prop for under powered sailboat

Discussion in 'Props' started by bcripps, May 2, 2011.

  1. bcripps
    Joined: May 2011
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    Location: usa

    bcripps New Member

    I have a 13hp diesel in a 12,600 pound, 34-foot sailboat. To say the boat is underpowered is an understatement. I get about 3.5 knots with the engine going full out. I run it full out most of the time… 2,600 rpm. Engine: Volvo MD7A with saildrive 110S. Ratio: 1.66:1. Prop: manufacturers supply, fixed two bladed; (I believe it is 16” diameter.) With the saildrive, there is plenty of tip clearance.

    I know there is no substitute for horsepower but for a couple of reasons, I’m stuck with this engine… but it is well maintained and runs great.

    Because I run the engine at 2,600 at all times, I don’t believe a variable pitched prop would be of much use but please advise.

    The only reason for a prop change is an increase in speed. I motor-sail mostly and drag or power in reverse are not an issue.

    Could someone recommend an upgrade… prop type and supplier (North America).

    Thanks in advance.

    Bry
     
  2. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    first question is is it running at 2600 because that is the governed top speed or is it held to that rpm by the load of the prop? Whats the prop made of
     
  3. bcripps
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    bcripps New Member

    Thanks, pistnbroke... I don't have a tach but the 2600 rpm is what is quoted in the manual. Diesel doesn't seemed strained... no black smoke. The prop is aluminum.
     
  4. pistnbroke
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    the point is if its running on the governer then its not fully loaded and you can take more pitch ( and more speed for the boat)....so find out what is the governed speed of the unit.....
     
  5. bcripps
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    bcripps New Member

    All I have to go on is the engine manual and I quote...
    "Max operating speed r/s (rpm)........................43,3 (2600)"

    I would guess this is the governed speed but can't say for sure.
     
  6. pistnbroke
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    pistnbroke I try

    I dont think you have the full handle on this ....you need to run it a little below the governed rpm so the engine is running max power ....but its the load which must pull it down to say 2300 not the governer which is cutting the fuel and hence the output ....you need a rev conter and more details on the engine
     
  7. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    its like a diesel car going up a hill in second ...you do 20 mph the engine is at 4500 on the governer and 20 mph is your lot..The engine revs its nuts off but is not fully loaded.
    In thrid you do say 32 mph the engine is at 3800 fully loaded...of course you use more fuel ..only way you will find what is going on is to fit a tacho to see if it reaches 2600 before the throttle lever if fully over to max.
     
  8. bcripps
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    bcripps New Member

    Thanks again, pistnbroke... I'll put this project on hold until I can get a better handle on the engine revs. Out cruising at the moment so it will have to wait until I return to civilization.
    All best... Bry
     
  9. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    To help it would be best if you get someone to calibrate your tach.
    The Volvos I have seen of that model took the RPM off a terminal on the alt which is can have major inaccuracy.

    Second use a GPS or one of the overboard speed sticks that are fairly accurate and give us a list if the speeds every 200 rpm from idle to full speed.

    WE have a conventional drive boat a Beneteau First 32 with the same engine , that runs 6K at about 1/2 throttle , no tach installed..

    Could your tranny not be solidly in gear and/or slipping?

    Push on the shift linkage at the tranny underway and see if that helps.

    For sure the boat could not have been sold with a 3.5K top speed.

    FF
     
  10. gonzo
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    gonzo Senior Member

    It's a sailboat with an auxiliary engine. 3.5Kt is plenty to get in and out of a dock and about what to be expected with that power. To check the RPM you need a tachometer; preferably a mechanical or laser type to measure directly from the crankshaft.
     
  11. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    It's a sailboat with an auxiliary engine. 3.5Kt is plenty to get in and out of a dock and about what to be expected with that power.

    With a 17 HP diesel any 5 ton ,34 ft boat should run 5 to 6K with ease.

    The only folks that would be interested in a 3K boat are today's "I'm greener than you are" folks , that think 20 miles at 3K is a fine days cruising.

    FF
     
  12. philSweet
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    There is something radically wrong here. You should be able to do 4knots at part throttle. Do you have any long term fuel consumption figures? 2 gal in eight hours should have you moving along nicely with a clean hull. The prop should be kept shiny clean. I run a 14X14 two bladed with a 2:1 reduction on a bigger boat, although some may think that is a bit undersized for my 27 hp universal. A 16" would need to be very flat in order to not lug the engine. No more than 10" pitch as a guess. You aren't underpowered, something is amiss. As previously noted, need tach and speed data or fuel consumption- speed data, preferably both. And clean the hull and prop ever so good when you gather the data. Just a bit of residue on the prop can reduce thrust by 25%. If you plan to motorsail, choose a prop that is smaller in diameter and steeper in pitch of the recommended choices because the thrust at any given speed is more variable due to the sail.
     

  13. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Can you post some more information on the boat? Waterline beam and length, displacement.
     
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