Traditional Main vs In-mast Furling

[Kenomac]

Originally Posted by UNCIVILIZED

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.

I don't agree... and my remarks regarding resale value of a boat without a furler are absolutely true and factual, just check yachtworld. Sorry your feelings were hurt by disagreement.

But now I'm finished with this cf forum arguing so... adios.

[minaret]

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.

Isn't this argument completely redundant except at anchor? Ie, isn't the weight of the sail aloft in exactly the same way when any boat is under way? Except with a bit more weight up high due to all that roach.

Seems to me when under way the only difference is in the weight of the stick.

Also, even if this were the case (500 kg ballast required to compensate), my ballast keel is about 18k lbs. Adding another 1000lb or so wouldn't affect things much if done from the factory. That's what, about a 7-8% increase in ballast? Plenty of info in my owners manual on the changes made at the factory to compensate for the all furling rig. Deeper keel, added ballast, taller rig. The differences aren't big compared to stock though.

Long and the short of it, in mast furling doesn't add that much weight aloft (except maybe at anchor, when it's a plus not a minus as weight aloft decreases roll). My roller furling genoa weighs about 180 lb, much more than my main, which runs about 120. Nobody hesitates to add that weight aloft. Why stop there? I've certainly never seen a big stability hit on boats which go from hank on to roller furling headsails. Perhaps in very small boats it's an issue. Maybe.

[Dockhead]

Originally Posted by minaret

Long and the short of it, in mast furling doesn't add that much weight aloft (except maybe at anchor, when it's a plus not a minus as weight aloft decreases roll).

We discussed this. More weight aloft, balanced by more ballast, decreases roll acceleration and the frequency of oscillation, but it INCREASES the amplitude of rolling -- which means you roll over further if you have more weight aloft. Let's get the facts straight. You can't say "decreases roll".

Originally Posted by minaret

Isn't this argument completely redundant except at anchor? Ie, isn't the weight of the sail aloft in exactly the same way when any boat is under way? Except with a bit more weight up high due to all that roach.

Seems to me when under way the only difference is in the weight of the stick.

Also, even if this were the case (500 kg ballast required to compensate), my ballast keel is about 18k lbs. Adding another 1000lb or so wouldn't affect things much if done from the factory. That's what, about a 7-8% increase in ballast? Plenty of info in my owners manual on the changes made at the factory to compensate for the all furling rig. Deeper keel, added ballast, taller rig. The differences aren't big compared to stock though.

The whole discussion assumes a boat designed for in-mast furling, which has more ballast to maintain the CG and righting moment. If you were to add an in-mast furling mast to a boat not designed for it, there would be all kinds of problems we haven't been discussing.

But yes, of course, more ballast or a deeper keel makes it easy to maintain righting moment. This makes the boat a bit heavier so slows it down a bit, but there is no reason for an in-mast furling boat to have less stability. What you can't avoid, however, is greater polar moment of inertia, which means you roll over further, when you roll -- exactly the same thing which happens with pitching when you add weight in the bow (or near the transom). Ask any boat designer -- weight aloft, like weight in the ends of the boat, is very undesirable.

[double u]

"...as weight aloft decreases roll...":
running dead downwind go up the mast & find out if that's true...the smaller the boat (& the bigger your weight) the more the lesson will sink in...
(Tip: wear protective clothing & maybe a helmet just in case you're wrong!)

[Dockhead]

Originally Posted by UNCIVILIZED

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.

This is exactly correct.

Why is this such a passionate argument? The facts are the facts.

Everything on a boat is a tradeoff.

Everyone likes what he likes, and we don't all have the same priorities.

In-mast furling has some drawbacks which don't matter to some people and matter a lot to others. That's normal.

If Unciv would sail 1000 miles with me on my boat, I bet he would come away appreciating in-mast furling much more than he does now. And he would not be the first racing-oriented sailor to learn to like the system, on my boat, being surprised at how fast and how well we can sail despite it.

