Mast builder and I had it out!

S830
Ixx=2538cm^4(rest are same units)
Iyy=884

IsomatNG80
Ixx=1780
Iyy=830


Just fyi in case anyone was curious..

 

BB:
I got 3 quotes. All the masts were lighter than original, varying levels of Ixx and Iyy, but usually Ixx around 40-45 and Iyy 15-18 (ft / lbs ^4)

The weight of the mast was measured by a crane at the yard that hauled it out. Three of my strongest friends and I tried to lift one end of the mast and it took all four of us to do it. We guessed it weighed about 600 lbs total.

The old mast is not an Isomat, the old mast can be seen deep in the rigrite page as a 'traditional oval section' by metal mast, it has no track and was completely oval with no other features.

Let me try to pose the original question another way:

If sailing with the original mast, the boat accelerates into a roll at .5 meters / second ^2, how fast will it accelerate into the same roll with a mast that weighs 40% as much? My logical brain would say, .83 meters / second ^2, or roughly 60% faster than before.

The question is regarding sea comfort and roll speed of the entire vessel. The heavier the mast, the slower the acceleration of rolling. These boats are prone to ROLL A LOT. I motored the boat without the mast in 1-2 ft seas and things went flying off the shelves, and I almost got seasick. I am a very good sailor, I single hand my Ericson 35' sloop regularly, I know how to properly trim sails and handle a big boat. The issue here is all about whether the boat with a lighter mast will be too uncomfortable to be used as a cruiser, especially at anchor, due to the missing resistance to roll acceleration from the inertia of a heavy spar. Sorry my terminology is wrong because I am not a naval architect.

I completely understand that with wind in the sails, the stability is not a factor, this is more about while motoring and while at anchor. Believe me, I was excited to spend my hard earned freedom units on a sexy new double spreader rig with shiny white paint, but if it makes the boat too uncomfortable to tolerate, it would all be for nothing.

Yes I understand a 5,000 weight at the top of the mast would NOT improve stability. My theory is that this boat is already OVER ballasted, and because of the 14' beam causing excessive lever arm from waves, the heavy mast was put their originally on purpose, with intention, to prevent the captain and crew from losing their lunch. That is why I got scared and asked the mast builder to explain it to me, which he did not. He denies that a heavy mast has any benefit whatsoever. I have already read and confirmed from multiple sources and it is intuitively obvious, that a heavy mast means slower rolling. Slower rolling means more comfortable boat. Less rolling is not the same thing as slower rolling. Less rolling that happens twice as fast could throw you against the cabin ceiling and cause your girlfriend to leave you. I do not want that

 

Take it out for a cruise on a choppy day, with no mast, and see how it feels.
That should ought to answer your question.
Let us know, please.

I can tell you it would be fine even with a $500k carbon fiber mast.
But, I'm pretty sure you wont believe me, so do the test.

Did you speak with a Naval Architect about your concern?

 

Okay, now I see where you're coming from.
No, it's not going to react as you describe.
What about the inertia of the rest of the boat?
The keel alone is going to dampen the roll response... 90% compared to the moment arm difference due to mast weight.
Another 8% damping by the rest of the boat hull, interior and rigging.

 

No. The math is all wrong. You are treating the mast as the only thing in the picture.

These can all be calculated.

The change to the roll period is minute.

There are far wiser men here to assist you.

All you need to do is provide the keel type and vessel weight and use the captains formula.

<removed error posted at 2am>

If it were such a great thing to have heavier masts, why did we see carbon instead of solid steel?

Just tryin to help. Hope one of the NAs steps up so I can go back to my corner.

 

Well, this is one issue where intuition leads you wrong. But you must understand the interaction between boat roll period and wave period. When the both periods coincide, you will have a resonance situation. The input energy from the waves, causing the roll must be killed by friction and turbulence around the hull ( eventually aided by a storm sail), or the roll amplitude will increase ad infinitum.

Now, with a heavier mast, the boat has a slower roll period, which will come in resonance with a longer wave, and longer waves deliver more energy that must be destroyed. There is then a limit as to how much roll energy the hull can cope with; exceed the limit and you go down, practically no matter what hull shape. This is what caused disaster among Alaskan crabbers years ago (ok, not sailing vessels, but the physics is there all the same). There were reports that extra ballast water was taken on board in order to "stabilize" the vessel in a heavy roll just before capsizing. So, a slow roll in lazy weather may feel comfortable, but it may kill you when it gets rough.

