Pocket Drive - modifications - photos

Discussion in 'Hydrodynamics and Aerodynamics' started by Mike Goebel, Jan 27, 2022.

  1. Mike Goebel
    Joined: Jan 2022
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    Mike Goebel Junior Member

    Hi, Trying to improve performance of a 30 Luhrs express open with prop pockets. Performs poorly.
    Props have been fined tuned and a sight cup added. This added 1 knot. Very high cavitations up to 2600 RPM. Trim tabs recessed into stern – full tab needed always needed to get bow down.S uspect props are not getting enough inlet flow. See photos.
    Specs:
    LOA 31' 6" 34 w/pulpit
    Length on waterline 27'7"
    Draft 2'3"
    Dry weight 14,000
    Tested weight 16,000 (fuel & crew0
    Yanmar 6 cyl turbo 6LP (315HP) 300 Hp @3400 / 250kw
    Props 19" x 27" pitch 4 blade, slight cup
    Gear 2.48:1 Prop pocket aperture 20-1/4" at transom
    1" to 1-1/4" prop tip clearance with hull pocket inside
    RPM
    1000 @ 4.4 kts
    1200 @ 5.8 kts
    1600 @ 7.1 kts
    2000 @ 7.9 kts
    2600 @ 8.6 kts
    3000 @ 13.6 kts
    3200 @ 15.6 kts
    3800 @ 22.0kts

    At 1100 - 32600rpm bow goes straight up; wallows; and digs a giant hole. Little trim tab response until 3000rpm. Large wake between 1400 – 2600 RPM. At 3200-3800rpm she runs OK, slightly bow up, but slow. Boat does not achieve published 27 knots at WOT 4,000RPM.

    Potential solutions (See attachment with photos):
    - Adding an 12-18 inch permanent trim tab, with a slight down dip, in between the pockets
    - Extend existing trim tabs a few inches (if the hydraulic cylinder can handle the load)
    - Reposition and install larger trim tabs to aft transom (old school).
    - Install a fairing where shaft exits hull – eliminate awkward mini-pocket in front of props.
    - Flatten and taper downwards the upper section of the tunnel outlet to minimize prop wash into the air.
    - Narrowing the upper pocket outlet at the transom, ie, taper the outlet, like a fighter jet exhaust / thrust nozzle.
    - Installing a "fence" just forward of the outboard hull pocket entrance, to channel any transverse side-flow of water into the tunnel (like a reverse chine). Same device used on some STOL aircraft wings to keep the air from running sideways off the wing tips.

    Any thoughts on best option(s) ?
    Thanks, Mike
     

    Attached Files:

  2. baeckmo
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    baeckmo Hydrodynamics

    Hi Mike, there's something not adding up regarding prop dia and gear ratio in your figures. With a 19"x27" prop you are far from the maximum engine power. And 27 knots @ 4000 rpm can't be for the diesels. So, yes, the tunnels are really bad design, but before discussing that, the above issues have to be cleared.
     
  3. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    The front of the tunnel is an abortion.

    Ah, just saw @baeckmo 's reply.
     
  4. Mike Goebel
    Joined: Jan 2022
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    Location: Conroe, Tx

    Mike Goebel Junior Member


    Hi, the numbers are correct. Yanmar 6LPs run at 4,000 rpm on the pins - I have owned 4 of them.
    Prop calculator at 35% slip indicates a theoretical speed of 26.8 knots. Engines have 400 hours.
    As such, suspect the prop pockets and the stubby rear end / short water line on this boat are the problem.
     
  5. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Is the weight measured or calculated?

    Despite the abrupt pocket entry, I have seen more than one boat with a dry weight that doesn't come close to the real one. Wet hull or transom or just years of junk thrown in back lockers, or worse, where someone has modifed the original boat and plunked 300 pounds of batteries in the stern, or a thousand pound t-top aft of center.

    So, also, is the boat level at static rest? If so how far off on a 4' level?

    I cannot help with pocket redesign, but there are some functional starting points to verify as well.

    You seem thorough, but it never hurts to ask and especially since you did not mention static rest.

    ps...lucky for you baeckmo is an expert

    The other thing to consider is cfd analysis which would be far wiser than piles of fiberglass scaps
     
  6. baeckmo
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    baeckmo Hydrodynamics

    Ok, you wrote 300 hp at 3400 rpm, thought it was semicommercial setting.
     
