Prop pockets and surface-piercing propellers

Discussion in 'Boat Design' started by cracked_ribs, Feb 27, 2021.

  1. cracked_ribs
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    cracked_ribs Junior Member

    I was looking at pictures of a boat the other day with a prop pocket; it was a high-speed sport fisher that ran around 60 knots full tilt.

    Just looking at the way the pocket was designed, I wondered if it would aereate at planing speed. It was about exactly the radius of the prop, so if you got flat laminar flow off the forward edges of the pocket, I thought maybe it might operate as a surface-piercing prop, and that that might be contributing to the speed of the thing.

    I don't really know anything about prop pockets but I had always thought they were more of a low-speed thing to reduce draft.

    Does the idea of an aereating pocket and surface-piercing prop have merit, or does that just not work? At low speeds I assume it'd be less efficient; the prop would be fully immersed but presumably its design would be maximized for high speed, not low. And maybe the hump would be really vicious as you throttled up, waiting for the pocket to aereate.

    Is this
    A) already done and I've just never seen it because prop pockets are uncommon here
    B) not done for fairly good manufacturing reasons, or
    C) not done but potentially interesting, if you want a really specific boat, that nobody really wants enough to make it commercially viable
    D) just dumb for reasons I would know if I knew more about props or prop pockets
     
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  2. jehardiman
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    jehardiman Senior Member

    I'd need a picture to be sure, but based upon you description the prop pocket on that high speed planing vessel serves two purposes. First, it allows a larger wheel for draft, lowering blade loading. Second, it slows flow into the wheel, increasing thrust for a given rpm. Generally, pockets on planing boats are specifically designed NOT to aerate, much like the inlets for water jets, and are used to manipulate the pressures at the prop to retard the onset of cavitation.
    FWIW, aeration actually reduces the thrust of all propellers, so even surface piercing blades need solid water flow to work at their best efficiency.
     
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  3. BlueBell
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    BlueBell "Whatever..."

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  4. cracked_ribs
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    cracked_ribs Junior Member

    Interesting, thanks guys.

    I'll see if I can find the pics I saw; the prop pocket struck me as shorter and steeper than usual and I wondered if the leading edge of the pocket was acting as a mini-transom at planing speed; I'm not sure if I explained that well. Or else I did, but the idea is stupid!

    I was guessing they had laminar flow as the water separated from the front of the tunnel like a transom, so the prop was conventional at low speeds, but as speed increased there was hard laminar flow and the prop was piercing clean water. Am I making sense here?

    I've done a terrible sketch but I don't think it makes things any clearer:

    [​IMG]

    I think this is just a bad drawing of a prop in a pocket, which nobody was confused about. Anyway the only information here that wasn't in my initial description was that that pockets looked like they'd been designed to have a "mini transom" at the front at speed, so they'd become almost like surface drives. But maybe that was not the intent at all; it may have been just the photo that gave that illusion.

    I'll read through the full Cajun Pocket Tunnel thread - that looks really interesting and probably a related idea?
     
  5. jehardiman
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    jehardiman Senior Member

    Ok, lets start with the term "laminar flow"...there is no laminar flow at the speeds that either of the discussed vessels run at. What you mean is clean flow, there is still a turbulent boundary layer, but the water movement is linear along the streamlines.
    Second, the bateau the the sport fisherman work in totally different methods and the tunnels are for totally different reasons. The bateau is a flat water, flat running boat where the aeration of the tunnel is to reduce squat. Notice that the cavitation plate on the OB is below the level of the tunnel and the venting of the tunnel is to restore pressure over the stern run, but that flow does not end up in the prop. The sport fisherman on the other hand, is an offshore V-design made for fairly rough water and running with some trim. The volume of the tunnels slows the inflow to the props and recovers some pressure, but is not intended for atmospheric aeration next to the skin plating.
    Finally, it is important to remember that the water is (generally) not moving at any significant speed relative to the vessels advance and it is the propeller "slip" that is causing most of the low pressure flow over the stern of the hull. OB's, I/O's, and surface piercing propeller outdrives generally approach this issue differently than IB's, where the wheels are tucked up under the hull. Propellers fitted aft of the hull generally have cavitation plates to deal with the propeller generated low pressure in the fluid, while props under the hull must mitigate that "thrust deduction" through various methods. Often this meant high shaft rake angles and large prop clearances. However, carefully shaped tunnels could avoid having you props hanging out so far beneath the hull.

    Edit to add a link.
    Prop Tunnels https://www.yachtingmagazine.com/prop-tunnels/
     
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  6. Barry
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    Barry Senior Member

    This fellow installed the fittings under the guise of equalizing the pressure in the tunnel. He assumed that there is some miraculous gain in equalizing unsubstantiated pressure zones. When you watch his
    video, and it is not often clear, there is air in the clear hose. BUT if there was a pressure differential as he thinks, then there would be water moving in the pipe. And there is little to be seen.

    Of course the boat works, but there does not appear to be any pressure equalization evident and no baseline to tell if his addition is of benefit.

    There was not any ventilation of the tunnel.
     
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  7. gonzo
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    gonzo Senior Member

    Do you mean anti-ventilation plates? There is a great deal of confusion on those terms.
     
