Adding a Third Pontoon

Discussion in 'Powerboats' started by sundeckbentley, Mar 10, 2008.

  1. sundeckbentley
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    sundeckbentley Junior Member

    I just realized I posted this under Powerboats again......will post this to Design as well.

    I posted this question under the Powerboats section so I thought I might want to post it here as well.

    I am new to this forum and if my wee 24 ft pontoon boat is out of this leagues forum, please let me know.

    I am adding a custom sun deck to at Bentley 240 Cruise pontoon boat that has a 90 HP Merc 2 Stroke outboard. I want possible add more bouyancy to the boat so am thinking of adding a new log in the center. I do not plan to have an integrated transom on the log....just a 16 to 20 foot log the same diameter as the other two to add bouyancy.

    I have heard very mixed reviews about altering a boat in this fashion...from its a great idea to its a horrible idea.

    My thoughts are that if the third toon is the same diameter as my other two logs and it is positioned close enough to the outboard I should see at least the same performance (speed) but have more stability and capacity. I have heard the third log "interupts" the flow of water and might partially aspirate the water reducing efficiency and performance.

    Anyway, the other two popular pontoon forums I visit have very wide ranging answers to this question, so I was hoping folks here could give me a more definitive answer from a design perspective. I think there is a right way to do this.

    Thanks for your help.
     
    Last edited: Mar 10, 2008
  2. edjunior
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    edjunior Senior Member

    Well, I think anytime you start trying to change "capacity", this would be something you would have to have certified (by the manufacturer or the CG probably). You can't just add something and say you now have increased the capacity. This is done at the design and building phase of the boat. Technically you probably would be able to increase the capacity, but legally?? I kinda doubt it. I expect there are other considerations that goes into the calculation of the capacity than just the number of "logs".
     
  3. charmc
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    charmc Senior Member

    Edjunior is right about pleasure boat capacity. Regardless of any custom alterations, the capacity plate, representing a formal review of the design and supporting data by the USCG, states the maximum capacity of the vessel. You did say you want to add bouyancy, but the implication is that you're looking for that because you're increasing the load. Exceeding the nameplate capacity while operating the boat is illegal and would make you de facto liable in the event of any injury or accident. There are ways to alter the capacity legally, but it would probably be less costly to just buy another boat.

    I'm a little confused as to your plans. I understand a pontoon boat to be a floating sun deck; so I'm uncertain how you add a sundeck. A more detailed description would help generate constructive feedback.
     
  4. charmc
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    charmc Senior Member

  5. KCook
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    KCook Senior Member

    I'm pretty sure this sun deck is another name for a hard top. Which has its own railing and stairs so it is accessible to guests. ie, a 2nd deck.

    Kelly
     
  6. charmc
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    charmc Senior Member

    That's a pretty major refit, with a major change in CG.

    Bentley announced a new 3 log model recently. Probably a good idea to run any plans by them.
     
  7. sundeckbentley
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    sundeckbentley Junior Member

    Done the Haitian

    Yes, and have done the Haitain a few times.
     
  8. sundeckbentley
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    sundeckbentley Junior Member

    Correct

    It is. Sorry about the confusion. It's a deck you can climb on via a ladder.

    My main reason for posting this question is how the boat will perform when adding the third log in front of the existing transom. As you can see from the previous post I have pretty good capacity on the boat already. Adding bouyancy is a secondary benefit but I do not plan to go over or use the boat in any way that violates the plate. I'm adding about 450 everyday lbs spread over 11 ft from stern to mid.

    Thanks for your responses....perhaps this sheds a little more light.

    Dan
     
  9. timothy22
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    timothy22 Junior Member

    Dan, you are venturing into dangerous territory

    Not saying you shouldn't do it, mind, just be aware that if your hard top is full width and a half dozen curious folks go to one side along with a dozen folks on the same side below them you may not have enough buoyancy in the back half of the log under them to keep from tipping over. a 30 inch diameter log 10 feet long will support 4000 pounds when fully submerged, and a dozen people could easily weigh half that, just in folks, let alone the weight of the boat. Imagine how the boat wll look with one log nearly under.
    Still, it can be done if you keep the width of the sun deck to 6 feet or so- just wide enough for seats, feet, and a little walking room between. The third log in front of the outboard might do more harm than good. It would affect the prop, and where you need the buoyancy is out wide and aft. You may want to consider a rectangular aluminum box say 18" wide under the deck each side next to the 'toons and just deep enough to be clear of the water at rest, ~10 feet long and shaped like a water ski in front. Leave an inch or so of room for shims between the boxes and the deck so you can correct the running angle at speed if needed, and be sure your guests won't mind if it does tip.
     
  10. sundeckbentley
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    sundeckbentley Junior Member

    The capacity is definitely something that has to be managed. 99% of the time it is my wife and 3 kids on the boat, which are all small currently.

