riverbus hull rebuild

Discussion in 'Boat Design' started by captainstick, Aug 26, 2021.

  1. captainstick
    Joined: Nov 2013
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    captainstick Greg

    A few years ago I made a post on this forum (Can my boat go faster? https://www.boatdesign.net/threads/can-my-boat-go-faster.60759/) and some of you guys were very helpful. Thank you for that! So I did indeed rebuild my strange boat with your suggestions. I tore it all apart and put it back together again with two 30ft long hulls as opposed to the three 20ft hulls I had on there previously. The first time back on the lake, I clocked her top speed at 19.6 mph. I never thought it would get over 15, so I was very happy to see that. Here's what it looks like after the rebuild...
    [​IMG]
    [​IMG]

    Anyway, a few years have gone by and I again have the itch to tear apart my boat and rebuild it. Haha. Seriously, there are still some nagging issues with the hulls that I want to fix. First, these plastic hulls are very prone to leaking. I have had them perfectly sealed, only to develop new leaks after a few more trips bouncing around on the trailer. I think it might be all the plastic welding that makes the plastic a little more brittle each time I heat it up. But I’ve also had a few leaks caused by rocks on the bottom of lake powell as I beach it, as well as a few bonks launching and retrieving. Since the hulls have extra buoyancy and are filled with foam, it’s not a huge problem to get these leaks, it’s just that I end up carrying hundreds (thousands?) of pounds of lake water around with me. It’s mostly annoying, but I also don’t feel comfortable launching in my local lake until I let her sit and drain for a month or more due to the invasive mussel issue out west where I live. Secondly, I’m not convinced the current hull shape is the best shape for my craft.

    So my idea is to create new hulls that are shaped similar to a modern catamaran type hull. My tentative plan is to pull the current hulls off - they are just bolted to the frame above. Then I will cut the plastic off and add some new flotation foam to the existing foam to reshape them a bit, and then wrap the foam in fiberglass. My specific ideas for the new hulls are as follows…

    1. This is my main concern and most of the reason why I'm seeking some expert guidance. I want to add about 4-8” more freeboard by vertically extending the hulls, but keeping the same width. There is some bow slapping in rough conditions as I only have about 10-14” of clearance (loaded vs unloaded). I’ve read that catamarans are generally designed with a clearance of 5% their length. So that would be 18” for a 30ft hull. Should I be worried about stability issues raising the boat that much? It feels very stable currently, but raising the cog concerns me a bit. The beam is 8.5ft so I don’t have to mess around with oversize permits while towing. My estimate of the current cog is about 4-8” above the deck (loaded vs empty). It's a little difficult to know the cog of the vw bus on top, but I do know that the bottom vw frame has a lot of steel that I never removed and all the seats weigh a lot, which are all on the bottom half. I guessed the cog of the vw to be about 1/3 from the bottom, so about 2ft above the deck. I guessed the current weight of the vw to be 2500lbs (3500lbs factory curb weight minus engine, suspension, wheels etc). I have a lot of tanks - 80g water, 50g waste, 140g fuel - installed below the deck, so the cog goes down as I load it up. Overall the boat weighs about 7000lbs empty, 9000lbs loaded. The total buoyancy of the current hulls is about 20,000lbs.

    2. I'd like to change the entry at the bow so that it is more like an axe bow like many catamarans.

    3. I was thinking I should reconfigure the stern so that it has a little more support for the engines, with some rocker. I like being able to trim my props above the bottom of the hulls. There are lots of opportunities to destroy props in the lakes that I visit.

    4. I am also thinking I will diminish or remove the flat planing or lifting sections about 1/3 of the way up on the current hulls. I think it does help my top end speed, when the hulls aren’t full of water, but I’m not sure it’s really the shape I need for the best fuel efficiency, which is a little higher priority.

    5. I’m thinking about epoxying some aluminum sheet onto the bow sections and bottoms to add a little extra protection. I don’t see any other boats with this, so is there some reason people don’t do this? The extra protection seems like it would be worth the effort and weight. Would extra fiberglass in those areas be a better strategy?

    What do you guys think? I appreciate the help - and jokes if they are funny.
     
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Nice done Capt.stick :)

    So, in answer to your Qs.

