Prop Sizing 4.3L Mercruiser?

[3 Card]

Bought a 1995 Wellcraft Excel 19SX bowrider last summer (Approx. 2500 lb boat I think)...Engine is the 1995 165hp 4.3L Mercruiser with Alpha 1 drive...Boat came with a 14X19 pitch prop...We used it all last season and it was great for skiing but never goosed the boat b/c it would jump above the 4400-4800 RPM range recommended at WOT. I recently made the great novice move of backing into my deck and dinging up the prop. I bought a Michigan Wheel Vortex exchangeable system with a 14X23 prop. I figured this prop would get me down to the low end of the prop range and be great for early season cruising on the Potomac...Took the boat out Sunday and it is running right against 4800 RPM with 14X23 and hitting about 53mph trimmed out on smooth water. Can this be right or is my tachometer off? If the tach is right I should be able to turn a 14X25 within the RPM range?! I've never even seen a 14X25. Does anyone think a shop tach would be a good investment to check my boat tach accuracy?

 

[QC]

Re: Prop Sizing 4.3L Mercruiser?

There may be a switch on the back of your tach that sets it for 4, 6 or 8 cyl. I would check that first.

 

[3 Card]

Re: Prop Sizing 4.3L Mercruiser?

Quietcat,<br /><br />Thanks for the tip...The Tach is original to the boat but I'm sure the switch could easily have been tripped. I'm on the road the next couple of days but will look at it on Thursday.<br /><br />3 Card

 

[Chris1956]

Re: Prop Sizing 4.3L Mercruiser?

3 Card, My neighbor has a similar boat/outdrive (mid 90s 20.5' Bayliner bowrider w/ 4.3L Alpha) package. He runs a 23" pitch Rapture SS prop. My boat is also similar('88 SeaRay 19 Sorrento Bowrider, 43.LX/Alpha). I can only turn a 21" pitch Rapture SS prop. I think the Alpha gear ratios changed between 1988 and the mid 90s, which explains whi I need the less pitch prop.

 

[tommays]

Re: Prop Sizing 4.3L Mercruiser?

if your tack is correct anything bigger than the 23 is just going to be hard on the motor and will probley slow the boat down to much load<br /><br />4800 rpm is PERFECT and 23 in is a huge prop for a 4.3<br /><br />tommays

 

[Mahoney]

Re: Prop Sizing 4.3L Mercruiser?

tommays has it right on, sounds like you do sking with the boat, but didnt mention if you had anyone else in there during your new run with the 23 prop. With a skier and one or 2 other people, oh and some beers, anything larger will definitely be working the motor really hard.<br /><br />you didn't mention your hole shot with the new prop either, I am guessing that engine is going to have to work a bit harder for the rpms to get a skier up with that pitch.

 

[3 Card]

Re: Prop Sizing 4.3L Mercruiser?

Thanks for the info guys...<br /><br />To answer Mahoneys question I did make the first run with about half fuel (11 gallons or so) and just myself in the boat. The hole shot was still pretty strong but of course I'm comparing that to 19 pitch with 4-5 people and 22 gallons of fuel<br /><br />Sounds like I might want to get a 21 pitch for skiing to maintain some hole shot with more folks in the boat<br /><br />Appreciate the insight...

 

[3 Card]

Re: Prop Sizing 4.3L Mercruiser?

Well I borrowed a shop tach and compared it to the boat tach at idle in gear (on the water) and out of gear (on the trailer w/ muffs). <br /><br />Looks like my boat tach is reading about 200-300 RPMs too high. This makes sense and suggests a 23 prop for cruising (4400 RPM @ WOT) and a 21 prop for skiing (4800 RPM @WOT). Time to head to West Marine for a 21 pitch as skiing season is upon us in Maryland...

 

[garycinn]

Re: Prop Sizing 4.3L Mercruiser?

Consider a 4-blade 20P for skiing.

 

[achris]

Re: Prop Sizing 4.3L Mercruiser?

Originally posted by Chris1956:<br />.... I think the Alpha gear ratios changed between 1988 and the mid 90s, which explains whi I need the less pitch prop.

Correct. The early 4.3l Gen II ratio was 1.65:1. Merc changed it to 1.84:1 because people were putting the engines into boats far bigger then the Merc engineers expected, hence a lowering of the ratio. When the engines went EFI Merc moved back to the 1.62:1 ratio, because those engine produce a lot more torque and can cope in the larger boats. Around the mid 90's Merc made a slight change to the gears in the lower gearbox, and the ratio in that went from 1.65 to 1.62, which is why all the ratios changed.<br /><br />1.5 -> 1.47<br />1.65 -> 1.62<br />1.84 -> 1.81<br />2.0 -> 1.96<br /><br />Hope this makes sense.<br /><br />Chris.........