Heavy sailboats are slower and

[Whokickamoocow]

Originally Posted by Don C L

So, correct me if I am wrong, but once the boat is sailing and heeling, the inertia offered by the mast is fairly minimal relative to the force on the mast (from the sail) balancing against the righting moment of the hull and ballast, no? The advantage of adding weight aloft (or already having it) really only applies to boats that are hove-to and/or not sailing, is that right?

Hi Don,
The inertia caused from increasing weight up the mast:
- slows the period (time) it takes for the boat to 'complete' a roll;
- decreases the magnitude (angle) of the arc the masthead will make as it rolls;
- will not effect damping, which is how long it takes for an oscillation or roll to stop.
So its more comfortable as a general rule.
But this holds true only up to a point. If you take it too far you will lose stability and make it more resistant to coming back upright. Taken to extreme you would capsize a boat even at the dock. But it will be comfortable as it goes .
Cheers,
WK

[Adelie]

Originally Posted by Don C L

So, correct me if I am wrong, but once the boat is sailing and heeling, the inertia offered by the mast is fairly minimal relative to the force on the mast (from the sail) balancing against the righting moment of the hull and ballast, no? The advantage of adding weight aloft (or already having it) really only applies to boats that are hove-to and/or not sailing, is that right?

For any body there are there are 6 degrees of freedom:
3 translational: surge, heave, sway
3 rotational: roll, yaw pitch.

A mast is 2-3% of the mass of the boat and its affect on the 3 translational degrees of freedom are proportional.

The mast is near the center of mass when viewed from above so it's affect on yaw via yaw moment of inertia is probably under 1% compared to the rest of the boat.

Masts are usually somewhat longer than the length of the boat and all of its mass is above the center of mass for the boat. It probably contributes about 10% to the pitch moment of inertia, significantly more than the 2-3% of total mass the mast represents.

In general monohull masts are significantly taller than the vessel's beam. This means the mast contributes a vastly inordinate amount to the roll moment of inertia of the boat, according to testing following Fastnet 1979 about 50% of the roll inertia.

So how does this affect stability and capsize resistance and rolling in winds.

A heavier mast will decrease stability and increase roll inertia.

In normal sailing conditions each reef or change down in jib size will need to be done in slightly less wind than otherwise, or the boat will heel more than desirable for maximum speed.

In higher winds, gale and up, the boat will be heeling away from the wind.
The heel of the boat is caused by the wind. Rolling is caused by passing waves and changes in wind strength. If the wind is perfectly steady the rolling is all wave induced. As the boat rolls changes in wind speed and apparent wind direction will dampen the rolling. Let's say you are heeled over 20* and a wave tries to roll you further. As the boat starts to roll the apparent wind angle to the sail decreases so sail lift which is also trying to roll you also decreases. Also as the boat rolls away from the wind the apparent windspeed decrease. Since the force on the sails goes as the square of wind speed, this can result in large changes in force on the sail. At lower wind speeds the percentage change in force is much higher but the absolute change is much lower. The changes in apparent direction of water flow over the keel and rudder will also dampen rolling.
The higher roll moment of inertia of a heavier mast will not dampen the roll but it will slow the start of the roll and the end of the roll, that is the start of the roll will be less of a jerk as will the end. Basically a heavier mast will make the rolling less uncomfortable.
The change in apparent wind speed and in apparent wind direction will vary with the height of the mast because the roll speed of the mast varies with height.

Breaking seas is when you really want a heavier mast. There are 2 types of stability: form/initial & gravitational/ultimate.
-Form stability comes from the shape of the hull. The wider the hull at the waterline and the harder the bilges the stiffer the boat is initially, at low to moderate angles of heel. Multihulls are form stability taken to an extreme.
-The deeper the center of mass for the ballast and the heavier it is the greater the ultimate righting moment at moderate to high angles of heel.
The thing about form stability is that it depends on the the surface of the water, if the surface of the water starts to slope, then form stability tries to heel the boat to match that slope. Initial stability will try to roll the boat along with the face of the wave. Ultimate stability will still be counteracting that.
In breaking seas the slope of the water will go past vertical and capsize becomes a risk. In these conditions roll inertia slows the roll so that the boat has not rolled as far when it reaches the crest of the wave, at which point slope reverses and form stability tries to right the boat. For a given boat a heavier mast will mean that a somewhat larger breaking wave is needed to capsize the boat. A narrower boat will help with capsize resistance because there is less of a rolling force on the boat.

