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My friend (he is not on here) is looking at multis and he is essentially retired, a very experienced mono sailor and wants to salvage a hurricane boat in the Caribbean.

He says the biggest issue he is hearing is finding a used mast for a cat. He is open on length, 30'-38'.

Can someone please explain to me why a used monohull mast cannot be used, modified, to work? I believe it has something to do with load amounts? Any professional riggers here to explain or have any good links to articles that do?

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I wouldn't say its absolutely impossible but it has a few issues that need to be engineered:

Usually monohulls have spreaders.  The wide sidestay attachment on the floats of a multihull gets you that angle without using spreaders so you need to deal with that change.  And the wide boat also is capable of more righting moment than the mono, which heels / falls over with less righting moment.  So, in theory you could load up the mast with more force and, without spreaders, it may buckle.  Dropping masts is dangerous business.  You can hurt people and a disabled boat can wind up on a lee shore.

Get the specs on the original mast and hire an engineer to calculate the capability of mono masts available.  I'd say if you can find a similar mast with an equivalent load carrying capability, that it could work.

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Here's a sweet cat listing with a ton of pictures of the rig. Study the SPREADER arrangement, as well as the shrouds and stays. This will inform the questions you ask a professional rigger and how to approach it.

http://www.yachtworld.com/boats/2005/Schionning-Waterline-15.2-2613984/New-Zealand?refSource=browse listing&refSource=browse listing#.WvI3bYVE3YU

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Thank you both!

That Schionning, omg talk about a floating masterpiece, just stunning, must sail like a dreeeaaammmmm. Never seen spreaders like those, btw. Are they common on big modern cats? I try not to look at boat porn too far outta my league....so that could be why I wouldn't know.

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The swept back spreader arrangement will help keep the mast in column better without a backstay. 

image.thumb.jpeg.4d0ed7b4349b149b4587942e5fd301f5.jpegHere's what a former cat sailor did with a mono. A true classic. I'm out for now but anxiously await other's response. (Inertial force might be one place a pro spar maker might point to different fabrication considerations) But a modified rig could easily compensate especially at the size of boat your 'friend' is considering salvaging. Good luck!

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Thanks. Well there are a handful of Maria damaged Leopard 39s on yachtauction currently, as an example. I am trying to sway him towards a friends older good to go 38 (partly because I know they took care if her) but it's on the left coast. Honestly, he doesn't know what he wants but he does know he wants to know his options and because he says other cruising friends say cat masts are hard to find right now that's why I am researching for him about alternatives. 

I was told by a multi sailor that mono masts will snap on you when put on a multi. I don't know if she was meaning a 35 monos mast on a 35 cat would do that? But that say a 45 mono mast (thicker wall, beefier eh) would be ok? I'm not a rigger and hoping for one to show up here and school me on it! :)

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Early multis were rigged like monos up until the 80s because that's all there was. A stout mono rig will work but the multi has higher righting moment so higher loading than an equivalent sized mono, as long as that's taken into account then it can work. Complications might be changing to a non rotating step (assuming that you intend to go that way), chainplate locations and strengths, boom clearanxce and sheet points, and platform flex vs forestay tension.

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It will depend more on the cat and its original rig setup, rather than just mono / multi rigs.

If the cat had a three-point rig, with three shrouds holding the rig up and diamonds etc to hold the mast in column, as found on most modern multis, it will be a bit of work converting a mono rig to suit.

If the original rig had a more conventional setup with chainplates for caps, intermediates, lowers, in-line or swept, there's no reason it can't be done with a replacement rig that reflects the extra loading that a cat will have.

I'm not a rigger but I've done this three times.

And got it wrong once, when an aft lower broke and the rig went over the side.

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11 hours ago, sailorcherry said:

My friend (he is not on here) is looking at multis and he is essentially retired, a very experienced mono sailor and wants to salvage a hurricane boat in the Caribbean.

He says the biggest issue he is hearing is finding a used mast for a cat. He is open on length, 30'-38'.

Can someone please explain to me why a used monohull mast cannot be used, modified, to work? I believe it has something to do with load amounts? Any professional riggers here to explain or have any good links to articles that do?

This is a long shot but.... There is a cruising multi up on the hard in Christiansted, St. Croix post hurricane. Sounds about the same size as your man's prospect. This particular boat is f-ed, never to be one with the water again. The rig is laying on the top, though, looks to be pristine. When I say it's on the hard I mean the crane deposited it on a property next to the water because there was space. It is at the end of the boardwalk on the Angry Nate's end. (If you know the area at all) I am sure someone should be able to make a deal for the mast on that boat. It is the only thing worth a dime left. 

