Mercruiser 5.0 vs. twin 7.4's

[briggs09hp]

Hello,

I am currently looking to upgrade boats. I have a 1995 Searay 20' cuddy with a 5.0L and Alpha 1 outdrive. It is very nice and I have had no problems with it, but I was riding around in the car with my dad the other day and we passed a boat sitting outside an auto repair garage that really caught our eye. We stopped and looked at it and I am considering buying it. It is a 1988 Sea Ray Pachanga 32. It has twin 454's.

So anyways my question. Now I know I will use more fuel if I get that boat. But I guess I'm wondering how much. Its probably a question that's hard to answer. I mean my boat now has a 29 gallon tank. This 32 foot has a 160 gallon tank. That's 5.5 times as large! So does that mean that boat uses 5.5 times as much fuel?!?! Or does it have a larger tank to give the boat a longer range?

On an average day for me, in my small block 5.0, I would say that an 8 hour Saturday on the lake with 5-6 people on board, some cruising, some just floating around, some wake boarding, and tubing. I use about 10-15 gallons. So if im gonna use like 20-30 gallons with the new boat, I can live with that. But if its gonna be 55 gallons, no way! I know it all depends on how you drive and lake conditions. But assuming pretty flat conditions, and just light cruise. Can anyone give me an estimate on how much fuel I would use in a 32 foot boat with twin Magnum 7.4's? Anyone have twin 454's? I cant believe that. That's 908 cubic inches! That has to be crazy to drive.

I love the boat, but if its going to suck fuel like crazy and make me poor, I'll skip it and look for a single engine boat.

Thanks for any help,

Matt

 

[HT32BSX115]

Re: Mercruiser 5.0 vs. twin 7.4's

Howdy,


In a nutshell, YES. any boat with twin 454's will "suck fuel like crazy".

My 21ft boat with a 454 burns 10 gal per hour at 3000 RPM and about 15 GPH at about 4000

3000-3500 is a pretty good cruising power setting for any boat with a big V-8. You can expect to burn 10-15 gph PER SIDE cruising around.

If 20-30 gallons per hour sounds good to you, go for it!

 

[tpenfield]

Re: Mercruiser 5.0 vs. twin 7.4's

I 'had' a Formula 242 w/ 7.4 Mercruiser . . . averaged about 7 gallons per hour, as I am pretty light on the throttle.

I am in the process of getting a 33 foot Formula with twin 7.4 Mercruisers. Probably gonna get about 14 gallons per hour. .

The thing about a longer boat with twins is that it stays on plane easier, so you can back down on the throttles to save fuel. With a single engine 20-something boat, you have to stay up at the 3K+ rpm level in order to stay on plane.

Gauging fuel consumption by the size of the fuel tank would not be a good approximation, it would be more related to the size of the engine. So, you will probably burn about 3 times the fuel, not 5 times.

 

[HT32BSX115]

Re: Mercruiser 5.0 vs. twin 7.4's

I measured mine with a Lowrance LMS-200 fuel flow gage. It's pretty consistently 10gph at 3000rpm/30mph (3mpg) and I have a Bravo III drive with 1.81:1 ratio and 26p props. If I had a single prop drive, I would use more fuel. Knowing what I know now, I wouldn't get a twin that didn't have either Bravo III's or Volvo Duo-prop drives.

A heavier larger boat with the same engines turning the same RPM (and propped correctly) will do the same fuel flow (each side, 20gph total) and the MPG will naturally be less. (it's sure not going to go 60 mph at 3000!)

Now, maybe he can operate at a much lower RPM like 2500, but it (of course) will have to be at an RPM that keeps the boat on a plane.

Your fuel flow will entirely be in your hands (in this case, most likely your right hand!)

Learn to trim the boat with the drive trim controls. You cannot use the trim gage since they are not very accurate, and drive trim requirements change greatly with speed and rpm. You'll have to adjust every time you change the speed if you're cruising. Having an accurate speed measurement(GPS) and fuel flow gage will allow you to adjust for maximum nautical miles per pound of fuel. (or MPG!)

