Proper Prop Specs?
 

I Have A 1996 Mariah Shabah 18' Open Bow With A 4.3lx Mercruise Alpha One I/o.
Current Prop Is A 14.5 X 17 Pitch. Which Yields 56-58 Hundred Rpm At Top End.
Any Suggestions On Proper Pitch Prop For The Above.
Ty All Vm
Steve

5800 rpm for the 4.3V6 is way too high. It should run between 4400 - 4800 rpm. I'd almost suspect the cylinder setting on the back of your tach is wrong though so you might want to check that first. I use a 14.25 x 21 - 3 blade aluminum on mine but am thinking of dropping to a 19 pitch prop or a 20 pitch - 4 blade for a little more bite. I think 180diablo (Ryan) uses a 19 pitch prop on his 18'.

its in my sig :)
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However I dont have a 4.3L in my boat. I think my 3.0 runs around where MikeyT mentioned, 4400-4800rpm range approx. I Agree with what MikeyT said. Check the gauge, or have someone inspect it for proper operation and then I would probably change out the prop.

There might even be a proper size selector chart on Mercruisers webpage??

I have a '97 Talari 179se with the same engine and outdrive. I'm running a 17 pitch(14 3/4" I believe) ss Ballistic prop. I can reach a max RPM of 4900 by myself and half a tank of fuel with a top speed of 49mph.

When I'm loaded with fuel, cooler, wife, myself, and all other gear I usually max out at around 46mph@4700RPM.

I agree with all the others that you have a problem with your tachometer. Also, please post your top speed with your current propeller.

~J

I think that merc optimum rpm is 4600-4800 rpm at any size motor. I asked my mechanic about my prop and he gave me the 4600-4800 rpm info. I'm running a 21p on my 7.4L. seems to be the merc sweet spot.

I know i have seen a few posts in my travels saying, never use the gauge on the dash as a 100% figure in your speed. Verify it with a GPS unit for more precise units.

As for me, from what I remember back in September I was running around 4000-4200rpm range. That seemed to be the noise / rpm / plane sweet spot. I have had it run around 35-36mph (dash) at 4800rpm range, but the engine noise is loud. I dont want to see a piston rocket to the moon ... heh.

After all, I purely have the boat for cruising and anchoring. :wink_thumbup:

That 4800 rpm figure is the maximum rpm figure. Running at anything more than that causes premature & excessive wear on the engine so if you are propped correctly that shouldn't be an issue. The engine rpm 'sweet spot' is a completely different animal. That point is where the engine is running comfortably with a reasonable noise level & fuel consumption. If i remember correctly (it's been 3 mos now..) mine was around 32-35mph & about 3700rpm.

All that said....yep, never completely trust your speedo or your tach until you check them out against calibrated measuring devices.

Our '93 18'10" Barchetta Z has the same Mercruiser 4.3LX Alpha One. Our prop is 17 x 21 pitch, with top rpm being 4800.

Sorry. Ours is a 14.5 x 21, not a 17x21. My husband says you would have better performance with a 21 pitch. We've used this size prop since the boat was new.

So even with your boat being a 4.3L and mine being a 3.0L, the 4000rpm range still stands out being the average.

I should have added in my previous post, that when I push it to 4800rpm, I only leave it there for about 5-10 seconds max and then drop it down again. Mostly I was doing it to just see what happens. Once I know, I dont need to use it there again.

I would love to get mine calibrated / checked out for accuracy :)