Helping my friend select correct Bravo III props

markrinker

markrinker

New Member
Jul 19, 2011
325
Lake Union - Seattle, WA
Boat Info
2000 410DA
Engines
Caterpillar 3126
A friend of mine and his wife left this fall from Minneapolis on the 7000 mile Great Loop. They have been river boating for years and have an affinity for the simple layout of their 2003 Bayliner 265. It really is a spacious, tall cabin with full queen bed in the rear berth. They have converted the forward berth to storage, which also helps with on plane performance.

The boat is powered by a single Mercruiser 5.7L carbureated I/O (250hp rating) with Bravo III drive. Current stock props are 22p and in good shape. He reports that they run on plane most of the time from point to point, but have only averaged 1.7mpg so far on their journey which has them at Evansville, Illinois.

I sold Jim my set of 24p Bravo III props as spares, but the prop selector at http://www.mercurymarine.com/propellers/prop-selector/# keeps pointing us to 19p props? This seems counter to what I would expect. His goal is obviously MPG (economy) over top speed (performance) but would slowing the boat down, while increasing RPM result in fuel savings?

My thought is the 24p set would give him more distance per RPM, and that the boat with two people, fuel, water, and gear (8000# max) is not overloaded for a 250hp engine, regardless of how you prop it, within reason.

Thoughts, experiences, wild ass theories all welcomed!!!
 
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I don't know a whole lot about props but I did want to mention that apparently there is a new design Bravo 3 prop out now that has 4 blades instead of 3. It is supposed to help with planing and performance. One person here switched to them and I think he saw a difference. Might want to do a search for his thread.
 
I think some more info is needed. What is his typical on-plane cruising speed (GPS) and RPM? What is his WOT RPM? WOT speed is not really needed, but WOT RPM is very important to know to determine how hard the engine is being worked & loaded at ALL RPM's.

If he is currently at the very top of his recommended RPM range at WOT, then the 24" propset might drop his cruise RPM while giving the same GPS speed, and improve his fuel economy noticeably. Plus, he will still get into the lower end of the RPM range at WOT, and not overwork the engine. If he only hits the lower end of his recommended WOT RPM range now, then the 24" will overwork the engine, and economy will either stay the same or get worse.

If he can't even reach the bottom of the WOT RPM range, then dropping to a 20" propset is needed to take some load off the engine at all speeds. Economy MAY get better here.

If he currently is able to get into the recommended WOT RPM range at all, I don't see how dropping to a 19" or 20" pitch propset will help cruise economy with a relatively light load.

I think that boat should be capable of 2 mpg. 3500 RPM should give around 22kts/25mph, while burning 12 gph.
 
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To an extent, yes, increasing pitch can increase MPG. But once you're out of the engine's efficiency zone, the MPG will drop.

Think of it this way... when towing on the road, you typically the select the highest gear possible, yet a gear that still retains some power. Let's just say that it's "5th gear". Now, the road conditions/trailer weight doesn't change but you select 6th gear. What's going to happen? You'll need to supply more "go pedal" to retain your momentum. Keep increasing the gears and you'll be lugging the engine which, I know you are aware of, not only hurts MPG but is bad for the engine.

I know this isn't a perfect comparison to a boat, but it's close. The point above about finding Max WOT RPM is the number one thing you (or he) can do to figure out the proper pitch for his application... ie: the way he is using the boat. Keep the pitch such that his WOT RPM is in the 4400-4800 RPM band. Generally speaking, then move towards the lower end of that spectrum with a higher pitched prop to gain some economy. After that, he'll need to experiment with cruising RPM's.

BUT, gotta find out where he is right now (max RPM).
 
4000rpm is top attainable @ 32mph. Minimum speed of 3000rpm @ 19mph to maintain on plane. Cruise is somewhere between those two, depending on wind and wave conditions.

Damn I think he needs a 300hp MPI Mag...I think he is out of fuel rate with that 2bbl setup, or maybe he is governed to 4000rpm?

Sounds like to raise his max RPMs, he'll need to move to a 19p or 20p prop, correct? I guess the Mercury prop selector was spot on.
 
No governors.

4000RPM MAX WOT? Yup - way inefficient. Kinda like missing third gear during a shift and hitting 5th, instead.

