So are you basing your theory on drawing out (lengthening) the flat center section of keel towards bows and adding flip in the nose anticipating the trim for the flat section by displacement? This would make sense on long hulls as you say due to pitching is less likely due to length and over all weight of hull from wave. The actual working area of the hull with its lengthened flat center section (sweet spot) achieving more buoyancy towards each bow through primary (static) and secondary (planning) adding lift forward, for a given displacement.

How does this compare in a no rocker hull with the buoyancy carried out to bows this way. More as a (vertical narrow column) stretched forward in a wave piercing form. Where primary flotation and secondary flotation seem to have no difference.

Todd

--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> A flat bottom hull has some advantage over a semicircular section to
> limit trim changes for a proa hull without moving weight. The
> objective would be to keep the boat close to level trim throughout
> its speed range and reduce the need for heavy handed control
> surfaces. With a high length to displacement ratio I believe this is
> achievable and have reasonable tools to predict how the boat will
> trim through its speed range.
>
> Once the length to displacement is reduced it leads into a slippery
> slope and there will be trim changes that cannot be managed without
> shifting weight and/or relying more on control surfaces.
>
> I previously analysed a 1t hull designed for 25kts. The lowest drag
> hull is 12m long but it cannot produce enough moment to counter the
> pitching from the sail needed to drive it. Getting up to 15m it
> becomes possible to produce a flat bottom hull that will stay in trim
> up to 25kts.
>
> In the case of a length constrained hull the extra drag of the
> shorter hull requires more sail. It therefore will be a taller rig
> for the same efficiency so CoE is higher. Lowering the aspect of the
> rig by increasing span reduces efficiency so this would push up
> lateral loads. No matter how it is done, the rudders need to be
> bigger to counter the higher sail moment so this shifts the resultant
> drag deeper. The pitching moment from sail to drag is therefore
> increased so the bow gets further pressed.
>
> You are correct about the higher water level at the bow than stern as
> it is the wave formation that is causing the changes in trim.
> However it is preferable to generate all the lateral resistance from
> the rudders rather than relying on the hull in any way. The L/D for
> rudders having aspect ratio of 4 could be 20 or more whereas the L/D
> for the hull will be very low. So trim the rudders to avoid leeway.
> (An advantage of bow and stern rudders on a proa is that they can be
> trimmed to avoid leeway.)
>
> Keeping the sail upright by avoiding it pitching forward should keep
> it in balance longitudinally. Canting the mast and/or setting it to
> windward of the centeline of the lw hull so the CoE stays close to
> inline with resistance as the ww hull unloads should also reduce the
> input required from the control surfaces. With the proposed flat
> bottom hulls I would set a static list so both hulls run level at the
> design speed. You will see they do this on the amas of the big
> tris. The lw ama sits on its lines when the central hull is about to
> fly. For a cruising boat you would want some safety margin but same
> principle for nice balance apply.
>
> Rick
> On 09/07/2010, at 12:29 PM, robert wrote:
>
> > Thanks for this. It gives me some numbers to work with. Interesting
> > that the depth to width ratio went up so much for the shorter hull.
> > Assuming the wave dynamics reducing the bow down effect implies
> > that the bow is deeper in the water than the stern, but the wave
> > dynamics means the water level is higher at the bow than the stern.
> > There is still the same movement of CO lateral resistance forwards.
> > About 8% extra resistance for 15m against 23m is something I can
> > live with.
> > 2.9kN generated from 7m from the waterline on a hull with .8
> > prismatic is not that different from a couple of people standing on
> > a bow. A back of the envelope calculation gives in the order of
> > 100mm bow depression. On top of this is the induced drag from the
> > foils which clearly are considerable and the induced drag from the
> > leeway resistance of the hulls and the air drag of the stuff above
> > the water.
