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Thread: Help with cavitating 20 hp prop on a 12' 6" inflatable

  1. #1

    Default Help with cavitating 20 hp prop on a 12' 6" inflatable

    I have a 12.5 foot mercury quicksilver inflatable boat and a 20 hp honda 4 stroke that I bought used. It seemed like a great deal and the owner was honest when he said that the package did not perform as well as he had hoped and that was why he was selling it. I figured I would have no problem figuring out what the problem was but I am stumped. Basically, whenever the boat gets on step it cups the water out so much the it creates a cavitation and I have to back off on the power. This turns into a series of fast/slow or I am simply stuck ideling along. I have tried over inflating the boat, all sorts of motor adjustments and different load configurations and nothing has helped. I have had people say that a planeing fin might help? I am thinking about cutting the transom down? I have thought about trying to find a lift that would help me lower the motor a little more plus it would help me run a little shallower. Any suggestions, ideas, history with a similar problem?

    Thanks
    Brian

  2. #2
    Member Bullelkklr's Avatar
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    Where does the existing motor "fin" sit in relationship to the bottom of the boat?

    It really sounds like your motor is too high. Is your motor a short shaft or a long shaft - I bet it is a short shaft...

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    Member spoiled one's Avatar
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    Perhaps it is under inflated? My little tender used to do this if the tubes were not pumped up enough or the keel was soft.
    Spending my kids' inheritance with them, one adventure at a time.

  4. #4

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    The motor is a short shaft...I'm actually not sure where it sits in relation to the bottom of the boat. I have not gotten the boat inflated yet this spring. Maybe I'll blow it up and set the motor to see where it lines up.

    Spoiled one- I have really inflated the heck out of it because others have metioned that under inflation could be a problem as well.

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    Moderator stid2677's Avatar
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    Quote Originally Posted by Brian Jackson View Post
    The motor is a short shaft...I'm actually not sure where it sits in relation to the bottom of the boat. I have not gotten the boat inflated yet this spring. Maybe I'll blow it up and set the motor to see where it lines up.

    Spoiled one- I have really inflated the heck out of it because others have metioned that under inflation could be a problem as well.
    My Jet Ranger does the same thing with a shortshaft prop motor. I think you need a long shaft or shorten the transom. I'm going to make a shorter transom to use the motor I have on mine.

    Just one other thing to check, some engines have a key or slip hub in the prop and they can fail and let the prop spin on the shaft when under pressure.

    Steve
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    Member AK Ray's Avatar
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    The cavitation plate on the motor should be about 2 inches below the bottom of the hull. However, on a rubber boat that might not be enough. I have never dealt with an inflatable.

    On craigslist in the boat section there is a guy selling a heavy duty garlicky (sp) swing arm style jack plate that will allow quick height adjustment. However, from my own experience they can also add to a cavitation issue and need some serious thinking to get the right "on step" setting before you bolt it on.

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    Member Akgramps's Avatar
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    I have found the cavitation plate on the motor should be even with the bottom of the boat, I have also found that a aftermarket cav plate will allow you to run the motor a little higher as it will help prevent blow out over the top of the prop.
    There are many to pick from, I have used a perm-a-trim and found I can run the motor a inch or so high w/o issues. Every set up will vary. You will need to find the right combo, experiment and observe whats happening.
    Where does the plate currently line up with the bottom?
    “Nothing worth doing is easy”
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    Member AKwanderer's Avatar
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    Try checking your trim level, and make sure its fairly even, or just slightly tipped up. Too much trim will make it cavitate. Also you might try cranking the throttle slower, as it might be spinning the prop up too fast. Look at the propeller and if it has severe pitting on the inner edge of the blades, it may have had that problem for quite a while.

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    Default Been there with exact problem with exact set up

    You need a long shaft motor.

  10. #10

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    Putting a long shaft motor on a inflatable is no answer, had to do it once since all I had was a long shaft on a borrowed boat, I think it was worse. Inflatables ride higher in the water. The top cavitation plate on most rides just under the bottom of the boat. (used inflatables for 30 years) half of them I had to lower the transom a inch to get that plate a inch below the bottom. To cheap to buy jacking plate but thats the best way to go. Keep enough weight in the back, gas/battery.

