Adding to keel

Discussion in 'Boat Design' started by kapnD, Dec 14, 2021.

  1. kapnD
    Joined: Jan 2003
    Posts: 972
    Likes: 206, Points: 43, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    I’d like to extend the existing keel on this 50’ vessel, primarily to reduce roll in rough seas, secondarily to protect the propeller.
    My plan is to build it upside down with a 6” flange to attach it to the hull using epoxy paste, then glassing over the flanges, and finally filling it with ballast. The shaft will run wet through a fiberglass tube, and existing strut/bearing will be left in place.
    It will probably require some mechanical fasteners initially, but I’d like the primary attachment to be a chemical bond. Will this method of fastening be adequate to support the keel and ballast, or is thru bolting necessary? D3D3E4B9-59C4-426F-AB74-67CA7763599F.jpeg
     
  2. bajansailor
    Joined: Oct 2007
    Posts: 2,660
    Likes: 985, Points: 113, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

    Here is a slightly closer view on the keel / propeller area -

    KapnD lobster boat keel extension proposal.jpeg

    I know that there are some amazing adhesives available nowadays, but even so, I would be rather dubious of a new keel built as you described above with no mechanical fasteners, just a bonded joint.

    Apologies for being dense here, but I think this means that you will build the keel itself upside down - or do you intend to turn the boat upside down?
    If the latter was possible then that would make life much easier for sure!
    If keeping the hull the right ways up, you are going to be asking a lot of that 6" flange re bonding it to the hull with epoxy paste.
    And re glassing over the flanges, you are talking about overhead laminating - it is possible, but not much fun, and pretty difficult really, with always the worry of the laminate drooping re gravity.

    Re mechanical fasteners, bear in mind that most sailing boat hulls nowadays have bolted on keels - none of them are simply 'bonded on'.
    And they seem to work ok :)

    If you did not need the extra side profile area of keel to resist rolling, and you simply wanted to add extra proptection for the propeller, then instead of the existing 'P' bracket you could perhaps have the propeller shaft supported by an 'A' Bracket (with fairly widely spaced arms for strength and stability).
    And then have an additional strut below the 'A' bracket connected to a fabricated keel (maybe a rectangular box beam or channel section?) bolted to the existing keel (?)
    The box beam keel could be extended aft of the propeller to provide a landing for a new rudder, instead of having a cantilevered spade type rudder as shown.
    You could perhaps add some extra strength to the arrangement with a fabricated pillar or skeg forward of the rudder, connecting the keel extension to the hull above (?)
    The system described above would still be vulnerable to sideways loadings - or if you run aground at speed, the aft end of the keel, being the deepest, will come into contact with the ground first and put a huge bending moment on the joint where the keel extension is bolted to the hull, as it has a very long (relatively) lever.
     
  3. kapnD
    Joined: Jan 2003
    Posts: 972
    Likes: 206, Points: 43, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    I’m definitely not considering flipping the boat, I was thinking to shape the part from foam, then fiberglass it on a bench, upside down.
    I have considerable fiberglass experience, and I know how to tab a part upside down, but if thru bolted, that should be minimal anyway.
    It was suggested by a commercial boatbuilder that I build the keel out of steel plate of a sufficient thickness to yield the desired amount of ballast, but I’m not anxious to add steel components under a fiberglass hull.
     
  4. Will Gilmore
    Joined: Aug 2017
    Posts: 704
    Likes: 292, Points: 63
    Location: Littleton, nh

    Will Gilmore Senior Member

    Unless you expect water intrusion and a circuit connection to ground and close proximity to a more nobel metal, an enclosed steel keel should present no issues.
     
  5. wet feet
    Joined: Nov 2004
    Posts: 866
    Likes: 175, Points: 43, Legacy Rep: 124
    Location: East Anglia,England

    wet feet Senior Member

    That is my feeling also.I would think a good size box section,attached to the keel at the forward end and ideally attached to the P bracket would be a good start.Such things are fairly common locally on hire boats that may be lodged on mudbanks on a falling tide.It isn't unusual for the rudder to have a lower bearing attached at the aft end,which can be unbolted if dropping the rudder is necessary.On this particular specimen that is less of a requirement as there is a hole for the shaft to pass through.
     
