i/o power vs. outboard w/ actual numbers!

[hibbert6]

I've always heard that the 3.0L Mercruiser was an excellent power train for a 17'-19' boat, though possibly a bit underpowered at 135hp. On the other hand, a 135hp outboard is pretty ballsy on that size boat. So I figured the performance issue had to do with the increased weight of the i/o engine/drive combo. But I just read a Mercury ad that gave the weight of its 3.0 with Alpha Drive as 625lbs, vs. a Verado 135hp O/B at 510lbs.

Now, the 115 lb difference is a lot, but not THAT much more weight - one skinny teenager or a trophy wife, maybe. So what explains the "power disparity"??? Is that a myth, and a 3.0L has plenty of power for water skiing, etc? I keep reading that if I get the 3.0L that I'll soon wish for more power, but I can't imagine wanting more HP than a 135hp outboard! (Mind you, the largest o/b I've skied behind was 115hp )

Thanks,

Dave

 

[Don S]

Re: i/o power vs. outboard w/ actual numbers!

To Boat topics and questions.

 

[tashasdaddy]

Re: i/o power vs. outboard w/ actual numbers!

i don't care for i/o's but i would never have anything smaller than a 4.3L. 3.0 with a loaded boat is a dog imho. i have a 5.7 volvo.

 

[JustJason]

Re: i/o power vs. outboard w/ actual numbers!

sooo.... what's your question???? 135hp is a 135hp no matter how you slice it. With merc's they are both rated at the prop.

 

[45Auto]

Re: i/o power vs. outboard w/ actual numbers!

I've heard that same thing Hibbert. About the only direct comparison I've found is here:

http://www.popularmechanics.com/outdoors/adventures/1277216.html?page=1

They test a 135 HP I/O, OB, and diesel in identical boats. The performance numbers are within a half second or so up to about 30 mph for the gas engines. But the I/O tops out at 34 mph while the O/B keeps going up to 42 mph.

Assuming the HP ratings are correct and comparable (both at the prop), and that the boats were each propped for max speed (looks like they were), then about the only explanation is that the I/O boat had more drag. Weights on the boats ended up identical due to the extra rigging and beefed up transom required for the outboard.

I'd like to see any other real data anyone knows about!

 

[dingbat]

Re: i/o power vs. outboard w/ actual numbers!

I've always heard that the 3.0L Mercruiser was an excellent power train for a 17'-19' boat, though possibly a bit underpowered at 135hp. On the other hand, a 135hp outboard is pretty ballsy on that size boat. So I figured the performance issue had to do with the increased weight of the i/o engine/drive combo. But I just read a Mercury ad that gave the weight of its 3.0 with Alpha Drive as 625lbs, vs. a Verado 135hp O/B at 510lbs.

Now, the 115 lb difference is a lot, but not THAT much more weight - one skinny teenager or a trophy wife, maybe. So what explains the "power disparity"??? Is that a myth, and a 3.0L has plenty of power for water skiing, etc? I keep reading that if I get the 3.0L that I'll soon wish for more power, but I can't imagine wanting more HP than a 135hp outboard! (Mind you, the largest o/b I've skied behind was 115hp )

Thanks,

Dave

...

 

[crb478]

Re: i/o power vs. outboard w/ actual numbers!

Many of the 2 cycle outboards weigh around 300 lbs + or - a few in either direction. Most of the outboards also are run at higher RPM's than the I/O's. Even though they both develop simular horsepower it is at different rpm's. A 3.0 may redline at 4500 rpm and an outboard may go to 5500-6000rpm at WOT. In my opinion a 3.0 will get you out on the water and you can ski behind it, but when you add weight or seas you will appreciate a little more power. A 4.3 feels like a big jump in power over a 3.0 but still gets decent gas mileage. I have owned both and I like the 4.3 much better.

 

[steelespike]

Re: i/o power vs. outboard w/ actual numbers!

If I was buying a boat in the 120 to 150 hp range it would be an outboard for sure.Better weight balance, easier maintenance,better hole shot and top end,
more options and easier to fine tune the setup.more interior room.

 

[Mischief Managed]

Re: i/o power vs. outboard w/ actual numbers!

The gearcase on the I/O is designed to handle the torque of a 5.7 liter engine and I'd bet the O/B gearcase is not. Due to this, I imagine the O/B gearcase is quite a bit smaller, but I've never made any measurements. Drag increases with the square of speed so a small reduction in drag can yeild a great speed increase.

The O/B obviously weighs less, but it also carries it's weight much further aft which would have the same effect as reducing weight in the bow area and that typically leads to more speed.

The O/B trim pivot point is higher on the transom than the I/O so it has a longer lever with which to lift the (lighter, since there's no engine in it) hull when trimmed. The more hull out of the water, the faster you go.

