adding a prop pocket?

Discussion in 'Boat Design' started by sidetracked, Dec 16, 2015.

  1. sidetracked
    Joined: Dec 2015
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    sidetracked Junior Member

    I have just picked up a 32'6" 1982 Cary Blackfin boat as a project boat. The boat currently has gas engines and I want to convert to diesel. I have engineering experience sizing the running gear, so got that covered. However, limited experience with shaft angles and no experience with prop pockets...

    boat is a twin inboard configuration with 24 degree deadrise at transom.

    Here is the issue:
    *Current shaft angle is 13 degrees
    *Current prop is 17" with just over 2" tip clearance at shortest point

    wanting to design for 380 hp diesel and feel the shaft angle needs to be reduced (not increased) and would like to swing a larger diameter wheel. Seems like adding a prop pocket would solve all my issues.

    Does anybody have any inputs or suggestions on designing the prop pocket? Can it be as simple as just glassing in piece cut out of a fiberglass pipe?

    thanks,

    Michael
     
  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    The shaping of propeller tunnels was discussed in the thread "DIY tunnel drive". In post #46 I have given some basic sizing relations that work. Note that a smooth longitudinal shape is important, but having a circular transverse shape is not.

    Also note that the transition between bottom and tunnel must have a generous radius, since there will be transverse flows at some speeds. If the edge is sharp, there will be flow detachment and disturbed flow into the propellers.

    Good luck!
     
  3. sidetracked
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    sidetracked Junior Member

    thanks, interesting read and definitely enough to get me started!
     
  4. Village_Idiot
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    Village_Idiot Senior Member

    I thought I read in Blount's Tunnel Design that blade tip clearance was critical and should be theinimum possible?

    Also, i am not sure, but it seems that it may be desirable to have a bulbous protrusion just fore of the tunnel entry to enhance flow into the tunnel.
     
  5. tom kane
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    tom kane Senior Member

    Installing a prop in a tunnel or pocket can be a bad experience for any boat owner, running the prop in less dense water changes many things from thrust loss, handling in heavy seas where the prop may loose grip on water and spin to loss of good steerage control.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Did he mention blade tip clearance ?
     
  7. sidetracked
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    sidetracked Junior Member

    not sure what you are trying to say? I have seen quite a few boats, in particular most of the new Vikings, that have prop tunnels - there has to be some advantage to it?

    My assumption is with that with the modern trend of higher speed inboards, larger props (more blade area) are needed. Therefore I am assuming the tunnel is more advantageous than the steeper shaft angle required to accommodate the larger props.
     
  8. Village_Idiot
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    Village_Idiot Senior Member

    Yes, it is discussed several times throughout the paper, and also includes a figure showing performance increase with decreased tip clearance.
     
  9. Village_Idiot
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    Village_Idiot Senior Member

    There are significant problems with using pocket tunnels. If you can sort them out thoroughly, you can see benefits. However, the payoff is often not worth the hassle of sorting everything out.

    There are other workarounds if you are experiencing poor prop performance. If you can find a prop with more cupping, or find someone that can add cupping to your prop, that has the same effect has increasing prop diameter. If draft is of little concern, you may also consider a v drive to reduce shaft angle.
     
  10. philSweet
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    philSweet Senior Member

    Sidetracked, you haven't given us anything that would give us a clue as to how this might pan out.

    What are the boat's specifications?
    What are the engines currently installed.
    What was it's performance curve (fuel burn and speed vs rpm)
    What were the props on it
    What was the gear ratio and shaft size.

    What are the engines/gears/props you are considering.
    What are your performance goals
    What is your budget
    How many hours of operation each year and estimated annual fuel usage.
    Is the 13 degrees the angle between the shaft and the hull, or shaft and the waterline? Can you provide the other angle too, please.

    Unless draft is crucial, I really can't see a situation where you aren't better off just taking a brute force approach and installing enough power to get it done without the pocket.

    Why do you think the shaft drop angle is a problem, or would be a problem for your new engines?
     
  11. sidetracked
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    sidetracked Junior Member

    Phil,
    Good point, so I will try to give as much information as possible:

    What are the boat's specifications?
    Boat has 24 degree deadrise at the transom and pretty much all the way forward (though it does get steeper towards the bow). Overall length of 32'6", waterline length of 28'. Beam at waterline 8' (9'9" overall). Current dry weight 10,500 lbs with twin gas engines - loaded weight estimated 14,000 lbs.

    What are the engines currently installed.
    460 cu. in. Ford Gas engines - hp unknown


    What was it's performance curve (fuel burn and speed vs rpm)
    Unknown, boat didn't run when acquired. Power Boat reports claim a 36+ top end with this configuration.

