Increasing seaworthiness for a 7 m. motorboat

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

  1. kapnD
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    kapnD Senior Member

    Not a great hull for the work you have outlined. The modifications you are contemplating will do nothing to improve seaworthiness. The narrowness of the hull is to your advantage, it will move with less horsepower, and be less prone to flying off waves.
    The worst feature of this type of boat is the broad low bow. Boarding seas in the bow are not acceptable!
    If I had to regularly use this boat in rough water, I would first increase the freeboard at the bow, adding a wraparound wave deflector and carrying it along the gunnels for at least half the boats length or more.
    That’s a lot of high, heavy cabin for a small boat, will you need all that? If so, try and determine its weight, as a heavy structure up high will degrade stability.
    A lightweight aluminum framework with canvas stretched over would offer good shelter and be significantly lighter.

    Edit: just refreshed browser to find that I based this post without seeing later posts, sorry!
    Extending the cabin is just batshit crazy, it’s already far too large, and likely too heavy for the boat anyway.
     
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  2. chileflora
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    chileflora Junior Member

    Are you talking about something like this? This is my small motorboat (4 meter) to which I added a wave deflector, added front windshields and some superstructure to navigate lakes; it works pretty well despite being low for up to 1 m. waves (never had a chance to experience higher waves on lakes).
     

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  3. chileflora
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    chileflora Junior Member

    Modifying bow is probably not going to be a first priority, but it can be done at any point later. Doing it "esthetically" is very difficult, but doing it "practically" is quite straightforward. But I would first make sea tests to see if I really need it.

    I am still more worried about lateral stability, with waves which may induce broaching. Also, since the boat is very narrow, and I need storage space for cruising, there is really no space for flotation. The only reasonable solution are external floats, both for stability and volume for extra flotation.

    I accept the idea that in movement a narrow hull may run more efficiently, and less thumpy, but with bad weather and making slow displacement progress (5-6 knots) the wider hull to me would seem much safer.

    The floats I am thinking of could be placed slightly higher, so that in calm water they really would not touch water when planing; they would come into play only when in displacement mode. I added a picture of boat which is more or less in theline of my thinking. They are used by navy here, and similar smaller boats are also used. The floats are rigid foam, not inflatables.

    As far as the cabin is concerned: YES, it is on the high side, I would have preferred to have it slightly lower. The problem is that the steering is inside, and to have a good view forward you have to have this two story structure. I love the inside steering, after sailing on open deck of Catalina it is one of the things I long for. It is not flimsy, but not too heavy either. Rather than weight, it is the windage area which is of concern...

    Canvass is out of question: navigation area +5 + 15 C, very often rain...
     

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  4. fallguy
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    fallguy Senior Member

    Just don't extend the cabin!
     
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  5. Mr Efficiency
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    Mr Efficiency Senior Member

    If the boat is structurally sound, (it does look like it has had a hard life, looking at the bow region), I can't see much of a problem using it as it is, certainly positive buoyancy ought to be a high priority, and adding some belts along both sides, but keeping the width within the legal trailing width, could be an idea. But the idea of being routinely out on a rough sea does not make any sense, it should be avoided wherever possible, both for safety and comfort. I'd suggest not going on any grand expeditions until you have trialled it under varying conditions, and you have a feel for how it performs, under different conditions. In other words, become familiar with the boat and its characteristics. I'm tipping your desire to go far in rough water will be limited by what those trials reveal, and I expect by what would be a rather firm ride in choppy conditions.
     
    Last edited: Dec 1, 2018
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  6. kapnD
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    kapnD Senior Member

    So you bought a boat that you don’t like and doesn’t come close to fulfilling your SOR, what else could go wrong?
    Run it, as is, in some protected waters, gradually moving into rougher conditions without getting too committed, to see what it will do while not getting too far away from calmer waters.
    If you start modifications without first testing it, you will have no basis for comparison.
     
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  7. chileflora
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    chileflora Junior Member

    The bow was damaged when unloading from truck... I just pulled it off with jeep from behind, and the bow got scratched on

    The legal width limit in Chile is 2.6 m, going to 2.65 would not bother anyone anyway. This is my width...

    I want to do long-range semi sheltered navigation (whatevere this means in the roaring forties...). I do not seek out problems, but occasional rough weather with 4 m. is unavoidable. I do not need it to run well in these conditions, but it must survive it well..

    I obviously can make test runs on a lake, but taking a boat out to sea is legally complicated. It has to be inspected, a lot of things has to be on board, etc.

    I did not say I do not like the boat, from studying here the market of used boats which can be trailered I would say this is a pretty good option. Other alternatives would be some Bayliner of similar size (22-25 foot), with an engine most probably with problems at about 30.000 USD and that would be one engine, which is less safe than having two brand new ones.

    My problem is that I have to invest heavily in the boat to take it to the ocean (at least 5000 USD plus the new engine from the other boat), and this still would involve considerable time in doing installation of required safety equipment. That is the reason why I am asking for opinion about the suitability of this hull for my purpose...

    I can always sell this hull without loss or with small gain, but this was not the idea when I bought it.
     
  8. kapnD
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    kapnD Senior Member

    upload_2018-12-1_14-14-32.jpeg This is what Boston Whaler has evolved into from a hull that was originally quite like the one you bought. It is vastly changed from the original, but is still not highly recommend as an offshore, heavy weather cruiser.
    You need to understand the limitations of what you have, and stop comparing it to your sailboat!
    Maybe some background reading would help you in your endeavor, how about “The nature of boats” by Dave Gerr for starters.
     
  9. chileflora
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    chileflora Junior Member

    This is open boat, this is very, very unsafe! Also, the bow is much flatter compared to mine... The cabin serves also as a protectin for flooding. Not at all sure it would withstand a 90 degree keelover, though.
     
  10. fallguy
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    fallguy Senior Member

    Some truth and some not.

    The Whaler is full of foam. If you bury her; she will still come up and float.

    The fact she has an open bow is an issue, but the design is well tested.

    The cabin on your boat has a few problems. If it were on a better hull; it'd not create horrible roll. But while it may be true the cabin protects the hull from flooding; it does make rolling much more. You will probably rename her Dramamine. And sponsons won't likely dampen the roll without a lot of trial m error.
     
  11. chileflora
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    chileflora Junior Member

    For me, the sponsons are not for dampening the roll, but to prevent a roll-over...

    And I always enjoyed bobbing on the sea. The best one was when I had three waves on anchorage, 3 m swell from southeast, a wind sea of 1.5 m from the north and a 0.6 m sea which came from a wind which just turned from south.. The night was unbelievable...
     
  12. fallguy
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    fallguy Senior Member

    But you won't bob on that mv. You roll. The movement in a narrow beam hull with a lot of cabin will be rolling back n forth. After a few hours; it may be trying you.

    Ironically, I find certain sea states enjoyable as well; on the right boat. I was on a large fishing boat in May and seas were ...3M swells. A lot of pax got sick. My son included.

    I enjoyed riding on the bow when the seas were head on. My hip did not like it as much- a personal matter.

    I am sorry to tell you since I am not a naval architect and lack the credential, but I cannot see sponsons saving this boat from calamity as she is...

    A rollover is probably not the biggest fear, but filling the massive cockpit and cabin with water is...can happen quick in a breaking sea or coming through a bad current or surf in some rolling. This is why positive buoyancy and scuppering would be better and staying out of such conditions wiser.

    If you want to raise the aft deck; you would have room for some flotation foam. And creativity will probably give you other spaces.
     
  13. chileflora
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    chileflora Junior Member

    Thanks a lot for the input...

    My conclusions from this discussion are these:

    -The boat is not the best for semi-offshore, but probably manageable for what I want.

    - I am still very hooked on the idea of installing lateral floats, both for stability and as a means of making flotation collar (this will eliminate the need to install flotation inside the hull taking little space which is available on this boat) and also make it much more stable in case of flooding (foam inside the hull would prevent sinking, but almost guaranteed to turtle the boat over.).

    I will install floats in such a manner that the lower part of them would be very close to the waterline (maybe immersed by 2 - 3 cm.) so as to reduce friction when motoring in calm sea. When planing, they would defintively be above the waterline. In heavy sea they would work. Most probably, the ride quality would be affected, but the safety increased.
    .
    - I definitively will raise the cockpit floor by about 20 cm., making it self-baling, small in volume, with closed storage spaces, basically, similar to sailboat cockpit. Underneath I plan to install main gas tank(s) of about 200 -250 l.

    - I looked at the bow, and I feel that if it really works badly in heavy sea, I can modify easily the upper part of it (basically, extending the total length of the boat by about 30 cm., raising the bow by about 20 cm., and making the upper part of the bow sharp-cutting. Modifying lower part is really impossible, a different hull would be needed.

    - I will listen to the advice not to to extend the cabin aft... One of the reasons for acepting this is being that I need to have room in the cockpit for the gas tanks; if I move cabin rearwards, the volume available for gas tanks will be reduced by almost half. Still I feel that I do not need open cockpit area and would have preferred to have it covered by the cabin.
     
  14. fallguy
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    fallguy Senior Member

    How would positive inside flotation result in turtling over?

    The foam on the sides will be plenty tough to build. Any ideas how you would go about it?
     

  15. chileflora
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    chileflora Junior Member

    The positive flotation unless installed along the sides of of the boat and near the flotation line when flooded will act like a destabilizing factor; if it is installed very low or near centerline, it will act as a "negative" keel. There is very little space inside the hull to install sufficient flotation correctly. However, the hull (as is now) has air spaces below deck of about 500 - 600 l, but it is all near centerline.

    My idea is the following:
    - Cut half-ribs from 9 or 12 mm. plywood (size: about 40 x 50 cm.)
    - Glue them in using fiberglass cloth and epoxy to the topsides of the boat, every 50 cm. (=Total weight about 15 kg., 12 ribs per side)
    - Maybe run 1-2 stringers along all the length for strength (about 20 x 30 mm.)
    - Put in expanded poliestyrene foam and shape it, using the ribs as a guide (Weight = 15 kg.), without gluing it in.
    - In the rear, cut out semisphere from eps and glue it to the rearmost rib.
    - Fiberglass everything with epoxy. Aproximate thickness, 3 - 4 mm. Exact thickness is another debate, I am not sure if Gerr's formula should be used (3.5 - 4.5 mm), or slightly thinner (3.0 mm) fiberglass could be used. (Total weight around 50-65 kg.)
    - The fiberglass will have a glue contact near the chine of about 3-4 cm. wide and at the toe rail (4 cm).

    As an alternative to reduce costs, I might cover the ribs with 6 mm. plywood, and then fiberglass it with 1.5 mm. This will reduce the cost of the resin. Weightwise it should be about the same.
     
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