Propellers

Discussion in 'Downeast Engine Room' started by captainlarry84, Aug 24, 2013.

  1. captainlarry84

    captainlarry84 Captain

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    I think the following might be helpful to some. The propeller is where the rubber meets the road so to better understand propellers, the following might be helpful:

    Diameter: This is the overall width size of the propeller or as stated the diameter. The diameter is measure from one outer tip to the other and directly through the center. On all propellers the diameter and pitch is always stamped either between the blades on the hub or on the face of the hub. The first number is always the diameter. Therefore a 24 X 22 stamped propeller means that the diameter is 24 inches. Even we as layman can measure the diameter. Diameter of the propeller plays a big roll in overall performance. On most vessels when wheeled correctly they have the maximum diameter allowable to get the largest blade surface. Large blade surface is important in order for the propeller to catch enough water and allow the pitch (twist) of the propeller to do it job and push the vessel forward. When purchasing a vessel you only pay for diameter, as the larger the diameter the greater the amount of material which equals cost.

    Pitch: Pitch is what pushes you forward. In a perfect world with zero slippage for each inch of pitch and every RPM your vessels shaft turns the boat should move forward 1". So a 1 shaft RPM with 22" pitch and 100% efficiency in theory you move forward 22 inches. But as we all know there are many variables which get in the way and reduce this number. Things such as keels, struts propeller pockets and just a few that lower our 100% efficiency number. Pitch is the second number on the stamp of a propeller; so on the same 24 X 22 propeller, the second number of 22" indicates 22" of pitch or twist in the blade. Unlikely diameter pitch cannot be measured by a simple ruler, as pitch is the gradual swipe or twist of the blade. On a flat plan and inch of pitch would be about 1/8" of rise at the outer edge. The rise of pitch however changes through the blade because the blade must end at the root of the hub of the propeller.
    Pitch must be measured by a propeller shop. The wheel is place on a jig and a dial indicator is dropped down the blade to measure the pitch. Pitch is usually what we need to adjust when reconing or resizing to gain or lose load on the blades.
    On many of the newer boats due to high horsepower and diameter clearance problems pitch may exceed diameter by as mush as 12" or more. So a 24" X 36" propeller is not uncommon on some of our high performance Sportfishing Boats to swing. Propellers such as these are not commonly seen on keeled bottom or displacement bottoms, as these types of hull bottoms could very provide enough water flow to feel propellers with such massive pitch.

    Cupping: Cupping is the gently bending of the trailing edge of the propeller blade. In a way cupping is in simple terms a way to add a little pitch to the very edge of the blade. Cupping is used to help hold the water on the face of the blades so that pitch and diameter can do there job better and push the vessel forward. Cupping is measured in numbers starting with #1 to smallest and then going to # 9 the largest. Cupping does not always help a propeller; in fact it can have a reverse effect if not needed. If the blades of a propeller hold water well without cavitation or slippage cupping will only then slow down the propeller and cause unwanted load on the propeller blade.

    Cupping in addition is always done at the propeller shop as it is not casted into the wheel it is done manually in a blacksmith method. In closing once a propeller is properly sized with diameter and pitch cupping may be added for just a little fine tuning

    Bore: The bore of the propeller is the taper hold in which the shaft slides through. The bore is also done at the propeller shop, it is would be too costly to carry the propeller pre-bored as there is many different size shaftings that a propeller may require. Bore size is based on a formula which goes with the shaft sizing. Example a 24" diameter propeller has a minimum shaft size of 1 3/8" and a maximum shaft size of 2". If a 24" propeller is bored over spec to 2 1/4, there may not be enough material on the hub left to support the blade load, which in turn could cause a propeller blade to snap off. If the shaft is under spec at 1 1/4" the propeller may have too much load and it can cause the shaft to snap. Therefore is important before buy a propeller new or used that you are award of what the specs for the bore call for.

