Nitro Z6 Motor Problem?

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DJRazorback

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I have a 2015 Nitro Z6 with the Mercury Optimax Pro 115hp and a Mercury Marine Laser 2, 48-16992 22P prop with 2 plugs in and one out. It also has a SE Sport 400 Hydrofoil that I added shortly after buying the boat to help it get up on plane.

I have an issue that I haven't heard of before so I'm hoping someone on the forum has run across this and can help. The boat has always run about 40mph at 4,000rpm when properly trimmed out. The last 2 times I took the boat out it came out of the hole just fine, planed out and cruised at the 4,000rpm that I normally run. But after a few minutes the rpm jumped to 4,500 and the speed dropped to around 35mph. The trim was still set the same and the throttle had not been touched. The motor sounds fine (other than being at higher rev's) but the boat is definitely going slower. I pulled into a cove to check the prop and nothing appeared to be wrong. So I started the motor back up and got the boat back on plane trimmed it out and set the throttle at 4,000 rph and was running just fine at approx. 40mph. Then as I turned out of the cove it did it again. The rpm jumped to 4,500 and the boat slowed down. I don't want to speculate but could it be lower unit? Or the prop slipping? Doesn't make sense to me being as it comes out of the hole, planes out and runs fine before the problem appears.

I have a special trip planed for few day in October on Lake Ouachita with my son and it's very rare that our schedules align so the we can both take off from work and family. Need to get this fixed before October. Help!!!
 

birda40

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I'm not a mechanic , but my Tempest had one plug two open when I bought it a few months ago. The cavitation was impossible for me , the guy I bought it from just could feel it and work around it. I called the marina and they sold me two more grommets. It helped my 225 Merc.
 

JBoutfishn

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Sounds like a prob hub issue. I had the same set up with a 2009 Z6. I ran all plugs in drilled out to 1/2 inch. Good hole shot, top end 50 mph on cold day at 5200 rpm.
 

Don Smith

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Jim is correct, I think. Try a new hub kit! I had a similar problem that my mechanic could not solve. So I called the prop company and the gentleman told me to buy the rubber insert for the prop. It only cost about 30 dollars and anything else you try is a lot more than that. Well, he was right and that was that!! No one tells you but you should replace this part every couple of years. It is a safety feature that saves the prop from stripping if it hits something. I hope this is it. Mine looked fine but it was slipping just a little. Let me know if this helped and good luck!
 

DONAOL

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Jim is correct, I think. Try a new hub kit! I had a similar problem that my mechanic could not solve. So I called the prop company and the gentleman told me to buy the rubber insert for the prop. It only cost about 30 dollars and anything else you try is a lot more than that. Well, he was right and that was that!! No one tells you but you should replace this part every couple of years. It is a safety feature that saves the prop from stripping if it hits something. I hope this is it. Mine looked fine but it was slipping just a little. Let me know if this helped and good luck!
This is the first motor I’ve had that the prop has plugs. All my plugs had been taking out before I got the boat. You say the plugs are a safety feature and possibly make the motor run better. Why would they remove them? Now wondering if I should install the missing plugs.
 

Don Smith

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No not the plugs, The whole center of the prop! It has a rubber center. Take the prop off to get to the center part. It is not the plugs!
 

birda40

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DJRazorback

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Hey Ya'll this is all great info. I have ordered 2 hub kits, (one to replace the one in my motor and one for a spare in the boat) as well as 3 sets of different size prop plugs. I'm going to replace the hub first and take the boat out to see how it does. If that fixes it I'll probably just keep the prop plugs incase myself, or one of my buddies, ever needs them. If the hub kit doesn't fix it then I'll start playing around with the plugs. I'm big one doing my online & you-tube research before tackling a new project so hopefully it will all go well and solve my problem. I'll let you guys know how it turns out. Thank you so much for the quick feedback and great advise. I donated $20 to the forum before I started the thread. I'm going to give another $20 when my boat is back in good shape. Nitro & Tracker Owners forum is an awesome resource!!
 

DONAOL

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The grommets are in the outter back of prop, mine was spinning and not moving due to much air. Cavitation, the grommets were a couple bucks apiece. Altho what Don is explaining is also possible a spun hub which is also easy cheap fix. Please let us know how it works out, call your parts guy hopefully in stock

Just learned some new. Thanks for the link.
 

Andrebass

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The rubber hub insert protects the lower unit from doing internal damage and protects the motor from damage. I hit some rocks on my old boat and stripped that rubber insert and was dead on the water since I had no spare.
 

birda40

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Had a buddy hit a trot line, spun it out quick, never know what lurks below.
 

