What is best Prop selection for new engine?

Discussion in 'Inboards' started by Gilaroo, Jan 4, 2009.

  1. Gilaroo
    Joined: Dec 2008
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    Gilaroo Junior Member

    I have a small boat with inboard board engine which I am replacing with a new Yanmar 21 HP , 3600 rpm, reduction ratio if 2.21. I have to change my prop too as it is Left hand direction while the new Yanmar is clockwise. The current volvo penta MD11C 23 Hp, 2500 rpm, gear reduction ratio of 1.91 with current prop is stamped and reads "12.78 C 7 LH" which I asssume is 12.78 '' diameter and pitch of 7'' and Left Hand (anyone know what the "C" means?) .

    I have used CastleMarines http://www.castlemarine.co.uk/ prop calc application which does the calculation to get info on what is best prop for new set up. In summary:

    Boat is 18.2 ft
    LWL 14.6
    Displacement weigh is approx 1400 KG
    Max RPM is 3600, reduction ratio 2.21 giving 1629 rpm at the prop

    The results from the prop calc are as follows

    1. If Full Displacment = Diameter 13.34'' pitch 8.68 DAR 55%
    2. If Semi Displacment = Diameter 13.76" pitch 11.56 DAR 55%
    3. If Planning Hull = Diameter 13.76" pitch 10.97 DAR 55%

    My question is:

    Having posted on another thread most feedback indicated that this is most likely a semi displacment boat and possibly could be a planning hull. But because I have only 21 horses, it will never get to planning speed. However, will there be any chance I can get to a SLR ratio somewhere between displacment and semi displacment situation with 21 Hp to improve on the max displacement speed of about 5.35 knots?

    If I can improve the SLR ratio then I will order as per
    Semi Displacment = 13.76" pitch 11.56 DAR 55%, otherwise I guess I should
    just stick to the Full displacement configuration 13.34 pitch 8.68 ???

    The prop calc indicates that therotically I could get to 9.91 knots which
    would be a great improvement on the current max hull speed of 5.35.

    Any input advise would be very welcome before I order the new prop.

    Please see profile photo of hull and stern

    Paul
     

    Attached Files:

    Last edited: Jan 4, 2009
  2. Guest625101138

    Guest625101138 Previous Member

    Do you have any idea of the weight of the boat?

    Rick W
     
  3. Joakim
    Joined: Apr 2004
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    Joakim Senior Member

    Are you sure about the old propeller? If those values are real, the propeller would have been VERY badly chosen. It takes only ~4 HP to turn a 12.78x7 propeller at 1300 rpm shaft speed, which is also enough for more than 5 kn speed for the boat you have.

    I have no real idea what your boat could achieve with 21 HP. Certainly not more than 10 kn, but possibly only ~6 kn. It is heavy and short, thus it probably needs lots of power. All this would be much easier, if we really knew what happened with the old motor.

    I would not thrust the propcalc, escpecially since your boat is not the most common one. Propcalc rutines are very simple and have been shown to produce very inaccurate resulsts in several cases. E.g. for my sailboat it says I need 13 HP for the speed I get with 9 HP. And also it suggests a 14x10 propeller, when I have a 14x8 propeller and I'm not getting the very full rpm.

    Joakim
     
  4. Gilaroo
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    Gilaroo Junior Member

    Rick, it's about 1400kg fully loaded
     
  5. Gilaroo
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    Gilaroo Junior Member

    Joakim

    thanks for that, the stamp on the old prop definately says 12.78 and I measured the diameter and its very close to 13'' so that metric is correct. The pitch I can't say for sure the stamp says "12.78 C7 LH" , so I assume C7 refers to pitch of 7 and LH (left hand which it is).

    The old engine maxed out about 2000 rpm never got to 2500 rpm and this got it to max hull speed.

    The boat might be a bit lighter than 1400kg maybe 1200-1400 don't have
    spec, it looks like an old late 70s french boat called Chiberta but I cant find anything on it on web.

    On prop I can only go to 14'' max no more room, but Im still confused as to whether it might get into a semi displacement position with the the new engine, give the shaft rpm at 1620 . What yeh think ?

    Paul
     
  6. Guest625101138

    Guest625101138 Previous Member

    That is much lighter than it looks. In that case I would class it as semi-displacement. I expect you would need around 40HP to get it to plane. With 21HP you should get nice bow up and starting to get some lift. I would be surprised if it could not get 8kts.

