1.81 to 1 on a 3.0 ?

[droptheline]

Will a 1.81 to 1 gear ratio with the right prop work on a 3.0? motors an 88, alpha one gen 1 outdrive, i know it will fit but will it work??? thanks ahead of time!

 

[Don S]

Re: 1.81 to 1 on a 3.0 ?

There is no right prop for a 3.0L with a 1.81 outdrive. The 1.98 drive was designed for the 3.0L because they couldn't get the proper response out of prop changes alone.
If you had a V6, the 1.81 would be great, but you have an inline 4 cylinder. Lot smaller engine.

 

[45Auto]

Re: 1.81 to 1 on a 3.0 ?

I have a Mercruiser 2.5 in a 16' trihull that had a dead 1.98 upper and an aluminum 19 pitch prop. Replaced it with a 1.65 upper ($27 off Ebay) and a 16 pitch SS prop ($50 off Craigslist).

Performance is identical with either combination - 33 MPH at 4200 RPM, can't tell any difference in acceleration. Averages 5.9 miles per gallon over a 12 mile run to the lake.

It also runs 33 MPH with the 1.65 upper and 19 pitch prop. However acceleration is SLOW and it can't get past 3400 RPM. Also averages 5.9 MPG on the 12 mile run to the lake.

 

[freak007]

Re: 1.81 to 1 on a 3.0 ?

there is no "right prop" for the wrong gear ratio... I know from experiance...

 

[45Auto]

Re: 1.81 to 1 on a 3.0 ?

The 1.81 is a 10% "taller" gear ratio than the 1.98. Drop the prop pitch 10% and you won't be able to tell the difference from my experience.

 

[freak007]

Re: 1.81 to 1 on a 3.0 ?

The 1.81 is a 10% "taller" gear ratio than the 1.98. Drop the prop pitch 10% and you won't be able to tell the difference from my experience.

Let us agree to disagree

I have a 16.5" bowrider with a 120 merc. When I got it (unknown to me) it had a 1.81 upper in it. It also had a 15 1/2 x 21 prop on it... loaded to 2200 pounds it would pull 3600 RPM... it would also take about 2 minutes to come on plane...

We went to a 14 1/4 x 17 and loaded it to "normal" weight of 2500 +200 on a tube... It would take it a good minute to come up, and would only pull 3200... by dropping 1 1/4 in diameter and 4' in pitch we should have gained 1100-1200 RPMs... even with the extra load I should have been (able to) over revving like crazy...



I went to the correct 1.94 ratio and now pull 4100 RPMS @ 2500 pounds with the 14 1/4 x 17 and it is on plane in under 5 seconds... with or without a tuber...

 

[45Auto]

Re: 1.81 to 1 on a 3.0 ?

You'd think that if the ratios were that critical then Volvo and Merc would have to run the same ratios and prop pitches on their outdrives. I believe the Volvo SX uses a 2.17:1 on their 3.0 vs 1.94:1 for Merc, and Volvo uses 1.97:1 on the 4.3 while Merc uses 1.81:1.

Glad you found what works on your boat!

 

[wca_tim]

Re: 1.81 to 1 on a 3.0 ?

what pitch prop are you running now to get your rpm's right at wot? All you have to do is take that pitch, decrease it by 10 percent and see if you can get similar props. If you're running a 19 now, will be no problem, if you're running a 15, will be doable, but harder to find props in the 13 range.

as 45auto indicated, outdrive gear ratios are combined with engines based on the best changce of getting a cheap, easy to find prop that will work in the application. there are actually a lot of applications where the gear ratio supplied with the drive / engine combination is not the best.

As an example, i get better performance out of my small light boat at high speed running a 1.8 gear ratio behind a v-8 - why, because props are more efficient at higher speeds when they have pitches of 28"-30".

yes, for the average (whatever that is) situation with alpha drives v-8: 1.47; small v-8 / v-6: 1.65; v-6: 1.81; 4: 1.98 (sorry if I got decimal places wrong, tired tonight...). Note that this is for mass produced equipment to meet the typical use of the broadest base of the market according to the company engineers.,.. and sometimes has a historical bias that is not representative of the current times. In other words, it's what is done, but not necessarily what is the smartest thing to do in all situations, nor does it mean that if you change gear ratios by 10% you're going to lose much or any performance once propped right...

1.98 to 1.81 isn't s huge difference and you should be able to find props that'll work. while people talk a lot about props not being available outside of the 17-23 pitch range, it isn't that hard to find them, etc...

no offense intended, just my opinion and experience...

