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  • prop question

    I'm getting ready to figure out what prop my project boat needs. I've heard when going from a 3 blade prop to a 4 blade prop that the rule of thumb is to drop one size in pitch. Is this accurate?

    So far I've tried a 14.25x17 & a 14x19c & it doesn't like to get on plane very well with the 19 pitch prop. Once it did it ran like a scalded dog.

    The 17p seemed to do alright but I did not have my shop tach with me to see what the rpm's were with either prop. Going back out this weekend with tach & taking notes.

    Jim

  • #2
    A couple of things that would help what boat and engine and gear ratio? You need WOT rpms to make an educated decision. Yes with 4 blade generally you want to drop pitch a little as 4 blades will keep the engine from getting as many rpms as a three blade

    Also what is your primary use ? Watersports, cruising, holeshot vs top end

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    • #3
      It's a 79 century 19' open bow deep v hull with a 260 mercruiser (I just rebuilt the engine & it's 350 hp) It's a 1.5:1 gear ratio MC-1 drive.

      The title says the model is a 180 but everything I've found online does not show a 180 being a 19' hull. So I'm not exactly sure what it is yet. I'm still digging through the interweb on that one.

      Not sure on WOT rpm's yet as I didn't have my shop tach with me & I know the tach in the boat is off.

      Plus I have not really ran it wide open yet. This weekend I'm going to see what it does with these two props. I'll take notes as to how it performs with each & note the rpm's & post back.

      We do a little tubing but mostly just cruising the lake. My other boat we usually cruise about 3000 rpm but I'm not sure of the characteristics of this particular boat yet as I haven't ran it much. I bought it as a project with a bad motor & have only had it out a couple times so far mainly to make sure everything was working right & you know, make sure it would even float. Lol. It does by the way.

      I was just curious about the 3 to 4 blade difference so when I figure out what 3 blade works I'll know what I need in a 4 blade. Maybe. Lol.

      One other thing, I had to replace my lower unit cause the case was cracked. I picked up a complete 888 mercruiser, motor & all, anyways, the 888 had the little trim tab anode fin thing on the lower unit & my 260 just has a flat plate with out the fin to adjust the steering torque or whatever.
      Is mine supposed to have the flat one on it? Is the difference perhaps because my 260 has power steering & the 888 did not???

      Thanks.
      Jim.

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      • #4
        Regarding...: Is mine supposed to have the flat one on it? Is the difference perhaps because my 260 has power steering & the 888 did not???

        Yes, trim fin not needed with PS.
        _

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        • #5
          Thank you Maclin.
          I thought perhaps the PS was the difference

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          • #6
            I finally got everything ironed out & got this old girl on the lake for some testing. Took it out Monday late afternoon & had the whole lake to ourselves. Water was as smooth as glass.

            It was my wife & granddaughter & myself. I had the 14 x 17p prop on.

            Leave the no wake zone after that 30 min trip thru the marina & nailed the throttle. She jumped right up on plane & I trimmed her up where she felt good & the GPS said 45mph & the tach read 5000 rpm's.

            So I've got the 14 x 19p prop on her now & gonna get her back on the water probably Friday & see how she does. Should get me close to the 4800 rpm mark.

            Remember that "whole lake to ourselves" thing I mentioned?. Yeah, that's nice, until you run out of fuel in the middle of the lake that nobody else is on.

            Needless to say the wife was not impressed with the boat or my abilities to operate said vessel. Finally got a tow to the dock & had mother in law bring us some fuel so we could idle back to the ramp we launched from & we finally got the boat out 4 hrs after running out of fuel.

            The good side is now I know exactly what the fuel tank to fuel gauge relationship is. When it gets to "E" it's empty. Lol.

            It ended up being a good time after all & I was able to get the info I needed. And the boat ran like a dream. She'll idle nicely across the lake for as long as you want & doesn't get loaded up on fuel & foul the plugs like my outboard does. And then when you hit the throttle she's ready to get with it & get on across the lake. Provided someone stopped by the gas station & put fuel in the tank that is. Lol

            All in all I'm more than pleased with how this project is turning out.

            Big thanks to all of you on here that help us less experienced folk be able to have an enjoyable time on the water with our families in a boat that is dependable because of the knowledge we've gained from this site. My 4yr old granddaughter loves this boat that her & I have been working on. She was right out in the shop helping me build the engine & right in my way trying to put it in the boat, ha ha. She's helped with about every aspect of this project which really means a lot to me as well as her so again I thank each & every one of you!
            Jim

            Comment


            • #7
              Sounds like my old 1970 ford f100,when it hit e for empty the truck died,no leaway atall,lol.

