Center Pontoon Log Position

Discussion in 'Boat Design' started by Demian Miller, Feb 17, 2021.

  1. Demian Miller
    Joined: Feb 2021
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    Location: Tampa, FL

    Demian Miller Junior Member

    I have come into possession of two 22' x 24" round pontoons and a larger (mismatched) elliptical center toon. The center toon is 26' long (including integral transom) and is 32" wide x 25" high. The outer toons have full length inboard lifting strakes and the center toon has lifting strakes for about the first 16'. My goal is to use these to construct a reasonably "fast" (~25 mph) tiny house boat with an 8'6" beam.

    For floorplan reasons, I am considering mounting the bow of the center toon 2' - 4' behind the bows of the outer toons. This will put the transom of the center toon 4' - 6' back from the ends of the outer toons. The center toon transom is rated for 250hp but I would most likely power it in the 150hp range. I will also drop the center toon a few inches below the outer toons to help keep them higher out of the water at speed.

    I have seen threads that discuss having the center toon protrude slightly forward of the outer toons, but I have not been able to find a discussion of pulling the center toon back behind the bows of the outers. I have some flexibility in placement of various tanks and house batteries to adjust the center of mass fore and aft and I can work out a budget for the superstructure weight, but I don't have an understanding of how the hulls will interact or a good way to field test and adjust once the boat is built and launched.

    I would really appreciate some thoughts on how the boat could handle. Are there likely to be serious speed/efficiency penalties incurred by placing the bow of the center toon aft of the outer toons? Potential unsafe handling issues in turns?

    Pontoon Tinyhouse.jpg

    Operating in protected waters (Tampa Bay, freshwater lakes, rivers, intracoastal waterway, etc.) for day trips and weekend camps.
    If I had to guess typical operating weight will be 4,500 - 5,000 lbs.
    --Toons & deck 1,200#
    --4-stroke outboard 500#
    --Cabin & furniture 1,000#
    --Fuel, water, batteries 1,000#
    --4 or 5 People 800#
    Buoyancy with 1/2 of tubes submerged should be ~8,000 lbs.

    Thanks in advance,
    Demian Miller
     
    Last edited: Feb 17, 2021
  2. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Welcome.

    Pictures, drawings, anything visual would help.

    And, what are you going to do with it and in what kind of waters? SOR (Google it)

    What will the final weight be?
     
  3. Demian Miller
    Joined: Feb 2021
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    Location: Tampa, FL

    Demian Miller Junior Member

    Good advice. I have updated the original post with a sketch, weight (guesses) and operating info.
     
  4. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I think that your estimate of buoyancy available with the three pontoons (approx 8,000 lbs) is in the right ball park - however I think that your weights estimate is probably a bit optimistic.
    It would be worthwhile doing a more detailed weight estimate, and then adding on a 20% margin for 'extras' that you have not thought about.
    And weight is especially critical on a pontoon boat, as you ideally do not want to be loaded down to half the diameter of the round pontoons - you want to keep something in reserve.
    And I also think that it will be very optimistic to expect a speed of 25 mph from a 150 hp O/B motor that is pushing 4+ tons through the water.

    Re the position of the centre pontoon - intuitively it does not seem to be a very good idea to me to have the bow of this pontoon aft of the two outer hulls - if anything, I think it would be better to have it forward of the two outer bows.
    I think that you might get some 'interesting' wave patterns if the middle bow is further aft than the outer bows. No scientific basis here, just a gut feeling.
     
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  5. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    Your dimensions don't add up.

    You said the outside tubes are 22' and center tube is 26'.

    This puts the center tube 4' further back even if the the tubes are even in the front.

    It's probably not a good idea to reduce buoyancy in the bow of a style of craft that tends to not do well in large waves to begin with.
     
  6. Demian Miller
    Joined: Feb 2021
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    Location: Tampa, FL

    Demian Miller Junior Member

    Thanks for the catch. If bows are even, transom will be +4. If center is set back 2-4’ aft, transom will be +6-8’ (not +4-6’ as originally stated). Any concrete thoughts on how center log position affects hull interactions?
     
  7. Barry
    Joined: Mar 2002
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    Barry Senior Member

    google tritoon boats you tube and you will see that the front of the three toons will be out of water so that it will make little difference if the front of the toon of the center is SLIGHTLY ahead of the outside toons
    Ie the three toons will be meeting the water at the same longitudinal measurement

    Re should the middle toon be further in front. Again, Google tritoon boat manufacturers and look at the pictures. Most have the center either slightly ahead or even with each other.
    BUT one had the center a bit back. But as the engagement of all three will occur at the same spot, it will not matter at the speeds that you are thinking about

    Albeit having the center toon at a different height can make the engagement measurement different re your quote below

    Before playing around with toon heights, it would be worthwhile for you to spend a masked Saturday and visit a few tritoon dealers to see what their height is.
    If the consensus is the same height, I would probably stick with it. Would there be a tendency at higher speeds and the center toon to be carrying most of the weight due it lower immersion
    to become a little unstable and "chine walk" or in this case "toon walk"?
     
