Announcement

Collapse
No announcement yet.

prop test

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • prop test

    How do you perform a prop test? I have a new boat and motor and the dealer sent me home with 3 props to determine which one I like best. But I am not clear as to do it. I am just looking for good all around perfomance for sport fishing. Boat is a jetcraft and the weight is 4600lbs with a single 225 Honda.
    Thanks

  • #2
    WOULD ASK THE DEALER! or read owners manual? talk with shop that sells props.
    with inboard engine- ( not sure if same but not sure why it would not be)
    know what max rpm's are for engine, under load
    load the boat the way it would be normally be . (full fuel, water, #of people and gear) after warming engine appropriately
    run it full out, what do the RPM's max out at? change of pitch will change maxed RPM's
    my understanding is your top speed should max at predetermined max rpm's.
    if no tach. #1 i woiuld get one
    but run boat and see how it preforms loaded.
    also with full load is it getting up on step well.
    had set for my boat top in speed MPH was great but hardly would get on step:mad:changed
    spare prop or two are good. if you have different sizes you can change prop for different situations.
    RETIRED U.S.A.F. CAPT.; LIFETIME MEMBER NRA; LIFETIME MEMBER ALASKA BOWHUNTER ASSOC.
    MASTER BOWHUNTER EDUCATION INSTRUCTOR; MEMBER UNITED BLOOD TRACKERS; POPE & YOUNG MEASURER

    Comment


    • #3
      A few things to ponder

      According to honda's website, normal operating range is 5K-6K. I highly recommend hitting the upper 25% of that range (5800-6K) with a full load. You do not want to lug these motors. Those motors hit max HP at 5500 rpm. It has been my experience that most dealers really do not have a clue about propping boats correctly. Run it with the prop or props you have and get a baseline, then post your numbers and we can arm chair quarterback for you. If you are running the salt, consider going stainless. I noticed a huge improvement in overall performance and mpg when I went stainless on my old hewescraft. I highly recommend you get a fuel management system, too. The aftermarket guages are cheap and reliable. You will pay for the savings in one season. Isn't it fun spending other people's money?
      sigpicSpending my kids' inheritance with them, one adventure at a time.

      Comment


      • #4
        I did have a fuel flow meter installed in it, and I did order it with a stainless prop but the dealer wants me to try some different sizes before he orders the stainless one. All of my experiance has been with jets until now. It is real nice in Valdez right now and I plan to put it in the water on Saturday and get the engine break in period over before shrimp season starts.

        Comment


        • #5
          Good deal. Post some pictures of the new ride, too.
          sigpicSpending my kids' inheritance with them, one adventure at a time.

          Comment


          • #6
            A prop is like gears. The lower the number the more power you have, the higher more speed. A 17inch prop will move the boat 17 inches through the water in one rotation. A 17 is a little small but allows me to carry heavy loads and gets on plane very fast. I carry heavy loads often and run WOT very little. The 19inch prop gives me about 5 to 7 mph more speed, but gets on plane slower and is harder to plane with a heavy load. When running the 17 I have to be mindful not the over rev the engine because it will pull about 6200 rpm with it and my max is 6000. With the 19 I max out at about 5800. I very rarely ever run at top speed for a couple reasons, I burn more fuel at higher RPMs and water conditions rarely allow max speed in PWS with my flat bottom sled. I too use a fuel flow meter and it is amazing the difference a couple hundred RPMs make in fuel use. So for me the lower pitch prop is the right prop, allows me to carry scary weight and gives me plenty of power to get on step in a hurry. PWS has many rocks and reefs that like to hide just under the surface, a stainless prop will take a lot more abuse than an aluminum prop. I've bounced mine off a few while cruising the shoreline looking for bears, so far nothing more than a couple scratches. An aluminum prop will usually bend badly or break off. Sounds like you have an awesome dealer, I've bought a few boats and all they ever ask me is what size prop I want, slap me on the back side and send me on my way. I would start with the smallest he gave you and work up from there. With the smaller size all you have to worry about is over revving the engine, if you start off too large it may not get on step. I'm running a Honda 150hp with 21ft sled.



            Hope this helps, Good luck

            Steve
            "I refuse to let the things I can't do stop me from doing the things I can"

            Comment


            • #7
              Spoiled one nailed it. The prop should be sized so that you are within a few hundred rpm of the redline at wot. You can certainly run the next size up prop and typically will have no problems with getting on plane or won't notice much difference in normal opperation.

              But, when you are overpropped, you will put more wear and tear on the powerhead, and in adverse conditions, heavily loaded or climbing up waves you'll be putting even more strain on the engine.

              Stainless will give you better performance, but is less forgiving on oopses. I'll burn a tad more fuel and replace a dinged prop vs. having the rebuild a lower end when a stainless prop hits something it shouldn't.

              Here was my lesson on carrying a spare last summer



              The funny thing is, I ran the first year on one prop without a single ding. Last summer I picked up a spare, just in case, and tweaked it on my second trip of the year.

