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Porting a question over here from a thread in fix-it anarchy, where it was kind of off-topic.

Any idea roughly how much stiffer (if at all) it would make a sloop to move the mainsail down 1 foot? (on a 27' boat with roughly a 33' mast (stepped about 5 feet off the water).

My rough calculation was that, assuming the sail's center of resistance started out about 20 feet above the water, and the keel's center of mass about 4' below it, moving the center of the sail's mass 1 foot downward would mean about 5-10% percent more force on the lever would be required to achieve the same heel.

But since wind resistance goes up with the square of velocity, that increase in force seems like it would be achieved with a relatively small increase in windspeed (as in, a few knots or less).

Am I in the right ballpark?

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The CE of the sails is easy - draw lines from the corners to the middle of the opposite side - where they intersect is the CE of the sail.

Do it for both then draw a line between the CE's, then mark a point on that line that is proportionate to the areas of the sails - if the jib is 60% of the combined area then the mark goes at the 60% point - towards the jib.

RM of the hull ain't so simple - check Skenes or similar.

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16 hours ago, Breamerly said:

Any idea roughly how much stiffer (if at all) it would make a sloop to move the mainsail down 1 foot? (on a 27' boat with roughly a 33' mast (stepped about 5 feet off the water).

It won't make the boat stiffer has stiffness is from the hull and its weight.

Nevertheless, it will make the boat able to carry more sail. You need to know where the metacentre of the boat is (hard if you aren't a naval architect). https://en.wikipedia.org/wiki/Metacentric_height

Then make your lever arms calculations around this point. I think that at high angles, the point moves to make things harder. On a yacht it is somewhere above the water.

If we say it is at deck level (optimistic, very stiff boat), that your mainsail P is 30ft and boom to mast connection is 3ft above deck, CoE will go from 3 + 30/3 = 13 to 2 + 30/3 = 12 so the gain is 8%

If we say that it is a water level (pessimistic, boat is borderline dangerous), it will go from 8 + 30/3 = 18 to 7 + 30/3 = 17 so the gain is 6%

So in short it is a small but non negligible gain....

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15 minutes ago, Panoramix said:

It won't make the boat stiffer has stiffness is from the hull and its weight.

Nevertheless, it will make the boat able to carry more sail. You need to know where the metacentre of the boat is (hard if you aren't a naval architect). https://en.wikipedia.org/wiki/Metacentric_height

Then make your lever arms calculations around this point. I think that at high angles, the point moves to make things harder. On a yacht it is somewhere above the water.

If we say it is at deck level (optimistic, very stiff boat), that your mainsail P is 30ft and boom to mast connection is 3ft above deck, CoE will go from 3 + 30/3 = 13 to 2 + 30/3 = 12 so the gain is 8%

If we say that it is a water level (pessimistic, boat is borderline dangerous), it will go from 8 + 30/3 = 18 to 7 + 30/3 = 17 so the gain is 6%

So in short it is a small but non negligible gain....

Great info. Thank you.

In re: stiffness, My understanding is the term refers broadly to a boat's baked-in ability to resist overpowering in higher wind. So, most primarily determined by built-in characteristics like ballast, hull shape, keel/rudder profile etc - but also determined by the standing rigging configuration, of which boom height is a part. I think we would say that, all other things equal, a boat with a shorter mast and smaller-area sailplan, was stiffer than the same boat with a taller mast, yeah?

Likewise, all other things equal, a fractional rig (CE moves downward) would be stiffer then a masthead rig?

Similarly, I've heard folks refer to repowering the main by increasing backstay bend as 'stiffening' the rig.

Obviously somewhere in there is a delineation between changes to the 'hard' configuration and temporary changes to the working sailplan, but it seems.... A bit amorphous.

Have I got that wrong?

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22 minutes ago, Panoramix said:

we say it is at deck level (optimistic, very stiff boat), that your mainsail P is 30ft and boom to mast connection is 3ft above deck, CoE will go from 3 + 30/3 = 13 to 2 + 30/3 = 12 so the gain is 8%

Would that be able to withstand 8% more force/effort at the same amount of heel? 

