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View Full Version : bravo 3 drive-correct gear ratio?



magman
05-17-2012, 07:20 AM
I am rebuilding my Magnum 25-1974, bought as a project boat, never had it in water. I just picked up a bravo 3 older style drive with 220 gear ratio. I am running a stock mercruiser mpi 300 hp for power.Does this gear ratio seem correct for this powerplant? any idea on what prop pitch to start with?

Morgan's Cloud
05-17-2012, 08:00 AM
Hi Gerard ,

As I've mentioned before , mine is set up as such ..
502 Mag (415 hp)
2.0:1 ratio
26 inch propset (lightly cupped)
+ or - 4950 rpm wot

With what you have there , I think you'll be looking at starting with a 24'' prop set . You could find a dealer with a decent prop testing program so as to avoid dropping big bucks into the wrong prop set .

Ghost
05-17-2012, 10:17 AM
Ditto that--I'd definitely go with a testing program so you can dial in without buying multiple propsets.

As for the right ratio with that boat at 300HP, I think your 2.2 probably is the best choice.

(As an aside, MC, you're reaching 60 GPS on good days, right? So a little under 2% slip? Less than I'd have expected but very useful info. Is yours a 3/3 or 4/3 set? I may have to talk to you more about props in a few weeks...)

Mike

Morgan's Cloud
05-17-2012, 12:03 PM
Mike , it's been a l o n g time since it's been in the water , but hopefully very soon now .
I really want to finish the testing that was started prior to the hurricane Fabian haulout in '93. That was needed because of the addition of the Hi-Tek's and the change in breather and fuel pressure. But Fabian got in the way and the rest is history now .

If I recall , we were knocking on 60 on a good day ..
It's the 3/3 propset . I read on an OSO prop thread just the other day that someone said that it's now been determined that the 4/3 set is better for the heavier/slower cruisers and the 3/3 set is better for the faster boats.
I never did find anything out about the supposed 'hi - perf ' Bravo III props that were mentioned a while back . Do they really exist , or was it the hyped up new 4/3 sets that were being referred to back then ?

Ghost
05-17-2012, 12:55 PM
Crossing my fingers you splash the Magnum again soon, as hoped.

Here's something from Brett at BBlades, which may be of interest to both you and Gerard. In my surfing, it's one of the more detailed discussions I've heard that addresses 3/3 and 4/3 sets and cupping differences, as well as the Mercury Racing B3 props you mentioned. (Even if they're not any different, they add some pitch options I suppose.)

The thread is here: http://midwestboatparty.com/forum/archive/index.php/t-20568.html

The most interesting part I'll paste in for posterity...

In 1999, while still employed at Mercury Racing, we began development of a new set of B3 propellers. The reason we did this was Formula began mating 500 EFI engines to B3 drives on their sport cruisers. The second generation B3 props (still a 3/3 set up) was cavitation burning so fast we couldn't get 10 hours on the props before their would be a hole in the front prop blades. The new development produced 2 finalists. One set had a 4 blade front which truth be told was a mis communication accident. However, after much testing it was determined the 4/3 had the better cav. burn resistance and the performance was equally as good as the other new development set. What we found was the 3/3 original set was slightly faster still. However, the new 4/3 burned much less and had a slightly better mid range. We decided to release them under the Mercury Racing logo and it took till 2004-2005 to get it done. This is the time frame slightly after my leaving Mercury Racing. The decision to Hi Polish the props came from the management that took my place. Hi Polish looks pretty but isn't a performance related issue.

If a guy wants low end and mid range specifically, the 4/3 is the better choice. If bow lift and speed are the main concern, working the 3/3 is still the better choice. Unfortunately, I believe the 3/3 is going to go away for good.

Now, as far as the new 4/3 versus the Merc Racing 4/3, there doesn't appear to be much difference other than the finish and a bit of a geometry and cup change. Main line propellers has basically created a new propeller option to continue moving forward with new offerings in the market place. It has a duller finish and is priced to offer the consumer a option. And a Hi Polish price point to match the Racing XR version. The whole thing is kind of goofy in that main line has created a product to directly compete with the Racing product.

We have worked with the XR propellers here at BBLADES. We have yet to work with the main line 4/3.

Again, bottom line, 4/3 for low end, 3/3 for top end. Also, go down 1" of pitch if you go from the 3/3 to the 4/3.

Brett

Morgan's Cloud
05-17-2012, 04:06 PM
Mike , I read your paste and will go back later and read the thread/link shortly. In the earlier post where I mentioned what I had come across the issue with cavitation and prop burning was not mentioned , just the basic performance difference between the 3/3 and 4/3 sets .
I have to admit though that I have not at this point seen any prop burning on mine with about 160 hours of use .
Because there seems to be such a dearth of info on the entire B3 unit from a 'performance' perspective from the experts, any research/info is good to get. Otherwise , we're just left to sharing personal experiences .

BTW , I made an error in my earlier post. The boat has been inactive since Sep 2003 .. NOT 1993 .. It just seems like it .:biggrin: