So basically once hull over powers bow wave all bets are off. Your analysis is more in favor of trim at displacement speeds!?

What is it exactly that beach cats are doing its not exactly plaining like a 49'er dinghy and not exactly sailing like a lido 14? Or is it?

I wouldn't stand on the beach and say wow look at the hobie 16 planning. Like I would say watching a 18" foot skiff race.

On the cat hulls narrowness and slippery shape would seem to easily over come any displacement waves being made. Hulls finesse ratio's and coefficients making it so.

Having rocker on the extreme end of those figures for max displacement would almost seem to trip it up causing to much push foward or unnecessary turbulence from the rockered keel bow section? Holding speed back until lifting clear. Having the bows (vertical buoyancy) on a nonrockered hull and being double ended pointy at both ends, placing buoyancy in a more vertical column on a hull with a hi Cp allows for the expected pitch movement foward vs rearward and improves drag coefficient vs tail dragging and kicking up a rooster tail which a no rocker hull will do trimmed to squat rearward.

Here's a clip showing my conclusion for vertical bouyancy in bows on hi Cp hull and its effects.
http://www.youtube.com/user/tsstproa#p/u/50/426M9YYxfsQ

Here's a clip showing a slightly reshaped hull. Granted the boat is larger and 4'' longer The shape was achieved from the model in the first clip and its out come.
http://www.youtube.com/watch?v=OzqXqDlDwTI&feature=channel

If either hull were to squat rearward could you imagine the drag from the flat rocker aft and square chine.

Just my thought anyways.

I thought maybe you had bridge the gap between flat rocker wave piecering and planing speed hull trim. What would the effect be with a High Cp hull at planning speeds with a drawn out flat rocker middle section with a nice nose flip be? The tip of the nose flip being level with displacement draft the rest of the curve being under water?

Thanks for putting up with me and my armchair designing back yard tinkering.

Todd


--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> Todd
> The analysis is based solely on the wave formation around the hull.
> The planing forces are not included. These will increase the bow up
> moment if the bow trims up due to wave formation at the speed of
> interest. However for slender hulls in the size being considered the
> dynamic lift is not particularly significant for speeds under 20kts.
>
> The analysis takes a bit of time. I have looked at various hulls in
> two ranges. These include the 90kg range for up to 10kts and the 1
> tonne range for up to 25kts. Given the time it takes it needs to be
> for something definite that might be of real interest for building
> rather than just for the exercise.
>
> A limited version of the Flotilla software is available for free and
> can be found here:
> http://www.boatdesign.net/forums/design-software/flotilla-demo-2-07-
> a-32276.html
> You will get some idea of the difficulty in using it if you have a go
> at it.
>
> Leo is working on a further development to take in planing forces. I
> do not know if this is for the AIS work on rowing hulls or something
> for the navy. I doubt that it will have much application for a round
> section hull though.
>
> Rick
>
> On 12/07/2010, at 11:56 PM, tsstproa wrote:
>
> > Your analysis for bow up trim is more in favor of a planning hull
> > or displacement hull and at what speeds?
> >
> > Yes I know sail forces along with having a flat run to stern causes
> > bow down trim. So what are the effects by adding slight rocker to
> > the deeper narrower hull form? Say on a 12'' vs 6'' draft for same
> > displacement?
> >
> > I have a great 10 minute clip unedited I need to post of Square
> > harry thats the flat bottom hull I designed with the old blue dyna
> > rig in a fair amount of wind and chop. The chop is almost perfect
> > scale to the craft looks like 1-2 foot chop with a rolling larger
> > swell. Showing a great visual analysis of hulls flat bottom no
> > rocker characteristics.
> >
> > I just have to reformat it and post it.
> >
> > By adding rocker you say the trim could be better , sail faster ?
> >
> > Todd
> >
> > --- 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
> > >
> >
> >
>
> Rick Willoughby
> rickwill@...
> 03 9796 2415
> 0419 104 821
>