My little Lifeboat Project needs help... - 21 ft steel lifeboat -

Discussion in 'Metal Boat Building' started by husnutolga, May 25, 2012.

  1. husnutolga
    Joined: May 2012
    Posts: 2
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Istanbul

    husnutolga New Member

    Hi Everybody,

    I have an 21 lifeboat (made in NY in 1955) riveted steel hull. I bought it from a fisherman. she was well used for fishing for 20+ years in aegean sea between Greece and Turkey. When i bought it it was already a converted lifeboat. I try to furnish it according to my needs and the outcome was not quite satisfying. now i am taking it out for repairs and some extension. I have some pictures of the boat attached. I plan to strip everything inside, extend by 3 - 3,5 feet, restructure a low cabin on front, raise the deck for draining directly into sea. I also want something like a swim platform or a ladder type of something and get rid of what i have at the back now. the current platform at the bac is extremely useful but, i don't like the appereance... I have an 27hp 3cyl diesel already installed but somehow the boat can not make more than 4 knots. I would like to ask the experienced ones here;

    - do you advise to extend the lenght? (I have welding capabilities.) I believe it will help with the speed. i need something a little more than 6 knot to go in channel that has a current around 4 knots...

    - on the certification it says certified for 30 people corresponding 75kg x 30 = 2250kg of ballast. I have stability problems. If i arrange a total load of 2250 kg will my problem be solved?

    - I can make 3d drawings myself. where should i try to locate the Centre of gravity after all equipmants are in place?

    - refer to 2nd question, should i extend the keel downwards to create more stabiliy?

    PS: I will be taking her out from sea and place in my yard in a few days i can post more pictures of the current water line. Beige boat is the condition, when I got it. than i painted in blue/white...

    Thank you very much in advance...

    Pictures can be seen in my post by clicking following links;
  2. MikeJohns
    Joined: Aug 2004
    Posts: 3,192
    Likes: 208, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    If you are going to model your hull in 3D you can run a basic smooth water resistance prediction.
    Look at the power required for 6 knots, take off your efficiency losses back to the engine add maybe 20 % for adverse fouling windage and small waves. Then see how the power requirement relates to your engine. Note get the resistance coefficients matching the existing setup prior to predicting the new requirements.

    Then you have a few options that can be discussed in more detail. Also if you want specific help on resistance calculation you'd be better off posting in the boat design section.

    Stability is also easily set with a 3D model and a free design package. Play with total displacement and CoG and look at the static stability curves. You can add some ballast in this simulation too.

    If you post easy shorter queries you'll get answers. Otherwise you need to engage one of us professionally.
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