Again, long shot but maybe worth looking into for your friend.

Thanks.

Mark

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 Hey @sailorcherry

"Danger Will Robinson; Danger!!" 

I say that not about your friend's idea but rather about my following back of the envelope uneducated speculation which lacks any sound basis... :ph34r:

Likely obvious to you is that multihulls have much more initial stability (and a wider based upon which to support a rig) and so imparts loads on the mast and standing that can be quite different from a similar sized monohull.  For a similar length, the mutihull mast is likely to be shorter than the monohull mast for similar design briefs (ie comparing multihull performance cruiser to monohull performance cruiser of same length).  From your comments I assume your friend is talking about a charter cat and that might be useful... because he, she or you should be able to fairly easily find specifications for the original multihull mast and standing rigging on-line.  Once you know the mast height, section, wall thickness, materials, etc... as well as the specifications for the standing rigging for the original multihull mast design you have a reasonable starting point and basis to say if any replacement mast and standing rigging is of same, greater, or lesser strength.

How hard can it be... says the guy who has no skin in the game!  ;) 

I would wing it if its a cheap fun project that is staying inshore.  On the other hand if taking family and friends into harms way off shore I would consult somebody smarter than me which would be just about anybody who has experience in the field!!

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Thanks Mark, I will tell him and if he gets truly serious I will PM you his contact info. 

Wess, right, good point, it will prolly be a charter cat so finding specs should be easy. TY! I was thinking that as well, that if you can find a mast with the same wall thickness, than why not?

But like ya say, you and I are not riggers, sooo....

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10 hours ago, Bruno said:

Early multis were rigged like monos up until the 80s because that's all there was. A stout mono rig will work but the multi has higher righting moment so higher loading than an equivalent sized mono, as long as that's taken into account then it can work. Complications might be changing to a non rotating step (assuming that you intend to go that way), chainplate locations and strengths, boom clearanxce and sheet points, and platform flex vs forestay tension.

Is this one of the reasons a lot of people said cats from the 80s were dangerous (prone to capsizing)? Or was that because of narrower beams?

Could one salvage the rotating step off the cat (assuming it dismasted just above the deck) and use it with the mono mast? Like I keep saying, I understand the basics, at best, about rigging.

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It’s about righting moments/force. Mono hulls heel and spill, Multi’s don’t. So your going to need a mast from a bigger mono to match the righting moment of your multi. 

Definitly seek professional advice. 

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the mast itself doesn't care about the sort of boat underneath it. it DOES care about the loads, and the loads on a multi are much higher.  Just talk to a mast guy about what section sizes can handle your boat of choice. It's just math, but really important math....

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Don't mean to be a nay-sayer with all you folks convinced that multihull loads on the mast are higher than on a mono mast....but most lead draggers have backstays that put huge compressive stresses on the mast PLUS, they also usually have a kicker (boom vang) that add another component that "most" multis don't have.  Don't know how much money it is saving using a mono mast, but if substantial and wall thickness is the same, same anodizing, then I'd say go for it but expect to change the spreader sweep.  Give a shout out to Buzz Ballenger and ask, Buzz will tell you straight.

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2 hours ago, Redreuben said:

It’s about righting moments/force. Mono hulls heel and spill, Multi’s don’t. So your going to need a mast from a bigger mono to match the righting moment of your multi. 

Definitly seek professional advice. 

Its not always so. I had the pleasure :blink: of moving and replacing both a Corsair F27F trimaran mast as well as an Alberg 30 (monohull) mast.  Similar length and same material for both masts but the monohull mast was MUCH stiffer and heavier.  I could have safely put that monohull mast (after changing the spreader angles) on my F27F trimaran but no way would the lighter and relatively noodle like F27F mast survived on the Alberg.

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You just said it's going into charter work, that's a different ball game. Spend the money.

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5 hours ago, Bruno said:

You just said it's going into charter work, that's a different ball game. Spend the money.

I think he meant that the stickless multi was a charter boat, not that it was going into charter. 

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5 hours ago, Bruno said:

You just said it's going into charter work, that's a different ball game. Spend the money.

No, no, there are some charter cats that were wrecked by Hurricane Maria for sale at yachtauctions....this would be for a personal cruiser, not a charter.

However, that does lead to a similar point....will insurance care if it's not a multi mast?

