29ft Fylbridge with narrow beam - add hull extension

Discussion in 'Boat Design' started by PE_01, Nov 6, 2023.

  1. PE_01
    Joined: Oct 2023
    Posts: 6
    Likes: 0, Points: 1
    Location: Australia

    PE_01 Junior Member

    Hi All,

    I have an Australian built 29ft Blackfin which has a blown outdrive and i'm now looking to change from sterndrive to outboards for various reasons. I'm looking to engaging a naval architect but timing is an issue, they're all very busy and can't assist for some time. The boat details are as follows

    29ft Australian Built Blackfin
    Beam at water level is 2400mm - 7 feet
    draft is Approx 800mm
    Dry weight 4500kg (with original engine, which was 531KG)

    So my ideas are as follows - as noted though i am trying to get professional advice.

    Replace the existing transom and stiffen stringers and bulkheads
    Remove old sterndrive and do a full width hull extension and add outboards.
    Add underfloor fuel tank approx 1000l - 250 gal in centreline (900kg full)
    Move the existing 120L water tank from the port side to behind a bulkhead and low centreline
    Add underfloor storage/tanks against the sides of the boat and create a flooded ballast system in these tanks if necessary
    Widen the existing chines (currently 50mm each side) to 150mm this can be easily achieved

    Possibly add sponsons above the water line if required to stop or slow down roll in a side on sea

    Any information is appreciated - thanks
     

    Attached Files:

  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum PE.

    What was the power of the old sterndrive, and what size outboards are you planning to use as it's replacement?
    Will they be mounted on a bracket, or on the transom itself?

    How long is your proposed hull extension?

    Will this be located just forward of the new transom, in the extension?
    Is it in addition to the existing fuel tank?

    How are you going to widen the existing chines? By adding spray rails?
     
  3. PE_01
    Joined: Oct 2023
    Posts: 6
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    Location: Australia

    PE_01 Junior Member

    No, there is a step-in along the length of the hull, my intention is to fill this with composite materials to essentially thicken the sides of the hull and extend it all the way to the chines.

    upload_2023-11-7_14-46-22.png
     

    Attached Files:

  4. Alexander Peter Bromley
    Joined: Apr 2020
    Posts: 128
    Likes: 66, Points: 28
    Location: South Africa

    Alexander Peter Bromley Student Naval Architect

    Hi, 1000L or 900kg worth of fuel on a boat with a dry weight of 4500kg might be a bit excessive. Are you looking have a big range on your fuel capacity?

    I'm thinking your fuel consumption might improve with two outboards and the hull extension, as long as the Hp on the outboards is not underpowered for the boat.
     
  5. PE_01
    Joined: Oct 2023
    Posts: 6
    Likes: 0, Points: 1
    Location: Australia

    PE_01 Junior Member

    HI Alexander,

    Thank you for the response.

    Yes i need the long range, i'm in North Queensland and want to be able to travel to the outer reef and continental shelf in excess of 120nm each way. The idea of the boat is to be capable of long distances and off grid style work.
     
  6. PE_01
    Joined: Oct 2023
    Posts: 6
    Likes: 0, Points: 1
    Location: Australia

    PE_01 Junior Member

    Hi bajansailor,

    Sorry i didn't see your comment properly at first thank you very much for the welcome and response i hope you got my answers. Cheers
     
  7. Alexander Peter Bromley
    Joined: Apr 2020
    Posts: 128
    Likes: 66, Points: 28
    Location: South Africa

    Alexander Peter Bromley Student Naval Architect

    I see. By how much are you planning on extending the hull?

    Post some pics of your swim platform and transom a bit more zoomed out.
     
  8. PE_01
    Joined: Oct 2023
    Posts: 6
    Likes: 0, Points: 1
    Location: Australia

    PE_01 Junior Member

    Here you go,

    a few photos hope this helps.
     

    Attached Files:

  9. Alexander Peter Bromley
    Joined: Apr 2020
    Posts: 128
    Likes: 66, Points: 28
    Location: South Africa

    Alexander Peter Bromley Student Naval Architect

    Done right it looks feasible, a lotta work though. To fair all that in is quite a major.
     
  10. PE_01
    Joined: Oct 2023
    Posts: 6
    Likes: 0, Points: 1
    Location: Australia

    PE_01 Junior Member

    Yep, I've already removed the floor, transom and some stringers to rebuild and strengthen it's going to be a lot of work but hopefully well and truely worth it.
     

  11. kapnD
    Joined: Jan 2003
    Posts: 1,395
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    Location: hawaii, usa

    kapnD Senior Member

    Your boat will benefit from the increased length of a full bottom extension, but widening the chines by 50mm will be futile, besides loosing the benefit of the spray rail.
    I’d do the extension and test the results before tackling the chines.
    chine width would have to be increased substantially to have a measurable effect.
    That type of boat hull is primarily designed to go fast in seas, and has no substantial keel, so it is to be expected that it will roll when off plane.
     
    bajansailor and BlueBell like this.
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