My hulls will have 150 degree keel with slightly rounded chines,vertical sides for the bulk and slightly reverse flare at the ends above the waterline with reverse stems at the bows. The 150 bottom blends into a 90 about 300mm from the stem. I am more concerned with the dynamics as the boat goes to ww through chop and waves and has a nice entry after coming off a wave as my observations are that this is where light multihulls have the most trouble. The 90 v should allow a low drag exit as well as giving some hydrodynamic lift. The small difference in drag from the hull optimisation for flat water I feel is over ridden by handling in sloppy condition.
A slight flare from the bottom with rocker and a lower cp will be more susceptible to hobby horsing
--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> Todd
> GODZILLA is a hull optimising program that can search out the lowest
> drag hull for design displacement, design speed and any prescribed
> constraints. Its limitation is that it does not allow for squat and
> trim changes or any planing forces.
>
> For a 500kg hull designed for 25kts with a minimum length of 12m
> after 14,000+ iterations it arrived at a hull with the following
> parameters:
> draft 198mm
> bwl 398mm
> close to round section
> wetted surface area 5.6sq.m
> Cp 0.65
> It is asymmetrical fore and aft
> the drag at 25kts is 980N
>
> Adding the constraint of a flat bottom with slightly flared sides
> after 11,000+ it arrived at:
> draft 192mm
> bwl 373mm
> wetted surface area 5.6sq.m
> Cp 0.65
> It is asymmetrical fore and aft
> the drag at 25kts is 997N
>
> So the flat bottom hull is not much different in drag to the round
> section.
>
> Removing the flat bottom constraint but setting a maximum beam of
> 300mm after 11,000+ iterations it arrived at:
> draft 198mm
> close to round section
> wetted surface area 6.07sq.m
> Cp 0.83
> It is asymmetrical fore and aft
> the drag at 25kts is 1010N
>
> Not a lot more drag but worse than the flat section.
>
> This gives some idea of how various constraints impact on the drag.
>
> Once the trim and squat is taken into account the relative
> performance between flat and round section can be a bit different but
> that is a much more laborious task.
>
> Rick W
> On 18/07/2010, at 4:20 AM, tsstproa wrote:
>
> > Same 12m hull with half the displacement would allow reducing the
> > beam width to tune the draft back to where it was on the 1 tonne or
> > close to it. Also losing even more fiction drag due to less surface
> > area on the narrower hull. Draw out buoyancy by lessen rocker. See
> > what I'm saying? I know you wouldn't want to go to narrow on such a
> > long hull. But see were the difference could be made up on shorter
> > hull forms if not producing enough lift. By having no rocker narrow
> > high buoyant hulls? Or not, I (guess) the numbers say different.
> >
> > It would seem your achieving or trying to achieve semi displacement
> > performance from displacement narrower flat bottoms vs the typical
> > wider semicircle semi displacement hull forms.
> >
> > On my hull program I can't get any hull above .640 Cp It has
> > restraints on the curve output, making really full midsections
> > impossible without widening the stems or going with a reverse flare
> > meaning wider keel than the sheer width. With reverse flare I can
> > get waterline finesse and Cp of 1.345-1.465 I actually built a 18''
> > 1/12 scale model of a mono shunter. 36'' wide keel 30'' wide sheer
> > with reverse stems and 9'' rocker. Penalty is wetted surface area
> > in this form. Sailed really well but tippy as a mono hull and two
> > wide as anything else. Maybe draw that out to 75- 80 feet. I'm
> > leaning more towards 80 feet.
> >
> > I did a real simple 80' main hull 24'' keel width 48'' sheer width
> > 60'' height at center, with a 24'' rocker Cp of 619, displacement
> > 6,465 lbs. @ 14'' draft, with a (wetted surface area of 214 sqft).
> > I could go wider on the keel widths and achieve high factor numbers
> > example Same hull but with a 48'' keel width, Displacment
> > 11,724lbs. @ 14'' draft , Cp .626 waterline Coefficient .871
> > midship coefficient 1.000. (wetted surface area 321 sqft.)
> >
> > I haven't ran the numbers for sail area to be carried just did what
> > looks right to my eye for know, sail area high aspect 2,000 sqft.
> > split 50/50 on a 70 foot mast. jib foot 30' mainsail foot 24' roughly.
> >
> > Todd
> >
> > --- In harryproa@yahoogroups.com.au, "tsstproa" <bitme1234@> wrote:
> > >
> > >
> > > Sure 25 knots or there abouts. To get over that constraint,
> > reduce beam width which will lower hull in water for same
> > displacement and drop rocker to raise Cp back up this automatically
> > pushing buoyancy foward to aid in countering bow down trim. Getting
> > us back into a light weight slippery displacment hull with minimum
> > drag.
> > >
> > > Todd
> > >
> > > --- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@>
> > wrote:
> > > >
> > > > You need to specify the target speed as well, but assuming
> > 25kts, the
> > > > lowest drag hull will be very short and designed to plane. The
> > drag
> > > > for an optimised planing hull at a guess will be around 800N.
> > > >
> > > > Constraining the length to > 12m produces a hull requiring 990N
> > to do
> > > > 25kts. The hull has a lot of rocker and will generate wave making
> > > > lift as well as significant planing force if flat bottomed with
> > rocker.
> > > >
> > > > This is really getting into planing regime rather than
> > displacement
> > > > and analysis without account for the dynamic lift would not be
> > > > realistic for 500kg at 25kts on a flat bottom.
> > > >
> > > > Dropping the design speed to 14kts for a 500kg hull gives the
> > lowest
> > > > drag hull of 11m. Drag at 14kts is 380N. Interestingly drag at
> > > > 25kts is 987N so about the same as the 12m.
> > > >
> > > > It takes a lot more effort to actually analyse the trim but I
> > would
> > > > be surprised if you could not get it to maintain level trim
> > while in
> > > > displacement mode up to 14kts. After that the planing forces come
> > > > into play and if it has some rocker the nose lift should overpower
> > > > the sail moment in the same way you see with a sailing dinghy
> > when on
> > > > the plane.
> > > >
> > > > So the 500kg hull to go fast creates an interesting dilemma. Do
> > you
> > > > want to go reasonably short and aim to plane or do you want to
> > keep
> > > > long and slender to achieve wave piercing. It also brings to
> > question
> > > > what the best shape of an unloaded ww hull might look like at a
> > > > particular design speed.
> > > >
> > > > Rick
> > > >
> > > > On 16/07/2010, at 1:25 PM, tsstproa wrote:
> > > >
> > > > > What if the same 12m. hull was only 1/2 tonne displacement?
> > > > >
> > > > > Todd
> > > > >
> > > > > --- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@>
> > > > > wrote:
> > > > > >
> > > > > > The length constraint or non-constraint is a function of
> > design
> > > > > speed
> > > > > > and weight.
> > > > > >
> > > > > > As an example a 1 tonne hull with a flat bottom designed
> > for minimum
> > > > > > drag at 25 knots results in a length of 12m. However this
> > hull will
> > > > > > not generate enough bow up moment through wave making to
> > counter the
> > > > > > moment from the rig to drive it at that speed. Extending
> > the length
> > > > > > to 15m will get close.
> > > > > >
> > > > >
> > > > >
> > > >
> > > > Rick Willoughby
> > > > rickwill@
> > > > 03 9796 2415
> > > > 0419 104 821
> > > >
> > >
> >
> >
>
> Rick Willoughby
> rickwill@...
> 03 9796 2415
> 0419 104 821
>