              4 blades are an option and I found out that they have some interesting differences. Comparing the 3 blade 14X19 to the 4 blade 13 7/8 X 17, they both provide the same fuel consumption at the same speeds, in my boat. But, the 4 blade does it at ~300 more rpm. Both props give me the same top speed (4 blades are more of a mid range prop) The big difference is the 4 blade has more lift, so it planes my hull out more level, which is nice as I can see over the cuddy. I just picked up another 4 blade today, and the 3 blade will become the spare, and hopefully last more than a couple of trips
              Those that are successful in Alaska are those who are flexible, and allow the reality of life in Alaska to shape their dreams, vs. trying to force their dreams on the reality of Alaska.

              If you have a tenuous grasp of reality, Alaska is not for you.

              Comment


              • #8
                inches vs pitch

                Stid talked about inches on his prop. And how the number moves the prop forward per rotation. As I recall, Pitch is the number that moves the prop forward one revolution. Inches is the diameter of the prop.
                Stainless and aluminum props, same size, same pitch, will often perform differently due to the weight difference. Sometimes you'll find a pitch works a bit differently if the props are different diameters. More blade tends to equal more resistance.
                I like stainless. I have never lost a lower unit due to running stainless, and my props get beat up plenty.
                The advice about maximizing the rpm power band is solid. Good analogy on the gearing. It makes it easy to see why a lower number is needed for heavy loads.
                That much HP should run a pretty big pitch I would guess, but again, stick with the testing. If you have somebody to go with you, time the hole shot. It is a bit subjective, but can give you some good info too.
                You guys running the ocean with a motor fixed on the transom is a bit different than my application. I use a CMC lift w/ a 150hp prop. I can run a 19 pitch with over 3,000 lbs in the boat because I can lift the motor and reduce drag. My hole shot suffers a bit, but I take into account how much room I need to get up. I need the extra pitch for long distance cruise.
                Somewhere in all this there should be a grant that I could get to study it all. You know, like a thesis. Travel all the water I can in AK testing props, motors, and boats. Hmmmm

                Comment


                • #9
                  A big whoaaaaaa

                  Originally posted by stid2677 View Post
                  A prop is like gears. The lower the number the more power you have, the higher more speed. A 17inch prop will move the boat 17 inches through the water in one rotation.
                  Whoooooaaaa Stid. The pitch number has nothing to do with how far you will travel in one rotation of the prop. If you were talking of a screw and wood you might be correct. (I always thought it was the degrees of pitch in the blades.) But water being fluid, it doesn't quite work that way. There are some other variables with a boat such as the shape of the boat (is the bow flat? pointed? flat bottom? v-hull?), the weight of the boat (how far down in the water are you sitting?), the horsepower (A 10 horse outboard will not push your boat as far @ 1,000 rpm as a 100 horse one will), the diameter of the prop, the way your boat is trimmed and more.

                  Spoiled one gave the correct answer. Find the recommended rpm range for your engine. Then try the different props and find the one that lets you hit the upper end of that range. If your engine turns up less than that, your engine will be working too hard, straining so to speak and that's not good. You also don't want to be over that range as the engine isn't engineered to turn that fast and you'll wear it out sooner than later. The recommended range will give you the longest engine life.

                  There are certain situations where you might want less pitch, but you still want to be able to hit your rpm range. If you consistently haul heavy loads or do a lot of heavy towing, less pitch allows the engine to turn up higher rpms under stress so it isn't working as hard to pull the extra weight. With more pitch and a hard tow or heavy load, you'll lug the engine down. Not good!

                  You also want to mess with blade diameter. The larger the diameter, the more water it grabs so you'd probably want a smaller pitch. A smaller diameter grabs less water, so you can use more pitch. Smaller diameter with more pitch is more of a speed prop, and larger diameter with less pitch is more of a towing prop, but each boat has a different prop it likes the best.
                  An opinion should be the result of thought, not a substitute for it.
                  - Jef Mallett

                  Comment


                  • #10
                    Originally posted by Paul H View Post

                    Here was my lesson on carrying a spare last summer


                    Heck, that a good looking prop Paul. It still has all four blades! A little hammering and some filing and it will be good as new!
                    An opinion should be the result of thought, not a substitute for it.
                    - Jef Mallett

                    Comment


                    • #11
                      Diameter is the distance measured across the propeller hub line from the outer edge of the circle that is made by the propeller's blades during rotation. Pitch is the distance, theoretically, that a boat will travel for each revolution without any slippage. (The actual distance the boat moves forward for each propeller revolution is somewhat less, depending on the amount of propeller slip.) The diameter is listed first and the pitch is second. Therefore, a 14" x 21" propeller would have a 14-inch diameter and a 21-inch blade pitch.

                      http://theoutboardwizard.bizhosting....propeller.html

                      I agree that RPM is the key indicator. When I wrote 17inch, I should have wrote 17inch pitch. I did not mention diameter because that should be the same on all three props.

