Mast builder and I had it out!

Discussion in 'Stability' started by Farlander, Mar 24, 2021.

  1. Farlander
    Joined: Mar 2021
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    Location: Berkeley CA

    Farlander Junior Member

    I have a '74 Morgan 41 Out Island
    4' Draft
    10,000 lbs. ballast
    22,000 lbs. displacement
    14' beam
    Full keel with forefoot cutaway

    The old mast was damaged by the previous owner and I opted to have a new one fabricated by a local builder who I will not mention by name.

    He insisted that a lighter mast is better in ALL cases.

    The original mast weighed 720 lbs. and was 55' 4" tall, keel stepped.

    The proposed new mast would weight approximately 280 lbs. and also be 55' 4" tall.

    I confronted the mast builder about the issue of loss of dynamic stability due to the lack of inertia aloft, and he got upset and canceled my project.

    Am I wrong to question the lighter mast weight affect on roll acceleration? I am concerned the boat may be extremely uncomfortable with too light of a mast, getting thrown and jerked rather than the gently rolling.

    Thoughts and experience appreciated,
     
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Welcome to the forum.
    A lighter mast will lower the CoG of the boat, which is good for stability and will probably make the roll period shorter. A lighter mast can have the same inertia (in its cross section) as a heavier one. Another very different thing is that the type of material allows, or not, to support the same stresses to both masts. But there are other factors that can tell if one mast is better or worse than another, for your boat.
    My advice: make peace with the mast builder and study with him the why's of everything.
     
  3. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell "Whatever..."

    Good advice from TANSL.
     
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  4. Farlander
    Joined: Mar 2021
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    Farlander Junior Member

    How can a lighter mast of same length have the same inertia as a heavier one?

    The relationship with the builder is scorned, I flamed him on Google reviews out of principal, whether I am right or he is right he did a poor job of handling the customer service part of the interaction.

    Regrets? Maybe. Fixable? Definitely not. Am I still screwed with a mastless boat? Yes. Any leads on used 55' masts accepted.
     
  5. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Inertia is a property of shape, not mass. A certain shape of paper can have the same inertia as that same shape in high-strength steel.
     
  6. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    I don't see any downside to having less weight aloft.The roll period,as has been mentioned,may well be reduced and so may the pitch period.
     
  7. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell "Whatever..."

    Was the boat designed for this mast weight?
    You need to replace the mast, correct?
    How many estimates did you get?
    Have you spoken with a Marine Architect?
     
  8. Scuff
    Joined: Nov 2016
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    Location: Richmond VA

    Scuff Senior Member

    The original mast weighed 720 lbs. and was 55' 4" tall, keel stepped.

    The proposed new mast would weight approximately 280 lbs. and also be 55' 4" tall.

    Wow that's an incredible reduction in weight. What was the new mast to be constructed of?
     
  9. Farlander
    Joined: Mar 2021
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    Location: Berkeley CA

    Farlander Junior Member

    The original mast was 6061 aluminum alloy, with at least a .125" wall thickness.
    The shape was oval, approximately 9.5" x 6".

    The replacement mast is also 6061 aluminum and would have been about 9" x 5", rectangular, with rounded nose, sides, and corners, and an integrated sail track.
    The replacement wall thickness was never mentioned, but due to the similarity in overall outer dimensions, I can only assume the extrusion wall thickness would be closer to 1/16", thus accounting for the roughly 50% weight.

    TANSL:
    "Inertia is a property of shape, not mass. A certain shape of paper can have the same inertia as that same shape in high-strength steel."
    I believe this to be incorrect. The only time the shape matters is if the object is being rotated. There is no rotating of the mast around it's longitudinal axis, in the case of a sailboat mast. There is only the length of the mast, which is the same in both cases and thus moot. The minor differences in overall diameter of the extrusion will not be very important. Basically what I am saying is that a 1" solid rod, 55' tall and weighing 500 lbs, will have roughly identical MoI to a 10" hollow tube, 55' tall, weighing 500 lbs., if we assume that both are levered about an axis which is at the end of the object (the keel). The moment of inertia of these two items should be roughly identical. Please somebody disprove this with some math or formulas, as it is the only way that I will believe that the new, lighter mast could somehow behave like the old mast simply because the shape is different.
     
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  10. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Sorry, I'll try and keep it down to one question at a time.

