Alaris

Keel bulb weight to crew weight equivalent calculation?

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I’m sure there’s a formula for this but I went to law school to avoid doing math so I don’t know how to describe what I’m trying to Google.

We are currently investigating the idea of increasing the ballast in our bulb and wanted to get an impression of how the difference in ballast would compare to additional crew weight. Meaning X pounds of additional ballast in the bulb equals how much in extra crew weight. The goal is to increase righting moment to reduce rail meat requirements.
 

About the boat:

VCG is at waterline. This makes calculating the arm for the location of crew weight pretty easy. Beam is 11’ and freeboard is 3’ so the arm should be appx 6’4” at max beam, if my math checks out.

Keel is all lead, 7’1” draft, with a bulb that has a flat bottom. Looking at adding ballast via adding lead below the bottom of the bulb. This leads me to believe that we are not looking at a huge difference in the arm of the additional weight being added compared to crew weight.

If we were to add, say, 500 lb of lead below the current bulb (we estimate this would be about 5” increase in span), is it going to be like adding, for instance, 550 lb on the rail, or more like 700 lb on the rail? That’s the essence of what we’re trying to find out. (One reason we want to know the crew weight equivalent is so we can use the ORC VPP to estimate performance by simply adding to the crew weight figure).

The boat has been measured for ORC so if additional measurements are necessary to answer this question I can provide them so long as they would be on that certificate. 

Bonus question: The boat’s limit of positive stability is currently ~109 with a stability index of ~106. How do I figure out how much more ballast is needed to get it to 115?

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To figure out self righting need existing bulb ana foil weight.

Rule of thumb one kg crew = one kg bulb, maybe a bit more lead needed in your case as boat is quite wide and light.

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Just to confuse the issue: a given keel weight adds more stability as the angle of heel increases, to a max at 45 deg of heel. But the sails & hull shape (and crew) don't want to sail like that. So you also have to determiner the best angle of heel for your hull shape & sail plan, then look at keel produced stability.

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The keel currently weighs 3100 lb, all lead. Boat displaces 8100. 

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The longer the moment arm the more force it provides for the same given weight - this is why deeper keels with smaller bulbs can exert the same RM with a longer arm. 

Because of this - you can’t willy nilly add more weight to your keel or increase its depth without consulting a naval architect. This is how keels fall off. 
 

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Just now, Miffy said:

The longer the moment arm the more force it provides for the same given weight - this is why deeper keels with smaller bulbs can exert the same RM with a longer arm. 

Because of this - you can’t willy nilly add more weight to your keel or increase its depth without consulting a naval architect. This is how keels fall off. 
 

This keel used to be 18” deeper than it currently is. It was cut off due to draft issues at the dock. Structure is not an issue.

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4 minutes ago, Alaris said:

This keel used to be 18” deeper than it currently is. It was cut off due to draft issues at the dock. Structure is not an issue.

I thought you said you were bad with numbers and didn’t know the right terms to even google the subject. 

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2 minutes ago, Miffy said:

I thought you said you were bad with numbers and didn’t know the right terms to even google the subject. 

That was a preface to my statement that I do not know how to calculate this. I’m trying to ballpark this to gauge potential benefits without having to consult/pay an expert.

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Very approximately, 225 lbs should solve the stability test.

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Just now, Frogman56 said:

Very approximately, 225 lbs should solve the stability test.

Thank you! Like I said we are looking at around 400-500 lb. Where would that put it? How is this calculated?

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4 minutes ago, Miffy said:

You’re really a lawyer?

I am, what difference does it make? It was a dumb comment about the cliche that lawyers are bad at math. 

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13 minutes ago, See Level said:

Okay so I entered my 500# and 5” depth increase (I know the depth will increase more because of the displacement but I don’t have that kg/mm figure in front of me). I guess I could use some interpretation of these results. Is this significant enough that I’m really going to notice it? Is it worth the trouble?

The goal is #1 to get to weather better than we do now (the boat is optimized for downwind and relatively overpowered on a beat unless we have 6-8 on the rail) and #2 to get to weather better while *also* reducing the number of crew required.

CE8EF8F2-4648-4862-B18E-450197EFC31E.thumb.jpeg.4232ddf42195f75780962c4c735061a9.jpeg9D27E482-4A73-472D-AA1C-72E583039965.thumb.jpeg.74d4a04a25d34987edac8059cde54f9a.jpeg

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41 minutes ago, Miffy said:

I thought you said you were bad with numbers and didn’t know the right terms to even google the subject. 

He's an Attorney so he knows exclusions. 

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There id no ewuivslence. Rail meat is faster. Addded lead reduces capsize. Added lead increases structural loading.

Adding bulb is frsnkly nit a goif idea on msnyt boats.

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5 minutes ago, fastyacht said:

There id no ewuivslence. Rail meat is faster. Addded lead reduces capsize. Added lead increases structural loading.

Adding bulb is frsnkly nit a goif idea on msnyt boats.

