Conference unifix::sailing

re three blade solid propeller:

A rotating propeller can be perfectly balanced (ie, have no vibration
when rotating) whether it has two blades or three (or any number). The
drag of a fixed three blade propeller is MUCH higher than the drag of a
fixed two blade propeller. Don't get a three blade fixed propeller
unless it is attached to a large motorsailor. 

>>    Problem:
>>    I have been suffering a prop/shaft/engine vibration problem for 
>>    the whole season. I also noticed this problem some last season but 
>>    not the previous 2 seasons.
    
Well, something has changed. (Do I get a prize for stating the obvious?)

>>    At engine RPM above about 1800, when moving or accelerating, I get 
>>    a thumping that I traced to the shaft tapping the shaft tube thru 
>>    the hull (what do they call that tube?). The tube allows about
>>    3/16" clearance all round to the shaft. The actual movement is the
>>    engine on its mounts. There is no evidence of cutlass bearing slop
>>    or the strut moving. When tied to the dock the problem also exhibits
>>    itself in gear but there is no engine vibration in neutral.
    
The tube is the shaft log. Ye gods, the propeller shaft is moving 3/16"
sideways? No wonder you have vibration. One thought: Perhaps one or more
engine mounts are failing or have failed. If the stiffness of the rubber
in the mounts has changed, it would change the way the engine moves
under load. The engine tries to rotate about some axis (probably the
crankshaft) when it is running. The engine mounts have to resist that
rotation. There are usually four engine mounts, two on each side. If one
mount is much stiffer or softer than the others, then the direction in
which engine's rotational axis points will change as the load on the
engine changes. This could result in very significant propeller shaft
misalignment, especially at high engine speed when the load is greatest. 

If the propeller shaft is really moving sideways as much as you 
indicate, I would think that the strut bearing would be badly damaged 
and would have to be replaced. Strut bearings (cutlass bearings) don't 
tolerate much out of alignment.

The propeller would have to be badly damaged or the propeller shaft 
quite bent to cause what you have described. No vibration in neutral 
indicates that the engine itself is running smoothly (on all three 
cylinders). 
    
Another thought re propellers: At 1800 rpm our propeller gives about 85%
of hull speed (ie, about a knot less than hull speed). Using the fuel
consumption figures in the engine manual, changing the propeller to give
hull speed at 1800 rpm would roughly double our fuel consumption. With
the current propeller our range under power is about 450 miles (at 1800
rpm, more at lower engine speed). I prefer the lower fuel consumption to
greater speed under power. 

Good luck,

Alan

    I will look at some other things on the next trip to the boat,
    like how much the shaft moves up-down, left-right, whether the
    thump occurs on every revolution of the shaft (I think it happens 
    something like every other one), whether the movement builds up
    gradually with speed or occurs suddenly around 1800 engine RPM,
    and get a better idea of the shaft to shaft log clearance.  I will
    also play with the motor mounts (3), but I dont think I can tell much
    without getting a new one and comparing one at a time to the old
    (the broken motor mount possibility is a good one, especially since
    these type wouldnt "look" bad).
    
    Thanks,
    Walt

    I would never, ever, ever, ever miss-align the engine to shaft.
    As I have seen a number of your notes, I would assume that you used
    feeler guages to check shaft alinment. Not sure how the Tartan
    shaft lays in there. If mis-aligning cleared up the thump, you would
    have repaired a symptom rather than the disease.  Is it possible
    that your sacrificial is in motion? If it is between your prop
    and strut or shaft log, could there be some movement.
    
    If you run the engine and hold on to some massive part of it, do
    you "feel" the vibration as coming from the engine itself? or can
    you  sense it coming from the shaft?
    
    Have you made any structual changes around the engine bed, or
    added a drip pan, or any structual change that could contribute
    to a resonance?
                                        
    Is your engine mounted on sophisticated pads, or simply bolted
    with thru-bolts. If the fancy ones, that is floating types, then
    it is possible there has been some rubber torn on the undersides
    where they are not visible.
    
    You might also give Tartan a call......
    
    Don

    RE: .3
    
    I mis-aligned the engine only once, to see what effect that had.
    
    I always unbolt the coupling before hauling and only connect after
    launch, a weeks settling and checking the alignment.  That is except
    the spring I launched from Masthead Marina, powered up while the
    slings were being retrieved from the water and promptly left the
    marina...then I had the shaft connected before launch. I did dis-
    connect it after arriving at the Wharf and allowed the usual week
    settling.

