----- Original Message -----From: Rob DenneySent: Wednesday, June 23, 2010 2:07 AMSubject: Re: [harryproa] Re: Rudder lift?Hard to get really high prismatics with rocker, but they may not be required. Something else to test.
On Rare Bird in the Gladstone race we were hit with a pretty good gust and the boat took off on a broad reach. Huge amounts of spray from the rudder, but the bow barely went down at all and steering remained easy. It was pretty near the comfort limit if we had been cruising. Doug put a video of it on You Tube.
If you are immersing the bows further than this on Blind Date (which is lighter, but has the same lee hull), you must be putting some serious force into it. How much wind was there? What sail did you have up and were there any waves? Anything else that might be relevant?
On Elementarry moving my weight fore and aft does not make much difference to the bow trim as I can't move far and it sters pretty well bow up opr bow down, except when it is going slowly. Upwind, I sit amidships or aft if I need to be near the mainsheet cleat. Really pushing it downwind, I sit on the aft beam, but have still managed a summersault..
What was quite noticable was the difference in bow down trim between having the front rudder (reasonably well shaped and faired, but nowhere near perfect) up or down. Up and the nose lifted noticably and the boat went much quicker. This happened both up and down wind, which is one of the reasons I switched to using a single rudder. I guess the drag would also add to the tendency to luff, but the rudders are still pretty close to the hull, so it was not noticable.
Still plenty to learn!
rob
On Wed, Jun 23, 2010 at 6:51 AM, Rudolf vd Brug <rpvdb@freeler.nl > wrote:
The buoyancy would be there, but located differently.Changing the lateral profile can change the behaviour of a boat dramatically.It could be the boat would seem to have a more bow down attitude, but the truth would be the deepest part of the hull moved forward just a bit instead of all the way to the bow.High prismatic coefficient is good to prevent nose diving, that doesn't change if the hull has rocker.Leeboards have advantages, but I don't know of leeboards at high speed( say over 12 knots).Sent: Tuesday, June 22, 2010 5:28 PMSubject: [harryproa] Re: Rudder lift?I don't see that rocker in the lw hull really helps. It will still require similar extra buoyancy in the bow to counteract the sailing loads. (it would also give more vertical load vectors form the sail, but this is pretty marginal) A rockered ww hull would make it worse.
A higher prismatic coefficient giving greater extra bow buoyancy for less side area should make it better.I am working on 8.5-9 prismatic coefficient, depending on the payload
Your work with a single leeboard is dominating my ideas at the moment with the rudders at 9m apart just out from the hull, though I am wondering about two leeboards. This is starting to look a bit excessive.
--- In harryproa@yahoogroups.com.au , "Rudolf vd Brug" <rpvdb@...> wrote:
>
> Hi,
>
> The steering problems on BD to me seem related to the bow being pushed down so it becomes the deepest part of the boat.
> It took me some time to realise that this seems to be happening and that it relates to experiences on other boats. For a non-proa a solution may be to trim the stern down. For a HP that would mean pumping water ballast. Another solution for a proa might be to design rocker in the lw hull.
> A rockerless lw hull of a HP in my opinion needs its rudder as far aft as possible. The small ones like Harriette are easily trimmed by shifting weight, which is a instictive reaction when sailing a small boat at speed.
>
> regards,
> Rudolf
>
>
>
> ----- Original Message -----
> From: Gardner Pomper
> To: harryproa@yahoogroups.com.au
> Sent: Tuesday, June 22, 2010 2:58 PM
> Subject: Re: [harryproa] Re: Rudder lift?
>
>
>
>
> Hi,
>
> The leeway is not my primary concern. I am worried about steering. I have heard several mentions about Blind Date and also about Sidecar, that they lose steering control, can't shunt, or head uncontrollably up into the wind before their rudders were enlarged, moved, etc.
>
> I don't see the advantage of a shallow draft boat if I need to have my rudders down 5' in order to steer. I have been trying to find out if this is just a problem when the boats are overpowered, or strong winds or something, but no one has admitted to that, so I am worried that I won't be able to sail in less than 4-5 feet of water. I will be able to drag my boat up near the beach, but the last mile will have to be done under power.
>
> - Gardner
>
>
> On Tue, Jun 22, 2010 at 8:05 AM, robert <cateran1949@...> wrote:
>
>
> In shallow water, there is a some leeway resistance due to the water having a hard time getting out of the way. The shallower the water, the greater the effect for the same depth of boat.
>
>
>
> --- In harryproa@yahoogroups.com.au , Mike Crawford <jmichael@> wrote:
> >
> > Gardner,
> >
> > Your Mainecat/5200 idea is safe with me. I'm building a plywood dory
> > this summer and will be doing the same thing. Even if it doesn't work,
> > the theory is comforting./ /
> >
> > So now you're basically tripling your submerged foil area for a 1.5'
> > depth: two rudders at normal width, and two mini keels that I'm assuming
> > would each be twice as wide as a rudder. You won't have ideal steerage
> > and leeway prevention, but my guess is that this is a moot point.
> > Either you have two rudders, or you have rudders plus dagger boards or
> > mini keels. Anything you do beyond the rudders will likely help.
> >
> > I'd try keeping the keels as close to each other, though. This
> > wouldn't be idea for beaching, but as Rob pointed out, tires are
> > probably better than keels anyway -- that way there's no risk of gravel
> > sanding off your bottom paint, or of bouncing in the surf taking your
> > keels off. In keeping the keels close together, almost like Rudolph's
> > tandem keel, the keels shouldn't impact steering that much, and could
> > even conceivably help. Two keels far apart, on the other hand, might
> > make it harder to shunt, particularly when you don't have as much rudder
> > in the water as you'd like.
> >
> > At least you'll have the option of two counter-rotating rudders.
> > That's definitely going to help steering in minimal depths when compared
> > to a cat.
> >
> > - Mike
> > / /
> >
> > On 6/21/2010 1:53 PM, Gardner Pomper wrote:
> > >
> > > Hi,
> > >
> > > Yes, your description of when to use mini-keels is what I had in mind.
> > > I would rather not build them, but I am unclear as to what the minimum
> > > rudder area needed when sailing in light winds. Here is the scenario I
> > > have in mind:
> > >
> > > In the chesapeake, or the bahamas, there are many spots that are 2-3
> > > feet deep that I would like to glide along in relatively light winds.
> > > The waters are sheltered, and therefore flat, and the wind is likely
> > > to be less than 10 knots. When doing this, I would like to have a
> > > draft of 2' or less. I can't have my rudders extended to 4 or 5' depth
> > > to get the requisite 2% of sail area, so I thought that fixed
> > > minikeels, maybe 1' deep, would give the boat a 1.5' draft. That, plus
> > > the rudders raised to 18" draft would give me more foil area than the
> > > rudders alone. I would not be looking to sail at > 5 knots under these
> > > conditions, because i really don't want to hit anything.
> > >
> > > In that scenario, with a boat about the size of a harry, or a
> > > harrigami, would I be able to steer with just the rudders (total
> > > submerged area of 3 sq ft, vs 400 sq ft of sail)? Would I be better
> > > off with an additional 3 sq ft of mini-keels?
> > >
> > > My Maine Cat 30 had the mini-keels attached with 5200, for precisely
> > > the reason you mentioned.. in a severe collision, the keels were
> > > supposed to break off without ripping open the bottom of the hull. I
> > > have no idea if that would have worked or not, but it gave me peace of
> > > mind to believe it, so please don't contradict the idea <grin>.
> > >
> > > - Gardner
> > >
> > > On Mon,
> > >
> > >
> >
>