What prop next?

Discussion in 'Sterndrives' started by Poppajoe, Jun 16, 2006.

  1. Poppajoe
    Joined: Feb 2006
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    Location: South Wales, UK

    Poppajoe Junior Member

    :confused: Can someone help please?

    I have used various prop calculators and I am getting more confused.

    I have an 18ft Icelander fishing boat with a ford xld 1.8ltr, 60hp diesel through a Volvo 280 single prop drive.

    Details below, but I need to know what sort of revs I need to be running at.
    The prop calc suggests 14x12 or 15x15 depending what hull shape, tried both and with 15x15 she bogs down at 2300rpm at about 11 knots, with the 14x12 she runs at 2800rpm at about 14 knots.

    The calc suggested 17 and 23 knots respectivley, in fact with the details entered at their worst case senario I could not get a figure below 17 knots suggested.

    I would presume that smaller dia and larger pitch (ie 12x17) would reduce revs?

    I am not looking for any particular speed but want to obviuosly run at the highest possible. I am not concerned with fuel consumtion and do not mind wearing out an engine at around 500/1000 hrs.

    I can post a pic of boat showing hull if it helps?

    waterline length = 16ft
    max revs = 4800 (60hp)
    max torque = 108 NM @ 2500RPM
    drive ratio = 1.89:1
    boat weight = 1300kgs (max checked on weighbridge)
    max prop dia = 15"
    Hull form? = on plane would be very shallow "v" about 10 degrees?

    Any help would be much appreciated.
  2. Preformer 40
    Joined: Apr 2004
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    Location: Naples, Fl

    Preformer 40 Junior Member

    whats the ratio on the outdrive?
  3. Poppajoe
    Joined: Feb 2006
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    Location: South Wales, UK

    Poppajoe Junior Member


    It's in the post, if that's what you mean?

    It's 1.89:1

  4. alex fletcher
    Joined: Jun 2006
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    Location: Ettalong Beach NSW Aust.

    alex fletcher Junior Member

    What I would do in your situation check the props your working with find out what type and its condition not just its dia and pitch. You need a machine and a pricey one at that in Australia I would take the propellers to a "PorpTraker" agency. You get a report(printed & Autocad), Advice and repairs to the prop if needed with achievable theoreticals
    It is money well spent. Proping is now a science and you need the right tools.
    Also whats your draft , Beam, Hull config? and what type of smoke is the boat making underway? bit more info
    regards Alex
  5. Preformer 40
    Joined: Apr 2004
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    Location: Naples, Fl

    Preformer 40 Junior Member

    Hows the bottom of boat clean? if not you need to clean and get back to us.Did this boat originally come with this setup?
    Your numbers indicate that your prop has to be smaller in diameter and pitch.I dont think you can find much lower with the 280,if you can then drop down in pitch and diameter,your at 2800 rpm,1inch of pitch =200 rpm,1 inch of diameter= i think 400 rpm.Is your rpm guage in the right pole position?are you 100 percent sure your at 2800 rpm? if all these numbers are correct and boat bottom is clean and motor is running correctly, then you need a diffrent ratio on drive say 2.00 or 2.33.This will let you throw those props at a higher rpm.Im not an expert,but some are on this board and can tell you the exact ratio needed to help,you could go to a diesel forum, i think its called diesel boats .com, they would be my choice to solve,its 25.00 to join,but its cheap for the expert help with the motor and drive.This boat sounds like its awesome i hope you get it solved,whats the mpg claimed?
  6. Poppajoe
    Joined: Feb 2006
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    Location: South Wales, UK

    Poppajoe Junior Member


    Hey performer, thanks for the reply.

    Your are right about the numbers they just dont add up,
    I have since posting sent the prop to a reworker who says much the same, he's taking it down 2" in the pitch from 12 to 10 to up the revs, it has to be reworked as it's virtually impossible to get a pitch at 10 with a 14" on a 280 drive as you said.
    The boat is new antifoul, engine is a recent recon that runs like a dream, revs are correct as it gets to the listed level, 4800 out of gear (I only did it for a second mind you!)
    However, I have since discovered that the small steel bilge keels it has close to the rear end will definatly cause drag (did not mention them before as thought they were irrelavant) so they are coming off this weekend.
    This would explain why the calculations I have had previuosly suggested the 14x12 to be the prop but won't get the revs up.
    The theory now is after discussion with the prop man that it is nearly on the plane, which when it is the dynamics change and the speed/revs will improve. He expalined that the boat is sitting at that critical stage when it's just about to "fall onto the plane" and having been there with it it's true, if it could only just fall forward it would then be planing instead of the hull being like a wall against the sea?
    Anyway, if the prop rework fails I am going to have to regear the ratio as you said, thanks for the tip I had'nt thought of that one!
    By the way hopfully she should do at worst 3 gals (uk) an hour and on tax free red diesel here in the uk thats fine for me!
    Thanks for the reply, it's great to have someone agree with the logic I have so far and gives me confidence. I was just worried about taking the prop down too far and not then having enough to move adeqaute water, but then when I think about it outboards of simular hp (although higher revs/gearing) would run a prop of about 14x10 anyway on a boat this size and weight.
    So in short, its nearly on the plane, then it will change the outlook - I live in hope hey, cos next its the same size engine turbo'd! which will add 80% more hp, and then I am back to buying a new prop?
    cheers, I will let you know how it goes..

  7. Frosty

    Frosty Previous Member

    I did'nt see any one mention this If it has I apologise but are you sure the leg is going right down, I mean right down ,--right onto its max stops. I had a Mercruiser once like this and i filed alluminium away to get it down further --it helped.
    Any way try shifting wieght about like put a sand bag up front just for a test run.

    Sounds like your real close
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