Formulas for Prop, shaft, gearbox and engine mounting

Discussion in 'Diesel Engines' started by Simon TW, Jul 24, 2015.

  1. Simon TW
    Joined: Jul 2015
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Red Sea

    Simon TW Junior Member

    Hello all who view, I've done the searches to some questions and although I have learned from these I haven't found the answers. That's if there are answers.

    I'll try not to make this too long winded, I've bought a boat, not old in the terms of years but old in the way that's been built.

    I a Brit, living and working in Egypt and here they build wooden boats but mask them really well with filler and copy all the flash designs of the super yachts.

    My boat is 15 years old built with wood and it has two (Scania 112) 6 cylinder diesels.

    I'm refitting the boat and in the process of having the engines overhauled. I've been told that the boat has a problem (the Egyptian thing is to love to create mayhem and cause problems then they can look good solving them and squeeze you for more money).

    I'm told that the propellers are too far forward and should be nearer to the stern. That the prop shafts are too long and the engines too far forward in the boat and that the whole system needs moving more towards the stern. This would require new holes in the boat for the re-positioned propshafts and it would also make the angle of the propshaft steeper.

    Now this boat is 15 years old and has been this way and managed, I'm reluctant to believe the local "Engineer" who quoted 140,000LE ($18000) in a country where the average wage is $10 per day.

    What I need to know is the formulas used in boat building and then I can measure this.

    Where to the rear of the boat should the propellers be situated?

    What angle of rake should the propshaft have?

    Is there an ideal length of propshaft?

    Thank you for taking the time to read
     
  2. Ad Hoc
    Joined: Oct 2008
    Posts: 6,973
    Likes: 917, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Welcome to the forum Simon

    Ok..so first question is..why are you over hauling the engines and secondly..have you run the boat on the water yet?
     
  3. Simon TW
    Joined: Jul 2015
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Red Sea

    Simon TW Junior Member

    Thanks for the reply.

    It's not just the engines I'm overhauling. The whole boat has suffered from years of neglect. The previous owner used it once a month maybe 8 months of the year and tried to avoid spending.

    Both engines hadn't been serviced in years the oil was more like mud.

    There's a funnel on the steering when they kept pouring the fluid rather than fix the leak.

    I was present when we moved the boat from down the coast. It ran okay with a snaking that was suggested to be the props being too far forward.

    The previous owner used the boat for two days at a time and stayed local. I'm planning to be more adventurous. I can go to where ever he went on my RIB bit I need to go to sea for 10 days at a time, maybe even two weeks.
     
  4. Ad Hoc
    Joined: Oct 2008
    Posts: 6,973
    Likes: 917, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Well..in the absence of pictures and hard data...i would suggest, first, to just overhaul the engines...then take the boat out..she what she does and doesn't do..then you'll know what, if anything, needs addressing.

    Since it is just too vague to say one way or another. But getting those engines up and running properly again you can 1) eliminate the engines 2) find out the true performance of your boat and 3) if there any vibrations or quirks.

    Once that is done..things become much easier to pin down.
     
  5. gonzo
    Joined: Aug 2002
    Posts: 15,249
    Likes: 951, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    If the boat had a steering problem, it can be cause by several things. Do you have any photos? A boat that is sitting down by the bow will tend to oversteer, but that could be fixed more economically by shifting weights.
     
  6. Simon TW
    Joined: Jul 2015
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Red Sea

    Simon TW Junior Member

    Thanks for that post, yes the boat was sitting down at the bow as it had fuel and water tanks that were empty. It carries 6 tom of fuel and 6 tons of water in 8 tanks and three tanks need replacing and the others were empty, I had to put two tons of fuel in to move it.

    One of the reasons given for moving the engines more towards the stern is to have it sitting better.

    Yesterday I found an old crew member and he told me that the kitchen floor is very heavy. Apparently it was tiled but they decided to lift the floor so instead of doing any real work they laid 5 inch of sand and then tiled on top of that. That will be two or three tons.

    The plan now is to remove the kitchen floor, fit the new tanks replace the steering pump, finish the engine and generator overhaul then see how she sits and performs.

    I'd like to post some pictures but I haven't yet figured out how.
     
  7. gonzo
    Joined: Aug 2002
    Posts: 15,249
    Likes: 951, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Click on the "Go advanced" under the message box and it will give you the option to attach files.
     
  8. Simon TW
    Joined: Jul 2015
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Red Sea

    Simon TW Junior Member

    This is a picture from the previous owner.

    [​IMG]
     

    Attached Files:

  9. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Installing an aft set of tanks might help. to trim the vessel.

    The props and rudders if too far fwd is not ideal, but it is not uncommon.

    In the USA to keep charter boats from requiring a crew with an expensive license , the "stern post" and rudder (the legal end of the boat) may be 20-30 ft from the stern.

    These handle OK, and do their work well.
     
  10. Simon TW
    Joined: Jul 2015
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Red Sea

    Simon TW Junior Member

    Thank you
     
  11. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 482, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    If you have design questions, you should hire a professional to have a look at the boat and/or it's plans. No boat mechanic or marina worker is capable of "running the numbers" appropriately enough to make these types of grandiose comments about gear locations. Calculating trim changes is pretty simple if you have the plans, though you do have to have a clue about this process.
     
  12. Simon TW
    Joined: Jul 2015
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Red Sea

    Simon TW Junior Member

    have you ever been to a third world country?

    "Hire a professional" if the population of Egypt found out that I had money to hire a professional boat designer I'd be dealing with 85,000,000 boat designers. As it is they are all professionals that are telling me different things because they think that they can squeeze some money for the job, even if there is no job.

    When you go to hospital here they don't want to to we well, they want you to stay sick then you keep paying for treatment.

    The reason that I'm asking for help here is that I'll get advice from someone who isn't going to make money out of it, therefore impartial. I have received the required advice in the first few posts.
     
  13. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Your BILL is in the mail!
     

  14. Simon TW
    Joined: Jul 2015
    Posts: 7
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Red Sea

    Simon TW Junior Member

    Ahh Fred that would be a pleasure to receive. However we don't have post. Before we get post we need addresses. We don't have them either :)
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.