I/O vs. Outboard HP rating

[smokeonthewater]

Re: I/O vs. Outboard HP rating

Yeah but in my example above 500lbs makes a big difference. The reason a Glastron and most other boats identically equipped run by my sig boat at WOT...500lbs except its dry weight not engine weight.

Especially since that extra weight is at the stern...

Well if you want to go there, the weight of the I/O, while greater, is ALSO further forward in the boat.... Honestly, this debate is a little silly.... every boat reacts differently and without exact measurements of weight distribution and without knowing how each hull reacts you can't make an accurate blanket statement.

As for the O/P's question, YES the horsepower of an I/O and an outboard is the same and generally most hulls will perform comparably enough with comparable horsepower that really the question is one of personal preference and should be more about whether either engine is in your way and what you plan to do with the boat..... IE if you want a huge sun deck on the back of the boat, the I/O isn't in your way and if you want lots of room for fishing it might be.

Style of boat matters too..... My 30' cruiser would look kinda silly with a bunch of outboards outboards on the back blocking my swim deck when I have plenty of room under the floor for two 260 hp v8's

 

[tboltmike]

Re: I/O vs. Outboard HP rating

Sorry it took so long for me to revisit this thread.
Mike's Arrow Glass Cheetah 1, New Graphics - YouTube
Mike's Arrow Glass Cheetah 3, Full Throttle Run - YouTube

After restore, but wiith original engine
http://groups.yahoo.com/group/Arrow_Glass_Boats/photos/album/1603926236/pic/list

 

[QC]

Re: I/O vs. Outboard HP rating

All I could think was "please Mike, don't leave the fender out on the full throttle run" . . . Doh!!

Nice looking ride!

 

[tboltmike]

Re: I/O vs. Outboard HP rating

Thanks,
Yeah, I always get too excited when I get in the water and forgit something!
Mike

 

[QC]

Re: I/O vs. Outboard HP rating

Sorry to razz you. Couldn't help it :redface:

 

[emilsr]

Re: I/O vs. Outboard HP rating

Just to add fuel to the fire.....

Roughly, Horsepower = Torque x RPM / 5252

This means an engine that produces 300hp at lower rpm (like a Mercruiser 7.4) is going to have more "oomph" down low (where you need it) than a 300hp outboard making it's power at 6,000rpm or more.

It's an apples to oranges comparison IMO.

 

[southkogs]

Re: I/O vs. Outboard HP rating

Sorry it took so long for me to revisit this thread.
Mike's Arrow Glass Cheetah 1, New Graphics - YouTube
Mike's Arrow Glass Cheetah 3, Full Throttle Run - YouTube

After restore, but wiith original engine
http://groups.yahoo.com/group/Arrow_Glass_Boats/photos/album/1603926236/pic/list

Great looking boat! Viva la tri-hulls!

 

[QC]

Re: I/O vs. Outboard HP rating

Roughly, Horsepower = Torque x RPM / 5252

Not roughly, exactly . . .

This means an engine that produces 300hp at lower rpm (like a Mercruiser 7.4) is going to have more "oomph" down low (where you need it) than a 300hp outboard making it's power at 6,000rpm or more.

Don't miss that the RPM and torque is also calculable at the propshaft, so gear ratio can affect this quite a bit. A 300 at 6000 RPM can be the same as a 300 at 4500 RPM if the ratios are 2.0:1 and 1.5:1 respectfully, which is pretty close to how they might actually be. Both result in a propshaft RPM of 3000 at "rated". And don't forget that 2 Strokes deliver good torque even with lower displacement. I am not sure that that much is different in these two examples except for a big weight benefit to the OB. Theoretical, but still not that far off.

 

[emilsr]

Re: I/O vs. Outboard HP rating

Not roughly, exactly . . .


Don't miss that the RPM and torque is also calculable at the propshaft, so gear ratio can affect this quite a bit. A 300 at 6000 RPM can be the same as a 300 at 4500 RPM if the ratios are 2.0:1 and 1.5:1 respectfully, which is pretty close to how they might actually be. Both result in a propshaft RPM of 3000 at "rated". And don't forget that 2 Strokes deliver good torque even with lower displacement. I am not sure that that much is different in these two examples except for a big weight benefit to the OB. Theoretical, but still not that far off.