                      Diameter: The first number listed when describing a propeller. Is two times the distance from the center of the hub to the tip of the blade. Also can be described as the distance across the circle the propeller makes when rotating.

                      Pitch: The second number listed when describing a propeller. The forward movement (distance) of the propeller after one revolution, assuming there is no slippage.

                      http://www.evinrude-parts.com/boat_p...n_article.html


                      tough crowd, I hope we all agree now. :-)

                      Steve
                      "I refuse to let the things I can't do stop me from doing the things I can"

                      Comment


                      • #12
                        pitch

                        Lets do some math.

                        Say you have a 15 pitch prop. By the definition you provided above, with every revolution the boat would travel 15 inches. so at 4,000 rpm's the boat should travel 4,000 times 15" or 60,000 inches. Divide that by 12 to figure the feet traveled and you get 5,000 feet. 5,280 feet is a mile. So 5,000 feet is roughly .95 of a mile. Times that by 60 for minutes in an hour and you get a speed of 57 miles per hour. Don't know of many boats that can do 57 mph at 4,000 rpms.

                        As I said, in a solid piece of wood, that might be a practical definition of pitch. But in water, as you said, slippage plays an important part, and we haven't even mentioned cavitation!
                        An opinion should be the result of thought, not a substitute for it.
                        - Jef Mallett

                        Comment


                        • #13
                          Originally posted by twodux View Post
                          Lets do some math.

                          Say you have a 15 pitch prop. By the definition you provided above, with every revolution the boat would travel 15 inches. so at 4,000 rpm's the boat should travel 4,000 times 15" or 60,000 inches. Divide that by 12 to figure the feet traveled and you get 5,000 feet. 5,280 feet is a mile. So 5,000 feet is roughly .95 of a mile. Times that by 60 for minutes in an hour and you get a speed of 57 miles per hour. Don't know of many boats that can do 57 mph at 4,000 rpms.

                          As I said, in a solid piece of wood, that might be a practical definition of pitch. But in water, as you said, slippage plays an important part, and we haven't even mentioned cavitation!

                          Argue all you want. The definition of pitch that Stid "pitched" is the accepted definition in the boating industry. The caveat here is the variable slippage as you stated. That is why every prop calculator has slippage factored in. Slippage is accepted and expected to a degree. These guys seem to have the prop choosing down to a science: propgods


                          Peace.
                          sigpicSpending my kids' inheritance with them, one adventure at a time.

                          Comment


                          • #14
                            My apologies

                            I want to make it clear, I'm not trying to make Stid look bad. He was saying what the boating industry uses as a norm.

                            Here's the definition from one of Stid's links.

                            "Pitch: The second number listed when describing a propeller. The forward movement (distance) of the propeller after one revolution, assuming there is no slippage."

                            Here's one I found just now.

                            "Technically speaking, pitch is the theoretical distance a prop moves forward in one revolution - assuming there is no "slippage" between the prop blade and the water."

                            And here is another.

                            "Pitch is the distance that a propeller would move in one revolution
                            if it were moving through a soft solid, like a screw in wood."

                            The trouble is, we can't assume there is no slippage because a prop spinning in water is all about slippage. So we have to compensate for the variables. And also, that definition means how far the prop would travel by itself NOT how far it would travel attached to your boat! Your boat is a variable.

                            Besides all the variables I listed above, there are more, cavitation increases slippage, whether you are plowing or on step is a variable, and sea conditions can affect your speed. Even the rpm's you run at and your momentum can affect your efficiency.

                            My point is, there are too many variables that produce slippage for that definition (not necessarily Stid's, but the industry's) to be practical. It's something we can strive for, but will never achieve.

                            Heck, to go even farther with a calculator how about we take a 15 pitch prop @ 1000 rpm's, basically a fast idle.

                            By the industry definition, that would be 15,000 inches traveled per minute. Divide by 12 and you get 1,250 feet per minute or .24 miles per minute. Times 60 minutes equals 14.2 miles per hour. Anybody here do 14.2 miles per hour @ 1,000 rpm's?

                            The industries definition might be theoretically technically sound under lab conditions, but it really isn't practical in everyday use.

                            Kind of like using a computer to pick the National Champion Football Champs in College Football. It sounds good on paper, but there are so many variables, it rarely works.
                            An opinion should be the result of thought, not a substitute for it.
                            - Jef Mallett

                            Comment


                            • #15
                              The important thing is that jakim selects the correct prop for his rig.

                              1. Maintain RPM within manufacturers specified limits.

                              2. Use a prop with a low enough pitch to plane the vessel with your heaviest load.

                              3. Use a prop with high enough pitch to maintain your engines RPM within the correct power band, Lugging it down is hard on the powerhead.

                              4. If power and hole shot are you priorities prop to the low side, if greater speed and fuel economy is priority prop to the high side.

                              Enjoy your new boat, be safe and go get some fresh fish and shrimp.

                              Steve
                              "I refuse to let the things I can't do stop me from doing the things I can"

                              Comment

                              Footer Adsense

                              Collapse
                              Working...
                              X