Seeking Opinions...

[longshanks]

I have posted a number of times here, regarding the performance of my 2006 four-stroke 50HP Mercury. I just bought the motor as a lease-return, it has around 30 hours on it now, and although there were some major issues with the power head, Mercury warrantied a new power head, which was rebuilt by a local Merc. mechanic. Motor is under warranty until June '08, which is a darn good thing, considering the number of times I've had to have it back to the mechanic so far.

I have now had it on the water about ten times, and I'm simply not getting the performance I think I should get out of it. My latest step was installing a tach, to see where abouts I am sitting with RPMs. I am looking for opinions on what I should do next.

I am spinning a composite prop on the boat, an 11 pitch by 12" diameter, and getting 4500 RPMs at WoT, moving a top speed of 23mph. It takes around 10-20 (or more) seconds to plane, depending on load. My mechanic recommended a lower-pitched prop, so I tried a 12.25 by 9", and it won't come out of the water, spinning only 3800 RPMs. Mechanic has inspected the motor, and feels that all is working correctly.

Now it seems to me that this motor is either set up incorrectly, or there is something wrong with the motor, or it is vastly underpowered for my hull (15'). I'm guessing the weight of the hull alone to be around 1000-1200lbs.

I was hoping for an improvement in performance when I bought this motor. I also have a 2-stroke Mercury 45 jet motor, which I get out of the hole in 6 seconds with, and run around 28mph. Tests that I have seen online for 50 horse 4-stroke motors show top speeds of around 28-30mph, at around 5800 RPM, give or take. I'm nowhere near these figures, so I thought I'd post a couple of shots of my rig, and see if you guys have any thoughts what is wrong?

cheers

longshanks

 

[Silvertip]

Re: Seeking Opinions...

I don't think that engine is supposed to have a 12-inch diameter prop but 11 or 12 pitch sounds about right. I believe that engine is supposed to have a 10 or 10-1/2 inch prop. I also doubt the boat is 1200 pounds (perhaps fully loaded, but not bare). Two inches of diameter is adding a lot of blade area and that engine is simply not set up to swing that big a prop. On a bigfoot gearcase yes, but on a 1.83:1 gearcase.

 

[longshanks]

Re: Seeking Opinions...

Silvertip

Regarding the boat's weight, the builder estimates around 1000lbs hull weight. It is a heavy little sucker, with 3/16" bottom, and stringers, designed for river / OB jet use also. I estimate around 1700lbs total weight, with passengers, motor, and fuel. Even then, I might be a little shy?

I picked my prop by both talking with local boat dealers, and checking what's available online. The majority of recommended props available for this motor, using Mercury's prop selector, lie between 10 3/8 and 12.25 in diameter, and from 9-17" pitch.

I am presently using a pirahna composite prop, and I've tried a 12.25 x 9, a 12x11, and a 10 3/8x13. The 12x11 was the only prop that performed half decent, but I am still 1000-1500 RPMs under the recommended RPMS for a Merc 50 4-stroke.

longshanks

 

[sabastianunf]

Re: Seeking Opinions...

Different brand props have different characteristics - I would do one of three things 1) find the same brand prop as the 12x11 but get it as a 12x9 or 12x7 if they make it. Reducing the pitch will allow the motor to rev higher but as you get closer to the power band for that motor it will perform exponentially better 2) If you like the composite props then I know that some companies make them with an ajustable pitch - try getting one of those and play with it until you get the motor running the proper rpm's. West marine sells propulse composit ajustable props. for your aplication they suggest a 10.4" that is ajustable from 12-16 pitch part #6902. it seems like it would perform a little better since it is a smaller diameter but I still don't think it will get the performance you are looking for. 3) try using an aluminum prop - I have heard mixed opinions about composits - I personally don't like them. Finally I guess if all else fails you could always buy a bigger motor which would definately solve your problem.

 

[longshanks]

Re: Seeking Opinions...

Thanks for your input guys. The props I've tried so far are all by the same manufacturer, including 10 3/8x13, 12x11, and 12 1/4x9.

I need to get my RPMs higher than they presently are, my tach says 4500. Last time I raised my motor higher than it is, it ingested air and overheated briefly. The prop is not cavitating or ventilating at all, just not revving as high as it should.

As it stands now, replacing this (nearly new) motor is not an option. It seems to me that I am missing something here, as the boat functions perfectly well with a 45HP outboard jet. If I can get this 50HP prop motor to function similarly, I'll be content. Running this motor at the low RPMs will not be good for the motor over the long term, so my remaining options are a) diagnosing a problem with the motor, which my mechanic says doesn't exist, or b) finding the right prop.

The composite 12" by 11" pitch prop is the closest that I've come to reaching max RPMs. I know that by reducing pitch, I should be getting higher RPMS, but I've tried a 12x9 by the same manufacturer, and it seems as if the motor just won't turn it (not past 3800RPMs). As Silvertip suggests, maybe by going with a smaller diameter prop, pitched somewhere between 10 and 12, the revs will come up?

 

[sabastianunf]

Re: Seeking Opinions...

Diameter does make some difference but the biggest change will come from a smaller pitch. It seems very odd that the 12x9 reduced your RPM's - either the blades on that prop are cupped and the 12x11 is not. the other possibility is if the 12x11 is vented - or has small holes near the base of the blades. Even though they are by the same manufacturer they may still not be the same design.

 

[Silvertip]

Re: Seeking Opinions...

I think you need to verify the tach. 4500 RPM @23 MPH with a 1.83 gear set produces almost 18% slip according to the prop calculator.

 

[sabastianunf]

Re: Seeking Opinions...

Once again I think silvertip is right about this one - verify the tach is accurate otherwise you will waste more money on the wrong prop.

 

[longshanks]

Re: Seeking Opinions...

edited

 

[longshanks]

Re: Seeking Opinions...

OK guys, time for me to resurrect this post by adding some new information. I seem to be closer to a solution here, but my motor is still not operating as I expect it could.

Based on previous discussions, I raised the motor this weekend. I was able to raise it up two holes, with no apparent water pressure problems, or cavitation. Operating temperature of the motor has not changed from what it was when the motor was run in the lower position; 118-122 degrees. So this is encouraging, since my holeshot improved, and the RPMs came up around 300. Speed did not increase.

I have recently hooked up a Mercury smartcraft guage to the motor, and it tells me that I am getting max RPMs 4800-5000, depending on load. This is exactly 1000 or so less than I should be getting from this motor (5500-6000 WoT RPMs). With this setup, I am still taking around 10 seconds to get on plane, and max speed is about 25mph. I calculate this at around 13% slip, which is more in line with what I'd expect from this hull. This using a 12x11 pirahna composite prop.

So all of this is now making better sense to me, but I am still well below the recommended RPMs, and I feel the motor has considerably more potential. Due to some earlier overheating problems, Mercury warrantied a new powerhead, pistons, and oil pump. A local mechanic rebuilt the entire affair, and one possible solution is that the motor is not properly set up. When I ran the motor before the warranty work (although it overheated), it seemed considerably faster out of the hole, and the computer showed that it over revved (>6150RPMs). This is using the same prop as I am currently using, at the same mounting height.

I am now trying to figure out what the $%&* could be wrong? Perhaps throttle cable? The motor runs beautifully, but it seems to me that it is lugging. I would appreciate your input regarding what things I should be looking at, to bring back the 1000 or so RPMs. The only other possible explanation that I can think of would be that the motor is incorrectly propped, but I can't see that making a difference of ~1000RPMs...

Thanks for your help guys. It has been a long and winding road, getting this motor set up properly!