Changed gears 181 to 147 Which prop

"On a Merc 170 19ft thompson

"On a Merc 170 19ft thompson - Changed gears 1.81 to 1.47 -Which prop? I was running a 19p with the old 1.81s but went to some 1.47 gears when I got a deal on an old 888 uppper. Now the boat dies when hitting the gas to get out of the hole, or has no power...your thoughts if a 15p prop would solve the problem or am i in trouble? Thanks in advance and please be nice to the neebie (great site guys!!)"

"assuming the following and us

"assuming the following and using this calc
http://www.rbbi.com/folders/prop/propcalc.htm

Engine RPM 4000
Gear Ratio 1.81
Propeller Pitch 19
Propeller Slip 10%
= Speed MPH 35.8

Engine RPM 4000
Gear Ratio 1.47
Propeller Pitch 15.44p
Propeller Slip 10%
= Speed MPH 35.8"

"I went through a similar issu

"I went through a similar issue, going from a 1.65r drive to a 1.98 drive, this is what I came up with using the same calculations

http://kainonmedia.com/image003.jpg""

"Thanks Brian, Sounds like I n

"Thanks Brian, Sounds like I need the 15p prop then. I needed a new prop anyways and the deal on the used 888 1.47 was just too good to pass up. Happy Easter.

Anyone else have any thoughts?"

"A ratio change of 1.81:1 to 1

"A ratio change of 1.81:1 to 1.47:1 is a large change.
The factory and their engineers have spent countless hours designing and developing engine and drive ratio combinations for these boats. Only in a few limited scenarios can we take it upon ourselves to make arbitrary changes to an over-all gear ratio.
Compensating via changing to a different propeller size/pitch is not the answer in many cases.
While we can UP tire diameter on our trucks to raise a gear ratio, this does not work in a hydro-propulsion scenario.
Good luck."

Thanks for your input Ricardo.

Thanks for your input Ricardo.

Why though when you run bigger tires on your truck you want to put taller gears in to bring back torque? Its not the same in boats?

"I dont agree that it's a

"I dont agree that it's a large change, 1.98 to 1.47 yes. But I do beleive going from a 1.81 to 1.47 is going to make it harder on the engine

It will be interesting result.

I wouldn't use the 1.47 longer than you can, try to find some 1.8 gears for it, you may only need to change 1 gear, I haven't looked yet, and dont remember the number of teeth per gear required for that drive the information is available on the net and in mercruiser drive manuals.

I will tell you going from a 1.65r 19p to a 1.98r 23p prop that my boat accellerated harder and quicker, But I couldn't tell the difference with a 1.65r/19p and a 1.98r/23p ( didn't have an accellerometer and GPS at the time. )"

"Quote: [b]"Why though whe

"Quote: "Why though when you run bigger tires on your truck you want to put taller gears in to bring back torque? Its not the same in boats?" unquote.
I'm not an expert in this area, but have been doing this for enough years to know that only in very limited and only in certain conditions does this work out well.
Hydro-Propulsion is quite different from a rolling piece of rubber on asphalt.

Quote: "I dont agree that it's a large change, 1.98 to 1.47 yes. But I do beleive going from a 1.81 to 1.47 is going to make it harder on the engine"
Brian, a 1.98:1 to 1.47:1 is a greater change than from a 1.81:1 to 1.47:1 .... I'd agree with you there.
The point of the car/truck tires was just an example of how a tire diameter increase could offset a gear ratio and we'd probably not know the difference..... but prop change to accommodate an incorrect ratio seldom works.


Again, these guys spent years and many $$$ to figure this stuff out so we wouldn't have to!

Just to be clear, there are conditions where this works..... but ya gotta know just what to do. The smaller and lighter the boat, the more you could get away with, I suppose.

"

"And a couple LONG TIME mercru

"And a couple LONG TIME mercruiser techs said dont use a reverse prop on a std rot drive, but I did, just had to use the shifter like my Gravely Tractor, forward was reverse and reverse was forward, I used it maybe 10 times before I was able to replace the prop, it worked great !! that was the 1.98r, when I went back to the 1.65r uppers I had my drive guy checked the gears later and they were fine, you run your boat in rev dont you? well Ok.. not for long periods or 4K rpm, just think 4 times longer than 1k rpm & backing down.

