Wave-Piercing, Pitching damping or marketing?

[Steve Clark]

"that person" is me.
SHC

[Retired Geek]

Originally Posted by TTS

Bob,

one of the A-Class platforms to look at is the DK-17, designed by Dirk Kramer.

Tom, as I have never seen the DK-17, is there any chance you could post a pic here of it ? would be nice to see what we are talking about.

RG

[Steve Clark]

Two views:
USA222 is DK17
USA323 is A2.1



SHC

[Retired Geek]

Thanks Steve, nice looking boat
RG

[Retired Geek]

Originally Posted by bobdarbygso

RG,

Is the design optimized for a particular weight range?
Any thoughts on how it would respond to crew in the 100 kg range.

Bob

Bob,
we have now sailed the boat with up to 120 kg and using a Mk1 butt as a speedo, appears to go just fine, but its definitely sliding sideways and probably needs bigger boards. When you put it in the computer, the numbers tell a different story, none of which looks good. As luck would have it someone just appeared who is willing to produce a "larger" version of the boat and while the details remain to be dealt with, its possible we may have something sailing sometime in the Q1 2008 if all goes to plan.
As a preliminary estimate, I'd guess it will be optimised for a 105-110 kg crew.
RG

[bobdarbygso]

RG,

Thanks for the update.

I'm curious if these are designs you're doing on a commission basis, or will they be offered as plans?

Bob

[Retired Geek]

Originally Posted by bobdarbygso

RG,

Thanks for the update.

I'm curious if these are designs you're doing on a commission basis, or will they be offered as plans?

Bob

Bob,
the boats (either version) will only be available in the US thru John & Ian Lindahl as finished boats, you can contact them at jlindahl_lcd@yahoo.com
RG

[bobdarbygso]

RG

Thanks!

Bob

[Triman]

Hy to all,
I'm an A-Cat sailor and of course followed with big attention the discussion. First of all,congratulation for the nice LR2 project. This is really other than mainstream, that points at the moment generally to flat and mainly fat bottom, with a straighter scoop-rocker line (flyer MkII, The Tool, Scheurer S5, BIM XJ). I would really try once this cat! The only doubt I have is about the rear beam, that seems to be very near the water; with this thin stern I wonder if it will hit soon the water downwind in weavy conditions (this was a major problem with the flyer MkI).
I love very much all these new boats after flyer MkI opened the doors for new thinking in cat design; but I must recognise that people are still going very fast with older designs like Marstrom (http://www.marstrom.com), wich is all but a wavepiercer or a low-drag cat (people call it CC - Container Cat). So it seems we have very different hull designs with not so big differences in performances.
So I'm always wondering if the design of the hull can make a real difference between the new A-Cats, or if the mast-sail (and boards) combination and the weight (and of course the sailing skills) of the sailor have a much bigger influence.
The reasons could maybe be described from the practical experience: since A-Cat can't plane, upwind you reach soon the maximum speed (ab. 9.5 - 10.5 knots; 11 knots in flat water) and then you have to maintain it with stonger winds; so you must have a very adaptable rig. The differences in speed between good trimmed and sailed boats is then very little. Downwind is the effect of sail-mast combination and sailor weight much bigger: with a powerful rig and/or a light sailor you fly the hull much earlier, and then the speed difference is huge. In my personal experience, +/- 7-8% of body weight makes you flying or sitting in the water...
So we can guess that at the moment, a good trimmed and developed mast-sail combination wich can provide good allround performance, and a regular diet for the happy A-Sailor can be at least so decisive as the hull shape itself.
Good Wind!
Marcus

[Retired Geek]

Hi Marcus
while the pic's of the LR2 might make it seem like the rear beam is low, it is infact a little higher than the Flyer MkI and in the worse chop we have sailed in so far hasn't hit the water yet.
In designing the boat we concentrated on 4 things, damping the pitch rate, minimising the hull wake, improving the rudder and board performance and developing a sail profile for minimum induced drag. If you accomplish the first then the rear beam issue vanishes and you end up with more usable power, the 2nd and 3rd hopefully give you the difference in hull performance that seems be missing between all the other designs and the last part is an attempt to eek out a bit more horsepower.
Personally I think that breakaway performance for any design is going to be extremely difficult, the best you can do is probably incremental small improvements....if you do enough of those increments, then perhaps you'll win a few races.

