Ballpark speed reduction amount of small or full (lobsterboat) keel on planing hull

Discussion in 'Powerboats' started by Northeaster, Sep 12, 2013.

  1. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Hi Folks,

    I will be building the Glen L 23' Doubel Eagle in Aluminum this winter, and have a question around the amount of speed reduction I could expect if I go with the a small (outboard design) keel or full inboard design keel, when I build using an inboard, shaft drive setup.
    For example, a guy on the Glen L site stretched the boat to 25' and gets about 20mph with a Mercruiser 3.0l and 2:1 reduction gear (16" prop). He went with no keel, and therefore has no protection for the prop, along with some other issues in backing ,etc. If he added a small (say 5" deep keel, tapered to nothing at the bow) what would be your guess as to the effect on his speed?
    Same thing for a deeper, say 20" keel (tapered to flush nearing bow) that completely protects prop and is used to connect as skeg for rudder as well. How much do you think this would slow it down?
    Here is a link to design with pic of keel if anyone has a minute to look and give their thoughts, please!

    http://www.glen-l.com/designs/workboat/dsn-dblla.html
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Wants some keel imo, that boat is quite flat bottomed aft. I suppose much depends on where the boat is going to be used, if running aground and damaging the propellor is going to be a major incident, well away from help in a remote area, I'd sacrifice speed for peace of mind. On the other hand, if running aground is unlikely and help is near at hand if you do, and you don't intend to go surfing down big swells that might broach the thing, something more modest would be in order.
     
  3. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    Thanks for the reply!
    I will be using it in the Atlantic, in Mahone Bay, Nova Scotia. There are 365 islands here though, so it is not open ocean. (It is open if we go about 8 miles out, past the farthest large island)
    Definitely common to have 15-20kt wind and 2 - 4ft chop. I know most of the obtacles around here, but will likely opt for the full keel for rudder/ prop protetcion, and the ability to dry it out between tides, and make trailering easier (just twice a year to put in and out). I do want to expore some other posts too, that are lower water levels.

    I did want some idea though of how much drag or speed loss a keel causes.
    Although my future boat may do 20+ kts, around here there are only a few days (and early mornings) here there isn't some chop, so I think most time would be spent at an average speed or maybe 12- 15 kts, or it may become too uncomfortable quick. But, I am normally a sailor, so I could be wrong.
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    There was another thread started here recently by a guy wondering how to get more speed out of his inboard diesel planing hull fishing boat that had a monstrosity of a full keel fitted, as a rule I think you have to accept that there is a considerable penalty in performance loss for such boats with a full keel, which would be much less the case if it was a slow displacement hull. But you get the maximum protection for the propellor and rudder. So it's a compromise situation.
     
  5. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    You might try asking your question on the Downeast Boat Forum www.downeastboatforum.com I've seen discussion there of the effect of skegs/keels on the speed of boats with similar hull shapes.
     
  6. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    It depends on your powering. If you put a 50hp diesel on it, you will notice a bit of speed loss and a slight reduction of fuel economy. If you are running more than 100hp, I doubt it would affect speed, just economy and perhaps prop selection. The hull looks like it was designed for about 18 knots. It isn't going to go much over 20 knots no matter what unless it is completely stripped down weight wise. How were you planning to power her?
     

  7. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    lookin gat a Cummins 4BT (130HP)right now, but that deal may fall through so will be still looking.
    Read yesterday that another guy n the Glen L site gets 25 kts with the 4BT, and full keel. But, as you say, you may really start to sacrifice fule economy above say 18 or 20kts, not sure.
    I hope to be normally running 12-18 kts.
     
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