  8. baeckmo
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    baeckmo Hydrodynamics

    cr, you may have stumbled upon something that I think was presented as "Power-Vent" propulsion (or something like). It is/was a prop working in ventilated mode in a short tunnel. Forward the tunnel was "cut short" to create a clean flow separation, and the ventilation air drawn through "side channels" along the tunnel sides. It was tested by USCG (??) some years back. Way back I did a rough check on the numbers stated; it looked fair enough, but not exceptional, and I can't see that it has made its way into the market, at least not this side of the Atlantic. For special applications, it might find its place though.
     
  9. cracked_ribs
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    cracked_ribs Junior Member

    Having read through the CPT thread it does look like the goals there are different - I'm not sure if I understand the theory behind the fittings but that is more of a conventional tunnel design, intended to lift a column of water to the prop so the prop can run high, I think.

    But it sounds like my idea of a tunnel that gets you clean separation once the speed gets up towards planing speed is a non-starter.

    I guess if I were really well-funded and wanted to pursue the concept maverick-style, I could take a boat with Arneson drives, or some fixed equivalent with rudders, and build "squat boxes" on either side of the drives with a swim step on top, then slowly fill in the areas between them until I stopped seeing the water separate from the transom. That'd be the experimental limit of the concept.

    But probably a lot cheaper and less work to just listen to people who actually understand props, prop pockets, and hull design.
     
  10. cracked_ribs
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    cracked_ribs Junior Member

    Oh interesting, you posted as I was typing. Well, off to google!
     
  11. cracked_ribs
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    cracked_ribs Junior Member

    Well, knowing that the Power Vent is the thing I'm trying to describe, it turns out that it's been discussed here previously and it does work, and it is interesting.

    The upside seems to be "Arneson effect with simple, rugged inboard parts" and the downside seems to be "wow that hull is complicated and has to be built around the drive system so you better get this right the first time." I guess the secondary benefit is you don't have a pair of 300hp blenders hanging off the back where your fishing lines go and your drunk passengers fall out.

    I did find the picture I was looking at, but after looking at it again, I realized I wasn't looking at a surface piercing prop anyway. But it sounds like the concept works, it's just not common. Of course, as with every novel drive system, my assumption is that if the benefits were worth the costs, they'd already be on everything.

    But thanks for finding that for me; at least I have some reading material now. I do think it's an interesting concept for a fast boat with a couple of big diesels, say. But likely the gains over conventional inboard aren't big enough to bother with the hull customization.
     
  12. jehardiman
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    jehardiman Senior Member

    Yeah, they get called many things, I learned that name early before college, even though now I understand that they are actually anti-cavitation plates. Suffice to say most open propulsors mounted aft of the transom have some sort of device to manipulate pressure and flow around the wheel and many people call them by the name I used.
    Before starting any concept be sure you understand the difference of action between a conventional propeller and a surface piercing one. A conventional propeller operating at maximum efficiency sucks water towards itself creating a below nominal pressure area ahead of it. The driving force on the blade is split about 50-50 high pressure face, low pressure back. It is this low pressure in the fluid that allow conventional above the water prop tunnels (like on a tow boat) to fill and use the full disk area. Even super-cavitating propellers generate this low (cavitation) pressure area ahead of the disk and you can use it to draw flow into the tunnel. A surface piercing prop blade operating at maximum efficiency however is 100 percent face pressure; it acts more like an oar, physically pushing against the mass of the water and creating/needing a atmospheric pressure void on the back side of the propeller. If too much of the propeller is submerged, shaft torque is greatly increased with no increase in propulsion, so the whole idea of a surface piercing prop is to keep it at the proper submergence, not inundate the disk.
     
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  13. cracked_ribs
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    cracked_ribs Junior Member

    Yes, I get that; the whole point of this concept would be to allow surface piercing props to run as they are intended to, which sounds as though that's exactly what happens if you build one as I described.

    But not having hundreds of thousands of dollars to throw at experimenting with it, I was mostly just curious to see if it worked as a concept, which evidently it does.

    Which is not especially surprising or anything, I was just curious because I'd never seen it done. But Power Vent seems to be the exact thing I was talking about. Interesting idea - maybe a little less draft, props are protected, and you get the efficiency benefits of the surface drive.

    Thanks for the info everyone!
     
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  14. DogCavalry
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    DogCavalry I aim to misbehave.

    It is a readily graspable combination of known requirements. Given the advantages and disadvantages of a surface piercing propeller, and the way it must be accommodated within the system, I'm surprised we don't see more of them. I sketched out exactly this back in the 90's, and just like CR, didn't take it any further. Supporting a family on a carpenter's wages- I remember days when having a backup box of Huggies in the closet felt like a win.

    I wouldn't put any faith in the notion that if they were any good we'd see more of them. Marketing and fashion drive sales, not sound engineering.
     

  15. cracked_ribs
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    cracked_ribs Junior Member

    Yeah, on the face of it it seems like such an obvious design; it may just be that the cost of the complex hull combined with the unfamiliar drive type makes it a hard sell.

    A company called Smith Boats seems to use it on giant CCs. But there sure aren't a lot of examples out there.

    Still, for a fast sportfish I think it's really got potential.
     
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