Pontoon stern lift?

Discussion in 'Boat Design' started by jasonvi, Oct 30, 2016.

  1. jasonvi
    Joined: Oct 2016
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    jasonvi Junior Member

    I would deeply appreciate any advice from the experts here...
    I possess a large tri-toon pontoon boat for which the intended use is ferrying a 3000 lb stable load on inland lakes at relatively high speed (would like to reach 35 knots) & recreation. The original boat design was for a work barge.
    The current setup is:
    26' long, 36" diameter circular pontoons (0.125" thick aluminum). The center pontoon circular shape is carried all the way to the stern, whereas the outer pontoons have an integrated motor transom and the running surface flattens as a "cutout" in the final ~4'.
    Each outer pontoon has a 250hp motor mounted on a 10" deep hydraulic jackplate which gives 6" of vertical adjustment.
    The beam is 12' (18" between the pontoons) with "Sea Legs" mounted under the deck.
    At low speeds (up to ~18 mph) without any load, the bow rises like crazy and the stern runs with a draft of about 16". Increasing speed (without load) raises the bow more and buries the stern more.
    With a load on deck the boat performs well enough up to ~18mph without as much bow rise (it does seem to "level out" as if on plane above ~14mph). However, with increasing speed/rpm, the stern lowers to the mid-point of the pontoons (i.e. draft at stern is ~18") and the wake created between the pontoons is enormous, shooting out between the pontoons and trying to bury the motors.
    I assume that additional lift at the stern would improve the situation (I appreciate the limitations of round pontoons, but that's what I've got!). Various suggestions have included:
    1. Lifting strakes
    2. T.A.P. Fins
    3. Trim tabs
    a. Mounted between the pontoons at the stern to take advantage of the generated wave to generate lift as well as cut down on the spray
    b. Mounted on the outer pontoons in the "cutout", but with concern for affecting the water getting to the props
    4. Hydrofoil between the pontoons
    I'd really like to keep the SeaLegs, but they limit what can be mounted between the pontoons.

    I am unfortunately not an engineer or naval architect, so please accept my apologies in advance for my lack of expertise in hydrodynamics. I'd simply like the boat to "do better!" The attached photos are of the stern at ~18mph and at ~25mph (briefly!).
    Thanks for any advice...
    --Jason
     

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  2. messabout
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    messabout Senior Member

    The pontoons appear to have a pronounced amount of aft rocker. (Note to OP; rocker is when the ends of the hull....or hulls, turn upward toward the waters surface) That would be a plus factor for a boat intended to go slowly. That feature is a bad idea for a boat that is intended to go faster.
     
  3. JSL
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    JSL Senior Member

    messabout has made one point.. here's some more

    to start, you have 2 issues:
    1. For that kind of speed you need a planing hull form... not circular like tubes.

    2. Your 'hull' clearances (distances between tubes) is too small. They should be at least one hull width( 36")... you have half that.
     
  4. jasonvi
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    jasonvi Junior Member

    messabout: Would that imply that installing a trim tab in the aft rocker area of the pontoon would help? If so, would that be your suggested improvement?

    JSL: I'm unable to modify the distance between the tubes; therefore, do you think adding lifting strakes (thereby modifying the shape of the hull) would be helpful?
    Thanks,
    Jason
     
  5. messabout
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    messabout Senior Member

    Sorry to say that you have made one, as far as the boat is concerned, fatal statement: Originally intended as a work barge. There are vast differences in the design of a slow boat such as a work barge and a boat that is designed for rapid transport of 3000 pounds of cargo.

    You have some enormously expensive and competant propulsion units. Two times 250 HP is a lot of excessive power. The boat was obviously designed to use something in the region of 50 total horse power or less.

    Your most advised plan is to Keep the really good ( and expensive) motors and get a much different boat that is more likely to respond to all that power and come much closer to the 35 MPH that you wish for. .....Or go the other way with the pictured boat, and be satisfied with 12 to 14 MPH and ten times more fuel economy with a couple of 25 HP motors.

    Sorry to be the voice of doom but you did enlist the forum to get some more technical information.
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I'd be inclined to extend your central tube rearward, eliminating that chamfer/slice evident on the outside two. Maybe 3 feet longer. If that isn't enough, get rid of the rise at the rear of your outside tubes, completely, by filling it in with a wedge so you have a cylinder. Your jacking plate can be used to get the height down. Or just the "fill-in" for the two outside tubes might do it. What you have there are two heavy motors perched out there with bugger-all to support the stern underway. You could test a temporary fill-in of those chamfers with some high density foam glued on, and shaped so they are cylindrical tubes, provided the jacking plates can get the height right. But 35 knots really does sound too much to hope for, even 30 for a cruise speed seems optimistic.
     
