Who has testing results with outdrive gear ratios?
 

I am looking for some accurate info. I am wanting to know who has done this test.
Has anyone ran a 1.36 drive with a certain model of prop at a certain rpm. Then swithced to a 1.5 drive and use that same model of prop. The only difference in the props would be the pitch so that both drives are running the same rpm. On top of that. Was the test done back to back within the same afternoon.
I realize that this is a pipe dream test.
Tell me your story.
Did you gain or loose speed going from 1.36 to 1.5?

Mike,

If it helps, I went the other way, from a 1.50 to 1.35. Same props, speed increase of app. 2 MPH at top end but significant gain at various cruise speeds. I swtiched gear ratio as I was hitting the limmiter with the 1.50 and 36 inch on the center drive, 35 inch on outers. (Triple engine, app. 725 each, Bravo XR drives, 4 blade labbed Mercury props)

Basically the same amount of fuel and other weight on board for each test, and similar weather in terms of temp, dew point and air density, but not run back to back on same day.

Will be trying 5 blade hydromotives this weekend. 1/4 inch larger diameter, but 3 inch less pitch. Currently turning about 5350 with the Bravos (rev limiter is set at 5600-reprogrammed ECU from Whipple)

Will share data after the test runs.

Take care,

Steve

Steve, I would guess some of the speed increase was because of you hitting the rev limiter on the center engine with the 1.5.
I have found that all of the Bravos I have tested and owned are really not 15.25" in dia(as advertised). They are around 15.1 to 15.125". I asked Dennis from Mercy. and he agreed.
What was your top speed with the 1.35's. I have been told the 34 and up are not very eff. Do you know your slip?
I am very interested in your new test with the 5 hydros.

Mike, This is an ongoing discussion. Which prop is more efficient a 28" or 30"+. The arguement is which is better a 28" with 1.36 or 30" with 1.5.

Mike - good question. A friend of mine switched from 23 @ 1.36 to 25 @ 1.5 on a 1989 35 Fountain. At the same engine RPM they went faster, spinning the 25 slower. I think this must be due to prop efficiency. I've also heard that Formula used 1.65 gears and more pitch.

SteveDavid - I read on Hydromotive's website that they have a new round ear Quad IV for boats that need more bowlift. (More like a bravo one prop?) Have you tried these?

Hi Guys,

The round eared (4 blade) Hydromotives work fine once on plane but cavitate significantly getting there (Probably due to the weight of my boat) However, top end was the same with the Hydro's as the Merc 4 blades. The 5 blade Hydro's we're testing this weekend are similar in blade shape to the conventional Hydromotive prop. I'll know how they work after this Sunday.

As to slip with my current Bravo set up, it goes from app. 15% at 80 MPH to almost 25% at 97 MPH.

Good info on this thread and the similar one Tomcat started in this forum. Sharing this kind of data can sure help in selecting the appropriate parts and pieces. Thanks!

Steve

Bam has done this and found the 1.5:1 is faster than the 1.36:1 when they are propped correctly. Check their web site. Most people here go to 1.36 drives because the run out of prop. Some have gone to the new 1.25 on boats going over a buck. Merc props of 34 pitch blow out on these boats.

Randy

[ 02-02-2002: Message edited by: BigGrizzly ]

I went from 1.50's to 1.35's and from a 32" lab merc prop to a 30" merc lab prop. with The 32" prop the boat (DCB Mach 26) was hard to plane because of blowout. I need to drop to a 29" or 28" prop this summer to see what top end numbers I get. During the winter the 30" prop worked ok turning 5800 rpm at 104 mph on gps, but during the hot summer months at Havasu I can only turn the 30 about 5100 rpm.

From friends experiences the smaller props (28"-30") have been more efficient as far as slip goes. I have not done enough testing with props myself to see if it holds true on my boat, but the 30" prop does not blow out like the 32" did.

HavasuCat, You're at about the same as me. I run 1.36 and 30" for top end 100mph and usse a 28" the other time for midrange punch. I am thinking of going to a 1.26 and use 26 and 28 and if I get a bigger motor then go with the 30". Knock wood I haven't broken a HP bravo drive with 725HP.

All the prop guys and race guys I have talked to seam to think that turning a bigger prop slower works better . Less slip . It has a better chance of grabbing ahold of the water to push . As others have said the only time you run higher gears is when you can't get a bigger prop. I have 1.32's with 17 1/2x 31's and I need more !It would probably work better if I could use 1.50 and get about a 44" pitch ! :eek:

Maybe a tug boat prop ? :rolleyes:

[ 02-05-2002: Message edited by: boot ]