repowering

[scuppers]

my question is  theoretical. regarding fuel economy. is it better to have a lower hp outboard that has to work harder to propel the hull or is it better to have a higher hp outboard that works less? i have asked this question and recieved opposing answers. don't know what to conclude is correct?

[ddd222]

i prefer higher hp myself, but for optimum fuel efficiency, i would go for a good mix of hp and technology, but DON'T underpower, you'll hate the boat. Four stroke is very close to di technolgy, but is much heavier. I wouldn't put a 4 stroke on an old aquasport w/ out modifying for the extra weight. I would go 2 stroke

[GoneFission]

Engines run most rich at wide open throttle (WOT).  For example, my trusty old Merc 200 runs about 11 gallons per hour at 4000 RPM, and 18+ at WOT and 5300 RPM - that's a big difference.  So, all other things being even, running a smaller motor wide open usually will not give better fuel economy that running a larger motor at partial throttle.  

Several other factors play into this - number of cylinders and piston size, carb versus fuel injection, etc.  In general, more cylinders with smaller pistons are more efficient that fewer and bigger - but fewer and bigger pistons give more torque.  More compression gives more efficiency, but is limited by fuel octane combined with RPM, cylinder head temperature, and cylinder pressures.  An injected engine is more efficient at partial throttle than a carb engine can be, but they will be about the same at WOT.  Direct injection (DI) best atomizes fuel (more efficient at 5,000 to 25,000 PSI injection pressures), but the DI pump adds some 50-60 lbs to the engine.  This is what makes modern diesels so efficient - very high compression, direct injection at 10,000+ psi, and diesels run very lean at all speeds except wide open.  

Four strokes are more fuel efficient that two strokes because they do not have the intake/exhaust overlap, but two strokes put out more power due to each cylinder firing on each engine revolution - four strokes only fire on every other revolution.  A four stroke engine must have either more displacement, higher compression, more technology, or all of the above to put out the power of a two stroke.  Also, piston driven four strokes require a valve train and camshaft setup that adds weight and complexity.  It's easy to get 200HP+ from a 2.5 litre two stroke - Mercury has been doing it for 25 years in an engine that weighs under 500 lbs.  However, a four stroke that puts out 200HP+ is usually going to be over 3 litres, use electronic fuel injection, maybe even run a supercharger (Verado), weigh in at 600 lbs., and will cost a lot more due to the additional complexity and technology.  However, the four stroke will use half the fuel and will run quieter.  

Figure a 2000 hour life for a modern engine, and you will find few make it that far, unless being used professionally on an every day basis.  You usually need to get over 1000 hours use in order to offset the price of a four stroke with fuel savings.  So my advice is to consider your use and choose an engine that suits your use.  If you make very long runs on a frequent basis, a four stroke may make sense because of the fuel economy in the long run.  If your use is shorter and less frequent, or weight is a major concern, a less expensive and lighter two stroke may be better for you.  

I know professional fishermen who are on the water every day.  Some love their four strokes, some don't trust all that technology and insist on two strokes and carbs, and some won't have anything but diesel.  Why such a difference?  Because motor buying is a little like boat buying, it's not all about reason and logic - it's about what you want and what makes you happy!  And if what makes you happy and you want that $20,000 Honda 225 four stroke with the high speed lower unit and custom paint job, go for it!  Hey, I'm still trying to figure out if I could have put that 450HP 4 litre OMC racing motor on my 22-2CCP!  

My $0.02, and probably worth no more than that...

[ddd222]