The problem is that frankly it’s easier to design and install 500# of lead than to find the same weight in reliable crew. We have been doublehanding since covid and anticipate a smaller crew (4-5 instead of 7-9) going forward. 

Let me just add that I’m not interested in debating the merits of whether to use more crew or increase the bulb weight. I’m asking how to compare the two in a quantitative sense when it comes to stability and speed.

Structural loading is not an issue for the amount of weight we’re talking about. It’s a custom boat designed for a 9’ keel that weighed considerably more than the 7’ one currently on it.

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1 minute ago, Alaris said:

The problem is that frankly it’s easier to design and install 500# of lead than to find the same weight in reliable crew. We have been doublehanding since covid and anticipate a smaller crew (4-5 instead of 7-9) going forward. 

I am a naval architect. It is not easy. Existing Fibergkass structure isvMystery Meat. And beyondvthst thev whoke standing rigging snd chsinpkts may need to be replaced. Adding ballast down low can start trouvle

Fir sailing righting moment, 100 lb on rail is ewuiv to 500 lb lead more or less.

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The real problem is the engineering.  Was your fin and hull flange, keel floors, ALL up to the task?    

 

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Just now, solosailor said:

The real problem is the engineering.  Was your fin and hull flange, keel floors, ALL up to the task?    

 

The inner structure is steel and rather over engineered. As I said it was originally designed for a 9’ keel that weighed more than what we’re talking about the new 7’6” keel weight being.

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1 minute ago, solosailor said:

The real problem is the engineering.  Was your fin and hull flange, keel floors, ALL up to the task?    

 

Probably not. But statistics and dtuff.

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Just now, Alaris said:

The inner structure is steel and rather over engineered. As I said it was originally designed for a 9’ keel that weighed more than what we’re talking about the new 7’6” keel weight being.

And you kniw this for fsct in documrnts witch can be vetted?

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4 minutes ago, fastyacht said:

And you kniw this for fsct in documrnts witch can be vetted?

Yes, since we put together the design brief, had the boat built, and shortened the original 9’ keel to its current 7’1” depth (canoe inclusive). Mars made the keel. Those documents all exist. For the sake of argument, since none of this is what I was asking, assume the structure is not at issue. 

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2 minutes ago, Alaris said:

Yes, since we put together the design brief, had the boat built, and shortened the original 9’ keel to its current 7’1” length. Mars made the keel. Those documents all exist. For the sake of argument, since none of this is what I was asking, assume the structure is not at issue. 

So do it then. Just make sure righting moment at 90 dorstnt exceed origimak design vajue

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Just now, fastyacht said:

But just re m ember. Railmeat akways faster.

Maybe I should have posted this in short handed anarchy but the idea is to make the boat faster without weight on the rail.

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Posted (edited)
17 minutes ago, fastyacht said:

Fir sailing righting moment, 100 lb on rail is ewuiv to 500 lb lead more or less.

This ratio seems awfully high for a boat that only has 3k of ballast to begin with. The distance to CG is nearly the same (keel bulb being the further distance)—how could that be possible?

Edited by Alaris

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23 minutes ago, Alaris said:

Yes, since we put together the design brief, had the boat built, and shortened the original 9’ keel to its current 7’1” depth (canoe inclusive). Mars made the keel. Those documents all exist. For the sake of argument, since none of this is what I was asking, assume the structure is not at issue. 

Did the new keel weigh the same as to the old one?

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Just now, See Level said:

Did the new keel weigh the same as to the old one?

Same keel. It was shortened. So it weighs several hundred pounds less now than originally. 

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48 minutes ago, fastyacht said:

 

Fir sailing righting moment, 100 lb on rail is ewuiv to 500 lb lead more or less.

It depends how the weight is distributed

 

0CEpK6-1414498153.jpg

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6 hours ago, Alaris said:

This ratio seems awfully high for a boat that only has 3k of ballast to begin with. The distance to CG is nearly the same (keel bulb being the further distance)—how could that be possible?

"more or less"

all depends on specifics. Its a trig problem. What heel angle? Etc.

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7 hours ago, Alaris said:

This ratio seems awfully high for a boat that only has 3k of ballast to begin with. The distance to CG is nearly the same (keel bulb being the further distance)—how could that be possible?

It's not the distance between the ballast that's important - is the horizontal distance at the angle of heel you're considering. So let's say for simplicity that when the hull is upright the crew are 2 meters horizontally or from the centre of gravity, and the keel is 2m directly below the centre of gravity. So when upright 100kg of crew provide 2 kNm of righting moment (2m x 0.1tonnes x 10m/s) and the keel provides zero righting moment. 

A typical sailing angle might be 15 degrees. So 100kg of crew provide 2 x cos15 x 0.1 x 10 = 1.93 kNm of righting moment,  100kg of keel ballast provides 2 x sin15 x 0.1 x 10 = 0.52 kNm of righting moment. So in this case, you need 400kg of ballast to replace 100kg of crew at 15 degrees of heel.

