Check out "Lifting Rudders" in the Files section.  There are other ways of doing it but this one looks best so far. 

My ideas on shallow keels are based on watching the rudders during shunts (a big advantage of rudders not under the hulls).  If you shunt and sheet on quickly with the rudders pointing fore and aft, they stall and the boat luffs into the wind.  If you turn the rudders so that as the boat luffs, the flow stays attached, you can then use them to steer the boat.  A keel would stall, and small rudders would not have the power to turn the boat unless they were at the ends, which introduces a lot of practical problems.    Once the boat is sailing in a straight line, it makes little difference. 

Having said which, Rudolph reckons the keel made a difference on BD, so maybe there is more to it than I have figured out. 

rob

On Thu, Mar 3, 2011 at 2:10 AM, carlos <carlosproacarlos@yahoo.com> wrote:

 

I always wondered for a solution for shallow draft sailing. That is the number one issue that has stop me fro building a harry proa since my sailing grounds varies a lot but I want to be able to sail in the bay side of the Florida keys where depth range from 6 ft to 3 ft.

If my assumptions are right - if a foil cannot be deep then it can be wide - so is there a way to design a rudder that telescope in an out of the water - so lets say you have a wide rudder that can be submerge at 3 ft, and inside of that one you have a deep rudder that can be submerged at 4 ft -being skinnier - both a retracteable. So then for shallow sailing you deploy the wide side of the rudder and once you are in deeper waters you deploy the deep section.

Sounds complicated but if what I assume holds true - then it could be a solution for the shallow draft issue - and it might also help in trimming the boat by keeping the front shallow rudder in place and the back deep rudder for steering

Have anybody tried this or a variation of this?

I look at the proa Equilibre and wonder if the build in design of a central shallow keel may help with the steering issues of the Harryproa - true it is not as efficient but for crusing it might help sinplify and reduce the size of the rudders

--- In harryproa@yahoogroups.com.au, "Rudolf vd Brug" <rpvdb@...> wrote:
>
> Thanks Mike,
> As usual your summary is to the point and helpfull.
> Rudolf

>
> ----- Original Message -----
> From: Mike Crawford
> To: harryproa@yahoogroups.com.au
> Sent: Tuesday, March 01, 2011 4:04 PM
> Subject: [harryproa] Re: New harryproa design - with every buzzword ever discussed <grin>
>
>
>
> Ben,
>
> There has been much discussion on Blind Date's steering and luffing. For a full recounting, the best thing to do would be to search for Rudolf's posts in the past. He's better than most in terms of being concise.
>
> As a general summary: Blind Date is a shallow draft boat. Extra shallow. As a result, there's not as much foil area beneath the water as there could be. The boat was non-optimal in its original design, with the rudders on the lw hull, and when the rudders were switched to the beams (closer together, further from the lw hull), steerage was reduced and luffing tendency increased.
>
> There is an argument that deeper foils would solve the problem by providing the ideal amount of underwater surface area, but going deeper is not an option.
>
> A daggerboard could be a solution, but in a shallow-draft boat, probably not the best idea. Even if there's a crash box, and the boat doesn't sink, who wants to repair the damage? It could be an expensive proposition just to build it in the first place, at least in terms of finding an ideal design that works in both directions, doesn't get unstable or ventilate, and somehow manages to gracefully handle a grounding.
>
> A leeboard might work, but there's still the issue of designing it to be strong, efficient, stable in both directions, and able to kick up.
>

> Rudders with more lift probably won't do the trick with the steering issue, which really depends upon having a larger moment arm (rudders further apart) to generate turning force. And if the rudders are losing their laminar flow with the current design, higher-lift versions are likely to lose that laminar flow even sooner.

>
> A partial solution was a bi-directional tandem keel, which provides most of the lift of a deeper keel, but without the depth. Since it doesn't have to be lifted or flipped on each shunt, the keel is a plus when taking blind sailors out for a day with a limited sighted crew. This has helped the problem, but not fully resolved it.
>
> Rudolf definitely knows what he's talking about; he and Arrtu have more time on the larger Harryproas than anyone, I believe.
>
> There are many suggestions about what to try next, but the budget for modifications is limited. And since I haven't sent a check to Zeilen met Visie ( http://www.zeilenmetvisie.nl/EN/index.html ) to fund my personal theories, I can't complain.
>
> - Mike
>
>

> bjarthur123 wrote:
>
>
>
>
> rudolf: have you thought about replacing the rudders with ones that can generate more lift, either by using a different NACA section and/or by lengthening them?
>
> relatedly, with the existing rudders and "keels" (dagger boards you mean?), how much leeway do you make? if the leeway is not too much, then you're right-- would be better to move the rudders further towards the end of the boat. but if it is too much, then making them generate more lift would fix both the leeway and the luffing tendency simultaneously.
>
> ben
> weta #358
> and thinking hard about a big harryproa
>