    I never knowingly run at speeds which cause the thumping. The onset
    is quite sudden and impossible to ignore (sounds a bit like a low
    RPM diesel one lunger - like those Med fishermen...).  Up to that
    point there is nothing noticable from topside (actually if it werent
    for the thump, I dont think I would notice this) but that doesnt
    prove anything.
       
    Funny thing is I have never really found the engine out of line,
    except at the start of the second season, and then only slightly.
    Last season, when begining to try to sort this out, I found the
    shaft was very close to the bottom of the shaft log, attributed
    this to settling (mounts, etc.) and simply raised the engine and
    re-aligned it.
    
    The motor mounts are rubber-doughnut-in-a-cup (there is probably
    a real name for these too) with the cast metal cup secured to
    timbers with 2 bolts and a large threaded rod extending from the
    donut up to a flange on the block.  I wouldnt call them sophisticated
    like those silicon filled super compliant jobs I have seen, but
    they arent solid steel either.
    
    I havent changed anything, here at all.  A nice sump (white fiberglass,
    nicely finished so its easy to keep clean) is already under the
    engine. It covers the timbers and will hold everything the engine
    could spill (oil and antifreeze). I did have new heat exchanger
    brackets welded up last year because one of the old ones (a piece
    of bent mild steel) cracked at the bend, but that is minor (the
    whole heat exchanger is only 3" x 12").  
    
    Good idea about calling Tartan, maybe they have chased this type
    problem before.
    
    I guess you can get an idea of how little use an engine gets on
    our boat.  The only reason I am doing anything about it now is
    because we motored out to Block one windless night (40 miles)
    and couldnt make more than 5.5 knots.  Thats not bad, but everything
    on the water passed us that night...the same boats we do horizon
    jobs on when there is a breeze.

    RE: .1
    
    Thanks for the input on 3 bladers.  I know a couple folks at the
    marina who have 'em and said the reduction in vibration was impressive
    (lots of theories and counter arguments here). None of them seem
    to give two hoots about performance under sail (one guy seems truely
    amazed that wind makes a boat go at all).
    
    
    Walt

    
    I think that the answer to the two vs three blade prop issue depends
    on how much deadwood the prop is hiding behind.  As my boat is full
    keeled, there is a lot to hide behind and without a three blade
    wheel, I get very poor powered performance--especially in reverse.
    
    I have one thought other than the engine mout suggestion made by
    Alan (which sounds the most likely).  Some marine transmisions -
    particularly Paragon - have a splined shaft within the transmission.
    The main shaft is prevented from running out too far by a "c" ring
    but there is usually some play in the run-out.  This is apparently
    to dampen the forces on the engine mounts when the engine is shifted
    from forward to reverse at speed mush the way a shock absorber would
    (a no-no that many people commit from time to time).  If there is not 
    much clearance between the coupling and the stuffing box on your boat,
    this designed in run-out may be causing the coupling bolts to
    occasionally bang into the stuffing box.  This could explain the fact
    that you only have the problem at speed and that it appeared for no
    apparent reason after the boat was a few years old (the splines are
    very tight on a new transmission but loosen up over time).  

    I have a slight correction to .0.
    
    When at the dock, it does NOT thump.  There is a very slight sense
    of something, but there is no shaft motion (except rotating).  The
    thump (and sense of something) occures at a fraction of the actual 
    shaft rotation.
    
    The something is hard to explain.  Its like a swoosh, or perhaps
    water from a blade thrust at the hull.  Could this be some kind
    of cavitation (both the swoosh at the dock and the thump underway)?

    The boat was prep'd for hauling and storage last Sunday...a sad
    part of the year.
    
    Walt

    
    	I had originally thought this problem to be engine mounts and
    had expressed that thought in a mail message to Walt.  This correction
    to the original note would lead me to look for a balance problem,
    though I still wouldn't rule out the mounts.  I'm not familiar with
    the particular engine, but I'd be willing to bet that a trouble
    shooting session with a strobe light would show something.  An ordinary
    auto timing light isn't enough since there is no way to vary
    the strobe's delay after the sync, which is needed to detect movement.
    BTW, since its a diesel, a sync adapter mounted on the crank shaft
    nose is needed.
    
    	Disclaimer:	As I already pointed out to Walt in my mail message,
    I am not a diesel sailor, just a reasonable motor medic.
    
    	Reg
    

    Well, its out of the water now.  I have pulled the prop and inspected
    the cutlass bearing and shaft.  Everything seems in order, but I
    will wait for the prop shop to give me word on the prop before I
    start pulling motor mounts and bothering Tartan (I like to do the
    obvious stuff first).
    
    I will let you how this progresses.
    