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Okay I think I am finally starting to understand. That helps. 

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For instance, railmeat on a yumblehome early ior sailing at 20deg is less effect than on a flared topsides contemporary sailng at 15 degrees. The difference can be very striking.

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3 hours ago, Jackett said:

It's not the distance between the ballast that's important - is the horizontal distance at the angle of heel you're considering. So let's say for simplicity that when the hull is upright the crew are 2 meters horizontally or from the centre of gravity, and the keel is 2m directly below the centre of gravity. So when upright 100kg of crew provide 2 kNm of righting moment (2m x 0.1tonnes x 10m/s) and the keel provides zero righting moment. 

A typical sailing angle might be 15 degrees. So 100kg of crew provide 2 x cos15 x 0.1 x 10 = 1.93 kNm of righting moment,  100kg of keel ballast provides 2 x sin15 x 0.1 x 10 = 0.52 kNm of righting moment. So in this case, you need 400kg of ballast to replace 100kg of crew at 15 degrees of heel.

Except where ever I've said 'centre of gravity' it should read 'centre of bouyancy'. Not sure why I can't edit my post.

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Sure, in teo

17 hours ago, Frogman56 said:

Rule of thumb one kg crew = one kg bulb, maybe a bit more lead needed in your case as boat is quite wide and light.

Sure, but my “keel kgs” have never been late or forgot to show up, bitched about the weather, dropped sails and winch handles overboard, got drunk, or quit to sail on faster boat... 

 

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Was the grid structure in that modified/custom 105 changed? Or was it originally just the normal 105 hull? What was beefed up when you added the custom carbon mast and deep keel? 

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16 hours ago, fastyacht said:

I am a naval architect. It is not easy. Existing Fibergkass structure isvMystery Meat. And beyondvthst thev whoke standing rigging snd chsinpkts may need to be replaced. Adding ballast down low can start trouvle

Fir sailing righting moment, 100 lb on rail is ewuiv to 500 lb lead more or less.

Has Snaggy hacked your keyboard? :P

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Just now, trimfast said:

Was the grid structure in that modified/custom 105 changed? Or was it originally just the normal 105 hull? What was beefed up when you added the custom carbon mast and deep keel? 

Keel sump was cut out and replaced with a heavily over engineered steel grid.

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5 minutes ago, BOI Guy said:

Water ballast

Many orders of magnitude more expensive and complex than adding some lead to the keel. Not to mention there is essentially nowhere to put it in the cabin. 

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11 minutes ago, Alaris said:
17 minutes ago, BOI Guy said:

Water ballast

Many orders of magnitude more expensive and complex than adding some lead to the keel. Not to mention there is essentially nowhere to put it in the cabin. 

Maybe a transverse stretcher across the berths with a lead sled on it? :ph34r::P

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3 hours ago, weightless said:

Maybe a transverse stretcher across the berths with a lead sled on it? :ph34r::P

I like helium antiballst myself.

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5 minutes ago, fastyacht said:

I like helium antiballst myself.

Takes up a lot more room though.

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4 minutes ago, weightless said:

Takes up a lot more room though.

Yes, it does indeed!

USS_Los_Angeles_moored_to_USS_Patoka,_19

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I sail an S2 7.9 which has 600#'s of lead in it's dagger board and 1200 #'s of lead in the floor of the cabin. Hull shape adds great deal of stiffness. We can go upwind in 25 knots of wind higher and faster than CS30's. So, I'm wondering if a cheap, quick and dirty test might be to carry lead weight in the bilge as an experiment. G&S , naval architects on my boat, were very clever guys. Boat won MORC national championships right out of the mold.  I know ratings play a role but sure seems they must have known something. Another design of theirs, the S2 9.1 also won MORC championships with a conventional keel. For whatever it's worth.

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There is no equivalent answer.  You will end up with different righting moment curves.  You can say you want the same righting moment at 15 degrees heel without 3 guys on the rail, so how much lead do you need to add at x depth to get to that righting moment.  You gotta generate the curve by calculations or computer program.  

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23 hours ago, VWAP said:

It depends how the weight is distributed

 

0CEpK6-1414498153.jpg

what was the question again?

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7 minutes ago, ExOmo said:

what was the question again?

Would this be better weight distribution on the rail?

 

_big_ass_bikini_by_bigassbelly_dcdgbqb-p

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The extra weight is on a longer lever in the previous picture if they’re hiking properly.

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On 7/31/2020 at 9:35 PM, mad said:

Has Snaggy hacked your keyboard? :P

Do not interfere with the professionals, since you are unemployed and despicable as in the game.
Exercising your despicable musle everyday don't you.

 

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3 hours ago, Schakel said:

Do not interfere with the professionals, since you are unemployed and despicable as in the game.
Exercising your despicable musle everyday don't you.

 

Where's Poopie Pants when we need him. Oh yeah--he's already here.

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