Efficiency? Getting the most bang for the buck.

[newbster]

I'm seriously considering ditching my MCM 470 motor and re powering. My question is a matter of bang for the buck.

Question 1: Am I correct in assuming that one will get the same efficiency out of different sized motors at cruise speeds? Say I'm running a 200 hp motor and cruise at a clean plane speed of 25knots. If I'm running a 250 hp motor can't I also run at the same gph at the same speed? You have maybe 100 lbs extra weight but are running the other engine easier since its spinning a bigger prop. I'm thinking it takes x amount of HP to deliver a given speed so unless you are pushing a motor hard you should be getting the same efficiency within a few percent.

Question 2: Am I better off going with the latest and greatest? More modern technology should be the most efficient. The extra cost can be amortized in getting better efficiency. If I save 10% in gas per trip and burn 100 gallons then each 100 gallon run saves me 10 gals x 3 bucks = 30 bucks per trip.

Its been a while since I have had a boat but think I'm on the right path. In my case I went out of my way to get an aluminum boat to save weight. I'm looking for the best path since if I'm going to spend money I might as well get the most out of it in the long run.

 

[skeat]

Re: Efficiency? Getting the most bang for the buck.

It's possible that a bigger engine could use less fuel if it's operating in a more efficient mode as you describe and last longer as well. ie: running a bigger engine at 2500 rpm instead of 4200 in a 4 cyl. But engines are not 100% efficient so as displacement goes up so does the amount of fuel that is not used to propel the boat. There is heat, friction, incomplete combustion etc. You can then go too big and lose any savings you were hoping to gain. There is a ton of testing that goes into new boats/engines and guys are paid a lot of money to figure this stuff out. The new stuff with fuel injection, electronic ignition is definitely more efficient even if for the sole reason it doesn't get out of tune as easily as the older carb/points system. Best bet is to talk to owners of the same boat and find out what engine/props they are running. From there you should be able to make a better decision as to what you want to do.

 

[Mischief Managed]

Re: Efficiency? Getting the most bang for the buck.

There will be some variation, but yeah, two different sized engines will use the same amount of fuel to push a given boat at a given speed, assuming the boat is propped correctly and there's not a huge difference in weight.

I think you'll discover very little difference in fuel consumption with fuel injection vs. a carb on a boat. FI works better than a carb in a car because of the fairly small throttle openings and low fuel flow a car uses at cruising speeds and the closed loop nature of modern car FI. Boats operate with much larger throttle openings than cars and just swill air and gas at cruising speeds. The open loop design of boat FI means it cannot make adjustements to the mixture on the fly and the norm is to err on the side of rich with boat FI. Carbs are completely in their realm when mixing large amounts of fuel and air at medium to large throttle openings.

That said, if you plan to spend a lot of time at idle or plan to change altitudes a lot. FI is a good idea. It's also really nice to have and adds resale value.

 

[QC]

Re: Efficiency? Getting the most bang for the buck.

Well my experience is that EFI does make a difference in a boat on fuel. Also, sometimes the smaller engine gets better fuel efficiency than the large, but not always. Diesels almost always use the same amount of fuel when compared to load, even larger and smaller engines. Throttled engines are a different animal . . . Anyway, to think this is ultimately an economic decision is, uh . . . well, a pretty tough case to make. 10% is a stretch, but even so, if you save $30 and the repower costs $3K, that's 100 trips. My guess is that you will "save" $10 . . . so even if the repower only costs a grand that's still 100 trips

 

[wca_tim]

Re: Efficiency? Getting the most bang for the buck.

It has been my experience that repowering anything in the name of efficiency typically costs much more than I will recover in efficiency. by the time done with repowering, will easily be a few thousand bucks - especially if you count your time as worth anything at all. that'll buy an awful lot of gas...

 

[newbster]

Re: Efficiency? Getting the most bang for the buck.

I'm going to get rid of the motor one way or another so the questions revolve around which repower choice is the best. I'm not talking about repowering just for a more efficient setup. The current motor will need to be replaced. If I'm looking for something in the 225-250 hp range, whats my best choice...

I'm even debating converting with an offshore bracket to outboard.

It comes down to the best bang for the buck over the long term as well as most value added.

 

[dcg9381]

Re: Efficiency? Getting the most bang for the buck.

I'm going to get rid of the motor one way or another so the questions revolve around which repower choice is the best. I'm not talking about repowering just for a more efficient setup. The current motor will need to be replaced. If I'm looking for something in the 225-250 hp range, whats my best choice...

I'm even debating converting with an offshore bracket to outboard.

It comes down to the best bang for the buck over the long term as well as most value added.

Outboard? I thought you wanted an efficient boat?

There are tons of choices in the 200-250 hp range.
I don't know the 3.7L Merc 470 very well so I don't know if repowering means that you've got to repalce the outdrive or not... Certainly rig up new motor mounts and do the engine alignment.

In that power range the bang for the buck is probably a very basic smallblock GM motor. 305 or 350.

The 4.3L V6 (which is 3/4 of a V8) can also be brought to low 200hp range - especially in the Vortec/MPI versions. For ease of install, I'd probably look at taking a Vortec head motor and bolting a carb on it. I believe they are roller cam and you can get roller rockers for them - decreasing internal drag a bit.

I'm a fan of the Ford 5.0L motor - it's right in that power range, the distributor is a little easier to deal with and it has a lot of aftermarket support. It's not quite as solid as a 4-bolt 350, but you're not going to be pushing it to past 300hp.