    Walt

    Buffeting from the water slipping off the tips of the propeller
    blade may be a cause, although there is no obvious explanation of
    two years of good health.
    
    The books I've seen say this is bad if blade tip to hull is less
    than ahead of the prop is less than 30% of diameter and aft of prop
    less than 15% of blade diameter.  Axial distances for clearances
    aren't specified, but I'd guess that 2 prop diameters is good.
    
    Each pass of a blade has a high pressure in front and low in back,
    and these pressure fields act on the large surface areas of the
    hull, causing them to flex in and out.  At 1800 rpm & 2:1 gearing,
    2 blades, you've got 1800/60 or 30 hertz driving force, just perfect
    for a low "thump."
    
    Still doesn't explain the "change" unless a bulkhead bo, or other
    modifications have changed the natural resonance on some part of
    the hull near the prop.  Probably worth making a very careful internal
    structural inspection.  
    
    If there are large area hull panels near the prop you might consider
    adding a foam core stringer or mini bulkhead to change the resonance
    patterns, if this line of thinking makes sense.
    
    

Note 677.9                 Power plant/prop vibration                     9 of 9
SSMP03::SAVIERS                                      25 lines   2-DEC-1987 23:11
    
>    The books I've seen say this is bad if blade tip to hull is less
>    than ahead of the prop is less than 30% of diameter and aft of prop
>    less than 15% of blade diameter.  Axial distances for clearances
>    aren't specified, but I'd guess that 2 prop diameters is good.
 
    
    Thanks for the input here. I vaguely remember some rules of thumb
    about prop clearance vs. prop diameter, but since I dont design
    boats, I tend not to remember stuff like that. 
    
    One question though: Is there a typo in the above?

    This is how I read it.
    
>    The books I've seen say this is bad if blade tip to hull ahead of 
>    the prop is less than 30% of diameter and aft of prop
>    less than 15% of blade diameter.  Axial distances for clearances
>    aren't specified, but I'd guess that 2 prop diameters is good.

    One correction in a previous note. The Tartan manual says the boat
    has a 16" prop, but in fact the boat is equipped with a 15". It
    may be that they found some above clearance too close.
    
>    Each pass of a blade has a high pressure in front and low in back,
>    and these pressure fields act on the large surface areas of the
>    hull, causing them to flex in and out.  At 1800 rpm & 2:1 gearing,
>    2 blades, you've got 1800/60 or 30 hertz driving force, just perfect
>    for a low "thump."
    
    When I first noticed this problem, one of the thoughts I had was
    similar (a wave pressure front was hitting the hull).  I guess the
    thing that convinced me otherwise was the day we powered back from
    Cuttyhunk last year and I went below to tweek the engine mounts
    to see if something was misaligned at 1800 RPM that didnt show up
    at 0. I had already adjusted the engine so that it was true to the
    shaft flange and the shaft was centered in the opening.  When I 
    raised the front of the engine between �" and �" the thumping stopped.
    Unfortunately when I slowed to see what I had done, it was apparant
    that the flange on the prop shaft was working (the engine was out
    of line), so what I had done was preload the shaft to stop the
    vibration at the expense of the shaft to flange coupling. I also
    crawled around the engine area and steerage listening and feeling
    for some resonant panel and could find none (for what its worth
    the hull is balsa cored and this area has several glassed in stringers
    athwartships in addition to the shallow skeg which ties forward
    into the engine mounts).
    
    By the way. I removed and cleaned the prop after she was hauled.
    I found the very slight surface cracks I saw on the base of one
    blade seems to have grown and now there are some at the base of
    the other.  I dont know much about props, but this doesnt seem right
    (even though bronze is supposed to be soft and stuff like this
    can be cosmetic).  I plan to get the opinion of a prop shop on the
    condition, but I suspect I will replace it. I still wont know if
    this is the result of the vibration, or the cause untill next year.
    
    I also plan to call Tartan one of these days for more guidance.
    
    Walt

    In addition to Grant's exposition on the effect of propwash on the
    hull, two other effects have to be noted:
    
    1. There may be a shrouding effect due to the way the shaft exits
    the hull. If there is some sort of protrusion of the hull holding
    the cutlass bearing, then this will shroud the flow over the propeller
    at certain points in its rotation. This can have a powerful vibratory
    effect, particularly if the cutlass bearing is a little loose or
    poorly supported.
    
    The same thing happens to to a lesser extent if there is any kind
    of strut with a cutlass bearing. Obviously, you had better have
    one or the other.
    
