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Thread: Suggestions for a cavatation plate

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    Member Akgramps's Avatar
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    Default Suggestions for a cavatation plate

    Finally got around to trying out the prop on my 20 foot River boat.
    powered with 140 Johnson 4S.
    Anyway, I seem to be having some cavatation/ ventilation issues, its not real bad, but I think a cavatation plate would help, I see several to pic from, permatrim, sport marine, doel fin, cobra edge.

    Any recomendations or suggestions????? Thanks, John
    “Nothing worth doing is easy”
    TR

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    Have a SeSport on my zuk 140 4s.
    It is one of the older ones where you have to drill holes in the plate.
    New ones you have a choice of a clip so you dont have to.

    Sting rays aren't bad either.
    Same thing on no drilling.

    Have doel fins on a 85hp zuk that have never broken with lots of chewing rocks.

    Have you tried lowering you motor down a little?
    Might help with cavitation.

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    Its on a lift, so I can lower it down, and it does settle down, but when iI do it looks as if there is quite a boil going on in front of and above the prop?

    When I raise it up the water settles down but ventilates now and then.

    I have never run a prop before, but from those that I have talked to, it sounds like The cav plate on the lower unit should line up with the top of the tunnel or even slightly higher.

    I dont know that I can tell the diffrence between ventilation and cavatation? but is seems like it is ventilating as I can hear it more so then see it on the tach.

    The boat ahas a jet tunnel and while I like the advatages of tunnels, they seem to create a lot of turbulence that would not be there with just a V or D-pad at the transom. ???
    “Nothing worth doing is easy”
    TR

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    Quote Originally Posted by Akgramps View Post
    Its on a lift, so I can lower it down, and it does settle down, but when iI do it looks as if there is quite a boil going on in front of and above the prop?

    When I raise it up the water settles down but ventilates now and then.

    I have never run a prop before, but from those that I have talked to, it sounds like The cav plate on the lower unit should line up with the top of the tunnel or even slightly higher.

    I dont know that I can tell the diffrence between ventilation and cavatation? but is seems like it is ventilating as I can hear it more so then see it on the tach.

    The boat ahas a jet tunnel and while I like the advatages of tunnels, they seem to create a lot of turbulence that would not be there with just a V or D-pad at the transom. ???
    You need to lower the cavitation plate lower. If you can get it even with he bottom of the boat, not the tunnel, it will be better. The water traveling thru the tunnel like you have noticed is very turbulent and may actually be traving in an upward direction causing a little bit of prop slip. The addition of the fin may actually make it worse. Those fins help to raise the back of the boat/help to plane quicker. Not a huge mod but may not solve the problem.

    Running both prop and jet will take some experimenting to find the right combo, it's not always a straight plug and play deal. I have worked a couple of different boats and the simplest one to rig to date was TBloomA's with the Bay Kit basically adding 5 inches to the shaft. For a long time lifts did not provide enough travel to run both or the setback didn't work well for the jet unit. Trying adding some/most of your weight to the back of the boat before making any adjustments. This will make the back run deeper in the water. If it solves the problem the engine needs to go down.

    Ventilating you will see the paint starting to get eaten away on your blades most times towards the hub from air. Cavitation is where you no longer get forward thrust from the prop, kind of like spinning a tire on ice all of a sudden.

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    Dupont,
    Thanks for the info, at least I know what to look for on the prop.
    The lift has over 7" of travel so there is no problem with adjustment.
    So far I have experimented with for about a 1-1/2 hours, I do ntice some increase in speed as I raise it up, also less probs under full throttle.
    I just expected the flow at the cav plate to be a little cleaner?

    I am really pleased with the performance w/ jet, I just need to fine-tune the prop arrangment.

    I picked up about a 5-7 MPH increase w/ the prop, however that is running at 6300 and the jet is maxed at 5400, so the improvement is not as great as I expected.

    I was light, but maybe I am under proped? I was using a 3 blade aluminum 14 X 19, maybe need to try a 14 X 21?
    “Nothing worth doing is easy”
    TR

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    Top rpm is 6200 for the 140 so you should be able to pull the 21 with you getting 6300 outa the 19. But with heavier loads have to go back to the 19 prop.

    Do you have weight up front?

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    No load in the front, but it does not ride bow high.
    It is a windshield forward type boat, and the fuel tanks are long so the weight is spread out fairly well.
    Two people and some 70 gallons of fuel, I would call it a light load.
    “Nothing worth doing is easy”
    TR

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    Quote Originally Posted by Akgramps View Post
    No load in the front, but it does not ride bow high.
    It is a windshield forward type boat, and the fuel tanks are long so the weight is spread out fairly well.
    Two people and some 70 gallons of fuel, I would call it a light load.
    Alot of that is over center. Try a 160 quart cooler full of water to the back and see what happens.

