Converting to RIB

Discussion in 'Boat Design' started by jambo1967, Dec 30, 2008.

  1. jambo1967
    Joined: Dec 2008
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    jambo1967 New Member

    I am considering converting a 20' speed boat with a cuddy cabin into a RIB. Could anyone offer any advise & possible pitfalls I may encounter?? Thanks
  2. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    Are you planning on cutting the hull down so the tubes are close to the water? If not you could just use a D tube at the top with screws into buried inserts for emergency flotation.
  3. jambo1967
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    jambo1967 New Member

    I was planning on cutting the hull close to the water
  4. cap
    Joined: Feb 2009
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    cap Junior Member

    I would suggest cutting the hull so that the tubes touches the water when lying dead in water and when in planning, they should be clear of the water (calm water of course). If the tube sits too low (typical Zodiac design), one can experience chine walking and also easier rupture of sponsons (>40 knots). Do you have a picture of the boat? Do you want to glue it to the hull or use a rail system? If you will glue it, the best thing is to mold a little lip from the hull to the sponsons.
  5. jambo1967
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    jambo1967 New Member

    thanks I can cut the hull close to the water - i would then plan to mould a flange around for attaching the ring. Obviously I would be removing a lot of weight so concern will be stability - will this be a problem - i can measure the hull & produce an autocad drawing
  6. TollyWally
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    TollyWally Senior Member

    I'm extremely interested in your project. Please keep us posted on your progress with lots of pics.
  7. jefflee2k
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    jefflee2k Junior Member

    why not? tubes.htm

    Glad to see I'm not the only one who thinks this is possible. I started a similar thread a couple of days ago. The links above are for companies that do rib replacements and custom ribs for new boats. The guys at Gemini also sell innertubes that you can insert into existing tubes. I am considering this option and constructing my own outer tubes or salvaging the sponsons from a used soft hulled inflatable (sib). Also check this forum for rib enthusiasts:

    As for my project, I am not cutting down my freeboards. I am going to sacrifice the extra weight for the confidence of maintaing the existing balance and handling. I plan to mount the sponsons so that they rest on the water when the boat is at rest, and clear the water except near the transon while on plane.

    For tube attachment, the following was taken from a thread on the rib site: "The tube set on my boat simply bolts on, there is a flap of material at the bottom of the tube running the length of the sponson, this flap is through bolted to the boat using clamping bars on the outside and a flange on the hull which is drilled out about every 6 inches. It takes about 1 1/2 hrs to bolt them down or remove them. This is an effective and simple way of securing them to the boat, any manufacturer could do this if they wanted to."

    Good luck on your project. I look foward to your progress.
  8. jefflee2k
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    jefflee2k Junior Member

    d-shape tube pic.jpg

    Mechanicaly attached pic.jpg

    These are diagrams from the link above. Each shows one of the two methods I am considering for this project. I am also in the process of deciding between air in the form of bladder style inner tubes and EPS foam. It looks like the price will be about the same for each. I can get custom made tubes for about $1000us, and there are a number of foam companies that supply custom shaped foam for about the same price. Or, I can shape it myself for less. I am even experimenting with making my own tubes from large truck/tractor tubes. Cutting them straight and liquid vulcanizing the ends closed.
    As for the foam, densities between 1 and 2lb/ft3 are common for marine flotation. The lighter foam is cheaper, more buoyant, and obviously weighs less, but is less durable. The is a product that is used in construction for epoxing over this type of foam to make it rigid. So that may be an option before placing it inside the outside tube.
    For the tube collar I am going to use 40mil pvc or a new product fro Firestone called EPDM (Ethylene Propylene Diene Monomer) in 45mil. I am waiting on a sample, but this product is similar to pvc in flexibility, and also easy to patch and work with. However it is more abrasive resistant, less affected by temperature, and much more UV resistant.
  9. Submarine Tom

    Submarine Tom Previous Member

    Sounds Good To Me


    Thanks for clarifying. I think you're on the right track then.

    Sounds like a great project.

    I would copy Zodiac style tubes. They typically use five cells with cones

    separating each. As I recall, a 30' boat will get about 12 000 pounds of

    buoyancy out of them. The only Zodiac I've ever heard of sinking had a

    crane fall on it puncturing all the tubes.

    Good idea to enlarge those drain holes/tubes. Self-bailing is a wonderful

    thing if you can keep the boat moving. You may want to read up on driving

    such a boat in big water as some of the techniques are quite counter-

    intuitive. There are some good RIB magazines and probably good web sites

    too. I wonder if you may need to beef up the hull where the tubes will join.

    It would be a shame to bust it open in ugly seas. Your attachment diagram

    looks good to me but that's not really saying a lot because I've never

    designed one, only maintained an existing one (Zodiac 810, 733 and a

    Mark V). The tubes on a RIB are right out of the water when stopped and

    are only involved at low speed negotiating BIG water.

    Try Googling "RHIOT Bamfield BC Canada", should be some good photos

    and perhaps some helpful links. I was a student there in January 2000.

  10. jefflee2k
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    jefflee2k Junior Member

    Looks like a blast. I wonder if there is anything similar for civilians?