 

[wca_tim]

Re: 1.81 to 1 on a 3.0 ?

I suspect that when you went from 21 15 to 14 17, you were using a totally different prop design. Some props slip a lot more than others. Had you played with it a little, never know how well it might have worked.

Let us agree to disagree

I have a 16.5" bowrider with a 120 merc. When I got it (unknown to me) it had a 1.81 upper in it. It also had a 15 1/2 x 21 prop on it... loaded to 2200 pounds it would pull 3600 RPM... it would also take about 2 minutes to come on plane...

We went to a 14 1/4 x 17 and loaded it to "normal" weight of 2500 +200 on a tube... It would take it a good minute to come up, and would only pull 3200... by dropping 1 1/4 in diameter and 4' in pitch we should have gained 1100-1200 RPMs... even with the extra load I should have been (able to) over revving like crazy...



I went to the correct 1.94 ratio and now pull 4100 RPMS @ 2500 pounds with the 14 1/4 x 17 and it is on plane in under 5 seconds... with or without a tuber...

 

[45Auto]

Re: 1.81 to 1 on a 3.0 ?

I agree, WCA_Tim, there's no "magic" single gear ratio for a particular engine. Your experience agrees with what people on the "go-fast" forums report from playing with gear ratios. I don't think my 4 cylinder comes close to qualifying as a "go-fast" , it doesn't have enough performance to make much difference which gear/prop combo it's turning as long as something is spinning in the water enough to keep the engine from over-revving!

BAM Marine says the same thing on their website:

In my experience, a boat will run the fastest with a higher numerical drive ratio assuming you can find a prop with enough pitch to limit the engine to the desired RPM at full throttle. This does not take into account any handling or bow/stern lift issues.
The theory is that the faster a prop rotates the more power it takes just to turn it in the water. A slower turning prop with more pitch is more efficient than a faster turning prop with less pitch even though the calculation for theoretical speed is the same for both.
The downside to a higher reduction in the upper is that there is more torque on the lower gears, vertical shaft and prop shaft.

http://go-fast.com/sterndrive_gear_ratios.htm

 

[freak007]

Re: 1.81 to 1 on a 3.0 ?

I suspect that when you went from 21 15 to 14 17, you were using a totally different prop design. Some props slip a lot more than others. Had you played with it a little, never know how well it might have worked.

[HIJACK]You are absolutely right... As you were typing here, I was trying to figure out why I only gained 150 RPM when I dropped 4" of pitch and 3/4" in diameter... I retract my previous post and have started a new thread to help me choose the correct prop [/END HIJACK]

 

[wca_tim]

Re: 1.81 to 1 on a 3.0 ?

funny thing, when I swapped from a 4.3 to a 383 stroker in my current boat, doesn't really qualify as a go-fast boat in light of a lot of the folks I see on the river, but will hit 75+ with a 29" hydromotive prop on it and a little over 70 with a 27" mirage, but hooks up out of the hole better with the mirage (but planes at lower speed with the hydromotive to pull skiiers). I'm thinking a labbed bravo in 29 or 30 will beat them both...

sorry to ramble, my point was that I assumed I would be better off with a 1.5 gear ratio. I have a number of props in the right range and that worked for the same boat with the 1.81 and v-6. Guess what, none worked as well with the 383 and 1.47 gear ratio as the first prop I tried with the 383 and 1.81 gear ratio. Not even close in fact.

I found the explaination in an old magazine article comparing prop pitch and gear ratio in outboard set-up. The bottom line was that once you get to 50 or 60 mph, props with approximately pitch = 2x diameter yield the best balance between reducing blade surface speed through the water (and hence drag / power consumed) and keping the amount of force lost in the radial direction to a minimum. 60+ gear to hit pitch 2x diameter. for the most common 14-15 inch diameter props for pleasure boats affordable by the average joe, that's 28-30 pitch... and tere are a number of props available from 17 through 30 or even 34+ for just that reason.

I have also read that there may be an added benefit in terms of heat build up with the 1.81 gear, and will let you know now that I've got temperature guages installed to measure lube temp on the upper drive. The engineering books (yea, the ones with waaaay too much math in em for my taste) indicate that under a similar load a greater gear reduction spaces the same amount of torque over a greater amount of gear mating surface area and makes a little less heat both overall and especially on the wear surfaces of the gear teeth...

food for thought...