68 Merc 500 50 HP trouble

[Edgar F Hodges]

While cruising at about 25 Mph, my engine suddenly lost power and went into what I would term "hi-rev", as if i had engaged the clutch on an automobile with the gas pedal down. Leaving the engine running I put it in gear and was able to limp home, unable to go any faster than about 5-10 Mph. After about an hour and a half, the power finally gave out. The engine idle sounds just fine, not running rough at all, idleing beautifully, just no forward motion. I was able to turn the prop by hand out of the water, engine in gear - I did hear a clicking sound, what I assume to be normal when not powered with the prop moving. I have heard of "shearing the cotter pin" but the prop assembly seemed firmly attached. Any thoughts? How much to fix? I love this engine and dont want to have to replace it. Much thanks - Edgar

 

[Tracy Coleman]

Re: 68 Merc 500 50 HP trouble

Edgar<br />Take the prop off and take/send it to a prop shop and have them put a new hub in it. If the prop is beat up, have it repaired at the same time.<br />Backfire

 

[Edgar F Hodges]

Re: 68 Merc 500 50 HP trouble

Backfire<br /><br />you were right on the money re: your assessment of the problem. The hub needs replacement. All the rubber was all melted and gooey. Can you recommened a new prop? The one I have now is pretty beat up - it's brass - two-blade. 1968 merc 500 50 HP. The boat is a 15.5 ft fiberglass (Astro-Glass). I don't have a tach so I can't deterine the WOT. I don't do any water skiing etc., just cruising and fishing. 2,3, or 4 blade? Steel or alumium? Or should i just do as you had previously suggested? <br /><br />Much thanks again - you can email me at bkeso@earthlink.net<br /><br />Edgar

 

[tmcalavy]

Re: 68 Merc 500 50 HP trouble

Edgar,<br />I have a rig much like yours. Its a 72 fiberglass trihull with a mid-70s Merc 500 50 hp. With the right prop (correct diameter and blade pitch) your engine should be wide open somewhere between 4800 and 5500 rpms...I think that's what my Clymer's manual says, pretty close anyway. Now you really don't want to run it that fast all the time, normally about 75 percent of that to get best MPG. Anyway, the prop on my engine was pretty toasted when I got it and the hub was spun. That prop was a 10.25 diameter (distance from blade tip to blade tip) and the pitch was 15 (pitch number is normally stamped between the blades or on the hub part). I bought a replacement, 10.25 by 13 pitch, rebuilt, for about $40 at the local boat shop. Still need to have the original rebuilt, cause I keep it around as an emergency spare. The 10.25 by 13 prop pushes my boat at almost 23-25 mph at wide open throttle, and the Merc seems to settle in and run best at around 20 mph. Haven't checked to see if I'm in the ideal rpm range cause my tach hasn't arrived yet. Both of my props are 3-blade, probably aluminum. Hope this gives you an idea of what kind of prop to look for. I'm still confused on all that's entailed in "propping'" a boat correctly, but I've heard that going down in pitch, say from 15 to 13 like in my case, adds rpm and speed to your top end. Maybe someone else can tell us the net result of changing prop diameter. I'd like to know myself.