I Found a Rig

[chris95040]

Originally Posted by smj

I could understand this train of thought if the boat was a monohull, but would this also be applicable for a catamaran?

I believe so. Although maybe in some respects it is easier with a cat since ‘eyeballing’ righting moment between two cats is likely easier than between two monos. I’d guess that two cats that look alike probably have similar righting moments, but I’m not a naval architect.

You make a great point, btw. My experience is 100% mono, even more reason for the OP to pick up the phone and call an expert. Maybe there’s a whole suite of other design considerations when rigging such a stiff platform.

[FAST FRED]

Go for it , cats usually do not ghost well, so when cruising when the boat is doing 3K the iron topsail is usually used anyway.

[fourlyons]

Of course righting moment applies to catamarans, it's just a boats resistance to heeling. On a catamaran it's just much higher for a given length, and peaks much sooner, but the mast must still convert that load to compression without buckling.

[Chotu]

Originally Posted by chris95040

Lol. If you crack a book on rig design righting moment will appear on page 1.

The ‘righting moment’ is what the mast is pushing against. Imagine a mast on a canoe. It would not have to be very stiff because the canoe will fall over before the rig breaks.

Your stiffer boat will need a stiffer mast.

You are right that a mast from a similar cat is possibly up to the task. Hire someone who has made it past the first page of a rigging textbook to help you evaluate that. [emoji3]

I KNOW what righting moment is. Jeez.

What I don’t know is how anything you’re going on about has to do with a catamaran with a 25’ beam.

Have YOU ever cracked a book to see the righting moment of a catamaran?

Cliff notes: it starts out almost immediately from full righting moment and fades to zero/negative approaching 90 degrees. Which is why nothing you’re saying makes sense.

Why TF did you answer the same question I’m asking here with an admission you don’t know when SMJ asked it without elaborating the points, but when I asked the same thing in full detail, you belittle me and treat me like an idiot?

We asked you the same damn thing

People like you have me seriously reconsidering putting up a huge build thread here. I don’t have time to deal with abusive a holes and they’ll probably be out in droves if I put the whole interior build up.

[GRIT]

Well, everyone's got an opinion, so I'm going to give mine too.

If the measurements of the donor boat are similar, including the weight, I'd jump on it before someone else does.

I wouldn't worry about the height, but if you're set on lengthening the mast, I'd definitely do it at the top. On a fractional rig, that's the easiest way, to save all that rigging. Mine is spliced just above the lowers, but most cats don't have lowers, so it's in the middle of the mast, just above the spreaders. If it wasn't for the lowers, the splice would look pretty random. I was surprised they didn't splice at the spreaders, which seems to be the consensus here on CF, of where one should be spliced. I'd have thought so too.

Take your mainsail foot length (about a foot less than boom length), and the hoist height (about 8 inches less than the length from boom to the halyard sheave), multiply them then take 72% of that number for your approximate sail area.

Boom 6m x hoist 17.3 x .72 = 75m2

Boom 6m x hoist 16m x.72 = 69m2 Not that different, and slightly larger than an outremer 45 mainsail (67m2).

You could go with a larger flat top. Mine's 4 feet, and could go to 6 feet.

.72 is a reasonable size cat sail, you could multiply by more, I just don't know how much more.

To sum up. Assuming similar size and weight boat. I'd buy that rig without a second thought.

Congrats on your find.

Cheers.
Paul.

[fourlyons]

Relax. I think it's perfectly reasonable to wonder if you do understand righting moment when you make the quoted statement.

Originally Posted by Chotu

I’m not understanding this.

The boat doesn’t heel (more than a few degrees).

Why does any of this matter if the rig is about the same size and from a similar size boat?

The righting moment of my busy comes from the beam and to a lesser extent, the beam of each hull.

My boat doesn’t roll over in a gust an neither does the catamaran this rig came off of.

How does anything you’re saying apply when a boat doesn’t heel?

[GRIT]

This may help, if you're going to do measurements on the "new" mast.