    Rick W
     
  7. Gilaroo
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    Gilaroo Junior Member

    Thanks Rick that's a bit more encouraging, I'll talk to the prop supplier also to get their advice on the prop size they would recommend.

    Paul
     
  8. Frosty

    Frosty Previous Member

    We got 2 threads going here!! Ive just replied to the other.
     
  9. Joakim
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    Joakim Senior Member

    I just found some data for 18' day cruisers weighing 1000-1200 kg. These are quite far from this boat, but gives you some idea of the needed power for a planning hull of this size and weigh.

    At about 7 kn a 130 hp gasoline engine had a consumption of 9 l/h and a 190 hp diesel over 6 l/h. Both suggest clearly over 20 hp of power. At 12 kn the consumptions were roughly doubled, thus 40 hp would probably not get those boats to plane and 20 hp would not get them over 7 kn. The propellers of these stern drives are typically more than the 13" max here, thus there will probably be no gain in efficiency.

    Joakim
     
  10. Gilaroo
    Joined: Dec 2008
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    Gilaroo Junior Member

    Thanks for new data Joakim,

    I spoke with an experience prop shop in the UK and they are recommending a 14X11 prop, so I think I'm just going to go with that and hope I can pull 1 or may 2 knots on the displacement speed of 5.35K. If not I'll just have to live
    with displacement speed, at least I should get a much more effecient ride given I measured the existing prop which was 14X17 way too much pitch which explains why I never got more than 1900 RPM on the last engine which had a 2500 max rating. Really appreciate your input and advise, it helped greatly. When I get out and about I'll drop a n update on this thread to say what I actually managed and how the ride was.

    For the money Ive spent on this little boat I could now have had bought a small 18-20ft new fisher boat with everything spanking new, but there is something special with an old wee boat and the unique challenges it offers + you learn a lot more hands on stuff which is all part of the fun. :)

    Paul
     
  11. Joakim
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    Joakim Senior Member

    How did the old propeller change from 12.78x7 to 14x17??? You should really be more careful about your data before posting questions. Were the clearances also wrong?

    However, now the data makes much more sense. A 14x17 propeller requires over 16 HP at 1000 rpm shaft speed and delivers a thrust of about 1600 N (with cavitation).

    Why did the boat have such a propeller? Maybe the designer/previous owner discovered, that the boat is not going to be fast and just looked after low rpm at cruising speed? The boat probably did over 5 kn at 1200 rpm (engine) and the rest was never planned to be used.

    A 14x11 propeller will be somewhat more efficient and less prone to cavitation, but the thrust will not be over 2000 N, thus not a dramatic change and I would not expect a dramatic change in speed.

    I hope there is still something wrong in the data and you'll get more speed.

    Joakim
     
  12. Frosty

    Frosty Previous Member

    If its too big and too close the the hull it will damage the hull and will vibrate. Similarly it needs to be away from the rudder or it will vibrate.

    In your photo it look like that prop to hull distance is inadequate.
     

  13. Gilaroo
    Joined: Dec 2008
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    Gilaroo Junior Member

    Joakim

    just to clarify the data I give on the old prop was the best info I had from what was stamped on the prop, however when I spoke to the prop shop they said 12.78 was not the diameter and 12.78 was prob the date 12/78 (dec 78 which fits with the age of the boat) , when I took the prop off it actually measured 14'' . They also knew what the "C7" is a reference was which no one was able to clarify for me, it is a ref to a blade which has more DAR than a standard 52-55% DAR and as we all knew the 'LH' was left hand. I then used a manual measurement procedure to measure the pitch using a straighline method I found and protractor which give me circa 17 pitch.

    As for the new prop , well clearly will be the same as old prop so keeping 14'' makes sense to go with. From these posts and other reserach I've done including buying the book you recommended by Dave Gerr , my I instincts tell me that a 12'' pitch would be the optimum prop for new engine. But who am I to argue with a guy who has made props for 30 years and has a lot of happy customers ?? He made a good point which was not to get too obsessed with the maths, within 2 seconds of giving him the data he was very convinced of a 14X11 as the best option, at the end of the day it's not going to be miles off.

    Im going to decide this afternoon, and again appreciate your posts. I still may order the 14X12 :)

    Paul
     
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