> > It would be interesting to see the the drag from a 10m long hull
> > with a bow down attitude of 1:20 at 1 tonne and 2 tonne
> > displacements and how much the rocker affects the drag in these
> > circumstances. I am going with 600mm hull waterline beam but was
> > wondering how much an extra 50mm would make
> >
> >
> > --- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@>
> > wrote:
> > >
> > > You need to determine how various constraints are going to impact
> > the
> > > drag and shape before you overconstrain.
> > >
> > > The lowest drag 4t hull for 20kts will have a LWL of 23.1m, BWL of
> > > 840mm and draft of 363mm. The drag will be 2.67kN.
> > >
> > > Applying a constraint to get a hard chine results in a hull with LWL
> > > of 22.8m, BWL of 813mm and draft of 342mm. The drag increases very
> > > slightly to 2.69kN.
> > >
> > > Applying an additional constraint to limit LWL to 15m results in BWL
> > > of 783mm and draft of 463mm for lowest drag. This hull has zero
> > > rocker. The drag is now 2.91kN.
> > >
> > > The problem with the 15m long hull is that it will not generate
> > > enough pitching moment to counter the moment from the sail required
> > > to propel it to 20kts. So it will sail with a bow down trim. I have
> > > not checked the trim with the 23m hull but it is likely it could
> > > generate enough bow up moment to counter the sail. The lowest drag
> > > hull has nice rocker in the ends that helps with the bow up trim.
> > >
> > > There would be now point in analysis the trim for the 15m hull
> > but it
> > > would be worth seeing what could be done with a 23m long hull
> > >
> > > Rick
> > > On 08/07/2010, at 8:33 PM, robert wrote:
> > >
> > > >
> > > > A powered craft is very different from a sailing craft due to the
> > > > sailing loads. I would be interested in a 20 knot 15m double ended
> > > > minimum drag hull with a 900mm stem and a .85 prismatic
> > coefficient
> > > > with 4 tonne displacement with the bow just submerging and the
> > > > stern just on the edge of lifting.
> > > > At the ww side it needs a 10m length hull varying between 3 tonne
> > > > at slow speeds and .8 tonne at 20 knots and bow down attitude
> > of 1:18
> > > >
> > > > I am not sure why you have flare in the bow. I feel a better
> > method
> > > > is to have more buoyancy down low and a reversed stem
> > > > --- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@>
> > > > wrote:
> > > > >
> > > > > Todd
> > > > > If you give an idea of the speed you would like to achieve
> > with that
> > > > > displacement then I can give you an idea of what the lowest drag
> > > > hull
> > > > > would look like and then what lift can be achieved with
> > various flat
> > > > > sections.
> > > > >
> > > > > The wave piercing is no problem going upwind if you keep the
> > draggy
> > > > > bits above the wave crest. With the slight flare in my hulls
> > I get a
> > > > > bit of extra lift in waves because the volume immersed for
> > average
> > > > > draft increases on an irregular waterline. The increase in
> > drag is
> > > > > of the order of 5 to 10% with waves unless you are dragging
> > unfaired
> > > > > parts through the water.
> > > > >
> > > > > It can get very wet on my boats though because the seating
> > position
> > > > > is not quite high enough to get me above the waves that I plough
> > > > > through. In larger waves the boat tends to rise and fall with
> > the
> > > > > waves. On a proa it comes down to how high the bridge beam is
> > set,
> > > > > how much of the windward hull is going to be forced through
> > water
> > > > and
> > > > > the fairing of all the bits that could be submerged.
> > > > >
> > > > > Going down wind I have not yet managed to eliminate diving if I
> > > > press
> > > > > hard down a wave. I can drive my hulls into the back of a
> > wave to
> > > > > the point where I am pedalling in water with the bow fully
> > immersed.
> > > > > In my latest hull I am playing around with the deck shape
> > with the
> > > > > aim of making it easier to lift when submerged. At present I get
> > > > > more down force with the deck submerged than lift from the flat
> > > > > entry. The faster I go the deeper it gets.
> > > > >
> > > > > Some recent power boat designs are fully wave piercing - eg
> > > > Earthrace:
> > > > > http://www.youtube.com/watch?v=dxJOJDGchTs
> > > > > There are others with same concept.
