To keel or not to keel

Discussion in 'Boat Design' started by rfleet1066, Mar 6, 2012.

  1. rfleet1066
    Joined: Feb 2012
    Posts: 137
    Likes: 10, Points: 18, Legacy Rep: 46
    Location: New Kent, VA USA

    rfleet1066 rfleet1066

    My new vessel is still on blocks. She is a 61 foot pontoon vessel. The hulls are round. Should I be looking at welding on a keel? I once piloted a kayak without a keel and it was miserable. It would be easy enough at this stage, but if not necessary...very cool. I have seen other pontoon designs that have no keel. Surely someone here know the whatfore.

    Ryland
     
  2. Ad Hoc
    Joined: Oct 2008
    Posts: 6,510
    Likes: 560, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Keels/skegs are usually added for 1 of 2 reasons, or both.

    1) Fir protection/support of the prop and hull. (Hull if/when grounding)
    2) Directional stability.

    With seeing pictures of your hull and knowing what your SOR is....I doubt anyone, unless familiar with you and your boat, can say for sure. Since sometimes it can be a personal choice too, just a whim!

    If you added a skeg and too much and in the wrong location..it would make handling much worse..beware.
     
  3. rfleet1066
    Joined: Feb 2012
    Posts: 137
    Likes: 10, Points: 18, Legacy Rep: 46
    Location: New Kent, VA USA

    rfleet1066 rfleet1066

    directional stability

    I have good hull integrity and don't think I need more structure there, but directional stability is certainly something to be desired.

    Ryland
     
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 6,510
    Likes: 560, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Is the boat you building known to have directional stability issues?..or other boats very similar to yours...if unknown...that makes the issue more difficult without seeing any profile views
     
  5. Submarine Tom

    Submarine Tom Previous Member

    Nah, don't bother.

    Two, 61 foot pontoons are going to give pretty good directional stability.

    -Tom
     
  6. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 474, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    A "61 foot pontoon vessel" sounds like a powerboat, likely a displacement mode, powerboat. In this case you could probably benefit from small appendages, commonly called skegs, which could be seen by some as "keels". They will offer directional stability (it'll track in a straight line) and can offer some prop protection. As John has mentioned, their placement is fairly important.

    Since we know very little about this design, further speculation about adding things isn't possible. How about a better definition of your 61' boat. General dimensions, weight, power, anticipated speed, expected uses, etc.
     
  7. rfleet1066
    Joined: Feb 2012
    Posts: 137
    Likes: 10, Points: 18, Legacy Rep: 46
    Location: New Kent, VA USA

    rfleet1066 rfleet1066

    keel?

    She's a 61' pontoon vesel, 25' beam, 22" draft, 8 knot hull speed, stern wheeled.

    The hulls are finished, but still solid modeling and stress testing the connections.

    Ryland
     
  8. souljour2000
    Joined: Aug 2009
    Posts: 481
    Likes: 13, Points: 18, Legacy Rep: 195
    Location: SW Florida

    souljour2000 Senior Member

    By and large...when someone comes on this site with something like you have...pics are a huge plus and even more importantly are more likely to engender a more helpful informed response from one of the professionals in here...i'll say it...Pics please...if no pics..then schematics/blueprints/etc. are just as good...
     
  9. rfleet1066
    Joined: Feb 2012
    Posts: 137
    Likes: 10, Points: 18, Legacy Rep: 46
    Location: New Kent, VA USA

    rfleet1066 rfleet1066

    photos

    Noted and logged. Will try to post photos. Hey yall watch this.

    Ryland
     

    Attached Files:

  10. Ad Hoc
    Joined: Oct 2008
    Posts: 6,510
    Likes: 560, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Now seeing your hull piccies, this is good advice.

    How is she powered, outboards I assume?...if so when the outboards are on the stern, how much distance is there from underside of the hull to the bottom of the outboard?
     
  11. Tad
    Joined: Mar 2002
    Posts: 2,303
    Likes: 185, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    It's tricky, is directional stability (and predictable maneuverability) more important than minimum draft, less important, or equally important? The amount of windage added by a deckhouse will control how much keel you need to counter that windage with.

    To me that pontoon ship looks like she'll be just as happy going sideways as forward. A paddle wheel is mentioned as propulsion, is speed (and thus expected maneuverability) low or high? What sort of voyages are to be undertaken? IE you need better maneuverability to safely enter and exit a crowded marina in a crosswind on a regular basis. On the other hand a semi-controlled cross drift may be just fine for the intended use.

    Most of the old stern wheelers were flat-sided barges amidships, that (relatively) deeply immersed chine provided some directional stability and resistance to cross drift. And they had big ganged balanced rudders (which could be viewed as directional skegs) forward of the wheel. You don't have that immersed chine with these round tubes.

    I happen to like boats that remain in control even while drifting. If you can afford the draft increase I would add a keel starting perhaps 30% of waterline length aft of the bow. Maybe 3-4" high at the forward end and 6" at the stern.
     
  12. rfleet1066
    Joined: Feb 2012
    Posts: 137
    Likes: 10, Points: 18, Legacy Rep: 46
    Location: New Kent, VA USA

    rfleet1066 rfleet1066

    Good advice

    Tad, that's good. It's a small thing to add a keel when in drydock. I tried a kayak without a keel some years ago and felt as if I had a football bat. I am drawn to your suggestion. Thanks.

    Ryland
     
  13. PlaningWheel
    Joined: Nov 2010
    Posts: 78
    Likes: 2, Points: 0, Legacy Rep: 5
    Location: Canada

    PlaningWheel Junior Member

    This vessel will track like it's on rails because of it's length to beam ratio and the fact that there's no camber to the hulls.
    Because the bow and the stern are fully submerged a great deal of water has to be moved sideways at the bow and stern to turn this vessel.
     
  14. Village_Idiot
    Joined: Oct 2007
    Posts: 382
    Likes: 14, Points: 18, Legacy Rep: 138
    Location: USA

    Village_Idiot Senior Member

    I'm with Tad on this one. The boat will track fine as long as the wind isn't blowing. Add a little wind and she will start to crab sideways, especially at low to zero speeds...
     

  15. Tad
    Joined: Mar 2002
    Posts: 2,303
    Likes: 185, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    Another thought is just weld a big angle (equal) or channel on the bottom centerline with the open side against the tube. That would be nice and strong when ashore........
     
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.