    Was this the original mast as designed for the boat?
     
  11. Farlander
    Joined: Mar 2021
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    Farlander Junior Member

    Hi BlueBell,
    Yes the original 720 pound mast was the original, put on the boat by the boat builder, Morgan Yachts.
    It is possible that it was way overbuilt, as these boats were mainly used for chartering in the Caribbean by novice sailors.
    I can't help from thinking though, why use such a heavy mast, unless there was some intentional reason behind it.
    Even in 1974, there were probably lighter spars available.
     
  12. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell "Whatever..."

    Okay, thank you.
    How many estimates did you get for this replacement?
    BB
     
  13. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Farlander, the popular nickname for your boat is an Outhouse 41 - derived from the saying 'it is as strong as a brick outhouse'.
    And these Out Island 41s were built fairly massively.
    Which was no bad thing really when so many headed south to join in on the booming bareboat charter trade in the Caribbean.
    https://sailboatdata.com/sailboat/morgan-out-island-41
    12 tonnes design displacement, with 30% of this weight in the ballast in the keel.
    Many reefs in the Grenadines and the BVI's have sustained major headaches from OI 41's bashing them at speed, and invariably the effect on the OI 41 was barely noticed.
    Hence one reason why they were so popular with the charter companies. Sailboat Data mentions that 1,000 were built.
    And I am sure that not many (relatively) ever lost their masts.
    The CSY 44 followed on later, as a sort of development of the OI 41, using these same principles of heavy construction.
    https://sailboatdata.com/sailboat/csy-44-mid-cockpit

    It is very possible that lighter spars were available in 1974 - but they would probably not be so forgiving in the hands of novice charterers.

    Re dynamic stability, you will gain much more dynamic stability by simply having the sails set and the wind filling them - and if you have a lighter mast, then (all else being equal) you will be heeling less, and you will probably be sailing faster and more comfortably (when compared to a sistership with original 'heavy' mast).
    I will endorse Tansl's suggestion to make amends with the mast builder, bury the proverbial hatchet, and ask him for further information - you might be pleasantly surprised.
     
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  14. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    "Inertia" has two different meanings when naval architects and engineers discuss masts and similar. One meaning is the moment of inertia which affects rotational accelerations. It is a function of both the shape of the mast and the density of the mast.

    The other meaning is the "moment of inertia" of the cross sectional shape of the mast which is used in calculations of mast bending based on beam theory. This can also referred to as the second moment of the area. It depends on the shape only and is independent of material proporties.
     

  15. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    A sailor I am not. I am the happy owner of a cutup S830 sparcraft mast. The size is about 6x10 oval with a track. A very similar mast to the one you mentioned; they are made about 19 meters long or so, I forget..

    The weight is about 6.05#/ft. For 55 feet; the weight is 332.75#. Wall thickness is close to 1/8".

    This mast per RigRite replaces the Isomat NG80 which is 5.38#/ft or 296#@55'.

    The critical question you have not mentioned and comments made show a lack of wisdom on the issue.

    If you first realize that a boat with a 5000 pound weight on top of the mast is not good, that is a start. It is conceptually an error to suggest your boat is less stable with less weight up. And you got this wrong, and in so doing, you red flagged the poor fellow into worrying about you suing him after you dump your boat. Noone enjoys a wise idiot; they can cause a lot of grief.

    According to RigRite, the Morgan 41 (no model) uses the Isomat mast.

    Now, if you had some factual information that proves your existing mast has higher moments of inertia and you provided some reason for wanting a stronger mast; your arguments would make sense, but you have not done so here, nor with the builder, based on your testimony here.

    But I suggest you deliver the apology even if he says goodbye; chalk it up to stupidity, or sheer confusion over the 720 pound mast. It does sound like a 290 pound mast is like 1/3rd of the strength, despite it being 40% of the weight (because of the 2 and the 7 in the hundreds places).

    What I am curious about is A) whether you have the original mast weight right and B) whether you'll need to add some ballast, but those are out of my league to answer. In order for the mast weight to be double; it would need to be nearly double the thickness unless the properties of the alloys vary that much, which I doubt. And since I know the wall thickness of a very similar mast, double would be somewhere around 0.200" which seems almost unreasonably thick for the length. (I could be wrong)

    If I may, who told you it was a 720# mast? I bet they are wrong.
     
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