1.62 Gear Ratio on Mercruiser 5.7

[cptbraine40]

Boating Brothers and Sisters,

Many of you on here know that I bought a boat that had the old 5.0 replaced by:

Mercruiser Pro Series Roller Cam Long Block 5.7: Serial Number OF298867

Again, I am getting to know my boat and realize that the Alpha One, Gen 2 outdrive on there has a gear ratio of 1.62 vs 1.47. I know the 1.62 was for the 5.0. It currently has a 3-blade stainless steel prop and the WOT RPM is 4800 (which is where is should be). Top speed is about 51 mph (GPS).

So my question is, do I need to change the upper half of the drive to a 1.47 and just leave as is? My concern is will I wear out the outdrive faster or cause unneeded stress on the engine with the 1.62.

Thank you again for any information and advice you can offer me. Everyone on here is great and I really appreciate it.

 

[bman440440]

me personally I would leave it alone... If your getting 4800 with the new 5.7 then you are fine... just remember that when you get to the 300 HP mark with those drives you need to take it easy with "slamming" the throttle... that's what stresses the drive.

 

[alldodge]

IMO, If you have 1 19 pitch prop then your slip number is around 5% which is great, and I wouldn't change a thing. Your not going to wear the drive out, so not issue

 

[cptbraine40]

My engine is running (about) 260-275hp. No desire to go anything higher. If I did go above 300hp, I would need to swap out the outdrive for a bravo..................and going from an alpha to a bravo on the same boat..............nope. LOL. It will be "buy a new boat time"

 

[cptbraine40]

AllDodge, it has a 3 Blade, 21 pitch Stainless Steel prop.........sorry, forgot to add the pitch

 

[cptbraine40]

I was messing with the boat prop calculator and I noticed if I went to a 1.47, my top speed should increase about 5 mph (give or take). I just want to make sure I am getting all of my boat's performance potential...............and I have OCD.

 

[alldodge]

With a 21 your slip is up to 13% and unless this is a heavy boat, that's no to good. If it is heavy then it could still be OK.

Yes you would increase the MPH around 5, but only IF (the big if) the slip did not decrease, and IF you could turn the 21 at the same RPM. In most all cases, changing the drive ratio will only reduce your WOT RPM.

Going to a 1.47 will spin the prop faster for the motor RPM, so my guess is you will need to go to a 19 pitch, the WOT RPM will stay about the same and might get a tad more speed because of less pitch, but your hole shot might suffer . Still just guessing.

The Alpha can handle more HP then rated so long as your easy with it (don't drop the hammer).

 

[Bondo]

it has a 3 Blade, 21 pitch Stainless Steel prop

Ayuh,..... That gives ya only 1 more jump in pitch easily available, before ya get into "Custom" props,....

The idea is to prop up so the motor runs at or very near the top of it's rpm range, to make the best/ biggest power,.....

Boat speed is, what it is, when ya get there,.....

Prop calculators can only Guess whether yer motor has the hp to get where it says it might,....

 

[alldodge]

As an example, bought a 1.81 lower during repower at the suggestion of the engine builder. Went from 415 to 530HP and the boat weights 10K pounds. With the 1.81 B3 drive and 22 pitch props it would barely come out of the water. Replaced lower with original 2.0 drive and 26 pitch props and she comes right up

 

[cptbraine40]

These are all great points. The boat is kind of heavy. It is a 1995 Searay 200 Signature, I think my paperwork states it is about 3500lb dry. So by the time you add beer, gear, gas, and people, probably around 4200lb. Again, I can get to 4800-4900 RPM WOT and my hole shot is great (for a 3 blade). I don't see any problems with the current configuration, I just have OCD and want to make sure my boat is tip top...............after my last post, I think Mr. Bondo understands me.

Thank you again Mr. Bondo, AllDodge, and Bman for all the advice and information. I think again I am just going to leave it alone and go boating.

 

[Scott Danforth]

leave lower, go boating

 

[thumpar]

If you changed ratio your engine would no longer be able to push the same RPM so you would end up dropping pitch to get to were you are now.

 

[QBhoy]

No is the answer. Some 5.7’s mist have came with a 1.62 Drive. My friend has a maxum 2300 cuddy with a 5.7 250hp Merc and Alpha 1.62. He runs a 19” really well.
Also, some 5.0’s came with 1.47 drive. Mine is one of them.

 

[kenny nunez]

The worst thing that you can do is over prop it where the engine cannot turn enough. I always advised my customers it is better to be on the higher end of the rpm, that way there is less stress on the entire drive line.

 

[cptbraine40]

The worst thing that you can do is over prop it where the engine cannot turn enough. I always advised my customers it is better to be on the higher end of the rpm, that way there is less stress on the entire drive line.

My WOT is great.......right between 4800 and 4900 (depending on water chop and outdrive trim). I am currently running a stainless steel, 3 blade, 21 pitch prop. My hole shot seems great, top speed is about 50mph (GPS). I am just going to stick with what I have and not change anything. I will probably pick up a stainless steel, 4 blade, 19 pitch prop for when I have the whole family on the boat vs just my woman and I. I know those 4 blades help a lot with hole shot but reduces top speed some.