> > > > >
> > > > > Even with wave piercing to the degree shown in the Earthrace
> > > > > modelling the water drag does not go up much if all the bits are
> > > > > faired. When dolphins want to travel fast they fly and dive
> > > > > repeatedly. They get deep enough to avoid wave drag and then get
> > > > > airborne to avoid water drag. This results in lowest overall
> > drag at
> > > > > their high speed. It also gives them the opportunity to
> > breathe of
> > > > > course.
> > > > >
> > > > > There are some interesting videos on Youtube of amas on big tris
> > > > > driving through waves.
> > > > >
> > > > > Rick
> > > > > On 07/07/2010, at 7:24 AM, tsstproa wrote:
> > > > >
> > > > > > Hi, Rick nice work.
> > > > > >
> > > > > > What about narrow flat sections with a deep draft 14-18'' with
> > > > very
> > > > > > pointy bows with large displacement 2,500-3,500lbs.
> > > > > >
> > > > > > Doesn't this negate the whole wave making drag and lift theory
> > > > even
> > > > > > when rockered 12'' lift from center keel to bottom bows?
> > > > > >
> > > > > > I Know that if you have a high displacement shallow draft hull
> > > > > > without rocker or very little the stern can cause problems
> > for the
> > > > > > bow, especially for high prismatic coefficient hull, unless
> > its
> > > > > > extremely slim usually meaning deeper draft Piercing hull vs
> > > > riding
> > > > > > over(heavily rockered low draft under 12''). Where's the
> > cut off
> > > > > > for Rockered depth of draft and non rockered piercing hull
> > > > depth of
> > > > > > draft?
> > > > > >
> > > > > > It almost seems you can't have you cake and it too. Commit
> > to wave
> > > > > > piercing for coastal waters and rockered for off shore
> > sailing.
> > > > For
> > > > > > large hull proas 40-60 feet. Difference in wave heights to be
> > > > > > encountered while sailing.
> > > > > >
> > > > > > Anyone seen the deadliest catch can't imagine a 50 foot wave
> > > > > > piercing sailing hull smoothly cutting through those kinds of
> > > > seas?
> > > > > > There are two distinct boats types I can pick out on the
> > show .
> > > > > > Ones a barge style and the other Norwegian boat sharp bow with
> > > > > > flare sits deeper in the water. Not sure on the entire bottom
> > > > > > shapes but seeing them both punch through 30foot seas one
> > can see
> > > > > > the difference in how the ride through the waves.
> > > > > >
> > > > > > Todd
> > > > > >
> > > > > >
> > > > > > --- In harryproa@yahoogroups.com.au, Rick Willoughby
> > <rickwill@>
> > > > > > wrote:
> > > > > > >
> > > > > > > Problem with a large proa is you are not going to shift a
> > large
> > > > > > > amount of weight each time you shunt.
> > > > > > >
> > > > > >
> > > > > > > The simple wetted surface argument for a round section hull
> > > > is not
> > > > > > > valid once wave drag comes into the equation. There is very
> > > > little
> > > > > > > difference in drag between round sections and flat sections
> > > > but the
> > > > > > > flat sections will lift more and trim more bow up. This
> > > > should be an
> > > > > > > advantage on a large proa where the weight distribution
> > > > cannot be
> > > > > > > easily adjusted.
> > > > > > >
> > > > > > > Rick
> > > > > > >
> > > > > > >
> > > > > >
> > > > > >
> > > > >
> > > > > Rick Willoughby
> > > > > rickwill@
> > > > > 03 9796 2415
> > > > > 0419 104 821
> > > > >
> > > >
> > > >
> > >
> > > Rick Willoughby
> > > rickwill@
> > > 03 9796 2415
> > > 0419 104 821
> > >
> >
> >
>
> Rick Willoughby
> rickwill@...
> 03 9796 2415
> 0419 104 821
>