Adding ballast to counteract mast weight

Discussion in 'Sailboats' started by captjj, Mar 20, 2013.

  1. captjj
    Joined: Oct 2009
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    captjj captjj

    Hello group,
    I have a Bruce Roberts 43 in GRP. The boat was fitted with in-mast roller furling, both masts. The masts are quite heavy and double walled. The boat seems to roll a lot going down wind and I suspect that the mast weight to be a factor. My question is: would adding ballast to the bottom of the keel be detrimental? I'm thinking of adding about 10% of the original ballast to the flat underside of the keel.
    Thanks for any thoughts.
     
  2. motorbike
    Joined: Mar 2011
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    motorbike Senior Member

    I may well be incorrect but I would think that the rolling is not solely due to mast weight, but some other factor. The reason is that the weight of the masts provide inertial mass which would tend to inihibit rolling. I doubt extra ballast is the solution and may cause other problems.
     
  3. gonzo
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    gonzo Senior Member

    A heavier mast will cause a longer but slower roll. Adding ballast will also add stress to the structure. Bruce Roberts is in business and would be the appropriate person to ask; he designed it.
     
  4. PAR
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    PAR Yacht Designer/Builder

    You need to contact Bruce Roberts International (Fine Line), which actually isn't Bruce Roberts or Bruce Roberts-Goodson. They are the owners of the Roberts plans, which has changed hands twice in the last 20 years. There are few different Roberts 43's that I know of, which one do you have? If it's the early Mauritius 43 or Norfolk 43 shoal draft ketch versions, they had issues with steering and lateral area, possibly compounding your rolling concerns. The plans where updated and these up dates are available for cheap. Contact Fine Line boat Plans at >bruceroberts.com.au<. If you contact BRI, they'll just tell you to replace the masts with the ones spec'd in the plans, though maybe a phone call might help >61/7/4159-4982<
     
  5. captjj
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    captjj captjj

    Thanks Par, I did not know about Fine Line, it may be just the resource I need at low cost.
    The build year was 1989 with a draft of 6 feet. Steering is a definite issue and I've considered a larger rudder but the expense is prohibitive.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Yep, ownership of 43' yachts isn't for the faint of heart.
     
  7. MikeJohns
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    MikeJohns Senior Member

    As Par said contact Roberts for a start.

    If you want to go further there's a lot of detail that's required to answer this.
    Start with the ballast, Roberts boats have been built with all lead, steel puchings, iron blocks and even just concrete. It's the joy of low budget home building or cheap yards.

    Rigs are probably designed for all lead ( worst case rig loads ) and this will give the stiffest craft. Who built it and are they contactable, or how detailed are the building records.

    My experience with older Roberts designs is that details like Delenbaugh angle or GZ curves are non existent. But the designs are not highly stressed performance craft and there is usually a lot of leeway in scantlings and rig strength.

    Under mainsail and genoa, What wind strength do you find you need to reef at to windward?

    Have you ever measured the natural roll frequency ? That's a basic test that can be revealing and is measured alongside the jetty on a calm day, the stiffer the boat the shorter the period 3 seconds would be very stiff 5 seconds would be concerning.
    A more involved inclining test can be useful, even a simple stability test will reveal whether the vessel is overly tender. Usually a 43 foot cruiser like that should not move much stepping onto the gunnel from the dockside if its a standard keel.
     
  8. captjj
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    captjj captjj

    Natural roll frequency test? how is that done?
     
  9. kvsgkvng
    Joined: Jan 2012
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    kvsgkvng Senior Member

    Roling downwind maybe a function of sails



    http://en.wikipedia.org/wiki/Vortex_shedding

    If you have sails extended as much perpendicular to the wind direction as possible there may be another factor in play. It could be vortex shedding. Next time you run try not to extend the sail too much outside the hull. Indeed you may lose some speed, but if my assumption is correct the rolling should be significantly reduced. The idea of vortex shedding came to my mind after you mentioned that your boat rolls only when going downwind.
     
  10. daiquiri
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    daiquiri Engineering and Design

    This is correct. The Karman vortex street is the reason, or the force which induces the roll. It is a natural phenomena which occurs when the boat speed and wind speed are matched to give a certain value of Reynolds number on the downwind course.

    When the frequency of the vortex shedding coincides with the natural roll frequency of the boat, the resonance occurs. The aerodynamic exciting force then works in perfect synergy with the boat's inertia and hydrodynamic damping, and give large roll angles, which the author of the OP has witnessed.

    A contributing factor may be the hull shape, which may cause some loss of stability when underway. Basically, in correspondence of the speed where you have observed the rolling phenomena, the hull might be running with a wave through under it's maximum beam, causing a decrease of the effective wet-surface beam - and hence a decrease of metacentric height and of righting arm. But that's just a secondary cause, not necessarily present, which increase the roll amplitude when concomitant with the aerodynamic excitation.

    Captjj, there's imo little you can do to eliminate this vortex shedding with your current sail set without hampering your boat's performance on other points of sailing. You should modify the shape of your sails' trailing edge and hence break the vortex formation, but it is not an easy thing to do on a flexible membrane, which a sail is. You could do it on the leading edge, introducing some vortex-breakers, but they would have to be pretty big and would cause an excessive drag on other points of sailing.

    Adding ballast below the actual CoG (you have to check where it is now) will increase the righting arm (or metacentric height), which might help to some extent, but will also increase the wave drag of your boat and will do nothing to eliminate the primary source of rolling excitation force, which is aerodynamic. A quick fix could be to decrease the sail angle (either by sheeting in or by changing the course) until the rolling phenomena is eliminated, or at least made less severe. The result is not guaranteed, but at least it's a cheap fix. ;)

    Cheers
     
  11. sean9c
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    sean9c Senior Member

    How about instead of Delenbaughs, Reynolds, Karman, vortex breakers, GZ curves, metacentric height, roll frequency etc. it's just rolling because he's sailing dead downwind and it's common for boats to roll when sailing dead downwind. So instead of sailing dead downwind try reaching up just a little and that'll likely cure your problem.
     

  12. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Well, yes. It is the same as said above, less the explanation of why it is happening.
     
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