Prop question on 50 hp elpto Merc

[nuffsaid]

I have a 2008 lowe fm 165 with a 50 hp merc elpto two stroke. I have added a full windshield and tend to run with a full tank of gas (26 gallons) and a moderate amount of gear, two batteries, trolling motor, down riggers and tackle. I usually fish with one or two people. I live at 4200 ft elevation and fish a lot of my favorite lakes at about 7000 ft. Every year I spend a week in northern Saskatchawan fishing at about 1500 ft elevation. My boat came with a 15 pitch prop and it will only run at about 4500 rpms at 4200 ft elevation and about 4200 rpms at 7000 ft. I was very doggy. I have ran several solus props, an 11 pitch with about a 12 inch diameter and last year ran a solus 10 pitch with a 12 7/8 diameter. It worked very well until I hit a submerged rock in northern Canada last year that wiped out my lower unit. When I ordered my new lower unit, I ordered a new mercury 10 pitch prop with about a 12 inch diameter. I works pretty much spot on for my high altitude lakes and will allow my motor to run at about 5200 rpms. I wanted a spare prop for my up coming Canada trip and thought that a smaller diameter prop would give me a little more safety from hitting rocks up north. So far I have wiped a prop on every trip. I bought a Michigan wheel Vortex 11 pitch with a 10 3/8 diameter. Today I took my boat out with a buddy to my favorite 7000 ft lake and found out that when we were done fishing I wanted to run back to the dock and the boat motor would only pull 4300 rpms and wouldn't get the boat up on plane. My max speed was 11 mph. When I took the boat out two weeks ago with just me in the boat, it seemed a little sluggish getting up on plane and would only run 24-25 mph at about 4600 rpms. In the past when I used an 11 pitch prop, I could easily run 25 mph at about 5000 rpms. Is it possible that I got a defective prop? Would going to a smaller diameter hurt my performance? Any other suggestions? Thanks in advance.

 

[Barramundi NQ]

The problem you're describing doesn't sound like a prop problem. Defcective props.....rare as rocking horse poo. If a prop is defective it will be oout of balance., and the vibration will be enormous.
It sounds more like a fuel problem. It could also be a electrical. Check compression first. Check plug condition as you do this. Look for a wet or very dark plug, one that is different from the rest. Check plug gap. Are the carbys opening fully from the cable? Can you open them fully with the butterflies flat in the throat? Sometimes the cable stretches and carbs don't open fully, this is usually a slower process, over a few trips.

 

[nuffsaid]

I will pull the plugs and check them. I don't notice any dead throttle response, but maybe I better check the fuel pump output and the fuel filter. This motor has pretty low hours on it, probably less than 100 hrs, so I don't suspect any compression problems, but I will check it just the same. I will also check to see if I am actually getting wot. Thanks.

 

[nuffsaid]

I will pull the plugs and check them. I don't notice any dead throttle response, but maybe I better check the fuel pump output and the fuel filter. This motor has pretty low hours on it, probably less than 100 hrs, so I don't suspect any compression problems, but I will check it just the same. I will also check to see if I am actually getting wot. Thanks.

 

[nuffsaid]

I pulled the plugs and the top plug was tan on the insulation, the bottom two were a little whiter, but not bad. Compression was even across the board. I checked the throttle and it opened 98%. I am beginning to think that between the altitude and the warm temperatures, I have lost enough hp that the 11 pitch prop won't get the boat up on plane at the high altitude. I might take the boat out later this week to a lower altitude lake and see what rpms it will pull. If that doesn't work, I will just stick with the 10 pitch prop. My main reason to run this 11 pitch is for the smaller diameter to give me a little more clearance over hidden rocks. You might say I am a little gun shy after destroying my lower unit last year. I also have a prop now that is supposed to spin the hub if a hard object is struck to save on breaking parts.

 

[Barramundi NQ]

With low engine hours like yours compression testing is to eliminate that basic problem. With each area checked off as being in good working order, you will get closer to the real source of the problem.
By reducing the size of the diameter of the prop the bite is far less, so it should rev higher. Do the hubs from the old and new prop match in diameter?

 

[Barramundi NQ]

Also do you have a water separator type fuel filter on board?

 

[nuffsaid]

The hubs on the props match. I don't have a water separator/filter at this time. According to calculations, I am losing 21% of power at my high elevation lake. The boat will jump up on plane with just me in it, but it wouldn't do it today with two of us and all our gear. I need to try again with my 10 pitch prop and record the results.

 

[Barramundi NQ]

My boat came with a 15 pitch prop and it will only run at about 4500 rpms at 4200 ft elevation and about 4200 rpms at 7000 ft. I was very doggy. I have ran several solus props, an 11 pitch with about a 12 inch diameter and last year ran a solus 10 pitch with a 12 7/8 diameter. It worked very well until I hit a submerged rock in northern Canada last year that wiped out my lower unit. When I ordered my new lower unit, I ordered a new mercury 10 pitch prop with about a 12 inch diameter. I works pretty much spot on for my high altitude lakes and will allow my motor to run at about 5200 rpms.

So if I'm reading you correctly dropping 5 inches and losing RPM's (or not picking up an average of 500 rpm for each inch of pitch) IMO there is another problem going on here.

 

[steelespike]

The plug color seems too clean.If you are at high altitude it would be running rich which should produce some black mild fouling.
​ The light color seems to indicate a lean condition.Also a "too clean"plug might indicate water infiltration.
Does the motor seem to produce white "smoke" that dissipates quickly?
99.9% of motors have a "safety" hub that slips with a hard strike.Very likely if you damaged your gear case you hit with more that a prop blade(s)
Does the small diameter prop fit the gear case correctly?

 

[nuffsaid]

The prop hubs that I was running were solid molded rubber with no give. When I hit the rock at 4000 rpms, one blade of the prop got torn 1 inch deep. The splines that drive the lower unit sheared off right at the crank shaft. The props I have now have centers in them that are supposed to spin if a hard object is hit. I am going to go out and see if I can check my fuel pump pressure before the filter and after and see if there is a restriction and then take the boat out to a lake that is at 4200 feet and see how it runs.

 

[nuffsaid]

I took the boat out to the lake and the elevation was 4800 ft. I left the 11 pitch prop on and gave it a go. Boat jumped right up on plane and jumped right to 5000 rpms. Playing with the trim, I was able to get 5100 rpms and 26 mph. I filled the live well 1/2 full and lost 1 mph. Right now I am convinced the altitude is robbing too much power to be able to run an 11 pitch prop. The engine is fine. This prop should run great in Saskatchewan at 1300 ft elevation.

 

[nuffsaid]

I also weighed my boat and trailer with most of my gear in it and a full tank of fuel. 2350 lbs, so subtracting about 450 lbs leaves me with around 1900 lbs without people.