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I am not an naval architect and my native tongue isn't English....

My understanding of the term is that the stiffness of the boat (at least how we use it in French) is how much a boat wants to self-right once you've inclined it. So you could measure it with all sails down in a harbour just by pulling the top of the mast and measuring the force (how much the boat is lifting you). So a boat with a taller will loose in stifness only because the mast will have a weight that wants to make the boat capsize.

The stiffer the boat, the most sail it can carry in a set wind.

Stiffening the rig is something different, in high wind you don't want your mast to move in all directions so you want it well tensioned to stop it "pumping" (that is moving like a bow used to shoot arrows!) and make it stiff. There are many things to know about this and entire books have been written about it so I can't say it all with just a few words!

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2 minutes ago, Breamerly said:

Would that be able to withstand 8% more force/effort at the same amount of heel? 

Yes, remembre force is proportional to square of wind speed, so that's about 4% more wind (reefing in 19 instead of 18 knots of wind for instance).

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6 minutes ago, Panoramix said:

Yes, remembre force is proportional to square of wind speed, so that's about 4% more wind (reefing in 19 instead of 18 knots of wind for instance).

Indeed. Got it. Thank you.

That lines up with my even rougher scratchings - something like ~5% windspeed change, or 10% in the most wildly optimistic reading.

Not huge.

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14 minutes ago, Panoramix said:

I am not an naval architect and my native tongue isn't English....

My understanding of the term is that the stiffness of the boat (at least how we use it in French) is how much a boat wants to self-right once you've inclined it. So you could measure it with all sails down in a harbour just by pulling the top of the mast and measuring the force (how much the boat is lifting you). So a boat with a taller will loose in stifness only because the mast will have a weight that wants to make the boat capsize.

The stiffer the boat, the most sail it can carry in a set wind.

Stiffening the rig is something different, in high wind you don't want your mast to move in all directions so you want it well tensioned to stop it "pumping" (that is moving like a bow used to shoot arrows!) and make it stiff. There are many things to know about this and entire books have been written about it so I can't say it all with just a few words!

interesting/excellent. Thanks.

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Your'e in the right ball park.  Moving the sails down a foot will reduce the heeling moment by the % lowered at least at first.  As the boat heels over the location of the center of effort approaches the old location until it is the same as the old location when the mast is horizontal.  So, that means as the boat heels the lowered CE location becomes less effective.  With all that said, it will reduce heel.   

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26 minutes ago, weightless said:

Folks have coded the calcs for guesstimates of sail force and moment. eg.  https://cdn2.hubspot.net/hub/209338/news/SailPowerCalc/SailPowerCalc.htm You could play around with your numbers in one of them.

Great calculator, but something is weird about it. Its listed value for sail heel (which it defines as heeling force exerted by the sails on the mast) does not change when you modify the boom height, even when you use crazy numbers. I think it might have a bit of wonk.

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31 minutes ago, Panoramix said:

So you could measure it with all sails down in a harbour just by pulling the top of the mast and measuring the force (how much the boat is lifting you)

I think this would just be righting moment, wouldn't it?

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1 minute ago, Breamerly said:

Great calculator, but something is weird about it. Its listed value for sail heel (which it defines as heeling force exerted by the sails on the mast) does not change when you modify the boom height, even when you use crazy numbers. I think it might have a bit of wonk.

Could be. It was just the first hit from google for "sail power calculator". There should be others and they may meet your needs better. IIRC, Larsson and Eliasson's Principles of Yacht Design has all that's needed to code up your own.

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5 hours ago, bgytr said:

Are you chopping off some amount from the bottom of the mast?

I say this in all politeness: You know that sails slide - like, slide up and down - right?

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Don't forget wind shear. I suspect moving more sail area into the disturbed region near the hull and the water's surface outweighs your Rm gain.

That and smacking your head / the water with the boom ;-)

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Just now, Snowden said:

Don't forget wind shear. I suspect moving more sail area into the disturbed region near the hull and the water's surface outweighs your Rm gain.

