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First thing is to slow down a bit, step back, and ask the question "OK, what was worked on or changed since it last ran reasonably?"

Then look at each of those things carefully to make sure a new problem wasn't introduced while fixing an existing problem. New prop can be problematic, hence my advice to make sure every new prop you buy has been put on a prop block to make sure the pitch/rake/cup is as it should be. Two new XXX props out of the box might perform significantly different, but two new XXX props put on prop blocks before use will be almost perfectly identical in performance..

If you don't have any luck there, then start looking for missing horsepower.

If the motor is running smoothly, it is always possible that you have an issue in the spark system. My XR6 once was acting like a 115 and it took a couple of us, including a good merc shop to finally figure out that I had a bad coil that worked most of the time, but at WOT would have problems producing a hot enough spark to fire every time. Motor ran smoothly, but was just weak. With that in mind, first thing I would do is replace plugs on your motor. And use what merc recommends, not what Auto-Zone will try to sell you. You want OEM-type replacements to avoid any problems.

It is hard to imagine it running smooth if the fuel system is broken. It will run lean and not run very well. I'd save fuel-related issues as a last-resort unless something (say the primer bulb) was replaced and possibly put in backward. Non-merc primer bulbs have been known to cause lots of fuel-related issues, from leaking check-valves to stuck check-vavles to restrictions.

One other thing to check is to remove the cowl and make sure every bolt and screw and nut is tight. The switchboxes use the bolts that hold them to the block as a ground. Loose bolts provide a poor ground which can cause issues up to and including melting a piston because the timing can get way out of whack.

I'd bet on either plugs, or something that was changed during the repairs... dropping from 75 to 63 is a _huge_ drop and it is hard to imagine a prop that far out of whack. And motor height is not going to make that big a difference unless the motor is so low it would look obviously wrong.

BTW, that's a good motor, but the 2.5's are not known for torque unless you go the racing division route of much higher compression. Which means that any little glitch can prevent it from getting up to its peak horsepower RPM which is well over 5,000 RPM.

OK, things are maybe a bit simpler. Some math...


I am assuming you are running a 1.75:1 gear ratio lower unit, which is typical for 200hp mercs.

Theoretical top speed with a 25" prop is

speed = 5700 / 1.75 * 25 * 60 / 12 / 5280 => 78 mph at 0% slip

You are running 63, so slip = 15mph = 19%+. So your slip is roughly double what is reasonable. The most likely candidate to cause that is that the motor is _way_ too high, wetting so little blade area that it is slipping badly. Also this will likely kill hole-shot as the prop should ventilate at that height like crazy letting the motor wind way on up in the RPM band.

If the prop really is a 25" pitch, and there is nothing wrong with it in terms of blade area (prop shops can reduce blade area, and also can thin the blades enough that they flex under load and lose pitch) then this has to be a height issue unless you have some serious issue in front of the prop. For example, a gash on the pad that is disrupting the water flow off the pad and onto the gearcase/prop. Or a (somehow) damaged gearcase that is disrupting water flow into the prop blades. I've even seen the old-style speedo pitot that mounts on the transom cause this mounted directly in front of the gearcase, as can a transducer puck mounted there. Assuming nothing was added in that area, check the height. They might have changed holes on you and raised it up higher than normal.

OK... back to the math. theoretical top speed would then be 72.5 with zero slip, which then begs the question of how you ever passed 70 as that would be impossibly low slip. But in any case, 72.5 theoretical with 10% slip gives 65 mph. So now you are not far off. However, 25p sounds odd if you were running 75 before, unless you were running 6500 RPM.

Were you giving GPS speeds or speedo speeds? It is not uncommon for speedo to be 7-12 mph above the true GPS speed. The speedo is simply measuring pressure, which optimally would just be the pressure from the lower unit moving thru water. But it isn't. The boat is displacing water in all directions at high speeds, so the pressure in front of the lower unit is actually higher than the corresponding water speed would produce, so you get high readings that are ok for comparing different props and such, but not OK for comparing different boats.

Is it possible also that you were using speedo/pressure reading, and are now using GPS reading? Smartcraft can do either and they could have changed the setting since the GPS reading is way more accurate.

I don't know what to say, other than 74 is absolutely impossible with a real 25" pitch prop.

Again,

speed = 5700 / 1.86 * 60 * 25 / 12 / 5280 = 72.55 mph at zero slip. Zero slip is not possible, unfortunately, because of the water involved. You can get zero slip in a car except when you spin the wheels. But in a boat, to get zero slip, the hull would have to have zero friction moving through the water.

So, I have no idea how you ran past the theoretical max. I can only think of two possibilities.

(1) your tach is wrong and you were running closer to 6300 than 5700. The stock 2.5's don't like to run that fast, as power drops off quickly after 6,000.

(2) your prop is not a 25p. A 27p would come closer to that top speed, and in fact, I think that the BWB 600 horse stampede article referenced 27p prop for the merc that ran around 75, if memory serves me correctly.

In any case, something is definitely up with the original numbers as either the speed, the rpm, or the pitch has to be wrong.

sure... first you need to convert crankshaft RPM to propshaft RPM. So you divide the crank RPM by the gear case ratio. Which BCB said was 1.86 for your motor. Now you have the number of revolutions per minute the propshaft is turning. But we measure speed by the hour, so multiply that value by 60 and now you have propshaft revolutions per hour. Your prop is a 25p, so multiply 25 by the revolutions per hour and you have inches per hour. I divide that by 12 to convert to feet, then divide by 5280 to convert to miles to end up with miles per hour.

Hope that helps... And note that the above number assumes you go a full 25 inches for each revolution of the prop, which is what would happen at zero slip. 10% is a realistic slip number so after you do the above calculation, reduce the theoretical top speed by 10%. It is very hard to get better than 10%. I've done it with a carefully worked chopper, but add even 100 pounds to the boat and slip would go through the roof. It is easy to get worse than 10% slip with a motor that is way too high, or a load that is too heavy, or a prop with ragged blades, etc.