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  #51  
Old 12-27-2012
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by PCP View Post
A First 40 or a J122 will be more stiff than a Catalina 400 simply because they need to be more stiff to carry more sail and going faster. That does not mean that a First 40 will not put a rail in the water faster than a Catalina.
The 400 will get to 8 knots with a heel angle of 7-8 degrees. The First will get to 8 knots with a heel angle of 15-20 degrees. By most any measure the Catalina will be more stiff than a First 40.

Given the Catalina is almost a foot wider, gives it 13% more righting moment for the same weight. However, it is also 3000 pounds heavier adding to the RM. The winged Lead keel is 700 pounds heavier and gains another 400 pounds of effective weight due to the higher density of lead. The keel CG is placed considerably lower in the water as it is attached to a fiberglass skeg that extends almost a foot below the hull. It is doubtful, the CG of the First is lower, so if we assume the CGs to be 1 foot below WL, the Catalina has 3000 ftlbs more mass righting moment. All and all, I expect the Catalina 400 to have about 25% more righting moment.

The reason a light performance hull heels more is because at low heel angles, the hull design focuses on reducing drag rather than creating moment. This can only be acomplished by reducing weight and wetted surface. This creates exactly a hull that is not stiff because the lowest wetted surface is cylindrical rather than flat.

On the other hand, cruisers invariably demand boats that do not heel much. Such boats are much more comfortable, but are slower in light wind because they have more drag.

It is well documented that LESS STIFF boats require more sail area because the force on the sails goes as the cosine squared of the heel angle. So the First will have only cos(20)^2 or 88% force at a heel angle of 20 degrees. The Catalina 400 will have cos(8)^2 or 98% of the wind force at 8 degrees. The Catalina can get by with 10% less sail area for the same force. The 100% SA is 808 for the Catalina and 848 for the First.

I think this stuff is well known and proven out by Catalina owners.
Bryce

Last edited by BryceGTX; 12-27-2012 at 09:55 PM.
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  #52  
Old 12-27-2012
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by BryceGTX View Post
... By most any measure the Catalina will be more stiff than a First 40.
....
It is well documented that LESS STIFF boats require more sail area ...

...Bryce
I will not even try to discuss that. Anybody that does not see that is an absurd statement will not deserve my time or effort.

Regards

Paulo

Last edited by PCP; 12-27-2012 at 10:55 PM.
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  #53  
Old 12-28-2012
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Re: Blue Jacket 40 (new racer/cruiser)

+1, makes me wonder if the 'experts' have ever sailed a bene First 40.7.

There's a lot more to it than just the numbers Bryce is posting.
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  #54  
Old 01-03-2013
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by zz4gta View Post
+1, makes me wonder if the 'experts' have ever sailed a bene First 40.7.

There's a lot more to it than just the numbers Bryce is posting.
Could be.. performance boats tend to carry more sail. On the other hand.. stiffness is not the design criteria for sail area.. rather RM is pretty much the design criteria for sail area. However, to say that a more stiff boat requires more sail area is absurd. Rather a less stiff boat requires more sail to achieve the same drive.

If we look at the Lagoon 380 which has similar weight and LWL as the First, it only has 883 square foot of sail. However, no doubt it is considerably stiffer, yet carries less sail. So stiffness clearly is not the design criteria for sail area.

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Originally Posted by PCP View Post
I will not even try to discuss that. Anybody that does not see that is an absurd statement will not deserve my time or effort.

Regards

Paulo
Anyone who thinks otherwise deserves my attention. However, you can easily prove me wrong by producing the RM diagrams for a First 40 and a Catalina 40. It would be interesting to see who is absurd.

From a yacht design book:
"The true reason for giving a crusing yacht more beam: In the first place more initial stability... It is very desirable for comfort that a crusing yacht should be kept as upright as possible.."

For those that don't know the term "initial stability" translates to stiffness at low angles of heel. The end result is that cruising boats are more stiff than performance boats at low angles of heel.

So then the question becomes, why are performance boats more tender at low angle of heel. It seems the performance boat manufacturers do not subscribe to the same design philosophy that Paulo is expecting.

The reason is quite simple, the design goal of a performance boat at low angle of heel is the reduction of drag, not high stiffness.

This intuitively makes sense because low angle of heel with high speed occurs during down wind sailing. Or similar in light wind. You can only get high boat speed in light wing with minimal drag. Minimal drag is light weight and minimal wetted surface. So as expected, performance boats are light weight.

When sailing broad reach with a performance boat, heeling is required to create righting moment. So stiffness only increases as a means to create righting moment at some heeling angle.

