Shanty Boat Hull Modification Questions

Discussion in 'Boat Design' started by Bing, Nov 12, 2022.

  1. mc_rash
    Joined: Aug 2020
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    Location: Netherlands

    mc_rash Junior Member

    As bajansailor said you can set it up in a straight line, with the keel extended to the "new" stern. Perhaps you want to cut the skeg a little bit away to improve waterflow to the propellor when turning (see the transparent part at the end of the skeg). This setup would not be too difficult to build, for you since you did a complete conversion to the interior of this boat.
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  2. Bing
    Joined: Jun 2006
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    Location: New Hampshire, USA

    Bing Junior Member


    Yes, I can build a straight angled bottom extension, no problem.

    You will definitely have less drag at displacement speed, but you will also lose some buoyancy at the back, which is probably a good thing, as you ideally want her to trim slightly down by the stern.

    Why would I want her to trim by the stern? When sitting still, the sheer is almost completely level with the surface of the water, which means the surfaces of the bunks, tables, and counters are level, which is a desirable thing. It is a houseboat after all. I'll do some calculations to see what buoyancy I'd lose with the rockered bottom compared to the flat bottom extension. I want to carry a little more fuel, will have a larger engine, and will have increased storage back aft, plus a larger deck equals more people hanging out there.

    If you do not give her some rocker in the side elevation, then you will have a lot more turbulence behind that transom, which equates to more drag / less speed.
    Re if you would pick up a couple of knots of boat speed - I think that would depend on how much power you decide to install.
    I'm not particularly looking for more speed, but if having rocker at the stern gains a bit, I'm all for it. The NY State Canal speed limit is 10mph on most sections. It's all I can do to go about 6mph now. I want to repower to a 40hp motor from the existing 30hp motor, and may look for a hi-thrust model. Extending the waterline by about 3ft should help too. All that said, any way to know what I should end up with for hull speed?
     
  3. Bing
    Joined: Jun 2006
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    Location: New Hampshire, USA

    Bing Junior Member

    Does this look about right? Turns out that the existing center skeg is only 3.5" deep, not the 5.5" I mentioned earlier, and it ends 42" from the stern, so it's no wonder that it wasn't effective. I've shown a 7" skeg made by attaching an additional 1.5" x 3.5" timber below the existing skeg, then sheeting the assembly in 1/4" ply and glass and resin.

    [​IMG]

    [​IMG]
     
  4. mc_rash
    Joined: Aug 2020
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    Location: Netherlands

    mc_rash Junior Member

    I would try to keep your boat untrimmed after the extension wether you extend it with a flat bottomed or angled stern. As it is a houseboat I would prefer to sleep on level ;)

    With the angled extension you would earn about 350 - 400 kg extra displacement which gives you enough space for more fuel, a larger motor - and your desired increase in deckspace.

    I would suggest the following:
    Extend the hull and keel to the aft but let the keeldepth as it is right now (3.5'' as you mentioned in your last post). Put your boat in the water, make some trials. If there are minimal or no changes in directional stability take here out again and make some changes to the keel (increase the depth by a few centimeters). This would be iterative, could be more work than needed, and maybe not possible due to taking her out of the water again; but I think the extended hull does contribute to the steering handling and in my opinion the keel incl. skeg will be good enough.
    Intuitive I would say after hull and keel extension without changing the depth you will have a directional stabler boat.
     
  5. Bing
    Joined: Jun 2006
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    Location: New Hampshire, USA

    Bing Junior Member

    Well, I suppose I could make a slightly deeper keel for the hull extension, say 5.5," then try that for a season. It's quite a chore to get the boat off the trailer and on to stands to work on the hull. Adding plywood cheeks to the keel at a later date, should it become apparent that the keel was too shallow, would not be a problem.
    [​IMG]
     
  6. Bing
    Joined: Jun 2006
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    Location: New Hampshire, USA

    Bing Junior Member

    Here are the dimensions of the underwater portion of the extension options, assuming that Terrapyn's draft remains at 7" from bow to stern.
    [​IMG]
    I think the rockered bottom gives me roughly 523 lbs of buoyancy to compensate for the added weight of the extension, bigger motor, added fuel, extra storage, etc., before the stern would begin to settle (or add weight towards the bow and even out the draft), whereas the flat bottom extension provides 831 lbs. Is this more or less correct?
     
  7. clmanges
    Joined: Jul 2008
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    Location: Ohio

    clmanges Senior Member

    mc_rash suggested adding a smaller skeg with the option of adding to it. I suggest the opposite approach, since, IMO, it's easier to remove material than it is to add more to it. Start with the longer and deeper skeg from post #18, do a test run and see how she tracks and turns.

    The problem with either approach is in not knowing how much to add or remove. I think that possibly overshooting on the large side is better because each time you add a layer, there's a possibility of some part of it failing to bond or seal completely.
     
