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Old 05-21-2013, 08:08 PM   #1
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Default Repropped....big difference

1997 2800 SCR
single 8.1L (496)
Bravo 3 (2:1)

During winter maintenance this year I had removed the props and were cleaning them up. The splines on the prop hub were cracked and some were sheared off. These were both 3 blade 24pitch props. I have always felt they were overpropped despite many people claiming this was a great setup for the B3 with a big block. I hit something 2 seasons ago that sheared the gears on the lower end of my old B3. I suspect this is residual damage that wasn't so obvious immediately after the hit.

I could never seem to get the boat above 4000 - 4100 RPM MAx RPM is supposed to be 4800rpm. I could never get the boat above 30-31 kts.

So I did a bunch of reading and ran the numbers through the Merc prop calculator. I even called US Marine who confirmed they never shipped my model with 24 P props.

I replaced with:

FWD: 4 blade 15.75" 22.5p
AFt: 3 blade 14.25" 22.5p

RESULTS:

-> Time to plane cut in half
-> Max RPM 4750
-> got speed up to about 35 kts but too choppy to continue.

The odd part was, i was expecting the rpm to increase in order to maintain my regular cruisng speeds. It didn't really change. I used to get about 22-25kts at 3800 (depending on tide, wind, current, etc (especially tide)). Now I'm getting about 25kts at 3850. I expected to have to go higher.

What I didn't expect is the difference in the sound of the engine. It just sounds like it's running smoother. Like it's not all loaded up. These things don't make any sense to me. But I am extremely happy with the improvement in performance.
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Old 05-21-2013, 08:48 PM   #2
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It is amazing what with the right equipment your situation has changed for the better.

Alittle bit of investigation on your part and look what happened....

Roger
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Old 05-21-2013, 09:10 PM   #3
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That's great news, Shrew. Will probably even improve the mileage a bit.

Mine is also a 2:1 BIII with the 3 blade, 24P props. It has no problem getting on plane or reaching max RPM range but I've read that the new 4/3 BIII prop set up still makes a lot of difference. Unfortunately those prop sets are a little expensive to conduct an experiment.

Dan
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Old 05-22-2013, 10:38 AM   #4
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That's great news, Shrew. Will probably even improve the mileage a bit.

Mine is also a 2:1 BIII with the 3 blade, 24P props. It has no problem getting on plane or reaching max RPM range but I've read that the new 4/3 BIII prop set up still makes a lot of difference. Unfortunately those prop sets are a little expensive to conduct an experiment.

Dan
My 2600 cruiser with a 350 mag / b3 came stock with 3x3 20p props. It has always roared onto plane, but the top end was limited and the mid range cruise rpms were too high. It also had a lot of prop blowout in hard turns and too much lower unit trim. I replaced them this year with a custom set of Hill marine 4x4 22p props. What an amazing difference. Still comes right on plane, but the top speed increased while the wot decreased. The mid range cruise rpm went from 3980 at 30 mph to 3600 at 30 mph. Minimum plan speed lowerd a few mph. Also, the prop blow out went away completely in both hard turns and when trimming the lower unit. And, the mpg went from 2.1 at 30 mph to 2.3.
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Old 05-22-2013, 01:40 PM   #5
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That's great news, Shrew. Will probably even improve the mileage a bit.

Mine is also a 2:1 BIII with the 3 blade, 24P props. It has no problem getting on plane or reaching max RPM range but I've read that the new 4/3 BIII prop set up still makes a lot of difference. Unfortunately those prop sets are a little expensive to conduct an experiment.

Dan
I wonder if the weight or beam start to impact performance. I probably have a couple thousnand pounds on you. Seapuppy used to claim to he loved the 24p setup, but at the time he was still running the 2700. That boat is similar size in length with a big block, but it was an 8' 6" beam. Maybe I changed too many variables with pitch and blades making it tough to tell where the benefit came from.

Hey, maybe I'm realying on 6 months of memory from when the boat ran last and a tinge of optimism after spending $1,100 dollars on an experiment and a hunch.
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Old 05-23-2013, 02:07 AM   #6
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I'm going to be experimenting with my Bravo 2 props this summer. I used to run a 3 blade 21" pitch and WOT at about 4100 RPM, planed at about 3000 RPM.
I just bought a 3 blade 19" pitch and am hoping that I will plane off faster and be able to bump the RMPs up.
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Old 05-23-2013, 12:38 PM   #7
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here is the link to the Mercury Prop Calculator. They have had variations of this for some time now, however this is by far the best version so far:

http://www.mercurymarine.com/propellers/prop-selector/


1994Maxum2700SC (That is quite a handle) I tried to run it through the calculator, but I'm not sure of your top speed or the gear ratio of the B2. It came up with both a 21P and a 19P. Though gear and weight will impat it as well, so if your only getting 4100 then the 19p seems like the logical choice.
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Old 05-23-2013, 11:50 PM   #8
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Originally Posted by shrew View Post
here is the link to the Mercury Prop Calculator. They have had variations of this for some time now, however this is by far the best version so far:

http://www.mercurymarine.com/propellers/prop-selector/


1994Maxum2700SC (That is quite a handle) I tried to run it through the calculator, but I'm not sure of your top speed or the gear ratio of the B2. It came up with both a 21P and a 19P. Though gear and weight will impat it as well, so if your only getting 4100 then the 19p seems like the logical choice.
Shrew,

Any idea on how to work this out for Shaft V Drive?, I have the exact same concerns as you with the props on my 3500, seems to only pull 4100 RPM max with the 7.4's and always seems to be laboring
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Old 05-28-2013, 12:42 PM   #9
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Shrew,

Any idea on how to work this out for Shaft V Drive?, I have the exact same concerns as you with the props on my 3500, seems to only pull 4100 RPM max with the 7.4's and always seems to be laboring
The same principle applies. In your case, you don't need worry about proper drive trim, however, minimal tab trim should be used in calm conditions. If you're still under the max rpm range,
with the ideal target being as close to but not over (I prefer just under) the Max RPM of the rpm range. So you'd drop pitch approx. 1" for every increase of 200 (I believe the rule is a change of 400 rpm for every 2" change in pitch).

It also helps to look around to see what you have for unnecessary weight. Do you need to carry so much fuel? Water?
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Old 05-30-2013, 12:34 AM   #10
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Quote:
1994Maxum2700SC (That is quite a handle) I tried to run it through the calculator, but I'm not sure of your top speed or the gear ratio of the B2. It came up with both a 21P and a 19P. Though gear and weight will impat it as well, so if your only getting 4100 then the 19p seems like the logical choice.
Lol thanks for checking for me man, I ussually have the boat weighed down so hopefully the 19P will be just right.
And I know, i was a forum rookie when I made my handle lol, there's no way to change it though I assume?

Thanks,
- Joe
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Old 06-03-2014, 07:33 AM   #11
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Hey guys, I just put my boat in the water the other day and took her out for a spin.
Running the 7.4L single engine carbed, bravo 2 with a 19 pitched prop (3 blade aluminum).
3 passengers aboard, 1/2 tank gas, and water tank only 1/4 full...
WOT at 4500 RPM - approx 26knots (30mph) from gps speed
planed out at about 3000RPM and cruises around 18knots (20mph)

Do these numbers seem on point to you guys?

Thanks,
- Joe
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Old 06-03-2014, 04:20 PM   #12
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Max RPM is in the right spot. Cruises @ 18kts (3000rpm) seems low and slow to be planing. I'd have to have the tabs all the way to come close to that.
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