If it doesn't have Bennett Trim tabs (not fixed, spring-loaded tabs....they just add additional drag) on it, I would plan on installing them


If you're even remotely worried about the price of gasoline, you might consider staying with your SBC powered boat....


ymmv.......


Rick

 

[briggs09hp]

Re: Mercruiser 5.0 vs. twin 7.4's

Alright let me ask this question. Lets take the same exact boat, but with 2 different engine configurations. 1988 Sea Ray Pachanga 32. One has twin 454's. One has twin 350's. But they are operated in the same way, at the same speed.

Do you think there would be much difference in fuel consumption then? Im thinking that the small blocks would use less fuel. But i've also heard that the big blocks have more power, and therefore have less demand on them, and therefore don't use as much fuel as you would think.

THh whole reason im asking is because I found 2 boats. One is very nice, and it has the 454s. But the other not in quite as nice a shape, and it has the 350's. The 350's boat is $10k cheaper though. And thats alot of fuel money.

What do you think?

Thanks,
Matt

 

[QC]

Re: Mercruiser 5.0 vs. twin 7.4's

The 5.7s will use a little less fuel, maybe . . . At same cruise speed, almost identical and not worth counting at this point. Are these Bravo 1s? I am going to disagree with Rick re: Bravo 3s as this boat may be over 60 MPH depending on the horsepower ratings.

Totally agree with Rick on Trim tabs. May have k-planes even.

 

[tpenfield]

Re: Mercruiser 5.0 vs. twin 7.4's

Alright let me ask this question. Lets take the same exact boat, but with 2 different engine configurations. 1988 Sea Ray Pachanga 32. One has twin 454's. One has twin 350's. But they are operated in the same way, at the same speed.

Do you think there would be much difference in fuel consumption then? Im thinking that the small blocks would use less fuel. But i've also heard that the big blocks have more power, and therefore have less demand on them, and therefore don't use as much fuel as you would think.

THh whole reason im asking is because I found 2 boats. One is very nice, and it has the 454s. But the other not in quite as nice a shape, and it has the 350's. The 350's boat is $10k cheaper though. And thats alot of fuel money.

What do you think?

Thanks,
Matt

I think if you have an opportunity to get a boat with 350's versus 7.4's . . . and the boat is easily powered by 350's . . . I would go for it.

If the boat really needs the 7.4's, you are better off with them.

 

[HT32BSX115]

Re: Mercruiser 5.0 vs. twin 7.4's

I am going to disagree with Rick re: Bravo 3s as this boat may be over 60 MPH depending on the horsepower ratings.

I meant at 3000 RPM with 7.4L V-8's

If it's got 800HP diesels installed, it might do 60mph at 3000 RPM!

One has twin 454's. One has twin 350's. But they are operated in the same way, at the same speed.

Do you think there would be much difference in fuel consumption then? Im thinking that the small blocks would use less fuel. But i've also heard that the big blocks have more power, and therefore have less demand on them, and therefore don't use as much fuel as you would think.

This has been tested by Boating magazines many times.

The exact same boat loaded the 'same' with different engines, will burn approximately the same fuel to go the same speed. The RPM might be different because the SBC's might need to turn faster but they're going to flow about the same amount of fuel.

Both engines will exhibit approx the same BSFC (Brake Specific Fuel Consumption)

There's no free lunch!

 

[tpenfield]

Re: Mercruiser 5.0 vs. twin 7.4's

Another factor to consider, particularly with an older boat, is that SBC's are far less expensive on a re-power than the BBC engines.

As stated, fuel consumption between the 2 engines on the same boat is not going to be much different. A more important issue is to understand if the boat is underpowered with a smaller engine, as that will be problematic when you go to sell. Just remember, when you buy a boat, you have to think about "how the heck am I gonna sell this thing?"

 

[TilliamWe]

Re: Mercruiser 5.0 vs. twin 7.4's

briggs, I'll confuse you even more, okay?

My dad's 1988 Sea Ray 268 cruiser had a carbed 454, and it got 1.5 miles per gallon at a 3200rpm cruising speed.