Nevertheless, my next boat will not have it. My priorities are just a little different from Kenomac's -- I sail further and harder and would like to sail just a little bit harder still.

To each his own.

[Dockhead]

Originally Posted by double u

"...as weight aloft decreases roll...":
running dead downwind go up the mast & find out if that's true...the smaller the boat (& the bigger your weight) the more the lesson will sink in...
(Tip: wear protective clothing & maybe a helmet just in case you're wrong!)

Ha, ha!

Been there, done that! It's like being a human metronome

Good illustration!

[double u]

...in the end I made the photos, but the more pressing thing was how to get down again without harm (would be less pronounced in a 65'er of course)
(this here excellent forum, that gets me through many a dreary landlocked day-12° outside & pouring rain right now - is really a mixed bag, isn't it? reflects the composition of the human race: real experts, selfproclaimed experts, ...the lot!

& uncivilized: you're not missing anything, this is exactly how it is.
for all disbelievers:
https://www.amazon.com/Principles-Ya...s=yacht+design

[Kenomac]

Originally Posted by double u

... - is really a mixed bag, isn't it? reflects the composition of the human race: real experts, selfproclaimed experts, ...the lot!

The doers vs the dreamers.

[Pete7]

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.

I know Ken said "100 kgs" but I am curious to know were this weight is. Certainly on my yacht at 31ft LOA with a 45ft mast there doesn't appear to be any additional weight, other than the 45ft hollow aluminium spar the sail rolls around and that isn't more than a few kgs. Also my mast appears to be the same cross sectional area as an identical yacht with slab reefing, it certainly isn't any larger.

Now at this point I have to be careful because I have a twin spreader rig on a 31ft yacht which is quite unusual, other Moody 31s have a more traditional single spreader rig, so I do have some additional weight aloft but its in the tens of kgs not hundreds.

Perhaps when you scale up in mast reefing to larger 50ft plus yachts the loads require significant increases in materials to take the loads.

DH mentioned earlier that having the main neatly rolled away inside the mast helps maintain the sail. Couldn't agree more and this is a major benefit, afer al mos yachts spend 6 out of 7 days a week stationary. To illustrate the point I have just replaced my in mast main. The receipt for the old sail dates it to Jan 2001. the sail has never been repaired and never been cleaned. The only reason for replacement was each cross cut panel has stretched badly and this is more down to the lower spec material choice the sail was made with.

Finally the only time we need to leave the cockpit whilst under way is to put the fenders away and get them out again on arrival. Yes it was fun at 20 changing hank on head sails whilst the bows dived under the solid warm waves of the med or Canaries. Now a few decades on carrying out all reefing from the cockpit when its blowing a hooly is a major advantage particularly on a smaller yacht were you don't have the room down the side decks for a promenade

Sceptical at first until we had been for a test sail, now 10 years on I consider it to be a major advantage even on a smaller cruising yacht.

Pete

[Dockhead]

Originally Posted by Pete7

I know Ken said "100 kgs" but I am curious to know were this weight is. Certainly on my yacht at 31ft LOA with a 45ft mast there doesn't appear to be any additional weight, other than the 45ft hollow aluminium spar the sail rolls around and that isn't more than a few kgs. Also my mast appears to be the same cross sectional area as an identical yacht with slab reefing, it certainly isn't any larger.

I think the 100kg came from me, and it's just a plug number. Different boats will be very different, and of course on your boat it will be much less.

My mast is 23 meters long and weighs about a quarter tonne, so the difference between the thicker in-mast furling mast and a normal one is going to be at least 100kg, maybe more. For a carbon mast, probably several hundred kg.

The difference comes from the in-mast furling mast's being physical larger, and with an internal bulkhead to take the forces which the back part of the mast can't take because of the slot.

The downside is weight, but there is also an upside -- in-mast furling masts are stiffer.