    The folks fabricating the deck are going to test the boat once its done to see how the dynamics has changed. These guys fabricate Fire and Rescue boats as well as work and utility boats in mono hull and multi hull configurations up to 50 ft in length. He is confident my boat will maintain the extra weight and the center of gravity changes.

    It currently has a 3000 lb capacity with 25" toons but I understand that that much weight up high can really affect the boats ability to stay upright in extreme situations.

    Thanks for the reply.
     
  11. timothy22
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    timothy22 Junior Member

    You bet it can--

    the sneaky part is that as the boat tips, the weight on the edge of the wide top deck actually is out over the water, so as it is pushing the toon down, it is also lifting the opposite side up, thereby transferring even more weight to the partly submerged 'toon. Beyond a certain point they just go all the way over. That is the reason for the narrower top deck-to keep the weight inside the center of buoyancy of the 'toon under it.
     
  12. SamSam
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    SamSam Senior Member

    The cavitation/ventilation plate on the motor has to be at or just below the surface of the water so you would probably have to lower the motor to bring the plate to the bottom of the new log, which is where the water will be at speed. That might open up the possibility of the engine itself getting swamped, when not at speed, unless you have a long shaft motor, 25" or more.
     
  13. sundeckbentley
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    sundeckbentley Junior Member

    Thank you SamSam. I appreciate the feedback. I think you are correct and will probably go the route of adding more bouyancy to the inside of the toons, or even outboard the toons.
     
  14. tom28571
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    tom28571 Senior Member

    I just got around to reading this thread and see that a major factor has not been addressed by anyone. There was a question about the effect on stability of adding a third pontoon hull in the center of the boat.

    It should be obvious that this will reduce the lateral stability, maybe dangerously so. At the same time, building a second level that will be accessible to passengers raises the CG significantly and also reduces lateral stability. I don't know whether the boat will then be unsafe to do what you want but I would be very concerned about it. You should get someone who can run the numbers to check this out. My suspicion is that you will not be happy with the answer.

    At first blush, this sounds like a bad idea.
     

  15. Ike
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    Ike Senior Member

    Not to disagree with the previous posts here, but the capacity of pontoon boats is not regulated by the Coast Guard. See http://newboatbuilders.com/pages/load.html So you would not be violating any Federal regulations. It is an American Boat and Yacht Council standard. Manufacturers use the ABYC standard to calculate the maximum capacity which is partially based on load but also partially based on stability. The manufacturer then puts a capacity label on the boat which looks just like the label required by the CG but does not say US COAST GUARD MAXIMUM CAPACITIES on it.

    Also, the manufacturer will certainly no longer be liable for any capacity or stability problems you may have. Additionally, in some states it is a violation of state law to exceed the maximum capacities on the label. So if you exceed those you could possibly be cited by a state water cop.

    Adding a third log will add some capacity but not any additional stability. Adding an upper deck will definitely change the stability of the vessel. You need to do capacity test based on stability to find out what the capacity should be with a second deck.

    The ABYC standard - H-35 Load Capacity for Pontoon Boats is not free. See http://www.abycinc.org. However the Canadian Regulation is based on the ABYC standard and only slightly different and it is free on line.

    In Canada these are covered in TP1332. http://www.tc.gc.ca/BoatingSafety/regs.htm
    Section 4, Hull Design Requirements.

    Sub Section 4.3 covers Capacity and flotation for multihull vessels, that is, pontoon boats.

    The Basic idea is you place weights on one side of the boat until the water covers the top of the pontoon on that side.

    The following is from the course I teach on capacity and flotation:

    Add weight to the pontoon boat until all pontoons are awash over their entire length.

    Mwc = Maximum Weight Capacity =

    Total Weight – Boat weight/Square root of pontoon diameter - 2.35 (1.35 for three log pontoon boats)

    Persons Capacity By Test:

    For transverse stability: Place weights for engine, battery and fuel tank in the location of each item. Add weight on one side of the pontoon boat evenly distributed fore and aft and as far outboard as practicable, within one foot from the edge of the platform, until the top of the platform on the loaded side is awash. Repeat this on the other side. Record the weights for both side.

    For longitudinal stability: (fore and aft) Add weights on the platform, with the center of gravity on the boats centerline. They shall be placed 1/4 of the length of the deck from the forward end. Add weight until the forward edge of the deck becomes awash. Repeat this with the weights placed 1/4 of the length of the deck form the aft end of the deck. Record the weights.

    The smaller of the weights in the transverse stability test or the longitudinal test shall be used to determine the Maximum Persons Capacity. The maximum persons capacity is 90% of the weight.

    Example

    Transverse tests Port = 2500 lbs Starboard = 2550 lb (1020.6 kg)
    Longitudinal tests Forward = 2600 lbs Aft = 2650 lb (1202 kg)

    The lesser of the above is 2500 lbs. 90% of 2500 = 2250 (1020.6 kg)

    I think that putting a second deck on it without performing a stability test is foolish if not negligent. And if you do, and determine how many people can be up there, you are going to have to constantly police the number of people on the upper deck. So I would give this a serious rethink.
     
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