PDA

View Full Version : What drive ratio/rpm works better ???



Lenny
09-30-2005, 03:31 PM
Lets say we have a Bravo1 and 1:65's spinning a 25" prop, and THEN, lets say we have an equivalent leg spinning 1:50's and a (say) 23" prop. Given similar rpms, which is a better way to go or is there a difference?

My reasoning is, Wes's Bravo with 1:50's spinning a large prop and apparently it was faster with a taller prop and 1:65's ???

If you had a choice in an 18, and were buying new equipment, what would you buy? on a Bravo. 350 MAG MPI.

RickSE
09-30-2005, 03:59 PM
I was told once that the big pitch/big gear combo is more efficent, less slip=more speed. The down side was that the thrust forces on the drive go up with the big pitch. This guy was mainly refering to the 1.8 ratio.

A lot of high altitude stuff here comes with the 1.6 ratio so some guys would take it a step further up to 1.8.

Lenny
09-30-2005, 04:00 PM
...also, this will be a "Sea Level" argument. :D

txtaz
09-30-2005, 04:40 PM
Lenny,
My boat did 74.5 with a 1.65 and 25 Mirage+ at sea level according to MrX. Now it has 1.5 and a 23 Mirage+ and does around 68 at 1000ft. I'm not sure if a gear change, altitude and pitch can account for 6 mph but it does handle and accelerate better with the 23. If I ever decide to put it in salt water (more dense air and water) I'm sure the higher gears and pitch would be better.
Wes

Ranman
09-30-2005, 04:41 PM
It's my understanding that it is more efficient to spin a big prop slower than it is to spin a small prop faster. I'm a 350 MAG with 1.65:1 Bravo and a 25P Mirage+. I've never been outrun by another STOCK 350 MAG. I'm typically 1-2MPH faster than the new MAG 1.50's (Alpha or bravo). In my case, I think it's my oddball MAG motor and the 1.65 gear that does it.


If you had a choice in an 18, and were buying new equipment, what would you buy? on a Bravo. 350 MAG MPI.

1.65:1 with a 25 pitch whatever :biggrin.:

BaldEagle
09-30-2005, 05:08 PM
My new 18C has a 350 MPI with Bravo-1 and 25P Mirage. It runs well I'm told. With 17 hrs I ran it up in flat seas and scored 65.7 Gps at 4900 rpm @ 600'ASL. Its my understanding it takes about 50 hrs to finish break in and it will get the best top speed in 6-12" waves.
I thnk Last Tango has a 350MPI with 1.5 Bravo-1 and 23P Mirage. He's pleased with the way his runs at 0'ASL. He also runs headers.
BE

smokediver
09-30-2005, 05:16 PM
mark has a 6.2 in his boat . I chose to run a 1.62 in my lower when I changed power . I went from a 1.81 that I had originally . I did that in order to run the same prop that I had . 25 laser2 . I am able to turn that to 4900 rpm 63 mph about 12% slip .

Mr X
09-30-2005, 05:23 PM
My new 18C has a 350 MPI with Bravo-1 and 25P Mirage. It runs well I'm told. With 17 hrs I ran it up in flat seas and scored 65.7 Gps at 4900 rpm @ 600'ASL. Its my understanding it takes about 50 hrs to finish break in and it will get the best top speed in 6-12" waves.
I thnk Last Tango has a 350MPI with 1.5 Bravo-1 and 23P Mirage. He's pleased with the way his runs at 0'ASL. He also runs headers.
BE

Nope, it's a 6.2