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
>