Merc 15 hp cavitates

[Bill kubiak]

I have a 12 ft scat cat tunnel hull with a 1998 merc 15 hp 4 stroke on it. I have the motor parellel to the water when sitting still. The height from the top of the transom to the bottom of the tunnel hull is almost 20 inches. I have a 20 inch long shaft motor. As soon as it starts to plane the motor start to cavitate and bang against the hull. The motor is also making a slight popping sound like it is not reving up all the way.<br /><br />What an I doing wrong????????

 

[achris]

Re: Merc 15 hp cavitates

Originally posted by Bill Kubiak:<br /> ...As soon as it starts to plane the motor start to cavitate and bang against the hull.

That's not good!<br />Silly questions time. <br /><br />Are you sure the motor is bolted up properly? <br />The tilt pin is in the right place?<br />Nothing loose under the hull that could be flapping around?<br /><br />Hope this is some help.<br /><br />Chris.............

 

[Bill kubiak]

Re: Merc 15 hp cavitates

This is the question, is a tunnel hull motor tilt setup different from a conventional hull, since there is two hulls making a stern wave, is it possible the turbulence is making the motor cavitate or am I doing something else wrong.<br />Yes the motor is very tight against the transom, The banging comes as the motor bites the water and then cavitates and then bites again and so on.

 

[jfm1]

Re: Merc 15 hp cavitates

are you sure your motors not jumping in and out of gear. like a worn clutch dog or forward gear

 

[Bill kubiak]

Re: Merc 15 hp cavitates

Good question, when we approached the dock with it in gear I cut off the motor and I could hear a clicking sound coming from the motor until we stopped, what could that be????

 

[jfm1]

Re: Merc 15 hp cavitates

You may have a bad clutch dog and foward gear. if it were mine I would tear down and inspect.

 

[Bill kubiak]

Re: Merc 15 hp cavitates

I just went and looked and I can turn the prop by hand with it in forward gear and it clicks as I do so. In reverse I cannot move the prop.<br />is it possible the linkage may be out of adjustment

 

[jfm1]

Re: Merc 15 hp cavitates

that don't sound good. drain your l/u see if you have any metal in the oil. see if you can adjust shift linkage, if not time to inspect l/u

 

[IBFishing]

Re: Merc 15 hp cavitates

Originally posted by Bill Kubiak:<br /> Good question, when we approached the dock with it in gear I cut off the motor and I could hear a clicking sound coming from the motor until we stopped, what could that be????

Originally posted by Bill Kubiak:<br /> I just went and looked and I can turn the prop by hand with it in forward gear and it clicks as I do so. In reverse I cannot move the prop.<br />is it possible the linkage may be out of adjustment

Just a thought - I don't know if this applies to the newer outboards. When I place mine in forward gear I can turn the prop clockwise (standing behind enine) and get clicking, but if I turn it counterclockwise, it's solid. If I try this in reverse, I cannot turn the prop either direction and also no clicking. All of this is normal. If you were to shut the engine off while still going forward and while in forward gear, the force of the water flowing would tend to turn the prop clockwise resulting in this clicking sound.

 

[Bill kubiak]

Re: Merc 15 hp cavitates

I have several other small outboards and I can not move the prop on any of them when it is in forward gear.

 

[jfm1]

Re: Merc 15 hp cavitates

When your motor is in gear and you try to turn the prop, your also turning the flywheel.<br />IF your able to turn the prop, your gears could be slipping. (not fully engaged)

 

[Bill kubiak]

Re: Merc 15 hp cavitates

tomorrow after work I will disengage the shift linkage and see if the props locks when it is fully in forward gear.<br />What can I do if it still turns? Are there special tools involved to pull that mechanism out of the lower unit or can regular shop tools do the job??

 

[Moody Blue]

Re: Merc 15 hp cavitates

Don't go tearing apart that lower unit just yet B-K. What you are describing is normal. No sense creating more work than is really necessary. IBFishing describes what is happening exactly. <br /><br />A picture or two would really help everyone get a better understanding of your setup. Don't understand how the motor can bang against the transom. Can you see this happening or are you guessing ? <br /><br />Have you tried adjusting the trim pin one hole closer to the transom ? Where is the anti-ventilation (not cavitation) plate relative to the bottom of the hull ? Can you hear the engine RPM's changing as this ventilating is happening ?

 

[Bill kubiak]

Re: Merc 15 hp cavitates

It feels like banging when the prop loses it's grip on the water, the motor revs up and then it suddenly grabs the water again.<br />When I look back the motor seems to shake as this happens.<br />It is pouring rain so I did not get a chance to disconnect the shift rod<br />I have a honda motor and an evinrude motor and neither one will allow me to turn the prop when they are in forward gear

 

[IBFishing]

Re: Merc 15 hp cavitates

Originally posted by Bill Kubiak:<br /> I have a honda motor and an evinrude motor and neither one will allow me to turn the prop when they are in forward gear

Honda, Evinrude, and Mecury are all manufactured by different companies, thus different designs. Just because one does a certain thing, doesn't mean the other will also.<br /><br />When it is in forward gear and when you turn the prop and hear the clicking, is it fairly easy to turn? If it was the clutch dogs slipping, I would assume it would have quite a bit of tension to it when you actually got it to turn, slip, and click. I assume this because you are able to get the boat to plane, thus being able to put torqe on the prop. Also, if it was the clutch dog, I would also assume that it would also slip and click in reverse gear. When you turn the prop and hear the clicking, which direction are you turning it? When in forward gear, does it click when trying to turn in both directions, or is it solid one direction and freely turn/click in the other direction?<br /><br />To me, this sounds normal.<br /><br />Just my .02<br /><br />As a side note, I'm not saying that you don't have a slipping clutch dog/gears, under a load, but the mercury's I'm familiar with freely turn and click when turned clockwise in forward gear. To me it sounds like you are ventilating when you get on a plane, and with it catching and slipping, the motor is vibrating.

 

[Bill kubiak]

Re: Merc 15 hp cavitates

it turns very easy by hand with the engine in gear. Facing rhe prop from behind it turns to the right, the direction the prop turns normally going forward. It will not turn to the left at all.<br />It will not turn in either direction with the shift in reverse

 

[IBFishing]

Re: Merc 15 hp cavitates

That is normal for mercs.<br /><br />I think the only thing wrong is the setup on your boat. You need to find the solution to it ventilating. Hopefully someone with experience on the set-up on your type of boat will reply. If at all possible, post a picture of your set-up. Could be a prop issue, but the engine is probably sitting a little too high. Sometimes a good prop will allow you to run your engine at a higher setting. Where is your anti-ventilation plate relative to the bottom of the hull (above it, even with it, or below it)? From my understanding on this, it should be even with the bottom of the hull or slightly above.

 

[Bill kubiak]

Re: Merc 15 hp cavitates

how do I include a picture with my reply

 

[IBFishing]

Re: Merc 15 hp cavitates

You will need some place that will host a picture for you. You may try this site:<br /> <br /> http://photobucket.com/ <br /><br />I haven't used them, but it looks like it is free. After you get your picture hosted, come back here and post a reply. When you use the "POST A REPLY" button, you will see a section below the "Add Reply" and "Preview Post" buttons that says "Instant UBB Code". Click the "IMAGE" button in this section and a dialog box will appear where you type the full URL of your image. Then it will place it in the text box after you click "OK". Then just continue with the post as normally.