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  8. ondarvr
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    ondarvr Senior Member

    It's during the slow speed cruising that the pontoon being back a bit will have an affect. With less flotation at the bow it will be more likely for water to flow over the deck.

    And what is the advantage of having the center pontoon sticking so far out the back?
     
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  9. Demian Miller
    Joined: Feb 2021
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    Location: Tampa, FL

    Demian Miller Junior Member

    The advantage of positioning the center toon further aft is to increase LOA of the boat for a roomier cabin and walk-around access to the engine, mooring cleats, and sun-deck ladder (see sketch above). Some of this deck structure would be cantilevered out from that part of the center toon that protrudes back beyond the outer toons, but I've got a buddy that welds aluminum so all things are possible.

    I could shift the cabin forward but I'd like a bit of a "front porch" and am sensitive to having too much weight over the bow. I guess this is the inverse of the buoyancy issue many folks have keyed in on...

    I could also tuck the sleeping compartment further forward into the main cabin but then it would overlap with the lavatory so I'd need to go with a narrower mattress which would likely end with me sleeping at the dinette. 10' beam was another option I considered, but that triggers oversize load permits and lots of trailer mods.

    All that said, it is probably unrealistic to expect a rigorous engineering answer to a question with so many (unknown) variables. Since I lack the capacity to do the actual math on this, what I'll do is tie the two outer pontoons together with a dozen or so 2 x 4's and then test run the boat with the center log at various positions while taking notes about speed and handling. I can also get the exact weight of the test rig and get a pretty solid estimate of its center of mass along the keel. I only have a 50hp available now to test with, but the test rig should be about 1/3 the weight of the finished boat, so maybe that's OK...

    Without a deck in place, I should also be able to get some good video of how the wakes interact, where the hulls engage, and other pertinent details at various running speeds that I can post for the good of the order. It will be a few months since a couple of other projects around the house are ahead in the queue but maybe I can get it done in time to recycle into the kid's next science fair project ;-)
     
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  10. ondarvr
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    ondarvr Senior Member

    I would build it so the center pontoon is bolted in place, but has several options on its position.

    This way it doesn't require any advanced testing, you can build the entire boat and do real world adjustments.

    What you will probably find is that adjustments to height and position don't create a big difference at these lower speeds.
     
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  11. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    I too think your weight estimates are a bit low.
    There are so many overlooked items.
    Provisions, wiring, plumbing, etc, etc, etc. add up at alarming rates.

    Then there is the total buoyancy available.
    ~8600 pounds total buoyancy in the two toons.
    ~6900 pounds total in the single, centre hull.
    So, about 15 000 pounds total buoyancy available.
    Multiply that by 1/3 gives ~5000 pounds of usable, design buoyancy.

    Addressing your handling question:
    I don't believe you're going to get 25MPH out of 150hp outboard.
    So long as you don't go too low on that centre hull, I think your handling will be fine.
     
    Last edited: Feb 18, 2021
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  12. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    DM,

    I hope we haven't confused and discouraged you.
    I was going to add more depending how you responded.
    You haven't responded.

    I can appreciate you want to use the three pontoons
    although for 25MPH you're going to need at least a 225hp outboard.
    And there's going to be nothing economical about it.

    I have a 25' 12 000 pound houseboat with a barge hull (10' beam).
    It does 5MPH with an 8HP and 6 MPH with a 25HP outboard at 60% throttle.

    Why do you want to go so fast when your house is travelling with you?
    I'd suggest you lower your target speed by 80% and use the 50HP you already have.

    BB

    EDIT: Getting back to your original concern: I would split the overhang evenly on each end between the centre toon and the outer two.
     
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  13. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Would it be feasible to add an extra section of elliptical tube to the middle of the centre hull to make it a bit longer?
    Although this could be quite difficult to fabricate - you would have to get the two halves of the ellipse shape rolled to the correct shape, and then weld them together, and then weld them into the hull.
     
  14. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Now there's an idea I hadn't considered, Bajansailor.
    It would be a good opportunity to add tanks+ too.
     

  15. Demian Miller
    Joined: Feb 2021
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    Location: Tampa, FL

    Demian Miller Junior Member

    Not confused or discouraged, but thank you for asking. I've more-or-less concluded that the question of wave interaction between the toons will best be resolved through experiment using a test rig. Whether in 2, 3, or 6 months, once I've collected some data, I'll dig up this thread and post it.

    As for the 25 MPH goal, it's just a ballpark based on wanting to be able to get from my house in Tampa, FL out to the ICW and barrier islands without spending the day in transit. My neighbor has a 24' tritoon with a 150 HP that readily gets 35 MPH. Suzuki and Yamaha have tons of boat tests on their websites showing 150 HP engines able to get 24' pontoon boats into the mid 30's. Of course the neighbor's boat and the engine test boats are lighter than my target weight and no doubt have less wind resistance.

    Sort of academic at this point since the current 50HP will be doing the work for the foreseeable future. It *might* get the stripped-down test rig fast enough to make use of the lifting strakes but will no doubt push the built-out boat through the water like your barge ;-)
     
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