Changing drive

[ericcarr2000]

Ok surly noted. Looks like I think I found the problem, at least I'm hoping anyway. This was the first time I wasn't able to get her up on plane. I'm thinking since it was just me and my dad on it, no one in the cabin area to put some weight up there, we were both at the helm, I'm thinking there just wasn't adequate weight distribution to get her up on plane resulting in me probably spinning a hub since it did go right back into gear. Since the drive is pretty pitted I'll probably just replace it with a remanned b1 with s new prop. Any thoughts on a pitch selection? I'm pretty sure it had a 19 ss on it when this happened.
In the past there usually are kids in the cabin to put some weight up towards the bow.

 

[alldodge]

Your boat should be able to get up with the 7.4 if the motor isn't tired. Replacing the drive with a newer B1 and stainless prop will not change much if any. Your having trouble getting up, this will continue until you change or fix something else. I suggest refinishing your current drive before buying another one of the same, if it does go bad get one then.

Your same boat with a 5.7 shows WOT at about 32 mph, you should be able to do better then that. Things that can cause your issue could be, tired engine, incorrect tune up, Hull has taken on water, Boat is over loaded with gear.

From what your stating I would weight the boat, check the engine out, and then go with a B3 lower if everything checks out

 

[HT32BSX115]

Yes....all things considered....I would STILL put a Bravo III on that Searay.

94 Formula 27PC Custom 502MPI MEFI3 , B3 XR

Hey Dodge, What is your WOT speed and RPM? (prop pitch and ratio?)

Love those Formulas!!

 

[alldodge]

Yes....all things considered....I would STILL put a Bravo III on that Searay.



Hey Dodge, What is your WOT speed and RPM? (prop pitch and ratio?)

Love those Formulas!!

4x4 24 pitch at 5200 rpm and 47mph

 

[ericcarr2000]

The thing is I really don't think it's a tired motor because it runs great, doesn't drop off the rpm's at all. It was running at around 4600 rpm's. If the motor was going it would probably be jumping around on the tack and revving. The oil pressure has always been solid at between 40-55 psi. I'll probabky just replace the drive and go from there because like I said it probably is the original one anyways.

 

[ericcarr2000]

Is it true that a bravo 3 drive would loose some top end power vs the bravo 1?

 

[thumpar]

Is it true that a bravo 3 drive would loose some top end power vs the bravo 1?

Yes. You have some power loss from the friction in the drive alone.

 

[ericcarr2000]

Ok, I think I'll just replace it with a newer bravo 1 and go from there then.

 

[alldodge]

The question should be, how much WOT do you do? Next question, do you want to come up on plane easier without folks moving forward and weight distribution.

If your not doing that much WOT and lets say you might loose 2 mph top end, and you burn a slight less amount of fuel, then B3
If you want to continue to have your boat do just what its doing now then B1

 

[HT32BSX115]

Is it true that a bravo 3 drive would loose some top end power vs the bravo 1?

Extra friction in the drive and extra (water) friction from turning 2 propellers vs 1..........

In reality the loss of speed is insignificant.

The gain in holeshot and better cruising fuel flow makes it absolutely worth it.

 

[HT32BSX115]

If the motor was going it would probably be jumping around on the tack and revving. The oil pressure has always been solid at between 40-55 psi.

No it wouldn't. If that 7.4L engine will turn 4600 under a load (correct prop, boat speed appropriate and at WOT) ,it's fine.

The RPM wouldn't be "jumping" if the engine had a "problem:........it wouldn't make rated RPM at WOT ...........A "jumping" RPM would be a sign of an electrical problem with the tach.

Oil pressure is not generally related to engine performance if it's high enough ( approx 10psi per 1000 RPM) If you only made 20-30 psi at 4000 RPM for example, assuming the gage is NOT faulty, the engine (rod, main, cam bearings) would probably be simply worn out............Power output is more related to good cylinder seal (ring and cyl condition) and timing.

 

[muskyfins]

Eric-the whole prop/gear ratio/engine HP/RPM thing can be confusing to say the least. I also think we've jumped around in your thread without maybe giving you the fundamental understanding from the beginning. But my opinion (which is worth 10X more than you paid for it 10 x 0 = 0) is to start at the very beginning. Be patient and wait until spring. (or get her done now-there's still time most places) Put her back in the water. Time how long it takes to get on plane. Record your wide open throttle (WOT) engine RPMs. Determine your drive ratio. With that data, any of these guys, or an experienced marine mechanic should be able to recommend the correct prop.

The above not withstanding, I also do not believe there is an issue with the drive. The drive on my last boat was 35 years old and functioning flawlessly behind a newer boat. They are designed to last almost forever. They don't wear out and need to be replaced in the sense you mentioned. Plus they are costly. A used B3 around here goes for $2500 and up. Remans are more. New is laughable. All to try to fix a problem it doesn't appear you have???

Ultimately, yes a B3 will get you on plane faster than a B1. But that won't solve your underlying problem. You'll end up with a new outdrive and on old problem.

 

[ericcarr2000]

Ok if I were to replace just the lower unit with a 3, what's that going to do with the gear ratio? Also is that ratio really in the upper or the lower unit?

 

[ericcarr2000]

Also where do I find the numbers on the drive to be able to identify the ratio?

 

[thumpar]

They should be stamped on the side of the drive. My B1 is stamped on the port side.

 

[alldodge]

Ok if I were to replace just the lower unit with a 3, what's that going to do with the gear ratio? Also is that ratio really in the upper or the lower unit?

For the standard Bravo Ratio is determined by upper and lower sections. If you have the 1.50 ratio then just getting the standard 1.50 B3 lower (15/19) will get you back to the 1.5 ratio B3

 

[ericcarr2000]

Where at, upper or lower unit?

They should be stamped on the side of the drive. My B1 is stamped on the port side.

 

[alldodge]

Up'er

 

[HT32BSX115]

Ok if I were to replace just the lower unit with a 3, what's that going to do with the gear ratio? Also is that ratio really in the upper or the lower unit?

You very likely have a 1.5:1 Bravo I.......... So you could choose a lower tooth-count (16/27) that would give you a 2.00:1 ratio according to the above chart.

I don't think I would do a 1.81:1 ratio. It would would require you to start with either a 20 or 22P propset. A 2.00:1 would likely allow you to use a 24P propset which is very plentiful on eBay.

If I am not mistaken, your 7.4L engine is WOT rated at 4600RPM.......with a 2.00:1 drive and 24P props, you should see around 40-45 MPH at WOT.

I used http://www.go-fast.com/Prop_Slip_Calculator.htm to calculate your expected approx speed using an estimated 15% slip.

15% is somewhat high for a Bravo III in a run-a-bout type planing boat ...........but with big cruisers, the slip is always higher due to additional weight (hydrodynamic drag) .

My best speed with a 1.81:1 drive, 26p props and 4600 RPM was 57 which calculated out to about 9% slip........but my boat only weighs about 4200 lbs or so......

When you use the GO-Fast calculator, enter the ratio, pitch and RPM. THEN press the Theoretical Speed Calculate button. Then enter the expected (hoped-for) slip and press Actual Speed Calculate to get the expected speed.

If you do some measuring with the boat, you can use your GPS derived speed to calculate the actual observed slip.

I

 

[ericcarr2000]

Ok I just got done looking all over the drive and can't find the markings to identify it. All I found was a Mercury sticker that has drive ratio and drive serial number on it but nothing's stamped on it.