gonefishin GREAT explanation and i agree wholeheartedly on every part but one, and one thing i mentioned quickly before but needs to be repeated. I esecially liked your explanation of compression, piston size etc., goes along w/ my explanation in the past of my boat when repowering. My mechanic explained it to me as such and made perfect sense, lower rpms for most purposes will lead to longer engine life. The person who started this thread didn't say boat size, make, etc., but lets use my repower for an example. My old evinrude 225 use to plane my boat at 35-3700 rpms,but need a little more for optimum lift and cruise and really needed 3900-4000 to cruise efficiently. My 250 yamaha planes at 3100 and cruises well between 34-3500. Seems at a glance to be not that big of a difference, but that small 2-500 rpm difference over the course of time will translate into longer life and negate any perceived fuel efficiency of the smaller hp motor. Have heard people here suggest to people w/ the same 222ccp as mine to repower w/ a 150 or 175hp for fuel efficiency. I originally had a 200, moved up to a 225, then bought new w/ the 250. With the added deep v of the ccp and a bit more weight of this model, anything under a 200hp would be a big mistake in my opinion. Putting a 150hp for example on my boat(rated for 240hp according to plate)I would venture to say that 2000 hour engine life to be trimmed down by at least 25%, maybe as high as 50%.
     The one item i disagree w/ you on was the 50% fuel savings w/ four strokes. Some data i've seen(maybe 1-2 years ago)had di and 4 strokes w/in 5% of each other, NOT including 2 stroke oil burn, just straight up gas to gas. Don't recall the source of that article but i think it was salt water sportsman or something like that. They also said that wasn't the case in every comparison but said a more realistic # would be between 10-20% savings. Now i could be wrong and i think this was and article from 05', but i don't think the di or 4 stroke efficiency has improved that much. I think there may be certain boats and applications where it may save as much as 30%, but i've never heard 50% fuel savings(once again i could be wrong). That being said the one thing i wanted to restate since this is a "classic" site is the weight of 4 strokes. Since most of our boats were not designed for the weight of 4 strokes there may be some "ass heavy" issues to deal w/ that likely will change the way an older boat sits at the dock, drifts, cruises etc. I think the bigger the boat the less issues u may have, but in my case my 250 efi weighs 480 if i remember right, whereas the same 4stroke was in the high 500's or low 600. Would have made a big difference in my 222 as it sits way ass heavy already. There are ways of moving weight forward like batteries etc. forward, heard off guys mounting console forward, reglassing transom to move scuppers higher. I work out on the water and i've seen older boats repowered w/ four strokes so its not impossible. Maybe some people don't realize a difference and are such recreational users the difference is minimal to them. But likely on an older aquasport or mako its gonna change the boat. Moving weight forward may band aid the issue in my opinion and address the scupper issue but the WHOLE boat will logically sit slightly lower in the water, raising the waterline slightly but different than it was engineered to do. Just a thought, maybe a long one but...

[GoneFission]

Right on - When I said a four stroke gets 50% better fuel economy, I was comparing it to a carbureted two stroke on an average economy basis.  An electronic port fuel injected (EFI) two stroke is usually 20-30% better than a carb model, a direct injection (DI) is usually better than port fuel injection, and a modern 4 stroke with fuel injection and variable valve timing (think Honda VTEC) will be better than DI.  

So if you were to compare my old Merc 200 carb model with a new, shiny Honda 200 four stroke - I think the Honda uses about 6 gallons per hour at 30 MPH, where my Merc uses 11 gallons per hour -  that's pretty close to 50%.  

I agree with you that a 150 would be underpowered on a 222CCP.  If I were repowering my 22-2CCP, I would probably go the route you went - with a 250HP two cycle - either Merc or Yammie, with EFI.  But I don't spend every day on the water and only put a couple hundred hours on in a good year.  I also like the under 500 lb weight compared to over 600.  

By the way, what's top speed for you with the 250?  My 200 max'es out at 40MPH and 5300RPM with a 19 pitch prop...  

See ya on the water!

[John Jones]

Yeah, what Capt. John said.  If I could replace my tired old 225 Oceanrunner with a new DFI I would expect a significant increase in economy.

I can get 40 MPH on my 22-2 with the 225 at about 5800rpm but I have more wind resistance than most.  The Navman fuel meter shows 25-26 gph at WOT.  

Cruising at 3800-4200 burns 13-14 gph.  On a normal fishing trip of say 50 miles round trip which would be mostly steady cruising speed I get about 1.8 mpg (when I remember to reset the gps trip computer).

[GoneFission]

Here's my mileage chart - best is just over 3 MPG at 25MPH and about 3750RPM - my boat does not really plane well below 3000RPM:



See ya on the water!

[ddd222]

top speed i've gotten w/ my 250 yami is 51 mph