Cruising RPM???
 

Hello, I just bought a 1998 21 foot Shabah Bow Rider this past weekend. I am looking for advice on a proper cruising rpm. The boat is equipped with a 5.7 litre small block Chevy motor and I believe a Bravo I outdrive. Your advice would be appreciated. Also, how do I confirm which outdrive I have? Thanks in advance for any info you are able to provide.

Re: Cruising RPM???
 

Congrats! I have a '98 Talari Z211.

RPM's is to subjective away to gauge it. Depends on prop pitch, out drive gear ratio, and weight in the boat. USUALLY shooting for the low 30's @ 2700 RPM's is average for a 5.7l. For cruising the idea is to go just about as fast as you can before you fall off plane. Get her up to speed and slowly back off while watching the tac. Should be pretty obvious when the back of the boat starts squatting and you start pushing too much water with the bow, note the RPM's when that happens then speed up and back her off again to that point.

The big key is if you have a 4bbl carb on her you want to cruise before the secondaries open up or you will CHUG the gas. :D

Usually 5.7's came with an Alpha drive. There are some visual queues that you can look for to verify its a Bravo, but its almost just as easy to pull the SN off the out drive case and call Merc. Pretty quick phone service there. Usually only takes me 5 minutes to get that info. I will also tell you what ratio it shipped with.

Re: Cruising RPM???
 

i think i run about 25 mph at about 3000 rpm...thats dif then what you said sterio... what do you think is my problem? 5.7 a1 4 blade 21p, 21.5 talari.....

Re: Cruising RPM???
 

That's not too bad without knowing your drive gear ratio, and is that GPS'd? Only a few % off of my general rule of thumb. To really do the math correctly I'd need your drive ratio, a GPS speed and the full prop specs. (Diameter and pitch)

Seems like you should be a little lower on the RPM's, a small diameter low cup prop would let more slippage if that is what you have, but that 4th blade should make up for that somewhat with a little more bite... Hmmm...

Just curious, what is your WOT (wide open throttle) top speed and RPM's? I have a feeling that you just might have a low drive gear ratio. Lets start with getting some more numbers.

Re: Cruising RPM???
 

wot she will go about 48. i just bought a 14.25 21p ss 3 blade. havent put it on yet. what do you think that will do for me?

Re: Cruising RPM???
 

thats a little to much pitch for me, my RPMs are not in the "specified" range on the top end. I do good with a 19 aluminum but I think a 19SS would be perfect for me as far as RPMs go.

Re: Cruising RPM???
 

you really think a 21p ss is too much pitch for a 350?

Re: Cruising RPM???
 

Not at all. That's what I run on mine. A 14.25 21P. Like I said before, drive gear ratio is a HUGE factor, as well as hull design, weight, etc.. Picture your drive gear ratio as the difference in driving your car in 1st vs 3rd gear ALL THE TIME. You never get to shift. If you have "3rd gear ratio" you might only be able to turn a 19P, but a "1st gear ratio" would let you turn a 21P.

What where your RPM's at that 48MPH? Did you have it trimmed out? (The outdrive not all the way down. As that just buries the nose.) Also see if there is a ratio stamped on the top of your outdrive. Something like 1.47:1. Knowing that would be VERY helpful. There are calculators that you can plug all this data into and it will tell you how a certain prop should behave.

Last, what is the diameter of the 4 blade and is it SS or aluminum?

Re: Cruising RPM???
 

the 4 blade is a 14x20p not 21 and it is alum. not too sure what the rpms are wide open. ill look next time. but thats trimed out. as far as the stamp on top ill have to take a peak in the next couple days.

Re: Cruising RPM???
 

In that case I could definitely see you being a little hi on the RPM's. Aluminum flexes quite a bit, so a 20 under load can un-cup down to a 18-19. That one of the reasons that SS works so well, its very rigid and holds it true rating. Remember 1P = 200 RPM's.

I bet you'll be surprised what that 21P SS 3 blade will do. Just keep in mind that its always a trade off. That 4 blade would be nicer for hole shots (skiing, tubing, etc), but a 21 will be better for cruising and top end. Just something to keep in mind.

Re: Cruising RPM???
 

yes it is for mine. I dont remember exactly what my rpms were during my testing. With the 21 aluminum trimmed up, it wouldnt turn more rpms than the lower end of the spec. I removed my drive to inspect the gimble bearing and it all checked out great. I should have everything ready for the water this weekend so I will report my numbers.

Re: Cruising RPM???
 

now when you say 1p
= 200 rpm, you mean wot i should be 4200?

Re: Cruising RPM???
 

Steroeman405, Thanks very much. This is great info. I will be out at the lake again in two weeks and I will try to get to know the characteristics of my boat a little better. Thanks Again, Badold98.

Re: Cruising RPM???
 

1P = 200 RPM Change

So if your boat was WOT @ 4200 RPMs with a 21P, it would increase to 4600 RPMs if you dropped to a 19P. (2 drops in pitch = 400 RPM gain). Inversely if you stepped up to a 23P you would LOSE 400 RPMs and be at 3800 RPMs.

Keep in mind this is how it should work mathematically, but is all depends on the motor / gear ratio. Your motor has to have the enough horespower / torque to move the weight of your boat at any given speed. If I put a 23P on my 19' Blue Water with its V6, it might not have enough power to get up on plane to ever get to 3800 RPM's. (It would be like starting your car from a dead stop in 4th gear.)

Also cupping of the prop and diameter plays a big part in this. A higher diameter prop has more surface area which = more friction = less slip = more bite. So a 21P x 13.75 might give you 4400 RPMs @ WOT and a 21P x 14.75 might only give you 4200 RPMs @ WOT AND cause porpoising (bow bounce) because it has too much bite. See isn't this fun how much is involed in this?? :D

For 350's usually sicking in the 14.25 range is pretty good and I HIGHLY recommend getting SS props unless you are always in the shallows snagging things. SS is so stiff you can plane quicker, cruise at a lower RPM, get more top end all while SAVING GAS. They pretty much pay for themselves.

I did a side by side test on my dad's 20' Maxum with a v6. Swapped between a 21P x 14.25 SS and a21P x 14.25 aluminum. The results? The boat used to over rev by 500 RPMs and could hit about 50MPH, just by switching to SS it sucked the RPMs down by 450 and ADDED 6 MPH on top end. Better hole shot and cruising speeds. All those RPMs were just wasted $$$$$'s.

Re: Cruising RPM???
 

No prob. :wink_thumbup: