Advantages of a taller rig/30% larger sail area?

[Adelie]

Originally Posted by Dockhead

You've got this backwards! Increased polar moment of inertia ("roll moment of inertia") increases risk of capsize. We discussed that above.



Extra inertia will not prevent the energy from the breaking wave from being transferred to the boat. No damping takes place due to inertia. More inertia simply reduces the acceleration and the frequency of oscillation. More inertia means the boat will keep going over longer and further after encountering a breaking wave.



The extra weight aloft probably does not cost righting moment -- assuming the tall rig version has extra ballast. If it does not, then the OP should run, not walk away from that one. But most likely it does have extra ballast.

Tank testing following Fastnet 1979 showed increased roll moment of inertia increased capsize resistance.

[double u]

I clearly remember the same!
one reason why the slower response of the boat to the heeling force of the wave is beneficial could be: the amount of energy transferred from the wave to the boat is limited by the time it takes for the wave to pass under the boat - & this time is quite short. the less the boat heels during that time the smaller the ultimate heelangle that is reached.
Marchaj was adamant I remember...

[Dockhead]

Originally Posted by Adelie

Tank testing following Fastnet 1979 showed increased roll moment of inertia increased capsize resistance.

I found this:

http://www.marin.nl/upload_mm/1/0/3/..._capsizing.pdf


Which confirms what you say, so I stand corrected.

I think this is a specialized case, however, which is not applicable to what we were talking about. They are not talking about oscillating rolling, but rather being knocked all the way over by a single steep breaking wave.

In oscillating rolling -- where there is not enough force to put you all the way over with one wave -- you will swing back and forth at a greater angle -- greater amplitude -- but slower -- if there is more inertia. If you replace your heavy in-mast furling mast with a light carbon one, and reduce ballast to keep the same righting moment, then the same waves will produce a faster oscillation (which might be more uncomfortable), but you will not swing over as far.

But in any case I am grateful for the correction, which has expanded my knowledge. Thanks.

[UNCIVILIZED]

In the '79 Fastnet race, a huge proportion of the boats which were abandoned because after they lost their spars, they were much more prone to multiple roll overs. And the motion onboard was horrid even without their rolling over.

[jtsailjt]

Originally Posted by Dockhead

I found this:

http://www.marin.nl/upload_mm/1/0/3/..._capsizing.pdf




In oscillating rolling -- where there is not enough force to put you all the way over with one wave -- you will swing back and forth at a greater angle -- greater amplitude -- but slower -- if there is more inertia. If you replace your heavy in-mast furling mast with a light carbon one, and reduce ballast to keep the same righting moment, then the same waves will produce a faster oscillation (which might be more uncomfortable), but you will not swing over as far.

.

But why is this relevant? How often do boats roll over due to too high amplitude oscillations caused by a tall mast? I've never heard of it happening but would be interested in learning if anyone knows of a boat with a tall rig roll oscillated all the way to capsize. The taller mast will cause you to heel a degree or two more in a blow but it's not going to cause you to roll over unless the ballast is too small so ultimate stability is compromised. The taller rig with appropriate ballast will yield a slower, less twitchy roll and a more comfortable experience for its crew, both at anchor and underway, with the only exception I can think of being at anchor if the period of the waves happens to match the natural rate of oscillation of the boat, but that could happen to a shorter rig as well. Since neither rig is likely to cause you to roll all the way to capsize, why do you care if the amplitude of roll is slightly higher, if, as you say, it's more comfortable? Isn't the crews perception of the roll what's most important rather than the actual angles involved?

[Dockhead]

Originally Posted by jtsailjt

But why is this relevant? How often do boats roll over due to too high amplitude oscillations caused by a tall mast? I've never heard of it happening but would be interested in learning if anyone knows of a boat with a tall rig roll oscillated all the way to capsize. The taller mast will cause you to heel a degree or two more in a blow but it's not going to cause you to roll over unless the ballast is too small so ultimate stability is compromised. The taller rig with appropriate ballast will yield a slower, less twitchy roll and a more comfortable experience for its crew, both at anchor and underway, with the only exception I can think of being at anchor if the period of the waves happens to match the natural rate of oscillation of the boat, but that could happen to a shorter rig as well. Since neither rig is likely to cause you to roll all the way to capsize, why do you care if the amplitude of roll is slightly higher, if, as you say, it's more comfortable? Isn't the crews perception of the roll what's most important rather than the actual angles involved?

