Take a look at All about drive ratios and see if it helps any.
We are still dialing in my boat and I need some clarification and some assistance in understanding the advantages and disadvantages of the 2 gear ratios.
I am running 1:36 gears in my xr drive now. We have gotten best speeds to date out of a 32 labbed bravo I, 95mph at 5100 rpms. So it looks like our orgional thought of going with the 1:36 gears because we would run out of props was wrong.
What advantages or distadvantages are there running 1:36 gears over the 1:5's. I have heard that the lowers on the 1:36 are stronger because less teeth to break, but I have also heard that they maybe slower than the 1:5, and the 1:36 do not have as good of throttle response? So hopefully some of my fellow oso brothers have already been through all this and can share your experiences and some of those who are knowledgeable on the topic.
I think that the boat has the potential to be faster than it is and I am looking at the gears/drives as one of the places that is robbing power. I am wondering if the lowers on the xr's are dragging to much, the gears slowing us down, etc....
Last edited by thunderdan; 06-13-2002 at 07:33 AM.
Thunderdan, I think it will come down to personal preference. I had a 26' Thoroughbred with a 765 HP blower motor. I originally started out with 1.50 gears and a 28" Hydromotive. I was turning 5800-5900 RPM. I saw radar speeds of 91-93 MPH. I really wanted to slow the RPM's down because of the cam size. So when I went to 1.36 gears with the same prop size I then turned 5300-5400 RPM and saw about the same speed, maybe 1-2 MPH faster. It does seem to take more power to turn the 1.36's but if you have a strong engine you won't notice much of a difference. It also seemed to take a little longer to get stretched out on top end, but it wasn't much difference. For me personally I liked the 1.36's for the reduced RPM, and for a faster cruise because I was able to turn the same size prop. As far as the strength, because of less teeth there is more "meat" on the 1.36's. If you have the 1.50's readily available I would test them and compare. As you know all boats, props, engines etc. like different combinations, and setups!! Hope this helps.
Last edited by Hessdk; 06-13-2002 at 08:59 AM.
Depends on where your motor likes to run, how its cammed, etc. You will accelerate harder with a 1.5 over the 1.36. I know several people who have tried both and generally agree with the above statement. Typically people go to a 1.36 because they run out of prop or want to pull down the r's, like blower guys who want to turn 5400.
the other? I they both require about the same amount of HP to turn the same RPM. The key is the prop. Yes, if you keep the same prop and go to a 1.5 ratio there will be an RPM increase in the motor, but is the prop turning more RPM? That is what determines how fast you are going. Let's say you turn a 32" 5100 through a 1.36 gear and go 91 MPH. If you change gears to a 1.5 ratio you would have to spin the motor 5625 RPM to turn the prop the same RPM as the 1.36 at 5100. Would there be a speed increase? It depends on where your motor is designed to run. If you keep the same prop and go to a 1.5 ratio and the motor turns it 5900 RPM because it is getting into a "happier" point in the power band, then it will be going faster. Here is the thing. You can do the same thing with a prop change. Drop down to a 30 or maybe a 30 lab and see what happens. Again there will be an increase in RPM, and if it moves the motor into its power band there will probably be a speed increase. In my opinion the gear ratio chosen is personal preferance. If you are running a 1.5 ratio and turnung a 36" bravo 1 prop to the max of your RPM limit, switch to a 1.36. This will give you the ability to use a lot more props. There are tons of props out there in the 26 through 32 pitch range and if you can't use them you are going to be limited to what is available. Just my opinion ans usual. Good testing!!!
the 1:50 is generally faster. we used to use the 1:36 for rough races because they would not break as much. that was before the creation of the xz and xr. a 1:50 xr is a pretty tough drive. if your racing sombody it's not all about top end. you also need accleration. as posted in the previous post you will get with a 1:50. also yes, you went to a 1:36 if there was no prop left. now they make large enough props now you don't need them.
