Mercruiser 170 question

[Jersey13]

I'm thinking of buying a boat - '86 Wellcraft 192 American cuddy - and i'm trying to figure out how the exhaust works on i/o's. I'm pretty familiar with straight ob's but this seems to be a completely different creature all together. The reason i'm asking is because i couldn't get the prop to bite. I had to keep easing her up to speed, trim, little more throttle, trim, back and forth. When i ran it out of the water on the ears, it seems like a lot of the water shot out between the transom mount and the outdrive (Alpha one). Is this right? Am i just seeing the water thats being sucked up for the heat exchanger being blown out? Just figure maybe this is causing cavitation before the prop and in turn not being able to get anywhere on the water. Help is appreciated in advance.<br /><br />-Jersey

 

[fishinchips]

Re: Mercruiser 170 question

Jersey,<br />I used to own a 86 EBBTIDE with the same Mercruiser 170 Alpha One. The exhaust (and water) vents out two ports (one on each side of the outdrive knuckle) as well as through the prop. I never really had a problem with cavitation with the standard vented props. I had to keep it trimmed all the way down until I got up in speed and then could put the trim all the way up without any cavitation. Make sure your prop is sized right for the rig. We ran a 23 (cupped) on our 18 footer.<br /><br />Fishin'Chips

 

[ODDD1]

Re: Mercruiser 170 question

Jersey, how is the fit between the prop and the gearcase? If someone ran the boat without a thrust hub[big washer] in front of the prop the prop pushes forward and chews the rear of the case up[and the front of the prop]. this extra space allows exhaust gas to leak out around the outside of prop hub and the prop cavitates...having the incorrect thrust hub will do it too.

 

[Jersey13]

Re: Mercruiser 170 question

Thanks for the response fishinchips and ODDD1. I see the two ports on either side of the outdrive knuckle. Thanks. I would start from trimmed all the down and try to work my way up (my bass boat w/150 merc o/b bites great) but this thing just didn't want to grab. I really don't know much about i/o's but i didn't think they were much different when trying to get up on plane then straight o'b's. Now that you explained about the exhaust, and it seems right, i think it has something to do with the prop. The prop that was origanilly on it when i picked it to was pretty banged up so before i took it out on the water, i put a spare on that i found under the seat. And honestly, this is only my second boat so i really don't know too much about props, but i'm wondering like ODDD1 mentioned that maybe it has something to do with the thrust hub. The only thing i did ODDD1 was take the old banged up prop off and put the spare on. They look like much different props (size & pitch) The original has a # 48 832832 A45 21p stamped on it and the spare has a # 48-58424-21 stamped on it. Both are black. Not sure if this helps. Not sure exactly what fit you're trying to explain put both props fit on the splines perfect and i put the boat in gear and tried to turn each prop to make sure niether was spun. both are fine. Is there suppose to be a washer that goes on the spline before the prop slides on? I didn't look to see if there was one but the front of the original prop looked fine. I would call the guy i'm thinking of buying the boat from but he overdosed on drugs two years ago! What a shame at 35. So i'm buing it from his sister (next of kin) that's had it sitting in her driveway for the last two years and she knows nothing about the boat whatsoever. Thanks again for the help guys. As you can see, i'm pretty much lost on this. Thanks for any info you can provide.<br /><br />- Jersey

 

[fishinchips]

Re: Mercruiser 170 question

jersey,<br /><br />The 21 pitch prop sounds right for a 19 foot cuddy. I am pretty sure the numbers you listed are for the proper vented prop. The 48 832832 A45 21p is for sure. There should be one thrust washer ahead of the prop on the shaft. I'm afraid I'm at a loss too.<br /><br />Fishin'Chips

 

[Jersey13]

Re: Mercruiser 170 question

Hey ODDD1 - I went home for lunch and checked out the thrust hub (big washer) you talked about. It was there but i don't think it was the right one for the spare prop. The diameter of the thrust hub was about 3/8 inch smaller (all the way around) than the diameter of the hub on the prop. I checked the washer against the original chewed up prop and it fit right together and the diameters matched. I pulled the thrust hub off of my other boat and it matched the spare prop diameter. Also, the # on the thrust hub i pulled off my other boat and the # on the spare prop matched. Could the smaller thrust hub that i used on the spare prop the other night cause the prop to cavitate so much? Thanks in advance.<br /><br />-Jersey

 

[ODDD1]

Re: Mercruiser 170 question

Well, jersey how was the fit between prop an the case? if the way you ran it had a air gap between prop an case it probably was part of prob......testing will tell...