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              • #8
                That's exactly right rude! My wife (being the smart mouth she usually is during times like this) says "well, doesn't the 'E' mean it's empty? What do you expect?" And I guess she's right. It definitely means empty in this boat. Haha.

                When it first happened she says don't these have a reserve like a motorcycle? I said no they don't & she says well don't you think they should?

                She's a funny one.

                Jim.

                Comment


                • #9
                  Hi Big Jim....forgive me for playing devils advocate...
                  Just looking at your specs and the props you are trying, I'm not sure it all makes sense to me.
                  If you engine is 350hp and that size of boat and the 1.5 ratio....I would have thought a 19" would be lightly pitched for that set up.
                  What did you do during the rebuild to get so much more HP ? Would I be right in thinking nearly an extra 100hp ?
                  I'm only asking because I have a similar boat in terms of size, gear ratio and presumed weight (just over 3000lbs) with a slightly modified 5.0mpi (estimated between 260-300hp). My boat is ideally propped with a performance 19" pitch. Can almost get away with a 21". Good on top end, but nothing else...and the 17" revved right out almost instantly and hit the limiter in seconds at about 5100rpm.

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                  • #10
                    Hey guys, not trying to high jack your forum. I have a similar question but can't figure out how to start a new forum? Any help would be appreciated!

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                    • #11
                      QBhoy, as for the engine, bigger cam, fully ported & polished heads, bigger valves, fully balanced, intake ported & gasket matched, complete MSD electronic ignition system. It doesn't take much to get a small block to 350 hp especially when all the components are matched to make power in the same (and appropriate) rpm range.


                      I initially figured it would end up needing an aggressive 19p or a 21p based on buddies boats of similar size & it still might. I suspect the hull is full of waterlogged foam & someone thought it would be a good idea to paint the hull below the water line with what looks to be rolled on epoxy type of paint. It's super thick & rough as a cob & I'm sure I'm losing a bunch performance wise because of those two factors. Plus it does take on some water & the bilge pump needs reworked so it sits lower in the hull & pumps more out then it does now. Still trying to figure out where the water is coming in at but I suspect the transom assembly needs resealed??

                      I haven't weighed it yet but am going to take it to local gravel pit & see about putting it on their scales. I think it's supposed to weigh 2850 lbs dry & I'm sure it's way over that with wet flotation foam.

                      Is there a easy way to check what the actual gear ratio in the drive is? The serial number on the gimbal housing is of the correct range & shows it to be a 1.5:1 ratio but there is no serial number on the outdrive itself & who knows what may have been done to it previously??

                      One of my "winter" projects is to pull the floor & remove the foam & fix any rotted stuff as well as get the hull cleaned up & painted.

                      Jim

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                      • #12
                        Also QBhoy, both the 17p & the 19p props I have are cupped & seem to have quite a bit of cup to them.

                        Wetnwired, there is a button on the top of the forum page that says "start new topic" that you click on to start a new thread.

                        Also, lots of good info in the stickies at the top of the page.

                        Jim

                        Comment


                        • #13
                          Hi Big Jim
                          Sounds like a machine there.
                          I think you must be carrying some weight in the hull. Has it been kept in the water and possibly water logged ?
                          Anyway...
                          As for checking the gear ratio...apart from looking at the stamp on the drive (it may not be visible anymore), there is a way...I'm not able to write it down specifically right now, because I have had a few beers and out with the other half right now.....but I imagine it is something to do with putting the boat in gear, cranking over the engine by hand and counting the revolutions of the prop.
                          I'll look it up and get back to you

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                          • #14
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                            That's what I'm thinking. I'm gonna try to get it weighed soon. I have a feeling it's quite a bit heavier than its supposed to be.

                            It hasn't been kept in the water, but it had been sitting outside in the weather for atleast 6 or 7 years. It had a cover on it but it was pretty well torn up. About all it did was keep the sun off of most of the interior.

                            Yes, it is a strong running motor. it was repowered with a new mercruiser crate motor in 2010 & showed 35 hours on the new hour meter they installed with the new motor but it had water in it & was trashed.

                            As for checking the ratio, that's what I thought about doing. We used to do cars in a similar fashion to get a close idea as to what the gear ratio was.

                            Thanks.
                            Jim

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