Wave-Piercing, Pitching damping or marketing?

Wave piercing beam, the next step ?

RG, Tom

I guess the picture shows the volumes not the loads ?
It is a great idea to think outside the box. Since a few years it is nearly monotype, and it seems that in a near future a new Martin Fischer A Cat shape is on the track. I have been told hull could be flatter and larger than the flyer.
Instead your design is in another direction and I can tell you that the Hobie Tiger designer Mr J; Valer has a new shape for F18.
Big difference with the Tiger are a narrower hull, the maximum draft has moved foreward and the stern is much thinner in order to allow the crew to put bow up easily moving its weight aft.
As a result the boat is much more foregiving and safe with spinneaker in strong winds. The hull has more freeboard than the Tiger and is not pierce-wave.
Despite 5% more wetted area than its competitors, thinner hull allow the boat to fly a hull earlier with spinneaker in light wind.

A special question for you RG, do you think pierce wave beam would provide more freedoom for the hull design (beam could move aft, or less freeboard at the beam:hull connection..)
Here is a link for a such a beam build for F18 HT, the picture is far from perfect and the beam are circular with 100 mm diameter in the middle and become flat elliptical with 60 mm thick and around 200 mm long.
In addition the beam is curved and the middle is 120 / 150 mm higher than the sides.
http://18ht.free.fr/Avancement/V016/Calculs/DetailsCalculsRDM.html#PartieRDM1

 

I have been thinking recently about how the new trend of fitting T foil rudders to catamarans might affect the hull design. I think your hull might be a good way forward. You don't need large bows as the pitch control is done by the T foils. Tall bows are a prblem with drag from windage.

Gareth

 

Tom,
program we use was developed as a collaboration between 16 people for applications like this and others you haven't seen.
Gareth
The program sorta negates the need for a T-foil, not to mention that its illegal in A-Class (google the hydrofoil debate in A's). See the attached files (sorry they aren't too clear as I had to zoom and crop some pics for examples)..the 1st thru 3rd show increasing speeds, note how we have achieved no bow wave in the traditional sense, but that it climbs slightly with speed and provides variable displacement at the bow and only on the leeward side of leeward hull. This solution appears to work quite well downwind, the jury is still out on how well this works to windward....this is all part of the solution to keeping the bows more or less vertically fixed regardless of sea state or driving force...again jury is out as I don't think the boat has seen much more than 12 knots of wind so far, but its encouraging that it does what it was designed to do up to this point so far.
Erwan
Our hull while thinner than a flyer, also has less wetted area but not by much (2% or so). Curious as to why you'd say thinner hulls fly earlier? (assumming all other things are equal)....whats your rational there? As for the beams, they don't affect our hull shape at all...we are designing to specific pressure distributions which in turn determines the shape and the forces in play...the hard part is getting the flow to behave as the pic's show over a range of speeds and chop/wave sizes.
Hope that answers most of your questions
RG

 

Thinner Hull fly earlier

RG

Sorry my former msg was probably a bit confusing.
That is true only for F18 with spinneaker,in light winds; its seems that thinner hull provide more lateral "grip" at low speed with high aspect ratio daggerboard, than other fatter hull sections.
That is why I guess you could expect to sail "the wild thing" earlier than the Flyer shape.
In addition, as far as I have understood correctly some theory, I believe that thinner hull creates less "spray drag" which is supposed to change according to the cube of the hull's width, above the waterline.
So your thinner hull than the Flyer's will be faster downwind, and your little volume in the aft part of the hull and the related ability to put bow up should enable the crew to push the boat further downwind, in strong winds.

For windward speed, I think that the Aerodynamic drag is the point and hull drag plays a minor role, but stabilizing the rig for a better average efficiency.
Hight aspect ratio centerboard combined with the rig provide most of the
efficiency windward.

It is what I have discovered so far, studying A-Cat.

But of course I will not compare my research to your team work of rocket scientists, I am unabled to create a spreadsheet with some Navier-Stock formula, or pressure distribution ...I am a second league player.

Congratulation for this great job, I look forwards for news after sailing tests.

Best regards

EK

 

EK
first...no one is "second league" .... there are only those who try and those who whine <grin>....and if you don't know the answers at first, at least by the time you try a second time your a whole lot smarter for having tried
As for our effort...it might look cool, but believe me, it has asked more questions than it has answered.
As for narrow hulls, by definition for a given displacement, a narrower hull is deeper, this will of course increase the lateral area which will generally increase the windward drag unless you design around that. Our narrow transoms are about drag, not allowing the bow to rise...in fact ours don't unless you stand on the transom...our optimum crew position for downhill is approx half way between the fwd beam and the centerboard....at least till its blowing 12 knots.
As for high AR boards...ours are very high....about 17 if I recall correctly...and the rudders are only slightly smaller....look at a Discus 2 sailplane....we use a similar planform....compared to tacticalz boards, you can get about 30% better LD,s and more lift, improved sections help a lot also with the use of 3-4 different sections over the span also.
While your correct about the aero drag of the hull being important, its not the major drag element, so don't ignore the hulls hydro contribution ever.
RG

 

RG,

Any reports on how the racing is going at Silver Lake?

