newbie - planing?

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

  1. IMP-ish
    Joined: Jan 2011
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    Location: united states of america

    IMP-ish powerboater

    I don't follow, your #5 is right.
     
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Sorry rosbullterrier, I didn't intend to upset you.
    Because your boat is only 27 ft long and was designed for stern drives the engine bay isn't long enough to accommodate a straight prop shaft. You'd have to change the layout completely or extend the stern (that is what I did). Compared to that, converting car engines is a simple operation.
    Assuming you have two OMC legs in good condition, I would leave things the way they are.

    If you want real economy you have to abandon the idea of planing and be satisfied with a little more than hull speed. Only then, the match between props and engines can provide optimal efficiency. In your case, two 40 hp engines would probably be enough.

    But changing from planing at 28 kt to displacement at 8 kt has the psychological effects of impotence. My family members made remarks that really hurt!
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Those old OMC Cobra drives have a number of known issues (and recalls) on them. Some have had good luck with them, but I never have. If they've not been well maintained, you'd be best advised to consider something else, which is a significant expense.
     
  4. rosbullterrier
    Joined: Feb 2005
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    Location: Cornwall, UK

    rosbullterrier Junior Member

    A reassuring message CDK.
    I think I have seen advertised details for half submerged propellers (I cannot remember their correct term) which stick out some way from the stern with a sort of platform above. I realise they would be racing props.
    My engine bay does extend about 4' from the transom forward - not enough room for a small gearbox and a V engine?

    In the meantime I shall continue to read and dream re engine marinising, propshaft installation and give up all life enhancments to fund the petrol account.

    Thanks for the Lancing info Frosty, but really don't like Ford engines, they are a bit like miniature archaic versions of American iron. Toyota diesel sounds good. How come a chap who lives somewhere warm and wet knows GB so well? I have been trying to persuade Her Indoors to move to South Portugal (with me) but no luck . . .
    ps Foey is spelt Fowey, pronounced like boy
     
  5. Frosty

    Frosty Previous Member

    Ah well that was just a example I said from Jags to fords.

    I Know Fowey well was there last July crab sandwich to take out 6 pounds.95

    If you live there you WILL know my sisters company.

    Charlestown is up for sale again.
     
  6. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    These are surface piercing props or 'surface drives'. Lancing offers a simple, fixed type with rudders, there also are expensive articulate designs.
    Because they all share the partly submerged mode of operation, noise and vibration are real issues and limit their acceptance. Most people also do not like the idea of having some sort of meat grinder behind their boat.

    Your 4' engine bay has room for a 4-inline engine, gearbox and a short U-joint shaft; the rest protrudes behind the transom. I've built a GRP extension with oil filled stern tubes, search for DIY tunnel drives on this forum if you are interested.
     
  7. Frosty

    Frosty Previous Member


    CDK please,--- don't,----- if you don't know --Im sorry but they are vibration free. I run a pair,--- the smoothness has to be felt to be believed and where is the noise coming from.

    They don't cavitate like a normal prop even though they work half submerged.
     
  8. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I'm not sure of the system you're running Frosty, but they're likely clevers, which work by bashing their way through the water. Most are offshoots of the supercavating prop designs (hybrid cousins, so to speak), though because they spin at slower speeds and are partly submerged, they entrap air instead of vaporized water, like that on a suppercavating prop. True surface piercing props could be called ventilated, rather than cavated for this reason, but they are noisier than submerged props. Early versions of these systems did have vibration issues, but many are well isolated now, as yours may be. The noise comes from the wedge shaped blades smashing their way through the water. They're good setups for high speed craft, not so much for modest speeds. What system are you using? Prop spec's?
     
  9. Frosty

    Frosty Previous Member

    I have seafury diamond backs even though the patent is held by Levy a friend of mine. The wedge is not the leading edge,- the entry is razor and yes they are 4 blade cleavers.

    The diamond back of the levi was to help reverse. . As you say there is no cavitation but airiation.

    If you have vibration it is not the surface prop design. Multi blade surface props are obviously better

    My system was designed by Seafury and the engines were supplied to thier specs along with Gb ratio and shafts.

    Noise is none existant, only engines , vibration above idle is none existant.

    Surface propulsion is the quietest and vibration free there is and not the issue previously posted.

    I have no reason to assume other systems are any different.

    Slow speed surface drives are possible ,Sony talked of tugs with surface drives.

    I achieve higher speed than manufacturer with 100HP less.
     
  10. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    You are the only one with such a positive opinion about surface drives. Before I started designing my own drive system, I communicated with Mike Bellamy of Lancing Marine. At the time he just had his own surface drives installed and tested. He used words like "acceptable" and "promising" describing vibration and noise but could not convince me to place the order.

    Later I met a guy in our bay who was in the "import/export" business between Montenegro and Italy. He had a 39ft RIB equipped with twin V-8's and impressive surface drives. Enormous prop shafts and 5 blade cleavers to reduce vibration, but the boat still shuddered at certain rpm settings and I could hear him approach when he was still miles away.
    He took it all for granted because he needed speed and the ability to operate in very shallow waters making his deliveries.
     

  11. Frosty

    Frosty Previous Member

    The best thing about surface drives is the smoothyness of them . What you have is tunnels if Im not mistaken.


    My shafts are just 4 feet long 2 inches thick with a certificate of balance on the props

    Now you say your an engineer so what is vibrating? what is making noise.

    I most definitely do not have either.
     
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