[Don C L]

Originally Posted by Whokickamoocow

Hi Don,
The inertia caused from increasing weight up the mast:
- slows the period (time) it takes for the boat to 'complete' a roll;
- decreases the magnitude (angle) of the arc the masthead will make as it rolls;
- will not effect damping, which is how long it takes for an oscillation or roll to stop.
So its more comfortable as a general rule.
But this holds true only up to a point. If you take it too far you will lose stability and make it more resistant to coming back upright. Taken to extreme you would capsize a boat even at the dock. But it will be comfortable as it goes .
Cheers,
WK

Thanks! Yes I get that the weight aloft slows the roll, but I am wondering about how being under the press of sails changes the equation. In other words, is it safer to keep sailing, and heeled, than to say run with warps or lie to a sea anchor with sails furled?

[Don C L]

Originally Posted by Adelie

For any body there are there are 6 degrees of freedom:
3 translational: surge, heave, sway
3 rotational: roll, yaw pitch.

A mast is 2-3% of the mass of the boat and its affect on the 3 translational degrees of freedom are proportional.

The mast is near the center of mass when viewed from above so it's affect on yaw via yaw moment of inertia is probably under 1% compared to the rest of the boat.

Masts are usually somewhat longer than the length of the boat and all of its mass is above the center of mass for the boat. It probably contributes about 10% to the pitch moment of inertia, significantly more than the 2-3% of total mass the mast represents.

In general monohull masts are significantly taller than the vessel's beam. This means the mast contributes a vastly inordinate amount to the roll moment of inertia of the boat, according to testing following Fastnet 1979 about 50% of the roll inertia.

So how does this affect stability and capsize resistance and rolling in winds.

A heavier mast will decrease stability and increase roll inertia.

In normal sailing conditions each reef or change down in jib size will need to be done in slightly less wind than otherwise, or the boat will heel more than desirable for maximum speed.

In higher winds, gale and up, the boat will be heeling away from the wind.
The heel of the boat is caused by the wind. Rolling is caused by passing waves and changes in wind strength. If the wind is perfectly steady the rolling is all wave induced. As the boat rolls changes in wind speed and apparent wind direction will dampen the rolling. Let's say you are heeled over 20* and a wave tries to roll you further. As the boat starts to roll the apparent wind angle to the sail decreases so sail lift which is also trying to roll you also decreases. Also as the boat rolls away from the wind the apparent windspeed decrease. Since the force on the sails goes as the square of wind speed, this can result in large changes in force on the sail. At lower wind speeds the percentage change in force is much higher but the absolute change is much lower. The changes in apparent direction of water flow over the keel and rudder will also dampen rolling.
The higher roll moment of inertia of a heavier mast will not dampen the roll but it will slow the start of the roll and the end of the roll, that is the start of the roll will be less of a jerk as will the end. Basically a heavier mast will make the rolling less uncomfortable.
The change in apparent wind speed and in apparent wind direction will vary with the height of the mast because the roll speed of the mast varies with height.

Breaking seas is when you really want a heavier mast. There are 2 types of stability: form/initial & gravitational/ultimate.
-Form stability comes from the shape of the hull. The wider the hull at the waterline and the harder the bilges the stiffer the boat is initially, at low to moderate angles of heel. Multihulls are form stability taken to an extreme.
-The deeper the center of mass for the ballast and the heavier it is the greater the ultimate righting moment at moderate to high angles of heel.
The thing about form stability is that it depends on the the surface of the water, if the surface of the water starts to slope, then form stability tries to heel the boat to match that slope. Initial stability will try to roll the boat along with the face of the wave. Ultimate stability will still be counteracting that.
In breaking seas the slope of the water will go past vertical and capsize becomes a risk. In these conditions roll inertia slows the roll so that the boat has not rolled as far when it reaches the crest of the wave, at which point slope reverses and form stability tries to right the boat. For a given boat a heavier mast will mean that a somewhat larger breaking wave is needed to capsize the boat. A narrower boat will help with capsize resistance because there is less of a rolling force on the boat.

Thanks! Especially this part:

"The thing about form stability is that it depends on the the surface of the water, if the surface of the water starts to slope, then form stability tries to heel the boat to match that slope. Initial stability will try to roll the boat along with the face of the wave. Ultimate stability will still be counteracting that."

which is a much better way to explain something about flatter and beamier boats than mine, that I have tried to say before!

It's really a bit counterintuitive since tippy boats seem more likely to suffer a roll than a beamier, stiffer boat.

But I am confused about the last sentence. Once a beamier boat is on a steep or breaking wave, how would a heavier mast help prevent the capsize? If the sea surface is vertical, it seems it is the weight of the mast that is directly competing with the ballast for the location of the center of gravity, and thus the righting moment, no?

