newbie - planing?

Discussion in 'Powerboats' started by rosbullterrier, Dec 11, 2012.

  1. rosbullterrier
    Joined: Feb 2005
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    Gents, I've just finished my first and only 27' powerboat.
    OK - apart from someone phoning me from the bank - how do I tell when it's on the plane?
    It has a rev counter for each engine and trim tabs to keep the bow down.
    And I have a GPS to show the speed.

    Looking for that economic (!!) efficient speed . . .
     
  2. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

  3. Petros
    Joined: Oct 2007
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    Petros Senior Member

    economic efficient speed? go slow. there are a lot more factors at play than planing, but also fuel consumption vs. engine speed, prop pitch, etc.

    easiest way would be for you to make a simple chart from each time you take the boat out: measure fuel consumed (by topping up the tank after you get back), hold a constant speed, different for each trip, and as accurately as possible determine the distance you traveled on each trip. each time you go out pick a different speed to hold steady. This will provide a speed vs. fuel economy chart.

    Likely you will find the slower you go the less fuel consumed. So you have to choose what is the slowest speed you want to cruise at. If you were running a commercial business than you have to weight off cost vs. number of paying passengers or cargo to deliver. Usually the faster you go the more revenue you make because you can take more revenue generating trips per day/week/month. different consideration than for personal use.

    good luck
     
  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    When you're doing 14 knots, you're up on plane, but the boat will tell you this. Technically, you start full plane mode around 12 knots, but the bow will still be coming down and she'll still be "settling in", until you're in the 14 knot range, where her trim will level out.

    As to economic speed range, well the boat will tell you that as well. Run her up to WOT then back it off about 20% - 25% and this will probably be the sweet spot, of course depending on several factors.

    My point is the boat will do what she can, dependent on hull form, weight distribution, volume distribution, gearing, prop, power, etc. You can make some predictions about preformance, but you'll need to supply more information, such as hull type, full up weight, including fuel, crew and stores, HP on tap, prop specs, etc. I'm not sure the predictions are really very important, as you'll find all this out pretty quickly on launch day.

    Lastly, congratulations on your project and post some pictures, so we have something new to drool over.
     
  5. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    "Looking for that economic (!!) efficient speed . . ."

    You will quickly learn that these qualifications do not apply to powerboats.
    There certainly are throttle setting being more wasteful than others, like just below minimum planing speed, but 'economy' is not an option.
    This is clearly demonstrated when you suddenly pull the throttles to neutral and watch what happens. The boat's mass will keep it gliding for a few feet, but then it plunges back in, slows down to just a few knots, then gradually comes to a full stop.
    That last part you may call economical: any speed between 0 and hull speed requires little effort, but unfortunately your power train is designed for planing, so below hull speed the props have too much pitch and insufficient surface area to be efficient.

    When my boat still had planing capability supplied by two gas guzzlers I found out that there was very little difference in fuel consumption between going at 5 knots and planing at 20 knots. Even running on one engine saved no more than 15%.

    The dramatic change came when I changed from stern drives and gasoline to diesels and prop shafts. Now every mile covered costs only 10% of what it was before.
     
  6. rosbullterrier
    Joined: Feb 2005
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    I'm very grateful for your interest. Not a question to inspire great technical thought I'm afraid. However your general thoughts around the power boat subject are useful for starting me burning fuel on the briny . . .

    Everyone who has discussed my project has shuddered at the thought of petrol(gas) engines. Well, the boat isn't worth a fortune (1981 Fletcher Zingaro Express) and I have enjoyed rebuilding the twin OMC 3.8 V6's with hydraulic tilting engine rams!

    I kept thinking the cost of diesel replacement would be three times the value of the boat.

    But eventually the distance it will travel will be limited by the fuel in the tank so diesels will be necessary. Propshafts and diesels do sound good for the future.

    If I start looking for used diesel engines - any suggestions to replace my heavy old obsolete iron lumps? ( the 4 manifolds must weigh the equivalent of a modern engine!)
     