A very general rule of thumb is 150RPMs for each inch of pitch.
 
There's really nothing he could do to get more power out of that 2 barrel -- it's restricted by CFM's of air. If he REALLY wanted to, he could install a new performance intake manifold & 4 barrell carb -- prob get a few more horses and pull slightly higher RPM's. I think the best bet is just drop to the 20" propset and live with it.
 
I do not concur. The 2bbl setup is completely capable of flowing enough air to make 5000+ rpms as evidenced by the numerous engines out there doing it. Emissions era engines of all kinds are set lean to meet regulations. I'm betting more fuel rate would bring up RPMs under same load - (...to a point, but in this case we are only looking for another 500rpm).

Back in 2003 I did the exact upgrade you mentioned - 2bbl to 4bbl on a 4.3L V6 in a Larson 180 runabout. My mistake was not repropping for the extra RPMs, and I floated a valve after running the boat too hard. It did add about 5mph to the top end. Just think what it would have done with the right prop!
 
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Really? Modifying the OEM 2 barrel carb for more fuel & power is a rediculous proposition that will NOT result in increased performance and/or economy. The OEM 2 barrel is 500 cfm, with OEM jetting matched to that airflow. Sure, that 350 2bbl can turn over way over 5000 rpms -- as long as the load on it is doesn't exceed engine power output. Put your friends 350 2bbl in a 3000lb boat with the 22" propset and it'll be on the rev limiter. The Quadrajet is 750 cfm. On 350's with 4 barrels, the vacuum secondaries open up somewhere around 3500 rpm.

Now, is it possible that the OEM carb just needs a rebuild, and you might see slightly improved performance and economy? Yes.
 
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EricW said:
Really? Modifying the OEM 2 barrel carb for more fuel & power is a rediculous proposition that will NOT result in increased performance and/or economy. The OEM 2 barrel is 500 cfm, with OEM jetting matched to that airflow. Sure, that 350 2bbl can turn over way over 5000 rpms -- as long as the load on it is doesn't exceed engine power output. Put your friends 350 2bbl in a 3000lb boat with the 22" propset and it'll be on the rev limiter. The Quadrajet is 750 cfm. On 350's with 4 barrels, the vacuum secondaries open up somewhere around 3500 rpm.

Now, is it possible that the OEM carb just needs a rebuild, and you might see slightly improved performance and economy? Yes.
Click to expand...


So you are saying that moving up one size jet would make no difference in WOT throttle RPMs or top speed? Care to place a wager on that 'rediculous proposition' ? :smt021

Granted, if the primary goal is to find max economy over their long trip, the answer is certainly to first move to the 19p props recommended by the Mercruiser prop selector. That, and to reduce as much weight on the boat as possible, but I've been aboard and they are very frugal when it comes to gear.

Lastly, how much is a new set of props vs. the cost of a new set of jets? What harm is there in testing the least expensive (and most easily reversible) test? Ever tried to return a set of used props when they don't meet your expectations?
 
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Fuel mapping is not my specialty... that's why I always referred to Kennedy... :smt001

But... If I look back at some old Sea Ray dealer handbooks ('96 happens to be the one I have in my hand), I can see the difference in HP between the 5.7L and the 5.7LX. The LX was a 4-barrel - I don't think they changed the intake, but I'm not positive. This is one of the last times that Sea Ray offered these engines as they then transitioned to simply a 2-barrel and EFI option and within the next few years, most of their boats only came with fuel injection. However, the 5.7L was rated at 210HP and the 5.7LX was 250HP. 40HP difference.

Is the bottom of the boat clean? The reason I ask is that they "roughly" have the same boat I have. If memory serves, I have either 22P or 24P props. I can easily run up to about 4,900 - 5,000 RPM's. I suppose it could just be due to a more fuel efficient hull design, though.
 
I'm not much of a gambler......

Rejetting the OEM 2 bbl carb is appropriate to compensate for altitude changes, but I can't imagine a positive result otherwise. I guess the carb could currently be jetted for high altitude operation, and jets for standard altitude are needed. Strange things do happen.....and I've been wrong before.

Either way, a drop in prop pitch is definitely needed and a positive result IS certain, based on the WOT RPM number you gave. I do hope you and your friend are able to get the results you're looking for, regardless of what you try.
 
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