    Blades & Blade Area: This is where the work gets done. On our powerboats propeller blades usually are either three (3) or four (4) blades depending on there use. On some vessels even five (5) blades are used. In theory the more blades the smoother the propeller, as the load is disperse over a larger area, thus making the load more manageable. As rule the fewer the amount of blades the faster the propeller is at top end. Most boats leave the factory with three bladed wheels because of this factor.
    With propellers blades pitch always changes. For example if you can turn a 24 X 24 three bladed propeller to full RPMs and wanted to go to a four bladed propeller, your four bladed propeller would most likely be a 24 X 23. The pitch being reduced to make up the load difference with the extra blade. This also explains why the three bladed wheels are faster, as it has one additional inch of pitch, which as stated earlier is the force that move your vessel forward. As you come down in RPMs to the Cruise RPMs on most vessels you pick up efficiency due to less slippage with the extra blade.
    In today selection there are also propellers made with different blade area. Blade area is the amount of surface on each blade. The surface can be increased or decreased depending on the shape. For example the newer models called DQX have a larger blade area and shape then a conventional DynaQuad blade. On a 24 X 24 dynaquad the blade area is approximately 77 square inches and on at 24 X 24 DQX the blade area is approximately 83 square inches. Increased blade area is important on a propeller when diameter clearance become a problem. If you need more surface area but you are limited in clearance you can get the sample blade area using a 24" DQX as opposed to using a 26" Dynaquad. So in essence you get the diameter of a larger wheel on a smaller propeller.
    This may should a little confusing but it is really pretty basic math due to load. As you therefore can see a DQX maybe a real good way to good if you need more propeller size in blade area and you have already maxed out on pitch.
    The more blade area the more grab and therefore the less slippage. Slippage is important to reduce, however you do not want too much blade area as it then can become drag and then reduces your pitch.
    On a keeled boat if you have enough diameter then a DQX may give you no improvement. On keeled vessel the diameter is very important because the area on the blades near the keel do not get a lot of water and therefore the field of the blades and the edges need to do all of the work. On some of the newer New England Boats you will notice a trend in tapering the keels thinner to allow more water flow to the blades.
    However in some applications the four bladed propeller will give the vessel a better cruising speed which in the long run is more important to fisherman because that is where we spend most of or time at cruise speed.

    Material: Material for propellers varies. Aluminum is used primary for light duty outboards and I/O drives. These are cheap and easy to repair and also easy to bend. Stainless Steel also primarily for outboards & I/O drives. This are much harder to bend and repair and they are also less forgiving on the drive unit should there be a collision. I n addition outdrives & outboards have housing, so in some cases stainless can cause electrolysis.
    Stainless is sometime, but rarely used in inboard boats. In larger sizes it is very hard to work with, very unforgiving in a collision and hard to repair. The metals of choice are Bronze, which is brass and copper. Nibral which is brass, copper & nickel. A nibral propeller is twice the coat of a bronze propeller in the same size. Bronze at half the price is good value. Bronze propellers are easier to repair; they also bend easier because they lack the nickel component which adds strength. Bronze propeller work fine on most inboard boats, providing they are not carrying to much load due to excessive.

    Efficiency: The efficiencyof the propeller is what we strive for 100% efficiency means that as stated above 1 of pitch per Revolution equals 1 of forward movement. However 100 % is nearly impossible to achieve. It can however be improved if all of the above factors are carefully planned during your propeller selection.
     
  2. lobstercatcher

    lobstercatcher Captain

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    I guess in all fairness as in the classifieds and what posts not pertaining to downeast boats.. A propeller isn't down east. a propeller goes on a plane or a outboard motor. A wheel goes on a downeaster

    That is a nice little tid bit to know !
     
  3. captainlarry84

    captainlarry84 Captain

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    KristenFormer Charter Captain
    Sharing knowledge such as this and so many other post. That is your response... pretty poor in my book.
     
  4. powderpro

    powderpro Captain

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    Larry- Good information. How much, if any performance gain do you see in a Nibral DQX versus a Bronze DQX? If a boat is propped correctly with a 28x30 DQX Bronze, would you use the same pitch if you went to a Nibral? And would the Nibral have any performance difference? Is the Nibral lighter weight? Please expand your discussion on the Bronze versus Nibral. If Nibral is worth an extra knot on the top, it may be worth spending the extra money for some guys. Thanks!
     