    I think I will reinforce the hull with strips of epoxy coated 1/2" marine

    plywood on each side of the fiberglass where the tubes join. This should

    distribute load the along the length of the hull.
  11. Submarine Tom

    Submarine Tom Previous Member



    Load distribution is important. You may want to consider stepping your

    plywood reinforcing and/or bevelling the edges. By stepping I mean using

    thinner sheets and making each layer wider than the last so the load is

    distributed more evenly onto your existing hull. Bevelling the edges would

    add to this effect too. Otherwise, you could end up with a failure where the

    plywood abruptly meets your hull. If you're set on reinforcing both sides,

    you may want to consider staggering the alignment for the same reason.

    I'd go narrower on the outside and wider on the inside.

    Stagger your steps too, inside from outside.

    RHIOT was a blast. I don't know of any civilian schools but the info's got to

    be available. You may gain a huge amount of confidence with your new

    seaworthness in your modified boat and (no offence, I'm sure you're a very

    capable mariner) suddenly find yourself in over your head (literally).

    As a Transport Canada certified tow boat captain with years of experience

    in 2000, I thought I had a pretty good idea how to handle rough seas.

    RHIOT school showed me otherwise. Pontoon RIB's allow a whole new

    dimension in boating that would scare most mariners.

    Do you own a dry suit?


    P.S. Personal message me with your email if this isn't clear and I'll send

    you a drawing of the load distrubution (I can't post pic's on this forum).
  12. jefflee2k
    Joined: Jul 2009
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    jefflee2k Junior Member

    I think I am going to go with the "D" tubes similar to the ones shown in the diagram from a previous post. Since my tubes will be completely outboard this will cut down on resistence. In a traditional RIB, only half of the tube is outside the hull. This should limit force acting on the hull connection and increase fuel efficiency. I also think that foam is the best choice because of the "D" shape. Its can't take the beating of air, but it should be much less maintenence.

    Tom, I like the idea of staggering the reinfocrement. I think the only reinforcement on the outside of the hull will be the laminated backing on the tubes themselves. Foam half-rounds mounted to 1/2" epoxied ply. This wrapped in tube material. Use stainless bolts (the heads oriented inside the tube next to the foam) passing through the tube backing and hull into the reinforced bracing. Place the bolts in a 2-1-2-1-2 pattern (like the dots on a 10 domino) to also stagger the load on the hull.

    No, I don't have a dry suit. No need so far. Winters here are faily mild. I've gotten by with neoprene waders and a waterproof coat. All my diving has been in warm water where the wet suit was more for buoyancy and abrasion protection. Kind of a wuss when it comes to the cold.:D
  13. jefflee2k
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    jefflee2k Junior Member

    Thoughts on tube material?

    Anyone have ideas on tube material. Since I am going with foam, I am not concerned with air retention properties. Here are my thoughts so far:

    PVC -
    easy to work with
    glues very well
    very short life span (uv)

    Hypalon -
    Very good abrasion resistence
    tends to stiffen with age (chemically, this is actually strengthening)
    easy to work with and glue (not as easy as pvc)
    VERY expensive ($60 -$100/yd)

    Polyurethane -
    Common in foam tube applications
    similar properties to Hypalon
    cheaper to produce (theoretically)
    I can't find a supplier

    EPDM -
    A rubber material with good a abrasion and excellent uv resistence
    very cheap
    100% water proof
    unreliable glued seams
    This is the common material for rub strakes on high end ribs

    There are a few more new products like EPDM. One is made from recycled tires. But all are rubber composition and difficult to glue. There is a cold vulcanizing compound for bonding rubber, but I don't think it will work with the epoxied backing.

    Side note: My son is getting brace$, so if anyone can help me avoid Hypalon I will literally be in your debt.
  14. jefflee2k
    Joined: Jul 2009
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    jefflee2k Junior Member

    unique boat design pics

    June 09 phone 2 022.jpg

    June 09 phone 2 024.jpg

    June 09 phone 2 020.jpg

    June 09 phone 2 018.jpg

    As someone familiar with the handling characteristics of a rib, how do you think tubes in this configuration would work for my application? The yellow boat is a close up of the flared bow of my boat. Since I am using a D shaped tube with a rigid backplate, the sharply flared sides are causing some problems. I can't mount the tubes flush to the boat in the bow section. I was thinking about shortening the tubes like the ones in the white boat, or using a thick piece of rubber as a bushing or gasket to help the tubes conform to the shape of the boat.

  15. Submarine Tom

    Submarine Tom Previous Member



    The distance between me guessing how this boat would handle and you

    taking it for a test drive is huge. I don't know that I could even guess.

    I can't see a water line so I don't even know where it sits in the water.

    I do see a motor on it. What are the chances of the owner taking you out?

    Of interest; we had a 34 passenger whale watching boat sink here off Maine

    Island yesterday. They claim to have hit a dead head at speed (probably

    30 - 35 knots) and it sank within 25 minutes in 100 feet of water.

    Their sister boat was able to transfer all the passenger off without incident.

    Only the captain got wet! Google it if you're interested as it was a "Zodiac

    style" pontoon boat but, obviously, one with insufficient floatation. I'll bet

    the captain was surprised when he saw it sink. Someone got cell phone

    footage of her going down.

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