Inboard vs outboard performance

[QC]

Re: Inboard vs outboard performance

Hint: It's a smaller boat, 1000 pounds lighter, and has LESS power. You would think it would get better MPG.

Edit: Nerver mind, let somebody else play . . .

 

[proshadetree]

Re: Inboard vs outboard performance

I/O are room eaters in a boat just as stated. O/B are in my opinion ugly hanging of the butt of a slick boat. I/O are normally faster and don't give much problems. When an I/O breaks is what will get you. I have seen many self-proclaimed mechanics not get and O/B to run for snot. I can work on them but the small parts and synchronization drive me nuts. Every boat I have owned for the last 25 years has been an I/O, get them close and they run great. Anyone know of an O/B that is as quiet as an I/O, as easy to work on for us blind boaters and will allow old car parts to be used to replace those expensive marine parts that O/B engines like? Maybe I can mount my 455 olds onto a 250 merc drive outside the transom for more room.

 

[Lou C]

Re: Inboard vs outboard performance

You may want to do a little research and understand what exactly a "propshaft" is .......

It's the thing the propeller is attached to. If power is measured at the propshaft, it doesn't matter how many changes of direction it takes to finally get there. It's the amount of power that is delivered to the propeller. The prop doesn't know if it's an inboard, outboard, 2-stroke, 4-stroke, Wankel, turbine, whatever attached to it.

You're confusing crankshaft power and propshaft power.

Ahh...I do all the maintenance on 3 vehicles in our family and the boat.....I think I know what a PROPSHAFT is....I've been wrenching on my own stuff for 38 years and have had to explain to some young uns how to start a boat with a carb, and how points work...

I am not questioning how the power is measured, but it is likely that 2 right angle changes in direction is less efficient than one, and this affects power transmission throughout the rpm range....we all know that hp ratings are hard to compare from one engine maker to another and even more important, there is rarely if ever any talk of torque in rating marine engines. In vehicles torque and the shape of the torque curve makes a huge difference in performance, yet you never (or rarely) see torque ratings for marine engines. Given the fact that all OBs and I/Os are running a single speed transmission, torque is even more important than in a vehicle with a multiple speed transmission. Here the pushrod I/O has some advantage, in that they typically produce more torque at lower revs than a smaller displacement OB...

The original poster wants a comparison of OB vs I/O performance, well for the same propshaft HP an outboard has to be the better performer for 2 reasons, one roughly 400 lbs less weight and for another, the ability to rev at least 1000 rpm higher than a typical pushrod I/O, either a 2 stroke OB or 4 stroke ohc OB will rev higher than a Chevy small block can, and maintain higher engine speeds with less wear than a pushrod I/O could.

The I/O does fare better in parts cost and the simple push rod engines are easier for backyard mechanics to understand, troubleshoot and repair. I'd rather deal with a simple Chevy based engine and the outdrive attached to it than a V-6 outboard with 6 carbs, or DI fuel injection, or the complexity of a 4 stroke OB. They are very expensive to repair if anything goes wrong. But they perform well and usually give good service with little maintenance.

 

[Mike Robinson]

Re: Inboard vs outboard performance

.... Maybe I can mount my 455 olds onto a 250 merc drive outside the transom for more room.

How about this?

http://www.panoramio.com/photos/original/6711740.jpg

 

[proshadetree]

Re: Inboard vs outboard performance

Im in I just need to work out and develop some mussels to tilt that BB monster.

 

[DuckHunterJon]

Re: Inboard vs outboard performance

I think Lou C hit it with the torque curves and the upper RPM limits. The I/O will produce torqure down lower, but also won't rev as high. Therefore, it will run a slightly higher pitch prop (and come out of the water quicker). The O/B won't pull as tall of a prop at the lower RPM's, but will spin it faster at the top end. Note, this may not apply to the 4 stroke OB's.

For me, my 200 Optimax gets the boat out of the hole quick enough, and still hits real close to 70 mph at the big end. As with everything, each has there benefits and drawbacks - at least we have a choice. Now, lets continue this discussion, only 5 more months of winter. We ought to have it hammered out by then!

 

[45Auto]

Re: Inboard vs outboard performance

The boat I referenced is a 19" Bayliner Trophy CC with a 125HP Mercury 2-stroke outboard. As you can see, despite being over 2 feet shorter and over 1,000 pounds lighter, it's still slower and uses more fuel than the Trophy that Dingbat used as an example.

Length Overall 19' 0''
Dry Weight 2,339 lbs.
Beam 8' 2''
Fuel Cap 55 gal.
Draft 2' 8'' (max)
Deadrise/Transom 19 deg.

http://www.boattest.com/boats/boat_video.aspx?ID=544

If you do any research at all, you'll find that there are no set rules for performance of an OB vs an I/O. Each boat is a unique set of circumstances. As QC said, weight distribution and trim make a huge difference in performance. That's why motors have power trim and higher-performance boats use adjustable trim tabs.

it is likely that 2 right angle changes in direction is less efficient than one, and this affects power transmission throughout the rpm range

No doubt there. However, if you measure power at the OUTPUT, how it got there is irrelevent. More power at the output wins, doesn't matter how many turns it went through to get there.