[Aerynt]

On a slightly different tack, if any of the other designers are in this forum, what are the things that they try and improve on when designing a new A-Class?

[Erwan]

Hi Everybody, Hi Triman,

You are in the right place to investigate solutions to improve A-Cat perf,
For the rig I highly recommend you to read the threads: "Soft Wing Sail"
and ' & "Sail Aerodynamic".

But you can get a good picture with a customized "Tornado Polar" and basic calculations:
1- You know at which true wind speed you just fly a hull winward, full trapeze.

2- You know your sailing weight, the boat speed, and the apparent wind speed, average angle of attack, and your righting moment full trapeze.
Of course, you assume that you trim the boat perfectly, especially the twist...and so on

3-If you are at constant speed (for the purpose of the computation, you hold everything constant especially the wind), the total drag=total driving force.

4- with your righting moment, you derive the lateral component of the sail lift

5- Then, combined with the angle of attack, you search the "implicit" lift coefficient which provides the appropriate aerodynamic force in order to balance your righting moment.

6-With the Lift Coef, and the angle of attack, you compute the total driving force.

7-With lift coef again and aspect ratio, and under the assumption you have the perfect twist, with elliptical ..... you can compute, the induced drag.

Of course, it is not VPP/CFD, it looks more like (KFD: Kitchen Fluid Dynamics) but when I did it, I realized that the outcomes were surprising for a basic cat sailor who has never computed anything.


Very quickly, if you consider the sail section drag + the sail friction drag+ the stays drag+ crew aero drag+ hull aero drag, you will find nearly more drag from air than from water (windward).

But hydro drag is alos important and Retired Geek provide us with interesting insights: The centerboard is made with 3 or 4 different sections. Probably, you need a special section for the part of the centerboard which is just under the hull, and has to deal with the boundary layer of the water flow around the hull.
At the rudder level, it is even more important as the boundary layer is thicker.
Then another section is required for the flow not affected by the hull boundary layer, the part with the largest chord.
Then for the centerboard elliptical tip another wing section is required for the large chord and a different one for the small one.

Good Wind

Regards

[TTS]

[quote] I love very much all these new boats after flyer MkI opened the doors for new thinking in cat design; but I must recognise that people are still going very fast with older designs like Marstrom (http://www.marstrom.com), wich is all but a wavepiercer or a low-drag cat (people call it CC - Container Cat). So it seems we have very different hull designs with not so big differences in performances. [quote]

Marstrom for one, is also testing a new wave peircing platform, Ben Hall has a new design being tested, Martin Fischer has a new design as well and all of them are one form or another of a wave peircing design.

[quote] So I'm always wondering if the design of the hull can make a real difference between the new A-Cats, or if the mast-sail (and boards) combination and the weight (and of course the sailing skills) of the sailor have a much bigger influence. [quote]

So now to the rig. I think that it is going to take a while for the hull shapes to settle down and work themselves out, so it is fair to say that the mast-sail dynamics are extremely important to the boat's performance. I think that Ian's boat is Hall Mast, Glaser Sail, Fred's boat has a Hall Spar with Fiberfoam parts (spreaders, gooseneck and so on), I do not know which sail he is using. My sails are Brewin, Ullman (Charlie Ogletree), Brewin, Goodall (Ashby) and will soon be a Glaser as well

I saw TTS's posting of the A class rules in another thread. I was curious if anybody in the class remembers how long the "hydrofoils are permitted" rule was in effect before the "hydrofoils are not permitted" rule took effect?

[TTS]

Doug,

I think that you can find that answer on the http://www.usaca.info/ website under the blog. If not there, follow the link to the international website and follow the postings back. http://www.a-cat.org/ one link is http://www.a-cat.org/id93.htm .