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Old 13-05-2008, 15:22   #106
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Quote:
Originally Posted by Therapy View Post
I get that when I think about it but I wish I could find that link I went to the other day. It was from someones post and showed several different hull designs and was inriguing. Partly because I have seen what I said more than once and don't understand it.

I know a sphere holds the most volume. Still not sure about surface area for mass displacement.

Only becaue I read it here - it has to be true!

Maybe someone will find the link again.

Just trying to get my head on it is all.

Not that I will design anything past my cat with twin masts (foam, alumimum and paper - 14in ) done 30 years ago!!
Perhaps you forgot that volume equates to weight? The volume that a boat displaces (below the waterline) is exactly equal to the weight of the water that it displaces. So when someone says volume....think weight. So in terms of a boat, a sphere has the lowest surface area to weight ratio. So if a boat has a very low volume to weight ratio, given everything else is equal, that boat will be faster because surface area equates to drag. So what its really working out to be is a weight (volume) to drag (surface area) ratio...right? Speed equals weight divided by surface area...in this relatively crude equation which could also be used as an indicator like sail area/displacement ratio.
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Old 13-05-2008, 15:48   #107
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Sink or swim

Weight equals volume, unless of course, the boat sinks. Wetted surface affects light airs performance, primarily. Weight and wave making affect higher speed performance more.
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Old 13-05-2008, 17:35   #108
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Quote:
Originally Posted by David M View Post
Perhaps you forgot that volume equates to weight? The volume that a boat displaces (below the waterline) is exactly equal to the weight of the water that it displaces. So when someone says volume....think weight. So in terms of a boat, a sphere has the lowest surface area to weight ratio. So if a boat has a very low volume to weight ratio, given everything else is equal, that boat will be faster because surface area equates to drag. So what its really working out to be is a weight (volume) to drag (surface area) ratio...right? Speed equals weight divided by surface area...in this relatively crude equation which could also be used as an indicator like sail area/displacement ratio.
Yea, I get all that.

I keep saying the same thing and hoping someone that posted that other link will do it again.
The cube has less surface area while displacing the same weight, not the sphere. A half sphere and a half cube (same inside volume) floating with 10lb in it. The cube has less surface area touched by the water.

Maybe I read it wrong.

I just wish I could find it.

I shook my head when I read it........like I am doing now.
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Old 13-05-2008, 17:37   #109
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Originally Posted by mikereed100 View Post
Therapy,

I think I recall seeing the link you are talking about. From what I recall the point the poster was making was that while a semicircle presents the lowest wetted surface for a given displacement, a square shape will handle an increasing load better. In other words, the wetted surface increases less for a square shape than a circular shape as weight is added. It was an interesting link, I wish I could find it.

Mike
Thanks, maybe that is what confused me.

I wish you could find it too!
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Old 13-05-2008, 18:11   #110
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Approximate Example:

A sphere with a volume of 4.19 cu.ft has a surface of 12.56 sq. ft.

A cube with a volume of 4.19 cu.ft has a surface area of 15.16 sq.ft.

Or a sphere has approx.18% less surface area.

Since displacement is equivalent to weight, a more rounded shape moving through the water with the same load would expose less surface area and have less surface drag. Of course many other variables come into play.

I just tested it in the bath tub

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Old 13-05-2008, 19:07   #111
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Originally Posted by ldrhawke View Post
Since displacement is equivalent to weight, a more rounded shape moving through the water with the same load would expose less surface area and have less surface drag. Of course many other variables come into play.

I just tested it in the bath tub

I don't question the math or your testing parameters . But unless the sphere has a pointy end, you're trading less surface area drag for water resisitance drag. Which I thought would be greater.

I also would have thought that the catamaran's much better Lwl/Bh would offset this. Yet, I agree with the original poster in that my boat needs wind to get going. I'm definately not in a performance class of boat with a Texel of 146
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Old 13-05-2008, 19:10   #112
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Quote:
Originally Posted by Therapy View Post
The cube has less surface area while displacing the same weight, not the sphere. A half sphere and a half cube (same inside volume) floating with 10lb in it. The cube has less surface area touched by the water.
It's the opposite, as others have alluded. A sphere is the most "efficient" volume holder. Cubes and other things with angles, rather than "continuous" sides, waste surface space making corners. Not much volume fits into a corner compared to a continuous arc.

Dave
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Old 13-05-2008, 20:07   #113
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Originally Posted by rickm505 View Post
I don't question the math or your testing parameters . But unless the sphere has a pointy end, you're trading less surface area drag for water resisitance drag. ..............
Exactly........the bottom of a catamaran hull is rounded with pointed ends. What we are saying a curved surface carries more load than a flat surface and produces less drag.
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Old 13-05-2008, 20:57   #114
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Take a look at the hulls on my boat. Photos are at the bottom of the first page and the link is in my signature. I have "V" hulls with tons of wetted area. These were typical of the earlier British cats of which mine is one.
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Old 14-05-2008, 02:36   #115
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Quote:
Originally Posted by ldrhawke View Post
You can do all the fancy math and calculations you want, but as someone said earlier, you can't believe any manufacturers published weight; so this argument establishes and proves nothing.
Hallo IdrHawke
we actually build our cats on load cells exact to 1 kilo in weight so I agree that most manufacturers are no very reliable in quoting weights but we sure are, specially since we build performance oriented sailing catamarans

Greetings

Gideon
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Old 14-05-2008, 09:34   #116
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Originally Posted by fastcat435 View Post
Hallo IdrHawke
we actually build our cats on load cells exact to 1 kilo in weight so I agree that most manufacturers are no very reliable in quoting weights but we sure are, specially since we build performance oriented sailing catamarans

Greetings

Gideon
Gideon,

Your enthusiasm for your FastCats is great. I find your posts and comments educational. But sometimes you sound like a used car salesman with very loose facts and omissions when describing FastCats. Let me explain a few of your "facts" I'm having trouble with. Basically, you've taken a St. Francis and put it on a diet.