Don Smith

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Thanks Andre, I never thought about putting a spare on my boat! Going to get a spare asap and put in my boat! This is a wonderful site to get great advice. Thanks to all for help.
 

DJRazorback

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I still need help. I checked the hub assembly and even though it looked fine I replaced it anyway since it is 7 years old. Took the boat back out and no help. Ugh!!!!

So, I started thinking, the boat has always cruised 40mph at 4000rpm, the engine runs fine and the hub is good, so what has changed?

I probably should have mentioned this before but never thought it could contribute to this problem. This summer I upgraded to a Terrova trolling motor and a 24 volt system. I took out a 50lb battery at the stern and added two 75lb batteries. To prevent increasing the overall load in the boat I took out a 25lb anchor I no longer needed and about 50lbs of old gear I no longer use. So the net increase in the load is was only about 25lbs. However, the weight I took out was in the middle and closer to the bow. The weight I added is all at the stern about 12" in front of the transom.

My question is, could this shift of weight to the stern cause my prop to over ventilate? If so, would lowering the motor on the transom help?
 

jerry shack

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I still need help. I checked the hub assembly and even though it looked fine I replaced it anyway since it is 7 years old. Took the boat back out and no help. Ugh!!!!

So, I started thinking, the boat has always cruised 40mph at 4000rpm, the engine runs fine and the hub is good, so what has changed?

I probably should have mentioned this before but never thought it could contribute to this problem. This summer I upgraded to a Terrova trolling motor and a 24 volt system. I took out a 50lb battery at the stern and added two 75lb batteries. To prevent increasing the overall load in the boat I took out a 25lb anchor I no longer needed and about 50lbs of old gear I no longer use. So the net increase in the load is was only about 25lbs. However, the weight I took out was in the middle and closer to the bow. The weight I added is all at the stern about 12" in front of the transom.

My question is, could this shift of weight to the stern cause my prop to over ventilate? If so, would lowering the motor on the transom help?

Where is the motor mounted now? You can take a pic and post it if not sure. However you can also check the position pretty easy with a straight board or some thing like that. Put the straight edge flat on the bottom of the hull and on the pad (it's the vey bottom flat part in the center) then measure the gap between the bottom of the cavitation plate (that's where you mounted your hydrofoil) and the straight edge. That's your motor height. You should be about a 1" or so gap. On a conventional hull there would be no gap. I don't think the weight shift is the problem.
 

birda40

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For slitz n giggles throw some weight up front , never know. Couple bags of sand and anchor... never know.
 

jerry shack

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This is the first motor I’ve had that the prop has plugs. All my plugs had been taking out before I got the boat. You say the plugs are a safety feature and possibly make the motor run better. Why would they remove them? Now wondering if I should install the missing plugs.

The plugs are for the hole shot, what they do is put exhaust air around the prop so it ventilates(slips)so the RPM'S go higher into the power range of the motor. Remember if you have a 90 or 225 or what ever that horse power is a max RPM's so you get more power the higher you go in RPM. So if you give it full throttle at the hole shot and the RPM is 2500 you may only be at 1/4 horse power. So if the RPM'S are at 3500 now you get1/2 to 3/4 of your horse power. So the vent plugs on the side of the prop is what regulates the RPM's at hole shot only. Once on plane most of the exhaust goes thru the hub of the prop.
 

birda40

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The plugs are for the hole shot, what they do is put exhaust air around the prop so it ventilates(slips)so the RPM'S go higher into the power range of the motor. Remember if you have a 90 or 225 or what ever that horse power is a max RPM's so you get more power the higher you go in RPM. So if you give it full throttle at the hole shot and the RPM is 2500 you may only be at 1/4 horse power. So if the RPM'S are at 3500 now you get1/2 to 3/4 of your horse power. So the vent plugs on the side of the prop is what regulates the RPM's at hole shot only. Once on plane most of the exhaust goes thru the hub of the prop.
The grommets helped my 225 Merc. trying to get up on plane the prop would just spin from to much air/ bubbles or what ever. With the grommets I can tell the difference when trimmed and on plane.
 

DJRazorback

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I did the best I could with the pics. Not sure which part of the hull should line up with the cavitation plate. The lowest part of the hull in the center of the boat (Bottom of Hull 1 pic) is about 2-1/2" below the cav plate (Hull 1 to Cav Plate pic). The larger/flatter part of the hull is off to the side about 8" (Hull 2 to Cav Plate pic) and best I can tell is about 3/4" below the cav plate (Hull 2 to Cav Plate pic). Does it look like there's room to lower my motor without developing too much drag?
 

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