I agree with you Ricardo it's not a good idea to use the 1.46 vs the 1.8 and understand the big/small tire diff, I've actually was forced to run 255(free) vs 265 tires on my truck showing 5mpg slow on the speedo, and yes i'm paying more in gas over the long run, but you gotta do what you gotta do to get by, 2 yrs later I'm still using them now with some bail-out money from Obama I can buy some new ones,

Granted if he was to use the drive long term gas consumption will go up and more strain on the bearings, gears(it is an 888 drive though, designed for a V8), coupler, crank and bearings, but how much? 10%. Yea It will depend on the weight of the boat overall and how much he keeps the throttle burried.

but to get the guy boating and if he has cheap and easy access to an $80 prop to get boating for a couple months till he can get the $200 gears swapped out and $400 for someone to re-shim the drive?

He might not do to bad. If I had a set of gears sitting around I'd send them to him, at least for $20

And conditions where this does or doesn't work, this may be where it works fine at least for the short term.

Yes, it's not a good idea but, as long as he's not in danger of hurting himself or others and if it's just short term, hopefully not too much harm.

We did our thing, which is why we are here right? giving opinions, trying to help others?

I wish I had someone to tell me "Don't use that OLD Crank Gear that was used with a Fiber Cam gear with that NEW Cam gear, it will be junk in months"..

guess what they were/wouldn've been right.

Well the cam was from a '74 Nova anyway.. and the engine block. ( I didn't change it, but I did partially blueprint and fully balance it, inlines don't need to race balanced by the way and not to 1/10th or 2/10th's of a gram, yea it's smooth)

Safe Boating!!!"

"Brian, You and I a both long

"Brian, You and I a both long winded..... we should be having this discussion over a cup of coffee! LOL
The Wright Brothers would have never flown had they not done some thinking outside of the Box!

But IMO, I believe that you were very lucky to have gotten by with running an incorrect hand prop on a standard Alpha drive with the intent of FWD propulsion......... and here is why:

Unlike the Volvo Penta single prop lower unit with one drive gear/one driven gear (pinion and ring gear in the automotive world)...... the A drive has one drive gear and two "driven gears" in the lower unit (allowing for FWD and REV).
It also has an entirely different style of bearing for the FWD "driven gear" (of which is the fwd propulsion gear) with regard to Bearing Thrust Control from an engineering and design point.
When you run the opposite hand prop on a A drive reverse gear (that is intended for the factory prop), this thrust surface is entirely too light for the load... nor does it place the thrust load on the correct component.

If that doesn't make sense to you, look at a cut-a-way view of one.
This is why M/C will suggest that the correct "Counter-Rotation" drive be used in a twin engine scenario.

Interestingly enough, here is an old post on this. According to sterndrive.cc/vbforum, "The reverse gear in your standard lower unit is very weak and does not have any thrust support."
https://www.sterndrive.cc/vbforum/showthread.php?t=148

Now contrary to that......, the Volvo Penta AQ series drive/driven gears are designed to operate either hand prop ....... and the thrust bearing supports the shaft, and as a result, the one single driven gear.
(This large "thrust bearing" is a double row ball bearing and it checks thrust via being positioned on the shaft, not on the actual gear.)


The Mercruiser B II drive does this in a similar fashion, I believe.

Anyway.... we're getting side tracked here so I am going to leave this one alone.

Good luck on the ratio change... I hope you can make it work.

"

"pre-alpha actually, yea. i di

"pre-alpha actually, yea. i did get lucky."

"thanks for the comments guys,

"thanks for the comments guys, Anyone have a 15p prop for sale?"

laughing at you guys while you

laughing at you guys while you take a breath. but one always does his engineering on what will work in most case its done with physics but when it comes to boating nobody anywhere says anything about what to use/try/ to get there craft or truck or lawnmower or washmachine to work just to get them by for just a short time. i say if it works then then do it if it dont then its lesson learned !! P.S il have another cup of coffe