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#1901
10-27-2013
 desert rat Senior Member Join Date: Feb 2013 Location: South of Albuquerque Posts: 500 Thanks: 7 Thanked 22 Times in 20 Posts Rep Power: 3
Re: Pros and cons of steel sailboats

TY Jeff
That helps me understand why there are not thousands of these boats on the4 water.
I will spend many days researching the evolution of the skipjack hull form, and I am
going to practice (at least try) calculateing center of effort, and center of resistance on different rigs. TY again.
WEW
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#1902
10-27-2013
 bobperry grumpy old man Join Date: Apr 2011 Posts: 3,929 Thanks: 1 Thanked 91 Times in 87 Posts Rep Power: 5
Re: Pros and cons of steel sailboats

"I am going to practice (at least try) calculateing center of effort, and center of resistance on different rigs"

Desert: I understand what you are trying to do but consider this as you proceed:

How are you going to do that? A sail is not a 2 dimensional shape laying flat on the boat's centerline. A sail is three dimensional and never laying flat on the boat's centerline. You can play with geometry and find the centroid of the different plans forms but I don't think this gives you an understanding of the center of pressure. It's the center of pressure that is at work. Not the center of the 2D area. I'm not sure what you mean by "center of resistance".
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Last edited by bobperry; 10-27-2013 at 11:23 AM.
#1903
10-27-2013
 desert rat Senior Member Join Date: Feb 2013 Location: South of Albuquerque Posts: 500 Thanks: 7 Thanked 22 Times in 20 Posts Rep Power: 3
Re: Pros and cons of steel sailboats

I will start with simple calculations and compare them with what really happens,
above (center of effort), and below(center of resistance), the waterline. As i add more and more variables it will quickly get beyond the reasonable effort for a learning exercise. Calculus with 2 or 3 variables begins to hurt my head. Jeff pointed me at oyster boats (skipjack) I will go through all the sailing designs i can find of skipjack hulls. I fear4 I may end up looking at drawings of some of those lovely racing boats PCP keeps posting videos of.
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#1904
10-27-2013
 Brent Swain Senior Member Join Date: Jan 2012 Location: British Columbia Posts: 2,376 Thanks: 19 Thanked 33 Times in 32 Posts Rep Power: 4
Re: Pros and cons of steel sailboats

Quote:
 Originally Posted by bobperry "I am going to practice (at least try) calculateing center of effort, and center of resistance on different rigs" Desert: I understand what you are trying to do but consider this as you proceed: How are you going to do that? A sail is not a 2 dimensional shape laying flat on the boat's centerline. A sail is three dimensional and never laying flat on the boat's centerline. You can play with geometry and find the centroid of the different plans forms but I don't think this gives you an understanding of the center of pressure. It's the center of pressure that is at work. Not the center of the 2D area. I'm not sure what you mean by "center of resistance".
Right on!
The 2D profile has nothing to do with the centre of pressure. We consider the centre of lift on an airplane wing ,a point 20% back from the leading edge of a wing, not the centre of area. The well rounded forefoot of a boat has almost zero lateral resistance , but shows up on a 2D profile cutout. A balanced rudder has the axis around 24% back from the leading edge, not in the middle.
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#1905
10-27-2013
 SloopJonB Senior Moment Member Join Date: Jun 2011 Location: West Vancouver B.C. Posts: 11,310 Thanks: 59 Thanked 65 Times in 62 Posts Rep Power: 4
Re: Pros and cons of steel sailboats

Quote:
 Originally Posted by Brent Swain There is a sistership of Angantyre for sale in Comox BC, possibly by Wills Marine. She is extremely well done, and recently given a mirror finish at Ocean Pacific in Campbell River. A very thorough , and practical welder fitter named Chuck Birchill bought the partly finished hull from a school teacher in Ladysmith BC and finished her extremely well .
I took a look at the Wills site - she looks nicely done but is hardly more than a "wet kit" - for nearly 1/2 \$mil.
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#1906
10-27-2013
 bobperry grumpy old man Join Date: Apr 2011 Posts: 3,929 Thanks: 1 Thanked 91 Times in 87 Posts Rep Power: 5
Re: Pros and cons of steel sailboats