I personally like a 1.5 ratio and a motor that can sing 6000 plus rpm. Awesome acceleration and if the motor can pull the R's the top end will be good too. On my Baja I turn a 29" Spinelli 6300 RPM for a top end of 97.8 on GPS. The top end is nice, but the acceleration is the real thrill. When crusing at 3000 RPM the R's go instantly to 5500 when the hammer drops. That surge of power is worth a few MPH on top to me. Probably not the best for endurance racing though.
Parasitic power loss through a drive comes primarily from one source: friction. There is friction at every seal, bearing, and gear face. At any speed above "motionless", friction increases with speed (there are odd applications where this does not occur, but for our purposes we can pretend this is fact).
With this firmly in hand, we can then assume that the numerically higher a drive ratio is, the slwer the output section will spin in relation to the input section. Theoretically, this would mean that a 1.36:1 ratio would require more hp to spin than a 1.50:1 which would require more than a 1.65:1 which would require more than a 2:1 and so on... The actual measurement of this power loss is up for debate.
I think a much more important factor in making a ratio decision is the available propellors on the market. While it is a comforting concept to believe that a 26" Brand "A" Style "Q" prop behaves exactly as a 28" or 30" pitch Brand "A" Style "Q" prop, but at different rpm, this is not fact at all. The dynamic nature of water is such that each prop pitch has a range of "sweet spot" in regards to rpm, hull efficiency, and vessel weight that makes it either a better or worse choice for a particular application. For example, on a particular hull and power combination, a 28" pitch "A/Q" at 5400 rpm may indeed be a faster and better choice than a 30" "A/Q" at 5225 rpm.
It truly gets confusing and science can not do the testing for us.
I have a 1991 F311SR1 with 502 Carb motors. I have 1.36:1 ratios. My boat has ALWAYS been faster than two identical 1990 F311SR1 boats equipped identically, but with 1.50:1 drives. The difference is less than 2 mph, but more than 1 mph. My boat was also exactly the same speed as a 1992 F292SR1 with identical power and 1.50:1 drives. Granted, I have never swapped drives for true comparison, but since I have blown 3 1.36:1 drives through the years, I have never been inclined to try the slightly-weaker 1.50s.
I hear that 1.50s give better acceleration and top speed but I can't understand why they would do so. If you use the same prop, then the acceleration component is a given. If you prop up for parity, then I would imagine that the edge would go to the unit that had the more efficient prop for the circumstances.
I used to dragrace an outboard. I had a 1.87:1 ratio and found a 26" pitch prop that worked really well. It worked better than a 24" pitch prop that I also had, but the motor wasn't turning up enough at the end of a run. I tried 22's 24's 26's and 28's. I had a sweet 28, but it was far too much prop for a qtr mile. Bottom line was that I could not find a way to make any 24 run like the 26 I had. I used the 26 in anything but glass water (where the 24 had an edge). When I grenaded the gears, I decided to go to a 2:1 ratio for an experiment. This put my 26 right in the sweet spot. Heaven (scary heaven, that is..). A fast mother. Could actually run with the 28 if I wanted to, but the 26 was the beauty. I dogeared it on a log in testing one day and was afraid I would never get it back right, but a rework brought it back exactly as it had been (luck, I'm telling you). This story is for mo other reason than to point out that sometimes a particular prop is the magic answer for a hull/power combination, and that you may even gear to suit a particular prop, as opposed to the other way around.
As always, testing on your boat is the only way to know for sure.
I see London, I see France...
Thanks for all the responses, we are trying a non labbed 30 bravo i, if the rain in florida stops. After that I am hoping to swap the lowers and try some 32's to see if there is a reaql difference. I will let you all know.
I am willing to give up a bit of throttle response for a little stronger drive.
Dan, this is a little off thread topic, but what were your findings when testing the 5 blade props??? Any info would be appreciated. My understanding on the strength issues involving the 1.36 vs 1.5 is when utilizing the 1.36 you are turning a smaller pitch prop, therefore decreasing load on said drive. Good luck and have fun. Thanks much.
Last edited by Neverfastenuf; 06-13-2002 at 04:05 PM.
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