Prop Pitch and Cavitation

Anne Racel | January 14, 2012 | COMMENTS:17 Comments »

After all this time, I’m still having prop problems. When the engine reaches about 800 RPM, I can hear cavitation. How do I know that’s what it is? Well, one of my regular crew is a Navy boy and he said that’s what the sound was. Not only that, when we revved it up more, there what a strong vibration to the boat – more than I’d expect to be normal.

So I asked Scott at Electric Yacht what he suggested. He said to try decreasing the pitch on the prop.

Pitch is the distance a propeller will advance with one turn. So, a 10″ pitch means that the propeller will move forward 10″ with one revolution.[1]

The prop is a 13″ 2-blade Max Prop. If you check the specs on the thing, you can see that it’s fully adjustable: pitch and direction of movement. I had based the pitch I set it at to what it was before I took the old inboard off. Or so I think. I didn’t know that I could look inside and find the settings. So I measured the angle and found a formula online to calculate what the pitch was. The pitch was 12″. So we decreased it to 10.

What’s so bad about cavitation? What is it? Well Wikipedia defines it as

… the formation and then immediate implosion of cavities in a liquid – i.e. small liquid-free zones (“bubbles”) – that are the consequence of forces acting upon the liquid[2]

The problem is, those bubbles can cause pitting on the prop. Enough of this and you have a non-functional prop.

After decreasing the pitch to 10″, the vibration at high RPMs seemed to go away. But the cavitation sound was still present. So I turned to the wisdom of the CAL boat and electric boat forums. The comments attached are the responses I received.

What now? Well, we’ll try decreasing the pitch to 8″ and see what happens.

[1] http://www.propellerpages.com/?c=articles&f=2006-03-08_what_is_propeller_pitch

[2]http://en.wikipedia.org/wiki/Cavitation

Category: Fixing It Up!, Learning My Lessons TAG: , ,

17 Responses

  1. Grae Morrison says:

    I replaced the prop on my 29 last season, there are a lot of folks in this group that have far more understanding of this than me so I’ll leave them to comment on how to calculate, however, on the recommendation of the prop people in Michigan I went from a 6pitch 12″ 2 blade which topped me out at about 3-4kts to an 8pitch 12″ 3 blade (because I mainly cruise not race) which works pretty well and gets me to 6kts easily (approx 3200 rpm) providing the hull and prop are clean, if they’re not it labours very badly so I have to assume it’s a little ‘over propped’ and may opt for an 11″ or a 7pitch next season (any comments gladly received). One thing that was pointed out was the gap between the hull and the blade tip and from what I was told there needs to be around the 2″ mark to allow for ‘flow’, with the 12″ on mine the gap is nearer 1.5″ so maybe 13″ is a little close – thoughts anyone ?

  2. Anne Racel says:

    Thanks, Grae. The prop was on the boat before, so I know it’s not too big. I can’t say for sure that I put it back exactly where it was. But I was told to confirm the clearance, and that was done.

    The prop is highly adjustable. I just need to figure out what adjustments are correct for my configuration.

  3. Edward Horn says:

    anne call PVI they are the MAX PROP dist for north america ..We have a three blade
    on our Cal 29 (1974) “Noah’s Kid” with Atomic 4 (30hp) … I would recommend staying
    with your max prop it is FULLY adjustable and will work for RT or LF rotations !!!
    on ours there series of numbers on hub a series of letters that correspond with what
    pitch you want.. call them they have charts and will give you thier recommendtion
    you mentioned that you redrilled your prop log ?? …what about your strut are you
    still using it?? …my buddy Ben has a Cal 29 (1975) with Fairymann and would like
    an 2 blade MAX PROP…max prop can cut down your blades …if your end of blade
    to hull clearance is too tight ?? one of my crew mates has a navtec elect. ericson 27
    and is really sharp …email us and we will send you his phone number edward(helen)

  4. Allen Edwards says:

    I had a bad vibration at certain engines before I realigned my engine. But I assume that had been checked.

  5. Chuck Lennox says:

    I agree with Helen. Call Max Prop.
    With the electric motor, max torque is available from the start. I would guess that you would use max pitch. The slower RPM of the prop the more efficient. Also I think a good diver can change pitch in the water? Good luck
    Chuck

  6. Anne Racel says:

    Yes, Helen, redrilled the prop log. The guy who fiberglassed it over redrilled it, too. Hopefully it was the same angle because yes, it’s the same strut.

  7. Anne Racel says:

    Thanks, Chuck. Yes, I’ll try calling MaxProp tomorrow. I have also heard that a diver can change the pitch underwater. But frankly, I doubt I can get someone to get into that Chesapeake water until spring. :-) Besides, the marina will haul the boat out and leave it sitting in the lift so I can do the work, then put it back in the water for me.

  8. Michael D says:

    We have a MaxProp on our Cal 2-27. It was installed by a previous owner. While sailing, we put the transmission in reverse to get the blades to feather. Yes, it can be removed and reinstalled underwater, but it really needs to be done in clear water on a sunny day. It takes more than two hands to hold the pieces parts in place while getting everything lined up. The markings on the hub and blades are stamped into the metal, and can be hard to see wearing a scuba mask. When we repowered Magic, PYI recommended an 18 degree blade pitch. We found it to be a tad “shallow” and later went to a 20 degree setting that has proven to be a much better setting. If you do the work yourself take the time to study the manual thoroughly. The prop also has two SS set screws that are removed and replaced with standard grease fittings/nipples for lubrication. I normally do this once a year or so.

    Michael
    s/v Magic, Cal 2-27
    Pompano Beach, FL
    Surf Temperate ~ 73 F.

