Conference vicki::boats

    What size is your boat? What are the pitches of the props?
    How much clearance do you have between the hull and the tip of the 18
    inch prop.
    
    You should have at least 10% of the diameter of the prop between the
    tip and the hull. Otherwise you will get a "prop wash" vibration cause
    the water coming off the tip of the prop hits the hull.
    
    Typical maximum RPM for the average v-8 inboard should be 4200-4400.
    Get as big a prop as you can and still hit that RPM. If you can't get
    it over 4000 then you are over propped.

    Its 34', pitch = 22".
    
    The 18x22 is the prop the factory shipped with this boat.
    
    I figure this is an oppurtunity. Doubt anyone wants to buy 
    a set of props, but maybe some other dia/pitch might be better..
    
    J.
    
    

    18X22 sounds about right for a 34 footer with twin 350 cu in v-8s.
    
    For reference, my 32 footer with twin 351 cu in v-8 will turn 4500 RPM
    with 18X20s yet drop to 3700 with 20X21s. The 20X21s make it fly but
    only have 1 and 1/2 inch clearance which causes a drumming sound as the
    prop wash hits the hull. 
    
    I would stick with the factory shipped size 18X22. If you know all the
    data like Shaft Diameter, Weight of boat, HP, etc., almost any prop
    supplier has a computer program they plug all this stuff in and
    recommend a prop size. It might be interesting to hear what they
    recommend. It is better to be slightly under propped than over propped.
    
    Unless you have a friend in the prop business, it is expensive to
    experiment.
    

    Here's who i've talked with..
    
    Oconnell's in Fall River. (Silverton dealer)
    Silverton                 (factory)
    H&H propeller
    Bayview Marine            (prop rebuilder)
    Silverton Marine Supply   (after market orig. equip. supplier)
    Boat U.S.
    Marina where i keep my boat.
    
    
    18x22 looks like the factory prop.  Funny the factory says "18x20"
    
    It can get expensive, but a diver charges $50, to pull and replace
    a set of props..
    
    But i don't want to experiment...
    
    keep ya posted..
    
    JIm.
    

    It isn't the changing that is expensive. An 18X20 Michigan Prop goes
    for $281.00 here in Seattle. 
    

    So far everyone has come back with 18" as the dia. if need i was 
    thinking if i had got a 22" pitch, it could be reworked to 21
    or 20. 
    
    Your point is well taken, rather get it right the first time.
    
    Michigan called back yesterday, they recommend an 18x19 4 blade.
    
    The dia./pitch didn't strike me, but 4 blade did. They explained
    that the weight of the boat had a lot to do with their decision.
    Also, when the factory tests the boat it is definitely not loaded,
    i.e. full fuel/water gear, people, in an effort to come up with
    "bragging" numbers. But after its sold, their owners tend to load
    them down. They claim the 4 blade will give it the proper thrust
    and there would not be any speed loss.
    
    interesting,
    
    JIm

    I have never seen a 4-blade on a planing boat. Has anyone else?
    

    	RE.7
    Not on a boat that size, but the bass boat folks use 4 and even 5
    blade props for a quicker plane.
    					B.C.

    4-Blade props are also available for in-board ski-boats now.
    Gives you more ooomph out of the hole (as if you'd need it) but no more
    on the top end.  Might be good for your boat.
    DW

    With the power they provided, 250 h.p Chrysler's, 270 Crusaders
    they are not typically fast. About 23knots is it.
    
    Michigan did say the 4 blade would provide thrust, more so than the
    3 blade, but no difference in top end. I've been under the impression
    that the 4th blade added drag. Well ya can't know everything.
    
    I;m still confused...
    
    JIm.

    Under "pleasure propellers":
    
    Equi-Quad (tm) Four blades for exeptional smoothness
    Designed for: Any normal cruiser where three-blade propellers have been
    used.
    Number of blades: 4
    Owners of pleasure cruisers who have never experienced the exceptional
    smoothness and fine performance of Equi-Quad will often be amazed.  The
    four-blade design gives extreme smoothness, plus the speed and "dig" of
    a three-blade prop.  Maneuverability is superb.
    Equi-Quad is available in most pitch requirements from 17" through 44"
    diameter.  It is offered as standard in our Federal-B alloy and is
    optional in Nibral or Stainless Steel.  It comes with the special
    offset and rake pitch originally incorperated in our Equi-Poise (R) and
    with a tapered hub for improved hydrodynamic performance.  It is
    strongly constructed and then balanced for reduced vibration.  Order in
    the same diameter as the correct three-blade size, but allow for 1"
    less pitch to maintain same engine loading.  Our engineers will assist
    on receipt of hull and engine details.
    