My original mast moments, as published on the original plans, before making all our modifications. Kurt Hughes 45 Mast moments, as an example:

Mast moments 22 inch4 x 44 inch4 or 915.7 cm4 and 1831.4 cm4

boom 12 inch4 20 inch 4. 499.4cm4, 832 cm4

Though we nearly doubled the sail area on the boat, and Selden calculated our new mast moments at: 1524 cm4 and 4686 cm4. That's quite a difference from original, which is due to the much larger sail area we've installed. The boat's beam and displacement at DWL are the same as in the plans, so those numbers haven't changed.

So, in my opinion, you can get a variety of masts to work with a boat. If the numbers are a bit shy, you could install less sail area to help compensate. Less sail, less force. There's a balance. The boat's righting moment is a big part of the calculation, but sail area counts too.

Both these masts were designed for my boat. Kurt's mast measurements are for a mainsail of about 48m2, and a jib of about 20m2, whereas our mainsail measures 75m2 and the mast design was assuming a mainsail size of 80m2. Our jib is 37m2.

I dare say on this same boat, I could use a mast from a hobiecat, and, if I reefed early enough, it would hold up just fine. Though I don't think my light wind performance would be very good. So, in my opinion, you can use a slightly lighter mast, if your dressing it with less sail area. So whatever that mast had on for sails, I'd bet you could easily use those sails on your boat (your boat's going to be lighter than pretty much anything else in that size range). If the length and beam are similar, I'd be shocked if you had an issue using that mast with it's original sails.

If the other boat is heavier, then your safety margin will have increased.

For what it's worth.
Paul.

[fourlyons]

Originally Posted by GRIT

This may help, if you're going to do measurements on the "new" mast.

My original mast moments, as published on the original plans, before making all our modifications. Kurt Hughes 45 Mast moments, as an example:

Mast moments 22 inch4 x 44 inch4 or 915.7 cm4 and 1831.4 cm4

boom 12 inch4 20 inch 4. 499.4cm4, 832 cm4

Though we nearly doubled the sail area on the boat, and Selden calculated our new mast moments at: 1524 cm4 and 4686 cm4. That's quite a difference from original, which is due to the much larger sail area we've installed. The boat's beam and displacement at DWL are the same as in the plans, so those numbers haven't changed.

So, in my opinion, you can get a variety of masts to work with a boat. If the numbers are a bit shy, you could install less sail area to help compensate. Less sail, less force. There's a balance. The boat's righting moment is a big part of the calculation, but sail area counts too.

Both these masts were designed for my boat. Kurt's mast measurements are for a mainsail of about 48m2, and a jib of about 20m2, whereas our mainsail measures 75m2 and the mast design was assuming a mainsail size of 80m2. Our jib is 37m2.

I dare say on this same boat, I could use a mast from a hobiecat, and, if I reefed early enough, it would hold up just fine. Though I don't think my light wind performance would be very good. So, in my opinion, you can use a slightly lighter mast, if your dressing it with less sail area. So whatever that mast had on for sails, I'd bet you could easily use those sails on your boat (your boat's going to be lighter than pretty much anything else in that size range). If the length and beam are similar, I'd be shocked if you had an issue using that mast with it's original sails.

If the other boat is heavier, then your safety margin will have increased.

For what it's worth.
Paul.

Quite right. I also pointed out early in this thread that sail area is the other half of the equation.

[chris95040]

Originally Posted by Chotu

I KNOW what righting moment is. Jeez.

What I don’t know is how anything you’re going on about has to do with a catamaran with a 25’ beam.

Have YOU ever cracked a book to see the righting moment of a catamaran?

Cliff notes: it starts out almost immediately from full righting moment and fades to zero/negative approaching 90 degrees. Which is why nothing you’re saying makes sense.

Why TF did you answer the same question I’m asking here with an admission you don’t know when SMJ asked it without elaborating the points, but when I asked the same thing in full detail, you belittle me and treat me like an idiot?