So we can have two different boats, Catalina 40 with high initial stiffness to reduce initial heeling angle. Then the First 40 with low initial stiffness for reduced drag at low angles of heel. Then both boats producing the same heeling moment at 15 degrees, where the First may be more stiff.

Probably at 25 degrees both boats have the same stiffness. Whenever we discuss stiffness, we need to be careful how we present it. Given this scenario which boat is more stiff?
Bryce

Last edited by BryceGTX; 01-03-2013 at 07:07 AM.
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  #55  
Old 01-03-2013
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Re: Blue Jacket 40 (new racer/cruiser)

Quote:
Originally Posted by BryceGTX View Post
...

Anyone who thinks otherwise deserves my attention. However, you can easily prove me wrong by producing the RM diagrams for a First 40 and a Catalina 40. It would be interesting to see who is absurd.
....
Bryce
Still not interested in discussing this with you but just not to confuse people:

To access a sailboat stiffness through a stability curve the one that should be used is a GZ curve (length of Arm) not a RM curve (righting moment) simply because a RM is obtained multiplying the displacement of the boat for all values of a GZ curve at different angles of heel. The values to be considered will be the ones till the MAX GZ with special incidence till 45º.

Two boats can have similar RM values till 45º and therefore have similar RM curves on that segment but one can weight almost the double. In that case the lighter boat would be much more stiff. It will be the case when we compare for instance a 40 class racer (or a First 40) with a Catalina 40:

A Catalina 40 weights about the double so for identical values on the GZ curve the RM at those points will be the double on the Catalina 40 regarding the racer, but the Class 40 racer will have a GZ curve 60% better, so even if the RM is smaller its Stiffness is much bigger because It needs a lot less sail to go at the same speed.

For the ones that these curves means little: The RM is the force that is making force at a given angle of hell to put a boat vertical and that is opposed by the wind force on the sails. If a boat weights half it will not only be needed less force to right itself up because it weights less but most of all, as it has a much smaller wet surface, it will need a lot less sail area to sail at the same speed of the heavier boat so the force made on the sails will be a lot smaller and it will be needed a much lesser RM to oppose that force.

That's why it is not the sail area that counts for a boat performance but the SA/Displ ratio. The same thing happens with stiffness that has not to do with RM directly but with the sail area the boat needs to sail at a given speed and the available RM. Of course, comparing boats with the same length.

Anyway, as I have said already, anybody that has sailed a race sailboat, or even a performance cruiser will know that the stiffness is bigger than in a cruising boat. For knowing that is not necessary to know a lot about theory, is evident to all with sailing experience in different type of boats.

Regards

Paulo

Last edited by PCP; 01-03-2013 at 08:05 AM.
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  #56  
Old 01-03-2013
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Re: Blue Jacket 40 (new racer/cruiser)

It is a pleasant change to see an extensive discussion/debate on matters pertaining to sailing on a Sailing Forum rather than politics. Both informative and educational. For my part I am not impressed with the BJ40, at all, and suspect it was merely an effort to broaden IP's market somewhat. Perhaps it will. Perhaps not. Good try in any case. (Of course, if one wanted a really good, affordable, performance cruiser, one would stick with a mid-'80's era Beneteau First 42--if one was fortunate enough to be able to find one, eh?)
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  #57  
Old 01-03-2013
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by svHyLyte View Post
For my part I am not impressed with the BJ40, at all, and suspect it was merely an effort to broaden IP's market somewhat. Perhaps it will. Perhaps not. Good try in any case.
I'm still reserving judgement until I see the BJ40's official introduction at the upcoming Strictly Sail in Chicago. I'm holding out hope that she's got the "it" feel and look in person.
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  #58  
Old 01-03-2013
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Re: Blue Jacket 40 (new racer/cruiser)

Quote:
Originally Posted by PCP View Post
Still not interested in discussing this with you but just not to confuse people:

To access a sailboat stiffness through a stability curve the one that should be used is a GZ curve (length of Arm) not a RM curve (righting moment) simply because a RM is obtained multiplying the displacement of the boat for all values of a GZ curve at different angles of heel. The values to be considered will be the ones till the MAX GZ with special incidence till 45º.
You are looking at this way too simplistically. The GZ curve can only be used to compare two boats if they have the same mass. Clearly the Catalina 40 weighs 3000 pounds more so the use of a GZ curve is useless.

On the other hand, since you can't provide the RM curves for the two boats, its not likely you can provide the GZ curves. So your discussion is not particularly useful.

Quote:
A Catalina 40 weights about the double so for identical values on the GZ curve the RM at those points will be the double on the Catalina 40 regarding the racer, but the Class 40 racer will have a GZ curve 60% better, so even if the RM is smaller its Stiffness is much bigger because It needs a lot less sail to go at the same speed.
Who cares about a class 40 sailboat... we are comparing two very specific boats the Catalina 40 and the First 40 (2012). Class 40 wasn't in the discussion..

