Traditional Main vs In-mast Furling

[Dockhead]

Originally Posted by Kenomac

Yes, on the Britican Oyster 56 in Sardinia. Spent time on their carbon fiber rig, no in mast furling. I doesn't make any difference regarding the amount of roll. Silly argument, increased freeboard makes the biggest difference.

It takes the Britican couple an hour to put their sail away and cover it up, it takes me 20 seconds to furl ours.

Ideal and totally simple storage of the sail, nicely rolled up with no creases, inside the mast, is a massive advantage of in-mast furling. In-mast furling mainsails last longer because of this, on top of the very large advantage of convenience.

But how does freeboard influence rolling behavior? Much less as the "biggest difference". Puzzled by this. A boat with huge freeboard will have more mass further from the roll center, but the deck and air underneath it are not that heavy, and they are not that far away from the roll center.

[Kenomac]

Very basic really. Low freeboard, lowers the center of gravity.

"Stability. The position of the centre of gravity of an object affects its stability. The lower the centre of gravity (G) is, the more stable the object. The higher it is the more likely the object is to topple over if it is pushed." basic physics wiki

[Dockhead]

Originally Posted by Kenomac

Very basic really. Low freeboard, lowers the center of gravity.

"Stability. The position of the centre of gravity of an object affects its stability. The lower the centre of gravity (G) is, the more stable the object. The higher it is the more likely the object is to topple over if it is pushed." basic physics wiki

Well, sure, but we are not talking about the position of the CG. Boats with more freeboard will have quantity and position of ballast suitable to place the CG in the desired position. You can't look at a boat with high freeboard and say necessarily that it has a higher CG -- generally it does not.


I'm not a fan of the monstrously high freeboard you see in some modern designs, and I'm not defending that -- I'm just pointing out the facts. A certain amount of freeboard is actually GOOD for stability -- because it adds form stability at high heel angles. So if you take two boats -- one with high freeboard and one with low freeboard -- the two will most likely have the same CG because boats are designed around that. But the high freeboard boat will have more stability at high heel angles because the rail doesn't go under as early as the low freeboard boat. Note the positive role that freeboard plays in the CE ratings, for example.

A boat with high freeboard will have somewhat greater polar moment of inertia because you're moving the deck out from the roll center, but moving a ton of deck 50cm will have less effect, than adding 100kg to a 23 meter high mass.

[poiu]

Originally Posted by Kenomac

Yes, on the Britican Oyster 56 in Sardinia. Spent time on their carbon fiber rig, no in mast furling. I doesn't make any difference regarding the amount of roll. Silly argument, increased freeboard makes the biggest difference.

It takes the Britican couple an hour to put their sail away and cover it up, it takes me 20 seconds to furl ours.

I know that boat and made an offer to buy it. They subsequently sold it two years later for less than I offered. I sailed with it and anchored next to in the Caribbean. I also didn't notice a difference in rolling, but it is a fast boat in light winds. (10to15kt). The mast is carbon, slab reefed and taller than usual. I reckon the extra speed should roughly, or maybe more than compensate for the extra messing around covering the sail up.

[Snore]

Originally Posted by Jim Cate

DH, when we were dismasted years ago, the motion of the boat was horrific as we made our way onward. The amplitude of the motion was reduced, but the frequency and the acceleration involved were MUCH worse... literally had to hold on to something at all times to avoid being thrown around.



So, I'm not so sure that reduction of weight aloft is always good in terms of comfort. I suspect that the relationship of the roll frequency (which changes with weight aloft) to the exciting frequency (wave interval) is critical to the amplitude of rolling that develops. Old timers used to advocate hoisting heavy weights aloft at anchor to change this relationship and to increase comfort. Never have tried it myself, but it makes some sense to me.



Jim

Jim

Can't argue with real world experience.

But for a similarity to exist, in-mast boats would have to be fitted with heavier or deeper keel than traditional. I do not believe manufacturers do that.

[Dockhead]

Originally Posted by Snore

. . .But for a similarity to exist, in-mast boats would have to be fitted with heavier or deeper keel than traditional. I do not believe manufacturers do that.

Oh, but they do. Ballast is usually added to versions of boats with tall rigs or in-mast furling. Standard design procedure.

Just like ballast is added to shoal keel versions.

