Props On 260 Sundancer

[araxton]

Hi All
Sorry for another prop thread - I've read many but want to post a fresh situation:
I recently replaced my original EFI 260hp with a new 350 Mag (300hp carb). Now bogging out of the hole around 2300rpm. There is about a 5 second delay before it finally kicks in and jumps on plane. Also only tops out at 4200rpm. Recently ran with 4 adults on board and would not get past the "bog" and up on plane.
Boat back in the shop now for second time. Twice mechanic checked everything and can find nothing wrong. One suspect all along has been the props. Currently running 24P. They put a set of 22P on and ran it up to 4650rpm (just mechanic on board). They recommend 20P. I am questioning whether this is too much of a drop. I have found a good used set of each, but no returns...so I want to make the best decision on which to buy.
A little further information, since I've owned the boat (about 8yrs), I have always had performance issues (never over 4200rpm and any junk on bottom at all will prevent planing). So, I have suspected all along that the boat had the wrong props to begin with.
Any advice is appreciated!
Thanks All.

 

[bbwhitejr]

If I remember right, my ‘06 260DA, 350mag, B3, came stock with 24 props. Your boat is lighter and should handle more than a 20. Sounds like you have something else going on to me.

Bennett

 

[Brite Idea]

Sounds like a fuel delivery problem

 

[Espos4]

I recently replaced my original EFI 260hp with a new 350 Mag (300hp carb). .

Is this a complete new motor?
Fuel injected or carbed? I thought the 350 MAG was fuel injected.

Physically Check your outdrive gear ratio too. Was that ever replaced with possibly a different unit with the wrong gear ratio?

 

[Lazy Daze]

If you've gone UP in horsepower at the PROP, since that's how the original would have been measured and some aftermarket companies advertise crankshaft HP, if anything you should currently be having a BETTER holeshot and over reving at WOT.

Like Espos4 mentioned, a 350MAG is a mulit port fuel injected motor. Please clarify what you have.

But, assuming higher HP, PROPS ARE NOT YOUR PROBLEM. But likely, as noted above, fuel delivery could be. How long was the boat sitting waiting for the engine? What's the fuel quality look/smell like?

 

[Thornton69]

I used to believe that Mercuiser "Mags" were all fuel injected. Maybe it was the case years ago when fuel injection was new to their marine engines and offered more hp. It seems to be getting thrown out there branded as Bravo even now with carbureted models like fluff with higher flowing Vortec heads and rpm's. You can see it listed on their reman line even today with carb'd "Bravo Mag". Believe in the later engines the non Mag's were the 2 barreled carbs with slightly lower hp ratings set up for the Alpha drives. Think it all changed when gm introduced the Vortecs with more hp available out of the same displacement but could be wrong.

 

[araxton]

Sorry all - 357 Mag w/4bbl - 325hp.
As mentioned, previous 5.7EFI (260hp) only turned 4200rpm - so I think the 24s that came with the boat were always too big. Marina has checked everything over twice now - specifically fuel delivery. Also consulted with Merc. All say Change the props. They tried a set of 22s and ran 4600rpm - so improved over current 4200rpm. They suggest 20s. The marina is lending me a set of 20’s from the same boat as mine in for reposed. I’m going to run them Friday and compare performance of all three. I’ve located a set of 20s and 22s for about $500/set. Will pull trigger on one or other once I run the boat this week. ( will have a set of 24s for sale if anyone interested.
Just curious what others are running with the same setup. Also wondering at what rpm should the boat be reaching plane.
Thanks all - really appreciate the input!

 

[Lazy Daze]

Oh, I missed the part where you said ALSO had trouble with the old engine.

I used to own the exact boat/engine combo as you. I had 22" on there and could achieve close to 5K RPM if I really pushed it. Could even pull a slalom skier up with ease.

It doesn't make sense that you are spinning MORE HP and would need to go lower in pitch.

 

[89P270]

My two cents. I had a twin 6.2LMX/BIII system with quicksilver 24P 3x3s. The boat weighed 12.5K Dry. The quicksilvers were stock. Boat took 19 seconds to reach plane. I wrung my hands for a year on proping down and/or changing types. I eventually went with Hill marine 4x3 22P. The boat performance was SUBSTANIAL improved. I got on plane in 11 seconds. Because I went down in Pitch the expectation was increased WOT. OEM props 4500 RPM (approx). Hill Marine 4700 RPM (approx). Top end only lost 1-2MPH (42-40). Very Very pleased. Hill will work you on Pitch. If you try 22 and performance not quite there, they will exchange for different pitch. Sets are somewhat expensive ($13 bills) but well worth the performance improvement. For others out there Hill also has Big Cow props for Bravo II. Not sure about Bravo I. When/if you call, ask for Ron and he'll give you an earful of how screwed up Volvo was in developing their duo prop design (later sort of emulated by Mercruiser) and how Ron was working on his design back in the day too.

Id say with the new engine I'd go Hill 4x3 22P to start.

 

[Nick70]

I’m eager to hear the results of testing the 22’s and 20’ s..
My money is on the 22’s. I think going to the 20P is too dramatic. I’ve been dealing with prop pitch also. My 260 had a replacement lower unit, and 28P props when I bought it.
We figured out the final drive ratio is something like 2.6:1..which is a steep gearing. Akin to 4:10 rear end on a car. They’re for high altitude. That lower was probably what the previous owner could find cheap and then needed or it came with the 28P props. Maybe you have a lower than usual gearing in your lower unit, since you’ve always had a issue getting up on plane. There were a few available.
My first 260 with a 350 had 24p props too btw..with people onboard and choppy seas, she really struggled to get up on plane too. Let us know what the results are.

