Prop selection

[HershBoat]

Hi, new member here, I have a 1993 20ft bayliner with a 4.3L mercruiser, 4BBL Weber, 1.84 outdrive ratio, nada says weight is 2475 (seems light to me).

I messed up and didnt winterize it last year and of course it cracked the block. (my bad) Anyway, after checking with several shops I decided to buy a remanufactured long block pull the engine and replace it myself. I am all finished and the engine is running great now, except for the fact that I have rpm's to burn at WOT. My tach goes to 6k and first test run out it pegged the tach. The original prop that was on the boat since I have had it is a 14.5X17P, So I got a 14.5X21P to replace it with. here are the numbers with just two people and no gear in the boat.

17P aluminum prop: 4800=32mph, WOT= 6K plus
21P aluminum prop: 4800=36mph, WOT=5400

I am planning on trying a 13-7/8X23P SS prop this weekend to see where that puts me. I am kind of surprised that a 4P change only brought me up 4mph, does that sound right? Also, is there anything else I should be looking at or am I barking up the right tree with the prop?

The new engine runs great, cranks right up and sounds really good. I am assuming that even though the old engine ran fine, it must have not been running right?

Thanks, Ed

 

[Maclin]

Re: Prop selection

Are the actual top speed numbers higher now than they were with the old engine? Or just now you have to hold it down to keep from over-revving?

The reason I ask is that it seems that your tach readings may be off. I am not sure how an engine replacement could have affected the tach, but I think you need to verify the RPM's. I see 30% slip with 1.84 ratio, 4800 rpm and 21 pitch at 36mph, that is a high slip especially for a new prop.

 

[Maclin]

Re: Prop selection

I see 23% slip with the 17pitch at 32mph, in the higher range.

 

[HershBoat]

Re: Prop selection

The speedometer reads about 5 MPH high, the readings I am quoting above are GPS. And before the by the speedometer reading would run about 42MPH with the old engine, as far as the rpm's... I don't know I never really paid attention because it was never that high. But after re-building the engine, I pay a lot more attention to everything. I guess you could say I was just a casual boater before.. that just wanted it to work. As far as the 21P prop I put on, it was a used prop off of craigslist with a fixed hub, where the 17P (original prop) has a removable hub. the 21P prop I bought looks good but maybe it's bad? While rebuilding the engine I noticed on the old engine ALL 6 spark plugs came right out with my fingers (compression loss I'm sure)

I haven't tried a different tach yet, but when it really does sound over revved when I do get anything over 5k. runs really smooth at 4800 though.

 

[Maclin]

Re: Prop selection

I think we need to verify the RPM's first or we will get false calcs.

 

[HershBoat]

Re: Prop selection

I'll do that this weekend, Thanks