Lower prop pitch or change gears on Bravo III (or both?)

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half_fast

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I have a 1999 270 searay sundancer, single engine with a Bravo 3 drive. The boat was repowered in 2005 with a 6.2 MPI. Original equipment was a 7.4.

I've owned the boat for a few years, and it has been great - however I've had one lingering problem (besides that damn cool fuel module!) .. My problem is with the WOT throttle range since I bought it years ago and I want to fix it before I burn the engine out.

When the boat was re-powered (by the PO) - the new engine was mated to the original bravo drive. The boat came with a bravo 3 from the factory with a 2.00 gear ratio with a 24 pitch prop set. (according to searay)

While the boat runs well enough (i cruise at 25-27 mph at 3400 rpm burning a little under 2mpg) - it tops out at about 4000 rpm - and it struggles to get out of the hole with heavy loads or later in the season with a season's full of marine growth added to the bottom. My concern is that eventually I'm going to damage the engine by operating it under too much load at lower rpms. Additionally, Ive met and read from a number of fellow 270 owners that their top speeds are much higher than mine, and that their cruise speed is closer to 30 and they are getting probably a little bit more than 2 mpg.

MY question is do you think I can fix this to some degree with different pitch props - or is it more of a gear issue in the drive and I'm stuck unless I invest a lot of money in the drive itself? It seems like I'd need to make an awfully drastic prop pitch change to accommodate 1000 rpm!

I called Mercruiser and they said the 6.2 MPI should really have a 2.2 primary gear in the Bravo 3 - as I mentioned earlier I am almost positive mine is a 2.0.

Do I try to go down to a 20 pitch set? (Thats the lowest I can go I believe) - or should I regear the drive. How much is something like that going to run?

Thanks for any input on this!
 

45Auto

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Re: Lower prop pitch or change gears on Bravo III (or both?)

Additionally, Ive met and read from a number of fellow 270 owners that their top speeds are much higher than mine, and that their cruise speed is closer to 30 and they are getting probably a little bit more than 2 mpg.

A Sundancer 270 is a LOT of boat for a small block, even a 6.2L. There's a reason they came with big block singles or twin small blocks. I test drove a 2005 260 with a 6.2L in it a couple of years ago and it was extremely unimpressive, seemed to be working very hard to get up on plane. I ended up with a Sundancer 300 with twin 5.7's. It cruises in the low 30's at 3000 RPM, about 1.5 MPG.

I can imagine you're probably bogging your motor pretty badly, which isn't doing it any good.

General rule for prop pitch is it affects rpm by 200 RPM per inch of pitch change. So dropping 4" of prop pitch would theoretically put you right at 4800 RPM at wide open throttle which is right where you want to be.

However, if you go with the 20" prop set, you'll probably find that the very ends of any range of props (lowest pitch or highest pitch) are not optimum for that drive set-up. It would definitely help, but it still may not quite get you where you need to be.

Changing the drive ratio by 20% (2.0 to 2.2) would gain you 20% more RPM, or about 400 RPM. That would put you about 4400 RPM with your current props. It would help, but you'd still need to drop a couple of inches in prop pitch to get your RPM where it needs to be.

Neither option is going to be particularly cheap. Buying a new upper and reselling your 2.0 upper would probably cost you about the same in the end as re-gearing your current upper, somewhere in the several thousands of dollar range. Then you would still need a slightly smaller prop set.

Buying a used upper and reselling your 2.0 upper would get you about even there, and re-selling your 24" props should offset the cost of the 22" set it would take you to get to 4800 RPM with the 2.2 outdrive. Used B3 prop sets run around $500, new ones are about $1200 I believe.

Solution depends on how much confidence you have in used versus new stuff. Going with used equipment and selling your current set-up should get you about even money-wise. If you feel you need new, then expect it to cost you $2000-$3000 minimum even after selling all your current stuff.

No matter what you do, the boat will probably still not perform as well as a 270 with a big block or twin 4.3's.