    2. Grant has described the time-varying forces generated on the
    hull as the propeller blades move past it, and the crucial effect
    of sufficient clearance (which greatly reduces the magnitude of
    the impinging pressure waves). Note that in any such circumstance
    that variable forces are felt by the propeller as well. This can
    induce vibration or thumping due to motor mount problem, shaft flex,
    struct flex - there are lots of possibilities. Those cracks you
    are observing in the blade roots sound suspiciously like impending
    fatigue problems.
    
    Ross Faneuf
    

    My configuration uses a strut with cutlass bearing.  I checked
    the strut itself and found it to be well anchored (on the other
    hand, while checking the feel of my cutlass bearing with other
    boats in my boat yard I noticed several that felt as though they
    should be leaking they moved so far).
    
    I also felt the cutlass bearing. It is not tight, as in difficult
    to turn. If I rock the prop (or the end of the shaft that the
    prop was on) I can feel a slight movement when the shaft goes from
    contact on one surface of the bearing to contact on the other. I
    did this with several other boats and found mine has equal or
    less movement than others (non of this visable).  I think I can
    measure this travel with a lever action dial micrometer I have. 
    Anyone know what is good and bad for a cutlass? 

    Walt

    Walt,
    
    I have a similar problem with my boat (Fisher 30, Motorsailer,1200h
    on the motor).
    This is now my third season.When the boat was new,I could run under
    full power with no vibration at all, but this summer at 70 % power
    I had already a lot of vibration ,getting worse and worse.So 
    I was following this conversation very closly.
    3 weeks ago I emptied the waterdrains on my Volvo Penta.2 cylinders 
    have been ok,but in the third cylinder,there was no water at all.
    By the way,I have direct cooling.
    I removed the cylinderhead.All coolingholes have been completly
    blocked with deposit.I opend the second cylinder and here as well,
    all except one hole has been blocked and the same in the third
    cylinder.
    Next week I will put the engine together again and I will install
    a heatexchanger to prevent it to happen again.I operate my boat 
    for 4-6 weeks in the Mediteranien and the rest of the year the boat 
    is on a lake.I have no idea where this deposit is comming from.
    
    I hope,my vibrations have been caused by an overheated engine
    (temperatur-gauge has always been normal? oilconsumption has been
    high). I do not have the result of the testing of the injectors
    yet.
    
    My previous boat (Fisher 25) had the same problem.And there is a
    third boat of the same type on the lake.The owner of this boat had
    vibrations as well.He installed a new propeller and a new
    bearing.As the problem did not go away,he installed one of
    this modern flexible couplings and since then his vibrations are
    gone,but he has motorproblems (stopping during manoeuvring or after
    a long run).
                        
    I'm not very hopefull,that my problem is solved with cleaning the
    coolingsystem.I got during the 8000 miles from the last 3 years
    some plastic bags,some nets and some lines in my propeller.My best
    guess would be a bent shaft.And I do not believe in any structural
    damage (my boat is extremly heavy built ,14000 lb for a 30' boat )
    
    Peter                                        
    

    I saw a Fisher 25 in NEwport last year. It looks to be a seaworthy
    boat.  I think the SW/FW heat exchanger is probably a good idea
    given the problems you have experienced.  My engine already has
    SW/FW exchange, and the engine has never run hot, so I dont think
    that is my problem.
    
    When I find the cure, I will be sure to post it here.
    
    Walt

    I talked with Tartan this week.  After describing things for about
    20 minutes, the folks there think the prop may have been the
    victim of electrolysis at the end of the season of '85 (Gloria).
    
    They suggested several possibilities, including loose struts and
    rudder posts (rubber vibrating in this case) but we ruled those
    out from the examination I have done.  They found the motor mount
    scenario hard to swallow based on the design of the mounts and their
    past experience with this configuration, and nearly impossible to
    test (other than putting new ones in).  So they only recommended
    that as a last resort.
    
    Walt

    This past weekend I polished up the prop (to see what I could see).
    
    It appears one blade is severely weakend near the hub, as evidenced
    by cracks on both sides of the blade. 

    I used a wire wheel on the prop and cannot see any obvious changes
    in texture or coloration pointing to electrolysis (they may be there
    but I am far from an expert here).  It is definitely new prop time.
    
    Walt

    
    	But what if the cracks are the symptom, not the cause ?
    
    	Reg
    

    That was my first inclination too.  I havent been able to find
    anything abnormal other than that blade.
    
