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  #51  
Old 04-19-2013
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Re: Bob Perry's take on Wolfenzee's dream boat

Thanks, Bob. Just being aware of the complications as the 3-dimensional changes from keel coming up, sails going over, hull heeling and wetted area/shape changing...and then add complications of speed and how it all sits in the water...I'm glad I don't know enough math to grasp whether particular physics is more or less complicated. I know that computers have made some incredibly complex things simpler, but given the limited audience for yacht design software, have no idea how far that has or hasn't come.
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by bobperry View Post
"I have read a recent study aimed to evaluated the performance of several types of keels upwind and it turned out that an Iron bulbed keel had a better performance than a lead fin keel with the same draft, both keels providing the same righting moment."

Paulo:
Those are your words, not mine. I disagree strongly with that statement. Plain and simple.

Please exlain to me how I am misinterpreting your initial statement. I'm not seeing it.
Yes, the study proved that in some circumstances that is true but I think you are assuming that the fin lead keel and the Iron bulbed keel have the same design. They provided the same RM and had the same draft but have a completely different design. We are talking about keel 1 and Keel 4.



As you know besides drag there is lift to be considered and keel 4 will provide a lot more.

from the study:


"The keels were also evaluated against each other on a race. The results from ten nautical miles on an Olympic track are presented in Table 8. Four different wind speeds have been chosen and the time is presented in minutes....The times for completing the race are extremely similar.

Keel 4 has the lowest lap time for the different true wind speeds. What’s most interesting is that at low true wind speeds keel 4 is even faster that keel 1.


Conclusions....

The results as analyzed throughout chapter 3 to 5 has
consistently shown that for both these cases the T-bulbed keel performs best of the bulbed keels, and also better than the fin keel in higher wind speeds."



Somewhat surprisingly but very clear. I was expecting that in higher speeds the lift provided by the iron torpedo keel would be able to provide more advantages than its superior drag but I find surprising that even with smaller speeds, on the simulated race, the Iron torpedo keel was faster than the fin lead keel.

I have no reason to suspect the seriousness of this study that evolved some very good CFD specialists and was presented by a credible University with studies in Marine Hydrodynamics.

Regards

Paulo
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  #53  
Old 04-19-2013
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Re: Bob Perry's take on Wolfenzee's dream boat

No Paulo, I'm not assuming anything. It is obvious that a lead keel and an iron keel, weighing the same and with the same VCG cannot possibly have the same design. This is stuff I know. I not new at this. I have designed a lot of iron keels.

Perhaps the T keel performed better than the lead fin because it had less frontal area and a more efficient foil in the fin. There are lots of variables. But there is no doubt at all in my mind that an all lead T keel would have performed better than an all iron T keel. They seem to avoid this obvious comparison. But I still have a problem.

Looking at the illustrations I find it hard to believe that keel 1 and keel 4 had the same VCG. I don't see that and I assure you my eye is pretty well trained.

However, to evaluate keels while keeping the VCG constant is an artificial approach. The reason you use lead is to lower the VCG. So much of any keel design is about lowering the VCG. I see no point in studying keels where the VCG must remain constant. It might be important to a college class but not in the real world of yacht performance. But we could apply that to a lot of things taught in college today.
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Re: Bob Perry's take on Wolfenzee's dream boat

Is it even legal for we peasants to own uranium, spent or otherwise?

I remember decades ago Eric Tabarly had it for ballast in a Pen Duick but he was a naval officer and had the connections and high level clout to get it from the French navy.
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Re: Bob Perry's take on Wolfenzee's dream boat

Quote:
Originally Posted by bobperry View Post
.....

... But there is no doubt at all in my mind that an all lead T keel would have performed better than an all iron T keel.
Bob, I have been saying that from the beginning.

Quote:
Originally Posted by bobperry View Post
However, to evaluate keels while keeping the VCG constant is an artificial approach. The reason you use lead is to lower the VCG. So much of any keel design is about lowering the VCG. I see no point in studying keels where the VCG must remain constant. It might be important to a college class but not in the real world of yacht performance. But we could apply that to a lot of things taught in college today.
keels serve to lower the CG but also to provide grip to sail upwind and they will be more efficient, assuming the same draft and RM, if they do that with less drag and more lift.

The objective of the study had not to do with lowering the CG but with studding drag, lift and upwind performance between several keels that provided the same RM with the same draft, but with different shapes, the ones that are more usually used on sailing boats. It is an hydrodynamic study regarding the efficiency of each shape in what regards drag and lift.

Regards

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Re: Bob Perry's take on Wolfenzee's dream boat

OK Paulo, you win.
I stand by my previous posts on this subject intact.
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Re: Bob Perry's take on Wolfenzee's dream boat

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OK Paulo, you win.
I stand by my previous posts on this subject intact.
Bob, this was not about winning, I am sorry if I have pissed you. Not intended. I was enjoying a conversation and it seems it is not mutual. I will go post elsewhere

Regards

Paulo
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Re: Bob Perry's take on Wolfenzee's dream boat

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Originally Posted by bobperry View Post
All boats are designed to sail flat. That's why we design them at a zero angle of heel. But they do heel and there is little we can do about it. But almost any boat, except in very, very light air will sail best if heel angle is kept to a minimum. The keel is more efficient that way and the sails are more efficient that way. But try as we might, the boat will heel.

People often ask me, 'What heel angle is my boat desigfned to sail at?" I always say zero degrees.

Bulbs per se are not fast. A clean tip to your fin is fast. But if a bulb can signifigantly lower the VCG then the bulb may contribute to boat speed in that it will help keep the boat more upright.
Reminds me of bashing north thru the NE trades from Tonga. Tie a reef in and the speed goes up substantially.
With all the changes, it would be far easier, cheaper and quicker to simply build a new modern hull from scratch, than to change and existing one with so many changes needed. I can put a bare hull together in a couple of days. The extremely short waterline and excessive overhangs ( dead weigh in the ends) can only be changed with a new hull.
Bill Garden had a very traditional looking boat with a very modern underbody. People would challenge it, assuming it had a full length keel, then be surprized when they got their asses kicked
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Re: Bob Perry's take on Wolfenzee's dream boat

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Originally Posted by SloopJonB View Post
Is it even legal for we peasants to own uranium, spent or otherwise?

I remember decades ago Eric Tabarly had it for ballast in a Pen Duick but he was a naval officer and had the connections and high level clout to get it from the French navy.
Then they covered the middle east with spent uranium ammo. No shortage there.
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Re: Bob Perry's take on Wolfenzee's dream boat

Paulo:
Don't go away.
You and I have far more in common than we have in,,,,,,,,,the other place.
We did not connect very well on this subject but I know we were on the same track.
My apologies if I offended you.

I work very hard at yacht design. I have done so for the last 52 years. I stand by what I have learned. But I'm a bit rough and crude in my approach to communication. The best I can say in that area is that my dog likes me.
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