Wet and unstable boat - what can we do?

Discussion in 'Powerboats' started by johnnythefish, Jan 23, 2021.

  1. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Take a look at this bow:
    http://modelshipworldforum.com/resources/Framing_and_Planking/bluff_bow_planking_primer.pdf

    As you can see, the bow is very full and is "large". In other words from the stem aft, the change in shape is huge - think of it like a rounded brick.
    This is classic displacement hull type of shape. So if you cut the hull at the waterline and look down in plan view...it goes from a stem to almost max beam in a very short distance.
    So we can say that the angle of entracnce, is large. Draw a line from the hull's CL at the stem...and follow the tangent line...it will be a large angle.

    Your hull, whilst not 'exactly' the same, it is not the correct bow shape for the hull's MO.
    The angle of entrance should be much less....so the hull is more like a knife, in that sense.

    Since you need to encourage the water to flow from the bow to being under the hull, below the chines in a smooth transition.
    If the hull is 'bluff' the high angle of entrance does not allow this. You're effectively "pushing" lots of water, rather than attempting to get on top of it.
    Does this help explain?

    Trouble is, not much you can do, without major surgery to the hull.

    Indeed that does not help one bit. I would get ride of it immediately.

    Would help considerably.

    Make sure the transition from the hull to the chine is hard and sharp..and the outer edge of the chine is also sharp.
    I would also have the chines at 5 degrees down, not horizontal.
     
  2. johnnythefish
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    johnnythefish Junior Member

    Just so we are on the same page, this “full bow” is a different issue to the “extended keel pad” that I am going to call that thing running up the keel towards the bow? Or are you saying the “keel pad” is creating this “full bow?”

    I don’t think it would be too hard to get rid of the keel pad. We have plenty of willing hands here.

    A couple of layers of biaxial fabric would counter any loss of strength.

    But if you are talking about the actual hull shape at the bow, you are right - that’s a giant task I wouldn’t go near with a barge pole.

    Regarding spray rails/ chine flats - how wide would you make them? And would you have them wider at stern and taper towards bow? So more “lift” at stern.

    Mr Efficiency - I haven’t been on the boat in a while - and I am in a lock down far away from the boat at the moment - but I do remember when I was on it a few years ago, the turns definitely made me feel like “something isnt right”.... maybe the most similar experience it felt like was a bit how a power cat turns? A very different sensation to most mono hull speed boats I’ve been in.

    She does jump on the plane just fine - so I have to assume that she is at least in principle a “planing hull”...

    She’s also pretty quick with those twin 50 Hondas - over 30 kts - so I don’t think she is inherently too heavy over all.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    So it does not bank heavily to the inside of the turn ? It is a fairly flat bottom, that tends to less bank, but narrow, that tends to more bank, and little in the way of a flat at the chine, which would reduce bank, if it were there, so a bit of a complicated equation, I think the best guide to whether the COG is high, is what happens in a sharp turn at low speed, it should not lean outward like a skinny old-style naval destroyer.
     
  4. johnnythefish
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    johnnythefish Junior Member

    It’s hard to explain this one. It definitely banks in a turn. I have just never felt particularly comfortable/ safe on it. It just feels odd. I will try and get you more detail. I wonder what easiest way is to test this - the sandbag idea?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Honestly, if the boat is wet, and trimming the motors out doesn't help that, then ballast will make it wetter. Try that 5 knot hard turn test, if it is leaning out madly, then it does point to a high centre of gravity, Could there be any appreciable weight in that canopy ?
     
  6. cracked_ribs
    Joined: Nov 2018
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    cracked_ribs Junior Member

    Just eyeballing it, I'd say:

    sharpen the chines and put a bit of downturn to them, but spray rails might be necessary on the stern third or so, depending on how deep it rides;

    It does look like it carries weight high, and unless you can lighten it, adding weight low is about the only fix for that I know;

    Twin 50s isn't much power so if it jumps on plane with that, it's definitely a planing hull all right.

    I don't know if that keel pad thing would make it much wetter or not. I would have to see it running to try to figure out where the water is riding up the hull. But the bow flares out so much, I'm inclined to think it's wet in the aft portions.

    If it were me working on it, I'd start by putting spray rails and loading in sandbags, because those are low-effort changes that will tell you quite a bit and probably get you halfway.

    Reverse chines would probably be a better thing to take on because the chines should be sharpened anyway, but if it's planing well with such low power - I think you said it's a 23' boat? - then I'd be less worried about trying to extract a more efficient running shape that would usually concern me.

    You know I'm wondering...does she ride high on plane? Maybe that planing pad at the stern has her teetering a bit, some boats with large planing pads can feel pretty tender at speed. That might explain it running well with just a pair of fifties... around here that would be half or less the power you'd see on most boats that size.
     
    Last edited: Jan 24, 2021
  7. Rumars
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    Rumars Senior Member

    The good news is that with this kind of construction (ply on frame), it is pretty easy to modify the bow of the boat. Just cut off the bow where the transition begins, then scarf long extensions to chine, stringer and gunwhale. Take the old stempost and move it around forward and back, up and down, until you have a good bowshape, then scarf in the new forefoot. The rest is like the original construction, shape the wood to its new form, glue on plywood, sand and paint. Maybe a week of work for a professional, a little bit longer for the amateur.
     
  8. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    When the sentimental value eclipses the annoyance value, your friend may consider acquiring a new boat.
    Until then, try moving stuff aft and see what happens.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Correct - they are separate issues.
    The "keel pad" is an appendage, so to speak. But as noted, it is horrible and does not help your cause!

    If you make them around 50-75mm or which ever dim fits into the existing shape really.
    Keep the width the same - although you can taper to a near fine edge (about 10-20mm) at the very bow from around 1/5Lwl from the bow. Since you don't want the chine to be "digging-in" when ploughing into waves. Thus just creates that classic graunching sound each time you head into a wave. It is not the chines that give "lift" - although many will attempt to argue it does. The chines provide the separation of water flow required for the hull, to give the lift you require.
     

  10. Pericles
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    Pericles Senior Member

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