  6. gonzo
    Joined: Aug 2002
    Posts: 15,769
    Likes: 1,196, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Adding ballast to the aft end of the boat will change the trim. It may end up making the boat squat and reduce speed and economy. Have you considered bilge keels? They are one of the traditional methods to reduce rolling.
     
  7. leaky
    Joined: Sep 2008
    Posts: 207
    Likes: 13, Points: 18, Legacy Rep: 14
    Location: nh

    leaky Senior Member

    I don't know why your boat behaves that way but if you are going to add a real keel. I assume this is a fiberglass hull with no core? Sorry if I missed a detail in a quick read..

    #1 I would look at pictures of full keeled downeast boats, like the one pictured below and modify to mimic the style - it's a much better setup than what you've got there (which is really goofy, or at least not what I'm used to seeing)..

    #2 skip the ballast, create and add a hollow keel. Since it's not going to be part of the hull other than the perforation where the shaft comes through on the inside, I'd probably fill it with foam after putting the shaft tube in place but prior to tabbing it onto the hull.

    #3 i wouldn't get all concerned with how it's glued onto the hull, I would grind into the fiberglass and make about a 1/4 inch ramped divot for the tabbing about 10 inches wide (deeper by keel and a sharp edge, shallower running out), I'd glue it on there temporarily using whatever resin system you are gonna tab with thickened into a glue, use jacks to hold it up square, let cure, run a fillet, tab, fair, paint etc.

    21319-194c8c4fd00b51f40de6909161cf5ce7.jpg
     
    bajansailor likes this.
  8. Will Gilmore
    Joined: Aug 2017
    Posts: 704
    Likes: 292, Points: 63
    Location: Littleton, nh

    Will Gilmore Senior Member

    The OP's boat is rolly because it's essentially flat bottomed and shallow drafted. The waves pickup the sides more easily because of this. It rides the tilting surface of a passing wave.

    The keel on that boat is for tracking, preventing slippage when turning or in the wind or on a wave. It probably doesn't provide ballast or reduce rolling. A hollow or foam filled keel won't stop rolling either, even if it is full. It might work to put wings on the keel so the keel traps water over the wings to help reduce the boat lifting unevenly by a wave. Heavy, low, centerline ballast is the only other solution, that I can see.

    -Will
     
  9. leaky
    Joined: Sep 2008
    Posts: 207
    Likes: 13, Points: 18, Legacy Rep: 14
    Location: nh

    leaky Senior Member

    Will I'm sure you know what you are talking about but there are a whole lot of boats built in Maine, of all sizes and varied shapes but similar designs, that essentially are like that but do pretty well in a sea - better than deep v's and such generally just not as fast or as efficient going 20+ knots anyway. Semi displacement hulls.

    I can't see from the OP's pic what his bottom really looks like, if at rest for instance midship it's deeper and if it narrows at the stern or not. But if you put a full keel on it, how does it greatly differ from these sorts of designs?

    20211215_233235.jpg 20211215_233223.jpg
    20211215_234455.jpg
     
  10. Will Gilmore
    Joined: Aug 2017
    Posts: 704
    Likes: 292, Points: 63
    Location: Littleton, nh

    Will Gilmore Senior Member

    Don't make that assumption.
     
  11. DogCavalry
    Joined: Sep 2019
    Posts: 2,125
    Likes: 1,075, Points: 113
    Location: Vancouver bc

    DogCavalry Soy Soylent Green: I can't believe it's not people

    Lol

    And doubly so if I'm talking!:oops:
     

  12. kapnD
    Joined: Jan 2003
    Posts: 972
    Likes: 206, Points: 43, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    Thanks all for the replies!

    The hull appears to be a Downeast design, and is fairly round at the bottom and bilges, clearly not a planing hull.
    I can’t imagine why the keel ends where it does, but it’s not to improve speed , as the boat just doesn’t have the horsepower or bottom configuration to go over 12 knots or so.
    In rebuilding the boat I removed nearly a ton of superstructure from the stern cockpit area, so I’m not afraid to add some weight back there.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.