  5. captainlarry84

    captainlarry84 Captain

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    When DQX came out the blade area was greater yet the cost was the same. The cost was the same because the amount of material used was the same. In doing so the QDX blades were made thinner. Making the blade thinner in Nibral was not that much of a factor in strength & flex. However in bronze it can be. The strength is put to the test not so much on diameter but on pitch. As pitch is what puts the forward load on the blade.
    The rule of thumb is if pitch excesses diameter go with Nibral. When using Nibral because the flex is less you may have to lose a little pitch in some case to keep your load correct. But yes a N ibral wheel will out perform the bronze.
    Once a boat is wheel or propellered correctly....or can I not say that? I prefer a Nibral wheel.
    Nibral wheels do have a draw back. Your bonding system must be spot on, or electrolysis can get the best of the propeller...OPPS I mean wheel.
    In the case mention for a 28 X 30 I would go with a Nibral wheel.
     
  6. offshore31

    offshore31 Captain

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    Larry, thanks for a good read.

    Joe
     
  7. lobstercatcher

    lobstercatcher Captain

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    lol Its a joke son..

    Are you always so stuck up?
     
  8. lobstercatcher

    lobstercatcher Captain

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    yea its all over the internet, He is excellent at copy and paste!

    But yea, lets give stuck up larry a hand !!!!
     
  9. Coastal

    Coastal Member

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    Considering that it is filled with spelling errors (no offense), I would say that he wrote it himself and did not copy and paste. I think he presented some very good information that many people don't know a lot about.
     
  10. lobstercatcher

    lobstercatcher Captain

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    I didn't read it so I wouldn't know.


    Becauses it was a joke response.
     
    Last edited: Aug 24, 2013
  11. Tuna Pursuit

    Tuna Pursuit Captain

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    Larry is an awesome guy and one of the most knowledgable on here in MHO, that was some pretty mean stuff.
     
  12. lobstercatcher

    lobstercatcher Captain

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    If people want to turn a joke into a pissing match. I can do that instead of being humorous. The support team seems to want to bring it in that direction.

    Doesn't anyone get jokes around here?
     
  13. powderpro

    powderpro Captain

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    Larry- you say the Nibral will outperform the Bronze, but in what way? Will cruise be the same or better with the Nibral? What about top speed? Have you any numbers or experience with a boat that goes from a Bronze to Nibral? Are we talking 1 knot difference? And what kind of a price difference in a 28" prop between the 2 materials? Thanks for bringing up this topic.
     
  14. blackdiamond296

    blackdiamond296 Senior Member

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    Stop being such a $hit stirrer and maybe your jokes will be better received.

    Seriously, looking at your past posts- how is someone supposed to tell the difference between your jokes and your soap box nonsense?
     
    Tuna Pursuit likes this.
  15. lobstercatcher

    lobstercatcher Captain

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    lol . funny you start off by stirring the $hit and then you end with stirring it?????:confused::confused:
     
  16. Blitzen

    Blitzen Captain

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    Consider Nibral and Bronze like Stainless and Aluminum for outboards. It all boils down to load on the blade area of the prop. Load will change with many factors so if you are looking of performance over a wide range of conditions opt for a stronger material to hold shape and form. If performance is not a concern than opt of the less expensive alternative of bronze which will flex. bend and distort more. In some applications Nibral is the go to material and in some application it is Bronze.
     
  17. powderpro

    powderpro Captain

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    Blitzen- do you have any experience going from a bronze to nibral where you could see the difference in performance numbers? If there is a good performance enhancement, I would be pretty serious about going to nibral after I get my bronze tuned into the correct pitch. My situation would be a 38'-40' downeast, 500hp, 17,500lbs, 4 blade DQX. Thanks.
     
  18. pugsley

    pugsley Captain

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    i figured it was a joke, but some ppl took it way serious. jeez.
     
  19. Hammer Slammer

    Hammer Slammer Senior Member

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    Larry,
    Great job very informative.
     
  20. lobstercatcher

    lobstercatcher Captain

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    Thank you. Maybe some of the readers weren't aware what was going on in classifieds?
     

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