Nol and Arto, you are very generous with your explanations. Those are quite eye-opening. Thank you. Luc
--- In harryproa@yahoogroups.com.au, Arto Hakkarainen <ahakkara@...> wrote:
>
> Slot makes both main and jib work harder if it remains right size and shape. Jib because there is faster airflow on the lifting side and main because the flow remains attached on tighter angle than otherwise. Since jib is at better angle to boat it will provide lot more forward thrust than mainsail when sailing upwind. Slot is very important for upwind sailing in traditional main and jib rig. The designers have taken this into designs by making jibs taller but not so much overlapping on most new designs on monohulls. Tall deck sweeping blade jibs with good slot and long luff seems to be the preferred solution for upwind sailing for most designers now. At least for monohulls that go slow and need lots of power upwind.
>
> Arto
>
> From: Nol Twigt <noltwigt@...>
> To: harryproa@yahoogroups.com.au
> Sent: Tuesday, August 30, 2011 1:06 PM
> Subject: [harryproa] Re: Wantoo vs Expeditionarry
>
>
>
> The set of railings on the lee hull is for seaworthiness.
> You do not want to flush overboard.
>
> I did not study Arvel Gentry yet.
> What I can say is just a mix of half cooked theory and experience (in broken English).
>
> The 'problem' of any sail sailing up wind is keeping the air flow on the lee side 'connected' to the surface of the sail. You may say a sail is always stalling a little. This is a hardly noticeable process because it does not happen suddenly like on a (solid) wing of an airplane or a rudder or a windsurfing finn ('spin out').
>
> The solution is adding more 'energy' to the airflow on the lee side of a main sail.
> That is what a good jib, or the air that goes trough the gap between jib and main, does.
> In fact a good jib and main combination works as one wing.
> The jib makes the main more efficient.
>
> All our former common beliefs about the 'Venturi effect' of the gap between main and jib are pure nonsense. A 'Venturi effect' would imply that the windspeed in the gap increases and the pressure decreases. Anyone can measure or even see and feel that this is not the case.
>
> The same positive effect of a good jib-main combination could happen with two main sails. The condition is, as I wrote before, that the leech of the front sail is parallel to the mast of the rear sail.
> This way the gap between the sails is the same from top to bottom and the result is a slow air flow that makes the rear sail more efficient.
>
> If the front leech and the rear mast are not parallel, there will be vertical movement of air that is compensating the differences in pressure.
> Instead of working together as one, (as they should on a love boat :) the sails disturb each other.
> This happens for instance with a classic main sail - mizzen combination. Based on theory and experience the Texel Rating compensates the mizzen for being only 75% efficient.
>
> On the balestron / aero rig / easy rig of the Blind Date you can see another bad combination of main and jib that is caused by a combination of not enough tension on the forestay, a short jib sheet rails and an awful jib. The jib should be much flatter to have a positive effect on the main sail.
>
> On a classic yacht with a keel you do not notice much difference between a sail that is really good or a sail that is kind of bad.
> On a fast multihull or on a windsurfing board even the smallest detail matters.
> The advantage of windsurfing is that you can feel everything a sail does directly in your hands. That is a great source of information. The air flow round a moving sail is very complex stuff. What is theoretically right and even things that work well in wind tunnels can be useless on the water.
>
> One more remark on the Expeditionarry and Wantoo and other Design Challenge III designs. I defined 'good design' and 'finished design process' for myself.
> A design process is 'finished' when you can not simplify the product any further.
> For example when I see a boat design with many bulkheads, I imagine what would happen if they are not there. Every little bit of material costs weight, work, maintenance and it takes away space. So the design becomes better if you remove everything that is not strictly necessary.
>
> 'Good design' means that if you change one thing, even the smallest thing, the result is worse. In a good design it feels like the decisions are 'chained'. If you change one, it affects the others in a negative way. You sort of have to start all over to find another good combination.
> That is why you can not combine two good designs. If the designs are really good, the result of the combination will be a worse design.
>
> My god, this all is really relation therapy, isn't it?
>
> Nol
>
> --- In harryproa@yahoogroups.com.au, "LucD" <lucjdekeyser@> wrote:
> >
> > Well, from Blind Date you went to Love Boat ;-)
> > I remember the Spartans went warring without women.
> >
> > Why is there a set of railings also on the lee hull?
> >
> > It would be interesting to be able to explain your experience with double sails surfboarding using Arvel Gentry's explanations.
> >
> > I agree that
> > > It is fun to see how one design decision influences all others.
> >
> > Luc
> >
> >
> > --- In harryproa@yahoogroups.com.au, "Nol Twigt" <noltwigt@> wrote:
> > >
> > > Hi Luc,
> > > Wantoo is not influenced by the mission of Blind Date.
> > > For sure my experience with the Blind Date influenced my choices. I probably would have designed a cat or a tri if Rob's Harryproa concept had not reached the Netherlands.
> > >
> > > I saw the concept I would like to design for myself reflected in the five main parameters of Design Challenge III: speed, simple, seaworthy, Spartanic accommodation for 2 or 3 sailors, trailerable.
> > >
> > > The difference with Rob's Expeditionarry is probably based on the relationship I have with my girlfriend. I could not imagine a ship without an (almost) full size double bed.
> > > With that in mind I designed Wantoo to minimum length, minimum weight, minimum everything.
> > >
> > > The position of the masts is a result of many hours I spend windsurfing on a tandem windsurfingboard in the seventies. This was the best opportunity you can think of to find out how two sails influence each other in a practical situation. We began with the old Windsurfer sails with long wooden wishbones. Later we tried rigs with short booms. We even sailed it with three sails. My impression was that the rear sail is usually a little less efficient. The best configuration was the smallest sail on the front, the biggest on the rear.
> > > Putting the sails close together did not work very well for the sails. It would make the rear sail less efficient. Very important for the performance was that the leech of the front rig was parallel to the mast of the rear rig.
> > > In the schooner rig of Wantoo I used these tandem windsurfing experiences.
> > >
> > > I expect that the first Wantoo will sail with a couple of different leeward hulls, to find out what the best length is.
> > > I am also still working on other rig options that bring the COE to the front.
> > > This will allow Wantoo to sail with only one rudder that is located on the bow of the leeward hull. Since we noticed that steering with the front rudder works as well as steering with the rear rudder, I find this an interesting option. It makes the concept and the handling a lot simpler.
> > >
> > > It is fun to see how one design decision influences all others.
> > > There is still a lot of work to do in the proa concept.
> > > The concept of the future.....
> > >
> > > So here is the real reason why I need a big double bed. I need plenty of space for me and my dreams :)
> > >
> > >
> > > Nol
> >
>