Merc V8 Re-Prop

[krakatoa]

Hello Guys I have this 1998-2000 Mercruiser V8 5.0L, when got the boat it comes with a 4 blade SS prop Offshore Series 48-825900 19. Iran the boat around 3 times with only my kid and Me aboard, 1/2 gallon tank so there is not much weight. i know the boanr weight is around 3221 pounds “dry” per specifications. The point is that can go more than 4100 rpms and it should go from 4400 to 4800 rpms. I believe that this V8 can perform faster and im think that this 4 blade isnt good for this setup. Maybe If I try same 14*19 but three blades or go 14 x 17 to go up additional 300 rpms... and go faster. Can tell exactly by MPH cause the gauge isnt working, maybe clogged hose. Anyway I think that this kind of 4 blade prop is for a bass boat with a big azz Outboard... Couldd you guys give me some advice on this one? Engine S/N 0L683151 Thanks

 

[krakatoa]

Its possible this prop could be RH instead LH. Per this specifications this engine is LH.......

 

[alldodge]

Staring with a 4-blade 19
To increase your WOT RPM you either go with a 4-blade 17 or a 3-blade 19

The motor is LH but the prop rotates RH for standard rotation

 

[Scott Danforth]

my 19' Sea Ray had a 15 x 19p and I would hit top RPM (just shy of 5000 RPM with just me) and about 50mph with it behind my 5.0/Alpha

you need to verify speed with a GPS (phone app works) vs the pitot tube speedo

make sure your boat motor doesnt have a flapper dropped in the y-pipe.

 

[krakatoa]

Staring with a 4-blade 19
To increase your WOT RPM you either go with a 4-blade 17 or a 3-blade 19

The motor is LH but the prop rotates RH for standard rotation

Never deal with 4 blades before but same size 3 blades could made any difference? Still same pitch on 19

 

[krakatoa]

my 19' Sea Ray had a 15 x 19p and I would hit top RPM (just shy of 5000 RPM with just me) and about 50mph with it behind my 5.0/Alpha

you need to verify speed with a GPS (phone app works) vs the pitot tube speedo

make sure your boat motor doesnt have a flapper dropped in the y-pipe.

Flappers? It have a rubber hose connecting raiser and y pupe obviously at the top of it but where this flappers are sitting???

 

[Scott Danforth]

flappers sit in the top of the y-pipe.

 

[alldodge]

Never deal with 4 blades before but same size 3 blades could made any difference? Still same pitch on 19

Rule of thumb (so to speak) the same pitch on a 4-blade is like 2 pitches down on a 3-blade. This is why I said your 4-blade 19 could use a 3-blade 19. More blades while increasing hold, it also increases drag/load

 

[krakatoa]

Rule of thumb (so to speak) the same pitch on a 4-blade is like 2 pitches down on a 3-blade. This is why I said your 4-blade 19 could use a 3-blade 19. More blades while increasing hold, it also increases drag/load

make sense, thats why I still saying this boat could perform better speed, it feels like holding/dragging....

 

[krakatoa]

flappers sit in the top of the y-pipe.

I will double check but will put my money in the (f word) 4 blades.

thanks for your replies, really appreciate it

 

[porscheguy]

The offshore series is now known as the vensura. Its been around for a while and Mercury is phasing it out. Depending on who you ask, its a great prop with a loyal following, or theyll tell you its obsolete and little more than a 4 blade Silverado/vengeance.

Check to see if your exhaust flappers/shutters are intact and not blocking exhaust flow which would restrict performance.

What is your gear ratio? I noticed on your engine label, there was no drive serial#. Your engine should be mated to a drive with a 1.61 ratio. But Im scratching my head because youre engine label has the engine serial and the transom assembly serial. In some applications they may have used the taller 1.5 ratio (the boat builder may have done this).

Youve got a 50mph boat (give or take 3). With a 1.61 ratio and a light load, you should be able to swing a 19p prop all the way to redline. And it should get a 19P offshore series pretty close.

If youve got a 1.5 ratio then your main issue is youre overpropped. First because the pitch is too high, second brcause its a 4 blade.

Most boats like yours use props ranging from 17-23 pitch.

Before spending money on props, spend some time making certain everything is running right. Spend time to collect accurate and reliable data. Gear ratio, speed, make sure the tach is correct.

 

[krakatoa]

The offshore series is now known as the vensura. It?s been around for a while and Mercury is phasing it out. Depending on who you ask, it?s a great prop with a loyal following, or they?ll tell you it?s obsolete and little more than a 4 blade Silverado/vengeance.

Check to see if your exhaust flappers/shutters are intact and not blocking exhaust flow which would restrict performance.

Will check first thing in the morning tomorrow, you are the 2nd fellow boater who mention about flappers

What is your gear ratio? I noticed on your engine label, there was no drive serial#. Your engine should be mated to a drive with a 1.61 ratio. But I?m scratching my head because you?re engine label has the engine serial and the transom assembly serial. In some applications they may have used the taller 1.5 ratio (the boat builder may have done this).

Per specifications the year ratio is 1.62 R (See Pic Below)
and DriveSerial is : 0L755795

You?ve got a 50mph boat (give or take 3). With a 1.61 ratio and a light load, you should be able to swing a 19p prop all the way to redline. And it should get a 19P offshore series pretty close.

