Need info on 1.36 and 1.65 Drive

[BacDoc]

Hi, I have a 22'Classic with a X 500 engine from Marine power. A local machanic is trying to sell me his 1.36 drive off his Valocity he's parting out(his engine has 1100 hp ). From a little research I think my set up would be better with the 1.65 drive. He also says he would like to put a small blower on my engine. Is the 1.36 the way to go if your getting a lot of HP with the blower and the 1.65 with the stock X500. Thanks for any help out there.
Dan

[Woodsy]

Dan...

First off, I would tend to shy away from any Bravo drive that came off the back of an 1100HP motor....

To figure out what ratio you need, first you have to know at what rpm the new motor makes its best hp. Now if this motor is an knockoff of the Merc HP500, your looking at spinning it up to 5000 -5200RPM or so. Right around 500HP at 5000RPM.

The 1:65:1 drive is eliminated almost immeadiately, as it will spin the propeller too slowly. This gear ratio usually allows a boat down on HP to spin a larger prop slowly, giving it better speed numbers... RanMan's 18 Classic is a perfect example. He has a 350MAG/300HP 1.65 drive and can spin a 25P Mirage+, a standard 18 with a 1.5 drive can only spin a 23P with the same power.

I would start with your standard 1.5:1 drive, and see what props it will spin. I think the HP500's came from the factory with a 29P Mirage Plus. I could be wrong there. One of the others I am sure will have that answer...

If you go the blown route, the 1.36:1 drive might be the better way, because you will have the HP to spin the prop alot faster. The slip on the prop goes up somewhat, but it is overcome by the increased prop rpm. George Cullotta has a blown HP500, approx 750HP or so, and he runs a 1.36 drive spinning a 27P Mirage+.

Anyway, I hope that helps...

Woodsy :D

[MOP]

Woodsy the 1.36 is stronger than the 1.65 due to the closer gear size match, would this not allow you to run lesser pitched props and get less slip and drive torque lift depending on rotation? Trying to learn more and reading to much.

Phil

[Woodsy]

Phil...

All things being equal, no they are not "stronger" gears. Gear strength for the most part is a function of metallurgy and tooth design. The difference in tooth profile between the different gear ratios is minimal at best. A Bravo 1 drive with a standard 1.36:1 gearset is not any stronger than a 1.5:1 gearset, a 1.65:1 gearset or a 2.0:1 gearset. The difference between them is how fast they spin the propeller at a given engine rpm.

In a simplistic world, the slower you spin a propeller the more efficient it is. You can't have zero slip as water is a liquid, and a rather forgiving medium at slow propeller speeds. Now effeciency is measured in slip percentage. The lower the slip percentage the better the prop is working.

Now, what you need to know is that essentially the sterndrive is a big gear reducer. The ratios determine how much of a reduction. For example, at 5000 RPM (Engine speed) a 2.0:1 gearset will give you 2500 propshaft RPM (PSRPM). A 1.65:1 gearset will give you 3030 PSRPM, a 1.50:1 gearset will give you 3333 PSRPM, and a 1.36:1 gearset will give you 3676 PSRPM.

What does this mean? You kinda had it backwards. A boat down on HP, can spin a BIGGER prop more slowly and thus effeciently and gain some speed. Ranman's 18 has a 350MAG/300HP and spins a 25P Mirage+ thru a 1.65:1 drive. He sees speeds darn close to 70 MPH. A standard 18 with the same engine and a 1.5:1 drive can only spin a 23P Mirage+, and sees low-mid 60's.

Now here comes the fun part... What alot of the high performance types are doing, and it works is spinning a smaller pitch prop faster. They do this by using the 1.36:1 gearset. thats 300 PSRPM over the standard 1.5:1 drive.Thats alot of extra propeller RPM. The key to this setup working is having the engine HP to pull that extra 300 RPM.

Now in BacDoc's case, he is close to the factory HP500 setup, so most of the guesswork has been done. If he decides to put a blower on his engine, HP600 could probably help him out on gear selection & prop pitch.

