Here's The true Sea Sled Story, circa1985

Discussion in 'Boat Design' started by tonydignity, Aug 23, 2004.

  1. Scott Nomates
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    Scott Nomates Junior Member

    Hi, when you say "old veterans" do you mean salty old boaties or actually returned military men? We launched today with a very experienced "boatie" helping. Seems like we haven't got it right yet and any input would be welcome...here's what it looks like in the water:

     
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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    You don't muck around, you have got the thing on the water and running pretty quickly. I got the sense that the boat is pretty heavy, and maybe trimming the engine down might have helped. Was there any cavitation/ventilation ? Engine racing ? Love the sound of the old straight six ! Is the prop stainless or alloy ? I would think stainless would be just about mandatory on a boat like that, where the prop is not working in the "cleanest" water. The best prop would probably be a five blade stainless.
     
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  3. Scott Nomates
    Joined: Dec 2019
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    Location: Queensland Australia

    Scott Nomates Junior Member

    I don't really know what to look for (listen to?) to identify cavitation or ventilation. When there was 4 adults in the boat, there were times where the prop sounded like it wasn't in the water - this wasn't the case with just 2 adults and 1 kid. The motor was not up (trim/tilt wise) at all...it was a low as it could go the whole time. As you may have heard, the guys filming reckon we need to add weight to the bow as they say it won't plane without it. The boat did "slap" the water a few times pretty hard when we had it at speed.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I say cavitation/ventilation because the two are difficult to distinguish, but both result in a sudden increase in engine revs, and if you have a tacho, the needle will jump wildly. Not desirable from any point of view. But not unexpected with this kind of boat. So is that an aluminium propeller ? You should check the height of the engine, specifically whether the cavitation plate is more or less level with the bottom of the boat. I really think you will not get satisfactory results unless you get the engine and prop set up correctly, these things are not "normal" boats. AS for the audio comment that the engine is too heavy, I'd say no, they are a relatively light engine, and if running well have plenty of power. If you are slapping hard in the Pumicestone Passage, then I fear the bay will be a challenge. :eek:
     
  5. Scott Nomates
    Joined: Dec 2019
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    Location: Queensland Australia

    Scott Nomates Junior Member

    Thanks a lot for your input...thats useful info. I do think we need to look more into the engine and prop set up. I don't recall where the cavitation plate sits relative to the bottom of the boat, I will check tomorrow. You know this area? Thats cool. Yeah I was thinking the same re Moreton Bay but it's early days - that was my first go at running the boat (or any boat really) ever...I figure we'll get to know it better with time and I'm still keen for a Moreton Island run (maybe one day).
     
  6. Scott Nomates
    Joined: Dec 2019
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    Location: Queensland Australia

    Scott Nomates Junior Member

    Oh and yes, Aluminium Prop...it had a big 90 degree bend on one blade when we got it but our mechanic (whom suits the nickname Mr Miyagi - as I affectionately refer to him as), repaired it well.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Is the mechanic satisfied the engine is running OK, good compression etc ? Most power cats run stainless props because alloy cavitates in turns too readily, and a sea sled is more likely to act up with alloy. I have been in cats with alloy props, and both cavitated in turns, no good. You have the extra complication of aerated water entering the stream, the cat does not have. Stainless (thinner, cupped blades) is a different world, but to make a good thing of it, I would go to a five-blade, the extra surface area is more tolerant of aerated water. Check what the prop pitch is, too, it should be stamped on the prop hub somewhere. You may get away with the alloy for test purposes, to see what revs you get at full throttle..
     
  8. Scott Nomates
    Joined: Dec 2019
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    Location: Queensland Australia

    Scott Nomates Junior Member

    Oh...I'll check that too...maybe it is a stainless prop. Yes, the mechanic is happy with the motor - it's tricky to start cold and not having a functional choke solenoid is a pain but, once warm, the motor is running super well so far (fingers crossed -it is 40 years old).
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I would say 90% likely to be alloy, especially if painted black. You should be able to pick up a choke solenoid from a wrecker. I had those old mercury straight sixes, good engine, but the alloy pre 1984 was rubbish, and most are long corroded out. Yours maybe survived through fresh water use ?
     
  10. Scott Nomates
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    Location: Queensland Australia

    Scott Nomates Junior Member

    Yeah, it looks like someone cared for it most of it's life. Yeah the prop is painted...what prop should I get? Stainless 5 blade...any thoughts on the correct pitch setting? This sounds like it might help a lot
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Maybe just experiment with what you currently have, before changing the prop, in terms of engine height, in-trim etc. I definitely though some in-trim would have been worth a try, looking at the video. But the odds are, you will need stainless. I would say 15-17" pitch would be about right. Is there a tacho fitted ?
     
  12. Scott Nomates
    Joined: Dec 2019
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    Location: Queensland Australia

    Scott Nomates Junior Member

    Thanks once again! I appreciate your patience and knowledge. There is no tacho fitted - none of the gauges are currently functional but there is no tach on there anyway...I've been thinking about which ones to add though.
    When you say "in trim" do you mean dropping the motor down from being on an angle? It's right at the bottom of that all through that run
    I like the idea of having more pitch in the prop - I'll check with Mr Miyagi and see about sourcing a prop
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    You want the right pitch, and too little pitch is preferable to too much. Without a tacho, selection is guesswork.It really does look as though the motor is not trimmed in hard, or there is not a lot of rake on the transom, or maybe you have a pin that is not in the hole nearest the transom. Maybe see if you can post a picture of the transom and engine at the same level as the bottom of the boat. Looking at the video you posted yesterday, it looks low enough.
     
  14. DogCavalry
    Joined: Sep 2019
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    Location: Vancouver bc

    DogCavalry Senior Member

    I agree Mr E. The engine looks like it could stand to trim in significantly. Change the line of thrust.

    Scott, he means adjust the engine so that the prop and leg are closer to the transom. If that wasn't already clear from context, or your knowledge.

    Massive congratulations Scott! You're well ahead of me. My sled won't sea water for another year, I think.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    He has done well, but my first impression was to think some more in-trim would help. Fortunately, this is easily accomplished by fitting transom wedges, a relatively cheap and easy contrivance, and you don't need to remove the engine to slip them into place.
     
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