Yes, I think no one can argue with your last sentence. What is comfortable is comfortable, and comfortable might be, in a given case, a lower frequency.

But you can't assume that the amplitude of the rolling is always harmless -- neither at anchor, nor certainly not underway. If the rolling is very mild, you might not care, and the "crew's perception" might prefer a slower larger amplitude roll, but this is unlikely to be the case if the rolling is stronger. At some point, stuff starts to get thrown around the cabin, and that is AMPLITUDE talking, baby!

And the lower the frequency, the greater the chance of resonance -- because the range of natural roll periods for small yachts is generally higher than the periods of waves. So the more you reduce that, the higher the risk of resonance. This might not be true for a 200' schooner, but for boats the size of ours, it definitely is. I didn't ever experience resonant rolling on my 37' boat with full batten main and short lightweight mast, but I certainly have on my 54' boat with heavy tall mast. If I added weight to the top of the mast, it would be even worse.

[GTom]

Originally Posted by Adelie

So I ran the numbers thru a formula and my guess is that you can expect to cover an extra 8.25nm/24hr on average when cruising.

Got some more reliable rig data from the Swedish Sailing Federation. They have established handicap ratings, showing ~3.4% advantage to the newer, taller model. The rig "P" value (mainsail luff) is 10.4m for the old model, 11.3m for the new. Respective SA's are 60m2 vs 55m, which is a 9% increase. Their displacement data sounds fishy though, saying 6t for the old, short rigged model, 5.5t to the tall-rig model while quoting 2t ballast for both. Anyway, SRS handicaps are actually measured in the Round Lidingö Race, and should reflect real performance differences.

[jtsailjt]

Originally Posted by Dockhead

But you can't assume that the amplitude of the rolling is always harmless -- neither at anchor, nor certainly not underway. If the rolling is very mild, you might not care, and the "crew's perception" might prefer a slower larger amplitude roll, but this is unlikely to be the case if the rolling is stronger. At some point, stuff starts to get thrown around the cabin, and that is AMPLITUDE talking, baby!

And the lower the frequency, the greater the chance of resonance -- because the range of natural roll periods for small yachts is generally higher than the periods of waves. So the more you reduce that, the higher the risk of resonance. This might not be true for a 200' schooner, but for boats the size of ours, it definitely is. I didn't ever experience resonant rolling on my 37' boat with full batten main and short lightweight mast, but I certainly have on my 54' boat with heavy tall mast. If I added weight to the top of the mast, it would be even worse.

When stuff starts getting thrown around the cabin, I'd say it's a LACK of amplitude talking because more amplitude slows down the roll so things (including people) don't tend to get thrown around as much as they would with a lower amplitude, but quicker roll that comes from a lighter or shorter rig.

I guess that I've just been fortunate because as far as I know I've never experienced resonant rolling in either of my hanked on mainsail boats or on my current behind the mast furler boat and I might not even notice that it was resonant if I encountered it next week. I've seen boats that seemed to roll a lot in a harbor when others stayed quite still but always assumed that it was much more a function of their hull shape, location and amount of ballast, and overall lack of displacement than it was the weight in their mast or height of their mast. To me, the whole issue of in mast furling boats or tall rig boats and the "negative" of a resulting greater amplitude but slower roll is about as close to a non issue as it possibly could be in regards to making the choice between one of them and a shorter or lighter mast. There are other factors that must be "weighed" while making that decision that are MUCH more important, at least to me.

[atmartin]

I'd be interested in a discussion of roll period/damping/etc as it relates to comfort at anchor. Whether it's wake or surge, even relatively small waves can initiate wicked rolling on some boats. There's not a whole lot of information in most yacht design books on the subject.

[Dockhead]

Originally Posted by jtsailjt

When stuff starts getting thrown around the cabin, I'd say it's a LACK of amplitude talking because more amplitude slows down the roll so things (including people) don't tend to get thrown around as much as they would with a lower amplitude, but quicker roll that comes from a lighter or shorter rig.

