Subject: Re: [harryproa] Re: Rudder lift? |
From: Rob Denney |
Date: 6/22/2010, 8:07 PM |
To: harryproa@yahoogroups.com.au |
Reply-to: harryproa@yahoogroups.com.au |
Hard to get really high prismatics with rocker, but they may not be required. Something else to test.
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.
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
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@yahoogrou
>
> 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@yahoogrou
> 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@
>
>
> 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@yahoogrou
> >
> >
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,
> > >
> > >
>
>
>