Is the mast filled with Helium?

 

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8 hours ago, Breamerly said:

Indeed. Got it. Thank you.

That lines up with my even rougher scratchings - something like ~5% windspeed change, or 10% in the most wildly optimistic reading.

Not huge.

Could you test your theory by reefing? This should lower your CE of the sail plan and give you an indication, it will decrease your sail area of course, but might be worth doing as a base reference. 
I get the thinking though, my gooseneck wouldn’t even be 2’ off the deck and I can feel a difference when hopping on opb with a boom 5’ off the deck.

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1 hour ago, Snowden said:

Don't forget wind shear. I suspect moving more sail area into the disturbed region near the hull and the water's surface outweighs your Rm gain.

That and smacking your head / the water with the boom ;-)

Dont frget endplate

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2 hours ago, Breamerly said:

I say this in all politeness: You know that sails slide - like, slide up and down - right?

Why yes, yes i do.  I'm trying to grasp the point of this discussion.  What miniscule gain do you hope to get by taking the same sail and dropping it down a foot?

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On 4/27/2021 at 4:01 PM, Breamerly said:

Porting a question over here from a thread in fix-it anarchy, where it was kind of off-topic.

Any idea roughly how much stiffer (if at all) it would make a sloop to move the mainsail down 1 foot? (on a 27' boat with roughly a 33' mast (stepped about 5 feet off the water).

My rough calculation was that, assuming the sail's center of resistance started out about 20 feet above the water, and the keel's center of mass about 4' below it, moving the center of the sail's mass 1 foot downward would mean about 5-10% percent more force on the lever would be required to achieve the same heel.

But since wind resistance goes up with the square of velocity, that increase in force seems like it would be achieved with a relatively small increase in windspeed (as in, a few knots or less).

Am I in the right ballpark?

You could try a shallow reef and see what happens?

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1 hour ago, bgytr said:

Why yes, yes i do.  I'm trying to grasp the point of this discussion.  What miniscule gain do you hope to get by taking the same sail and dropping it down a foot?

The point is that I'm curious.

That and I happen to have the boom and gooseneck fitting off the mast, which occasioned me thinking about how much extra room there is between the boom and the deck - far more than is necessary to accommodate standing headroom in the cockpit - and wondering how much that extra room adds to the heeling force on the rig. (I presume the trade off is achieving just a touch more 'reach' on light-air days.)

I say all this of, of course, in good faith, on the assumption that you aren't just asking what the point is so that you can heap disdain on it from 10,000 feet (rather than contributing).

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2 hours ago, shaggybaxter said:

Could you test your theory by reefing? This should lower your CE of the sail plan and give you an indication, it will decrease your sail area of course, but might be worth doing as a base reference. 
I get the thinking though, my gooseneck wouldn’t even be 2’ off the deck and I can feel a difference when hopping on opb with a boom 5’ off the deck.

I have definitely thought about the similarity of it to reefing. But I'm not sure it's comparable, given how much a reef *reduces* the sail area. I don't know how much to ascribe the heel reduction to the lowered center of effort versus the reduced sail area.

 

And yeah, my thinking at the beginning was that levers often seem to involve an exponential relationship type deal, so maybe a little change would make a big difference. Sounds like, though, a little change would just make a little difference.

 

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4 minutes ago, Breamerly said:

The point is that I'm curious.

That and I happen to have the boom and gooseneck fitting off the mast, which occasioned me thinking about how much extra room there is between the boom and the deck - far more than is necessary to accommodate standing headroom in the cockpit - and wondering how much that extra room adds to the heeling force on the rig.

I say all this of, of course, in good faith, on the assumption that you aren't just asking what the point is so that you can heap disdain on it from 10,000 feet (rather than contributing).

Is your downhaul on the deck?  Slide the sail down a foot or so (or more) without the boom and go out for a sail.  If you have enough luff tension, it’ll be a fairly close approximation.  You should saw off the top of the mast if you like it shorter, anyway.  Although if that messes with standing rigging and blocks etc, that will be some work....