I guess that I've just been fortunate because as far as I know I've never experienced resonant rolling in either of my hanked on mainsail boats or on my current behind the mast furler boat and I might not even notice that it was resonant if I encountered it next week. I've seen boats that seemed to roll a lot in a harbor when others stayed quite still but always assumed that it was much more a function of their hull shape, location and amount of ballast, and overall lack of displacement than it was the weight in their mast or height of their mast. To me, the whole issue of in mast furling boats or tall rig boats and the "negative" of a resulting greater amplitude but slower roll is about as close to a non issue as it possibly could be in regards to making the choice between one of them and a shorter or lighter mast. There are other factors that must be "weighed" while making that decision that are MUCH more important, at least to me.

Fair enough. But weight aloft is a bad thing in general -- which I think is a reasonable general statement. It means more ballast has to be added to counteract it. More ballast for the same righting moment means the boat is slower and less powerful, OK?

Resonant rolling is something which affects larger boats -- because the natural roll period needs to match the wave period for it to occur. I've experienced it in my present boat and in some other large vessels, including a Swan 90 I spent some time in. It is a nasty thing in an anchorage. Less weight aloft definitely reduces the risk of this. It's not all that common but it's a b**** when it happens. Happened to me once in March in Warbarrow Cove with a short sea rolling in from the Channel, and a couple of times in one bay in the Finnish Archipelago.

Resonant rolling affects ships underway. It's dealt with by changing course to meet the waves at a different angle. It's called "parametric rolling" and typically occurs when the ship's roll period interacts with the wave period. The Doppler Effect can alter the wave period to bring it into resonance with the ship's natural pitching or rolling frequency. Here is a good illustration:

https://www.youtube.com/watch?v=ewqaRMGv2mE

There is an interesting paper on it here: https://www.google.ee/url?sa=t&rct=j...0ReKa30wLVHZOA


When a ship is in calm water, any disturbance in transverse direction can cause
roll motions. When the equilibrium is disturbed, the righting moment or the hy-
drostatic restoring moment tends to return the ship back to the upright position.
When the equilibrium position is attained, the ship does not stop there but due
to inertia continues to roll at a decreasing velocity. When a maximum roll angle
is attained, the roll restoring moment [righting moment] tends to push the ship back to the upright position. Again when the equilibrium position is attained, the ship does not stop there but due to inertia continues to roll. Thus the ship begins to oscillate about the equilibrium point with a constant roll period and amplitude if the damping is neglected. These free roll oscillations, described in ABS Guide [2], are shown in Figure 2.6.


You can see clearly here that inertia is the enemy, which causes the dangerously large amplitude of this kind of rolling. It's the same thing which happens in bad rolling at anchor, and roll inertia -- weight aloft -- is what causes it.

[jtsailjt]

I agree that weight aloft is generally a bad thing for a racer but I'm not sure that's true for a cruiser who is more concerned with comfort over a long period of time than a racer is. From what I think I know about your future boat, it's going to be long and relatively narrow so will depend more on its ballast for stability rather than it's form and that means it's going to initially be a bit tender but once heeled over will stiffen up nicely and will yield a very comfortable ride at a near constant angle. So, you can probably "afford" to have a light rig with little weight aloft without compromising comfort. But for some other boats, they'd seem very "twitchy" without a certain amount of weight aloft to dampen their tendency to quickly roll back to (and beyond) neutral, but of course not so much weight that ultimate stability is compromised.

I think I generally understand the resonance you are talking about but I'm currently in China without a VPN so can't view your video right now. But I will the day after tomorrow.

One of the things I think is most exciting about your future boat is that it's going to be fairly narrow and very weatherly and I think that will make it a blast to sail. Once you've engaged a naval architect, possibly you can get him to run the numbers for various amounts of weight centered about 1/3 of the way up your mast and see how it affects speed, comfort, and stability? I'm sure he'll have a computer program that will tell you all that and more. I'm betting that, especially in a strong breeze, the difference in all 3 will be nearly imperceptible even if some ballast must be added to compensate for the hit to your CG. I believe that it's possible to have a lightweight, carbon fiber, mast equipped with in mast furling so you could have the best of both worlds. Once this boat is built you're going to have to post LOTS of videos!