In vehicles torque and the shape of the torque curve makes a huge difference in performance, yet you never (or rarely) see torque ratings for marine engines.

That's because torque is irrelevent, power is what matters. A 5.7L GM marine engine produces a max of 370 lb-ft of torque. I torqued a nut to 750 lb-ft last week using a 4:1 multiplier and a 200 lb-ft torque wrench. You really think I can deliver twice the performance of a 5.7L engine because I can easily produce over twice the torque?

Power includes torque and time. More power means that I can produce more torque for the same amount of time, or the same amount of torque for a longer time. The 5.7L produces much more power than I can, even though I can produce much more torque.

you never (or rarely) see torque ratings for marine engines

Maybe that's true for outboards, but GM has published complete power and torque specs for all their marine engines since the internet has been around. That's where I got the torque spec on the 5.7L I used above.

http://www.gm.com/vehicles/innovation/powertrain-technology/engines/specialized/marine/marine_engines.jsp

well for the same propshaft HP an outboard has to be the better performer for 2 reasons, one roughly 400 lbs less weight

You'll find that total weight is less important than weight distribution in the performance of a planing hull. See the example I referenced above, where one outboard boat weighs over 1,000 pounds less than the other, yet is slower and uses more fuel with the same power. The weight difference was irrelevent.

An outboard is a huge chunk of weight way up in the air at the very back of the boat, the worst place you can put it. The OB weight advantage is more than offset offset by the I/O weight distribution advantage since the weight of the I/O is carried much lower and further forward in the hull.

for another, the ability to rev at least 1000 rpm higher than a typical pushrod I/O, either a 2 stroke OB or 4 stroke ohc OB will rev higher than a Chevy small block can, and maintain higher engine speeds with less wear than a pushrod I/O could.

RPM has NOTHING to do with performance. Doesn't matter what RPM you're turning the engine at, what matters is the POWER that it's delivering.

You really think that roller bearings lubricated by a random oil mist in the fuel/air mixture will have less wear than full time pressure-lubricated (hydrostatic) shell bearings? You may want to do a little research on bearing wear. Here's a quote from Machine Design:

Hydrostatic bearings do not wear when operating properly because the bearing surfaces are separated by a film of oil.

http://www.bearings.machinedesign.com/guiEdits/Content/BDE_6_1/bdemech6_21.aspx

Wikipedia has a pretty good article on hydrostatic bearings:

fluid bearings require little or no maintenance, and have almost unlimited life. Conventional rolling-element bearings usually have shorter life and require regular maintenance. Fluid bearings generally have very low friction?far better than mechanical bearings.

http://en.wikipedia.org/wiki/Fluid_bearing

The original posters question was how a 140HP outboard compares to a 140HP inboard in power. In properly set-up boats, you won?t be able to tell the difference between them in performance (see post #8). As far as other advantages and disadvantages, everybody has an opinion, just like everyone has a certain bodily orifice!

 

[ThePartsMan]

Re: Inboard vs outboard performance

Outboards make the same power for less weight.
With the right prop they will have better performance.

You can't break the laws of physics.

I like OBs because they are easier to swap and work on.
(I prefer standing upright than on my head)

 

[QC]

Re: Inboard vs outboard performance

Outboards make the same power for less weight.
With the right prop they will have better performance.

If, IF, all else is equal, then I totally agree. Buuuuuuttttttt . . . the problem is that all else is not possibly equal. Again, weight balance, center of gravity, trim leverage (distance and angle from hull), yes, even torque curve differences (although I totally agree with 45's attempt at explaining this) will change performance attributes.

So I am not mucking up this torque curve discussion as it is a pet peeve of mine to get this right . . . let's take an example comparing two OBs so that we don't have the variables inserted by the I/O vs. OB discussion. If we have the same horsepower at the propshaft, say 227.9 bhp at 5000 RPM. Same weight, same gear ratio, same props, all same/same, heck same boat as we can easily swap OBs . . . with the one exception being that OB "A" has 100 lb/ft. more torque at 3000 RPM than OB "B"". And this torque benefit is not only at a single point i.e. there is a curve that shows this peak at 3000, but it is a nice smooth curve up to the exact same torque at 5000 RPM (has to be the same torque if they both make that 227.9 bhp at the same RPM). Then OB "A" will have better hole shot.

Soooooo if you are deciding between two OBs for a marginally powered and heavy boat, or you are concerned about deep water slalom skier starts, then OB "A" would be the better choice. If you are a leisurely fishing guy, or you have a reasonably light and free type hull and you just go out with the wife for picnics and sightseeing trips, and acceleration does not matter to you much, then you will not care (or use) that extra 100 lb/ft. Top speed of both examples will be exactly the same. If you disagree, please reread as both examples are EXACTLY the same except one as higher peak torque.

Again, ALL else must be equal, even the size of your butt If you want to discuss the possibility that the OB "A" with the extra 100 lb/ft design might exhibit better fuel efficiency at cruise then that is another discussion. It is not guaranteed, it is not automatic, but it might be true. Can't say without my theoretical example's data laid out for us to examine . . .