Woodsy

[BacDoc]

Thanks Woodsy for the info. Now I know why my machanic keeps saying my engine is crying for a small blower. I think that my 500hp engine with a blower getting in the 700hp range whould take the drive with the 1.36 gears. At this time I don't know if I'm ready for that kind of HP or speed ( I know,I know you can't ever have TOO MUCH HP or SPEED ) What about a 2" shorty drive. Does the Donzi HP500 have just a stock Bravo1 drive with the 1.5 gears? Maybe all I need is a bigger prop. I'm running a 25p Mirage and going maybe 73mph (on my speedo not gps) I've heard people on this board getting that with 454's. HP600 what do you think?? Or anybody
Thanks
Dan

[BigGrizzly]

Let me jump in here. I have a blown 502 with 690+ hp with 700 ft lbs tq with a closed cooling system and is varifiable with dyno sheets. I have a TRS drive with a 1.5 ratio and spin a 28 Turbo lightning at 5300+ rpm at 86 MPH. My motor is not a stock motor with a blower, but a modified one. The sister of this motor has broken 11 bravo drives in one summer both 1.5 and 1.36. What we discovered was the 1.36 is infact weaker maby due to the load imposed on it with a deep drive, it lives about the same with a shortie drive due to less bite but does spin an inch to 2 more pitch. The only reason we down here use the 1.36 is some boats on this lake can't get big enough props that works. after all this I would go with the 1.5 ratio. Now let me throw in durability I have well over 200 hours on my engine and and still doesn't use any more oil than a stock 502 unit.

[RedDog]

Hey Big Griz - just to be sure another variable is discussed / considered, if YOU were the driver of the Bravo boat that busted 11 times, would it have busted any / some with you as the pilot?

[Rootsy]

Griz,

what was the failure mode of those bravos? those torque numbers are kinda beefy for that drive, especialyl if it's pushing weight.. what boat was in front of it...

[Walt. H.]

Simplifying the ratio numbers for the application:

1.84:1 ratio for a V-6 engine :(

1.5:1 for a small block :p

1.36:1 ratio for a big block :D

1.65:1 for a small block pushing a heavy boat :(

W.H :)

[terry]

Well that sure makes it easy for everyone!

[BigGrizzly]

Red Dog I was not the driver of the boat. the engine was in a 24 Python that is the same weigh as the 22 clasic. The python is a knock off of a knock off of the clasic hull with some length added.

Rootsy, there were three different causes one was the upper bearing and gears another was the vertical shaft and the third i sad was rhe lower gears came out of rhe side of the foot. than they put ImcoS on it and the vertical shaft and bearings were letting go. My info says the first were new Merc drives , not rebuilt. I don't know who put the rest of the drives together. My dyno sheet puts the torque a 700 ftlbs at 3977 rpms and dropps to 649 at 5200 rpms. the engine has about 500 ar 3100. These motors were Procharged, mine is the only closed cooling system one. Last I heard of that engine it was pulled out of the Python and put in a HTM with added 2 psi of boost and was headded to Floridia

[DonCig]

I have read elsewhere that the Bravo 1 drive was built to handle maximum engine rpm's in the low 5,000 range and that higher engine rpm's will lead to a shorter life expectancy for this drive. How much of this statement is truth? Is there a design limit on the Bravo 1's as it relates to engine rpm's?

Thanks,

Don

[Rootsy]

the 1.36 appears to be "stronger" but in reality it may totally NOT be accurate.. sure gear strength ultimately comes from the tooth contact, material, gear tooth design (helical, spur, etc), thickness, etc... shaft strength comes from diameter, material, stress risers, machined features, deflection, etc... but what it really boils down to...

a gear ratio is a torque multiplier. reducing the shaft speed increases the torque output relative to a given torque and shaft speed input. THEREfore with a 1.36:1 you are not loading the gears and shafts as highly for a given RPM and torque input as you would be on a 1.65:1... whala...

THUS with less torque output at the propshaft you cannot spin as much pitch therefore you must decrease prop pitch and increase rpm... the difference between a 1.65, 1.5 and 1.36 is somewhat proportional as far as selecting a prop pitch amongst them. all things being equal you will be at nearly the same speed with each gear ratio, if propped correctly to achieve the same motor rpm. now different pitches will act a bit differently and give better or worse overall efficiencies in terms of slip...

adding to this a bit more, it takes a certain amount of thrust to move a given load a certain speed. when you reach your top rpm... your thrust at the prop is equal to the load of the hull moving through the water... hence no more acceleration... gotta love Mr. Newton... this is a function of torque output and rpm. the smaller pitch with the higher numerical gear ratio will spin more rpm at the propshaft.

to be concerned with rpms at the sterndrive is somewhat subjective... outboard guys spin em 9000+ rpms... i spin my alpha SS 5300 - 6000 depending upon what prop i have on for testing, etc. my gen II has seen 5400... some mercury racing motors which run bravo's run at higher rpms in the mid 5's...

the break down... gear and shaft speed at a certain torque translate into a horsepower number... torque gets you there.. hp keeps you there. run good gear lube, check and replace it regularly and don't get it too hot and you shouldn't have much of an issue... lastly... if you pop it out of the water throttle back... cause shockloading gears and shafts breaks things in a hurry... ouch...

Root -