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I have a 1998 Mercury Optimax 135 on a 1995 Outrage 19. This engine had a replaced power head under the extended warranty and has 350 hours. I assume the lower unit is original.

After putting the boat in the water after winter storage and typical winterization the boat lacked power/thrust. The symptoms were that the engine sounded OK, but lacked typical thrust at RPM above 3500 to get boat on a plane. This was worst with more weight. The boat could get on a plane with one or two people, but not 3 and required higher RPM that normal. This was slightly non-consistent.

First thought was slipping prop. This was not the cause. Brought it into shop, they replaced plugs. Engine ran better, but did not fix the lack of thrust problem. It seems like the engine is running fine. No problem hitting high RPM, sounds fine and no alarms. It seems like something "slips in the transmission at 3500 RPM. Any ideas?

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Your outboard motor does not have a transmission.

If there is a lack of thrust, then the motor is not running fine.

If there is no problem hitting high RPM, then please tell us how fast you are going at "high RPM" and what you mean by "high RPM".

What propeller are you using?

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Check your fuel system, I had similar lack of power issues when I first dropped my optimax in the water, engine power would drop off at about 3000 rpm, fortunately I had two fuel lines from a twoway valve for a kicker engine that is not on the boat. I switched them while on the water and problem solved. The main fuel line must have developed a crack or something or a blockage.

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Thanks for the responses. This is my first post, so sorry I missed some details.

By the "engine running fine" I was trying to say that everything sounded fine in the powerhead, no vibrations and the shop I took it to replaced the plugs and did a sea test. I also felt like it is running fine in the sense that I can get to high RPMs (5000) with no problem and no alarms. Again it seems like something doesn't sound right at around 3500 RPM, but it doesn't sound like a hesitation in the power head.

These numbers are not exact, but let's say last year the boat would plane at 3800-4000 RPM, now it takes 4500-5000 RPM to plane. After reaching plane I could throttle back to 3200, now 4000. Top speed is also reduced by about 10 mph at same RPM.

Prop is a Mirage Plus 17P. The prop is dinged up and need to be repaired or replaced, but was not damaged between the end of last season when the boat ran fine and the beginning of this season. I took off the prop and checked the Flo Torq Hub and it looked fine.

I had thought this was fuel related and expressed that to the shop I took it to, but I don't think they really listened to the issues I told them about. I think they just went for the easiest fix, spark plugs, which were likely needed, but didn't solve the real problem. Part of my frustration is with the shop since they also did a sea trial after replacing the plugs and should have noticed the lack of power.

I did also have the shop replace the fuel line at the same time, so it shouldn't be that.

Thanks again for responding.

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Have you added anything to the hull, like a transducer or speed paddle wheel? Have you examined the bottom of the hull for any changes? Did you change the mounting of the motor? Sounds like something a Mako I had many years ago did and I discovered I had rubbed the antifouling paint from the keel just in front of the engine and had a small amount of growth there. Just an area the size of my hand of marine growth prevented the boat from getting on plane.

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I haven't made any changes to the hull that should have a negative effect. I did bottom paint the hull. I removed on old paddle wheel and old transducer and added a new transducer to the transom. I will take a closer look near the engine as you suggest, but I should have noticed anything while I was painting it. The motor mounting has not been changed.

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Was it bottom painted before, and you just re-coated it? Or was this the first time bottom painted at all? If the latter, that will add some drag, but this situation seems extreme.

Basically, if you have a lot lower speed at the same RPM you had last year, there are 3 possibilities:

1) additional drag
2) spun hub making the prop slip on the shaft
3 damaged prop that is keeping it from pushing water like it should.

At a guess, I'd say you're looking at a combination of 1 and 3. Maybe with smooth bottom your damaged prop was OK, but not now. I always like to fix what's obviously broken (in this case, your dinged-up prop) before looking for other explanations, so my recommendation would be to replace your prop and see where you stand.

Tim

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Where, exactly, on the transom is this new transducer?

What transducer is it?

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If the bottom paint is smooth without any lumps then it should have no, or very very minor effect on the performance. The problem with my old Mako was that the growth on the bottom was aerating the water just in front of the prop. If your mechanic would let you borrow a prop you could at least eliminate that.

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If you gave replaced the Flo-Torq II hub then it is not very likely the hub is spinning.

If the propeller has dinged edges, it needs to be repaired. NEVER run a damaged propeller. It is amazing how little damage it takes to degrade a boat's performance.

A dinged up prop could well be causing ventilation and/or cavitation, which would allow the prop to slip.

If a new transducer has been recently installed, and it is too close to the centerline, that can also disturb the water flow to the motor and cause ventilation.

A Mercury MIRAGEplus is a great propeller for that boat and it may, or may not, have the PVS vent holes. If it does, be sure the solid vent plugs are in place and none are missing.

There is no need to replace the prop is it is dinged; it can be easily repaired by a qualified prop shop, and should be before the boat is run again.

Also remember: NEVER run an outboard at more than 2000 RPM in neutral. To do so risks damaging the motor and with no load on the motor in neutral, there is no point in doing so anyway.