 

[Lazy Daze]

Curious... when you're testing for top speed and RPM... are you trimming up as far as you can go until either the props ventilate or speed no longer increases? As I said, I had the exact same boat/engine with 22's and absolutely ZERO issues with performance.

 

[Thornton69]

Sorry all - 357 Mag w/4bbl - 325hp.
As mentioned, previous 5.7EFI (260hp) only turned 4200rpm - so I think the 24s that came with the boat were always too big. Marina has checked everything over twice now - specifically fuel delivery. Also consulted with Merc. All say Change the props. They tried a set of 22s and ran 4600rpm - so improved over current 4200rpm. They suggest 20s. The marina is lending me a set of 20’s from the same boat as mine in for reposed. I’m going to run them Friday and compare performance of all three. I’ve located a set of 20s and 22s for about $500/set. Will pull trigger on one or other once I run the boat this week. ( will have a set of 24s for sale if anyone interested.
Just curious what others are running with the same setup. Also wondering at what rpm should the boat be reaching plane.
Thanks all - really appreciate the input!

I question how Mercury came up with these numbers. Higher rpm's? It is just a .040" over 5.7 Vortec with Tbolt 5 and knock control. I have one opened up at the moment that lost the block due to a cracked cylinder wall. There is nothing magical in it to get that 325hp and they appear to now be discontinued. Their 5.7 MPI's are only rated at 300 hp.
It's like they took their carb'd 5.7 (260 hp), bored it .040, spun it faster and added 65 hp in the specs.

 

[Lazy Daze]

I question how Mercury came up with these numbers. Higher rpm's? It is just a .040" over 5.7 Vortec with Tbolt 5 and knock control. I have one opened up at the moment that lost the block due to a cracked cylinder wall. There is nothing magical in it to get that 325hp and they appear to now be discontinued. Their 5.7 MPI's are only rated at 300 hp.
It's like they took their carb'd 5.7 (260 hp), bored it .040, spun it faster and added 65 hp in the specs.

I think I may have mentioned this above, but it could be because that 325 is crank HP, not prop HP like the 260HP?

 

[Thornton69]

I think I may have mentioned this above, but it could be because that 325 is crank HP, not prop HP like the 260HP?

Ya not sure but could be. When they were available, the alpha version of that 357 was only listed at 300hp. The only other difference between the 2 spec sheets was that the Bravo version came with the raw water pump mounted. Seemed odd as you would think that the RW pump would reduce the amount of available hp at the crank.
I am going to take all the internals (minus pistons) along with the bolt ons from my 357 this winter and do a build with them on a .020 over block. It will be replacing a prevortec roller cam 260 Mercruiser. Will be interesting to see what happens to my WOT infront of the same B2. Right now it is 4450. The vortec heads alone should probably add 40 hp I would like to think.

 

[89P270]

The rebuilt 4V alpha from Mercruiser was listed at 275HP. I put two of these in my Pachanga as replacements to the original 350Mags (260HP). I wonder then, Mercruiser is quoting at the prop (275HP) vs at the flywheel then? a pair of these outperformed the original 350 mag's I had by about 7 MPH at WOT (53ish vs 60ish) - Note Mercruiser refers to it as a 5.8L.
From Mercruiser brochure:
Power 275 HP (205 kW)
Full Throttle Range 4400-4800 RPM
Displacement 357 CID (5.8L)
Engine Type Remanufactured GM V8 Marine Iron Block & Gen+ Heads
Bore & Stroke 4.040" x 3.480" (103mm x 88mm)
Compression Ratio 9.4:1 with Hypereutectic Pistons
Rocker Arm Ratio 1.5:1
Fuel Requirement 87 Octane (R+M)/2
Fuel System Weber 750 cfm 4V Electric Choke Carburetor & Mechanical Fuel Pump
Induction System Dual Plane Aluminum Intake Manifold with Bronze Inserts
Ignition System Thunderbolt V with Knock Control
Charging System 65 Amp / 917 Watt Alternator
Water Drain System Multi-Point Drain - Hand Removable Blue Drain Plugs
Exhaust System Iron Manifold with Wet-Joint Exhaust Elbow
Cooling System Thermostatically Controlled Raw Water - 160 F*
Accessory Drive System Serpentine Belt
Lubrication System High Flow Oil Pump
Recommended Engine Oil SAE 25-40 NMMA Certified FC-W Synthetic Blend
Audio Warning System Oil Pressure, Coolant Temperature, Drive Lube
Alpha Ratios 1.47:1 / High Altitude 1:81:1
Dimensions (LxWxH) 32" x 30" x 22" (814mm x 740mm x 559mm)
Engine Weight (lbs./kg) 871 / 395
Optional Fresh Water Cooling Dealer Installed Kit
Boat Harness Type 10-pin

 

[Lazy Daze]

Since somewhere around the mid-90's, all Mercruiser HP has been published as propshaft HP... when INSTALLED in a boat from the factory (or at least that's the case with Sea Rays). In the case of crate engines, I "believe" they typically use crankshaft HP. Obviously one reason is for advertising purposes, but another could be because it depends on the application since if the engine is put into a straight drive boat, there is less parasitic loss.