Adding a second story to a displacement cruiser trawler

Discussion in 'Boat Design' started by Mobcat, May 21, 2020.

  1. Mobcat
    Joined: May 2020
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    Mobcat Junior Member

    Hi all Family is growing and its time to make a parents retreat im thinking but a bit worried about the boats stability when adding a flybridge and cabin with ensuite and small enclosed sallon behind the fly, My boat is a 42ft trawler style weighs about 20tonne , i built the boat from a old abandoned trawler its a cool sea boat but i feel it still lacks weight, power is by a ford lehman 120hp as new fully rebuilt by the photos attached you can see i have made the lower structure way over the top to accomadate a future top story, looking for thoughts on stability weight and balance etc not so much construction i have that covered thats the easy part, need to get the planning right first :)
    Thanks in advance
    MC IMG_7850.jpeg IMG_7851.jpeg
  2. Mobcat
    Joined: May 2020
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    Mobcat Junior Member

    Also im thinking of building aft to where the temporary solar support is with the second story with a small open deck at the aft of the boat as a deck to the top story cabin cabin and a davit for the tender.
  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Welcome to the Forum Mobcat.

    Re your idea about building a 'second story' - I would be very dubious about this.

    Not just because of the possible / probable stability issue, but to put it bluntly, the boat will look hideous with something like what you are proposing on top.
    And if you add a "flybridge and cabin with ensuite and small enclosed salon behind the fly" - this is going to take up most of the top deck surely? "
    If you are not into Photoshop ( I am not) you could print a copy of a side profile photo and then stick on a sketch of what your top deck cabin arrangement will look like.. Try various arrangements, including just a flying bridge, and see which looks best.

    I think you could get away with just having a flying bridge and this could look rather nice if you design it right - it could have a bimini awning with the aft end supported by that tubular gantry mast.

    Changing tack slightly, she appears to be rather 'light' in your photos, re where the immersed waterline is, and also where the bow thruster tube is (I presume that this is what the 'hole' is?).
    If so, is the bow thruster still effective even though it is so close to the water surface?
    Last edited: May 21, 2020
    fallguy likes this.
  4. Mobcat
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    Mobcat Junior Member

    i am not really into ascetics as long as its practical its been a labour of love for years this boat and space is more a need than asceticswe can sleep 8 atm on board with the day bed nine but we reall need to get to 10 comfortably , i think it will look ok with another story on her im not making it the full width of the boat just the width of the original wheel house with a walk way all the way from the bridge to the aft open deck should give it some lines from the sheer up, i have had a 1.5 tonne tender on the roof before and she performed well of shore on a trip to the whitsundays once from the gold coast and return, so i figure with a 2 tonne of old chain down low in the keel or under the lower bunks around the engine room it should balance the top story well , to be at the right water level i still need around 3 tonne onboard and all will be good, bow and stern thrusters all work well even now with only minimal coverage i couldnt install them any lower due to constarints in design, but all will work out in the fullness of time and a few more tonne on board, to be honest i am more worried about the extended freeboard (draft Air) of the second story in the wind on docking than i am of the weight of the second story, i had a carver 444 cpmy for years and that was beast around the docks in wind will be interesting to see how the trawler performs with the extra freeboard (draft Air).

    Happy Days
    Mobcat :)
  5. gonzo
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    gonzo Senior Member

    Considering that your life and that of your family is at stake, paying an engineer of naval architect to do a stability calculation is the best route. It is possible to use an inclination test to acquire the data necessary for the calculations.
    bajansailor likes this.
  6. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Thanks for the update Mobcat.

    Some more questions - what is the overall beam (width) of your boat at it's widest point?
    The hull draft amidships does not appear to be a lot - maybe a bit over 3'?
    Although I appreciate that with a fair bit of rake on the keel the maximum draft at the aft end is more like 4' (?)
    Does she really weigh 20 tonnes when floating at the waterline shown - ie where the weed line is?
    Did you measure the weight one time when she was lifted out by a crane?
    I will second Gonzo's comment above re doing an inclining test to assess the stability in her current condition before you start to add lots more structure and weight high up.
    Once you know what her current stability is like, then you can calculate what effect the addition of the extra weight will have.
    Including the positive effect of adding ballast in the bilge.
  7. Mobcat
    Joined: May 2020
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    Mobcat Junior Member

    Hi Baja she has a 12,6 beam and she draws 6ft currently probably 6,6 when finished completely, she currently weighs 20 tonne, she has everything onboard imaginable and is built old school rock solid ex Bass Strait cray boat before being double diagonal ply and fully fibreglassed and well faired,