We asked you the same damn thing

People like you have me seriously reconsidering putting up a huge build thread here. I don’t have time to deal with abusive a holes and they’ll probably be out in droves if I put the whole interior build up.

Righting moment is measured in torque, not degrees.

Righting moment changes as heel changes, which I suspect is the concept you are (inexplicably) referring to here.

Make sure you follow my canoe vs catamaran example: it’s harder to tip over an upright catamaran than an upright canoe. The righting moment of the upright catamaran is higher than that of the upright canoe.

To size a rig a designer looks at righting moment because *that is what the mast is pushing against*. Whether your cat is most stable at 0 degrees or 30 degrees or 180 degrees isn’t really the point, the point is what is the worst case load we can expect the mast to see, and that is estimated by a function of righting moment (among many other things).

Lucky for you you don’t need to dive into righting moments. Your designer did that and ultimately specified a mast stiffness, so, umm, use it.

Anyways, it’s clear you’re involved in a tough project that’s harder to complete than you thought and are going into fight or flight mode when someone brings up unexpected detours like the inexpensive and short term proposition of hiring some expertise. Sounds like it’s time to cut yourself some slack and take a break.

[longjonsilver]

Here is my cheapskate advice. Buy the mast, use it like it is. If you like it - great! If not, then later modify or buy new. You will always be able to sell the mast for something and meanwhile you get to use the boat.

jon

[Chotu]

Originally Posted by chris95040

Righting moment is measured in torque, not degrees.

Righting moment changes as heel changes, which I suspect is the concept you are (inexplicably) referring to here.

Make sure you follow my canoe vs catamaran example: it’s harder to tip over an upright catamaran than an upright canoe. The righting moment of the upright catamaran is higher than that of the upright canoe.

To size a rig a designer looks at righting moment because *that is what the mast is pushing against*. Whether your cat is most stable at 0 degrees or 30 degrees or 180 degrees isn’t really the point, the point is what is the worst case load we can expect the mast to see, and that is estimated by a function of righting moment (among many other things).

Lucky for you you don’t need to dive into righting moments. Your designer did that and ultimately specified a mast stiffness, so, umm, use it.

Anyways, it’s clear you’re involved in a tough project that’s harder to complete than you thought and are going into fight or flight mode when someone brings up unexpected detours like the inexpensive and short term proposition of hiring some expertise. Sounds like it’s time to cut yourself some slack and take a break.

Who said anything about the units of righting moment? Do you even know the units of torque without googling it?

Now it’s getting really obvious you actually don’t know what you’re talking about thinking me saying the magnitude of the righting moment goes zero/negative at 90 degrees on a cat has something to do with the units of righting moment. Funny you didn’t understand something so simple.

I don’t usually throw it out there because I don’t enjoy douching around, but I’m getting tired of your BSing your way through this discussion. I have a masters in physics. I don’t appreciate you misconstruing simple things like me describing the how the magnitude of the righting moment goes as the boat rolls and then trying to pretend I was talking about degrees being a unit of torque.

Nor do I a appreciate your canoe example BS used to talk down to me.

Where the cat exerts its maximum righting moment has EVERYTHING to do with what we are taking about here. At 90 degrees when a mono (simplified) exerts it’s maximum righting moment, there is no sail area presented to the wind. At 5 degrees or so, where a catamaran achieves its maximum righting moment, ALL the sail area is presented to the wind. That means the mast on a catamaran, all else being equal, is under significantly more load than a monohull.

Or put another way, the “worst case” load is enormous.

So given this mast came off a nearly identical boat with the same beam (beam and weight determine righting moment on a catamaran) and LOA, and the nearly identical boat has circumnavigated, it proves this mast stood up to the forces involved with my boat.

In other words, you didn’t make the right call and you didn’t understand how righting moment works on a catamaran. You admitted as much replying to SMJ.

Yet you’re belittling me and putting me down?

