Increasing seaworthiness for a 7 m. motorboat

Discussion in 'Stability' started by chileflora, Nov 30, 2018.

  1. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    You seem determined to do this regardless. So make it easy, buy sheet foam, glue it to the outside of the hull, shape it as desire and glass over it.
    Calculate volume of foam needed to float the vessel, and choose foam sheets thick enough to achieve desired number after shaping to fit hullsides and fair in towards the bow.
    No frames, ribs, or stringers needed, the boats structure is already sound, isn’t it???
    The downside is that the boat becomes heavier and wider, not necessarily a good thing in rough ocean conditions!
    My reference to the Whaler photo was to illustrate the changes they made from early whaler HULL configuration, not the open topside configuration.
    If you know what the 60’s whaler hulls look like, it’s a radical change from that, indicating that though popular, the original whaler hulls had their faults, and were reengineered to produce safer boats,
     
  2. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    If not installed low or big enough; these become their own hazard.

    Think one side floating high and the other side filling the cockpit.

    If you use xps; make sure and rough sand it. Just this week I built a part and it delaminated from the peelply if I didn't mention it already.

    You can glue the sheets together say 1" with regular glues, but then rough sand the entire thi g with 36 grit floor paper and fill it with thixo before glassing.

    I would also build a wooden rubrail socket for it and use the rubrail for fastening to the hull. Xps won't take any impact like a dock at all. It will delam instantly as it has zero crush atrength.
     
  3. kapnD
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    kapnD Senior Member

    Don’t use weak foam, sure the good stuff costs more, but it’ll pay you back in workability and durability of finished product.
     
  4. chileflora
    Joined: Nov 2018
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    Location: Chile

    chileflora Junior Member

    I found relatively cheap extruded foam, and I plan to use this one. I started working on the boat, it will take a few months to complete it. Was thinking also about this cathedral hull, looked at different existing models; I still feel it should be not too bad. The central bow is quite pronounced and sharp, it is considerably deeper than the lateral appendages (compared to other models). There is no doubt the boat will bang more when falling off the wave, but I doubt that would compromise safety. The many arched curves should make the hull much stronger compared to tradicional V shape. The upper part of the bow is dull, but this can be modified quite easily. Transom has reasonable deadrise. The central part is trimaran-shaped, and this is not very good.

    My general feeling is that: it should be quite efficient in calm seas, more so than a traditional V. In heavy seas against the waves it should be OK if driving at very slow speed (4-6 knots), with slamming when falling off the wave. But I still feel it should not be unsafe for this - uncomfortable - yes.

    The built is good and solid, it is a workhorse.
     
  5. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    This is a picture of laminating or rather delam with xps I did this week. It was 17 oz biax on unprepared, but clean xps. I started to sand the xps a bit before taking the picture for posterity. Xps has poor shear. The only way I have found to deal with it is to rough sand the surface well with 36 grit floor paper and hotcoat it before glassing. The delam started when I pulled the peelply and this was more of a test than anything. This is a livewell base and I am going to rough the xps and glue it in as there is glass on the locker base already. Look closely; the top of the base at the cutout for the drain is corecell M.
    DDA7D978-5310-4E23-A9B0-B62AE7F606F2.jpeg
     
  6. chileflora
    Joined: Nov 2018
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    Location: Chile

    chileflora Junior Member

    Took the documents for registration to the Port Captaincy. Happens that they know this boat very well. Initially it was used for coastal patrol duty exactly in southern Chile.

    However, they told me that since the kidneys of sailors were all beaten out rather quickly, the boats were moved to inland duty and/or sold. Also, there were cases where the boat got flooded and capsized, especially with two heavy engines - it sits back low, and since the cockpit is open and below waterline, once waves start coming in, there is not much to do...

    I am still pretty decided to keep the boat and run the mods I was thinking about. However, I decided to make sea trials before entering to modify hull.

