Hey ya'll, i recently had my '86 21' Checkmate Enchanter converted from origional thru-prop hub exhuast over to straight thru-hull exhuast and also added a set of performance GLM aluminum manifolds as well. The engine is a MerCruiser 350/260hp sb chevy with an alpha one outdrive. After all was said and done i've lost about 300 rpms from last season according to my tac. I was turning 4300 rpms before the exhuast conversion, now i'm only getting barley 4000. I have a GPS unit in my boat and at 4000 it says i'm doin almost 60mph, does that seem right?? I've always used a 23 pitch prop, however i just put on a new 23p prop that is a little bigger in diameter than my old prop, would that make that big of a difference?? With the exhuast mods i would think i could turn a slightly bigger dia prop right? Where do you think my revs went? I've heard also that my engine my not be properly tuned for the straight thru-hull exhuast and that i would have to completly re-jet and re-tune the carb due to all the added breathing potentional of the new exhaust, would that be why i lost 300 rpms? Just to give you an idea, i checked my rpm's at different speeds, at about 2500 i was running around 34 mph, the shop that worked on my boat said that seemed a little fast for the rpms and that my tac my be off, what do you guy's think? If i could turn 23p before why can't i now dispite the bigger dia.? The book on my motor says my WOT rpm range is 4200-4600, i may try my old prop and see where that get's me. I also added a new K&N marine flame arrestor to to help the motor breath as well, i'm just plain stumped as to why i lost horespower! Any suggestions or ideas would help a ton, thanks!
Try the old prop so you can test one thing at a time. The exhaust should help a little and yes the larger diameter prop can hold back the rpm's. I need more info about the props. Are they the same mfg? Different mfg's use different blade designs and there may be more than just a diameter difference. You also didn't say how much larger the diameter was.
I would have suggested more pitch instead of diameter if more prop was needed.
Try the old prop so you can test one thing at a time. The exhaust should help a little and yes the larger diameter prop can hold back the rpm's. I need more info about the props. Are they the same mfg? Different mfg's use different blade designs and there may be more than just a diameter difference. You also didn't say how much larger the diameter was.
I would have suggested more pitch instead of diameter if more prop was needed.
Thanks for the reply BDMar. My new prop is a Turning Point Express 23p SS with a 14 1/4" Dia. I have no idea what brand my old prop is or the dia, all it has is 23p stamped on it, i'll have to look up inside the prop ans see if there is any brand on it. That prop cleared the trim tab on the outdrive by quite a bit, we had to remove the trim tab for the new Express to fit.
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