design on it you might see what looks like rudder ventilation. I use rudders to test balance of hull to sail. Not to test foil designs. Except of course the large leefoil and windward foil design models.
Todd
Rick Willoughby03 9796 24150419 104 821
Rick Willoughby
03 9796 2415
0419 104 821
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