Wouter,
all your reasons aside you seem to have forgotten that no one else but you is going for the support tubes and with the exception of Phil's boat the ones starting to build right now are all stayed. I should also point out that the boomless rig was the idea of the people building the boat and they are paying for it, so I guess it's their choice.

BTW, your analysis of the small flap at the bottom is just plane wrong, I have recently done a lot of CFD work on just this subject for the A-Cats in the form of a mini sweeper (about 400mm chord) and it is extremely beneficial even without a seal to the tramp an I for one will be adding them to any design that the owner allows me to.

Cheers
RG

Hi Jeffs
Look on the leeward hull behind the trap wire, you can see the centercase there, and yes the boards and rudders work fine and you can out point most people most of the time.

Cheers
RG

Just to add my 2 pennies worth, I know that RG has 4 boats up north that will all be boomless plus our 2 will also be that way, not sure about the other 3 that are just starting, but I suspect they will go that way so we are all the same down here in NZ and just as a personal observation, I think Wouters arguments while probably meant well just look like rubbish to me.
Aerynt

Wouter, does this description ("angled upward booms") apply to the LR2 photo that RG posted.

I appreciated the effort Wouter went to to explain his thinking about the aerodynamic properties of the flap. Agree or disagree, since this is a community effort (right?) it would be nice if we had some dialog about the substance of his comments, rather than just dismissing them on the basis of some computer modeling that none of the rest of us have seen.

That said, my understanding is that leaving the luff length unrestricted will not actually penalize Wouter's design, since his argument I believe is that aerodynamically a design without the flap is more efficient.

BTW, on the issue of support tubes, I would be careful about making an argument just on the basis that noone else is using them. Don't forget Chris's "McDonald's" story. Eliminating diversity unnecessarily may not be doing the class as a whole a favor.

What makes it official is that all four boats will comply with 3,80 m length limit. A hammer in my hands only serves to open a hole in the head to make room for all that data.

I totally forgot about feathering. Optimists feather their sails when being towed and when rigging. Lasers do it when rigging. This is a usefull feature even for kids who prefer to wait in the capsized boat when things get rough (for quietness).

While we are at it, Optimists around here use a snap shackle (or similar) to fast-release the mainsheet block from the boom to feather. I guess we should copy this feature.

I like Wouter's mast support system. It makes sense from the structural point of view and others. My daughter justified her preference for the chined hull shape because "it is easier to grab in order to climb back to the boat". Those bars certainly help too. Also, the capacity to feather makes it easier to rig and tow. Both are important for kids.

Great ! Lets get into it. How did you model it, how was the flap modelled, what approximation routines did you (or the software) use. How did you model the wind flow. What variances did you use, if any. Does the software allow stochastic modelling at all or just lab style deterministic models.

Can you intepretate the data for us, as in explaining why the flap works when the flow detaches from the round mast section before it reaches the point where the sleeve seperates from the mast tube. This detachment is broadly documented in a score of scientific publications.

I'm really interested in being proven wrong cause that is the way we learn and progress design.

I'm also aware that no single person (including myself) can cover a whole field in engineering like aerodynamics. There is just too much detail there.

If I make a mistake here then I want to know about it.

Wouter

Ohh, I'm perfectly happy to race against any flapped design while sailing a non flapped design myself. It is just the perception thing that worries me and the fact that I do believe the flap obstructs critical visualbility in collision situations.


It is also my believe that without strickter F12 class rules then we have now we'll end up with mini A-cats. Which in my view is the same as the other expensive boats (F16, F14, A-cat) but then with less less performance and robustness. That is in my opinion not the right balance to strick with respect to the entry level cat that the F12 was intended to be.

But I'm happy to have the others decide to go down this route if they really want. I will however keep the orginal F12 concept going on the background just in case.

Wouter

A very large part of my scepsism against the flaps below the boom (and even the angled up booms) is derived from this :





Sailing is basically harvesting energy from the wind. If the wind at a lower altitude above the water contains less energy per kg (or volume if you will) then there is less energy to harvest there (less sail drive to produce)

Now compare the energy content at 1 mtr. above the watersurface to that of say 4 mtr above the water surface. The latter being at the centre of the mainsail luff when the boom is 1 mtr above the water.

In laminair winds (below 5 knots wind speed at 10 mtr altitude) the ratio is 1/15th, meaning that 15 times more energy is to be harvested at 4 mtr altitude then at 1 mtr altitude.

