Would adding a keel increase low speed turning

Discussion in 'Boat Design' started by Diver down, Dec 14, 2017.

  1. Diver down
    Joined: Dec 2017
    Posts: 2
    Likes: 0, Points: 1
    Location: USA MN

    Diver down New Member

    This is a 23' boat made by Kann with a 150 hp on the back. For the most part it's a flat bottom boat, a very slight V. It's used as a dive boat, and the problem we're having is that at low speeds it's got no control. We're looking at putting a 3-inch keel on the boat hoping it will improve turning at 2-4 mph Any thoughts would be appreciated.
     
  2. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 145, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    Adding a keel certainly improves low speed turning. The penalty of course is reduced turning capabilities at high speed.
     
  3. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,760
    Likes: 273, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    That is puzzling, unless the boat is particularly nose-heavy. Normally boats of that kind have a fair bit of submergence of the sides, and track well at lower speed, rather than wander around. Does the problem manifest itself in calm conditions ?
     
  4. jorgepease
    Joined: Feb 2012
    Posts: 1,614
    Likes: 46, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

  5. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 471, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I disagree in that low deadrise boats track well at low speeds. Most modern planing boats track like crap at low speeds, though the "Orca" ventilation plate does help a little. Instead of spending a few hundred on one, just put some 90 degree bends in a hunk of 3/16" - 1/4" aluminum plate and give it a try. If you'd like to keep most of the top speed on your 150 HP outboard, make it about 18" wide, between the bends and bend it up, not down. This will still help tracking, but less drag at speed. It's mounted just like any ventilation plate might be.
     
  6. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,760
    Likes: 273, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Most minimal deadrise boats already have a "keel", they need it to provide some lateral plane to control broaching tendencies, while planing. That "keel" on an outboard-driven boat, which is likely to be no more than 3" deep, combined with the near vertical sides being immersed, is less inclined to wander than deep vee boats, in my experience. There is minimal topside immersion with a deeper vee, and the gentle slope of the vee is not as effective as a vertical surface at maintaining a track. Most fishermen who have done a lot of trolling offshore would probably agree.
     
  7. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 471, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I can show you dozens of production boats with sea skiff, Garvey, Jon or shallow V's without any keel or skeg. Low speed maneuverability isn't a design concern for most, as they expect them to blast along on their patch.
     
  8. rwatson
    Joined: Aug 2007
    Posts: 5,839
    Likes: 276, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    This is very vague. what is "low speed", what is "no control" ?

    If you're talking 1-3 knots, of course there will be "no control". You have to have a water flow to act on any control surface, and a largely flat bottom boat will be like a Canadian canoe, blown around by every breath of wind.

    And what does "no control" mean. Putting the wheel over 45d doesnt induce directional control ? Of course it wont, the motor is a pretty inefficient rudder at low speeds.

    Can we get a bit more specific info before sounding off with suggestions?

    For dive boats, preventing being blown around by the wind, and making life hard for attempted boarding, it may be as simple as a 5ft length of plywood, hung over the side like a lee board near the bow, during minimum power application, to control direction into the wind and waves.

    Putting a 3" keel the length of the boat could have major performance implications in high speed turns.
     
  9. fallguy
    Joined: Dec 2016
    Posts: 2,141
    Likes: 143, Points: 63, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Sounds like you are corking to me. That's what I call it.

    I have experienced this problem with boats that have too much weight for'd. The "no control" you cite is probably more like a constant oversteering state?

    If so, a keel would do ZERO.

    Try moving some weight to the back of the boat before you do anything else. And make sure you aren't overloaded. If you are overloaded, a keel won't change that. When you slow down; you are no longer on plane and unbalanced loading really can affect the boat. I have this vision of 6 divers for'd with gear anchors, benches, and your driver slows down and is struggling with the helm because the boat is essentially like a fishing float.

    If you are simply slipping sideways; follow the rudder advice. But I find it hard to believe you have that much slip at slow speeds.

    Also have to say, the problem is a little undefined to give you great advice. Another fellow said, vague. I tend to agree.
     
  10. Diver down
    Joined: Dec 2017
    Posts: 2
    Likes: 0, Points: 1
    Location: USA MN

    Diver down New Member

    Thanks for the input guys. This is a search and Recovery boat that spends 95% of it's time running sonar between 3 to 5 knots. It's always loaded down with divers and gear and very seldom do we get it over 20 knots. Even though it's capable of 30. We're looking at options to increase its handling at slower speeds. We're okay with losing some performance at higher speeds but would still like to remain safe.
     
  11. fallguy
    Joined: Dec 2016
    Posts: 2,141
    Likes: 143, Points: 63, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Are you looking for better straight line tracking for the sonar?

    That is different from handling.

    Is the helm always unresponsive, or only when divers are onboard?

    Steering system?

    Are you overloaded?

    Even a prop change could help you.
     
  12. rwatson
    Joined: Aug 2007
    Posts: 5,839
    Likes: 276, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    So, to answer the question - IS IT the 3- 5 knots range thats the problem ?

    IS IT wind or wave induced ? - (because I bet it doesn't happen on dead flat water)

    Can you post a photo of the underside of the boat, and another of the trim in the water fully loaded ?
     
  13. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 471, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The common solution to low speed handling on dive boats here (Florida) is a fin, maybe two or the "Orca" thing, which is easily home made. The basic problem is the boat isn't very well suited (shaped) to tracking at these speeds, without a huge drag penalty doing higher speeds. If you can live with losing a couple MPH or three at the WOT, use the Orca style of ventilation plate and you'll notice a significant improvement. With it installed she's still going to be what she is, but your "chassing the wheel" efforts will be diminished. It's an easy thing to make from aluminum or stainless and any reasonable metal fabricator can bend one up in less than a half hour. Aluminum is easier to work, cut and drill for the guy in his garage. For your needs, make the fins point down, I know I said up before, but this was before I heard about your general use and operation. In this position it'll shed lines and can offer some prop protection too. If you'd need a drawing, I can provide one if you'd like.
     
    fallguy likes this.
  14. JosephT
    Joined: Jun 2009
    Posts: 841
    Likes: 98, Points: 28, Legacy Rep: 218
    Location: Roaring Forties

    JosephT Senior Member

    Diver Down, based on your original description it appears are are describing a Kann work barge that has been adapted for use as a dive boat. It's about as close to a floating cardboard box as you can get. We feel your pain. I'm a scuba diver myself and have seen other pontoon & flat boats used as dive boats. Steering performance is just bad at low speed. The driver of the boat has to be very skilled and know the vessel or you could be in trouble.

    Since an outboard is present a larger, custom fin on the lower unit might help. Any extra rudder authority you can get will be a plus. Also, I do see various bow thruster configurations for flat work boats & pontoon boats so those may be worth a look

    Google search: pontoon "bow thruster"

    I like the look of this particular unit as it can be mounted in line (less drag) and is less risk for divers with the embedded impeller (no open props to endanger divers & gear).

    Exturn Bow Stern Thrusters Found At Boat Show https://wn.com/exturn_bow_stern_thrusters_found_at_boat_show

    With the dive boats it's all about getting in/out of the docks safely and precise positioning over the reefs at slow speed. This allows dive crew to tie up to the mooring ball below and then you're all set.

    Happy diving!

    Ref: Kann work barge (with crane)

    [​IMG]
     
    fallguy likes this.

  15. fallguy
    Joined: Dec 2016
    Posts: 2,141
    Likes: 143, Points: 63, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Take PARs advice. A keel is the wrong idea.
     
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.