Desert:
I think if you establish a method of measurement and stay with that method and slowly accumulate enough results from a wide variety of boats you may have something useful for comparisons.
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#1907
10-27-2013
 PCP Senior Member Join Date: Dec 2004 Location: Portugal, West Coast Posts: 16,195 Thanks: 21 Thanked 102 Times in 85 Posts Rep Power: 11
Re: Pros and cons of steel sailboats

Quote:
 Originally Posted by bobperry Chuckles: But you are laboring under a false assumption. You are equating directional stability with having a long keel. That's just not so. The boats I know with the best directional stability have small-ish keels and nice, big spade rudders well aft. They turn on a dime and they hold a straight course. I've done dozens of boats like this. Look at the Valiant 40. It has a relatively small keel like STEPALAH's but it is renowned for its directional stability.

Quote:
 Originally Posted by Brent Swain Well said Bob! Right on! If a boat sails 5 keel lengths in the time it takes to broach, and wanders only 5 degees per keel length, that is still 50 degrees off course ! For a 30 ft long keel, that's only a sailing distance of 150 feet! Dont expect the relativey short comparative length of a full length keel to make any significant difference in that distance. Hull shape and hull balance makes a huge diference, keel length ; minimal difference, if any. A full length keel will do nothing to compensate for poor hull balance. What it will give you is around 350 pounds of useless weight in the stern of a steel boat, an area which is almost impossible to access for maintenance, and which is too far aft to use for tankage , without putting excessive weight in the stern, where you need it least . In a steel boat, it is also a lot more work to build a full length keel
It seems that you two finally agree on something and it is a thing that many, I would say less informed, members, would tend not to agree, I mean that a long keel boat is not necessarily more stable than a fin keel boat. Me and Jeff are also saying that for a long time.

I hope this settles the question, unless it is needed Tanton to say the same for you all to be convinced and put an end to that unsubstantiated dogma.

Regards

Paulo
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#1908
10-27-2013
 bobperry grumpy old man Join Date: Apr 2011 Posts: 3,929 Thanks: 1 Thanked 91 Times in 87 Posts Rep Power: 5
Re: Pros and cons of steel sailboats

I think if I owned an old fashioned full keel boat I would name it:
UNSUBSTANTIATED DOGMA

Great boat name.
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#1909
10-27-2013
 bfloyd4445 Senior Member Join Date: Sep 2013 Posts: 382 Thanks: 8 Thanked 5 Times in 5 Posts Rep Power: 2
Re: Pros and cons of steel sailboats

Quote:
 Originally Posted by Brent Swain I cant imagine it . I sure dont want to build any more boats. Just turned down a couple of boats . I'll stick to jobs I can do in a couple of days, max. Sale of books and plans , plus full pension soon, is all the money than I need. Cant imagine why he would want a boat that big!
off topic, sorry.
I asked myself the same questions but was polite enough not to ask. His 32 didn't have any electronics but he had sailed it to NZ and other places in the pacific.

sounds like you have a great plan. Since I retired I just play with my toys and wander here and there doing what ever strikes my fancy at the moment. Don't live high on the hog but I eat the best parts.....
#1910
10-27-2013
 bfloyd4445 Senior Member Join Date: Sep 2013 Posts: 382 Thanks: 8 Thanked 5 Times in 5 Posts Rep Power: 2
Re: Pros and cons of steel sailboats

Quote:
 Originally Posted by PCP It seems that you two finally agree on something and it is a thing that many, I would say less informed, members, would tend not to agree, I mean that a long keel boat is not necessarily more stable than a fin keel boat. Me and Jeff are also saying that for a long time. I hope this settles the question, unless it is needed Tanton to say the same for you all to be convinced and put an end to that unsubstantiated dogma. Regards Paulo
.....laughing so hard.....thanks Paulo...Please, I'm an uninformed member not less informed < Bob will most certainly agree with this>and trying to correct this handicap. However, I do understand how much Bob and Brent love each other.....chuckle....

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