Mast builder and I had it out!

Using the info found here:
Beam vs Ballast for Stability http://www.kastenmarine.com/beam_vs_ballast.htm
Formula about 1/3 of the way down the page, moment of inertia of the mast, i.e. resistance to rolling. In this case, it appears the heavy mast will be 182x more important in resisting initial roll than the keel ballast. The keel ballast would seem practically irrelevant.

300 lbs. mast @ 25 foot distance of CG from axis of rotation = 136,718,750 ft. lbs^4 resistance to roll
720 lbs. mast @ 25 foot distance of CG from axis of rotation = 281,250,000 ft. lbs^4 resistance to roll
10,000 lbs. ballast @ 3 foot distance from center of gravity to axis of rotation = 1,500,625 ft. lbs^4

I will try to make amends with the mast builder once I know whether I was right or wrong in presuming that the heavy mast vs. the light mast will have any significant effect on roll acceleration. As mentioned before, with no mast, the boat is so twitchy that navigating in 1-2 ft seas at 3 sec. period is practially impossible.

 

Re your photos, the mast appears to be locally buckled in various areas - I presume that this is the reason why you are now replacing it?
What were the circumstances that caused the mast to buckle in these areas?

 

The mast was damaged by the previous owner in an unknown way. Maybe we will find out in court next month. I believe that he intentionally concealed the kink (it was beneath the weather boot). The mast is so heavy and solid like a tree trunk, I believe the only way this could have happened is if inexperienced rigger loosened all the shrouds on one side but not the other, or a bridge collision. That is the $25k question. The kink is only in one place, right at the deck partners. I still have this mast though it is in the yard now, I am tempted to cut out the kink, splice and reinstall on the boat. I will raise the keel step to match the removed length. Would probably save me about $15k-$20k. I was excited about a sexy new rig but this whole situation has me at the end of my wits. I know, get a survey. I saw the mast was bowed but figured it was just out of tune. The previous owner was clearly a knuckle head (didn't know that he had a roller boom for reefing) but myself and 2 other captains and a yacht broker looked at the boat before purchase and none of them suspected a kink. The mast is so beefy whatever did that must have been serious.

 

Oh dear re that kink - it appears to be on the side of the mast. so something must have walloped the mast hard to cause it to buckle in way of the partners.

In your 'Curvy' photo looking up the mast, is it actually bowed to one side as appears in the photo?
If so, I would have thought that simply adjusting the rigging tensions would get it back into column?
Or is it still bowed, even when it is off the boat and properly supported?

Re the previous owner being a covfefe knucklehead, then maybe he honestly did not know about the mast being buckled locally underneath the boot?

Cutting out the buckled section locally and then sleeving the joint sounds like the easiest / cheapest method of repair in the given circumstances, especially so if nothing else is wrong with the mast.

 

Interesting on the roll inertia. It'd be nice to hear from one of the NAs why everyone wants ultralight masts instead of solid steel ones.

 

Excessive weight is always a problem, and if this weight is set too high, such as the mast, the problem worsens. All the "wonders" that we see now, for example in the America's Cup, are fundamentally possible thanks to the enormous reduction in weights that has been achieved.

 

Oh, you're taking him to court?
Or is he taking you to court?
Maybe that will help you resolve some of your issues.
Best of luck.
BB

 

I'm grateful for the all the responses and interest in this post.

I'd like to bring it back to the primary objective.

What happened to the old mast is not pertinent.

Why the boat builder put a 700 lbs. mast on the boat is the intriguing question.

This situation is unique to this boat because it has a 4' draft and a 14' beam, much like a power boat, and the ballast makes it extra snappy.

I don't want to fly above the water like America's Cup racers, this boat is not a racer! This sounds like repeat of the conversation with the mast builder.

I want the boat to be reasonably comfortable, as it was when it was built, and when I bought it.

Can anyone out there give me real factual data or experienced based data that would help predict the changes in roll acceleration of the vessel with the different mast weight?

In the above post I calculated the difference in moment of inertia of the masts as they pertain to roll resistance, but I don't have the basis of knowledge to know what a million ft lbs ^4 feels like as it pertains to the roll motion of a 14' wide boat.

Someone previously mentioned the roll period, which was almost identical between the two masts, which I find intriguing. Logically that does not make sense to me that the period would be that similar, I would expect the 2x weight mast to be 2x the roll period OR MORE due to the exponential force factor of the distance away from CG.

