Airplane Pontoon Hull as Trimaran base?

Discussion in 'Multihulls' started by Scotty Wisley, May 2, 2009.

  1. Scotty Wisley
    Joined: May 2009
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    Location: Papua

    Scotty Wisley Junior Member

    I am not a boat guy and I can't get any help/response on my questions about making a trimaran with a pontoon from a float plane....maybe everyone thinks I am crazy...actually, lots of people do say that so...
    If you have some advice I'd love to hear it before I make a decision to launch into this project.
    I've got 1 pontoon off a float plane that I can use. It was designed to keep about 750kg off the bottom and hydroplane to help the plane lift off. There is a small "step" right near the rear as well to break the suction.

    Cash is very short, I live on the island of New Guinea so going shopping isn't much of an option. My thought is to make a trimaran which would be 15 ft long and about 12-14 feet wide. I am thinking sail eventually but first mount a 15hp outboard on it. The pontoon weighs about 60lbs as is. It is about 2.5 feet high by 2 feet wid, fairly bulbous up front tapering down to nothing in the rear. I like it cause it has 5 sealed compartments that can each be pumped out.

    Here are my thoughts. Please get out the red pen!
    Outrigger mounting.
    There are two well positioned mounting brackets that go through the pontoon for the wheel strut from the plane. They are positioned about 5 feet back from the front and 3 feet from the rear with me cutting off about 6 feet of the rear of the pontoon. If I anchored a 4-6 foot length of 3 inch pipe (Alum or Zinc coated) centered in each mounting bracket. (really solid mounting bracket as it is designed to hit the water at 60plus mph I think)Then with 2.5 inch piple inserted into this continue out to the outrigger. This will slide inside with a pin to hold it in position when on the water but able to pull the outriggers off when transporting or slide em in to narrow the boat. Rough width would be about 12-14 feets?
    Outriggers seem to me to be pretty key. Most easily available material is PVC and I was thinking of 4 or 6 inch pipe bundles of 3 pipes for each side with one on the bottom and two center above it. Idea being that when the boat gets up to plane the outriggers come up with it and minimize contact with the water but provides stability in turns or waves...I have no idea if this is enough. Not much of a math guy either. The way the pontoon is designed is that it sits low in the water at the rear and high at the front and as speed increases the back comes out and it levels. So, the front of the outrigger will sit 10 inches higher than the rear until it gets speed. Wanted to use 3 pipes in case I break/crack one and to give strength while keeping the flexibility. Other thought was compartmentalizing one into 3 sections.

    Deck. Was going to get a 4x8ft sheet of alum diamond plate, cut it in half and put 2x8 on the outrigger pipes on either side of the hull/pontoon making a 6x8ft deck.

    OUtboard. Mount on the rear outrigger pipe/deck to one side of the hull/pontoon. 15hp is already going to max out my bank account but would like to pull the kids on a tube/wakeboard.

    Any advice

  2. peterAustralia
    Joined: Mar 2006
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    Location: Melbourne Australia

    peterAustralia Senior Member


    Sounds like a cool project. Which Island are you from, does not assist in your technical questions, but would be nice to know.

    To your pontoon, the weight seems low, so that is good. Using the existing supports to mount crossbeams seems the best way to go, as they would be natural strongpoints. The one at the front sounds a bit close to the front, but ought be OK. The step, well I think that would only assist in reducing drag at such high speeds that will never be obtained, so if it could be faired away that would be good.

    You can use 2 outriggers, yes, but realise that you can use one outrigger if you need to. This is called a tacking outrigger. I am biased as I have one.

    Ahh, I see your boat is a powerboat, not a sailer, well that changes things. Your use of three per side sounds ok, remember to curl them up at the bow so they do not dig in, I guess you will need ropes to get them to curl back.

    In some ways what you are describing is a banka. This is the traditional small craft of the Philippines. Where the outriggers are very light bamboo. Doing some background on bankas, may be a good idea. Maths is not needed, just trial and error

    750kg sounds ok volume. I have seen some PNG craft where the deck is raised above the float by 8 inches or so. I would not think that the deck need extend all the way out to the outriggers. I would think 5ft of width would be plenty.

    I assume you have a lot of timber out there in PNG, so using wood/bamboo for crossbeams or additional outrigger volume would be an option?

    Do any of the locals still make boats? Maybe paying one of them 15 kina a day for work and advice could work well.
  3. Scotty Wisley
    Joined: May 2009
    Posts: 6
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    Location: Papua

    Scotty Wisley Junior Member


    I live in a small village in the highlands called Bokondini and we are on the Indonesian side of the island. I am about 1 hour by small plane from the coast and about 4-9 hours by truck to the closest town in the highlands, Wamena. It is 50 miles away. Bokondini is NorthWest of Wamena. We need to get out of here at least twice a year with the family to get a good break. Idea is to go and camp at some isolated beaches and surf for 2-3 weeks at a stretch. We work in education and training with young people here.

    Thanks for the positive feedback on the potential here.

    I spent 4 years in the southern philippines... Zamboanga, as a boy and always messed with the little boats there and here as well. That is where I got the idea was looking at the local rigs and then looking at this pontoon.

    I was thinking that the deck would be max of 6 feet wide but the width from one outrigger to the other would be around 12-14 feet... that way I would have 3-4 feet of outrigger with no deck over it... try to keep everything centered... guess that the volume I'd need in the outriggers is dependent on the weight distribution as much as anything else?

    One thing we have done up here with PVC pipe when we don't have the right connectors is heat... could I heat the pipe and curl/bend the front up a bit or does that damage the pipe to, make it brittle? better to use rope maybe?

    From your experience do you think I should go with 3 4 inch or 6 inch pipes for each outrigger? Or just keep adding each time I flip over?

    There is lots of wood available. I am just trying to stay as light as possible and as maintainence free....

    thanks for the input and would appreciate any other thots or advice!

  4. ThomD
    Joined: Mar 2009
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    Location: TO

    ThomD Senior Member

    I would go with 4 inch there is virtually no difference in weight and it has lots more stiffness and bouyancy. I don't know if it is enough, which depends on your beam and other factors. Basically you have to think of the worst off-center load like someone standing on a shear and figure if the bouayncy of the pipe is greater than the person standing on the shear, accounting also for the fact the center of bouyancy of the floa is further outboard. Similar kind of idea as ballancing on a seasaw, when you have different weights. For the power part of that, there won't probably be much in the way of off center loads, but for the sailboat part, it is a whole other thing.

    The step on your hull is possibly a problem as it will create drag. There was a fast cat built to race across the atlantic on step hulls, but it was vastly powerful, in essence the power needs to be capable of planing the boat. But hey, you have your hull. Unless you figure the drag could affect your performance and get you into trouble, you should be fine. Should be ok for fair weather use.

    I always liked this Banca:
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