    After I replace it, I will look carefully for symptoms which
    could lead up to another prop problem based on the suggestions
    contributed in this note.   Tartan said the shaft between the strut 
    and engine would have to move an inch or so at the engine to cause 
    stress at the prop, and the strut or cutlass would likely be damaged 
    first (not to mention the hull because the hole isnt that big).
    They also said hull/prop interaction or resonance problems have
    never been reported on the 33 (except some worn rudder bearings
    that were excited by propwash), and the electrolysis issue has.
    While the AC power system contains ground fault interruptors, these
    would not detect very small ground leakage problems from shore power
    or any leakage on the DC system. I check the DC system regularly
    (for the sake of the batteries mostly) but I dont know the
    condition of shore power at marinas when I am not around (I dont
    always leave the boat connected, but when I am doing a battery
    charge, I am connected sometimes for a few days in my absence).

    
    By the way I measured the deflection of the shaft/cutlass and found
    a .004-.006" deflection (depending on how much force is exerted).
    I checked 3 boats immediately adjacent to mine and found all over
    .010".  One was about .020" and the strut itself could be moved side
    to side >�" and up/down about half that. This one also had the
    cutlass so loose in the strut it rotated and could be deflected.
    The owner said he didnt remember any problems (this is the boats
    first year).  I went looking for the best case and found a 2 year
    old Moody 33 where the deflection was probably under .002".  This
    boat has an upper thrust bearing and flex coupling between the engine
    and shaft.  

    Walt

    I have rebuild my engine during the last two weekends.The check
    of the injectors was positiv.One injector was bad and had to be
    repaired.
    
    My testrun showed no vibrations from the propeller anymore.
    
    Before I rebuild my engine,with no load there has been no vibrations.
    Going backwards or faster then 3 knots,looking at the stuffing box, you
    could see a movement (vibration) of at least 10mm .This movement made
    a noise like "kkrrrr" 1/2 seconds quiet then again "kkrrrr".
    Now looking at the stuffing box,it is nearly stationary and no "something
    is eating my boat" noises anymore.
       
    The overheating of my engine did not damage anything visible.But
    the one bad injector probably created this vibrations of the propeller.
    
    Peter
    
                                                                
    (installing an addon heatexchanger is an other story)

    Well, I have the new prop on the boat. It is in the water.  Last
    weekend I tried it out after aligning the engine to the shaft.
    
    The thumping still occurs but well above where I would ever
    cruise.  I was running 7 kts with the engine about 2200RPM and the
    stern was down about 12-15".
    
    I plan to realign the engine this weekend.  I think I will also
    pull and test the fuel injectors. I have never done the injectors
    on a Universal 3 lunger.  Any suggestions or tips?
    
    Walt

    As something of a final update here...
    
    I have not experienced as much problem with the thumping this season,
    because the spped needed to cause it is well above where I like
    to motor (the boat squats in the water like it is trying to plane
    before this set in).
    
    A month ago I had reason to haul the boat and pull the rudder. At
    the same time I had the cutlass bearing replaced (I couldnt get
    it out this past winter without pulling the shaft, which required
    dropping the rudder,...
    
    The new cutlass along with the prop seems to have eliminated all
    the problem, in fact the whole boat motors more smoothly.  If you
    will recall the side play in the cutlass wasnt much at all, so
    I guess one cannot tell for sure by wiggling things whether it is
    good or not.
    
    Walt

    To Zinc or not ?
    
    I want to put a zinc node on my shaft .... even though the prop seems
    in pretty good shape, however there is very little space.  I have a one
    inch shaft and the smallest one inch doughnut anode has a thickness of
    about 1.25 " and only about 3/4 of an inch to mount it.
    
    I can mount the zinc aft of the prop, over the locking nut.  This has
    some advantage, the bieggest of which is that it is actually mounted. 
    Unfortunately, given the ourside diameter of the nut, I need to put
    about a 1 7/8 - 2" doughnut on the nut, or I can put the 1" that I have
    and space it .... because of the angular sides of the nut, the 1 inch
    anode fits fairly well.
    
    Problem is that I now have an asymmetrical anode around my shaft.
    
    Asymmetrical = Out of balance
    
    I guess the question is does anyone have any sense if this will cause a
    problem in spite of the fact that the zinc is so close to the shaft ?
    
    Graham

    I have seen zincs that are made to put over the shaft nut in the marine
    stores. Find one of those. They look like a cap.
    

    Re "Find one of those"
    
    No can do .... we only have one store inthe city that sells boat stuff
    and it concentrates on fishing gear.
    
    I am afraid I have found whatever is available !

re: zinc over nut:

If the asymmetry is modest, it probably won't cause any vibration. Last year
my zinc was asymmetrically eroded, and I didn't experience any vibration from
that.

--RS