 

[cptbraine40]

No is the answer. Some 5.7’s mist have came with a 1.62 Drive. My friend has a maxum 2300 cuddy with a 5.7 250hp Merc and Alpha 1.62. He runs a 19” really well.
Also, some 5.0’s came with 1.47 drive. Mine is one of them.

That is very interesting.............I learn something new every day on here. My guess is that Mercruiser did that depending on the size and weight of the boat...that is just my guess. I am very picky about my boat and just want everything to be perfect on it and run great. Here in Tennessee boating season is really about from mid-April to about mid-September so i want to make sure I take full advantage of it on that water...........not in my driveway working on it. LOL

 

[Scott06]

You will be fine no worries. Your WOT is perfect , stop worrying about drive ratio and enjoy.

 

[porscheguy]

I just entered your info in the mercury prop slip calculator app and it shows you’re at 16% slip. On a 20’ sea ray runabout, that’s awful. My 19’ wellcraft w/4.3 and 1.84 gears will do 52-53mph @ 4900rpm, with a 23 pitch laser2. That’s about 9% slip. With a 21 pitch mirage+, I’m at 49 and on the rev limiter. With a 21 pitch revolution4 I’m at 48 and on the rev limiter. I’ve also got a 23 pitch mirage+. Straight from Craigslist this prop did 50mph at 4700rpm.

So your boat which is maybe 1’ longer and maybe 200-300lbs heavier than mine, but has a 75hp advantage over mine is performing at the same level as mine. Truthfully, it’s worse because it has such a significant hp advantage. I’m not bragging. What I’m trying to say is your boat should run circles around mine. It’s not, which means something’s up. In your case, what’s up is your 16% prop slip.

you say you’ve got a 21 pitch 3 blade stainless. Stainless covers a lot of ground as props go. Do you know what type of stainless prop it is? Who makes it? What model? If it’s a no frills vengeance/Silverado, that prop is a stainless casting of their standard aluminum prop. It’s hardly a performance prop. And if you want to go faster than 50, you need to start looking at performance oriented props. I’m not talking about dropping $1000 on something that’s been lab finished. I’m talking about a mirage+/thunderbolt. A 21pitch mirage+ should get your slip under 10%. Mine has extra cup and I’m at 6-7% slip. Your 75hp advantage should have no trouble with it. In all honesty, you could use a 23 pitch mirage+ for light weight cruising.

Are your carb secondaries even open at redline with your current prop? I’m going to send you a PM.

 

[QBhoy]

I just entered your info in the mercury prop slip calculator app and it shows you’re at 16% slip. On a 20’ sea ray runabout, that’s awful. My 19’ wellcraft w/4.3 and 1.84 gears will do 52-53mph @ 4900rpm, with a 23 pitch laser2. That’s about 9% slip. With a 21 pitch mirage+, I’m at 49 and on the rev limiter. With a 21 pitch revolution4 I’m at 48 and on the rev limiter. I’ve also got a 23 pitch mirage+. Straight from Craigslist this prop did 50mph at 4700rpm.

So your boat which is maybe 1’ longer and maybe 200-300lbs heavier than mine, but has a 75hp advantage over mine is performing at the same level as mine. Truthfully, it’s worse because it has such a significant hp advantage. I’m not bragging. What I’m trying to say is your boat should run circles around mine. It’s not, which means something’s up. In your case, what’s up is your 16% prop slip.

you say you’ve got a 21 pitch 3 blade stainless. Stainless covers a lot of ground as props go. Do you know what type of stainless prop it is? Who makes it? What model? If it’s a no frills vengeance/Silverado, that prop is a stainless casting of their standard aluminum prop. It’s hardly a performance prop. And if you want to go faster than 50, you need to start looking at performance oriented props. I’m not talking about dropping $1000 on something that’s been lab finished. I’m talking about a mirage+/thunderbolt. A 21pitch mirage+ should get your slip under 10%. Mine has extra cup and I’m at 6-7% slip. Your 75hp advantage should have no trouble with it. In all honesty, you could use a 23 pitch mirage+ for light weight cruising.

Are your carb secondaries even open at redline with your current prop? I’m going to send you a PM.

Is it maybe just that your wellcraft seems exceptionally quick in comparison. 53mph is really good going with a 4.3. Most will be sweating at touching 50. Sounds like a good set up ?

 

[bman440440]

Is it maybe just that your wellcraft seems exceptionally quick in comparison. 53mph is really good going with a 4.3. Most will be sweating at touching 50. Sounds like a good set up ?

My 1997 20' Maxum will hit 54 (GPS and 4950 +/- and no I dont keep running it there) swinging a 4 blade 21" turning point prop with a light load ( me and 15-20 gallons of fuel) and when loaded for fishing (yes I know its not a fishing boat) with 2 large guy's and 1 "regular" guy, 47 gallons of fuel, 32 gallon bait tank, 100 pounds of ice in the kill bag and all our gear I can get 47-48 MPH... but I did spend a lot of time getting it propped correctly... loaded for fishing I will max at 4700 RPM and light load 4900-5000 RPM