    Another example:
    
    "Cyclone Ski Prop
    Fourth blade means reduced vibration, smoother ride.
    Designed for: Ski Boats
    Number of blades: 4
    Ski boat owners have always searched for the "perfect" ski prop - one
    that helps the skier get up quickly out of the water, and that provides
    a stable, smooth ride.  Now the search is over.  While ordinary ski
    props feature three blades, the new cyclone features four.  This extra
    blade reduces vibration by increasing the blade frequency.  This is
    very important at shaft speeds.  The Cyclone is available in two sizes:
    13X12 RH, and 13X12 LH.  Both models are made of famous Federal Nibral
    for extra strength and are cupped to supply extra speed.
    
    Cyclone Feature                         Cyclone Benefit
    _______________                         _______________
    4th blade                               Reduces boat vibration
                                            for smoother ride
    
    Higher low end                          Skier up quicker
    
    Reduced wake                            More stability for skier
    
    Cupped Nibral swept-blade design        Top end near 3-blade cupped
                                            Super Cup (R)
    
    
    dw-H&H is a Federal Distributor
    
    BTW: This catalog has 96" props (for your shrimp boat) and
         132" nozzle systems (for gawd-knows-what) that weigh 23,000 lbs!
    

I owened a 37ft Chris, with 4 blade props, and the difference between 3 and 4 
blades becomes very obveous when docking, or manuvering with the engines. The 
responce from 4 blades is like haveing brakes, they respond to direction 
changes imediately (no slip like with 3 blades).

Go for the 4 blades, at cruseing speed, you will notice that they are smother,
at top end, you may lose a little, but when in close quarters, you will love
them.

    When you say "you may lose a little" how much is that?????
    
    JIm

    All this Prop conversation has hit me sqarely between the eyes !  I
    have been wondering why I can only turn 3200- 3400 RPM on my twin I/O
    Wellcraft Nova.
    
    Is it possible that I also have mismatched props?  Any help that you
    can provide will be invaluable !
    
    The boat is a '73 Wellcraft Nova with twin Mercruiser 188's (ford
    302's).  it weighs approximately 3000 lbs. dry.  I typically carry
    about 50 gals gas.

RE: Note 782.14
>> All this Prop conversation has hit me sqarely between the eyes !  I
>> have been wondering why I can only turn 3200- 3400 RPM on my twin I/O
>> Wellcraft Nova.
   
For more info on props, type "DIR/TITLE=PROP".

Note 575.0 gives the formula for figuring out theoretical prop efficiency.
If you know your gear ratio and prop pitch, you should be able to turn the
formula around a bit to tell whether your boat is correctly propped or 
whether you've got some kind of other problem.  Since a Nova is a planing
hull, for starters use an arbitrary prop slip of about 5-10%.

It could be that you've got something else wrong that is preventing your 
engines from going past 3400rpm.  But until you know the gear ratio and 
prop pitch it's all just speculation.

    Give us your numbers and we'll help figure it out if you'd like...
    
    Rick

    RE.14, 
    
    You have 2 engines, they both will only turn 32-3400 rpm.
    
    right?
    
    I don't think you have mismatched, (like mine).
    
    i think,
    
    1. You have the wrong dia. or pitch, or maybe both.
    
    2. Your engines have the flu, but i doubt both of them
       are sick to the same degree.
    
    
    You can call Boat U.S. in Clearwater Fla., the Product Info
    Dept. (they moved) there is a Naval Architect on board,
    he's very cooperative. He'll need the following info:
    
    Boat weight, H.P., trannie ratio. He can tell you what you should
    have for dia. and pitch.
    
    Go out and look at your props.
    
    Let us know what you find?????
    
    The phone No. is....   800-365-7732. i believe his name is Al...
    
    
    JIm.
    

    re .16
    
    I called BOAT US.  They were pretty helpful considering my non-member
    status.  Their best advice was that on a producton boat (like mine is)
    the manufacturer usually has the prop selection dialed in after about
    the fifth hull out of the mold.
    
    My next step is to get hold of a Mercruiser/Wellcraft dealer on the
    Jersey Shore and try to find out the recommended prop pitch and dia.
    
    I went to check out the winter cover this past weekend, but it was too
    cold to drag the stern drives out of storage to check my presetn props. 
    I'll let you know how I do in a couple of weeks.
    
    Thanks for your help.
    
    Tom