Gear ratio / direct drive / prop speed
 

Hi All,

Been lurking and reading here for a few months now and learning a great deal. Currently roughing out some ideas on my first go fast boat.

Specifically, I'm interested in gear ratios for surface drives. Most of the discussion here seems to be about Bravo's with a 1.5 gear. If I run an Arneson can the prop be run through a direct drive transmission at a 1:1 ratio?

A 1.5 geared Bravo will get you a prop speed of 4000rpm with an engine speed of 6000rpm. Obviously with a direct drive transmission it's 6000 for 6000.

If we do some simple math (ignore slip for now) we see that a 26.4" pitch prop at 4000 prop rpm gives us 100 MPH. A 17.6" pitch prop at 6000 prop rpm also gives us 100 MPH. I think I did the math right.

Is there a prop rpm limit? I see where those little cracker box racing boats have direct coupled props to the engine and they seem to work. So what gives?

This is a complex question ,and has a lot to do with boat weight and what sort boat ,example a F1 catamaran with an S3000 outboard engine is running 10.000 engine rpm,s ,and has a 16:17 ratio or a 15:17 something like that ,dont remember exactly ,but close to 1:1 ,this propellor is very small like 10 inch diameter or so ,it doesn,t have to be big cause its only pushing 1000-1100 LBS boat weight ,at this rpm ,the propellor can,t be much bigger in diameter cause it will shear blades ,and will have such an high speed at the tip of the blade that it will be inefficient .
pitch will be around 16/17/18 inch.

Now if you put this type propulsion on a heavy boat ,it will not work properly ,and overall performance would be bad cause the prop would not moving enough water ,thats why we see bigger diameter props when the boats are getting bigger .

Though I know this is just 1 example , but I have owned a Schiada 21 river cruiser V drive ,I had a 38% step up in the V drive gears ,and my engine was turning 5600 rpm,s ,so propshaft speed was aprox 7500 rpm,s (long time ago don,t remember exactly) this prop was also very small in diameter 11 1/2 inch and 3 blade ,and believe it was 15 pitch.

I don,t think this is the correct answer/examples to this question,maybe others can chime in .

It all depends upon what you are doing for power and boat hull and the application of course.

There are in line Arnesons that are 1:1 and there are reduction ratio Arnesons that could be between 1:1 to 1.63:1 depending upon the application.

Stirling,

I realize it's a complex question and I over simplified. Just starting to get my head around this stuff.

The project in question is for a 26 foot V bottom weighing about 5000 pounds. It will have a pair of LS engines at 450 HP each with Arneson surface drives.

I'm looking at transmission options right now and deciding whether to go with a direct drive or a reduction box in the driveline. If I do go with a reduction box it would most likely be a 1.5 as that seems to be where most Bravo's are and would afford the greatest number of prop options.

Thoughts?

Rik,

You answered as I was writing. The basic data is in my previous post. The application or use is simply a Hot Rod boat. I'm an ex-race car guy now getting into boats. I live in Texas and will probably see a 50/50 split between lakes and the gulf.

Pm me the boat hull particulars and I have software in the office that will give very close predictions, and can simulate how different combinations will behave. In general, you are going to want some gear reduction in order to swing a big enough prop to get the required thrust. Too small is inefficient due to high slippage and cavitation. Too big is bad due to extra drag. Arneson with no reduction would work with a diesel. It takes a ton of horsepower to wind a prop up to 6000 rpm.

Even I'm looking at transmission thing right now and deciding whether to go with a direct drive/reduction box in the drive line. If we go with reduction box it would be a 1.5 as they are the great number of props,and yeah there is some of the horsepower limit.
used cars

With a pair of small blocks, naturally aspirated use a 1.5:1 ratio.

Thanks.