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5 hours ago, Wess said:

Its not always so. I had the pleasure :blink: of moving and replacing both a Corsair F27F trimaran mast as well as an Alberg 30 (monohull) mast.  Similar length and same material for both masts but the monohull mast was MUCH stiffer and heavier.  I could have safely put that monohull mast (after changing the spreader angles) on my F27F trimaran but no way would the lighter and relatively noodle like F27F mast survived on the Alberg.

Good analogy.

Wouldn't you also say too that there's no way the F27 mast would work on the Alberg also because that's a waaaay more racing/performance oriented boat than the Alberg.

Same thing with this cruising project cat, that mast needs to come from a non performance rig, mono or cat, right?

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I know where there is a nice new 38' long aluminum wing-section mast blank for sale.

http://www.sfbama.org/scripts/trade.php

-MH

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3 minutes ago, Multihauler said:

I know where there is a nice new 38' long aluminum wing-section mast blank for sale.

http://www.sfbama.org/scripts/trade.php

-MH

38 may he too small....

Have a for sale link (that one just takes me to sfbama home page)?

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4 hours ago, sailorcherry said:

38 may he too small....

Have a for sale link (that one just takes me to sfbama home page)?

Click on the Trading Post link.

-MH

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The righting moments are much, much higher on a fat 39' catamaran than a 39' monohull. Like 2-3x more (roughly). So the mast needs to have a moment of inertia correspondingly larger. (That doesn't mean the mast has to be 3x bigger; moments of inertia go up as cross sectional dimensions to the 4th power. But it will be a lot beefier.

Here is the Righting Moment curve for 44' Halberg Rassey. Peak is around 11 700 kg.m

HR44-GZ-Curve_zpsl690cqhp.jpg

Here is a calculation of the mast beam for a PDQ 44 cat, a similarly heavy cat. Note the max RM = 192,000 ft.lb (26 607 kg.m) or 2.3x the H-R

mast2-6.jpg?zoom=2

I've also got a Fusion 40 mast drawing. On it says displacement 6000 kg / RM = 13 576 kg.m.
The Fusion is much lighter than a HR44, which is 13 300 kg EMPTY yet it still has more RM.

Digression into mast design. Lateral bending of the basic tube takes first consideration. The compressive forces from shrouds, while substantial, are of lesser importance. Then you consider local buckling and panel lengths (why we have multiple spreaders on taller masts).

A lot of mast design by riggers and yacht designers is pretty crude because they are indeterminate structures (multiple load paths due to lots of wires holding them up). Unless you do FEA, you're sort of guessing and using rules of thumb which are conservative and have worked in the past. At the high end of yacht racing, proper mast design is a lot more advanced of course and FEA is commonly done.

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Posted (edited)

I appreciate the lesson.  I still have questions, though.  The RM above is calculated at max (total weight of vessel at total length of righting arm).  Why isn't it calculated at max heel angle before capsize? Also, why total weight of vessel instead of half since other half is buoyed. 

Edited by MultiThom
Clarify

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A month ago, in another thread, I posted this, which is relevant here.  To elaborate slightly, gust loads on a multihull peak suddenly at low angles of heel (frequent) as opposed to a monohull where max. RM has it's peak when the mast hits the water (rare).

What is missing from this observation is the fact that righting moment peaks at very different angles of heel for multihulls vs. monohulls, as shown in this graph from 'AdHoc' at boatdesign.net (just one of many to be found):

Y-stab-2.thumb.jpg.e38572cbd02e0af8478d5362e85edc9e.jpg

One more from John Shuttleworth:

NESF3.GIF.0c3cf0ddb2773750dc50a26524d0be96.GIF

This fact of life is highly relevant to rigging loads because in this respect (and others), there is a big difference between a monohull and multihull.

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according to a friend, who is on the mast-building business, as a rule of thumb, considering a cruising cat and a cruising mono of about the same l.o.a. and for an equal lenght mast, for the cat rig he would go up at least one "size" on the tube

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2 hours ago, MultiThom said:

  The RM above is calculated at max (total weight of vessel at total length of righting arm).  Why isn't it calculated at max heel angle before capsize? Also, why total weight of vessel instead of half since other half is buoyed. 

For a catamaran, the RM peaks just as the windward hull is out of the water. This occurs at small heel angles as the graph from Principles of Yacht Design shows. (this also shows why flying a hull is exciting. As you heel more, the RM is decreasing steadily. Things are getting worse as heel increases slightly!) 

A monohull's RM is much trickier to calculate since you have to use a hydrostatic program that takes into account the shape of the boat; you need to figure out where the center of buoyancy of the underwater volume is located as the boat heels.

