Understanding Simplified Stability Test - Full Keel to Shoal Keel Conversion

Discussion in 'Stability' started by Jeff in Boston, Dec 12, 2020.

  1. Jeff in Boston
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    Jeff in Boston Junior Member

    I'm looking to turn a Pearson 26 One Design from a full keel sailboat into a shoal keel electric displacement powerboat. Yes, I know that is a odd approach, but the cabin and the cockpit and the price match what I am looking for. It is clear that other sailboat conversions to powerboats have been successful. There is a boatworks that has converted many Com Pac boats over the years, but I couldn't find it again using web search.

    I understand I will need to replace the keel with one of the same weight to get the boat to sit on her lines and have the intended beam and length at the waterline. I am hoping that the raising of the VCG due to the shorter keel is offset some by the removal of the mast and sail weight high above. With the reduced heeling moment due to wind pressure I am thinking it will work out.

    However, I would like to test the resulting hull. I am aware of the roll period test but I would also like to understand the simplified stability test. I've reviewed the USCG form, and there is ALOT there. The test isn't strictly applicable to this boat due to the huge cockpit, but I want to understand the test as a whole. The concept of measuring the change in heel / freeboard based upon a worst case passenger and wind load makes sense, but I'm having trouble finding on the form what the limit is - angle or change in freeboard. I will spend some more time with it, but I wanted to see if anyone could give me a hint.
     
  2. Jeff in Boston
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    Jeff in Boston Junior Member

    I just found this document:

    CG4006.PDF (uscg.mil)

    It is way easier to parse than the other PDF I was looking at.
     
  3. Jeff in Boston
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    Jeff in Boston Junior Member

    So I found out the simpler document is an older version of the same document. But I have enough of an understanding of the procedure to figure out what to do. I'm curious why you would pick the maximum of the offset passenger load OR the windload, and not the sum. I guess they think that is an unreasonable worst case?
     
  4. bajansailor
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    bajansailor Marine Surveyor

    I don't think that you should be too hasty and worried about adding ballast after removing your keel - Sailboat Data mentions that at your LWL your displacement is 5,200 lbs, and the ballast is 2,200 lbs.
    https://sailboatdata.com/sailboat/pearson-26-one-design

    Re that 2,200 lbs (not to mention the weight of the rig that you are removing), it sounds like a lot, but you will quickly 'use it up' when you add up the weight of your batteries, motor / propulsion gear, a larger water tank for cruising, solar panels to provide a charge to your batteries, perhaps a portable generator to help charge the batteries when required, as well as stores and boat equipment.
    Not to mention the weight of the crew......

    I think that you are getting unnecessarily worried about your stability - be sensible and pragmatic, keep your heavy weights as low as possible, and you should have better stability than many 26' power boats currently operating happily.

    Re how you are familiar with the roll period test, is this similar to the one in the attached scan?

    Once you have your 26' shoal draft electric powerboat, what do you intend to use her for? Long range cruising / living on board, or just weekend trips locally - or something in between the two?

    Stability check using roll periods.jpg
     
  5. DCockey
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    DCockey Senior Member

    Is the displacement published in the third party compilation design nominal displacement, design light ship displacement, actual light ship displacement, an arbitrary number that some one involved in marketing though was appropriate, or ????
     
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  6. bajansailor
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    bajansailor Marine Surveyor

    That is a good point!
    I have no idea - I am assuming that it is lightship displacement, but it might be the displacement loaded with typical crew and stores (?)
     
  7. Jeff in Boston
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    Jeff in Boston Junior Member

    Yes, that was the roll test I was referring to.

    I was thinking of only replacing about 1600 lbs or so and then expecting to add more ballast in the bilge as needed. I figured there was no way to nail the fore / aft blanace with the replacement keel the first time so I would need to leave some room for adding ballast to balance. And I would keep that ballast moveable to handle changes over time.

    That's a good point about the specifications being fuzzy. I am not sure how the exact displacement will matter, but verifying the keel is actually 2200 lbs after I pull it off is a really good idea.

    I am going to replace the cast iron keel with steel. Easier to have fabricated and should be easier to maintain as it isn't porous. Any ideas on the damping effect of the keel sideways resistance to rolling? I was thinking an 8" to 10" deep keel.

    Thanks folks!
     
  8. bajansailor
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    bajansailor Marine Surveyor

    This is probably a silly question, but would it be feasible to simply chop a chunk off your existing cast iron keel?
    That might be easier than fabricating a new keel, and getting new bolts, and lining them up with the existing bolt holes in the hull bottom.
     
  9. Jeff in Boston
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    Jeff in Boston Junior Member

    Not a silly question!

    These reasons:

    1) The keel is 33" deep and I want to get it under 12".
    2) The weight is concentrated on the bottom. I'd lose more than 2/3rds of the weight.
    3) The keelbolts are so rusted I need to take it off anyways.

    I'll attach some pics when I'm not on my phone.
     
  10. DCockey
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    DCockey Senior Member

    Weight and measure the location of everything which comes off the boat. With that information two simple calculations will determine the total weight and CG of what goes on the boat needed for the boat to float on the same waterline.
     
  11. Jeff in Boston
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    Jeff in Boston Junior Member

    image0.jpeg Rusted keel bolts.
     
  12. Jeff in Boston
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    Jeff in Boston Junior Member

    It will be used for day trips to start, and if the wife and kiddo are comfortable, overnights and maybe eventually short coastal cruises.
     
  13. bajansailor
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    bajansailor Marine Surveyor

    Copied below is the GA for the 'cruising' version of the Pearson 26 - the hull shape is the same as yours, but the quoted displacement is 200 lbs more.
    https://sailboatdata.com/sailboat/pearson-26
    What are your thoughts about the rudder - are you going to build a new one, or cut a section off the bottom to make it slightly less than your shallower keel?
    Or maybe build a transom hung rudder?
    Then you could have the propeller for the inboard electric motor further aft, perhaps where the existing rudder is - and by doing so it could be a larger diameter than if it is further forward.
    Or are you planning on simply having an electric outboard motor mounted in the well on the transom?
    And then you would not need any rudder at all.

    I would still consider the pros and cons of cutting down your existing keel, and then fitting new keel bolts, rtaher than building a new keel.


    Pearson 26 GA from Sailboat Data.jpg
     

  14. Jeff in Boston
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    Jeff in Boston Junior Member

    The rudder sold on its own is worth more than the boat. I plan to sell it and steer with the outboard.

    I'm looking at the Navy 3 from https://www.epropulsion.com/navy-3

    For the keel, maybe I could bolt a bulb on the stub. My understanding is that cast iron drills easily, but doesn't weld easily. Lemme think on that.
     
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