If you?ve got a 1.5 ratio then your main issue is you?re overpropped. First because the pitch is too high, second brcause it?s a 4 blade.

Most boats like yours use props ranging from 17-23 pitch.

Before spending money on props, spend some time making certain everything is running right. Spend time to collect accurate and reliable data. Gear ratio, speed, make sure the tach is correct.

Still thinking need to go to a 3 blade same pitch same diameter, maybe a 17 p. But definetely will have to look at the flappers. will came up with pics.

 

[porscheguy]

Your boat is the same length as mine. Youve got a fuel injected 5.0, I have a 2 barrel 4.3 vortec. Youve got at least a 40hp advantage on me. And your torque should be a lot higher than me. As for weight, youre probably a little heavier. Your boat should be at least as fast as mine, maybe a bit faster. I can turn a 21P rev4 right up to redline. Ive got a 1.84 ratio. I can turn a 23p laser2 up to redline with a light load.

Although it has 4 blades, your prop is fairly mild in terms of rake and cup. The diameter is a bit smaller than most modern 4 blade props. Youre leaving 700rpm on the table and I dont think your prop is the root cause of this. I plugged some info into the prop slip calculator. I went with a conservative 12% slip (based on the 8.5% I get from my rev4) because your prop is older, and well used. With your gear ratio and a 12% slip, your boat should do 47mph at 4800rpm. What did I say earlier? I said youve got a 50mph boat, give or take 3mph.

You dont need to go down in pitch, and you probably dont need to worry about having a 3 blade or 4. You need to figure out whats robbing you of 700rpm (I dont think its the prop.).

When was the last time you changed the fuel filter? How does the engine run? It should run hard and strong. Small block v8s like to rev and run hard. Have you checked ignition timing? What about the running temp? Where is it? Fuel injected engines are sensitive to operating temp. Yours should run at 160. If any temp sensors are bad, it will run rich and rob power.

From your pics, your boat looks clean and well cared for. I dont want to see you waste money on the wrong prop. Its likely whatever is causing you to lose 700 rpm is trivial like a broken exhaust shutter blocking the exhaust, a dirty fuel filter, timing, or some random sensor.

 

[Scott06]

I'd second the comments to make sure things are running right first. You should be able to hit 4600-4800 with that prop (you will most likely over rev if you go down to a 17) so in addition to the exhaust flaps, definitely verify the tach is working correctly and check out the ignition components and that you are getting full advance.

I run a Vensura on a similar sized boat. Mine is a 2004 Sea Ray 200 sport, 3200 lbs dry, with a 5.0 with an edelbrock carb. My boat original was a V6 so I kept the 1.82 ratio drive but stepped up in pitch from a 19" vengeance to a 21" vensura when I repowered. What the Vensura does well is give a strong holeshot and great cruise which is great for me as skiing, tubing, and wake boarding with a boat full of people is most of what I do. Lightly loaded and trimmed up It tops out at 44 mph at 4900 rpm with the vensura. I also have a 22" enertia three blade that pulls 47 mph at 4800 or so. I think my slip numbers are 16 and 12 respectively. I'd like, to try a revolution 4 if I can find one used, tend to like 4 blades because they do well for how I use my boat. If top speed is your thing get an enertia or similar 3 blade.

With your 1.62 drive and a 19" I'd think you'd post similar WOT rpm if your tach is correct and all is running well. It would be good to verify speed with gps on your phone for a sanity check.

 

[porscheguy]

Scott, you should look for a 23p mirage plus used. If I can turn 4750rpm with one (and Ive got a 4.3), you should be able to get it up to 4900. Ive got a rev4 but I keep going back to the mirages.

 

[Scott Danforth]

porscheguy the OP never mentioned his induction, most likely the 2-barrel mercarb and 220hp rating since its paired with the 1.62 drive.

 

[alldodge]

The OP said he doesn't know how fast he is going because its isn't working. The question is how to get the RPM's up, not how to get the speed up. So far there is no indication that the motor is not running t full power, only that RPM's are lower

 

[krakatoa]

Ok, here is what I found on the exhaust Y Pipe:

Starboard Looks fine the flapper is in place.

Now the surprise on the Port side, flapper was alllllll the way down in the Y Pipe as you guys mention.

 

[krakatoa]

When was the last time you changed the fuel filter?

The fuel filter was changed on October with all the fluids on engine and lower unit as well

How does the engine run? It should run hard and strong. Small block v8s like to rev and run hard. Have you checked ignition timing? What about the running temp? Where is it? Fuel injected engines are sensitive to operating temp. Yours should run at 160. If any temp sensors are bad, it will run rich and rob power.

The engine starts fine its running welll but I still thinking it could be faster.

Engine temp is firm between 150 to 160 maybe no more than 165 firm all day, no problem with overheating...

Also Oil Pressure is on 40 PSI when hammer down.

From your pics, your boat looks clean and well cared for. I don’t want to see you waste money on the wrong prop. It’s likely whatever is causing you to lose 700 rpm is trivial like a broken exhaust shutter blocking the exhaust, a dirty fuel filter, timing, or some random sensor.

Maybe we hit the jackpot with the flapper finded at the bottom of the Y Pipe on port side. Hope this will get the speed trouble done.

 

[krakatoa]

need to order the flapper, just wondering If can I use the boat for the weekend without that flapper...