[Whokickamoocow]

Originally Posted by Don C L

Thanks! Yes I get that the weight aloft slows the roll, but I am wondering about how being under the press of sails changes the equation. In other words, is it safer to keep sailing, and heeled, than to say run with warps or lie to a sea anchor with sails furled?

Hiya Don,
IMHO that comes down to the type of boat. With a 'normal' fin keel and no bulb I found heaving to worked well, or running with a minimalist sail plan with no warps or drogues. I had a Jordan series drogue but I only deployed it once for testing. It worked very well but I never used it as I never found the conditions to justify deploying it. If you go that route, make sure you have good hard points, the loads are quite impressive!

I had loads of RM, and a bigger keel with a decent chord length, so in really crappy conditions it was sailing with a minimal sail plan that was my go to. In the gusts the boat would kinda just sit on its chine and accelerate rather than heel or broach. Canted rudders gave you really good purchase and control, the deep draft and decent chord length added a lot of roll resistance, so in a lot of ways it was actually easier to sail in really fresh conditions.
But if I had a tender boat in the same conditions, ie: shoal draft keel and a little single rudder then that's a different ball game altogether. I might be thinking about warps or a drogue in that case, as I might not have enough underwater square meterage to maintain control.
If I had a heavy boat or a full keeler with a good balanced sailplan, heaving to might be the best outcome.

It really does come down to the boat. We found out the optimal configuration the old fashioned way, by simply going out sailing in crappy conditions and trying everything to see what happens. That's how we found out the boat could heave to and quite nicely, even though everyone told me that hull design would never hove to.
Cheers!
WK

[Don C L]

Originally Posted by Whokickamoocow

It really does come down to the boat. We found out the optimal configuration the old fashioned way, by simply going out sailing in crappy conditions and trying everything to see what happens. That's how we found out the boat could heave to and quite nicely, even though everyone told me that hull design would never hove to.
Cheers!
WK

Yes, I have one of those narrow, longer keel boats and she really loves to roll if given the chance; not a lot of form stability. If she is sailing and heeling, or if just the main is up while motoring, she is far more stable, except for DDW. So far I have found she likes to heave-to best with just the main up (maybe reefed) with the tiller tied off. Then she sits very well, balanced, rolling only to leeward. Backing the jib with the main works fine too, but she seemed to set better with just the main for some reason. Like you say, best to just go out and practice on a breezy, bumpy day.
Is that the Pogo you were heaving-to in?

I was actually thinking in terms of the roll speed of a boat that had a lighter mast and did not have any sails up at all; would that create a greater hazard, especially if you were hove-to in such a way you were beam-on to the seas. And in considering the case of the race that was mentioned earlier with boats with carbon masts, I wonder if that rolling speed may have been a factor as well.

[Whokickamoocow]

Originally Posted by Don C L

But I am confused about the last sentence. Once a beamier boat is on a steep or breaking wave, how would a heavier mast help prevent the capsize? If the sea surface is vertical, it seems it is the weight of the mast that is directly competing with the ballast for the location of the center of gravity, and thus the righting moment, no?

Yep, that's pretty much right. Weight up the mast might decrease the roll period and magnitude (more comfortable), but this also means the mast takes longer to stand back up again from being heeled over, which is not good in big seas when you're gyrating all over the place.
Ultimately, if you lift the VCG in the vertical plane by adding weight aloft, you are shortening the metacentric height (GM). If you shorten the metacentric height, you shorten your lever arm (GZ) which gives you your righting moment. Lessen the boats righting moment and yes, it is more susceptible to falling over.
It's a trade off between comfort and RM. My VCG (G) is under the boat and with a wide beam the centre of bouyancy (B) is way out to the side when heeled, so my lever arm (GZ) is really big. The tradeoff is the snappier motion can be more uncomfortable in really foul conditions.
Cheers!

[Whokickamoocow]

Originally Posted by Don C L

Yes, I have one of those narrow, longer keel boats and she really loves to roll if given the chance; not a lot of form stability. If she is sailing and heeling, or if just the main is up while motoring, she is far more stable, except for DDW. So far I have found she likes to heave-to best with just the main up (maybe reefed) with the tiller tied off. Then she sits very well, balanced, rolling only to leeward. Backing the jib with the main works fine too, but she seemed to set better with just the main for some reason. Like you say, best to just go out and practice on a breezy, bumpy day.
Is that the Pogo you were heaving-to in?

I was actually thinking in terms of the roll speed of a boat that had a lighter mast and did not have any sails up at all; would that create a greater hazard, especially if you were hove-to in such a way you were beam-on to the seas. And in considering the case of the race that was mentioned earlier with boats with carbon masts, I wonder if that rolling speed may have been a factor as well.

