Mercruiser 5.0 vs. twin 7.4's

[QC]

Re: Mercruiser 5.0 vs. twin 7.4's

If your boat doesn't get 3.5 MPG at cruise something is wrong. It may be OK at 2800, it's more about speed of the hull than the motor. Smaller boat, lower cruise RPM can be very efficient. With that said, most boats cruise best around 3000 - 3500 RPM as Tilliam notes. Heavier cruisers usually 4000. Lighter hulls with more power can be below 3000.

 

[HT32BSX115]

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

That 5.0L V-8 has something wrong with it if it's burning 9gal/hr at such a low power setting and speed.

The only way you can accurately measure fuel burn is to fill it up and run it at constant speed over a closed course for 1 hour and fill it back up again.

And the only way you will determine accurate speed is to use GPS (pitot speedometers are notoriously inaccurate!) While you're at it, get another tach and compare. Tachometers are frequently off too.

What is your WOT (flat out) speed and RPM?

 

[tpenfield]

Re: Mercruiser 5.0 vs. twin 7.4's

I am buying a 33 foot boat with twin 7.4's . . . So, I'll be able to tell youze guys for sure how much fuel you should expect to burn per hour.

However, it won't be until next Spring when I put the boat in the water

Based on BoatTest results, I am expecting something around 18 gallons per hour (total) Cruise and 12 gallons per hour overall.

 

[briggs09hp]

Re: Mercruiser 5.0 vs. twin 7.4's

My WOT RPM is about 4600. 48 MPH with the GPS.

I dont think my estimate is incorrrect about the fuel. I know the gauges are inn accurate. But Its always used that kind of fuel.

Im taking my boat to another lake today. I will fill the tank at a gas station on a trailer. I will track my overall run time and mileage with a GPS. Then i will fill it back up and tell you my findings.

I dont think my estimates are that far off. It always uses that much fuel. So im kind of used to it.

 

[TilliamWe]

Re: Mercruiser 5.0 vs. twin 7.4's

My WOT RPM is about 4600. 48 MPH with the GPS...

I dont think my estimates are that far off. It always uses that much fuel. So im kind of used to it.

That WOT rpm isn't too low, but it could be 4800. That speed isn't too low. Someone on here will run that through the propslip calculator for sure.

If you are used to that much fuel burn, that's too bad. It should be much better. A fuel flow meter might be a good idea/experiment for you.

 

[HT32BSX115]

Re: Mercruiser 5.0 vs. twin 7.4's

That WOT rpm isn't too low, but it could be 4800. That speed isn't too low. Someone on here will run that through the propslip calculator for sure.

If you are used to that much fuel burn, that's too bad. It should be much better. A fuel flow meter might be a good idea/experiment for you.

Can't do a prop-slip calc without the drive ratio. (can we assume it's a 1.5 or 1.47?)

And those numbers really aren't right with a 1.5:1 because the slip would be 25% with a 21p prop......WAY too high for most planing runabout boats....... (should be around 12-15 or so for a single prop)

When I had the 460 in my boat (below) with the 1.43:1 King Cobra and 20p prop, 5100 RPM would produce 60 mph........which corresponded to about 12% slip

How much does that boat weigh?

 

[QC]

Re: Mercruiser 5.0 vs. twin 7.4's

And those numbers really aren't right with a 1.5:1 because the slip would be 25% with a 21p prop......WAY too high for most planing runabout boats.......

If it is right, that could be where the fuel is going Maybe soaked foam, maybe dirty bottom, maybe munched prop, maybe cruddy prop, maybe that stuff plus bad tune, bad compression. I'll even say it could be a spun hub, but I never believe those will reach any decent speed although some have reported it here. All speculation of course, but two things aren't right with what we have. Fuel consumption too high, and prop slip (if a 1.5 ratio) is poor. They could be related.

 

[HT32BSX115]

Re: Mercruiser 5.0 vs. twin 7.4's

If it is right, that could be where the fuel is going Maybe soaked foam, maybe dirty bottom, maybe munched prop, maybe cruddy prop, maybe that stuff plus bad tune, bad compression. I'll even say it could be a spun hub, but I never believe those will reach any decent speed although some have reported it here. All speculation of course, but two things aren't right with what we have. Fuel consumption too high, and prop slip (if a 1.5 ratio) is poor. They could be related.

Yeah. I'm always sceptical of "spun" hubs and couplers running for more than a few minutes!

I think most (if not all) drives behind V-8's have ratios close to 1.5 or so......

 

[QC]

Re: Mercruiser 5.0 vs. twin 7.4's

I agree it's probably a 1.5. I also agree on couplers and hubs. Not only are they short lived if they're in trouble, they also don't reach any significant speeds. They can't transmit horsepower, that's why they're "spun"...

 

[TilliamWe]

Re: Mercruiser 5.0 vs. twin 7.4's

...

How much does that boat weigh?

"1995 Searay 20' cuddy with a 5.0L and Alpha 1 outdrive" I'll bet a nickel it should weigh around 3300lbs. That's what my 97 Chap 20.5' cuddy 'weighed', according to NADA. NADA says 3050lb for his boat.

I also agree, that should be a 1.47 drive ratio. But what if it's a 1.65? What would that make his propslip?

 

[QC]

Re: Mercruiser 5.0 vs. twin 7.4's

But what if it's a 1.65? What would that make his propslip?

13% which would be very normal if it is a 1.65 http://www.mercuryracing.com/propellers/propslipcalculator.php

 

[HT32BSX115]

Re: Mercruiser 5.0 vs. twin 7.4's

"1995 Searay 20' cuddy with a 5.0L and Alpha 1 outdrive" I'll bet a nickel it should weigh around 3300lbs. That's what my 97 Chap 20.5' cuddy 'weighed', according to NADA. NADA says 3050lb for his boat.