His 2004 Chaparral Signature 300 (weighed about the same, maybe a touch more than the Sea Ray) with Twin 5.0EFIs got 1.2.

So what do you think twin 454s are gonna get? 1mpg if you are lucky, at a reasonable cruising speed. A lot less if you run it hard, like most Pachanga owners would.

 

[briggs09hp]

Re: Mercruiser 5.0 vs. twin 7.4's

Thanks for the replies. Its nice to get some opinions. I was told that 8-10 gallons per hour, per engine would be a safe bet. I mean yeah, it has twin big blocks with thru hull exhaust. Of course I would like to run it hard sometimes and its gonna use a ton of fuel. I mean even my small block uses a ton of fuel when i have it at top speed. But i think most times I would be happy just cruising around. Or floating with engines off, lol.

But just another question. Been doing some research and i just wanted some feedback. I went to look at the boat yesterday and i couldn't tell cause both drives were painted black with no stickers on them. But did they have bravo drive back in 1988? What if they are alpha drives? I heard that a 7.4 will be too much power and torque for an alpha. I was also told that those boats had TRS drives?

I dont wanna get a boat thats gonna eat through out drives. Why would Sea Ray put drives on a boat that are not strong enough for the engines in it?

 

[babbot]

Re: Mercruiser 5.0 vs. twin 7.4's

For me a better way to compare is $/hr not gal/hr. These days it is over $4/gal at the dock so you are looking at about $64/hr-80/hr (which sounds conservative to me of a boat that size and power)
If a boat is worth that to you then go for it.

 

[TilliamWe]

Re: Mercruiser 5.0 vs. twin 7.4's

...But did they have bravo drive back in 1988? What if they are alpha drives? I heard that a 7.4 will be too much power and torque for an alpha. I was also told that those boats had TRS drives?

I dont wanna get a boat thats gonna eat through out drives. Why would Sea Ray put drives on a boat that are not strong enough for the engines in it?

No Bravo in 88.
If they are Alphas, you can make them last by not abusing them. (That Sea Ray I referenced had an Alpha that never broke in 15 years of pushing that HEAVY boat around)
More likely they are TRS drives.

There were only two choices in 1988, Alphas and TRS. TRS requires a transmission inside the boat, are heavy, are expensive, and have limited expensive props.

 

[tpenfield]

Re: Mercruiser 5.0 vs. twin 7.4's

Bravo's did come out right around 1988 or 1989, but 1987 boats went with alphas.

If you do take a look at the boat again, see if you can get a serial number off the outdrive somewhere, or even a picture or 2. The Bravo1 has a fairly distinct fin shape to it.

 

[briggs09hp]

Re: Mercruiser 5.0 vs. twin 7.4's

They are alphas. From what i can figure out, the owner did a repower in 1999. He replaced the 330 HP 7.4's with magnum 365 HP 7.4s. The motor covers say Bravo 1 on them, but they are not bravo drives.

My dad talked me out of buying the boat. I mean assuming I use 20 gallons an hour, that's $80 just to take it out for an hour. Fuel here is $4.09 at the pump. That would be alot of 5 gallon gas cans (which is how i currently fuel my boat) On the water its over $4.50. Could I buy the boat? Yes, its do able for me. But it would run me broke to operate it. I would have this beautiful boat, and not be able to run it.

Now I understand why all the larger boats in my marina never leave the marina. The owners probably don't wanna pay for fuel.

I think ill look for a Baja 24 or 26 foot with a single 7.4 and Bravo drive. Something like that. Maybe a nice cuddy with captains choice exhaust. I think the outlaws are cuddys and the islanders are bow riders.

Thanks for all the help,

Matt

 

[HT32BSX115]

Re: Mercruiser 5.0 vs. twin 7.4's

They are alphas. From what i can figure out, the owner did a repower in 1999. He replaced the 330 HP 7.4's with magnum 365 HP 7.4s. The motor covers say Bravo 1 on them, but they are not bravo drives.

.....................
Matt

That could be a problem.

