Bigger Motor?

Discussion in 'Outboards' started by valvebounce, Oct 4, 2016.

  1. valvebounce
    Joined: Dec 2010
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    valvebounce Senior Member

    I have a 14ft open boat with a planing hull.
    I put a renovated 1963 18hp engine on it,the engine runs well.
    The height of the cavitation plate is spot on with the lowest part of the hull.
    I tried it today on the river(slow flowing)
    Although the engine was putting out plenty of power,the boat would not plane.
    I adjusted the leg a couple of notches and it raised the bow and the speed very slightly.
    The boat was only achieving the same speed at full or half revs.
    Do I need a bigger motor to get it on the plane?
    I don't need a lot of speed,I intend to use the boat for close inshore fishing,but getting on the plane obviously has it's advantages.
    If I go to the expense of another motor what do you think the minimum size could be?
    For security reasons,I have to take the motor off after each trip and store it inside.
    So the lighter in weight it is,the better.(I prefer 2 strokes)
    The boat has a covered bow with a dashboard,steering and two seats,it has a decent freeboard with handrails and bow rails.
    Although the boat is fairly light in weight,I think it probably weighs twice as much as a 14ft Jonboat.
     
  2. Commuter Boats
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    I have very fond memories of those engines, as a kid I had three, two Evinrude's and a Johnson. I'd leave town with two of the transom and one flayed out, return a couple days later with still two on the transom but a different one flayed out.

    It be great if you could post a picture of the boat but my initial thought is propeller. If it was running poorly, I think you'd know.

    See if you can find some numbers on the propeller and post a picture of the boat and the propeller.
    Gerald
     
  3. valvebounce
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    valvebounce Senior Member

    I'll take a few pictures and put them up Gerald.
    The prop has been cleaned up and painted,so I can't see any numbers.
    The wake was very powerful,and the engine was running sweet.
    I have put new coils and points and spark plug in it,a new impeller and changed the gear oil,(which wasn't dicoloured)
    I think it is only reaching displacement speed,maybe because of the weight of the boat.
    It used a lot of fuel,which surprised me.I ran it at full revs most of the time,
    even though it didn't plane.When I throttled back,the bow dropped,but it did the same speed.(roughly 8mph)
     
  4. Commuter Boats
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    If you have too much propeller ( too much pitch ) your engine will not be able to develop enough RPM and can't develop its horsepower. You said you cleaned it up, was it mangled? It's not easy for the untrained eye to evaluate the condition of a propeller, I continue to suspect that your biggest issue is likely the prop.
    Gerald
     
  5. valvebounce
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    valvebounce Senior Member

    The prop is the original that came standard with the engine I think.It had been submerged on a boat on the river trent for a while(freshwater)
    There were pits starting to show so I filled them with epoxy and painted it.There were tiny dings on the edges of the blades,but not detrimental to the overall dia when I smoothed them out.
    Incidentally,I ran the boat with the 5hp yamaha spare engine,and the speed was not much less.
    The revs came up when I accelerated the 18hp,and the wake was what I expected,just wouldn't plane.
     
  6. Commuter Boats
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    The level of precision as to the shape and weight of the blades is such that it's unlikely that the propeller that you have is very efficient. So first you need a propeller that is hydrodynamicly capable of transferring the rotational energy of the engine into thrust in the water and secondly the propeller needs to be of the right pitch ( gear ratio would be a crude analogy ) to match the engine to the load / boat.

    A propeller to push a light fast boat is quite different than one that would push a heavy slow boat.
    Gerald
     
  7. valvebounce
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    valvebounce Senior Member

    That's a great bit of info Gerald,thanks.
    I'll get some pics up of the boat.
    So in your opinion,should the 18hp get the boat on the plane with the right prop?
    I'm 71,and can move the boat about on the trailer on the flat quite easily.
    I rope fed the boat and trailer into the water via the slipway by hand last time out.(some indication of the weight)
    Because of my age and storage capabilities,I looked for a 14ft boat.This one took my eye because of the nice lines and decent freeboard,plus the safety rails.
     
  8. Commuter Boats
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    Yes I do believe that the 18 hp should be adequate but just as important as the propeller is between the engine in the water, the bottom of the boat has to provide a reasonable planing surface to the water. There's lots of potential for the bottom of the boat to be distorted or altered in such a way that it isn't optimized which would further debilitate a situation that didn't have abundant power.
    Gerald
     
  9. valvebounce
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    valvebounce Senior Member

    The hull of the boat has no breaks or deviations,and has never had to be repaired,the original lines are still intact.There are no pressure dents from being on the trailer etc.
    I'll get some pics up.My brother takes the pics,so it could be a few days before he gets time.
    So,like Arnie says "I'll be back"Haha
     
  10. Commuter Boats
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    And that was a significant concern..

    The photo should be taken from the stern with the camera just barely below the bottom of the boat, the photo angles on the propeller that would be most helpful would be in line with the shaft and 90° to the shaft and reasonable close-ups but if they're blurry they won't be of much value.
    Gerald
     
  11. valvebounce
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    valvebounce Senior Member

    I'll pass the info on to my brother,he is pretty good at taking pics.
     
  12. Commuter Boats
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    Outstanding, I look forward to seeing them and you've only got a decade head start on me...
     
  13. Ike
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    Ike Senior Member

    If you want to determine what the maximum rated power for your boat would be use the following formula. Centerline length times transom width times 2 minus 90. thats' in feet (such as 14.2 feet) transom width is the maximum width including rub rails.

    What you get is the max allowed. But a 14 foot boat with remote steering should not have a problem planing with an 18hp. engine. I too, suspect the prop. But also check the angle of the engine to the transom. If it's tucked in, moved it out, and vice versa.
     
  14. Commuter Boats
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    The US Coast Guard formula that you describe is intended to be conservative on most common boats and is a great guideline but as to maximum allowed, does the UK have a similar standard to ours?
    Gerald
     

  15. Ike
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    Ike Senior Member

    The standard in the UK is in the Recreational Craft Directive which is taken directly from the ISO Standard, as follows:

    The maximum power capacity for boats with outboard
    engines of any length up to 26.2ft (8m) shall be determined by the manufacturer and if the maximum speed is greater than
    7 x (sq root of the hull length) in knots where L is the length of the hull,
    the maximum powe r shall be verified by the ISO Avoidance Line Test
    .
    Annex C is for guidance only and may be used for initial performance
    evaluation. Outboard craft supplied with both tiller and
    wheel steering should be tested in both configurations.
    (4.1)
    The maximum power capacity shall be stated in kW
    together with the word ‘MAX’ and a propeller symbol on a Capacity Label located in view of the helm.

    What that all means is that the manufacturers use the power formula (the same formula as the US) to determine the HP, but if the boat with the maximum rated HP exceeds the speed determined by 7 times the Sq root of L then they have to also perform an avoidance test.

    The square root of 14 is 3.74. that times 7 is 26. I doubt your boat will go 26 knots (29 .25 mph) so the formula is the rule.

    Also you don't have to put on the maximum. Any amount less is ok.
     
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