 

[45Auto]

Re: Inboard vs outboard performance

Outboards make the same power for less weight.
With the right prop they will have better performance.

You really believe that a couple of hundred pounds is that big of a factor and nothing else matters?

Try this example:

19' boat, total weight 2,339 lb, 125HP

21' boat, total weight 3,342 lb, 135HP

The second boat is 2' bigger and 1,000 pounds heavier. Which one would you predict is faster and gets better gas mileage?

(Hint: Post 31, 39, and 40 in this thread may give you a clue.)

 

[45Auto]

Re: Inboard vs outboard performance

Edit: Give this a minute, we're trying to use better numbers. I mucked it up with the 127.9 and a 100 lb/ft. difference in torque . . . :redface: - QC

Here you go QC, I made you a couple of charts of your Motor A and Motor B example if that'll help you explain! Motor A and Motor B both have 127.9 HP at 5000 RPM, Motor A has 100 lb-ft more torque at 3000 RPM.

That 100 lb-ft difference at 3000 RPM limits Motor A to a little over 200 lb-ft or it makes more power at 3000 than WOT. Means Motor B can only have about 100 lb-ft at 3000 RPM.

Good luck!

 

[QC]

Re: Inboard vs outboard performance

OK, let's change it to 227.9 bhp and all else stays the same. I come from a world of 1850 lb/ft torque at 1100 RPM, so 100 here, 100 there . . so what?

 

[DuckHunterJon]

Re: Inboard vs outboard performance

I think the big difference of opinions here is some are quoting different hulls, displacements, weights, etc - while others are saying with the exact same setup, everything else being equal, etc.

As for torque not mattering - I totally agree if we are only talking top speed. But if we are talking acceleration, that's just not true. Take a 500 hp high reving 4 cylinder and a 500 HP big block V8. Drop the two of them in IDENTICAL cars (yes, add lead weights to the 4 cylinder so the total vehicle weights are the same, and keep the transmission ratios the same). Both will hit the same top speed (assuming drag limited, not gear limited). But the big block (presumably higher torque) will out accelerate the smaller, higher revving 4 cylinder.

Again, everything being equal, wouldn't the same thing be true on the water?

 

[QC]

Re: Inboard vs outboard performance

Reread my post, I said the same thing . . .

 

[bear_69cuda]

Re: Inboard vs outboard performance

This is only from my personal experience, but I wanted to share my data.

1999 Bayliner
length: 19.5'
Beam: 89"
Hull: V hull
SS Smart Tabs 60 lb actuators. (~ 100 rpm drop @ WOT when I installed tabs, all data below is with tabs.)
Wet weight (23 gal of fuel) = 2143 lbs
Loaded weight with gear and 2 adults (200 lbs ea) = 2643 lbs
Engine: 135 hp Mercruiser
Prop: 14.24 x 17"p stainless Stelitto

Power to weight ratio: 0.051 hp/lb

Top speed @WOT 39.5 mph GPS at 4800 rpm


Boat: 2008 Hurricane 202 Deck Boat
Length: 20?
Beam: 102"
Hull: Modified V hull
SS Smart Tabs 60 lb actuators. (~ 100 rpm drop @ WOT when I installed tabs, all data below is with tabs.)
Weight wet: ~ 3500 lbs.
Weight with load: 4 adults, all gear ~ 4400 lbs.
Engine: 150 hp 2008 Yamaha F150TXR 2.0:1 (28/14) gear ratio
Prop: Yamaha Reliance 14.25 x 17"P (non vented)

power to weight ratio: 0.034 hp/lb

Top speed: 42.5 mph GPS @ 5800 rpm.

My Hurricane can run Circles around the Bayliner, even if the Bayliner has better power to weight ratio...

 

[QC]

Re: Inboard vs outboard performance

bear,

Do you know the deadrise of each? May explain the better performance of the Hurricane. I assume by your description that the Hurricane is around 16 degrees and the Bayliner closer to 20 . . .

 

[bear_69cuda]

Re: Inboard vs outboard performance

Hey QC,

I can't seem to locate deadrise for either, but I would think your estimate is very close if not spot on... I guess this means I need to re-power my Hurricane with a 3.0 Mercruiser for real comparison data... I just wanted to share what I have is all... Either way it's interesting...

 

[beckspop]

Re: Inboard vs outboard performance

" the sun revolves around the earth...."

You mean it doesn't?!

 

[45Auto]

Re: Inboard vs outboard performance

" the sun revolves around the earth...."

You mean it doesn't?!

All depends on what you choose to believe, since very few people have the education or skills to make the correct decision based on their personal observations of the sun.

Randomly drop people in two different worlds, one where the sun revolves around the planet or another where the planet revolves around the sun, and 99.999% of them won't be able to tell you which world they're in.

They'd be right 50% of the time just guessing randomly. Kind of the same way it works with boat performance ....

 

[bear_69cuda]

Re: Inboard vs outboard performance

" the sun revolves around the earth...."

You mean it doesn't?!

Not sure if this was directed at me?

Did I say something to offend you? I was only stating my observations....

So my earth orbiting the moon theory is toast also?