What gear ratio for a boat?

[SS-310]

Im building a boat that has 2 screws with an engine for each and cant get the right gear ratio for good fuel economy and moderate speed. Best i have so far is 9:5 at 6 m/s

[Ra-Ra-Rasputin]

Depends on the boat, the engine setup, the situation, the drive setup, the weather and everything else. There is absolutely no way to answer your question. For some boats it's 1:3, for some boats it's 1:6, and for others it's anything between 1:1 and 1:9.

The only general rule of thumb is that you want the engine to be around 6-8rps for economy, and 8-12 for speed.

[Booze_Rooster]

Alright, so here's my bread and butter.

If some of this is already known to you, just skip it.

Gear ratios indicate how many revolutions one side will do relative to the other. Your 9:5 box if oriented that way means for every nine revolutions of the engine, the prop turns five times. This is a rather inefficient setup unless you are looking for high torque and slow speed.

Gearboxes can be oriented either towards the engine or away from your engine. That arrow will indicate whether your 9:5 is a 5:9.

Gearboxes that point towards the engine will LESSEN the load on it by increasing the torque. Short gear ratios are good for getting a large boat moving, and once a certain speed and rps is reached, a microcontroller can be used to disengage the clutch, change to the next ratio, reengage the clutch- all without you having to do more than hang onto the helm.

Multiple gearboxes in mixed orientations can achieve better results. A single gearbox is limited to two ratios. Adding three or four boxes adds more potential to get maximum potential from your engine.

I'm no programmer, nor am I a computer whiz. What I'm good at is spilling oil, skinning knuckles and crossthreading bolts before road testing a hooptie with a dixie plate and hoping the local cop on duty is on the other side of town.

I had a twin engine, triple screw boat. I wanted to optimize my gear ratios. I plopped three gearboxes down between my clutches and my prop shafts.

I set each gearboxbox up with a temporary bank of instrument panel switches.

I then went in logical order. I set the OFF value of each box to 1:1. Spawned my creation and took it out of the dock. I recorded RPM (oh yeah. That's how we do engines) and speed in knots (because that's how we do boats) I pulled my trusty legal pad over and recorded my numbers in a chart.

1:1 across the board, 6 knots, 1200 RPM- bangin' the limiter the whole way.

Then I came to a dead stop and engaged the first box on ratio at the next setting. I think it is 6:5, but the box was pointed at the engine so my next recording was something like

5:6 - 10kn, 1000 RPM

continue in this fashion and add the ratios in sequential order in the ON ratio slots of your boxes. As you find good results, start adding the good ratios like 6:5 as the OFF ratios in the later gearboxes and start filling the ON ratio slots with new untried gear ratios.

Basically test them in order with multiple gearboxes and as you find positive results, start stacking your gears into the boxes. Plot the results on paper and you get a rudimentary power curve. THE POINT is to decrease engine RPS and increase propeller RPM. If you start in that tall gear, the engine will stall or bog, meaning slow acceleration. If you find a pathway through the gears, you can start in 1:1 or even less than 1:1 to get your boat or ship moving...and then finally when at speed, you are in a cruise gear that will maximize fuel consumption at speed.

You can pinpoint where you are no longer gaining performance and at that RPM (or RPS) you can have your Microcontroller clutch, bang the next gear and unclutch.

it takes awhile, but you will begin to get a feel for what gears your boat needs. Engine choice is nothing next to gears. Set your gears up right and control them efficiently and you can make something that doesn't drink fuel and holds speed.

That's why my buddy's 89 K5 Blazer on 35's with a 350cu V8 cant do more than 50 mph on the road with 4:10 gears in the diffs and a single factory transfer case and has to skinny pedal up everything on the trail, and my 86 Toyota pickup can hit 80 mph on the highway with a 4 cylinder, 37's, 4:88 in the diffs and two transfer cases bolted together that reduce the factory 2.28:1 through a second case regeared to 4.70:1. At idle it will crawl so slowly a speedometer radar won't pick it up, but will drive itself up a 30 degree incline. Gearing is most definitely a replacement for displacement.