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11 minutes ago, Amati said:

Is your downhaul on the deck?  Slide the sail down a foot or so (or more) without the boom and go out for a sail.  If you have enough luff tension, it’ll be a fairly close approximation.  You should saw off the top of the mast if you like it shorter, anyway.  Although if that messes with standing rigging and blocks etc, that will be some work....

Alas. The sail track stops where it is now. To slide it down further I'd have to extend the track (and I guess add a stoppered car to carry it/rest it on).

Ultimately what I was wondering was whether it was worth the work to make it adjustable as a heavy weather item - have the ability to lower the CE a bit as another arrow in the quiver along with reefing, active sheeting, pinching off, etc.

Boom has a kicker, so I'd have to also set a second, lower deal for that, to allow moving it up/down.

For a couple of knots difference in the overpower-threshold windspeed, it seems like more trouble than it's worth, honestly.

EDIT: oh I see what you mean. Interesting though. But the difference I think would be in stiff weather. Plus, like, no foot tension?

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38 minutes ago, Breamerly said:

Alas. The sail track stops where it is now. To slide it down further I'd have to extend the track (and I guess add a stoppered car to carry it/rest it on).

Ultimately what I was wondering was whether it was worth the work to make it adjustable as a heavy weather item - have the ability to lower the CE a bit as another arrow in the quiver along with reefing, active sheeting, pinching off, etc.

Boom has a kicker, so I'd have to also set a second, lower deal for that, to allow moving it up/down.

For a couple of knots difference in the overpower-threshold windspeed, it seems like more trouble than it's worth, honestly.

EDIT: oh I see what you mean. Interesting though. But the difference I think would be in stiff weather. Plus, like, no foot tension?

Upwind?  Tie the sheet to something on the stern.  6-8 k TW will tell you what you want to know....

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9 hours ago, Snowden said:

Don't forget wind shear. I suspect moving more sail area into the disturbed region near the hull and the water's surface outweighs your Rm gain.

That and smacking your head / the water with the boom ;-)

In case you haven't noticed, racing boats keep moving the boom lower... I am sure it isn't for aesthetics!

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33 minutes ago, Panoramix said:

In case you haven't noticed, racing boats keep moving the boom lower... I am sure it isn't for aesthetics!

Something like a foiling IMOCA is going to be above the disrupted layer of wind at the water’s surface anyway though?

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6 hours ago, Breamerly said:

The point is that I'm curious.

That and I happen to have the boom and gooseneck fitting off the mast, which occasioned me thinking about how much extra room there is between the boom and the deck - far more than is necessary to accommodate standing headroom in the cockpit - and wondering how much that extra room adds to the heeling force on the rig. (I presume the trade off is achieving just a touch more 'reach' on light-air days.)

I say all this of, of course, in good faith, on the assumption that you aren't just asking what the point is so that you can heap disdain on it from 10,000 feet (rather than contributing).

What kind of boat?  I am a naval architect, so might have some input if you are congenial about it.

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26 minutes ago, Snowden said:

Something like a foiling IMOCA is going to be above the disrupted layer of wind at the water’s surface anyway though?

Not really I suspect, even on a 12m boat (with say a 20m above water mast) you can see the wind gradient is still going on high up when you trim your sails. It is just I suspect a mix of better lever arm + end plate effect that make them favour low boom. I don't know if you noticed but sometimes on a reach in breezy conditions you have to have the main regularly depowered, you then reef, the boat heels much less so you think may be I am now under-canvassed, you tweak the sails for max power then realise that you are faster despite the lack of drama, same effect...

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5 hours ago, Panoramix said:

Not really I suspect, even on a 12m boat (with say a 20m above water mast) you can see the wind gradient is still going on high up when you trim your sails. It is just I suspect a mix of better lever arm + end plate effect that make them favour low boom. I don't know if you noticed but sometimes on a reach in breezy conditions you have to have the main regularly depowered, you then reef, the boat heels much less so you think may be I am now under-canvassed, you tweak the sails for max power then realise that you are faster despite the lack of drama, same effect...