[Dockhead]

Originally Posted by double u

. . . & uncivilized: you're not missing anything, this is exactly how it is.
for all disbelievers:
https://www.amazon.com/Principles-Ya...s=yacht+design

There is a good chapter on rolling there, explaining a lot of the concepts we've been discussing here.

The role of inertia is explained here:

"If a hull is given a heel angle in still water and is suddenly released, the righting moment will immediately tend to put the hull upright. The hull starts rolling back to its upright position, but due to its inertia it will not stop when the heel angle is zero. Rather, it will continue to roll over to the other side, where an opposing righting moment develops. The hull then rolls back and forth until the motion is damped out."

I hope this makes it crystal clear, that adding more inertia by adding weight aloft does NOT reduce rolling.

One point emphasized is the importance of DAMPING:

"Damping may be caused by three things:

* Friction between the water and the yacht
* Generation of waves on the water surface
* Generation of vortices from the keel, rudder, sharp bilges and sails."

Note that more polar moment of inertia does NOT dampen rolling. It merely reduces the acceleration and natural frequency of oscillation, but at the expense of increasing the amplitude. Damping is dissipating the energy to kill the roll -- inertia merely repackages the energy.

At anchor, the underwater foils do much less to dampen rolling, which is why rolling is more of a problem at anchor.

My previous boat had a long fin keel and short conventional mast, and did not roll too badly at anchor. The large area of the keel dampened rolling; less weight less high off the water meant less inertia. My present boat is much worse not only because of the tall heavy rig, but because of the much shorter bulb keel, which doesn't do much work with no speed on. The bilge is round and doesn't produce much friction.

Principles of Yacht Design also talks a lot about RESONANCE, which I mentioned. Changing the inertia by adding or subtracting weight aloft changes the natural frequency of rolling.

"From a theoretical point of view, the natural frequency may be changed by increasing or reducing either the stability or the inertia (or more precisely, the mass moment of inertia around a longitudinal axis). To avoid the resonance situation, the natural frequency can be either increased or reduced. However in conditions where the problem occurs it is better to move to the left in Fig. 4.15, either by increasing stability or reducing inertia. If weights located at a high position are moved down to the bottom of the hull, both of these effects are accomplished."

All of this starting at Page 46 in my copy.


Ships hauling water barrels up the mast are trying to kill resonance by reducing the natural frequency of rolling. Adding weight aloft is the worst way to do this -- for the reasons discussed above. But it might have been the only means available in some cases. Resonance can even capsize a vessel, so preventing that is more important than other issues caused by rolling. Resonance occurs when the vessel's natural frequency of rolling matches the frequency of the waves.

[double u]

" there doesn't appear to be any additional weight, other than the 45ft hollow aluminium spar the sail rolls around" - let's not forget the sail & the fact, that the mastsection is heavier than a normal mast

[Pete7]

Originally Posted by double u

" there doesn't appear to be any additional weight, other than the 45ft hollow aluminium spar the sail rolls around" - let's not forget the sail & the fact, that the mast section is heavier than a normal mast

Unfortunately I can't download Selden's catalog at the moment, but it would appear that for smaller masts the difference isn't much and only changes as the masts increase in size for larger yachts.

mast section - Seldén Mast AB - PDF Catalogues | Documentation | Boating Brochures

Sail weight isn't much of a factor, I can pick my main sail up with one hand and rode home with it on the back of my Suzuki. Sometimes there are advantages to small yachts.

Pete

[Pete7]

I had to go back to Phil's original post to see what the question was. He's probably wondering what he started with such a simple question. At lest he didn't ask about anchors

To answer his second question as Ken did, in mast reefing is now so common it isn't going to affect the re-sale price of a yacht and buyers could well be expecting it.

Pete

[double u]

rollerfurler mains may have all sorts of advantages - weight isn't one of them. it may be worth the compromise (imho @ 31' it's definitely not worth it, but that's personal, like wheelsteering)