    1. Yes, 5% of the Lwl is a minimum, thus increasing freeboard shall achieve this. Thus, how does it influence the stability?...well, in the absence of any real hard hull data, you can conduct a simple experiment.
    With the VW on the boat, add around 1.00 tonne of 'stuff'... could be bags filled with water or whatever... ontop of the VW's roof. That can simulate the same effect, by increasing the KG to reduce the stability. She how she feels in such conditions. If you feel safe....there is your answer.

    2. NO!... that's a red herring. Leave the bow roughly as she is.
    The axebow shape is just sales hype....which it seems many accept blindly.

    3. The stern, as she is, is the totally wrong shape, for the speeds you are now achieving. Your current Fn is over 1.0!
    So you need to have the transom to be roughly 60% depth of midships, and the end to be a vertical cut off or close to it (allowing a slight angle for your outboards).
    You can also improve the resistance by changing the chine to a round bilge from roughly 2/3rds to transom, as shown:

    upload_2021-8-27_7-40-27.png

    4. See #3.

    5. Ok, protection against what? As that shall dictate what to do or not to do.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    If there are thousands of pounds of water that has infiltrated into the hulls, then any repairs would have to involve getting rid of the soggy foam. Without an audit of what is actually the case, you can't proceed. The idea of "removing" the plastic hull material, leaving the foam which you then glass over, is fraught, to say the least, if water has corrupted that foam, and probably not the greatest of ideas even if it hadn't.
     
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  4. captainstick
    Joined: Nov 2013
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    captainstick Greg

    Cool, that helps a lot. Thank you very much for taking the time to help guide me.

    As for the ? Re No. 5, I tend to run into a lot of sandstone while beaching the craft at lake Powell. It’s unintentional, but they are generally hiding under a thin layer of sand. Seems like some aluminum sheet under the keel at the bow would be like a wear layer. Doubling up the fiberglass is probably a good idea too, but then the glass will get abraded quickly if there is no other protection. Not sure that’s a big deal, but I’ve never beached a glass boat so I’m not sure what the consequences would be after a few hundred beachings.
     
  5. captainstick
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    captainstick Greg

    The foam drains for a month or so after I find a leak. But yeah, I’m a little concerned about how much water is left once I get the plastic off. It’s very dry where I live so the air will undoubtedly suck the moisture out of the foam if I let them sit in my barn for a few months. I’m mostly trying to avoid paying for $5k in foam and dumping the existing stuff in a landfill.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    You might look at a flat-bottom punt instead, and put some more cushions on the seat of the Kombi to absorb any bumps in choppy water.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Ok...so, does it currently cause your boat an issue, do you get localised sections of the hull exhibiting ware/erosion?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    When you say "plastic", is it polyethylene ? That is good for abrasions and bumps, but for water to have infiltrated, have there been splits ? If so, it sounds like the material was too thin for the rigors imposed on it. Especially if it had the foam backing it.
     
  9. captainstick
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    captainstick Greg

    Yes, the plastic gets a lot of abuse on the keel of the bow sections. The nice thing about the plastic though is I can just weld some new plastic on every season and build it up again. I’m just concerned if I switch to fiberglass I’m going to have bigger problems if I keep beaching it. It’s too difficult to camp at lake Powell without beaching. It’s possible of course, it just takes more time to find spots that will work.
     
  10. captainstick
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    captainstick Greg

    Yes, it’s polyethylene. And yes the leaks are usually small cracks that appear on or near the welds. I mean there’s a car on top of them, so it’s probably user error haha. Like I said earlier there have also been a few punctures that caused leaks. Again, user error. But I think fiberglass would be a lot stronger and I would also add some vertically oriented plywood into the foam right below the 2 beams that the vw is welded on to. So I think that should help support the load much better than just having the foam/plastic as it is now.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Then it sounds like sacrificial rubbing strips/plates are what you need then.
     
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  12. Mr Efficiency
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    Mr Efficiency Senior Member

    If it was just abrasions, welding on some PE strips would be an obvious fix, but it seems the structure is a little underbuilt. Or, perish the thought, subject to "excessive operation" ! :eek:
     
    Last edited: Aug 26, 2021
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  13. captainstick
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    captainstick Greg

    Haha, yep
     
  14. philSweet
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    philSweet Senior Member

    You can add protective aluminum if you want to, but be sure to protect that with a wear-resistant epoxy coating:). All the aluminum airboats use epoxy to protect against sawgrass abrasion. The generic search term is "abrasion resistant overcoating".
     
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  15. captainstick
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    captainstick Greg

    Cool, thanks for that tip
     
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