Merc 470 sterndrive ratios

[Snostruck]

I have a question regarding the merc 470 drive ratios.
I'm have recently installed two rebuild 470 engines estimated at 200hp each. I know originally the boat had 170hp with 1.87 gears and used a 14.5x17 props
my question is can I use 1.50 gears with say a 14.75x15props. The units are brand new and very cheap to me.
The boat is 11,200 dry 11.5 beam and 29.5 feet

Tia

 

[Bt Doctur]

very doubtful with that size boat. 200 hp is still in the 1.84 range

 

[Snostruck]

I imagine getting itto plain would be the issue. I did the math @ 1 inch of pitch per 2-300 prop rpms. The difference from 1.87 to 1.5 should be about 550 prop rpms at 4600 engine rpms which is why j assumed dropping 2 pitches would get me close.

 

[thumpar]

Even the 5.0 use a 1.8x ratio in a boat that size.

 

[Snostruck]

These are new alpha 1 gen 2. 1995 and up era. Then ran the 1.62 with the 5.0 but there have been some with the 1.8.

 

[Snostruck]

5.0 are 200ish hp. Merc did use some 1.6 on those. I imagine it's not ideal as the manufacture used certain gears for a reason. Just wondering if anyone has been down this road. 2 sets of gears mathamatiy should be able to compensate each other to achieve the same prop rmp. I don't understand difference in prop thrust with less pitch and higher prop rpms and a heavy azz boat

 

[Scott Danforth]

I have a question regarding the merc 470 drive ratios.
I'm have recently installed two rebuild 470 engines estimated at 200hp each. I know originally the boat had 170hp with 1.87 gears and used a 14.5x17 props
my question is can I use 1.50 gears with say a 14.75x15props. The units are brand new and very cheap to me.
The boat is 11,200 dry 11.5 beam and 29.5 feet

Tia

No, you do not have enough motor to spin a 1.5. stop thinking HP, think torque. you do not have enough low end torque like a V8

you didnt increase the HP much on the 470 engines because unless you get into custom cam grinds (which do not work in a marine application) or forced induction, you cant.

stick with the correct 1.87:1 ratio

 

[Snostruck]

Can higher gearing not be compensated with different props? To say I don't have the power is untrue. The fact is the prop I would have to reduce to to gain proper engine rpm might not be in the optimal range of best performance.

Surely someone else and tested this rather than just saying in won't work based on unknows variables.

 

[Bondo]

Can higher gearing not be compensated with different props?

Ayuh,...... It would be nice if it were true, but life don't work that way,.......

The gear ratio has to match the motor, 'n it's load,......
Prop pitch is then used to fine tune the application,......

I have a torque-shift prop that I can adjust the max pitch on from 'bout 20" to 26",.........
Pitch at idle is 11",.... perfect for trollin',.....
With a 1.65:1 drive, I need to tighten the screws right in, 'n hold it to 'bout 20", to get 43/ 44 mph at 4800 rpms at Wot,......
I threw on a 1.98:1 drive so I could use the pitch at 25/ 26" at Wot, givin' me even slower trollin' speeds,.....
At 26" of pitch, at 5 grand Wot, I was runnin' in the hi-30s mph,...... Alota Noise, goin' nowhere slowly,.....
I later put on a 1.84:1 drive, but never got around to testin' the torque-shift on it,.... just ran a 21",.....

 

[Scott Danforth]

Can higher gearing not be compensated with different props? To say I don't have the power is untrue. The fact is the prop I would have to reduce to to gain proper engine rpm might not be in the optimal range of best performance.

Surely someone else and tested this rather than just saying in won't work based on unknows variables.

Believe us...or dont....you asked, we gave you the answer, however you still want validation to use the wrong gear ratios.... You wont get it, because it wont work. You may be able to prop a 1.62:1, however not 1.5's

your heavy cruiser should have twin V8's, not twin 4 bangers.

 

[thumpar]

Just as an example Merc used to pair the 454 with an Alpha. They had a ratio of 1.32 for them. They are 330hp. The 350 mag is 300hp an paired to a 1.5 ratio drive. That is only 30hp difference. The real difference is torque. You may have upped the hp on your engine but the torque is probably not much more than it was.

 

[Snostruck]

I'm not arguing. I just don't understand how yall are only focused on sterndrive gears only. No kind of calculations what so ever. Pitch and diameter act as a set of gears. I assure you i can achive the same engine rpm@ wot 1.5 gears as you can with 1.8. Its mathematics. 1.87 gears spin at 4800 engine rpms and produces 2566 prop rpms. 4800÷1.5 is 3200 prop rmp. 1 inch of pitch equals ~ at worst case betwee. 2-400 engine rpms depending on the engine. ie diesel vs gas or 496 vs 470.
The issue is not if I can reach engine rmp. Math says I can. The question now is more towards the volume of water moved by props relative to plane time vs mid range vs top speed. 14.5x17 is what was ran in the boat i mentioned. Mind you it had 170hp with 1.87 gears with pre alpha sterns. It was all junk. So I 100% gutted it. The 200hp are out of a frinds boat who owns a local boat shop. It has higher compression, and ported head, aftermarket tuned carbs, Mallory distributes. There brand new rebuilds. This boat ran 1.96 rations with 14"×17 Props. That boat ran 27mph.

HP has nothing to do with my math in the prop speed. Theoretically if I'm spinning the prop at wot with relatively the same engine rmp do different props produce the same amount of trust? I'd guess the simple question is:

Does a 14.75x15 prop produce the same thrust as a 14.5×17 prop.

 

[thumpar]

You might want to educate Mercruiser too. Do some real world testing and show them.

 

[AyeBoats]

This might be a collision of practicality and theory, overcomplication.

Read about propeller tip speed and cavitation. Theory gets deep, fast. Propulsion and traction are not the same thing. Pitch, tip speed, rpm, diameter, weight, drag, wetted area, etc, so many variables, so little time.