Are we talking about gradient or shear?  I can’t find it, but iirr there was a piece by the 1m rc class (?) that was indicating that the majority of gradient was done pretty close to the water, and was also changed by wind speed- over 6 knots shear was diminishing.  So what does that do to gradient or shear change, which, as you pointed out, also can change quite a bit depending on atmospheric conditions.  Maybe the square riggers were a bit more sophisticated than they are given credit?  

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5 hours ago, bgytr said:

What kind of boat?  I am a naval architect, so might have some input if you are congenial about it.

Whoa whoa whoa there mister, this is the internet, and I'm a stranger - exactly how else am I supposed to react to even the most miniscule ribbing other than by getting insanely butthurt and starting a flame war? lol.

It's a 1969 Santana 27. I haven't measured yet, but the boom looks to be about 7.5' off the cockpit sole, and about 2.5' (maybe a touch more) off the deck.

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1 minute ago, Amati said:

Are we talking about gradient or shear?  I can’t find it, but iirr there was a piece by the 1m rc class that was indicating that the majority of gradient was done pretty close to the water, and was also changed by wind speed- over 6 knots shear was diminishing.  So what does that do to gradient or shear change, which also can change quite a bit depending on atmospheric conditions.  Maybe the square riggers were a bit more sophisticated than they are given credit?  

AFAIU Wind shear is a temporary thing that tends to happen before a wind shift. I was thinking wind gradient which also implies that high up your sails are less close to the apparent wind as true wind is stronger, I think that you need to go quite high for the gradient to disappear (like owning a super yacht to witness it).

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J-24's also have a terrifyingly low boom - the bane of Learn to Sail. 

But what's that got to do with lever arms ? 

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13 hours ago, weightless said:

From the disdain for cutting the excess from the mast foot I take it this not a cunning plan to reduce the mass moment of inertia?

I didn't really take it as a serious idea I guess. I didn't ask originally about center of inertia - I asked about the effect of reducing the boom height.

I asked that specifically because I was contemplating adding adjustability, not making a permanent change (one that would require redoing all my standing rigging and having the genny recut).

As always on these posts, there's an element of balancing between enough context and too much. I was trying to keep it simple. Apologies if that added confusion.

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5 minutes ago, AJ Oliver said:

J-24's also have a terrifyingly low boom - the bane of Learn to Sail. 

But what's that got to do with lever arms ? 

Finns?

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12 minutes ago, AJ Oliver said:

J-24's also have a terrifyingly low boom - the bane of Learn to Sail. 

But what's that got to do with lever arms ? 

Nothing, in that scenario.

In my case, however, the boom is set relatively high off the deck, with a main cut to accommodate that. 

By extending the track downward a bit, I'd have the option to lower the boom a little more than foot (it's on a car), and hoist from that lower position.

Which would give me the same sail, acting on a shorter lever.

Thus my question: How much of a difference would that actually make (in terms of heeling force)?

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1 hour ago, Breamerly said:

Whoa whoa whoa there mister, this is the internet, and I'm a stranger - exactly how else am I supposed to react to even the most miniscule ribbing other than by getting insanely butthurt and starting a flame war? lol.

It's a 1969 Santana 27. I haven't measured yet, but the boom looks to be about 7.5' off the cockpit sole, and about 2.5' (maybe a touch more) off the deck.

In what type of conditions do you predominantly sail?  If it's mostly lighter winds, I'd leave the up where it is as it will be up where there is more wind.  Especially in lighter going, the wind down lower is usually a lot lighter.  If you are out there powered up and heeling 20 plus degrees every time you go out, then it could be worth lowering.

Pulled up the data on sailboatdata.com.  The benefit that you would see would likely be only in upwind or close reaching when you are fully powered up. You might get an extra tenth of a knot vmg upwind and a similar boost in straightline speed close reaching in strong stuff.  In lighter stuff and offwind you'd probably be better leaving the main up high.  I've not done any calcs to back it up, but just my gut reaction knowing the physics behind it and experience.