For the cat, total weight is used because you are heeling the whole boat :).  See the math in green on the PDQ44 diagram. Boat weight @ middle of boat x distance to center of buoyancy = R.M.

Quote

according to a friend, who is on the mast-building business, as a rule of thumb, considering a cruising cat and a cruising mono of about the same l.o.a. and for an equal lenght mast, for the cat rig he would go up at least one "size" on the tube

Only one size on the tube might be only 40% more moment of inertia. Useful starting point I guess, but I suspect two sizes might be more typical.

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On 5/9/2018 at 2:49 PM, Wess said:

Its not always so. I had the pleasure :blink: of moving and replacing both a Corsair F27F trimaran mast as well as an Alberg 30 (monohull) mast.  Similar length and same material for both masts but the monohull mast was MUCH stiffer and heavier.  I could have safely put that monohull mast (after changing the spreader angles) on my F27F trimaran but no way would the lighter and relatively noodle like F27F mast survived on the Alberg.

cats' and tris' masts, specially condomaran cats, are two very different beasts.

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27 minutes ago, Zonker said:

For the cat, total weight is used because you are heeling the whole boat :).  See the math in green on the PDQ44 diagram. Boat weight @ middle of boat x distance to center of buoyancy = R.M.

 

I don't see that.  When a mono static diagram is used to calc RM, the bulb/keel weight is used (depending on design), not weight of the boat.  For a cat, the weight that is lifted would be equivalent, not total weight.  Granted, my limited experience in this is keeping my submarine from inverting when submerging, but that's how we calculated (subs are very unstable going from surfaced to submerged since CB and CM are overlapping (ie, no RM) briefly-which is why you never do it stationary).  I don't disbelieve, I just don't see the physics.

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40 minutes ago, Zonker said:

For a catamaran, the RM peaks just as the windward hull is out of the water. This occurs at small heel angles as the graph from Principles of Yacht Design shows. (this also shows why flying a hull is exciting. As you heel more, the RM is decreasing steadily. Things are getting worse as heel increases slightly!) 

A monohull's RM is much trickier to calculate since you have to use a hydrostatic program that takes into account the shape of the boat; you need to figure out where the center of buoyancy of the underwater volume is located as the boat heels.

For the cat, total weight is used because you are heeling the whole boat :).  See the math in green on the PDQ44 diagram. Boat weight @ middle of boat x distance to center of buoyancy = R.M.

Only one size on the tube might be only 40% more moment of inertia. Useful starting point I guess, but I suspect two sizes might be more typical.

Here is an example or the righting moment peak on a cat. I use the term 'fly point' which I guess I got from Gino Morrelli, an describes it well.

image.thumb.png.12147d1293445fede39809903258c0bc.png

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4 minutes ago, MultiThom said:

I don't see that.  When a mono static diagram is used to calc RM, the bulb/keel weight is used (depending on design), not weight of the boat.  For a cat, the weight that is lifted would be equivalent, not total weight.  Granted, my limited experience in this is keeping my submarine from inverting when submerging, but that's how we calculated (subs are very unstable going from surfaced to submerged since CB and CM are overlapping (ie, no RM) briefly-which is why you never do it stationary).  I don't disbelieve, I just don't see the physics.

OH, lightbulb...OK, the diagram for the PDQ is wrong headed.  The RM comes about (the way I was taught) by multiplying the lever arm between the CB and CM.  Flat on the water, the CB is in the middle; but as the hull lifts the CB is moved way over to the floating hull.  So it ends up the same, half the boat weight times the boat width.

 

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35 minutes ago, MultiThom said:

OH, lightbulb...OK, the diagram for the PDQ is wrong headed.  The RM comes about (the way I was taught) by multiplying the lever arm between the CB and CM.  Flat on the water, the CB is in the middle; but as the hull lifts the CB is moved way over to the floating hull.  So it ends up the same, half the boat weight times the boat width.

You got it.  From Westlawn: http://www.westlawn.edu/forum/upload/dgerr/20073198539_multihullappendixupdate.pdf

Quote

For mast and rigging calculations, you always use the maximum possible righting moment, which is:

RM = half beam x displacement

westlawn2.png.c11290dc947048f71c1b258a20283b89.png

(where RM = Gz * Displacement) 

westlawn.thumb.png.5732ca0ecd677737ef619e2cd95fda80.png

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In the Navy, before they teach us anything, they ask Midshipmen to explain what stops a ship from rolling once it starts.  Then they show nifty diagrams showing a separation between CM and CB with CM as a vector pointing down and CB as a vector pointing up.  They usually ask someone who looks sleepy to tell them what happens if CB does not equal CM.  Basically it was a pretty good lecture back when I was 26 in 1976.  It also explains why multihulls are more stable upside down than right side up.  Then in Sub School they show what happens when a sub submerges--gives diving officers nightmares.