    What were the props on it
    Current props are 17" with 2" of prop tip clearance to hull - pitch, cupping unknown at this point

    What was the gear ratio and shaft size. Gear ratio reported to be 1.5:1, has 1.25" shafts

    What are the engines/gears/props you are considering.
    Cummins QSB 380 hp diesels, ZF transmissions, gear ratio and prop size TBD - will upgrade shaft size for a safety factor of 5 once gear ratio selected

    What are your performance goals
    Speed, isn't that everybody's?? I want to maximize cruise speed of course, but this will be a fishing boat and will make long runs offshore so want the props to perform well in all seas and not just at full throttle - current boat has prop cavitation issues in heavy seas, which I think I have determined to be from the prop. Works fine in calmer seas, but feel I need more blade area for the more extreme conditions.

    What is your budget
    More than any sane person's would be - I am part owner of a boatyard, as well as enjoy doing the work myself, so the costs won't be for me what it would be for others. I have done a full restoration on an old boat before, so I know whatever budget I have now is not enough.... I will have $50k in engines and running gear easily, so the hull costs won't be much compared to that. I fully realize the boat won't have a market value of what I will have in it. Had this same debate when I did my last boat 15 years ago. Still no regrets on that one either and fully realize what it will bring on the market when I do finally sell it once this one is done.

    How many hours of operation each year and estimated annual fuel usage.
    Not relevant if you are leading to make a gas vs diesel argument - this will be a diesel boat.

    Is the 13 degrees the angle between the shaft and the hull, or shaft and the waterline? Can you provide the other angle too, please.
    angle between the shaft and the hull. Not sure what it will be between the shaft and the waterline, but greater than 13 depending on how the boat rides. Here is a video of a sistership running I found on youtube https://www.youtube.com/watch?v=lhH-JC6HFc4

    Unless draft is crucial, I really can't see a situation where you aren't better off just taking a brute force approach and installing enough power to get it done without the pocket.
    I may be better off without the pockets, I don't know. Prop pockets aren't a definitive choice yet, just starting to evaluate my options and will then choose my path. I am wanting to maximize performance. I see the newer large Vikings with prop pockets all the time - just went out and measured a 64' we have in the yard. it has a shaft angle of 8 degrees with a tip clearance of 8.5% prop diameter

    Why do you think the shaft drop angle is a problem, or would be a problem for your new engines?
    very short answer is: My original train of thought is that the boat needs a 20" 4 blade prop (based off some limited study of cavitation), which would require increasing the shaft angle to 14 degrees to accommodate. Using Prop Expert software I can't get satisfactory cavitation results at that shaft angle - lowering it overcomes the cavitation issue.....

    Open for any comments (outside of gas vs diesel :D)
     
  12. sidetracked
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    sidetracked Junior Member

    from my experience and what I have been told by some (self proclaimed) prop guru's, the cupping doesn't have too much effect until the speeds get higher (above 30 knots +/-). My current boat has #5 cup in it, and though adding the cupping helped, it didn't solve all my current cavitation issues.

    not sure how a v-drive would reduce shaft angle since the angle is limited by the prop size and clearance and not any internal factors??
     
  13. sidetracked
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    sidetracked Junior Member

    this attached sketch may help.... (hint: scroll to bottom to see)

    yellow is the hull, green is the current prop configuration.

    on the port prop, the blue prop is a 20" prop, 10 degree shaft angle, and tunnel size required to get 10% tip clearance.

    on the starboard prop, the blue prop is a 20" prop, 14 degree shaft angle required to get 10% tip clearance.

    Using Prop Expert software, the issue appears to come into play as the hp increases - at 380 hp I haven't been able to come up with a configuration that doesn't have excessive cavitation at the 14 degree shaft angle. Hull speeds at full hp are anticipated to be 38 knots +/-.

    at 10 degrees, 1.5:1 ratio transmission, a 20" prop with moderate cup, in a tunnel, doesn't show to have any cavitation issues (Prop Expert Software calculations).....

    this is why I am thinking about a small tunnel. The other issue I am concerned about though is the tunnel would eliminate the lifting strake and I have no idea what effect that would have....

    thanks for all the input guys!!

    Michael
     

    Attached Files:

  14. Mr Efficiency
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    Mr Efficiency Senior Member

    How much extra weight will you be installing compared to the current engines ? That isn't mentioned so far as I can see, but might have a significant effect.
     

  15. sidetracked
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    sidetracked Junior Member

    loaded weight difference will be about 1,200 lbs heavier with the Cummins diesels than it is now. The boat was originally offered with CAT diesels as well that were about 2,000 lbs heavier.

    I got the weights above INCORRECT -

    CORRECT Dry weight with gas engines is 8,500 lbs (10,500 was the dry weight with the CAT 3208). Add 1,200 for the cummins, 2,100 for fuel, 200 lbs for water then another 1,500 -2,000 lbs for people and gear so I am estimating a loaded weight of about 13,500 - 14,000 lbs when I am finished.
     
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