    Try dropping to halfway on the tunnel...if still an issue even it up with the boat bottom.

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    my understanding (and i'm no expert by any means) is there is a different configuration for tunnel size for use with prop (taller tunnel) compared to tunnel for prop. If so you may not have the benefit of the tunnel when running prop on tunneled boat designed for a jet.
    with my old ocean boat we went thru 3-4 outdrives before we found out the builder had mounted the outdrive wrong , cavitation plate was 1" above keel, after adding 2" extention problem was solved. it should have been 1-2 " below the keel. I realize I'm not talking apples to apples here but would think some of the same concepts apply.
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    I do have a jet tunnel as that is my primary use, I ended up buying a Permatrim plate. and it works great. With the lift, once on step I can raise the motor quite high w/o cavatation, you can feel it speed up as I raise the motor, also see the speed increase on the GPS, not a lot but I do gain a mile or two w/o touching the throttle.

    The more throttle I give it, the more I can raise the motor, the lift sure makes for a lot of control on positioning the prop or jet. the cool part is the setup works well for both and is not a compromise.

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    Quote Originally Posted by Akgramps View Post
    I do have a jet tunnel as that is my primary use, I ended up buying a Permatrim plate. and it works great. With the lift, once on step I can raise the motor quite high w/o cavatation, you can feel it speed up as I raise the motor, also see the speed increase on the GPS, not a lot but I do gain a mile or two w/o touching the throttle.

    The more throttle I give it, the more I can raise the motor, the lift sure makes for a lot of control on positioning the prop or jet. the cool part is the setup works well for both and is not a compromise.
    I wish I could have found one of those a few years back as I would have not passed up a deal on a Grayling boat. This thing was a rocket with the prop on it and it would haul a butt ton plus, but the change over required an engine hoist to get the motor to it's corret location to be able to run jet. We had spent alot of money on different lifts, plates and other ideas folks had to try and get it to run both with never finding the 100% solution other then building a removeable transom adapter. We needed right at about 7 inches of travel............

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    Default Hey there gramps

    I think you mentioned getting 6300 out of the prop and were looking at a 21 pitch. Was that with the bow trimmed correctly and the motor lowered all the way, or was that with the motor lifted?
    I know on my set up that the difference in top end RPM is significant between motor lowered and motor lifted. I am running a 150 V-Max on my 22' boat and using a 19 pitch prop. Hole shot with heavy loads is still pretty good, yet when I get on step and trim, my RPM still goes way up due to lack of resistance when I raise the motor. I don't think I'd go to a 21 pitch on my rig due to the possible decrease in hole shot with loads. As I recall, I was also told by the dealer to check the rpm/pitch with the unit lowered all the way.
    I also run a hull extension plate for both the jet and prop due to the setback from the CMC lift. I do not get cavitation with this set up. I can raise the motor too high though, and get an increase in RPM and reduction in speed, but don't get cavitation per se.
    I've never run a 4 stroke, but my understanding is that when comparing hole shot on the same sized HP that the 2 stroke does better. I wonder how that translates to different pitches for the same type of set ups?

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    Quote Originally Posted by Ak River Rat View Post
    I think you mentioned getting 6300 out of the prop and were looking at a 21 pitch. Was that with the bow trimmed correctly and the motor lowered all the way, or was that with the motor lifted?
    I know on my set up that the difference in top end RPM is significant between motor lowered and motor lifted. I am running a 150 V-Max on my 22' boat and using a 19 pitch prop. Hole shot with heavy loads is still pretty good, yet when I get on step and trim, my RPM still goes way up due to lack of resistance when I raise the motor. I don't think I'd go to a 21 pitch on my rig due to the possible decrease in hole shot with loads. As I recall, I was also told by the dealer to check the rpm/pitch with the unit lowered all the way.
    I also run a hull extension plate for both the jet and prop due to the setback from the CMC lift. I do not get cavitation with this set up. I can raise the motor too high though, and get an increase in RPM and reduction in speed, but don't get cavitation per se.
    I've never run a 4 stroke, but my understanding is that when comparing hole shot on the same sized HP that the 2 stroke does better. I wonder how that translates to different pitches for the same type of set ups?
    AKRR, I was not able to run the 3 bladed 21, too much pitch. I think max RPM w/the 21 was 5700? I would have to look at my notes to verify, I know it was under 6000.
    With the 4 blade adjustable at 17 I could see 6300 before the rev limiter kicked in, I changed it to a 18 pitch and that seems about right for my boat, seems right even w/ a decent load.

    I always trimmed the motor to best speed, The more throttle, the higher I could raise the motor, usally would watch the tach and GPS, but can feel it when its right. On my setup the rpm would come up when properly trimmed, but I wouldnt call it significant, maybe 100 rpm, maybe thats significant?