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Do we also expect the cat mast to have to deal with a max load or near-max load event much more frequently than the monohull and that changes the calculations?

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On 5/11/2018 at 2:16 AM, MultiThom said:

In the Navy, before they teach us anything, they ask Midshipmen to explain what stops a ship from rolling once it starts.  Then they show nifty diagrams showing a separation between CM and CB with CM as a vector pointing down and CB as a vector pointing up.  They usually ask someone who looks sleepy to tell them what happens if CB does not equal CM.  Basically it was a pretty good lecture back when I was 26 in 1976.  It also explains why multihulls are more stable upside down than right side up.  Then in Sub School they show what happens when a sub submerges--gives diving officers nightmares.

What do you mean by CM?

MetaCenter is not a vector, so I guess you can't mean that.

On 5/11/2018 at 12:55 AM, MultiThom said:

OH, lightbulb...OK, the diagram for the PDQ is wrong headed.  The RM comes about (the way I was taught) by multiplying the lever arm between the CB and CM.  Flat on the water, the CB is in the middle; but as the hull lifts the CB is moved way over to the floating hull.  So it ends up the same, half the boat weight times the boat width.

 

If CB refers to Center of buoyancy as expected, then CM would have to be center of gravity, why such abbreviation?

There is no leverage between buoyancy and MetaCenter, in case M would refer to the only word I can think of starting with M and having anything to do with static lateral stability.

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On 5/11/2018 at 7:29 PM, Dex Sawash said:

Do we also expect the cat mast to have to deal with a max load or near-max load event much more frequently than the monohull and that changes the calculations?

I guess it changes the fuzzy factor also called safety factor, thus not calculations, just the results.

In reality the mast would see loads far exceeding the load based on static load calculations, because catamarans have high rolling moment of inertia which couples with rolling accelerations to cause that effect. That's the biggest reason for the higher safety factor for cats, even if called as a gust factor. Inertial effects on the mast itself are not that significant for alloy masts, having a constant cross section at all heights, because other loads are much higher. For fractional carbon masts, the top mast dimensioning would depend somewhat on the inertial effects of the mast itself.

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1 hour ago, NotSoFast said:

What do you mean by CM?

MetaCenter is not a vector, so I guess you can't mean that.

If CB refers to Center of buoyancy as expected, then CM would have to be center of gravity, why such abbreviation?

There is no leverage between buoyancy and MetaCenter, in case M would refer to the only word I can think of starting with M and having anything to do with static lateral stability.

Sorry, physicsspeak...center of mass. 

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22 hours ago, MultiThom said:

Sorry, physicsspeak...center of mass. 

In physics mass is a scalar not a vector. Center of mass is not a vector either, and neither is center of buoyancy. Although there is a related position vector to both centers if and only if you have defined a co-ordinate system. Buoyant force is a vector and so is weight.

In other words you are not using physics speak, just claiming to be doing so. You have every right to use whatever speak you like, but please stop claiming it has something to do with physics when it clearly does not.

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3 hours ago, NotSoFast said:

In physics mass is a scalar not a vector. Center of mass is not a vector either, and neither is center of buoyancy. Although there is a related position vector to both centers if and only if you have defined a co-ordinate system. Buoyant force is a vector and so is weight.

In other words you are not using physics speak, just claiming to be doing so. You have every right to use whatever speak you like, but please stop claiming it has something to do with physics when it clearly does not.

CM is a point.  Mass, while a scalar acts as a vector when in a gravity field (or when figuring orbits or even describing where you want to have your lifting body in an airplane).  CB is also a point.  Buoyancy, while also a scalar value acts as a vector when in liquid and in a gravity field.  I was describing a lecture from 40 years ago in the Navy where many (most) of the officers there had no real education in physics.  Myself, my degrees are in math and nuclear engineering; I do think I know what I'm talking about.  I understand your chagrin at not being able to figure out that CM meant center of mass...it is embarassing, but no need to get offensive.

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Perhaps I should have said force instead of vector...happier now?  On the diagram they look the same, a point with an arrow coming from it.

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I have a complete 53ft mast off the 40x27 trimaran Scout for $2500, includes dyform shrouds, double spreaders, foot,  in oriental nc. Do I need to buy an ad?

Stephen

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