I don't have any training in hydrodynamics but I have seen many different hull/bow types in action - especially hanging over the bow and watching.
Lots of power boats have 2 deflection systems - one just above waterline and the other at the gunwhale. Both work fine into the wind but once you get the wind off the bow these deflectors lift the water to where it can be blown back across the deck. 10 knots in a 1 metre sea makes for a real shower.
I haven't been able to see the latest RB video but the earlier one also showed spray from the WW hull blowing over to the cockpit. I think either a vertical or flared WW hull will result in spray raining over the cockpit if you are going to sail at 10-15 knots. Hence the cockpit cover.
Personally, if I can be sailing at 10, 15 or even 20 knots, I wouldn't care about spray.
I grew up with dinghy sailing - a very wet affair.
Just my 2 bob's worth.
Carl.
--- In harryproa@yahoogrou
>
> In a sailing cruising boat there is such a large range of immersion, load, trim, speed, sea state and wind, that I can only see a committee of a shape rather than something perfect for particular conditions. My strongest parameters are avoiding hobbyhorsing in the type of waves and slop we get out of Twofold Bay and not slamming in heavy conditions coming off the back of waves. The lee hull on a Harry needs less compromise than on a cat, while the ww bow immersion is quite different as it is most immersed when sailing slowly. For the ww hull I worked on the relative immersion from being at rest through to hull speed and there wasn't a great deal of difference, so trying for the the parabolic bottom didn't make so much sense. Wave resistance would be more of a limitation, so skinnier hulls with a flare above to possibly get a lift from skating over the chop. Therefore a bow was needed to make a good transition. What made sense to me is a cross between an F-Boat center hull and the wavepiercer hulls of a local runabout catamaran. For a racing version I reckon to more morph towards the power cat as you would be punching the hull through some waves at higher speed with less displacement.
>
> It may be a complete stuff up and worse than the basic Vis and Harry hulls, but I figure it is worth having a go. An experiment is good, even if you don't get the results you were hoping for.
>
> I did do some Hydrodynamics at one stage when studying maths and engineering, but am mainly working on years of observations while at sea, seeing how the various craft and sea creatures make a hole in the water, how the water gets away and how the hole is filled in again. I was especially interested in the large subsurface movers such as seals, sharks and whales and how little disturbance there was for the speeds they were going. My thoughts are that if you are going faster than the waterline length allows the water to move away in a relaxed manner, but not enough power or suitable shape to sit on the bit of water squirted out, then you are squirting out water nearly vertically and expensively. If you peel away the water first, the hole in the water is made with less drama. The water will rise up the bow and may be blown across, but it won't be squirted out vertically in sheets. If you can deflect he water back down again, without significantly interfering with its initial rise, then the spray problem should be substantially reduced
>
> --- In harryproa@yahoogrou
> >
> > Robert,
> >
> > Both you and Tarjan make a good argument for reverse bows with buoyancy
> > down low in terms of handling and safety.
> >
> > Melvin makes a good point, though, about the shape being boat-specific
> > and requiring some research and development. Tarjan also mentions that
> > it might make for a wetter ride.
> >
> > So, not being skilled in naval architecture of fluid mechanics myself,
> > I'm not really equipped to move the state of the art forward. At the
> > moment the design is a great idea that I can't wait for someone else to try.
> >
> > Thanks for keeping the idea alive and under discussion.
> >
> > - Mike
> >
> >
> > Robert wrote:
> > >
> > >
> > > Hi Mike,
> > > Welcome the discussion.
> > > --- In harryproa@yahoogrou
> > > <mailto:harryproa%
> > > wrote:
> > > >
> > > >
> > > > It's definitely great to see the performance of Rare Bird, as pointed
> > > > out, with no whitecaps to be seen. Especially since it's substantially
> > > > heavier than Blind Date. With the planned larger rig, that will be some
> > > > boat to behold.
> > > >
> > > > Some of the old discussions were annoying at best. I'm glad to see
> > > > that the real world has proven the design -- no need to argue any more
> > > > about the ww hull forcing the whole boat to round up.
> > > >
> > > > I think the new beam-mounted rudders are close to the center of the
> > > > boat. Without the hull-mounted bracket, they should also be less draggy
> > >
> > > There is the problem that the rudders have less resistance to rounding
> > > up being closer to the center and thus would need to be slightly
> > > larger, thus creating more drag and greater spray.
> > >
> > > > and create less spray. Being closer to the cockpit, though, it will be
> > > > interesting to see what happens to the spray they do create. At least
> > > > it's always to leeward!
> > > >
> > > > ---
> > > >
> > > > Robert: speaking of spray, how does this relate to your
> > > > reverse-sheer-
> > > >
> > > > You asked me for specific objections a while back, but I got busy and
> > > > forgot to respond. There were a few thoughts I had:
> > > >
> > > > - Spray. I think your planned design may be more suited to racing,
> > > > where no one minds getting wet, than to cruising or multi-purpose use,
> > > > where it's nice to have the option of being as dry as possible. It
> > > > seems as if it would be easier to deflect spray with topsides that were
> > > > angled down towards the water, at least slightly, and a deck and stem
> > > > that aren't going to throw water or spray up. That said, I'm no expert
> > > > in how your intended design would work in terms of spray, so I could be
> > > > mistaken.
> > >
> > > Spray is certainly a consideration. I would expect the quantity of
> > > water displaced to be less but would tend to flow further back over
> > > the bow but with a lower velocity. This is contrasting with water
> > > squirting out of the way with vertical or forward rake. I have worked
> > > in spray rails that start just back from the bow and end up as part of
> > > a 400mm flare that helps give greater bury to the crossbeams and allow
> > > attachment of rudders. The rails would theoretically transfer some of
> > > the energy of the spray into lift. I was working on the rails to be
> > > rising slightly to the bow at an angle.
> > >
> > > >
> > > > - Ease of construction. Rob's U-shaped bent panel seems to be about
> > > > as easy as it can get for a quick build. Switching to a compound shape
> > > > could add a lot of build time. Unless you're just talking about the
> > > > sculpted bows made of foam, which perhaps was your intention in the
> > > > first place.
> > >
> > > The compound area is only in the bows and anything difficult in foam.
> > >
> > > check out how much spray there is from Oracle bows.
> > >
> > > http://1.bp.
> > > <http://1.bp.
> > >
> > > Having the boat sail flatter with less pitchpoling would reduce spray.
> > >
> > > Some articles about the reduction in pitchpoling
> > >
> > > http://www.aeroyach
> > > <http://www.aeroyach
> > >
> > > http://www.synfo.
> > > <http://www.synfo.
> > >
> > > >
> > > > - Reserve buoyancy. I like having a bow that keeps on providing more
> > > > buoyancy the further it gets depressed. That said, this may be just
> > > > because I grew up with bows like this, and am therefore used to it. You
> > > > make a good argument for a bow that sheds water from the top down when
> > > > the going gets rough.
> > >
> > > I have probably beaten this drum a bit too much, but any buoyancy
> > > after the stern starts to rise is going to increase the chance of
> > > pitchpole as it increases resistance , thus increasing the pitchpoling
> > > torque. Low down buoyancy keeps the boat sailing flat.
> > >
> > > >
> > > > As you can see, none of these are definitive stances. But I thought
> > > > I'd pose the issues anyway for discussion.
> > > >
> > > > Any thoughts?
> > > >
> > > > - Mike
> > > >
> > > >
> > >
> >
>
Change settings via the Web (Yahoo! ID required)
Change settings via email: Switch delivery to Daily Digest | Switch format to Traditional
Visit Your Group | Yahoo! Groups Terms of Use | Unsubscribe