    The hole shot is so much better with a prop over the jet, I think it would be worth trying a 21 pitch if your rpm is high w/ 19. You may find what you really want is a 20.............I think on my motor that would have been ideal, not many choices for aluminum 3 blade 20 pitch prop. Thats the neat thing about the pro-pulse. You can adjust in 1" pitch increments.

    When I switched from 17 to 18, it lowered the RPM by 200-300 RPM. I noted the GPH on the fuel gauge and it lowered the fuel burn by 1/2 GPH or more at the same speed. My data is over a relatively short time, I tried to record as much as I can, but it can take some time to really get it dialed in. Hope this was helpful..........John

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    Default pitch questions

    AKG, when you gave the RPM numbers, what position was the motor? Again, my understanding is the motor should be all the way down, bow trimmed for normal cruise, then check top end RPM once you are on step and moving. Is that your understanding, and is that what you did?
    What is your motor supposed to hit at top RPM? I was comparing the numbers you gave to my 150 2S, so to me you seemed to be pulling too many RPM.
    When I said my motor did not cavitate, I should have said that at properly adjusted height, it does not do so. I can pull it up too high, and it starts to suck air. At that point, RPM's go through the roof, but then that is not what we are looking for.
    Both the 19 and 17 pitch I have are 3 blade. I've never been around anybody using a 4 blade. Interesting thought though.
    The 19 is a great light boat cruise prop and still, with the guts that motor has, will pull 3,000 lbs of my fat butt and gear up from a stand still pretty quickly. The 17 does not have the cruise, but hole shot is awesome with the loads. I did not try the 21 as I did some load testing and the 19 is on the edge, so more pitch would not help get me up, even though it might help my cruise. Oh yeah, top end RPM with the 19p, heavy load, motor down, bow trimmed to plane correctly, is right about 5200 or so. It's been a while since I ran it like that hard.

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    Those numbers are with the motor as high as I can raise it with the vertical lift once on step, conditions can vary whether or not I can get on step with the motor in that posistion, if smooth water and the boat is heavy than most likely yes, if choppy and light then I may have to lower the motor some to get up w/o and cavitation.

    Usally it will only cavitate very briefly once or twice while coming up, If so I usally lower the motor ASAP, most of this is from running around in Homer where I have plenty of room to get up and dont need to give it WOT, usally, what I would do, is push the throttle to 4600-4800 and it would be up fast enough.

    I do not think the motor needs to be all the way down? I think it should be up as far as you can run it w/o it cavitating, more lower unit in the water=more drag.

    If I give it WOT it will get up quick and sometimes I will need to lower the motor to keep it from cavitating. To find the right prop I was only concerned w/ WOT, I figured I would use a prop that got me just under red line at WOT.

    As far as the trim goes, I have it trimmed out slightly, on my gauge its about 34%, sounds like alot but its not quite vertical, and that percentage may mean nothing to you unless you have an identical setup. My boat does not seem to benefit very much from a "bow-up" attitude, it seems to prefer a pretty flat trim, (its the hull shape).
    I have experimented with the trim and watched the GPS looking for that increase, and it will make a small imrovement but will defintely cavitate out of the hole if I dont trim it back, maybe this is normal?

    The powerband for my motor is 5600-6200, I have seen 6350 before it bumped the rev-limiter, this was brief and I throttled back, this was with the 3 blade 19, I could not run the 3 blade 21? With a big load the 19 might be ok?

    Generally speaking I do not run very long at WOT, for prop testing and "just to see what she will do". What is the powerband or max RPM for your Yamaha? 5200 sounds low to me, but I am not up to speed on 2 strokes anymore????........John

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    Default Good Morning!

    You're going to make me put that boat in the water, aren't you?
    I said 5200 because it was conservative, and I could not remember off the top of my head. Top end is more likely supposed to be 5300 or maybe as high as 5400. Now my curiosity is going. I'll have to look in the manual or call Randy at the boat shop.
    If my boat is light, I can get it up on step with the motor raised depending on where people or weight is in the boat. Still, normally I lower the prop to get a better bite. And light I still may trim for bow down, which moves the lower unit closer to the boat and deeper yet.
    As I can lift the motor high enough to spin the prop, all my top RPM testing was done with the bow trimmed for cruise, motor lowered. That gives me maximum bite, and the most resistance. Keep in mind that cruising to and from moose camp I am heavy. Around moose camp, in the off chance I run the boat, the 19 still does everything I want it to. If my old mind has not failed me, I think I can run the boat with 2 or 3 guys and light gear at around 3900 rpm or so. Most of my local running is with the 4-blade stainless impeller in my jet unit, which puts the top RPM at around 5200 to 5300.
    Now you have me thinking about what a whale tail or something could do for me. I wonder if I could pull the motor up another inch or 2, reduce drag, maintain thrust, and increase speed. Hmmm. Darn it, now I just have to put it in the water and go play, I mean test it some more.

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