EDIT: A gear ratio of 2.0 with 24" props at 3400 RPM and 27 MPH gives a prop slip of 30%, which is extremely high for a normal prop, much less a dual prop B3. My B3 on my 22' boat runs with a prop slip of 2% to 3%. A 2.2 drive with your same speeds and RPM would give a prop slip of 22%, which is still high. Verify your current drive ratio before you do anything else.
 

half_fast

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Re: Lower prop pitch or change gears on Bravo III (or both?)

Thanks for the input!

I really think I'm heading for a gear ratio swap :(

As for the 6.2 - Its got about the same HP as the 7.4 had, actually supposedly has more shafthp than the original 7.4. The torque is apparently only slightly less - but maxes out at a much higher rpm range - (likely in the range I'm not getting to!)

I've met a few 270 owners with 6.2s that are running well into the mid 30s, mph wise - I don't think its incredibly under-powered.

I'd prefer to do it at the end of the season to avoid downtime and an extra haul/launch charge .. plus I can remove the drive for the winter and leave it at a shop to have it done when its slower, possibly saving me a few bucks. But I'm growing more and more concerned that the poor engine is not a happy camper running in the 3000s right now the way it is... not sure what to do.

Thanks for the estimated numbers though - frustrating to think that I'll likely need both a gear change AND a prop swap... !
 

half_fast

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Re: Lower prop pitch or change gears on Bravo III (or both?)

Changing the drive ratio by 20% (2.0 to 2.2) would gain you 20% more RPM, or about 400 RPM. That would put you about 4400 RPM with your current props. It would help, but you'd still need to drop a couple of inches in prop pitch to get your RPM where it needs to be.

wouldn't a 20% gain be 800 RPM, bringing me right to 4800 with just a gear swap?

EDIT: Actually I think 400 RPM is right, as 2.0 to 2.2 is a 10% increase in drive ratio, not 20% ! All comes out the same in the end! :)
 

45Auto

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Re: Lower prop pitch or change gears on Bravo III (or both?)

You're correct, I meant to type that a 10% change in drive ratio would gain you 10% in RPM, which would move you from 4,000 to 4,400 as you stated. I got the 10% RPM right, was just thinking faster than I could type!

Any idea what prop the other 270's with 6.2L are running with 2.2 ratios? Have you posted on the Club Sea Ray forums to see what they say?
 

Tail_Gunner

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Re: Lower prop pitch or change gears on Bravo III (or both?)

Here's a thought...Keeping that gearing should be a good thing. You would be reducing final gearing but more importantly prop size and the drag associated with large prop's.

I have never played with something like that but if you look at high speed drag boat's they use very small props and turn them to very high rpm for top results...The smaller prop will improve hole shot and midrange performance...Less pitch and less drag in the midrange speed.."Less Blade Area".

However will a smaller prop be able to push as effeciently is the question...Perhaps Dhadley or Walleyehed might chime in...it would be intresting to here there thoughts.
 

half_fast

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Re: Lower prop pitch or change gears on Bravo III (or both?)

You're correct, I meant to type that a 10% change in drive ratio would gain you 10% in RPM, which would move you from 4,000 to 4,400 as you stated. I got the 10% RPM right, was just thinking faster than I could type!

Any idea what prop the other 270's with 6.2L are running with 2.2 ratios? Have you posted on the Club Sea Ray forums to see what they say?

I posted in the gas engine forum over there a while ago but haven't gotten a lot of insight - I'm sort of learning a little more each time I post somewhere and each time I have a bit more to add to my question to get the best response! At first I wasn't even sure if it was a gear/prop issue but the more I'm ruling out other things and the more I learn about how I'm set up - I'm pretty certain it requires a significant change.

I'm going to post to the 270 dancer owners thread over there and see if I can find any 6.2L owners to post their gear/prop.

I also hope to manually verify the prop pitch again in the next two days or so, and I'll get some new readings with GPS speed and digital tach reading out of smartcraft. 30% slip doesn't make much sense. I'm not sure I'll ever know 100% for sure about the gear - I mean it could have been swapped already by the PO - and I'm not taking the drive apart to start a manual turn-and-count test - I'm only going on what it *should* be based on the history I have of the boat.