  9. John Boyce says:

    Michael,

    What is your prop diameter. I have a Martec 10″ dia 14 pitch.

    John B
    Cal 227
    Gotcha Again

  10. Michael D says:

    John,

    We have a two-blade 13″ MaxProp on Magic.

    –Michael–

  11. Edward Horn says:

    Anne … Haul out boat drop rudder mark mounts pull shaft (take off prop 1st)
    ‘borrow’ shaft that is stright wet cutlass bareing slide this shaft gently in up to
    try to borrow a speed boat shaft which is 3 or 4 feet longer look down the rudder
    post hole shaft should be in the CENTER of rudder post hole if not twick strut
    or remove and re bed strut …these three things need to be in line the rudder post
    hole , the sturt, and the shaft log… you adj the motor to this line …the shaft
    should turn , Freeley but allways wet cutlass bareing when your slideing
    or turning shaft… if shaft log is missed drilled you can glass and redrilled
    or some times drop strut down ..to back in line by rebedding sturt…
    make sure that your shaft is true and stright … one difference with elect motors
    is they go to MAX tork instansly … if shaft is out of line or bent you get vibratetions also
    you will need to put a “lock” on shaft to make the max prop feather out..

    if you do not you eng (motor) will slow you down 1kt and the cal 25s and cal 27s will pass you… (there is a westerbeck for sale for $1100 SFBAY CL…)
    good luck !!!

    I have been intriged by elect power but have noticed that these repowers take 2 or 3 seasons to complete … where and repower with same type model only takes 2 to 3 weeks ….also and old un relieable aux egine FORCES you to be a better sailor or aleast a better mech. Edward(helen)

  12. Wayne Steeves says:

    Couple of items to check:

    1) When you installed the new motor was the coupling in the same horizontal position relative to the boat? In other words did you move the shaft in or out at all to match it up with the motor shaft? On mine I had to pull the shaft inboard about 4 inches. In doing so, this moved the outer diameter of the prop to within 1.5″ of the hull. It really should have been 2+”. In your case it should be 2.25 or more inches but check that as I’m no expert (distance should be 17% of prop diameter). Having it too close can start to cause vibration and/or cavitation as RPM’s go up.

    2) No more than 40x the shaft diameter should be without the support of a strut. This will cause vibration as RPM’s go up. Also check your cutlass bearing. There should be no play when you move the shaft side to side for a new one. Too much play in a worn cutlass bearing would cause what you are describing and finally, make sure you have aligned the prop shaft to the motor shaft precisely. Any combination of the above can amplify problems. These things may not be the problem since you did not experience these issues with the old engine but worth a check since you can now notice this sort of stuff more acutely simply because the engine noise and vibration is not blocking it as you mentioned.

    Wayne
    Sequoia T30.

  13. Anne Racel says:

    Wayne -

    When I first put the motor in, the shaft had enough discoloration that it was easy to figure out where it was before, so that didn’t change.
    When I got the new shaft (old one was bent), I’d already drilled for the mounts, so, if there was a difference, it should only have been slight.
    I know there is enough distance between the prop and the hull – I’ve measured that since it was an early suggestion made to me.

    The shaft diameter is 2″, and I know there’s not a point where there’s 80″ of unsupported shaft. There is quite a distance between the engine
    and the coupler. I’m at home, not at the boat, so I can’t tell you exactly. But it seems there’s about 16 – 18″ there. Could that be part
    of the problem?

  14. Anonymous says:

    Wayne -

    When I first put the motor in, the shaft had enough discoloration that it was easy to figure out where it was before, so that didn’t change.When I got the new shaft (old one was bent), I’d already drilled for the mounts, so, if there was a difference, it should only have been slight. I know there is enough distance between the prop and the hull – I’ve measured that since it was an early suggestion made to me.

    The shaft diameter is 2″, and I know there’s not a point where there’s 80″ of unsupported shaft. There is quite a distance between the engine and the coupler. I’m at home, not at the boat, so I can’t tell you exactly. But it seems there’s about 16 – 18″ there. Could that be part of the problem?

  15. Wayne Steeves says:

    Don’t think so. The vibration would indicate alignment. It’s not as easy as you might think with the coupling tube that is used as opposed to the plate couplers normally used (or whatever their called). Any play in the shaft at the coupler end makes it more difficult.

    If there is any play at all (1/16″ is max before replacement) in the cutlass bearing any slight misalignment will be magnified and cause the shaft to flop around as RPM’s and load increases. This could be the problem if you did not replace the cutlass bearing when you replaced the shaft. The old shaft being bent will have provided considerable uneven wear to the bearing over time. I would run the motor disconnected from the shaft and insure it is not the problem (doubtful), then attach shaft without prop and run it. If no vibration problems then problem would likely lie with prop itself. Also make sure your stuffing box has been properly packed and did not wear the packing unevenly due to the prior bent shaft.

    As far as cavitation how are you detecting that – loss of thrust at RPM? If so, I’d be looking at the prop and it is experimentation at that point. If its a resonant noise or vibration it may not be cavitation but something else and I would focus on alignment while in the water and hope it corrects the problem. 1250 RPM seems awful low for cavitation to be an issue. Aeration and/or ventilation are more likely but I wouldn’t think so looking at your configuration or any sailboat hull with inboard drive for that matter. I’m not familiar with the max props or feathering/folding props in general. Good Luck

  16. Robert Garbe says:

    A 2in diameter shaft seems awfully big for a 29 ft sailboat. I would have expected a 1 in max. Just saying. To align a shaft properly you have to remove the flex coupling, mate the two metal surfaces together and check with a small feeler gauge all around to see if there is a perfect fit. Even a small misalignment can cause vibration.

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