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Shaft tubes done

Discussion in 'Technical Discussion' started by Bluefin, Mar 26, 2010.

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  1. Bluefin

    Bluefin New Member

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    Ok, the shaft tubes are on. VERY preliminary results show a reduction of 4 gal per hour at 1850, a reduction of 4-5% load and a gain of about 1 kt. Top end went from 2320 to 2345. Once I get full numbers I will post. Getting ready to take the boat on a trip so it will be a couple of weeks.

    Here is what was done.
    First, remove old bearings, then shafts.

    Then new bearing were installed. Full size bearing were put half in and half out of both struts, and another bearing was cut in half to fill in the rest of the strut area.

    Attached Files:

  2. Bluefin

    Bluefin New Member

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    next the tube was cut so that it would fit over the bearings. Also they were tapped for a set screw to keep the tubes from turning. And a small hole was cut in the bottom to allow water to cool the after bearings.

    Attached Files:

  3. Bluefin

    Bluefin New Member

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    then covered with ad tech and faired

    Attached Files:

  4. Bluefin

    Bluefin New Member

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    then painted and we were done. Quite simple.

    Attached Files:

  5. K1W1

    K1W1 Senior Member

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    Hi,

    That looks like a very neat install.

    I have to say when I only saw the first part of your post I was asking myself how that little bit of bearing sticking out could make any difference.
  6. YES!

    YES! Senior Member

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    I would be curious to know the condition of the running gear, bearings, wheels, bottom coating prior to the stern tube installation.

    Theses are the level of percentage improvements I typically see after after a thorough bottom job which usually includes work on the engine top ends as well.

    Hard to believe that 3 feet or so of stern tube located aft of a flow-damaging strut could have any significant impact on performance.

    Any clarifications would be appreciated.
  7. Bluefin

    Bluefin New Member

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    Bottom (including struts, etc) had been cleaned by a diver within two weeks.
    You can see the condition of the bottom in the first photos. Those are before the re-paint.
    Props had a small amount of cavitation burn but otherwise were in great shape.

    I agree that if the bottom and gear had been covered up that it would make a great difference.

    I couldn't imagine that the tubes could make any difference if they didn't go all the way to the hull either. But the guy supplying the tubes said that they had researched and that if you had clean water 3' or more in front of the wheels that's all you needed. He would gladly sell us more material but we didn't need it.
  8. Marmot

    Marmot Senior Member

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    It cost a lot of money, it has to work! :rolleyes:
  9. Bamboo

    Bamboo Senior Member

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    How do the bearings get water flow? That may cause the bearings to have premature wear- and in which case brings to mind- how do you change the bearings? Will that extra work at haul out time cost more than the fuel savings?
  10. Bluefin

    Bluefin New Member

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    there is a small hole in the bottom of the tube for water flow to the aft bearings.

    Yes, when the bearings have to be replaced, the tube will have to be cut off, and re-glassed after replacing bearings. Will this offset savings? Perhaps. I'll know a lot more after running for a while and getting good numbers.

    Obviously there are a lot of people that think this whole thing is hokum. Maybe it is. All I can say is that I will post my true numbers when I have gathered all info, and we'll find out. Did I waste my $$? Maybe, maybe not. We'll see.
  11. Silver Lining

    Silver Lining Member

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    Why not use aero tubing? In formula cars the suspension components are all made from aero tubing as used on older airplane construction and there is a measurable effect on drag and top speed. In the water, (reynolds number?) the flow separation effects and drag values may be substantially improved. Aero shrouds can also be effectively used to improve flow around round tubing known to have non-optimized flow properties.
  12. PropBet

    PropBet Senior Member

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    Subscribed.

    I'd be interested in seeing what the inside of that tube looks like after a season of growth in the water where a where diver won't be able to clean regularly. Additionally, how the water holes hold up to (again) growth, potential plugging, and if flow to the bearings is thereby restricted over time and use.

    What flavor boat is this being done to?
  13. Bluefin

    Bluefin New Member

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    It's a carolina boat. In the same yard, they just pulled a Paul Spencer that has had the tubes since new (2005). In that one, there is no intermediary strut. It has the same small hole for bearing cooling and no issues since 2005. It was built with the tubes so no idea what difference they make.
  14. Marmot

    Marmot Senior Member

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    Does that mean the tubes are in effect a part of the hull? Like an extension of the shaft log?

    If so then the water comes from the engine raw water circuit through the log under pressure and that small hole probably doesn't even need to be there since, if the tube is sealed at the hull end, all it does is reduce flow through the strut bearing.

    Given that even a zinc on the shaft too close to the strut can wreck the flow to a bearing I am curious where the water to lube the aft strut bearing is going to come from on yours.
  15. Capt J

    Capt J Senior Member

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    Interesting. The tube is going to have more resistance then the smaller diameter shaft, but it also is going have less resistance because the water can flow over the rear strut much more smoothly. I'm guessing it gets good waterflow through the front strut, and the small hole is additional and it is enough to cool the rear strut.

    Striker enclosed the shafts in a housing all of the way back on a few sportfish if I remember correctly......but then they never caught on. I would think one tube all of the way would be even more efficient, but who knows.
  16. Seafarer

    Seafarer Senior Member

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    This sounds like a Wall St. analyst call I was in on many years back, when that was my gig... "I expect that if traders view today's news as good prices will strike sharply upwards. If they don't, the market will shift down. If I'm wrong, we'll know quickly."
  17. Capt J

    Capt J Senior Member

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    Actually, nothing of any substance should grow inside the tube. In order for growth you need waterflow or current, and sunlight. I don't see any sunlight getting inside of the tube and while the boat is sitting at the dock, the water inside the tube should be stagnant, so I don't feel growth would be a problem.

    I ran a 58' Striker for a year or two while it was for sale and it had two seachests. The port seachest only had the engine and generator drawing off of it, and with the lid off you got sunlight in there coming up from underneath the boat. It grew NO barnacles. The starboard one had the starboard engine and generator, AND A/C pickup (never plumb the a/c pickup to a seachest), it grew barnacles like wildfire and in 2 months would look like a coral reef in there because it had sunlight and plenty of waterflow in the seachest. The port side, when you took the lid off, if the boat hadn't been run in a week or longer, the water inside was stagnant and rotton smelling.
  18. Bamboo

    Bamboo Senior Member

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    Won't the shafts have to be pulled also?

    Glad you are willing to be the test case.
  19. Bill106

    Bill106 Senior Member

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    It constantly amazes me that builders don't plan for maintenance that they KNOW will eventually have to be performed! Must be either a case "it's not my problem" or thikning it will generate future revenue, neither reason IMHO is sufficient justification.

    Shaft tubes CAN be made removeable so cutlass bearing replacement doesn't require pulling the shafts. They can be split down the middle into two halves, and have grooves are machined into them that fit two-piece collars (much like a shaft zinc) to hold them together. These collars are then flush to the outside of the tube and don't cause any additional drag or turbulence. Pulling them off for bearing replacement AND inspection is a simple matter of loosening a few screws.

    When you find it's time to cut them off, send them to a machine shop and have them modified and some collars made. It only costs about $750 per tube to do all the work (around here) which should quickly be recouped at the next bearing change.

    By the way, she's a pretty boat!
  20. Bamboo

    Bamboo Senior Member

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    Is there a update of how the performance has held up over time here? Thanks.