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Author Topic: Tumblehome as it relates to IOR Ratings  (Read 9808 times)
dburry
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« on: December 10, 2010, 07:34:07 pm »

How does IOR rating rules influence tumblehome designed in some sailboats? I own a Hughes 26 and many of the other Hughes boats I've noticed have a very pronounced tumblehome. The S&S 34, sailed by Jessica Watson is simlar with respect to tumblehome.  All these boats are designed by Sparkman and Stephens.
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Barney Post
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« Reply #1 on: December 11, 2010, 11:24:26 am »

How does IOR rating rules influence tumblehome designed in some sailboats? I own a Hughes 26 and many of the other Hughes boats I've noticed have a very pronounced tumblehome. The S&S 34, sailed by Jessica Watson is simlar with respect to tumblehome.  All these boats are designed by Sparkman and Stephens.
Not an expert on this but I think I can provide a somewhat educated answer:
The IOR measured beam at a certain percentage below the sheer (at max beam) instead of at the deck. It was thought that using this tumblehome would trick the rule into thinking the boat was beamier than it really was. After 1980, boats still had the bulge at the measurement point, but no tumblehome.  I think it was a change in what designers felt was fast rather than change in the rule. (also, human weight on the rail was made more effective?) 
None of this is really my own take on it, but from what I heard other, more knowledgable people say on the subject.
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dburry
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Boat Type: hughes northstar 26

« Reply #2 on: December 15, 2010, 05:53:12 pm »

Your observations makes sense.  The tumblehome took a little getting used to.  It seems okay in canoe designs, but I found it a little odd seeing it in a my Northstar 600.  Having my first summer with the boat, I'm getting used to it and it doesn't seem to look as peculiar as it once did.  Actually, I'm starting to accept it as one of those traditional design characteristics found in some older workboat designs.  If you haven't checked out the Galway Hookers you'd be quite interested in the extreme amount of tumblehome in them.  These were open decked workboats and the tumblehome helped keep the gunwales out of the water when heeled.  I'm curious to find out other peoples' opinion of how tumblehome affects the seaworthiness or performance of the modern sailboat aside from getting around some IOR rules.

Derrick,
Twillick
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ceeceebee
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Boat Type: Beneteau First 260, First 36.7

« Reply #3 on: February 04, 2012, 07:37:22 pm »

How does IOR rating rules influence tumblehome designed in some sailboats? I own a Hughes 26 and many of the other Hughes boats I've noticed have a very pronounced tumblehome. The S&S 34, sailed by Jessica Watson is simlar with respect to tumblehome.  All these boats are designed by Sparkman and Stephens.
Not an expert on this but I think I can provide a somewhat educated answer:
The IOR measured beam at a certain percentage below the sheer (at max beam) instead of at the deck. It was thought that using this tumblehome would trick the rule into thinking the boat was beamier than it really was. After 1980, boats still had the bulge at the measurement point, but no tumblehome.  I think it was a change in what designers felt was fast rather than change in the rule. (also, human weight on the rail was made more effective?) 
None of this is really my own take on it, but from what I heard other, more knowledgable people say on the subject.

Actually the other way around. The rule measurement was at the deck, where the
'beam' was less and effected the measurement number less. The tummblehome BELOW the measurement point allowed more beam without hurting the measurement. It was this type of 'rule breaking' scheme that doomed the IOR....

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sonosail
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« Reply #4 on: February 07, 2012, 09:23:40 am »

Yes, Right.

Free (unmeasured) beam.

Thanks for the correction.


rb
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rcamp
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« Reply #5 on: April 30, 2014, 09:02:01 am »

If anyone is still interested in the topic...
If have often wondered if pronounced tumblehome was part of a hull design to impart greater rigidity to the hull. I have seen many IOR influenced hulls with maximum beam at deck level that would start to bend with heavy cranking on a hydraulic backstay tensioner. Not being a NA or engineer I cannot say for sure, but I suspect the hull with tumblehome may be a little stiffer.
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poopdeck pappy
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« Reply #6 on: April 09, 2015, 07:20:55 pm »

I realize this is an old thread but I'm new to this forum and had something to contribute.

I worked at Jensen Marine when Ranger 37 (1 ton) hull #1 was launched.  It was one of the early, maybe the first, big tumble home designs of the early 70's.  Sea trials got pretty exciting when she heeled over laying the bulbous "free" beam into the water.  All that "free" beam also introduced added buoyancy when heeled and lifted the rudder clear!!

A deeper rudder was redesigned mas rapido.
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sonosail
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« Reply #7 on: October 19, 2015, 01:03:53 pm »

Yep.  I have a '84 3/4 ton. No tumblehome, but very hard bilges.  Initial form stability, but creates other problems. It doesn't help sailing in big waves either.
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SPB
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« Reply #8 on: February 17, 2016, 07:24:51 pm »

How does IOR rating rules influence tumblehome designed in some sailboats? I own a Hughes 26 and many of the other Hughes boats I've noticed have a very pronounced tumblehome. The S&S 34, sailed by Jessica Watson is simlar with respect to tumblehome.  All these boats are designed by Sparkman and Stephens.
Not an expert on this but I think I can provide a somewhat educated answer:
The IOR measured beam at a certain percentage below the sheer (at max beam) instead of at the deck. It was thought that using this tumblehome would trick the rule into thinking the boat was beamier than it really was. After 1980, boats still had the bulge at the measurement point, but no tumblehome.  I think it was a change in what designers felt was fast rather than change in the rule. (also, human weight on the rail was made more effective?) 
None of this is really my own take on it, but from what I heard other, more knowledgable people say on the subject.

Actually the other way around. The rule measurement was at the deck, where the
'beam' was less and effected the measurement number less. The tummblehome BELOW the measurement point allowed more beam without hurting the measurement. It was this type of 'rule breaking' scheme that doomed the IOR....

Sorry, but you are incorrect.  IOR B measurement was at a point BMAX/6.0 below the sheerline.  B (not BMax) was used in the girth calculations.
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sonosail
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« Reply #9 on: February 18, 2016, 09:29:49 am »

How does IOR rating rules influence tumblehome designed in some sailboats? I own a Hughes 26 and many of the other Hughes boats I've noticed have a very pronounced tumblehome. The S&S 34, sailed by Jessica Watson is simlar with respect to tumblehome.  All these boats are designed by Sparkman and Stephens.
Not an expert on this but I think I can provide a somewhat educated answer:
The IOR measured beam at a certain percentage below the sheer (at max beam) instead of at the deck. It was thought that using this tumblehome would trick the rule into thinking the boat was beamier than it really was. After 1980, boats still had the bulge at the measurement point, but no tumblehome.  I think it was a change in what designers felt was fast rather than change in the rule. (also, human weight on the rail was made more effective?) 
None of this is really my own take on it, but from what I heard other, more knowledgable people say on the subject.

Actually the other way around. The rule measurement was at the deck, where the
'beam' was less and effected the measurement number less. The tummblehome BELOW the measurement point allowed more beam without hurting the measurement. It was this type of 'rule breaking' scheme that doomed the IOR....

Sorry, but you are incorrect.  IOR B measurement was at a point BMAX/6.0 below the sheerline.  B (not BMax) was used in the girth calculations.

You don't have to be sorry.  We all like to hear from someone that knows the subject in detail. I probably won't check my copy of the IOR rule to see if you are right. 
Thank you.
rb
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