Sounds like you’re a person who pretends to understand things so they can spout off on the forum masquerading as an expert, yet when pressed, just drops sound bites and starts insulting people because you have no depth of knowledge.

Take a seat. You’re in over your head on this thread.

[Chotu]

Thank you, Grit. Much appreciated.

The dimensions of the boat the mast is coming off are as follows:


LOA: 48ft (mine is 50)
Beam:24.8ft (mine is 25)
Displacement: 19,000lbs (mines around 14,000 currently but more to add)

Sail area-
Mainsail : 76.0
Jib / genoa : 56.0

Shipyard: Bleu Outremer

[chris95040]

Originally Posted by Chotu

Who said anything about the units of righting moment? Do you even know the units of torque without googling it?

Now it’s getting really obvious you actually don’t know what you’re talking about thinking me saying the magnitude of the righting moment goes zero/negative at 90 degrees on a cat has something to do with the units of righting moment. Funny you didn’t understand something so simple.

I don’t usually throw it out there because I don’t enjoy douching around, but I’m getting tired of your BSing your way through this discussion. I have a masters in physics. I don’t appreciate you misconstruing simple things like me describing the how the magnitude of the righting moment goes as the boat rolls and then trying to pretend I was talking about degrees being a unit of torque.

Nor do I a appreciate your canoe example BS used to talk down to me.

Where the cat exerts its maximum righting moment has EVERYTHING to do with what we are taking about here. At 90 degrees when a mono (simplified) exerts it’s maximum righting moment, there is no sail area presented to the wind. At 5 degrees or so, where a catamaran achieves its maximum righting moment, ALL the sail area is presented to the wind. That means the mast on a catamaran, all else being equal, is under significantly more load than a monohull.

Or put another way, the “worst case” load is enormous.

So given this mast came off a nearly identical boat with the same beam (beam and weight determine righting moment on a catamaran) and LOA, and the nearly identical boat has circumnavigated, it proves this mast stood up to the forces involved with my boat.

In other words, you didn’t make the right call and you didn’t understand how righting moment works on a catamaran. You admitted as much replying to SMJ.

Yet you’re belittling me and putting me down?

Sounds like you’re a person who pretends to understand things so they can spout off on the forum masquerading as an expert, yet when pressed, just drops sound bites and starts insulting people because you have no depth of knowledge.

Take a seat. You’re in over your head on this thread.

Not only am I an engineer and math geek, I've also designed a new rig before - first by myself with 2 methods (a simple old timey method in skenes, and then using something called the nordic boat standard) and finally by hiring a rigger to crunch the numbers themselves.

All methods ended up in roughly the same place and that fact gives me a warm fuzzy feeling when I'm out and about.

I was trying to help you achieve the same level of warm and fuzzy feeling by involving a rigger or naval architect in your project. I won't make that mistake again!

[joelhemington]

Originally Posted by Chotu

I need all the devil’s advocates I can get on this one.

Definitely looking to avoid the $60,000+ bill for the rig if bought new.

I just don’t get how people buy used masts if something like this doesn’t work either. It’s the closest thing I’ve ever found.

If the beam is slightly different, the spreaders will not work as the angle of the shrouds will be different.

Sounds like a reasonable plan though I'm not sure if I'd trust the rest of the rigging after a circumnavigation. The chain plates are also something you probably don't need as I'm sure you've already installed the new ones by now. For the savings you're looking at, you could easily swap the headsail if necessary.

[Chotu]

Originally Posted by joelhemington

Sounds like a reasonable plan though I'm not sure if I'd trust the rest of the rigging after a circumnavigation. The chain plates are also something you probably don't need as I'm sure you've already installed the new ones by now. For the savings you're looking at, you could easily swap the headsail if necessary.

Actually, I still don’t have chainplates because I had planned to do composite chainplates before I was hit by the epoxy allergy.

So, now I have to bolt on some metal ones.

Agreed about the standing rigging. Hiring a rigger to inspect the shrouds, hounds, etc... is definitely on my list.