Tom

 

Erwan,

I would love to be brought into the loop as you continue development of the wing. What are you going to use as a platform for the wing when completed?

Tom

 

Tom
Ian just sent a quickie note saying he blew the Flyer away and he's very happy.
RG

 

Silver Lake Result

This is a cut and paste from Ian....

Today was 4-7 knots and the boat performed great. Flew a hull as quick if not quicker which is something I have not been able to do on any other boat in light wind like that. As soon as it would start to fly a hull you could really feel the boards start lifting to windward and I would separate from all other boats. The only other boat that stayed with it was the flyer. He is a good sailor and weighs 165 pounds so the fact that I was able to still keep pace and sail higher is very promising. Downwind I was evenly matched until the wind would get up to the 7 knot range and then I would start to pull away. I am sure my weight was a factor in that. As far as my bow down comments. It is an adjustment in sailing a boat with as much volume forward along with the rocker. After this weekend I have found that I have been over sheeting and not allowing the boat to drive down and ride on its lines. It is just a rigging issue and me learning to sail the boat. I can say this. I intentionally drove the bows down downwind to see if it would want to pitch and it does not. Waterline goes just beyond the forestay attachment and then pops right back with out slowing down a bit. Today was choppy also and it cuts through waves very nicely. All in all with a bit of time this thing is going to be a rocket ship.

RG

 

RG,

Way to go. Cannot wait until Ian gets comfortable with the boat, learns what it can and cannot do and figures its subtleties out. Then if it already is tearing up the course, it will even perform better.

tom

 

sailing shots & video

try this link...nice video
http://sl-acat.blogspot.com/

 

RG,

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

Thx
Bob

 

Bob,

I cannot respond to the LR2 on crew weight, but one of the A-Class platforms to look at is the DK-17, designed by Dirk Kramer. This is the email for the person who right now is in possession of those molds. pointfarm@verizon.net
This is a copy of the email he posted on the usaca.info website.
United States A-Class Catamaran Association List Server e-mail:
Some information:

The DK 17 was designed by Dirk Kramers for sailors who weigh more than 200 lbs ( 90 kg)

I bullied him into actually completing the design after his Boyer Mk4 died in an accident with Ben Hall’s A2.

We have built 5 platforms. Dirk and I have them, George Saunders has one (which was featured in a photo in Seahorse last month)

Oliver Moore has one and Bill Vining has one.

Mine has probably seen the most miles at this point, and I generally think the boat is very satisfactory. It does not make up completely for the fact that I weigh over 100 kg, but I think I am more competitive than I was on either the Mk4 or the Flyer.

The molds live in my shop, and Dirk, George and Bill have all come, pulled the 4 necessary halves and taken them away for completion.

Oliver, being my nephew, finished his in my shop recycling bits of old catamarans as much as possible.

I expect that I will be starting a new platform for Dave Penfield in the near future to replace his A2.

We did a day of boat swapping and he is pretty convinced that the larger displacement/ higher water plane inertia of the DK 17 is a better fit for his weight.

The boat will be Carbon/ Nomex/ Epoxy, we post cure things a bit, but the tooling can’t be allowed to get too hot.

All the boats have been well under minimum weight and remarkably stiff.

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RG,

Thanks. I'm actually familiar with the DK-17 and that lead to my question to you re crew weight. It also has it's displacement low in the hulls but your sections appear rather more elliptical and clearly you've moved well beyond Kramer in reducing volume above the w/l in both ends.

I assume the beams (on your design) are bonded into mouldings that are bonded in turn to the hulls i.e. no fasteners?

Thx
Bob

 

Bob,
The boat is designed around a 75kg crew and at a stretch might accommodate 100kg, but thats just a guess. To my knowledge no one who is that heavy has sailed the boat yet. One of the things we set out to do was model the dynamics of the boat in waves and get the pitch rate way down to just a fraction of that of the other boats which we appear to have achieved, but that coupling is very mass dependent, so I'm not sure how that would be affected with a heavier sailor as I never modeled that situation...perhaps something I can look at on a rainy day.

As for the beams, yes they are bonded in place and the sections of the hull change the entire length, they vary from ellipses with major axis in y & z to round to veed sections aft, plus we have chines aft also. In contrast to the larger waterplane area that was mentioned for Dirks design, ours has the smallest waterplane of any A-Class by quite a lot, that I'm aware of.
RG