In regard to what engine is most efficient at which speed, I'd say you want an engine that's operating in it's efficiency range, which is typically peak torque range. I don't agree with the statement that X speed requires Y fuel regardless of motor - from tuning EFI motors, I know that engine efficiency differs radically between motors and RPM ranges. It's going to be tough to figure this out up front, because you're manipulating weight, balance, and prop pitch... You could make some educated guesses based on similarly sized and weighted boats.

I love the MPI (EFI) motors - they're pretty easy to install and the EFI is stand alone... Pricey, unless you can get a good deal on a used one.

 

[dcg9381]

Re: Efficiency? Getting the most bang for the buck.

A simple solution for repower...
"http://austin.craigslist.org/boa/439875357.html"

 

[QC]

Re: Efficiency? Getting the most bang for the buck.

Currently OBs are the most efficient, small boat gasoline power plant . . . Weight matters . . . a lot. There is an ongoing debate between the Merc Optimax and the ETEC, but believe it or not, also due to weight and lower maintenance, all other things being equal the latest 2 cycles are winning in operating costs . . .

 

[backwater dawg]

Re: Efficiency? Getting the most bang for the buck.

So you bought a alum boat to save weight--alot depends on what size boat you have--if you are running a smaller boat I agree that big $$$ changes will never pay out---if youre looking for increased pref and speed??? maybe a different story???

 

[jackcaptjr]

Re: Efficiency? Getting the most bang for the buck.

Hello all. Not sure if the repower issue could be resolved with some brand new 470 long blocks. They have all internal parts including the water pump, oil pan and valve covers. I have 2 that I purchased in the late 80's as spares for my Sea Ray. They have never been used and have been in my shop sealed up in plastic bags ever since. If you have an interest shoot me an E-mail jackcaptjr@aol.com.

Thanks and have a great day

JR

 

[180shabah]

Re: Efficiency? Getting the most bang for the buck.

You got a couple of choices for I/O power, both of which are going to require new stringers/mounts be fabricated. It will add a few days to the project, but when you go to sell the boat, not having the 470 will make it worth it.

4.3L V6 depending on year of block and components used is good for 160 - 250 hp. Above about 180hp or so, you will want to change the gear ratio in the drive.

SBC 5.0-5.7L again hp can be anywhere from 200 to 300+ and will require a gear change.

Any of the above will be better than what you currently have, and will bring more money at resale.

Personally, I would go with a 5.7L The amount of work involved is the same no matter which of the above you chose. 250hp is a fairly light demand of this engine, you have room to increase later if you decide, will bring the highest resale, and will cost roughly the same as a 5.0 or 4.3 to build.

 

[Don S]

Re: Efficiency? Getting the most bang for the buck.

Here would be your first choice for performance, and effeciency for your size boat.
Not the cheapest, just the best.
Everything else is a compromise
.
.
.
.
IMHO


http://www.volvo.com/NR/rdonlyres/81BAD056-9AD2-4DAF-9B78-CD5827B4D446/0/43_DPS_08.pdf

 

[180shabah]

Re: Efficiency? Getting the most bang for the buck.

A simple solution for repower...
"http://austin.craigslist.org/boa/439875357.html"

Hello all. Not sure if the repower issue could be resolved with some brand new 470 long blocks. They have all internal parts including the water pump, oil pan and valve covers. I have 2 that I purchased in the late 80's as spares for my Sea Ray. They have never been used and have been in my shop sealed up in plastic bags ever since. If you have an interest shoot me an E-mail jackcaptjr@aol.com.

Thanks and have a great day

JR

Is it possible that two different people BOTH bought two spare 470's and are now trying to sell them on a forum that doesn't allow selling?

 

[coastalcruiser]

Re: Efficiency? Getting the most bang for the buck.

Researched the question and found out the best fuel economy is on a plane. Reason why is friction, less boat in the water the more efficient. At wot the engine can use 2x more fuel! What I do is get on a plane using the trim tabs, throttle back until i'm only turning 2900 rpm instead of my max about 4200.
Also weight is critical, get rid of heavy junk you dont use. and if you have water tanks don't fill them unless you need to etc.

 

[180shabah]

Re: Efficiency? Getting the most bang for the buck.

Actually, most boats get better economy ideling along at displacement speeds. It's just not much fun.

 

[Don S]

Re: Efficiency? Getting the most bang for the buck.

Boating, in General for those in these forums, is Recreational, and has absolutely nothing to do with efficiency or economy.

 

[newbster]

Re: Efficiency? Getting the most bang for the buck.

Generally, you would get the highest mpg at low displacement speed. Your next best efficiency level is a clean planing speed where you are fully up and not settling in... about 20-25 for most boats. Pushing up on step or faster than a cruising plane is going to cost you more due to friction although different engines/configs change those points. Thats my experience.

I'm thinking a significant increase to 90% of the max rated power would do me well since I would not be pushing the motor on a normal cruise speed. Given a 5.7 is easily capable of the 250 target HP would I not want to go as high of a ratio on the Alpha drive as possible? What the highest ratio and how painful is that to accomplish?

 

[wire2]

Re: Efficiency? Getting the most bang for the buck.

Actually, most boats get better economy ideling along at displacement speeds. It's just not much fun.

Less gal/hr but is it more mpg? I suspect not. And planing hulls tend to wander L&R constantly at idle, very tiring also.

 

[MikDee]

Re: Efficiency? Getting the most bang for the buck.

Here would be your first choice for performance, and effeciency for your size boat.
Not the cheapest, just the best.
Everything else is a compromise
.
.
.
.
IMHO


http://www.volvo.com/NR/rdonlyres/81BAD056-9AD2-4DAF-9B78-CD5827B4D446/0/43_DPS_08.pdf

There you go Don,,, Got me drooling all over my socks!