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  #61  
Old 01-08-2013
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Re: Blue Jacket 40 (new racer/cruiser)

Quote:
Originally Posted by PCP View Post
Stifness in a sailboat equals boat power.

Paulo
Stiffness has nothing to do with power. Perhaps this is your misunderstanding. Stiffness is simply a characteristic of the RM diagram.

The amount of sail area is related to the RM value not the stiffness. For 200 years, that has been the case and it will remain for the next 200 years.

Different types of boats have different stiffness at different points in the RM curve. One of the reasons the Catalina 40 is more stiff is that it has a significantly higher RM value than the First 40. All else being equal this simply produces a stiffer boat.
Bryce
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  #62  
Old 01-08-2013
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by PCP View Post
The First 40, because is lighter and has less beam (less wet surface) needs less sail are to go at the same speed compared with the Catalina and obviously can have more sail up so evidently is a more stiff boat.
Regards

Paulo
Performance boats will always have more sail area for the simple reason, that to go faster in light wind requires more sail. This to me is a no brainer and is widely known in the literature.

Bryce
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  #63  
Old 01-08-2013
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by PCP View Post
I am not discussing this with you and who says that I cannot provide the curves? It is just not worth it.

Paulo
LOL.. good plan.. you would hate to prove yourself wrong eh??
Bryce
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  #64  
Old 01-08-2013
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by PCP View Post
Generally mass production cruisers as the Catalina are less stiff then performance cruisers and performance cruisers less stiff then race boats. That is about the same to say that cruisers are less powerful than performance cruisers and this one less than racers.
For a given generation of boats, the cruisers always tend to be more stiff at low angle of heel because the most important person in our lives does not like to walk around on a deck that is not level.

My wife does not like the boat to heel above 10 degrees. Fortunately, our Catalina 40 is already doing 8 knots before this. If our Catalina 40 is heeling at 15 degrees, its because the boat is traveling at 10 knots.

Now looking at the polar diagram and heeling listing for the First 40, it is only downwind the First will not be 15-20 degrees doing 8 knots.

So you say the cruiser is less powerful at 8 knots??? Doesn't make sense, the Beneteau First 40 is lighter and less drag. So by definition, it will requires less power, not more power to run at 8 knots.

So since it requires less power, to go the same speed, but it heels more.. then by your definition of more power meaning more stiff, the First 40 must be less stiff.

Perhaps you would like to reformulate your definition of more power being more stiff.
Bryce

Last edited by BryceGTX; 01-08-2013 at 09:37 PM.
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  #65  
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by PCP View Post
To compare the energy that is needed to capsize a boat you use a RM curve, to see if a boat is well designed and have an idea of its performance, namely stiffness, you use a GZ curve.

Paulo
Stiffness is not usually used as a metric for performance. And clearly, the GZ diagram is not used at a criteria for stiffness. Perhaps this is why you are confused.

A GZ diagram is simply a moment arm length. You might never tell the difference between a 1000 Ton boat and a 2 ton boat by looking at the GZ diagram. So you would mistakenly think the 2 ton boat is stiffer.

Let see you put the 1000 tone sails on the 2 ton boat because the 2 ton boat has a longer moment arm.

Clearly, the GZ curve is useless as a measure of stiffness.
Bryce
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  #66  
Old 01-08-2013
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by PCP View Post
RM curves are also more important in what regards static stability and GZ curves in what regards dynamic stability (the AVS point is the same).

Paulo
ROG... I would love to hear this discussion.

Both curves are static curves. Perhaps someone else would like to explain to Paulo the relationship of these two curves.

Clearly neither curve can illustrate dynamics because neither curve embodies a value of inertia or damping. These are two characteristics required to define the dynamic motion of anything.
Bryce
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  #67  
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by PCP View Post
When you design a boat you only use GZ curves. what you want is the lighter boat for a given robustness and one with the better possible GZ curve that, in what regards the part that has to do with sailing, means the steepest you can get and with the bigger possible values till 35ļ of heel and that means normally a big Max GZ. The weight of the boat is a consequence of the weight needed to get the desired strength and the ballast needed to obtain that curve.
Paulo
Now there's is a mouthful..

Clearly GZ curves are not the design criteria for any sailboat. If it were, all 40 foot boats sailboats would be 40 feet wide. Making a very stiff boat. Similar to a Missisippi River Barge.

I have to question the words "the steepest you can get and with the bigger possible values". So you are saying the steepest the curve, with the highest value of the curve correct? Then it clearly is the 40 foot beam in a 40 foot boat. Such a boat results in this characteristic.

Do we want such a boat?? Clearly not. So what other criteria are used to design the boat? Well.. there are a multitude of conflicting criteria. Drag, weight, sail area, cost and most of all: racing boat rules .

Bryce
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  #68  
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by PCP View Post
It is because dynamic stability is more important for a boat stability than static stability that there is not a direct proportionality regarding boat seaworthiness and the amount of energy that is needed to capsize a boat and that's the reason why boats with a big GZ curve can be more seaworthy than boats that theoretically need more energy to be capsized ( represented by the area under the positive part of the RM curve).
Paulo
LOL.. another mouthful.. how do you get to a discussion about dynamic stability from a simple comparison of two specific boats stiffness??

How does this have anything to do with the stiffness of a Catalina 40 to a First 40? You need to stay on track with the discussion..
Bryce
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  #69  
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by PCP View Post
That is obviously the case with a 40 class racer (regarding much heavier boats with the same size) and that's why I have talked about it, just because it is a more extreme example and therefore it is easier to understand even if the principle is true to all boats.
Paulo
Clearly, the most extreme case would be to compare the USS Aircraft Carrier Enterprise with a Beneteau First 40.. On the other hand, such a comparison does not get us anywhere closer to comparing the stiffness of a Catalina 40 to a First 40 anymore than your discussion of the Class 40.
Bryce
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  #70  
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by PCP View Post
By the way, the Blue Jacket 40 is going to be also a considerably more stiff boat than the Catalina 400.

Paulo
I am going to guess that you somewhat confused about the stifness of the Catalina 40 and how it compares to a First 40. So lets just put it into laymans terms.

To get a First 40 to the stiffness of the Catalina 40, you need to duck tape 6 sea kayaks to the side of the First 40 at the waterline to get its beam, fullness and LWL the right dimension. Next, you need to drive a VW Beetle into the Salon of the First 40 to get the weight of the Catalina 40.

Perhaps now you understand how much stiffer the First 40 will become.
Bryce

BTW.. oddly enough.. I might be in a good position to discuss the BJ-40, First 40 and the Tartan 40 because my wife and I have actually seen these boats. And we know a bit about the Catalina 40.

Last edited by BryceGTX; 01-08-2013 at 10:41 PM.
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