Yeah, I must admit that comment about the race confused me too. A carbon fibre mast weighs less, so it will lower the height of your VCG compared to aluminium mast. This might make it less comfortable, but it's not 'worse' from a VMG point of view. When working to windward I am focused on keeping the heel at the same angle as it's draggy when your keel is rolling side to side. A lighter mast means you might have more steering inputs to keep the angle constant, but IMHO that's better than a heavy mast causing a a metronome effect where the keel is slowly rolling port /starboard/port starboard (drag) and you can't counter it with steering.

I've never found a CF mast to be a negative when working to windward, so not sure what that observation meant.

[Adelie]

Originally Posted by Don C L

....
It's really a bit counterintuitive since tippy boats seem more likely to suffer a roll than a beamier, stiffer boat.

They seem like they would be more likely to suffer a roll but aren't, beamier boats are more likely to roll over.



But I am confused about the last sentence. Once a beamier boat is on a steep or breaking wave, how would a heavier mast help prevent the capsize? If the sea surface is vertical, it seems it is the weight of the mast that is directly competing with the ballast for the location of the center of gravity, and thus the righting moment, no?

The effect of the heavier mast doesn't start when the boat is already rolled, it starts as soon as the boat starts to roll.
The effect of the sloping wave is to roll the boat, it is a force acting on a moment arm or torque. The longer the arm (beam of boat) the greater the torque applied to the boat. The roll moment of inertia resists the torque like mass resists a force. Higher roll moment slows the acceleration. So for a given amount of torque applied for a given length of time the boat will roll more slowly over time and will roll less in a given period of time. If the roll rate is slow enough, the boat will reach the top of the wave before the mast is horizontal or more accurately the point of no return, since the boat is still rolling towards a capsize even after the crest of the wave has passed and it takes time for the both the initial and ultimate stability to reverse the roll.

Actually the motion is a lot more complex than I describe above, the wave impetus varies as it passes below the boat, the center of mass of the boat is moving up and down which also changes the roll.

[sandy stone]

Very good explanation of a complicated subject.

[Bowdrie]

Originally Posted by Adelie

Actually the motion is a lot more complex than I describe above, the wave impetus varies as it passes below the boat, the center of mass of the boat is moving up and down which also changes the roll.

True, (IIRC,) in the book "Heavy weather Sailing", by Adlard Coles, he describes what Adelie is basically saying above.
The narrower/deeper boat, (as the sea passes beneath it,) will actually roll into the wave front.
Now this vid is kind of hard to see but shows a stern view of a Swan/PJ-43 in some nasty stuff.
She gets hit with a big one at ~00:37
Now by today's standards a Swan 43 is an "old narrow/deep" design, but for a 31ft waterline this old girl is taking care of her crew and working her way forward.

https://www.youtube.com/watch?v=cp0qvM98pZY

[Don C L]

Originally Posted by Adelie

If the roll rate is slow enough, the boat will reach the top of the wave before the mast is horizontal or more accurately the point of no return, since the boat is still rolling towards a capsize even after the crest of the wave has passed and it takes time for the both the initial and ultimate stability to reverse the roll.

Actually the motion is a lot more complex than I describe above, the wave impetus varies as it passes below the boat, the center of mass of the boat is moving up and down which also changes the roll.

Thanks! Now THIS I'd love to see a video of. I can imagine it, and thanks Bowdrie for that video, but I think I'd like to see steeper, bigger, breaking waves to demonstrate how different hulls respond. I don't know if I want to volunteer for this one yet though.

[Na Mara]

Originally Posted by Bowdrie

True, (IIRC,) in the book "Heavy weather Sailing", by Adlard Coles, he describes what Adelie is basically saying above.
The narrower/deeper boat, (as the sea passes beneath it,) will actually roll into the wave front.
Now this vid is kind of hard to see but shows a stern view of a Swan/PJ-43 in some nasty stuff.
She gets hit with a big one at ~00:37
Now by today's standards a Swan 43 is an "old narrow/deep" design, but for a 31ft waterline this old girl is taking care of her crew and working her way forward.

The Beam on breaking wave at the end of that video was quite something. It must have been at least equal to the beam to hit the camera like that and yet the swan hardly heeled to it at all. Impressive. A lot of boats of similar beam would have been knocked down by that kind of wave.

[DMF Sailing]

Originally Posted by Na Mara

The Beam on breaking wave at the end of that video was quite something. It must have been at least equal to the beam to hit the camera like that and yet the swan hardly heeled to it at all. Impressive. A lot of boats of similar beam would have been knocked down by that kind of wave.

What is really frightening to me about this video is where the heck they're going? I see nothing but obstructions ahead!