A whole new engine

[agallant80]

Re: A whole new engine

One other thing to think about is the fuel cell. Can you reuse the Diesel fuel cell or do you need to get a gas fuel cell. Remember you can drop a match in diesel without it blowing up, gas not so much so there may be a difference in safety equipment reguarding the fuel cell. As for the outdrive, I would replace it. You can sell yours and get one used with the proper ratio, you can even sell just the upper as that is where the ratio is made. Why put a new engine in the boat and have a strange configuration.

 

[Alumarine]

Re: A whole new engine

What's the cost of playing around with props for one of these?

 

[HT32BSX115]

Re: A whole new engine

Can the 3x 1:81 to 1 be used on a repower to gas? It seems the further I get into this process the more problems arise. I want to make intelligent choices. Thanks everyone!

Sure, depending on the size and weight of the boat and the HP and torque of the engine.



I removed a 340HP (@4600rpm) OMC 460 from my 21' Four Winns Liberator (roughly 4200lbs..... 2900lbs boat, 1200lbs engine and drive)

THEN I installed a 330hp(4600 RPM) Mercruiser 7.4L engine AND 1.81:1 ratio Bravo III with 24p props.

With the 24p props, lightly loaded (2 people, no gear 1/4 fuel) It was *just* "touching the rev limiter at 4700 RPM. With 26p props it will run ~4500-4600 RIGHT WHERE I want to be with a stock Merc 454.



IMHO, Your 25' Grady White with a roughly 4000lb hull weight will weigh a bit over 5000lbs with the engine/drive........with an SBC, a 1.81:1 drive even with 22p props will still be too high. because of the RPM the engine must turn at WOT (4800-5000)

From Prop Slip Calculator using 15% slip, that combination would run approx 50 MPH at 5000 RPM. A (stock) 350 is going hard pressed to push a boat that size and weight at 50mph.

Also, with a 1.81:1 and running that high an engine RPM, you take those propellers somewhat above from their optimum and efficient RPM. (this why Mercruiser has several different ratio Bravo III's)

Just about ALL the boat manufacturers put 2.0 or 2.20:1 ratio Bravo III's in boats that size with an SBC.

With 22p props, and a 2.20:1 drive, you would need to turn approx 3500 RPM (a pretty "normal" cruising RPM for a 350) to go approx 28 mph........ I am not sure a stock carbureted 350 will do that. you may do it with a 383, etc)


And you certainly will be able to do it with a 454 or 496 though.



The 6.5L in my 1994 Suburban has over 200,000 miles on it since I bought it new. It will still pull my boat with no problems, doesn't smoke and will run right up to governed speed (3300 RPM) with no problems.


Tell us a little more about performance with the 6.5L engine. When it ran good (new), what RPM and max speed did you get at WOT?
Would it run right up against the governor? at around 3300-3400 RPM? (AKA the REV limiter on a gasser)


With your current set-up, according to the prop-slip calculator, it appears that your boat would *just* make it up on a plane at 2400rpm (approx 20-24 mph?)


That Bravo IIIx is worth a LOT of money. You should be able to trade it for a good 2.20:1 drive straight across.

What's the cost of playing around with props for one of these?

You can buy bravo III prop sets (2) on ebay for anywhere from $450 UP. I see 24p props all the time, used & undamaged for $500-600 (I paid $600 for my used 26p props about 6 or 7 years ago. and the prices haven't gone up much at all)

One other thought, is it worthwhile going with fresh water cooling? I fish the New York Bight and all salt.

And if I didn't say it earlier, I wouldn't put a "new" engine in that boat WITHOUT closed cooling!

 

[dieselgrady]

Re: A whole new engine

Before I put the Bravo 3X on the boat, I had a regular Bravo 3 that was 2:00 ratio. I still have the 24P props from that drive, and they're in good shape. The WOT on the Marinediesel Hammerhead 300SC was 3600RPM. It made that with the 22P props. It went about 35MPH at those revs. I'm not in a rush so I typically run the engine at 27-2800 rpm which is the bottom end of the 27-3200 rpm recommended cruise for that engine. The fuel tank will have to be flushed, the diesel return line from the injector pump will have to be removed and plugged. I'll also have to reinstall a bilge vent fan, as I removed it after the boat was converted to diesel. I'm sure there will be surprises as I proceed, but hopefully not enough to make me sorry I began the project. And of course I don't know if the alternator on the new 5.7L will be able to handle the 2@ 31M Sears Platinum AGM batteries that I have. (1150 CCA each) Again let me thank everyone for their thought, there's a whole lot more to this than I realized.

 

[HT32BSX115]

Re: A whole new engine

The WOT on the Marinediesel Hammerhead 300SC was 3600RPM. It made that with the 22P props. It went about 35MPH at those revs.

Ok,

From: Prop Slip Calculator

Those numbers above are right in the ball park with 15% slip (might be a "tad" low for a large heavy boat)

I was quoted under $10,000 for a new Mercruiser 5.7L that puts out 325hp, which is roughly the same as the diesel.

Quite frankly, 24p props, a 2.20:1 drive and the above engine will give you similar performance.

Without looking it up, I think that engine will want to turn 5000 at WOT? (and a range of 4400-5000?)

You can experiment with the 22 and 24p props for the best performance. You can also possibly get a 4 blade "front" prop if you're someplace in the "middle" etc.

Did you ever try the 24's?

 

[Walt T]

Re: A whole new engine

I guarantee you will regret installing a 5.7 and not a big block.