  7. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    There's nothing wrong with gas engines. There are countless tens of thousands of gas powered boats world wide, most with little issue. Problems with gas power is operator/owner oriented, more so than the fuel type. Good boating practice and common sense, will provide long life for both you and the boat.
     
  8. Frosty

    Frosty Previous Member

    Your on the right track If your in Foey then your close to the sea and look at any sea going boat,--its certainly shaft and diesel.

    Where is the nearest petrol station in Foey --it must be up many hills to St Austell.

    Every American lump I ever had I blew it,-- even a 350 L82 corvette spun a bearing. Horrible push rod 60's crap.

    Your right on the weight too Yanmar 6LP for instance is 380 kilo all in with 250HP and parts at the Toyota shop.

    Rodley Motors in Bradford do all American crap if you decide to stick.
     
  9. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    It all depends on how much you will actually use it.
    Before you go planning on putting in diesels-use it for a year or two,and keep track of the hours and fuel costs.Keep in mind the usage will likely go down as the years go by...
    Most of the time,it would take many years to repay the conversion costs including the decreased fuel costs every year.

    I knew of a guy that did this- I figured it would take him 25-28 years ( a 45' boat)to pay off the diesels-by which time they'd need rebuilding and/or very expensive work.
    He was 55 at the time.

    AFAIK the only way to do it cheaply is via VW diesels like CDK did...
     
  10. Frosty

    Frosty Previous Member

    Your regular American push rod iron motors will have needed replacement many many times.

    American iron does not like boats especially the exhaust manifolds.

    Take that into account and the figures may work.

    American iron do good torque like on a car,--- RPM is not what they do for long.
     
  11. IMP-ish
    Joined: Jan 2011
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    Location: united states of america

    IMP-ish powerboater

    GM blocks are good for 800 - 1200 hours in a boat before a rebuild or replace.
     
  12. CDK
    Joined: Aug 2007
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    CDK retired engineer

    I do enjoy this pro and con discussion about gasoline and American iron, but that is only a part of the story.
    It was the word 'economy' that triggered me. Regardless of engine choice, a boat designed for planing is always a thirsty animal. Of course diesels use less fuel, but the initial cost is higher, so with limited (recreational) use, gasoline may turn out to be cheaper.

    My statement about pitch and prop surface area in post #5 is wrong: the props of a planing craft do not have enough pitch for fuel efficiency at low speed.
     
  13. Frosty

    Frosty Previous Member

    But Mercruiser do just that with their 3 blade overlap clover leaf props . Probably trying to absorb torque from the torqy American iron.
     
  14. rosbullterrier
    Joined: Feb 2005
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    "I do enjoy this pro and con discussion about gasoline and American iron"
    - it amuses you sir but I'm shell shocked . . .

    Well back to my original concept 'I can buy many gallons of petrol with the cost of new engines'. Now I hav'nt used this boat in anger or extensively so we will see whether it is the one to keep - however the idea of changing running gear inspires thought. Maybe I'm a mechanic more than a captain.

    I've read before about marinised VW engines, and obviously they are plentiful. You will say the marinising is expensive and not so simple. I did build a replacement exhaust manifold for the 3.8 OMC in stainless - couldn't get a replacement here. I was quite impressed with the finished product (and it works). Won't try another without a Tig welder though. Mig is not good enough.

    But how do you change a planing hull to propshaft drive? Surely the propeller needs a keel in front of it? Two keels? My low deadrise stern hasn't got one keel!

    ps my American iron was hardly worn at all, it was the corrosion persuaded the previous owner they were scrap. I thought they were seized beyond help.
    At 3000 revs they boat is flying . . . (to me anyway)
     

  15. Frosty

    Frosty Previous Member

    You don't need a keel infront of a prop infact you should not on a planning boat.

    Lancingmarine.com

    They are in Southampton on the back road A 36 I think. ALL the bits and pieces to marinise most engines from V12 jags to Ford Transit.
     
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