  7. Mike Goebel
    Joined: Jan 2022
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    Location: Conroe, Tx

    Mike Goebel Junior Member

    ---------------------------------------------------------------------------------------------------------------------------------------------------
    Hi,
    Thanks for the responses. I am a marine geek by profession, retired, studied Narc 101 &102 at school, sailed tankers for many years.
    When static, boat is trimmed slightly by the head approx 1 inch, with 3/4 fuel and 1/4 water. Water tank is aft. Fuel tank midships, close to the tipping center. Genset just aft of the main engines. I have put 2 x 175lb bodies on the bow while running, no speed change.
    The pocket inlet is way too abrupt, and edged. I am kinda certain there is a vacuum / air pocket, and/or perhaps a back eddy, at the pocket inlet.
    Suspect an angular transverse water flow across the pockets at slow speeds, when then reduces when boat gets up to 15knts.
    Let me research a CFD analysis / cost. Alas, I used to hang out at ship model tanks used for building ice breakers.
     
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  8. fallguy
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    fallguy Senior Member

    It would be good to hear follow up on your efforts as we now have two threads on this boat and no real solutions.

    My vote absent cfd is for a fairing as a simple test. If you get improvement; then that might indicate a hull rebuild of the entire tunnel worth it, or maybe it is enough; though I doubt it.

    I am ever so curious about this boat. There are advertised articles about a Luhr 30 getting 35kts. It might be good for you to understand the difference between a boat doing so much better.

    This boat claims a top speed of 35 with same engines.

    what is the difference?

    Luhrs 30 Open: Sea Trial - boats.com https://www.boats.com/reviews/sea-trial-luhrs-30-open/
     
  9. baeckmo
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    baeckmo Hydrodynamics

    I checked some of the numbers; it's the same old story over and over again. With the stated displacement on a 8,4 m waterline, the boat has a substantial hump drag due to too low slenderness ratio. In addition, those pockets add drag and reduce the effective planing area. On top of that, the props are too small diameter for the power and rpms, leading to a lower than normal inflow pressure into the propeller. The effect of that is threefold; first it causes early cavitation, second it reduces the static pressure in the pockets and third it increases the transverse velocities in front of the propeller, causing additional turbulence in the prop inflow, reducing prop thrust aso...

    The waterline length is at least 1 m too short for a decent behaviour with that weight, and the propeller tunnels are completely wrong shape. Both the side edges and the forward transition should have large radii to avoid vorticity and flow detachement, so how far are you prepared to go for a remedy? The bitter medicine is that you won't fix this with trim tabs or add-ons to the tunnel outlet.
     
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  10. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    No. Indeed not. Fill in the tunnels, put on a couple levi surface drives with props the size of garbage can lids.
     
  11. fallguy
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    fallguy Senior Member

    Wow. Sounds like a hull extension is the best option. I did not see that coming.

    what about the pocket fairing after the shaft comes out only; would it help and if so; any way to quantify? Very fun stuff.

    where did you learn all this baeckmo? Formal education?
     
  12. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

  13. fallguy
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    fallguy Senior Member

    I just wondered where he got his formal education.
     
  14. Mike Goebel
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    Mike Goebel Junior Member

    ------------------------------------------
    Hi Baekmo,
    Agree, too difficult to fix the hull, so more focused on a few tweaks to improve speed. "Hull Length" - add permanent hull trim tab, Water Flow Dynamics - add tunnel fairing , and, Reduce Transverse Water Flow - trim tabs to get the stern up. Saw a few more pockets designs a few days ago at Saunders Boat Yard, installed on newer mega-sportfish hulls. 7 blade props and trim tabs installed inside the prop pockets ! Makes sense - down thrust the prop wash and get the stern up, which also places back pressure on the props to reduce cavitation until the hull speed creates sufficient inlet flow to feed the front of the props.
     

  15. philSweet
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    philSweet Senior Member

    That was my first thought. You need to get the nose down to reduce transverse flow. I guess the pockets also reduced the shaft angle and ruined the pitch control during transition to planing and at modest planing speeds. Trim tabs outboard of the pockets would probably be most effective as they would tend to reduce transverse flow. That and buy a big dock box and get rid of some weight.;)
     
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