1976 Merc 1500 - Prop Replacement

[Kargh]

1. Performance issue you are trying to correct.
   • None specific. Just looking for the best prop for the boat. Current prop is pretty gnarly - old and chewed up. Needs to be replaced.
2. Current prop manufacturer, model, aluminum or stainless as a minimum.
   • ​​Mercury (48-65588-19), aluminum
3. Current prop diameter and pitch (required).
   • ​​I believe the diameter is 13", pitch is 19. I did not find any specific specs for this model number but I am not surprised.
4. Wide open throttle RPM and speed with an average load (very helpful)
   • ​​Max 5800
5. Engine/drive make, model, year, and HP
   • ​​1976/1977 Mercury 1500 (150 HP)
6. Boat make model, year, length and weight
   • ​​1977 Hydra Sport 178 (I believe), 18', 750-900 lbs. w/o motor and gear. I am unable to find a verifiable answer on boat weight anywhere except from iBoats. With people, motor, gear, gas, etc. boat probably weighs around 2000 lbs fully loaded.

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I am leaning towards a stainless steel prop but even with all the researching I've done, I still feel like I know just about nothing when it comes to picking the right one. I've read that I should drop down in pitch when going to SS but at this point, I don't even know if the prop that was on it was the right one. I bought the boat used and the previous owner didn't show the boat any care. Any suggestions, advice, experiences, etc would be greatly appreciated. The boat is used for fishing on inland lakes. If I can answer any other questions, I'll do my best.

Thanks!

 

[jimmbo]

What WOT rpm are you getting with the current prop? The range for that engine is 4800 - 5800. That engine does make its best power at the top end of that range.

 

[Kargh]

What WOT rpm are you getting with the current prop? The range for that engine is 4800 - 5800. That engine does make its best power at the top end of that range.

To answer that question might be a bit misleading but I'll give you the information I know. When I ran the boat the other day, the RMPs showed around 4800-5000. I distinctly remember that it was just under 5k when I looked -- BUT the prop that was on the boat was not new and though it was in decent condition it was still dinged up a bit. I do not know how much it might differed in size, weight, strength, etc. from a new one.

The boat was very loaded that day -- likely close to the max that I'll ever have in it, and we were heading directly into the wind on a fairly choppy day for the lake (10-15mph sustained winds).

 

[Chris1956]

Do you have power trim? An aluminum, 3 blade turning point Hustler will work very well on your hull. It will have better performance than all but the highest performance SS props, which will cost $500 or so. You need to work out the correct pitch.

Usually chewed up props have less pitch than new ones of the same type. So if you only hit 5000 RPM with your 19 P prop, when trimmed to maximum, you likely need to drop to a 17P prop. A 17P prop should give you 5400RPM, plus some other RPM gain due to being in a better power band.

 

[Kargh]

Sweet! Thanks for the suggestion on Turning Point. I checked out the reviews and information and was impressed by how many people rave about their props. I ordered 2, one 17P and one 19P and now I'm anxiously waiting until I can get them in the water and try them out. It'll be good to know how each performs when new.

I've never played with trim a whole lot when cruising to see if there is any difference. I usually set it all the way down and forget it until I hit some shallow water. I only get out a few times a year and most of my time is spent with my line in the water rather than moving. Hopefully I'm able to get out more often this year now that the boat is in much better working shape than years past.

 

[Chris1956]

Kargh, The next time out start with the trim all the way down. Put the boat on plane and note the speed and how the steering feels. Now trim it a bit at a time until she steers lightly. The speed should increase as well. Now trim it more and the steering should get heavy and the speed should increase more.

The correct trim is when you get good speed and light handling. All the terms I have used are relative, and based upon the design of your hull, motor mounting height, boat trim and weight distribution. The amount to trim is based in part upon the speed of the boat. More speed allows more trim.

Best of luck.

 

[Kargh]

Had the boat out fishing yesterday. Boat was fully loaded with people, gear, gas, livewell, etc. 13.25" 17P prop. At WOT, rpms hit ~4600-4700. I assume I need to step down in pitch? Of course, this is also assuming the motor is running in perfect condition and everything is set as it should be (motor runs well, sounds good, seems to be in good working order but I have no experience with outboards, much less one this old and finicky). Hit 43 MPH after adjusting trim -- 42 mph with it all the way down.

 

[Chris1956]

Those Merc 1500's were a bit finicky. I would retry the prop with 2 people and minimal gear and see what she will turn. If she revs up well, then the motor is likely running properly. You then need to decide what your normal "fishing" load will be and prop accordingly.

Your spark plugs are all the same shade of dirty, right? If one is clean (steam cleaned), you could have some water infiltration.

 

[jimmbo]

Your numbers of rpm and mph aren't quite lining up for a 17 inch prop. Are you mph from GPS? Also is your tach accurate?