Mercruiser did put the 330hp (crankshaft HP) BBC engine in front of the Alphas for a very short period. They wanted to be able to compete with OMC's (340hp) 460 and King Cobra but didn't have a drive to use so they used the Alpha. (The (460 model) King Cobra by the way, was MUCH bigger/heavier than the "smaller" Cobras that were used with the small block GM and Ford V-8's.)

Depending on how some (not all) people worked their throttle hand, some of them had problems with stock the 454/Alphas. I suspect that Mercruiser covered any problems with them quickly & quietly (under warranty) but after the next year or so, only offered the 454 with the "new" Bravo drive.

There are plenty of people here that will relate experiences running an Alpha with much more powerful engines than stock. And there are some that have rebuilt Alphas with (evidently available) tougher internal gears carriers, bearings, etc



Most agree that if you install a much more powerful engine in there (in excess of the approx 300hp "limit") and you will have to be even "that" much more careful to keep from breaking them.

If I had done the repower in that boat, I would have also put Bravo I's or Bravo III's in it as well.


Cheers,


Rick

 

[briggs09hp]

Re: Mercruiser 5.0 vs. twin 7.4's

Alright just to be sure. The numbers I could find on the drives are: 0B926198. Looks like the port and starboard drives are the same numbers?

So anyways I just did a test today. With my 20 ft boat, powered by a 5.0 mercruiser. 21 pitch prop. Cruising on plane at about 2800 RPM. I used about 9 gallons of gas going about 16 miles. Maybe about an hour, or hour and a half run time.

So thats about 9 gal/hour. Costed me about $40. About 1.8 MPG. I wish i brought my GPS, but my boat speedo said about 20 MPH

So if the twin 454 boat gets 1 MPG, i think that's not bad at all. The owner says he cruises at 38 MPH at about 2500 RPM. 48 MPH at 3500 RPM. 4 blade props. Max speed about 63 MPH. I mean id get 16 miles way faster in that boat at a lower RPM than my boat, so maybe its not so bad?

I mean yeah, if i go tubing or wake boarding in the twin 454, the fuel consumption would be outrageous, with all that stopping and starting.

Sound about right to you guys? Sorry, just thinking out loud.

I might reconsider buying a Pachanga. I found another one, but its 500 miles away and $10,000 cheaper. I would need to have it shipped. Any ideas? I just submitted quotes to about 10 companies i found on a Google search.

Thanks,

Matt

 

[TilliamWe]

Re: Mercruiser 5.0 vs. twin 7.4's

So anyways I just did a test today. With my 20 ft boat, powered by a 5.0 mercruiser. 21 pitch prop. Cruising on plane at about 2800 RPM. I used about 9 gallons of gas going about 16 miles. Maybe about an hour, or hour and a half run time.

So thats about 9 gal/hour. Costed me about $40. About 1.8 MPG. I wish i brought my GPS, but my boat speedo said about 20 MPH

You are doing something VERY wrong with MPG numbers like that. Even with a carb, your boat should get at least 3mpg. You either figured the gallons wrong, the mileage wrong, or your engine is sick.

 

[briggs09hp]

Re: Mercruiser 5.0 vs. twin 7.4's

The engine runs great to me. I mean this is my first boat, but i know alot about engines and automotive. Ive built race motors and things like that for cars. Plus I had my mechanic check it out and tune it up when i bought it. He says it runs great too.

I wonder if i should be running the engine a little faster? Maybe its not in the sweet spot yet and its working harder. Im just figuring the slower i go the slower RPM's would mean less fuel consumption.

I mean i can check the carb. Its the stock Mercruiser 2 bbl. This morning the fuel gauge said 1/8 tank. I put 10 gallons in using two 5 gallon gas cans. When i returned to the dock the fuel gauge said the same thing as when i started, 1/8 tank. I did about a half hour of idling though, cause i was helping a sail boat that tipped over. So i figure i used a gallon idling.

 

[TilliamWe]

Re: Mercruiser 5.0 vs. twin 7.4's

fuel gauges are not accurate in boats. You haven't figured the gallons used correctly.

But also, 2800 rpm is probably a little too slow to run the engine at "cruising" speeds. These V8 boat engines seem to be cammed to produce better power at about 3200 rpms.