It's a fundamental design parameter. Otherwise, the tall rig boats and in-mast furling boats would be much more tender than the normal versions.

[Kenomac]

Originally Posted by Dockhead

Well, sure, but we are not talking about the position of the CG. Boats with more freeboard will have quantity and position of ballast suitable to place the CG in the desired position. You can't look at a boat with high freeboard and say necessarily that it has a higher CG -- generally it does not.


I'm not a fan of the monstrously high freeboard you see in some modern designs, and I'm not defending that -- I'm just pointing out the facts. A certain amount of freeboard is actually GOOD for stability -- because it adds form stability at high heel angles. So if you take two boats -- one with high freeboard and one with low freeboard -- the two will most likely have the same CG because boats are designed around that. But the high freeboard boat will have more stability at high heel angles because the rail doesn't go under as early as the low freeboard boat. Note the positive role that freeboard plays in the CE ratings, for example.

A boat with high freeboard will have somewhat greater polar moment of inertia because you're moving the deck out from the roll center, but moving a ton of deck 50cm will have less effect, than adding 100kg to a 23 meter high mass.

A couple of solid "facts" in this universe and time period, our 54ft Oyster has a couple thousand extra pounds of ballast 2.3 meters underneath it more than a Moody, our teak deck is only 36 inches above the waterline, which means the entire weight of our boat sits much lower than on a Moody 54 which has a much higher freeboard. Our mast is a furling mast just like yours, but not the same as your boat in another time period.

Our center of gravity is lower to the water line, and as a result of this... we've never had an issue with roll. I simply added the stabilizers for increased comfort, no complaints prior to adding them.

When someone adds "100kg" to their mast by adding a furling system, the 100kg is not added to the very top, it's spread across the entire length with slightly more near the bottom. 100kg is nothing compared to the rest of a 50,000lb boat.

[Dockhead]

Originally Posted by Kenomac

A couple of solid "facts" in this universe and time period, our 54ft Oyster has a couple thousand extra pounds of ballast 2.3 meters underneath it more than a Moody, our teak deck is only 36 inches above the waterline, which means the entire weight of the boat sits much lower than on a Moody 54 which has more freeboard. Our mast is a furling mast just like yours, but not the same as your boat in another time period.

Our center of gravity is lower to the water line, and we've never had an issue with roll. I simply added the stabilizers for increased comfort, no complaints prior to adding them.

You can't know the CG of different boats, just by looking at them, without the design data. The Oyster 53 has a little more ballast than the M54, but the ballast ratio is the same since the Oyster is heavier, and the M54 has a little deeper draft. I don't think there's much difference in freeboard -- M54 has more spring in the sheer so a higher bow, but the rest of the deck is about the same level.

I guess the CG will be about the same, but again -- even a small difference in the shape of the keel can have a big effect; or the construction of the deck, and that's not visible. We don't know without the design data. The Oyster 53 is prettier, but that doesn't affect CG


P.S. -- the Oyster 53 has, if I'm not mistaken, the pre-Moody Decks screwed and plugged teak decks, which are 14mm thick and much heavier than the glued-down decks on later Oysters, made by Moody. Huge effect on the weight of the upper part of the hull. Roughly a half ton difference, back of the envelope. Just an example of the invisible differences, which affect CG.

[Kenomac]

Originally Posted by Dockhead

You can't know the CG of different boats, just by looking at them, without the design data. The Oyster 53 has a little more ballast than the M54, but the ballast ratio is the same since the Oyster is heavier, and the M54 has a little deeper draft. I don't think there's much difference in freeboard -- M54 has more spring in the sheer so a higher bow, but the rest of the deck is about the same level.

I guess the CG will be about the same, but again -- even a small difference in the shape of the keel can have a big effect; or the construction of the deck, and that's not visible. We don't know without the design data. The Oyster 53 is prettier, but that doesn't affect CG


P.S. -- the Oyster 53 has, if I'm not mistaken, the pre-Moody Decks screwed and plugged teak decks, which are 14mm thick and much heavier than the glued-down decks on later Oysters, made by Moody. Huge effect on the weight of the upper part of the hull. Roughly a half ton difference, back of the envelope. Just an example of the invisible differences, which affect CG.

You are mistaken. Our teak decks are 100% glued down, no screws. I refinished them myself.