[poiu]

I posted this in response to another thread. Here it is in the right place.

Sailing yachts at anchor have less of a rolling problem than similar displacement motor yachts in most circumstances. Bigger sailing yachts with more inertia have less of a problem than small sailing yachts. The difference is quite remarkable in general.

I have a fairly heavy yacht with a heavy mast and can anchor in places that few other boats, sailing yachts or particularly motor yachts can anchor in. Next to the main channel into port, where large motor yachts belt past creating wake for example. A boat wake will slap my side, but will knock a 20ft boat on its side. At sea, the damping from the hull and sails mean resonance is never seen

I think the focus on inertia as a continuum is applying the wrong emphasis on the problem, which should be on establishing resonance. The point is that it is not just about the degree of the problem, it is about preventing it entirely. With high inertia getting the boat into a resonant state is not easy and usually impossible. The higher the inertia the rarer the circumstances where it can happen.

For the few exceptions where it happens, changing the angle of the boat to the waves or a flopper stopper like Kenomac's is a good solution. The smaller the boat, the more it should be needed. I don't know why they aren't used a lot.

[Dockhead]

Originally Posted by poiu

. .. .With high inertia getting the boat into a resonant state is not easy and usually impossible. The higher the inertia the rarer the circumstances where it can happen. . . .

The only thing I would quibble with in this post is this.

More inertia does not necessarily make it harder to get into a resonant state. Talking about yachts, the opposite is more often the case.

Resonance happens when the natural roll period matches the wave period. Adding inertia increases the roll period. The normal range of wave periods is longer than the normal range range of roll periods of yachts. The overlap is at the lower end of wave periods and the upper end of yacht roll periods.

Small ships can be very badly, even disastrously affected by resonant rolling -- see the video posted above.


What is very true in this post is that larger yachts lie better at anchor in general than smaller ones. Like Poiu, I often lie in places where smaller boats can't or wouldn't. Normally there is no problem, even in somewhat boisterous conditions. My boat is fairly light for her size though, 20 metric tonnes light ship, so she gets tossed around more than Poiu's big heavyweight. But if resonance happens -- all bets are off.

[Dockhead]

Originally Posted by jtsailjt

. . . .Once you've engaged a naval architect, possibly you can get him to run the numbers for various amounts of weight centered about 1/3 of the way up your mast and see how it affects speed, comfort, and stability? I'm sure he'll have a computer program that will tell you all that and more. I'm betting that, especially in a strong breeze, the difference in all 3 will be nearly imperceptible even if some ballast must be added to compensate for the hit to your CG. . . .

I'll do that. I think the NA may be at work as early as this winter. We'll see what the pro says after all of our speculation.

[Kenomac]

Originally Posted by Dockhead

Yes, I think no one can argue with your last sentence. What is comfortable is comfortable, and comfortable might be, in a given case, a lower frequency.

But you can't assume that the amplitude of the rolling is always harmless -- neither at anchor, nor certainly not underway. If the rolling is very mild, you might not care, and the "crew's perception" might prefer a slower larger amplitude roll, but this is unlikely to be the case if the rolling is stronger. At some point, stuff starts to get thrown around the cabin, and that is AMPLITUDE talking, baby!
And the lower the frequency, the greater the chance of resonance -- because the range of natural roll periods for small yachts is generally higher than the periods of waves. So the more you reduce that, the higher the risk of resonance. This might not be true for a 200' schooner, but for boats the size of ours, it definitely is. I didn't ever experience resonant rolling on my 37' boat with full batten main and short lightweight mast, but I certainly have on my 54' boat with heavy tall mast. If I added weight to the top of the mast, it would be even worse.

"At some point, stuff starts to get thrown around the cabin, and that is AMPLITUDE talking, baby!"

Honestly..... as previously written on another thread, this has never happened on our Oyster in over six years of ownership. You might want to discuss this phenom with Mr. Dixon, the designer of your present boat. However, it did happen on a regular basis on our Hunter 450 in anchorages and was very uncomfortable. I attributed it to high freeboard combined with a shallow draft wing keel on the Hunter.