The O/B propeller maintains a constant speed (for a given engine RPM) regardless of trim. Due to the U-joints in the drive, the I/O prop varies in speed continuously through each revolution, unless the prop shaft is prefectly parallel with the engine drive shaft. Trimming "up" generally takes the prop shaft out of parallel and that increases the magnitude of the speed variation. This hurts efficiency.

 

[hibbert6]

Re: i/o power vs. outboard w/ actual numbers!

Wow! Good Stuff!

Dave

 

[MikDee]

Re: i/o power vs. outboard w/ actual numbers!

Did anyone notice the price of gas here in 2003?
$1.79 a gallon, I forgot how cheap it was, not that long ago! I wonder if we'll ever see gas at under $2 a gallon anymore!

 

[45Auto]

Re: i/o power vs. outboard w/ actual numbers!

Hi Mischief

As long as the driveshaft has 2 u-joints then the output is a constant speed. As one slows down, the other speeds up an equal amount. I know Bravo's have 2 u-joints.

http://www.custompistols.com/cars/articles/ts_ujoints.htm

 

[Boatist]

Re: i/o power vs. outboard w/ actual numbers!

I have a 3.0L 181 CI I/O on my 21 foot Aluminum boat and it is fine for me but I am a fisherman not a skier.
19 pitch stainless prop at 4200 WOT RPMs top speed is 38.
17 pitch stainless prop at 4600 WOT RPMs top speed is 35.

If your a freshwater boater then I think a 4.3L fuel injected is a good way to go.
If your a saltwater boater then the 3.0L would be my choice. Main reason is the cost of maintenance when it is time to change the exhaust manifolds and risers. The 3.0L has one exhaust manifold and riser. The 4.3 has 2 exhaust manifolds and 2 risers, so about double the cost.

 

[Mischief Managed]

Re: i/o power vs. outboard w/ actual numbers!

Hi Mischief

As long as the driveshaft has 2 u-joints then the output is a constant speed. As one slows down, the other speeds up an equal amount. I know Bravo's have 2 u-joints.

http://www.custompistols.com/cars/articles/ts_ujoints.htm

Pretty sure that's true only if the input and output shaft remain parallel. Re-read the last paragraph in the link you provided. When the drive is trimmed up, those yokes (and therefore the shafts they connect to) cannot be parallel and the velocity cannot be constant.

 

[JustJason]

Re: i/o power vs. outboard w/ actual numbers!

if your buying a boat then really what you have to think about is application... or "how your going to use it". and not price/performance. There are 3 general drive designs because people use boats very differently. If your an offshore fisherman a nautique ski boat wouldn't serve you very well. And if you are a watersports person... well outboards are not the best way to go because of the lack of tourqe at most towing speeds. If your going to do a little of this and a little of that then an I/O is a good choice.

 

[Texasmark]

Re: i/o power vs. outboard w/ actual numbers!

Mischief, that assembly 45 is talking about is commonly referred to as a "constant velocity joint"and it has two u joints and connecting bracket in the assy. The drive shaft which connects to the engine is on one joint, and the shaft to the lower unit is on the other. Front wheel drive auto's have the same thing.

Another caviot about I/O's is you have an additional 90 degree angle to transgress and the gearing always adds loss.

Additionally, the I/O is limited in it's manueverability on the transom, unlike the OB. You can jack em up, put jack plates on them, and notch your transom (which I/O's may do also, just personally never saw one.

Mark

 

[Mischief Managed]

Re: i/o power vs. outboard w/ actual numbers!

Mischief, that assembly 45 is talking about is commonly referred to as a "constant velocity joint"and it has two u joints and connecting bracket in the assy. The drive shaft which connects to the engine is on one joint, and the shaft to the lower unit is on the other. Front wheel drive auto's have the same thing.

Another caviot about I/O's is you have an additional 90 degree angle to transgress and the gearing always adds loss.

Mark

I have taken plenty of CV joints apart; two U-joints does not equal a CV joint. It comes close, but it is not the same and that's why the drive should not be run in the full up position. You'll not find a pair of U-joint on a modern FWD axle, they cannot handle suspension and steering angulation, only real CV joints can.

What I don't get is why I/O makers don't use a CV joint in the application. They can handle more torque and will allow lots more trim/turning angle. They can run without damage fully tilted up too.

Both the I/O and the O/B Hp are measured at the propshaft so the extra 90 degree gearing, while indeed lossy, is moot in this discussion.

 

[Texasmark]

Re: i/o power vs. outboard w/ actual numbers!

Well, the cv joint I am talking about is an assembly made out of 2 u joints with a connecting assembly, all right together. Only variance is what z(shafts) you put on the input/output. Worked with them on I/O shafts, drive shafts of rear wheel drive auto's and drives of front wheel drives.

You are correct on the prop rating, but there is still loss there that has to be overcome with horsepower at the engine.

Mark