Hi Rudolf,
You got me thinking .
I tried putting some numbers to the idea to see where it went.
Take a 15m Harry hull dis[placing in the order of 900kg when unloaded.
For simplicity make the cross section an 800mm U.
Give it a prismatic of .8.
Give it enough rocker for the bow and transom both just kissing the waterline.
The draught should be about 250mm.
Get the geometry absolutely perfect so that the extra displacement from righting moment while sailing exactly matches the for /aft weight shifting so that the transom stays at the same water level. If the transom lifts too much the centre of lateral resistance moves forward. If the transom depresses, it probably doesn't matter too much but suggests the hulls are a bit skinny for the displacement and the bows would need to be a bit higher causing windage problems
Drive the boat hard enough to depress the bow 500mm
Look at the profile.
Now take the same hull and deepen the bows to a soft V to take up the rocker.
This time have the geometry so that the stern keeps the same draught as the sailing loads increase. The boat would have to be marginally skinnier to have the equivalent geometry but not too far off.
Compare profiles
I see no significant difference in the centre of lateral resistance and the straight rocker one should have a kinder motion in a chop
For the ww hull I can see problems with rocker as you start to lose waterline length as the bows depress but the weight is taken off the hull.
--- In harryproa@yahoogroups.com.au, "Rudolf vd Brug" <rpvdb@...> wrote:
>
> The way I see it, if the lateral plane looks like a circle segment preferably with the bows extending above the water. Depressing the bow would mean the waterline is extended to the front, the deepest part of the hull is moving forward just a bit.
> In a straight keel design the depressed bow means the lateral area changes to a triangle with the bow being the deepest part.
> I think in any sailboat the bow is depressed more or less when sailing. Rocker doesn't automatically lead to more bow depression, that is a matter of design.
> It may lead to more forward pitch but there is much less forward shifting of centre of lateral area.
>
>
>
> ----- Original Message -----
> From: robert
> To: harryproa@yahoogroups.com.au
> Sent: Thursday, June 24, 2010 2:25 PM
> Subject: [harryproa] Re: Rudder lift?
>
>
>
> I don't understand how rocker improves lateral balance in a double ender. The way I see it is that rocker means that the stern rises more and transfers the plane of resistance even further forwards. Rocker leads to lower prismatic coefficient and therefore more bow depression. I am hoping for better lateral balance by going to a schooner rig. By driving the front rig more than the rear one, it should allow sailing in skinny water
> The only reason I can see for rocker in a multi is to make it easier to tack.
> There is something going on that I don't understand if the rudders further apart improve the stability as my vector analysis agree with rob that the main difference in the fixed foils is to move the drag to lw. This is why I am going for a leeboard rather than fixed foils attached to the bottom as it moves the drag even further to lee and allows kick up
>
> --- In harryproa@yahoogroups.com.au, "Rudolf vd Brug" <rpvdb@> wrote:
> >
> > The new rudders of BD seem to show that thew minimum distance for the large rudders is the old position.
> > Also I would say that lateral balance in a sailing hull is a nice thing to have, and that seems to lead to rocker imo.
> >
> > rudolf
> > ----- Original Message -----
> > From: Rob Denney
> > To: harryproa@yahoogroups.com.au
> > Sent: Thursday, June 24, 2010 8:09 AM
> > Subject: Re: [harryproa] Re: Rudder lift?
> >
> >
> >
> > The bows may be narrow, but they have far more buoyancy, much further from the mast than any other boat I can find with the same sail area.
> >
> > Most boats sail bow down when pressed hard. They don't have control problems.
> >
> > I can see how a daggerboard/leeboard/fixed keel have lower loads than an oversize rudder, but I cannot see how they do anything to the centre of lateral resistance that the oversize rudders don't, apart from moving a component of it (the rudder drag) a metre or so to windward. Given the extra work, cost and danger of the fixed foils, I will stick with oversize rudders, built strong. However, keep experimenting, the more data, the better.
> >
> > rob
> >
> >
> >
> > On Thu, Jun 24, 2010 at 8:37 AM, Arto Hakkarainen <ahakkara@> wrote:
> >
> >
> >
> > This reminds me of some of the model testing Todd did a couple of years ago. When the boat is pressed hard with high rig the bow seems to be pressed under and knocked down and/or luffs to wind. Seems quite uncontrollable and violent incident based on the model tests. You should find the test videos from youtube. My current connection is too slow for youtube search this week...
> >
> > It seems that the high rig and narrow wave piercing bows will cause the boat to sail bow down at high speed when it is not wanted. Bow down may be good for light weather work for boats with wide aft sections since it reduces wetted surface. In heavy weather and higher speeds bow up would be best choice but how to get that is the problem.
> >
> > My favourite boards for proa are the twin bruce foils developed by Fulgencio and demonstrated at the proa file. In his system the bow foil is canted to give more lift up for the bow and more lateral resistance at the aft end. Foils are linked so that canting one foil to more horisontal will cant the other foil to more vertical position. Canting forward foil to more horisontal will lift the bow and move the lateral plane aft which will lead to better balance and better hull position.
> >
> > Downside of the system is that IMO it needs also rudders for added control when sailing and all control when motoring. Also making the system strong and simple enough is a challenge and with also rudders means four moving foils on the boat which is hardly simple anymore.
> >
> > Arto
> >
> >
> >
> > --- On Wed, 6/23/10, Rudolf vd Brug <rpvdb@> wrote:
> >
> >
> > From: Rudolf vd Brug <rpvdb@>
> >
> > Subject: Re: [harryproa] Re: Rudder lift?
> >
> > To: harryproa@yahoogroups.com.au
> >
> > Date: Wednesday, June 23, 2010, 10:45 AM
> >
> >
> >
> > BD has a rockered ww hull while RB has a straight keel to ww. That is a difference that might change something.
> > On BD at the test sail with new rudders sailing with just the the main was hard enough. Lots of rudder angle to keep course.
> > Raising the jib (20 m2 on a total of 76m2) gave a lot of extra power, we felt the drag, the boat didn't accelerate and rounded up uncontrollable.
> > We have always been surprised by the bow down attitude of BD in a bit of wind, maybe from 14 knots upwards. Also we haven't seen speeds
> > over 14.5 knots even at winds where you would expect more.
> > With the old rudder set up we tried lifting the front one and lots of different positions but it didn't seem to make a lot of difference.
> >
> > Rudolf
> >
> >
> > ----- Original Message -----
> > From: Rob Denney
> > To: harryproa@yahoogrou ps.com.au
> > Sent: Wednesday, June 23, 2010 2:07 AM
> > Subject: 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).
> >
> >
> > ----- Original Message -----
> > From: robert
> > To: harryproa@yahoogrou ps.com.au
> > Sent: Tuesday, June 22, 2010 5:28 PM
> > Subject: [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@yahoogrou ps.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@yahoogrou ps.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@yahoogrou ps.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,
> > > > >
> > > > >
> > > >
> > >
> >
>