  8. Bing
    Joined: Jun 2006
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    Location: New Hampshire, USA

    Bing Junior Member

    Hmm...that's something to ponder, starting off deep then taking away if necessary. But then one wouldn't know if one added too much, unless the boat was difficult to turn or pivot. If steering was OK, I'd wonder if I could have gotten by with less keel -- not that it harms anything, as the effective draft is determined by the bottom of the motor skeg. If I first built the keel extension shallow, that would form the "core" of the keel and would be well bolted and glassed. If it worked as is, nothing more needs to be done. If tracking was still a problem, I might be able to install some temporary plywood test cheeks while it was on the trailer, just using some PT decking screws through the cheeks into the sides of the core keel (easy to repair screw holes). Experiments with different depths and lengths should determine the sweet spot.
     
  9. jbo_c
    Joined: Jul 2017
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    Location: Gainesville, GA

    jbo_c Junior Member

    FB2E0509-1755-4786-8213-7BFC4BF7E3C0.jpeg Love your little houseboat. I’m building one that’s very similar in many ways.

    I don’t have anything really to add to this conversation other than to say that I put three more or less full length runners on mine. They are 2” wide and 1” high(?). The center one is double height for the last 8 feet.

    I can’t report how effective they are(or aren’t). The hull is still upside down on the forms. But I hope it’s effective. :)

    (The taller portion hasn’t yet been added in the photo, but you get the idea.

    Again, love your boat.

    Jbo

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  10. jbo_c
    Joined: Jul 2017
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    Location: Gainesville, GA

    jbo_c Junior Member

    Not sure why the pic posted twice, but the hull isn’t wonky like the pic makes it seem.

    Jbo
     
  11. Bing
    Joined: Jun 2006
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    Location: New Hampshire, USA

    Bing Junior Member

    That looks to be a very stable platform for a houseboat. I don't know if you will have tracking problems or not without more of a skeg/keel, but there seems to be easy remedies if you do. Wave slap at the bow will probably be an issue. I think one of the reasons it's so bad on my boat is that the chamber in the bow has little in it to dissipate the noise, so it echos. The access is a small, maybe 18"x18" opening just behind the forward cabin step. We put stuff in there that we don't use that much, but it's mostly an empty space. I've wondered about laying in some sound deadening material. Next summer I'm going to try leading the anchor rode off center, per forum suggestion, to see if that helps. You have a much easier opportunity to address the problem once you turn over the hull over and think about how you will design the bow deck and interior.

    Despite limiting your cruising waters to rivers and calm lakes on nice weather days, I would caution that if you are planning to operate your boat where you may encounter boat wakes from larger boats (they don't even need to be going fast), that you plan for quite large splash waves rolling over the bow. If my experience is any indication, low freeboard scow hulls are prone to this. You can lessen the impact by turning the boat at an angle to the oncoming wake wave, or stopping altogether, but you may have times when an inexperienced person is at the helm who won't know what to do in the few seconds one has to react. And if you have a lot of boats speeding around, you can encounter quite confused wakes hitting you at several angles all at once [turns out that 45mph bass boats are much less of a problem -- they leave only tiny wakes at speed]. You will want to design the bow so the inevitable wake wave can roll over the bow and quickly run off, and not rush into the cabin, or fill up an open bow area. I know of at least one Adeventure Craft houseboat that went to the bottom because of this very scenario, and this isn't a scow hull (to be fair, the owner was moving along at a fair clip, as these boats can get up on plane, and not paying attention).

    [​IMG]
     
  12. jbo_c
    Joined: Jul 2017
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    Location: Gainesville, GA

    jbo_c Junior Member

    Thanks to this post and another somewhere else, I am working on a mitigation strategy for potential water over the bow, though I hope not to need it.

    My design will be lighter/have shallower draft, and (I think) have more hull volume forward, and will also sit a little lower in the stern/higher in the bow.

    It will have 100% positive flotation, but my wife will flip if she sees water come over the bow.

    My program is rivers and sloughs. Will only be in the lake when calm. I’ll have to go out in increasingly “adverse” conditions over time to figure out how she’ll handle what.

    Thanks for the feedback.

    Jbo
     
  13. jbo_c
    Joined: Jul 2017
    Posts: 67
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    Location: Gainesville, GA

    jbo_c Junior Member

    How high are the sides on your boat?

    Jbo
     
  14. Bing
    Joined: Jun 2006
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    Location: New Hampshire, USA

    Bing Junior Member

    Terrapyn has 15" to 16" of freeboard. It's a nice height as we can step off onto floating docks quite easily, and there's only one step down to enter the cabin. My wife doesn't like to clamber around on or jump off of things. As I mentioned earlier, for those of us thinking about boating into one's golden years, Terrapyn is about as good as it gets (as long as we leave the bottom of the dutch door up front closed -- I just noticed that the one picture of us underway has the forward door wide open).
     

  15. jbo_c
    Joined: Jul 2017
    Posts: 67
    Likes: 11, Points: 8
    Location: Gainesville, GA

    jbo_c Junior Member

    I should have 18-20” of freeboard. That’ll make a little difference(but not much).

    Appreciate the response.

    Jbo
     
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