  7. mydauphin
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    mydauphin Senior Member

    Consider hydrofoil between hulls to lift then out of water. Used these on a power cat successfully. I would check the pontoon hulls to see if they can take the pounding.
     
    kapnD likes this.
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Might help at speed, but not much help backed-off. It the thing is desired to be optimized for high speed, those chamfers need to go, but it might be difficult to get motor height needed.
     
  9. Barry
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    Barry Senior Member

    My first choice would be to add onto the area at the back of all the pontoons end to square up the ends. This will lift the back of the boat up. As pthers have suggested
    I have sent along a few pictures from Google Images, High speed pontoon boats

    Many of the boats do not have lift strakes but perform well. My neighbor has a 26 foot with a single 250 and does just about 45 mph. At 500 hp, you have lots to get this boat up to your desired speed.

    Then I would add on lift strakes to help the pontoons to provide more lift. Nice of the guy in the picture to show us the bottom of his boat, though I would rather run a set of strakes in such a way to in essence make the hull form more to planing than being pure round

    I would take a trip down your local pontoon boat dealer and poke around to see what they have then check the manufacturers websites as to the expected performance

    I would stay away from a foil as this takes more than a little bit of engineering and attaching them to .125 aluminum might cause problems with the attachment

    Not sure if I have ever noticed lift trim tabs on pontoon boats especially ahead of an outboard.

    Pict one: unusual orientation of strakes
    Pict two: round tubes like yours, obviously planing
    Pict three: Almost full length lift strake
     

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  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The simplest solution would be to make the aft portion of the outer tubes into a square bottom. This could be done by welding vertical flanks to the sides of the outer 'toons, with their bottom cut parallel to the centerline of the bottom of the round section. A triangular filler piece would then be welded in place to close up the bottom of the now squared off aft ~4 of the outer tubes. You wouldn't need to seal them in with an aft plate, just leave it open. At rest, this area will flood, but with good welds, no big deal, as they'll self drain, as soon as you touch the throttle. The end result would offer a bit more planing surface, of a more appropriate shape, better dynamics, etc.

    Agreed, that is a typical displacement speed 'toon workboat setup and not intended to go much past low double digit MPH speeds. The faster you push it, the more it'll trim down aft. Cranking the trim in all the way will help initially, but as speeds reach the mid teens, no amount of trim will help and the bow will progressively come up with speed increases.

    The center 'toon can benefit from the same treatment, offer even more bearing area and dynamic lift, so you'll get up on plane faster, easier and be able to hold it at lower speeds. Without these changes, 35 MPH is just a dream. Lastly, what is the weight of the boat? Workboats tend to be made much heavier and excess weight is a speed robber in any kind of boat. With the changes I've suggested, you should be able to easily reach the mid to high 20's, depending on how heavy she is. A bit like the rough sketch shown.
     

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  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I really can't see how you are fairing that box section into the round tube. :confused:
     
  12. PAR
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    PAR Yacht Designer/Builder

    The somewhat truncated as shown forward area, would bend it pretty easily with aluminum. Some scissor in hand time at the kitchen table, with a Christmas wrapping paper tube, will make it easy enough to sort out.
     
  13. ElGringo
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    ElGringo Senior Member

    Wouldn't two pontoons 36 inches in diameter support the 3,000 pounds of cargo? Can you remove the center one? It looks like it is trying to drag half the lake with it captured between the pontoons.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Lifting the load isn't the issue. The problem is the dramatic upturn to the aft portion of the 'toons, which causes the boat to be moderately efficient at displacement speeds, but really sucks in full plane mode.
     

  15. jasonvi
    Joined: Oct 2016
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    jasonvi Junior Member

    Thanks for the great feedback. The dry weight of the boat without rigging is 3500 lbs. If I eliminate the chamfer and convert the aft portions of the toons to a square bottom, should I also try to reduce the setback of the jack plates (that is, if I can't eliminate them entirely)? They are currently a 10" setback model, but also come in a 6" setback...do you think the 4" difference is of any significance?
    Also, how far forward would you suggest carrying the flattened bottom?
    Thanks again,
    Jason
     
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