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1 hour ago, bgytr said:

In what type of conditions do you predominantly sail?  If it's mostly lighter winds, I'd leave the up where it is as it will be up where there is more wind.  Especially in lighter going, the wind down lower is usually a lot lighter.  If you are out there powered up and heeling 20 plus degrees every time you go out, then it could be worth lowering.

Pulled up the data on sailboatdata.com.  The benefit that you would see would likely be only in upwind or close reaching when you are fully powered up. You might get an extra tenth of a knot vmg upwind and a similar boost in straightline speed close reaching in strong stuff.  In lighter stuff and offwind you'd probably be better leaving the main up high.  I've not done any calcs to back it up, but just my gut reaction knowing the physics behind it and experience.

Right. I get that. As I wrote up above, what I'm interested in is the potential utility of making my boom height adjustable, as a way to make the boat a bit more adaptable to heavy weather by giving myself the ability to lower the CE, along with typical heavy air strategies like reefing, active sheeting, and pinching off. To that end I'm trying to understand how much it would change the heeling force, and by extension the windspeed threshold at which the boat overpowers when going to windward.

Sorry, I posted a lot more specifics in my original on fix-it anarchy, and left this thread a bit bare-bones.

I am not contemplating a permanent height change. As I wrote up above, I understand that a higher main is intended to 'reach up to the wind' in lighter air, and that lowering it would be better for heavy air. 

Again, since no one is interested in hearing my entire use case and decision matrix/criteria list, I'm not asking for an overall recommendation about how to set up my rig. I'm trying to understand specifically how much difference this one change would make, in terms of one specific type of force.

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Sorry if I sound grumpy. Asking for advice on forums is damned-if-you-do/damned-if-you-don't. If you provide lots of specifics, people start answering questions you didn't ask, but if you ask a bare-bones question, people say, 'can't answer the question without more specifics.' And if you really provide a thorough explanation of the details and your whole thought process, you end up with a novel that no one wants to read!

Everything I've gathered so far is that lowering the CoE by a foot (about 5%) would mean it would take 3-4% higher windspeed to force the boat into the same degree of heel.

Or, it might buy me something in the ballpark of a knot or two more of wind I could handle before being overpowered.

Sounds like it's not really worth it.

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8 hours ago, Ease the sheet. said:

Why would you not lower the boom and just get a bigger mainsail?

Better in the light and you can always reef when it gets heavy.

That's a great idea. So great, in fact, that I talked to the sailmaker about it yesterday. I think that will be a next year spend, though. 

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19 hours ago, fastyacht said:

Do the Delkenbaugh calc befor and after...

Will give you exactly ehat.  You eant

That's a very straightforward formula that I hadn't seen before, as is WPC. Thank you.

If I'm understanding correctly, what's most relevant is the heeling moment formula, which is a component of both - Hm = sail area * Cos^2(heel angle) * Heeling arm * Wind pressure.

@bgytr is it correct to think I could get a sense of the effect of the changes by comparing the moment generated by the same sail area and windspeed with two different Heeling arm values?

Or in 20 knots of wind, at 20 degrees of heel, the Heeling moment with my current heeling arm and sail area (20ft, 158ft2) is about 1180 Lbs.

Shorten the Arm by 1 foot and the same moment is reached at a windspeed of.... Drum-roll please.... 20.6 mph.

If I'm doing that right, seems like a real wet fart of a modification.

(I do understand that to truly compare stability, I'd need to compare The induced heel of the two configurations at a given wind speed - but it seems like the way I'm doing it here is okay for a rough, rough comparison?)

 

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On 4/28/2021 at 11:02 PM, Breamerly said:

That and I happen to have the boom and gooseneck fitting off the mast, which occasioned me thinking about how much extra room there is between the boom and the deck - far more than is necessary to accommodate standing headroom in the cockpit...

Are you going to tell the crew you lowered the boom? Could make for a fun surprise first time out! 

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