new prop, lower max RPMs and speed?

[Nextelbuddy]

1996 sea ray 210 mercruiser 5.7 210 hp alpha 1 gen 2 with 1:61 gearing.

according to the sea ray sport boat PDF this is how m boat was originally specced

Year Model Engine Drive Ratio Prop Size WOT RPM
1996 210 BR 5.7L (210 hp) Alpha 1 1.62:1 13.75 x 21 4600

so it originally came with a 13.75 x 21 P prop 3 blade

previous owner installed a mercury quick silver 14 x 19p 3 blade propeller and with this configuration at 4600 RPMs i can only manage about 36-37 mph trim all the way down with full tank of gas. if i trim up, the RPMs will creep up to almost 5000RPMs and i will get maybe 39 mph BUT the boat BOW just hops up and down and its annoying and frustrating so i trim back down and hit my 36 MPH and smooth ride.


i thought i did a lot of research before hand and settled on purchasing a 14x21P aluminum prop 4 blade.... i figured/hoped that by going from 19p to 21p i would gain some top speed back AND some hole shot with the 4 blade.

instead the boat was a slug out of the hole and i can only get about 3000-3300 RPMs max. terrible i didnt even look at the speed on GPS, i could tell the boat was slow out of the hole and with only reaching 3000 rpms top speed was probably around the 25 mph range.


threw the quick silver 14x19p 3 blade and im back to where i was again initially.


so i suppose first thing first is maybe i just have a tired 5.7 engine.. i see other sea ray owners stating they are getting 45-50mph with their 5.7 alpha 1 210 boats. if i am only getting 36 mph at 4600 rpms, my engine must just be tired.

2nd thing is, why did going to a 4 blade 14x21p prop sow me down so badly? should i have tried to go with or find a 13.75x21p like it had from the factory? is there that big of a difference in regards to overall diameter?

what is the correlation between performance and prop diameter?

 

[Stinnett21]

If you're getting 5000 rpm out of the 19" that's about all she's gonna do. Boat dirty on bottom? Bottom is straight? Waterlogged foam? Trim tabs will help or maybe even eliminate the bow hoping up and down. If you don't want to spend $600 search Nauticus Smart Tabs. The 4 blade did worse because it has an extra blade, i.e. more drag. You have to drop maybe 2 inches when going to 4 blade. Maybe try a 17".

 

[dingbat]

2nd thing is, why did going to a 4 blade 14x21p prop sow me down so badly? should i have tried to go with or find a 13.75x21p like it had from the factory? is there that big of a difference in regards to overall diameter?

What is the correlation between performance and prop diameter?

In reality, the diameter has little affect on RPM. Generally speaking, assuming your comparing props of the same make and design, the two main drivers of RPM are Pitch (~100 rpm per inch) and number of blades. (~ 200 rpm per blade).

If your changing make and model of prop, all beats are off. It becomes a crap shoot

 

[QBhoy]

Hi

i think this is all over the place really. Your 1996 5.7 I’d be almost certain isn’t 210hp. Not sure there has ever been such a thing. I’d also expect it to usually have a 1.47 ratio perhaps too. Unless it was set up for high altitude ?
i haven’t ran the numbers for prop slip, but might be worth checking too.
Are you trimming up ?

 

[Nextelbuddy]

Hi

i think this is all over the place really. Your 1996 5.7 I’d be almost certain isn’t 210hp. Not sure there has ever been such a thing. I’d also expect it to usually have a 1.47 ratio perhaps too. Unless it was set up for high altitude ?
i haven’t ran the numbers for prop slip, but might be worth checking too.
Are you trimming up ?

According to the sea Ray sport boat PDF my boat hull ID it came with a base model 5.7 non efi non lx engine and the outdrive came with a 1:62 ratio and it actual says 1:62R on the outdrive unit itself near the serial number.

I believe the engine has 250 propshaft HP but to the prop ends up being 210 according to sea Ray.

 

[Texasmark]

It's not your prop, it's the physics of the hull vs angle/amount of thrust. You are Porpoising at high trim angles for a given speed....any boat will do this. To stop the phenomenon, you either apply more thrust, or change the angle. What the prop does is to help determine just how much and effective the thrust is when applied. High performance props (high pitch, cupping, high rake), with then ponies to twist them, for a given speed, will be able to pull on out of it...move it up to a higher speed, over the one currently running for a given RPM.

