3.0 merc prop size

[atistang]

boat is a 1989 cheetah 18ft bowrider
130hp 3.0 merc
1.98:1 drive ratio
max rpm 4200-4600rpm


with my 14" aluminum 19p i run 36mph at 4800rpm

with a borrowed 14" aluminum 23p i ran 37mph at 3900rpm

even with the 23p the boat still planed out pretty quick considering the large pitch, but the 23p prop was a little dinged up

heres the odd part, with the 23p on a plain at WOT if i trim up to the stop (where you have to use the trailer button to go any farther) it would airrate(sp) and over rev, with my 19p this does not happen. anyone have any idea why this is?

also i have a dolphin fin on the boat, any chance that is holding my boat back from planing out like it should to achieve max speed?

i'd really like to be able to hit 40mph, not sure where to go from here as the next step is going to involve spending money, so i want to get as much insight as possible before i buy something.

should i get a 19p stainless? 21 aluminum?

both props i have ran were mercruiser aluminum props

 

[SparkieBoat]

Re: 3.0 merc prop size

you should get a 21P alum or SS

 

[oops!]

Re: 3.0 merc prop size

and loose the whale tail !

 

[atistang]

Re: 3.0 merc prop size

and loose the whale tail !

i was thinking about this, once i get to about 30mph with either prop, i get a big spray from the center of my boat, i've never seen other boats do this. i wonder if the reason it i topped out at the same speed is the whale tail really digging in at that speed. I think i'll take it off and run the 23 one more time just to compare.

now i've always heard that stainless props are almost like adding more pitch, such as a 21 stainless would act more like a 23 aluminum than a 21 aluminum, is that true? also i've been looking at turning point hustler props, are they better than your run of the mill prop?

 

[steelespike]

Re: 3.0 merc prop size

The whale tail will help hole shot but can cause drag if they don't c;lear the water on plane.
they can also cause the boat to lay down too flat (perhaps the cause of your spray).
I suggest you remove the tail and do a run with the 19" prop and report the results to us.
you can let it briefly overrev to get wot rpm and speed.
Before making a prop selection tell us more about your boating activities,Typical load etc.

 

[atistang]

Re: 3.0 merc prop size

The whale tail will help hole shot but can cause drag if they don't c;lear the water on plane.
they can also cause the boat to lay down too flat (perhaps the cause of your spray).
I suggest you remove the tail and do a run with the 19" prop and report the results to us.
you can let it briefly overrev to get wot rpm and speed.
Before making a prop selection tell us more about your boating activities,Typical load etc.

most of the time its about 4 people on the boat, every once and a while i'll pull a tube, most of the time we just go out and run wide open, i figure if i get a 21 that will be better with lighter loads then i can always throw my 19 on when tubing. with the 19 we were able to pull a tube with 8 people (9 people total) on the boat lol, but we were only doing about 20 at WOT

i'll see if i can get out with both props and no whale tail next week

 

[steelespike]

Re: 3.0 merc prop size

Yes a 21 seems a logical choice but wait till you get a test without the tail.
with the load yuou carry you may want to consider a 4 blade even if only for when 8 or 9 people aboard.Is the boat rated for 9 people?

 

[atistang]

Re: 3.0 merc prop size

Yes a 21 seems a logical choice but wait till you get a test without the tail.
with the load yuou carry you may want to consider a 4 blade even if only for when 8 or 9 people aboard.Is the boat rated for 9 people?

no, 7 lol. i don't load it like that, but my friend who owned it before me used to

how does a 4 blade compare to a 3 blade? do you lose some top end? speed in turns? i figure there has to be some downside to it because i rarely see people run them

 

[steelespike]

Re: 3.0 merc prop size

4 blade will do about everything better than a 3 in low to mid range performance.
Should have better hole shot,stay on plane with less throttle,resist venting,
Run smoother,handle better at the dock.Usually but not allways the 4 blade will be slower.
at top end.

 

[atistang]

Re: 3.0 merc prop size

alright took the whale tail off, put the 23p back on, and went 38mph at 4200rpm

i'm gonna see about borrowing my friends ss 21p and see how that does. one thing about ss props, they are very heavy, i would think that would have a large negative effect on speed for boats with smaller engines like mine. right?

 

[steelespike]

Re: 3.0 merc prop size

Prop weight is of no consequence to speed.
Some believe the weight may cause premature wear on the clutrch dog.
So you should be sure the motor's is at proper idle and shift crisply at idle speed.
These procedures are true for all I/O's and outboards with al or ss props as well.

 

[tejastoolman]

Re: 3.0 merc prop size

I put a 21p on my 3.0 merc and have been very happy, 19.5ft capri does 46-48mph with just me in it..... but then my spedo could be off a tad lol