    Resolving the issue of flooding - this is covered by the idea of raising the cockpit and making it sailboat style.
    Resolving the issue of heavy stern - the same floats would resolve this issue; the only thing is I would make them now "fuller" at the rear end, so they would carry weight of the extra engine.

    AND... I looked at the bow and the hull shape and here my question:
    The bumpiness of this type of hull is due to:
    - flat bow (which would not cut the oncoming wave, but slam into it)?
    - wide trimarane looking front 1/3 of the hull (which would produce slamming when falling off the wave at slow or moderate speeds)?
    - moderate deadrise (which would produce slamming when jumping off the wave at higher speed)?

    What I have seen that it is actually quite easy to modify the bow section (starting about 1.3 m from the front of the bow and extending it by about 50 cm. beyond the current bow) and faring it so as to make it V shaped. This would definitively change the cutting ability. It would involve doing about 3 m2 of fiberglass and a weight penalty of about 30 kg, and could be done in a couple of days.

    What can not be done is to modify the main part of the hull (the trimarane with the two laterals). Now, in this boat I still think that the main part of the hull is much deeper than the lateral sections, so it may not be of such an issue. I looked at my Catalina, and in fact it is pretty flat in the 1/3 front and all the way to the rear... If this boat would behave like Catalina, I would be satisfied.

    What is your opinion, modifying the bow would improve considerably the "bumpiness" factor or not so much?
     
  7. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    There is not much you can do to improve the ride, as I mentioned earlier, just about everything in the cathedral-trihull genre slams pretty convincingly in rough water, largely because the the shape of them causes water to be "trapped", rather than thrown out of the way, like a vee-hull. The only exceptions were boats that had deep tunnels extending all the way to the transom, and a well vee-d central part. It really isn't a practical proposition to be altering the hull to gain a better ride. However, the boat is only intended for pleasure use, not coastal patrol duties, so picking the days you venture out, could alleviate much of the grief. I don't see a real problem with the stern height, if the engine well is properly set up, and it's not as if it has heavy engines you intend to fit.
     
  8. chileflora
    Joined: Nov 2018
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    Location: Chile

    chileflora Junior Member

    What I noticed in this hull, that the first 60 - 70 cm. of the bow are funnel-shapped which exactly should generate heavy slamming once the wave becomes so high that can not be pushed into the tunnels. However, exactly this can be eliminated easily by modifying the bow.

    My objective is to have a cruiser capable of navigating for 7 - 10 days in a row with two persons on board. Now, I am not really looking for trouble, but I still need the boat to be able to deal with 3 - 4 meter waves; it does not have to deal with such sea effectively and pleasurably, but it must not compromise the safety.

    In fact, the conditions I am talking about (3 - 4 m. steep waves) can be expected only 10 % of the time. 1.5 m- 2.5 m. - 40 % of the time. And going with the 2 m. waves should not be such a problem, as opposed to going against such waves. So basically if the boat can survive well 4 m. waves from time to time, I would be more than satisfied.
     
  9. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The "trouble" with the ride quality doesn't really emanate from the bow, but further back under the mid-body, anything that is essentially nearing flat there, or with a shallow gull-wing, will slam. If you are put into a position of having to make ground into heavy seas, you should not be out there in the first place, but slowing down to crawl speeds is the only tolerable solution. Or if turning to run down-sea, or beam-on, to a safe harbour, is an option, take that every time. I don't think the hull shape is unseaworthy, but we are talking about small boats on a big sea, and prudence dictates there is a limited number of days when it is wise to be out there. But for planing boats, it is often the case that open waters are less demanding of them than wide, shallow, windswept bays, where wind against tide can produce chop that can test the very best of hulls. Offshore, with longer wavelengths, can be surprisingly easier to handle.
     
  10. chileflora
    Joined: Nov 2018
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    Location: Chile

    chileflora Junior Member

    Thanks for the reply...
     

  11. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Just make sure the boat has adequate flotation, placed such that it remains stable when swamped, whether it be external, as you were contemplating, or internal, or some combination of both.
     
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