low and slow - Making the Albin 27 more efficient

Discussion in 'Powerboats' started by tranmkp, Jan 28, 2008.

  1. tranmkp
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    tranmkp Junior Member

    IM not in a hurry obviously. Just would like to hear some feeback on how to make the Van as efficient as possible --trim tabs? Absurd maybe? It has a Westy W70 with a HUrth 1-1.

    Next to a clean fair bottom, clean and balanced shaft and prop, keeping her light - what else can be done? Would a foil shaped rudder be better than a flat plate in the 7 -10 knt range?

    thanks
     
  2. longliner45
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    longliner45 Senior Member

    are these boats ,,,heavy in the stern?or dose the sailplan put wieght on the stern ,,or do I just not understand?...longliner
     
  3. tranmkp
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    tranmkp Junior Member

    little pocket trawler

    Not a motorsailor -(you might be thinking of the albin vega) Not a true HEAVY disp hull. About 7000#. Mid engine - maybe simi plane if enough power - but its not going to happen with a 70hp diesel. Not to heavy aft -
     
  4. Pericles
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    Pericles Senior Member

  5. tranmkp
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    tranmkp Junior Member

    Thanks for the links

    I had already checked out all those links - joined the albin group -

    not much advice as to bottom prep - still thinking about effects of rudder mod on this type of boat.

    As for the bottom, Im just going to treat it like my last sailboat, clean it, fair it, shoot it with vc offshore, balance and polish the prop and thats all I can think of.
     
  6. Pericles
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    Pericles Senior Member

    Well, you know what they say? Great minds think alike---------and fools seldom differ!!:D :D :D

    Good luck with your plans, use http://www.coppercoat.com/about.htm

    Regards,

    Pericles
     
  7. FAST FRED
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    FAST FRED Senior Member

    If you want the best fuel economy , replacing the 70HP (insane size!) engine with a more rational (and more modern ) unit will help.

    At 7000lbs the boat only about 3 tons and will run hull speed with about 10hp.

    A 15Hp diesel with suitable reduction gear and matched 2 blade prop is about as good as you will get. The weight reduction 500lbs? will also help efficiency.

    FF
     
  8. Guest625101138

    Guest625101138 Previous Member

    Fred
    I agree. For a boat of that size I expect there is a huge difference in drag between 7kts and 10kts. The latter speed would make a nice big wake requiring a lot of power. Far more sedate at 7kts and not much longer to get there unless the current is strong.

    Rick
     
  9. tranmkp
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    tranmkp Junior Member

    that makes a lot of sense

    Fred and Rick

    I like the way you think, with the 70hp (hauled it yeaterday - now clean bottom clean prop and clean rudder (flat plate) boat feels very slippery -

    7.5knts @ 1500rpm. On the cam (top for power curve) 2700/2800rpm 11.7knts - with a wake so big you could surf on it...

    on thought - from a drag perspective - wouldnt a foil shaped rudder be more efficient than a flat plate? Im not talking about turning efficiency?

    2 blade prop? expalin why nto a 3 blade - 2 blade is easier to turn? Less drag?
     
  10. Guest625101138

    Guest625101138 Previous Member

    Rudder - a flat plate is not a very good rudder. The best rudder has a thickness ratio of around 20%. If you reduce the length of the rudder, keep it at the same draft and make it thicker you can dramatically reduce the area and still achieve the same steering force. SO the thick rudder is ONLY BETTER if you reduce the area. The benefit will not be much - unlikely to measure any increase in top speed.

    Prop - The prop blades become more efficient as their aspect ratio increases. AR is blade area divided by maximum chord. However you cannot just fit a a two bladed prop with narrow blades and get a better result. If the blades are not acting on enough area they will operate at a high angle of attack resulting in high prop slip. So if the prop is heavily loaded, it is better to have more blades. Most boat props are a compromise to reduce draft so blades are heavily loaded. At full throttle your prop is heavily loaded and efficiency has dropped off. On the other hand 11.7kts is a good speed for 70HP in that boat.

    I would need to spend time to model the hull to determine the best prop but I doubt that you would have the room to swing it. At full throttle and speed of 11.7kts a 16" prop would have an efficiency under 60%.

    Assuming the boat requires 10kW (13HP) to do 7.5kts you need to go up to a prop 3ft diameter and turn it at 400rpm to get above 80% efficiency. Even with this diameter a 4-bladed prop is better than 2-bladed.

    There is not much point in trying to improve prop efficiency at the top end because the extra force generated is simply increasing the wake.

    If you are after fuel economy then a smaller engine swinging the largest 4-bladed prop that you can fit at slow rpm will make a measurable difference to fuel consumption in the 7 to 8 knot range. Of course top speed will NOT be much more than 8kts.

    The exercise to go through this is time consuming and I would need to know the current prop size, gearing and a good idea of hull shape just to confirm current conditions. Then it is a matter of looking at options like how big the prop could be.

    Unless you are using the boat a lot it is probably more cost effective just to throttle back on the current set up and tolerate the inefficiencies. The hull is not particularly efficient anyhow even at 7kts.

    These boats were designed and built in an era when hydrocarbons were cheap and use for liesure not regarded as anti-social. If you were designing now you would have a longer, more slender hull, canoe under water with a bigger prop and requiring much less power. How things used to be when engine power to weight was much less than now.

    Rick W.
     
  11. tranmkp
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    tranmkp Junior Member

    goodness - thats a lot of information to ponder...

    slender canoe underbodies... I cant think of any powerboats that meet that configuration (Im not an historian) - I can think of some sailboats, remove the darn rig - repower and reprop I suppose. Something like a Swede 41 for example.

    I almost feel guilty now.. my 1 gallon an hour guzzler.
     
  12. TeddyDiver
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    TeddyDiver Gollywobbler

    Around here we have river boats with trim planes to get them planning with outboard engine. The boats are 5m LOA 1,1m Beam 300kg Disp (fully loaded). They plan allready with 5hp engine but some are used up 20hp's with speeds accordingly from 8 to 15 knots, and thats a reasonable speed for a row boat :cool:
    These trim planes are placed horzontally just one inch above the waterline, made of 10mm plywood, 80cm wide and reaching about 30cm back from the stern. Front side is cut to fit agains the hull without any gaps. I will take a picture an place it in the Gallery later.
    By the way, what is Westy W70? a Westerbeke perhaps?
     
    Last edited: Feb 3, 2008
  13. tranmkp
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    tranmkp Junior Member

    long and slender

    Thinking about the above - really it sound as though you could actually buy a used shaol draft sailboat, strike the rig, build a pilothouse or a hard doger windshield ala HR, and change the prop/geabox and off you go?

    MKP
     
  14. tranmkp
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    tranmkp Junior Member

    you are correct.
     

  15. tranmkp
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    tranmkp Junior Member

    back on topic - afterplanes/transom extension

    So the guy I sold my sailboat to is going to add a sugar scoop to it - he has a shop in Costa Mesa , a go fast guy that seems to know what he is talking about. I also found a thread here that sort of mentions the same thing can be done on powerboats.

    So, would adding additional length to the hull of the Albin 27 actuall help? Imagine the teak transom step, rather than being just a step was actually structural. Adding about 20 more inches to the waterline length.

    Math types might help on this. original 24.4 w/l. Would the 20 inches really help - anything longer would just be stupid.

    thanks
     
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