I'll report back.
 

half_fast

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Re: Lower prop pitch or change gears on Bravo III (or both?)

Here's a thought...Keeping that gearing should be a good thing. You would be reducing final gearing but more importantly prop size and the drag associated with large prop's.

I have never played with something like that but if you look at high speed drag boat's they use very small props and turn them to very high rpm for top results...The smaller prop will improve hole shot and midrange performance...Less pitch and less drag in the midrange speed.."Less Blade Area".

However will a smaller prop be able to push as efficiently is the question...Perhaps Dhadley or Walleyehed might chime in...it would be intresting to here there thoughts.

Well, my wallet certainly wouldn't mind just swapping my 24 set for a 20 set @ a theoretical 4800. I guess the concern though is how that big a change will affect my cruising speed/fuel economy.

If I'm even just under the optimal WOT range (4800-5200 according to the 6.2 spec sheet) I'll sleep at night - as long as whatever change I make allows me to plane off easier under heavier loads - keep the engine in a more optimal rpm range, and most importantly - allow me to cruise at a similar or even slightly higher speed at the same or increased fuel economy.

If I gain rpms but its not helping me get her out of the water with a full boat, or it adversely affects my daily cruising economy - then I'm not really gaining anything.

I wish I knew a place on/near Long Island, NY that would let me test a set.
 

craze1cars

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Re: Lower prop pitch or change gears on Bravo III (or both?)

It's rather clear that the previous owner repowered the boat with a smaller high-winding engine, but never invested in repropping it correctly. And in order to make its horsepower, the new 6.2 MPI needs to run at SUBSTANTIALLY higher RPMS than the old 7.4 did...compounding matters.

Mercury says Bravo III's can be had down to 19 pitch. It appears they even offer the 19 in three different diameters:
http://www.mercurymarine.com/propellers/elite/bravo-three/

Don't ask me which diameter to buy. I haven't the foggiest....but my gut tells me mid-size or bigger is better for a boat of this size.

Why not try one of those 19's? Your RPMS will probably get up into the low end of your proper range, you'll have fantastic holeshot compared to what you have now, and it will be substantailly cheaper/easier than regearing for sure. And it'll take a heckuva load off your motor! You're gonna do some damage if you keep running it the way you are now...gotta be banging the heck outta those pistons unnecessarily...you gotta let those MPI's spin!!

If the 19 doesn't work and you need to regear anyway, I think you'd get at least 75% of your money back on reselling the gently used 19 pitch prop set.

I guess if I were in your situation, I'd get ahold of a 19 pitch set and run it, before doing anything else...even if it is simply written off as a cost of doing research and getting some firm answers on what you might need...and in a perfect world, it actually might just solve your problem.
 

craze1cars

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Re: Lower prop pitch or change gears on Bravo III (or both?)

and I'm not taking the drive apart to start a manual turn-and-count test -

Bear in mind that you do not need to disassemble to do this...ESPECIALLY if you suspect it's a 2.0...the easiest of all to identify.

Just mark the flywheel on the motor and also mark the prop. Put it in gear (engine off), and turn the motor one full revolution by hand. The prop should have tured EXACTLY 1/2 revolution. If yes, it's a 2.0. If the prop moved a bit less than that, it's probably a 2.2.

I see no reason this wouldn't work. No disassembly needed, except pulling the spark plugs will make turning the motor much easier...and this all depends on how easy it is to get ahold of your flywheel to turn it...
 

half_fast

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Re: Lower prop pitch or change gears on Bravo III (or both?)

Bear in mind that you do not need to disassemble to do this...ESPECIALLY if you suspect it's a 2.0...the easiest of all to identify.

Just mark the flywheel on the motor and also mark the prop. Put it in gear (engine off), and turn the motor one full revolution by hand. The prop should have tured EXACTLY 1/2 revolution. If yes, it's a 2.0. If the prop moved a bit less than that, it's probably a 2.2.

I see no reason this wouldn't work. No disassembly needed, except pulling the spark plugs will make turning the motor much easier...and this all depends on how easy it is to get ahold of your flywheel to turn it...

Thanks - I'll give that a try for sure!
 
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