 

I think the reason was that this design was the 'best fit' at the time (early 70's) for the boat, while taking into account that they wanted her to be fairly indestructible in the hands of charterers.
You could probably do a standing gybe in a Force 8 and not be at all worried about the mast coming down - do this in a fractionally rigged racing boat with runners, and the odds are good that something nasty will happen.

I have seen OI 41's sailing in the 70's and 80's, , and even under full sail in a good breeze they appeared to be fairly stiff - and I am sure that this would have been with the original 700 lb mast.

I think that if you put a lighter weight / more modern mast on her, the difference in roll acceleration is going to be not a lot really - and certainly nothing to be worried about.
Bear in mind that a much heavier mast will give you more longitudinal inertia compared to a lighter weight mast, so if you are motoring into a head sea you will probably be pitching more.

It is not as simple as this.
The weight of the mast is only a very small percentage of the displacement of the whole boat.
If you double the weight of the mast, then the effect on the roll period is going to be altered only slightly.

 

There are three factors, roll angle, roll period and roll dampening.
Roll dampening means how quick the roll will stop after the initial push, and has to do with surface area, both in the water and in the air. Big underwater keel and raised sails help stop the movement sooner, they act as a hydraulic shock absorber.
Roll angle means how far the boat will roll, and this has to do with the initial stability or stiffness of the boat. More mass up high makes the boat less stiff and roll more deeply, let's say 30° from vertical instead of 20°. A stiffer boat can better translate the wind force into forward motion, that's why racers put people on the rail, or use water ballast.
Roll period is how fast the movement is, meaning how fast the boat returns to vertical. High stiffness means quick movements, but short ones (small angles), while low stiffness means slow movements, but longer ones (big angles).
People often feel that long slow rolling is more comfortable than short quick rolling, so the designer has to adjust the metacentric height such that the crew is comfortable on deck.
The above also applies to pitching, but people normally prefer to pitch less.

Is a lightweight mast better? Well, it depends on what you want. For upwind sailing the increased stiffness will translate into better sail carrying ability and higher average speed. For comfort the same increased stiffness will make the boat roll less severely with a quicker return, producing higher accelerations at deck level. This motion can be percieved as jerky and uncomfortable, or sporty and fun.

As you can see, both you and the mast builder are right, a lighter mast is better for sailing, while it could make the boat more uncomfortable. Unfortunately comfort is a subjective notion and open to interpretation, at least until acceleration or roll angle become life threatening.

Modern cruisers tend to collect weight up high in the form of dodgers, solar panels, and other things the designer never intended to be there, plus all the stuffed lockers. It's very possible that a "cruise ready" boat has a completely different metacentric height and roll period from what the designer originally intended, and that a lighter mast, or lighter modern sails could potentially restore the initial behavior "as designed".
It's possible to translate the above into numbers, but it still takes an experienced NA to translate those numbers into "feelings".

 

Less than 2%
(See post #18)

 

I only recently learned, on this sight, about the adverse effects of motion on a sailboat that has lost its mast. It was explained to me that without the mast in the air, a weighted keeled boat can jump around quite uncomfortably with its center of rotational motion well below the waterline.

I learned this from these very same posters when I commented on the increased stability of finding yourself mastless, because I was thinking about how the center of mass and the forces put on the boat from a tall mast were more favorable. But they disillusioned me of this idea.

Now, I'm wondering, if cutting the weight of your mast in half doesn't affect the roll motion in a significant way, why would dropping the other half of that weight change it much?

-Will (Dragonfly)

 

I would think a halved mast weight would give you a halfway experience between "intolerable" with no mast and super stable "Outhouse41" with original mast?

 

I wonder if some of the appreciable difference in weight quoted by the mast builder is due to him quoting the weight of the bare mast.The mast shown in post #31 is festooned with rigging and has a radar scanner attached.I would be interested in knowing the weight of all the rigging and other hardware and would also think that if a slower roll is desired,you could always mount the radar a bit higher to move the C of G up.As was said earlier in the thread,under sail the sails themselves provide a measure of damping and it would likely be only when motoring that a bit more of a lively roll might happen.

 

So from this I gather the roll dampening is more from the lateral wind resistance of the mast and not from the weight of the mast.