[Glen Ballou]

I have reefed at all points of sail however would always prefer to reef into the wind to take pressure off every thing.

[Dockhead]

Originally Posted by Kenomac

You are mistaken. Our teak decks are 100% glued down, no screws. I refinished them myself.

Ah, good for you then. You have a Moody deck -- much lighter and better, and no leaks. I didn't realize Oyster started using them that early.

The Oyster 485 I had a contract on, built in 2001, had the massive 14mm thick screwed down decks, and they were in poor condition. Furthermore, the decks had been installed incorrectly with the screws penetrating the deck, so hundreds of small leaks, which had ruined some of the beautiful cabinetry. It was going to be a nightmare to fix; one reason why I ended up backing out of the deal.

[Lost Horizons]

Originally Posted by Jim Cate

Can't agree with this statement. When reefed, the rolled up head of the sail is still weight aloft. With slab reefing, the bunt of reefed sail is at the boom, many meters lower down.

Jim

Technically, you are correct. When a classic sail is reefed, a few kilograms of canvas are lowered by a few meters. Which result in the boat center of gravity lowered by a fraction of a millimeter. That is quite something, isn't it?

[UNCIVILIZED]

Originally Posted by Kenomac

When someone adds "100kg" to their mast by adding a furling system, the 100kg is not added to the very top, it's spread across the entire length with slightly more near the bottom. 100kg is nothing compared to the rest of a 50,000lb boat.

Ken, 100kg up high such as in an in mast furling system is pretty dang far from "nothing". It's the equivalent of adding a 500kg bulb to the bottom of your keel. So yeah it effects stabilty.

Originally Posted by Lost Horizons

Technically, you are correct. When a classic sail is reefed, a few kilograms of canvas are lowered by a few meters. Which result in the boat center of gravity lowered by a fraction of a millimeter. That is quite something, isn't it?

Adding all of that canvas & furling up high categorically makes a big difference. On the order of a loss of 5 degrees of LPS in boats up to 15m or so, more in smaller boats. Unfortunately I don't know the exact figures for bigger boats. But adding a furler & a sail, regardless of whether it's a jib, or the main inside of the spar has a big effect on stability. And in a huge percentage of the designs out there, absolutely nothing is done to the ballast package when changing rig types from standard to in mast furling.

[Kenomac]

Originally Posted by UNCIVILIZED

Ken, 100kg up high such as in an in mast furling system is pretty dang far from "nothing". It's the equivalent of adding a 500kg bulb to the bottom of your keel. So yeah it effects stabilty.



Adding all of that canvas & furling up high categorically makes a big difference. On the order of a loss of 5 degrees of LPS in boats up to 15m or so, more in smaller boats. Unfortunately I don't know the exact figures for bigger boats. But adding a furler & a sail, regardless of whether it's a jib, or the main inside of the spar has a big effect on stability. And in a huge percentage of the designs out there, absolutely nothing is done to the ballast package when changing rig types from standard to in mast furling.

Yes you're correct. Adding a furler greatly effects the financial "stability" of your marine investment (boat) and "makes a big difference" in your ability to re-sell your boat at a future time.

Makes your boat easier to sell with added value to a wider market.

BTW: Nobody is adding 100kg to the top of the mast, so your first statement is completely bogus.

[UNCIVILIZED]

Originally Posted by Kenomac

Yes you're correct. Adding a furler greatly effects the financial "stability" of your marine investment (boat) and "makes a big difference" in your ability to re-sell your boat at a future time.

Makes your boat easier to sell with added value to a wider market.

BTW: Nobody is adding 100kg to the top of the mast, so your first statement is completely bogus.

Smartass, uncalled for remarks not withstanding, the numbers aren't bogus at all, & here's why.

For the sake of simplifying the math, let's call your mast 20m tall. So we're adding 100kg at half of this height, which gives us a force of 1,000kg, if the boat's CG is at the WL & we take this lever arm into account. So then if your keel is 2m deep +/-, in order to balance this force out we'd have to add 500kg to the very bottom of it. Or is there something I'm missing?

Yes, there's more sail weight down low than up high due to it's shape. But then the bottom of the sail starts at what, about 2.5m above the WL? So, loosely speaking, the 2 balance each other out. Meaning the locaton of the CG of the sail is on the spar vs. it's CG in relation to the boat's CG.