On top of that, our own built-in balance system and the brain suspension are tuned to a high tolerance for frequencies around our step frequency; diversions both up and down will cause discomfort and ultimately illness. So, all in all I think the mast guy had it right; question is, do you have the decency to make it right with him?

 

A short, fast roll is uncomfortable and leads to seasickness.

 

I suspect that the heavy weight includes all the rigging, halyards and electric wiring.
The lighter weight is simply the necked tube.

 

I punched some numbers and here is what I came up with.

A change in the mast of 136kg changes the metacentre by 0.013m (about a half inch)using half the mast length as the distance and 10 degrees as the angle and original mass of 15649 kg which may be high.

Using the Captains formula and an arbitrary one meter for the original metacentre; the change results in a change in the roll period from 5.9738 seconds to 6.01 seconds...of course, I had to cheat and reverse the thing and treat it as an add to use the formula, but I believe cheating is allowed so long as we adjust at the end. I calculate the new mast will affect the roll period by 0.04 seconds and reduce the period to theoretical 5.93 seconds, not increase. As a percent, I get .66 % difference.

I just like math. And boats.

corrections welcome

OP-I am just a builder and make no claims of expertise. I just had a problem with the notion reducing the mast weight affected things to the degree suggested. In your defense, it would have been cool for the builder to show you some numbers, but is not what he does..

-my original post reflected the mass of the mast at full height-sorry

 

If you remove weight from the top, it seems logical to think that the CoG will drop somewhat, very little, of course, but since the inertia of the water plane will change insignificantly, the GM will increase so it does not seem logical to deduce that the balance period will increase. Maybe I'm wrong about something, what do you think?.

 

Yes. I believe you are correct. The calculation I provided uses a square root, which does not work with negative numbers is all. So, I ran the numbers as an added mass which must then be reversed. Faster roll period, but critically low percentage.

 

Farlander, I think that the general consensus here is to simply don't worry about it.

I saw a mention above that the new proposed mast has a double spreader rig - this will allow the thickness of the mast section to be reduced significantly, when compared to a single spreader rig.
The formula for calculating the buckling load for a straight column is (3.142 x 3.142 x E x I) / (L x L)
where 3.142 is that wonderful constant called 'pi'.
With a single spreader rig, the L is effectively L/2.
With a twin spreader rig, the L is L/3.
So L squared will be L/4 and L/9 respectively.
We have constants in the form of pi (3.142) and E (the modulus of the material).
So that leaves I as a variable.
For the two masts to have the same buckling load, then I on the single spreader mast will have to be 9/4 times the I on the twin spreader rig.
If both masts have the same section shape, then the single spreader mast will need to have a thickness of aluminium that is much more than that of the twin spreader rig, in order to achieve the required 'I'.
And has been shown above, the change of roll period is going to be very slight when comparing the two rigs.
So, if you otherwise like the mast, it might be best to make amends with the guy who you 'had it out' with, and ask him for a quote.

You can also think of it in terms of how you can now add extra equipment on deck (dinghy, water / fuel containers if required, davits, maybe a solid dodger and bimini over the cockpit) without any worries. Similarly, you could add a radar scanner at the mast.

In summary - 'don't worry about it'.

 

Just out of curiosity.
What prompted the mast change?

 

It is not that simple. That would work if the boat was a pendulum, but there damping by the hull shape and appendages which won't change.

 

This clears up one cause of misunderstanding.

If I remember correctly, it was Marchaj, in "Seaworthiness: The Forgotten Factor" who went on at length about how important roll moment of inertia is for preventing wave-induced capsize. And even though a monohull's mast is only a small proportion of its weight, it contributes a rather large proportion of total roll inertia because of its lever arm. It's been decades since I read the book, so my memory may be off, but I think it was up to 40%. Take that away, and although you increase stability, the resulting loss in roll inertia can make the boat more vulnerable to being rolled by a wave. I think that idea was empirically confirmed by tank tests, and boats that lost their rigs in the 1979 Fastnet Race were more likely to be rolled repeatedly afterwards.