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    That's a lot of motor for that boat. You say the motor cavitates. Does the boat porpoise as well? My Quicksilver didn't have near the vee as my older Zodiacs. That would help make it porpoise. The weight of a 4-stroke 20hp may be a factor, too. Try loading some weight in the bow. That may help. Find a 15hp 2-stroke to borrow and see how that works. Better is my guess.

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    I have a 14 foot inflatable and a 20 hp ss 4-stroke Yamaha, and my boat has a 15" transom and I have zero problems with cavitation. My motor weighs 115 lbs and I run it with just me, 200lbs, and 6 gallons of gas, 40 lbs, and no cavitation. My suggestion is to start simple. Measure your transon if it is 20" then you need a Long shaft. If it is 15" then you need a short shaft motor. Based on your description I bet you have a 20" transom, which means all you need to do is cut 5" off. Some inflatables are used as tenders and the transoms are 20" so that a person can use their kicker motor as the tender motor. Look up that model of inflatable and see what the specs are. I bet its a transom issue.

  13. #13
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    Quote Originally Posted by alaskapiranha View Post
    Putting a long shaft motor on a inflatable is no answer, .
    It's not about long shaft motor vs. short shaft motor. Its about the transom. The same amount of motor is in the water if a LS motor is on a LS transom (20") and vice versa for the Short Shaft motor on a 15" transom. You have to match your motor shaft to the transom height.

  14. #14

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    Thanks for all the good info. I'm going to get her on the water next week and will start simple and then continue to try some of the modifications that have been suggested until i get it figured out.

    Brian

  15. #15

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    One thing to check is the flap of rubber that laps over the bottom of the transom and forward onto the bottom of the raft. If that's peeled back even slightly you have instant cavitation. Approved glue from the manufacturer will deal with it easily, but it's a real common fault with inflatables. If your transom and shaft length are right and you're still cavitating, look closer at that transom.

  16. #16
    Member Bullelkklr's Avatar
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    I am able to bring my cav plate 1 inch up from the bottom of the boat on my zodiac successfully. That is right on the edge though...any higher and it cavitates.

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    Quote Originally Posted by Brian Jackson View Post
    I have a 12.5 foot mercury quicksilver inflatable boat and a 20 hp honda 4 stroke that I bought used. It seemed like a great deal and the owner was honest when he said that the package did not perform as well as he had hoped and that was why he was selling it. I figured I would have no problem figuring out what the problem was but I am stumped. Basically, whenever the boat gets on step it cups the water out so much the it creates a cavitation and I have to back off on the power. This turns into a series of fast/slow or I am simply stuck ideling along. I have tried over inflating the boat, all sorts of motor adjustments and different load configurations and nothing has helped. I have had people say that a planeing fin might help? I am thinking about cutting the transom down? I have thought about trying to find a lift that would help me lower the motor a little more plus it would help me run a little shallower. Any suggestions, ideas, history with a similar problem?

    Thanks
    Brian
    Brian, i went through this exact same problem with a 15' avon and 20 hp yamaha. everything you said is the same as what i went through. as soon as i installed a hydrofoil my problems were gone. at first i HATED the thought of drilling holes in my outboard but I am very pleased it completly fixed my problem, I am however bummed i bought the one i did, a searay i belive, they have a much better one at sportsmans warehouse. hope i helped.

  18. #18

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    Just thought that I would add that I was also having the same problem as you, Brian. Fought it all summer and finally fixed it this weekend. I installed a hydrofoil with a sport clip (didn't have to drill holes in my motor) and I also had to shim my motor to tilt it down more. Because my motor was tilted all it could be already, I used washers under the top portion of the plate to shim it to a more desirable angle. At least on my Achilles, it seemed like the transom wasn't at such a steep angle of other boats and I think that contributed to the problem.

  19. #19
    Member pacific23's Avatar
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    Maybe try a 4 blade prop

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