In turbulant winds (above 5 knots wind speed at 10 mtr altitude) the ratio is still a tad lower then 2/5th meaning that 2.65 times more energy is to be harvested at 4 mtr altitude then at 1 mtre altitude.

Of course a peice of sail area can never harvest more energy then 100% of the energy content of the given altitude layer (the real theoretical ratio is actually max 52 , but I won't bother you guys with that). So basically the same area of cloth at 4 mtr altitude can be several times more inefficient and then still outproduce the flap in the amount of harvested energy/saildrive. So why have the area (that comes out of the total) being "wasted" in the flap that way.

The only reason to justify that is when the flap reduced the overall drag of the design to such an extent that this amount is larger in absolute terms then the same cloth area placed higher up can compensated by produced sail drive.

I also contest that possibility but that is for a later posting.

By the way, the wind can only blow in two mutually exclusive forms, one being the laminair flow pattern and the other being the turbulant flow pattern. The transition from one to the other is abrupt and complete. The typical borderline between these two forms is found on average at 5 knots of wind (as measured at 10 mtr altitude).

The graph were developped by me based on data found in "High Performance Sailing" by Frank Bethwaite and various documentations that were used during my courses on fluid dynamics. The flow approximation is a 5th order fluid approximation. Reynolds numbers are not of interest at this time as we have explicitly modelled the laminair and turbulant flows. Of course the Reynoulds number is normally used to distinguish between these two situations, but experimental data had already fixed this transition at 5 knots of windspeed at 10 mtr altitude so using the Reynolds number at this time would be pointless.

Wouter

Silence ?

I was planning to post the follow-up messages to complete the arguing but it seems everybody has left.

Wouter

It's the holiday season... <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Waiting on my kit to arrive, Scarecrow is reworking it to fit the change in length. Wont have my new shed up this summer so had to clean out my other one, no point taking a photo now. Theres nothing so depressing as a clean shed.
regards

My guess is the silence in an indication that people have
decided to back up some of the many words with a bit of real work.
In my case I now have the ply required for the build and as my F16 arrives in a couple of weeks I'm putting up a 12mx6m carport which will allow me to move some of my other projects out of my workshop under cover with my F16 while I build the Blade 12. I just need to make sure I get all the materials in before the extended Christmas close down.

Hey Jeff. Good luck on the build.
Looking forward to hearing about your progress.
Regards,
Phill

Yeah, that must be it !

We're in the middle of setting the F12 framework and suddenly people decide to run ahead of schedule and do stuff that can actually be ruled upon later. I thought we just had agreed to set at least a few limits that suit everyone before people we are locked in by having build boats.

I'm beginning to understand why multihull sailors are looked down upon by the larger body in the sailing world. We couldn't organise our own rescue if our lifes dependent on it.

Wouter

The Youth catamaran effort has finally moved from just a discussion to an actual concrete effort. With so many personalities and agendas there are bound to be conflicts.
Various parties have chosen a path to take...Some are Leading, some are Following, and possibly some need to....

Regards,
Bob

Come on Wouter if we don't get organised and start now we wont have enough boats on the water for next summer. How do you get enough people excited without putting a boat in front of them. I have to build at least 2 boats, work, sail, maintain my current cats, repair the sailing school mono's and keep a family happy. I think your the only designer on the forum who hasn't moved to build yet, didn't you read in an earlier post how many boats were commencing the build, the race is on to build, then we need to get those cats together for a fun comp.
regards

have just finished cutting out all of our jig frames for our 2 boats, hope to have it all assembled and lined up sometime in the next week.
Talked to RG last night and it looks like some of the contacts I gave him might actually build some more also. He also mentioned that 2 guys up north are hoping to start their 4 boats early in the New Year, so all up NZ should have at least 9 boats building by the end of January 08.

As for the class fee of $50 per design, is anyone in charge of collecting that yet?
Aerynt

How long does it take to all agree on the rules. A day maybe or a week ?

We can't even wait with building that long ?

You guys have been avoiding this subject for months now. If you didn't then you could even have commenced with building months ago.

So please don't insult me with these stupid arguments.

Wouter

Then tell us, how you would like to be insulted?

The way you insult us? or do you have something else in mind?

Aerynt,
That is good news.
Possibly you could post some building posts/pic on the catsailor building forum unless you have a web site or blog where you will be showing off your handy work.
Good luck, I hope you enjoy the work ahead.

Regards,
Phill