I also agree, that should be a 1.47 drive ratio. But what if it's a 1.65? What would that make his propslip?

Well using http://www.go-fast.com/Prop_Slip_Calculator.htm

That would make it about 17% not out of the realm of possibility for a boat like that. Judicious use of trim might reduce it a little.....

EDIT: I got 17% using both of them (the go-fast site and Mercury) .... his numbers were 4800, 48mph, 1.65, 21p weren't they?

Wouldn't a 1.65:1 drive be the wrong one for that engine?

 

[TilliamWe]

Re: Mercruiser 5.0 vs. twin 7.4's

Only 4600 WOT. That gear ratio could be used with that engine, sure. Just not likely in that boat.

 

[QC]

Re: Mercruiser 5.0 vs. twin 7.4's

Yeah, Rick. 4600 was what OP reported.

 

[HT32BSX115]

Re: Mercruiser 5.0 vs. twin 7.4's

Yeah, Rick. 4600 was what OP reported.

I need to start wearing my glasses!!

 

[QC]

Re: Mercruiser 5.0 vs. twin 7.4's

Can you suggest same to Mrs. QC? You should see what she brings home from her gathering excursions . . . Can't read labels :facepalm:

 

[HT32BSX115]

Re: Mercruiser 5.0 vs. twin 7.4's

Can you suggest same to Mrs. QC? You should see what she brings home from her gathering excursions . . . Can't read labels :facepalm:

Um, if she's anything like Mrs "HT" I'm NOT going to go there!

 

[briggs09hp]

Re: Mercruiser 5.0 vs. twin 7.4's

Sorry it took so long to reply. I did do my fuel consumption test on my boat. I tried to find a gear ratio on the out drive but could not find one. The serial number of the drive is 0F47C133 if that helps

Anyways, I went 40 miles on 18 gallons of fuel. So thats 2.2 MPG. Cruising at about 27 MPH using the GPS. About 3200 RPM. Lake was pretty calm, so it was a smooth ride. I filled the boat up on the trailer at a gas station before i launched. Then filled it back up when i was done. 18 gallons.

So if what you guys are telling me is true, its sound like i'm already using as much fuel as a big block boat for some reason. Ill rebuild the carb for sure, but other than that, plugs are new, as well as wires, i checked the timing myself and it is right on. Ill check compression at the end of the season.

Now if if im gonna buy this other boat, i need to find a trailer for a 32' boat. Which poses another problem. Any ideas? looking maybe $3000 for a trailer?

Thanks,

Matt

 

[TilliamWe]

Re: Mercruiser 5.0 vs. twin 7.4's

...
looking maybe $3000 for a trailer?

If you're lucky.

 

[pachanga27]

Re: Mercruiser 5.0 vs. twin 7.4's

My .02 cents worth. Like Briggs09hp - I have a Sea Ray 20ft Seville Cuddy - older -1988- The original motor - a 4.3L with a two barrel carb - bless it mechanical soul - after 18 years of faithful service - I replaced it with the current motor - remanufactured 4.3L with the four barrel carb. The 4.3L with the four barrel or the two barrel is/was very economical to operate. However - some years ago - I spotted a 1989 Sea Ray Pachanga 27 ft with twin 350 Magnum Mercruisers for sale. From having the 1988 Seville - unless the Pachanga had been abused - the Pachanga would be a solid built boat. Engines can be replaced - but hulls are not as easy to correct. Keeping the Pachanga with the small blocks in mind - I looked at other boats with twin motors - specifically the big blocks - like a Formula with 454s - (Pachangas are NOT exactly on every block for sale) and tried these big blocks out - looking at gas cosumption, ride, holeshot, cruising, speed, etc. Since the lake where I boat (Lake Murray, SC) is large but not huge - I found out a boat with 454s was not the hot ticket for me. Taking the 27 ft Pachanga with the twin 350s for an extended ride - the 350s were the best combo for me and the lake. I bought the Pachanga 27 for a very reasonable price and it has proven to be a solid boat - the 350s are actually very economical to operate - Cruising speed - approx 40mph by the pitot tube speedo - gas usage is pretty darn good - even though I have not measured it - Just going on how much I have to fill the tank back up after a day on the lake and overall performance is excellent for this old boat. Gets on plane quickly - accelerates from 3500 rpm to WOT very nicely and is an easy boat to operate - the twin screws makes docking a breeze and handles rough water without a whimper. I looked for a Pachanga 32ft ..but could not find any available within reasonable distance for sale... and if there had been one available... I may have bought one. Bigger is always better...right? But in the end - I think I bought the best boat for my boating area. Ironically - I still have my 1988 Seville and the poor thing has not been on the lake in 2 years now... Thankfully I have it stored properly in a heated and air-conditioned building. In the end - depending on the size of your lake or where you boat - the Pachanga 32 might be a good choice. It's a pretty simple boat. I think they are pretty tough overall also. Sea Ray IMHO made a pretty solid boat 30 years ago. I did just replace the port outdrive with a SEI outdrive. It started to make ugly noises - but after 23 years of service - I can't complain. The starboard outdrive is still good. Both Mercruisers are still going well. No problems over the years - regular oil changes, impellers replace every two years, etc. My opinion - take a good look at the 32 foor Pachanga and if it has not been altered - the Alpha drives and the motors - if stock should be good for years of service. But... if you find a 27 foot with a the 350s... I don't think you can go wrong. Again ... if the boat overall has not been abused. Since i have not taken any measurements to see exactly much gas I use..I will do that next weekend.