You have mention weigh cells a number of times, yet I have never seen them in the construction photos you have sent me?

I appreciate you go to great pains to make your boats light, both in construction materials and equipment selection; but so do other builders and designers. You list the sail away weight of your new FastCat 455, which is 46' 9" in length as 12,760 lbs. In comparison, Chris White's states his Atlantic 48 sail away at 21,500 lbs.

Chris White also uses a two piece mold construction, epoxy resin, carbon fiber and tri-axial glass cored with cell foam, and AwlGrip poly paint for weight reduction: similar to your approach. He takes into account the weight of every part used and added to the boat to try and reduce weight. He is a minimalist in layout and design, with less living space than your FastCats and has no weight adding hull bumps. He doesn't even believe in AC or generators on his boats because of the added weight. Where is he going wrong being 4.5 tons heavier? Even, the additional 3' of bridge deck width doesn't add 4.5 tons when you compare boats.

Chris even states for every 1500 lbs you bring aboard you will get an additional 1 inch of hull immersion, which significantly starts to effect performance. Can you tell him how to save 4.5 tons? I know he would love nothing better

Photo of Atlantic 48 hull being removed from molds:


Also, you list the minimum bridge deck clearance of your FastCat 435 at 80 cm, the Atlantic 48 has 90 cm. But in your drawing you show a very large bumps in the hull to make room for wide bunks, which adds volume and weight. Hull bumps slam waves just like the bridge deck. At what point are hull bumps taken into account in your 80 cm measured clearance. You notice in the photo above there are no bumps in the Atlantic 48 for this exact reason.

I'm not trying to be argumentative. It just seems there is a wide discrepancy in how boat builders measure and weigh their boats. I find very few photos of finished FastCats on your web site to be able to make a more refined analysis.
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Old 14-05-2008, 10:12   #117
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infusion molding techniques?
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Old 14-05-2008, 10:16   #118
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Hallo IdrHawke

There are a few possibly a few differences between the weights of a FastCat and a Atlantic.
We build in epoxy with Resin infusion and get a resin to Basalt Fiber ratio of 31 to 69 %
( 31 % resin 69 % fiber )
we use a one mould system so the complete hull is infused in one shot at a vacuum of 98 % , that saves in attaching materials . we use Kevlar only to attach parts to each other , we do not use a dagger board and dagger board casing and this alone saves 900 lbs.
There might be a difference in what we call ready to sail and How Chris calculates
we take the empty boat with all needed to sail like equipment etc but empty tanks.
Light weight ready to sail is 1800 LBS more than empty taking into consideration 4 people on board 200 liters of water and 200 liters of diesel.
I am not a great photographer so that might be the reason that there are few pictures on the web site but this will change later this year.
The FastCat is not a St Francis and has no relation to it at all besides that the designer is the same Angelo Lavranos.
The St Francis 48 was designed in 1999 , the FastCat 435 originally in 2003 and redesigned in 2008 to become the 455 the way these boats are build are totally different and the shapes are also very different
Load cells are very small iron items with a size of only 5 x 5 x 5 in that send the info to a computer , we have 4 of these units mounted on a frame and the information is send to our computer system , if at night a toolbox is left on the boat we know about it .
We are presently moving to a new purpose build factory and after we have moved I can send you some more info on that if you are interested

Greetings

Gideon
p.s. I have attached a picture of the first FastCat 455 as she was loaded on the truck yesterday, the Karin Meredith took the picture , she and her Husband will be living on this cat from next month on .
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Old 14-05-2008, 10:18   #119
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Quote:
Originally Posted by rickm505 View Post
infusion molding techniques?

and the use of basalt Fiber saving another 15 to 20 % over glass.
and having a spread sheet and going over each item and see if there is a substitute that is as good but has a lower weight.

Greetings

Gideon
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Old 14-05-2008, 10:41   #120
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Halloooo Gideon ,
What is the length of a typical basalt fiber and what is its typical tensile strength per cross sectional area, density, strength to weight ratio and energy absorption before failure? Compared to E-glass? Compared to S-glass? Compared to Kevlar? Compared to carbon fiber?

Saying basalt fiber alone does not provide any means of comparison. Perhaps someone would be interested in running a chart comparing these properties? I know next to nothing about Excel.

I like the look of Butterfly above the waterline. With the hull below below the waterline being painted black, it is difficult to see below the waterline. Do you have another pic of the same model boat out of the water? Is this a boat a keel or daggerboard configuration?

Also Gideon, and I have been meaning to say this for some time and please take this as constructive criticism. AfricanCat's website could look a lot better. Your website is a reflection of your entire company. For most people, it is the first thing they see. As you know, first impressions are very important. It could make the difference between a potential customer looking around your website some more or going to the competition. I know your boats are better than your website makes them out to be. A little more professional looking website I am certain would help you sell more boats. At the very least, you need more pictures on your website. Your website is too verbage heavy and a bit boring to come right out and say it. Pictures sell boats as much as verbage, if not more. Also, hull plan drawings are important but you also need plenty of pictures of the product from different angles. I hope you take my suggestion the right way.

Cheers,
David
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