It is possible to get hydrodynamic lift on a rockerless hull by adding an inclined rail just above the waterline. It can also come from a chine
My suspicion is that the increased lift of the slight rocker in your models is at the expense of hydrodynamic stability in a chop. If there is increased lift due to the wave dynamics, then one would expect a greater lift as the bow moves into chop at a lower speed.
--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> Todd
> A hull with no rocker still has a pitching moment trying to force the
> bow up. Whether it will trim bow up under sail forces can be
> determined. From the analysis I have done, adding a little rocker in
> the ends increases the bow-up moment. It also means there will be
> some lift from planing forces.
>
> If a hull with no rocker trims bow down under sail loads it gets no
> benefit from planing force. It relies solely on the buoyancy.
>
> Also the trim/squat is not my theory it is something that be
> determined using the Flotilla software and something I have measured
> on my own hulls to verify the Flotilla output.
>
> Rick
> On 11/07/2010, at 10:11 AM, tsstproa wrote:
>
> > 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
> > >
> >
> >
>
> Rick Willoughby
> rickwill@...
> 03 9796 2415
> 0419 104 821
>