Clearly a Catalina 40 is not double the weight of the First 40.. so not sure where you are going with this discussion about Class 40.

You keep saying the First 40 is more stiff than the Catalina 40. Clearly no numbers support your opinion. And you provide no RM diagrams to support your opinion. Even boat designers do not agree with you.

On the other hand.. my Catalina 40 runs at half the heel angle as a First 40 at 8 knots. And you suggest the First is more stiff?
Bryce

Last edited by BryceGTX; 01-03-2013 at 09:15 PM.
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  #59  
Old 01-04-2013
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Re: Blue Jacket 40 (new racer/cruiser)

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Originally Posted by BryceGTX View Post
On the other hand.. my Catalina 40 runs at half the heel angle as a First 40 at 8 knots. And you suggest the First is more stiff?
I don't suggest, I say it is absurd to say that a Catalina 40 is more stiff than a First 40. That would be evident for someone that sailed both types of boats even if he does not understand a thing about GZ or RM stability curves.

The stiffness of a boat is not measured by the smaller or bigger heel angle that is needed for a sailboat to sail at a given speed. Anybody knows that narrower boats will need more heel angle to sail than beamier boats but that does not mean that the beamier boat would be more stiff. It depends on the type of keel and draft (regarding maximizing ballast) and the B/D ratio of both boats. Also it depends on drag and upwind, on wave drag. The beamier boat will need more sail than the narrower boat for the same speed.

Stifness in a sailboat equals boat power. A boat with more sail area will not be necessarily faster. Power in this case it is a relation between the sail area the boat can fly and the drag it has to overcome. Of course heavier boats and beamier boats produce a lot more drag and therefore need a lot more sail area to go at the same speed

The Catalina 40 (assuming you are talking about the 400MKII) has a beam of 4.11m, 9299 kgs and 75.06 m2 of sail area.

The First 40 has a beam of 3.98, 7536 kgs of weight and 98 m2 of sail area.

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.

This all subject is ridiculous, I am not posting for you (I don't care what you think) but because it may be of some help to other members regarding understanding better the concept of stiffness on a sailboat.

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.

The First 40 is a particularly stiff boat among performance cruisers. One of the situations were you need a stiffer boat is to go upwind with waves and bad weather. The First 40 is the most used performance cruiser on the Sydney- Hobart race, a race that is well known by upwind sailing in bad conditions and they are the most popular because they are the ones that performed better, having already won the race in compensated time and finishing in real time among much bigger boats.

Quote:
Originally Posted by BryceGTX View Post
You are looking at this way too simplistically. The GZ curve can only be used to compare two boats if they have the same mass. Clearly the Catalina 40 weighs 3000 pounds more so the use of a GZ curve is useless.

On the other hand, since you can't provide the RM curves for the two boats, its not likely you can provide the GZ curves. So your discussion is not particularly useful.
I am not discussing this with you and who says that I cannot provide the curves? It is just not worth it.

In what concerns comparing boats the GZ and RM curves are used to compare different things and it all depends on what you are comparing. 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.

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 35º 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.

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).

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).

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. But, off course, that would also be the case of a First 40, a very seaworthy and stiff boat, in my opinion more than any main market mass production cruiser.

By the way, the Blue Jacket 40 is going to be also a considerably more stiff boat than the Catalina 400.

Regards

Paulo

Last edited by PCP; 01-04-2013 at 02:04 PM.
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  #60  
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 stiffness of a boat is not measured by the smaller or bigger heel angle that is needed for a sailboat to sail at a given speed.
Really??

Two boats of similar size at the same speed in the same wind clearly gives an indication of stiffness. The stiffer boat heels less. In the example given, the Catalina requires more power to run the same speed because it has more drag. All else equal, we would expect the First 40 to heel less. Not sure how you could argue otherwise.

Quote:
Originally Posted by PCP View Post
Anybody knows that narrower boats will need more heel angle to sail than beamier boats but that does not mean that the beamier boat would be more stiff.
Here is the problem with your argument, the Catalina 40 is not just beamier, it is longer at the water line and fuller in the fore and aft body. And its got a hull design remarkably similar to the First 40 below the water line. Perhaps if you had ever seen these two boats out of the water, you would understand.

And of course the Catalina 40 is heavier. Not sure how you could argue a First is stiffer. Perhaps you should look at the RM diagrams to convince yourself.
Bryce

Last edited by BryceGTX; 01-09-2013 at 12:42 AM.
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