On the bow lift, as the boat comes up out of the water, less of the hull is touching the water to restrict performance. Semi-V hulls, lifting strakes on deep Vs, reverse chines, "pads" on bass boats, and other hull characteristics aid in reducing the wetted area which reduces the drag, increasing speed and rpms....as long as prop slip is constant, speed is directly proportional to rpms.....trim out lifting the bow, speed increases as does the screw, minus the prop slip percentage.

Once up and running out, trim positions effect this angle and amount of hull in the water and can increase speed and rpms to a point.....the point of porpoising or chine walking....boat rocks back and forth on flat water at a pretty good rate.....unstable.

To stop the porpoising, you either need to put more ponies to play or tuck in your trim till it stops....which will slow the boat and drop your rpms.

On some hulls, the transom angle is such that you can trim out till the prop starts ventilating....before porpoising occurs, whereby the prop is partially airborne whereby the rpms will increase and the boat speed decreases.....this point is your boat's max speed for the equipment-load-water conditions at the time. (Reference: my personal experiences)

 

[QBhoy]

According to the sea Ray sport boat PDF my boat hull ID it came with a base model 5.7 non efi non lx engine and the outdrive came with a 1:62 ratio and it actual says 1:62R on the outdrive unit itself near the serial number.

I believe the engine has 250 propshaft HP but to the prop ends up being 210 according to sea Ray.

So we have gathered that it’s a 250 v8 5.7. Glad of that. If searay are saying it’s 210...they are full of it. Really surprised at them saying that. It’s not the case at all with your engine.

 

[Scott Danforth]

So we have gathered that it’s a 250 v8 5.7. Glad of that. If searay are saying it’s 210...they are full of it. Really surprised at them saying that. It’s not the case at all with your engine.

its a 2 barrel. Mercuiser quit putting 4-barrel engines on the V8's when the EFI came out

 

[Nextelbuddy]

its a 2 barrel. Mercuiser quit putting 4-barrel engines on the V8's when the EFI came out

i'm so confused with my engine setup still at this point. when i run my HULL ID in the searay website it shows:
ENGINE 5.7L ALPHA I ( 210 PHP ) STERN DRIVE Serial #1 OF760044

I assume this means it is NOT an LX and does NOT have thunderbolt ignition either as i dont see any thunderbolt ignition module on the port side exhaust rider like i've seen on youtube videos. also my boat has 1:62 outdrive and the LX which originally came with a 4 barrel came with a 1:47 gear fro my research online.

however someone in the past has upgraded my intake manifold to an edelbrock intake manifold and it does appear to have a 4 barrel carb

ive been looking everywhere for th proper flame arrestor cover and can only find ones that either say LX or Thunderbolt but not one that just says 5.7 Alpha with nothing else lol.


i guess i got the super base model engine in my boat.

 

[QBhoy]

its a 2 barrel. Mercuiser quit putting 4-barrel engines on the V8's when the EFI came out

Still be 250hp though Scott.

 

[Scott Danforth]

Still be 250hp though Scott.

250 at crank, 210 at prop, however they wouldnt sell the EFI motor if they put the 4-barrel on it at 260hp (or more) if the EFI is only making 260-300. kind of hard to justify the $3k extra expense..... so ham-string and de-rate the carb motor

 

[Scott Danforth]

i'm so confused with my engine setup still at this point. when i run my HULL ID in the searay website it shows:
ENGINE 5.7L ALPHA I ( 210 PHP ) STERN DRIVE Serial #1 OF760044

I assume this means it is NOT an LX and does NOT have thunderbolt ignition either as i dont see any thunderbolt ignition module on the port side exhaust rider like i've seen on youtube videos. also my boat has 1:62 outdrive and the LX which originally came with a 4 barrel came with a 1:47 gear fro my research online.

however someone in the past has upgraded my intake manifold to an edelbrock intake manifold and it does appear to have a 4 barrel carb

ive been looking everywhere for th proper flame arrestor cover and can only find ones that either say LX or Thunderbolt but not one that just says 5.7 Alpha with nothing else lol.


i guess i got the super base model engine in my boat.

you have a 5.7 with the TB 5 ignition. (all mercury products have "thunderbolt" ignition - its a trademarked name)

with the 4 barrel carb, you are putting an honest 260hp to the prop about 290-300 at the crank.

the 1.62 ratio is fine

dont fret over a sticker on a vanity cover..... its about as meaningful as a wing on a honda civic