    IMG_8112.PNG IMG_8114.PNG IMG_89F4DC19950C-1.jpeg s

    she is still sitting high needing that extra 3 tonne of ballast to original rigged trawler waterline so thats why im thinking the roof addition should only be a tonne with the fly bridge and 2 maybe 3 tonne of old chain in the bildge should see her more stable than ever, thats my thoughts anyway:)

    Thanks for the input.
    Happy Days
  8. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Adding a second level will increase the windage a lot, already looks like considerably slab sided.
    That makes it very hard to dock in a crosswind, and downright dangerous in a big blow.
    Aside from that, it looks like a sweet hull, and capable of carrying a lot more load, just not up too high!
  9. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Sounds like a good way to knock tens of thousands of $ off the value, it would need a team of stylists from Pininfarina to make it looks something other than an atrocity.
  10. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    The motion sitting up high is only pleasant in relatively calm seas and only one swell running I'm led to believe.
    A bloody big job either way.
    it's a nice boat,you only need space for 2 more., a well designed raised step built into the forward deck with port holes could make better use of existing space below.
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Yes, that forward area does seem the most likely place to expand into. Going high-rise would be a gamble.
  12. Squidly-Diddly
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    Squidly-Diddly Senior Member

    how about a canvas solution? add a flybridge on top, then frames and canvas back from there. If you got a fly-bridge, then you can add more frames and canvas forward and still see over the tent to steer the boat.

    Makes the tents/frames able to serve as fully enclosed living space, or just sun shade, or just side privacy shields, or just bare frames to hang clothes, lights, speakers, fishing poles, drinks, hammocks, etc from. Naturally, the frames would be dis-mountable with a bit of effort. Design the canvas so they can be rolled up and secured to frames so as to keep handy when going through the various configurations.

    IMO a good tent and frame design would solve all the problems mentioned by others: excess weight, wind catching, de-valuing, bridge-burning. Only permanent changes would be fly-bridge and frame mounting fixtures, and all the frames and canvas could be done off-site or subbed out.

    IMO its OK to have ungainly tent add-ons, ungainly hard structure not so much.

    One of my back burner projects is to re-purpose the moving frames, canvas and glass-rear-window from an old American convertible on a boat, including the windshield frame that mates with the top's frame front member. On a boat it would be mounted backwards.
  13. wet feet
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    wet feet Senior Member

    Before going to far down the path,take a look at what happened to a boat that had a bit of stuff added quite high up.
  14. Mobcat
    Joined: May 2020
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    Mobcat Junior Member

    I do feel the weight on the roof of the structure being only a tonne with furnishings and mirroring the lines from the sheer up and adding the extended sun shade over the cockpit in the same structure that exists presently by just extending the roof line to cover a area so we can davit a tender, which is some thing I should of done years ago the temporary supports we have used to hold the solar panels have been a fantastic shaded area since installation and still very fishable in the cockpit.

    given I have so much room in the lower areas under the floor and not directly in the bilge centre line to put ballast to counteract the extension balance wise I think the plan is sound to extend, I just need to get the calculations right given the moment of the extra weight on the roof, at a guess I would say 4 to 1 ballast to roof structure and furnishings would be fine does anyone have the calculations or formula for such a project, as I would say there is a formula for this equation, and given I need still to put
    Weight some 3 or four tonne on board to meet waterline why not make it useable weight to obtain space, obviously you can build 2 story boats there everywhere,
    I Would just like to prove concepts before waisting money on marine engineering to give me weight limits against height etc.
    and then maybe the real job begins to make it all look half pie decent
    Thanks for all your input everyone it’s appreciated
    Happy days
    Mob at

  15. JSL
    Joined: Nov 2012
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    JSL Senior Member

    Gonzo is correct - Hire someone to check things over.
    However, if you really want to 'wing-it' a quick experiment might give you an idea. I've done it on a few of boats and it helps get a feel (and has probably saved some lives and money.
    1. estimate the weights & centers of the additions & outfit.
    2. place equivalent weight at same location. (drums of water are okay but could overload the deck structure) Don't forget to allow for weight of people (200 lb/person)
    3. Assess the boat & stability. You can do a roll test to guestimate the GM.
    4. Walk around to assess the vessel's motion & movement.
    5. Go out on a run to see how boat behaves or how quickly it capsizes.
    6. The added weight will reduce your freeboard (waterline to weatherdeck). When fully loaded try to keep it at least 10% of the beam
    Ilan Voyager and bajansailor like this.
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