I'm tracking. Problem is that thrust (not torque) makes boats move, and the torque/rpm/thrust equation is pretty elusive.

My theory, right or wrong, is the higher torque engine can spin a larger prop at slower boat speeds, pushing more water into a larger thrust column and therefore more pounds of thrust.

Ever see a small, high revving engine on a ski boat?

 

[tboltmike]

Re: I/O vs. Outboard HP rating

Sorry to razz you. Couldn't help it :redface:

Hey we have to take humor as positive reinforcement. We are here to better each other. Iron sharpens iron.
Thanks, Mike

 

[coolguy147]

Re: I/O vs. Outboard HP rating

I'm tracking. Problem is that thrust (not torque) makes boats move, and the torque/rpm/thrust equation is pretty elusive.

My theory, right or wrong, is the higher torque engine can spin a larger prop at slower boat speeds, pushing more water into a larger thrust column and therefore more pounds of thrust.

Ever see a small, high revving engine on a ski boat?

master craft pro star 200 outboard walkaround - YouTube

There you go

IMHO, I prefer outboards due to the ease in maintenance. I/Os suck to work on....speaking from experience. Too heavy.

I, however, do enjoy straight or v drive inboards. They're nice, but you can't beach them.

 

[QC]

Re: I/O vs. Outboard HP rating

Ever see a small, high revving engine on a ski boat?

Yes, they call 'em barefoot boats and they're the only ones that really run. I'm not a believer in this need for mega power for ski boats. I could yank your arms out with a 150 OB and a small prop any day, anytime. And I am an I/O guy. I am actually a believer in "power is power". As long as they are geared correctly, and the application is clear, I don't care if you make horsepower at 10,000 RPM or 2000 RPM. This is kinda my favorite debate. I contend we have been oversold the value of torque by Big Auto

 

[emilsr]

Re: I/O vs. Outboard HP rating

While I concede that "with a long enough lever you can move the world", you won't move it very far or fast.

Maintenance does suck on an i/o, as does the winterization dance. We missed several nice days of boating this year because we had already buttoned up for cold weather. Pretty sure I'll go back to egg beaters on the next boat.

 

[snowman48047]

Re: I/O vs. Outboard HP rating

It has to be a HP to weight issue. Have you seen the new Sea Ray 37' with twin 300HP outboards? Weight savings approx 622lbs over 5.7 L sterndrive 300-hp engines. Even spinning faster they still use less fuel too I think. Quieter. You can beach it. I see nothing but positives. With the gas mileage thing being forced on everyone the big engines are eventually going to become obsolete. My $0.02.

 

[QC]

Re: I/O vs. Outboard HP rating

While I concede that "with a long enough lever you can move the world", you won't move it very far or fast.

This is where we get into trouble. Horsepower is rate of work, so in effect, speed. So I say you go just as far, and potentially faster if the weight is lower.

 

[emilsr]

Re: I/O vs. Outboard HP rating

This is where we get into trouble. Horsepower is rate of work, so in effect, speed. So I say you go just as far, and potentially faster if the weight is lower.

Maybe so, but then you wouldn't have one of these: Seven Marine - The Most Powerful Outboards on the Planet I think that's a reasonable compromise between outboard and i/o.

 

[QC]

Re: I/O vs. Outboard HP rating

For some reason I have a mental block on those. I feel like they went too far. And again, I am an I/O guy for the boating I do. Philster use to say, date an OB, but marry an I/O. I think that's sums up my feelings for the type of boating I do now. Lake, psuedo go fast with friends and family.

 

[emilsr]

Re: I/O vs. Outboard HP rating

For some reason I have a mental block on those. I feel like they went too far. And again, I am an I/O guy for the boating I do. Philster use to say, date an OB, but marry an I/O. I think that's sums up my feelings for the type of boating I do now. Lake, psuedo go fast with friends and family.

Same here. We usually spend more time floating than boating.

I love it, but then again I'm a fan of the small block chevy.

Basically I'm an outboard guy who likes the rumble of a V-8. The 557 fits my "needs" perfectly....for the low low price of about $60k per motor. For that, I'm pretty sure you could buy a Mercury racing 565/Bravo XR drive combination and still have money leftover for gas.

Either way, it's an interesting combination; automotive type motor as an outboard powerhead. I don't know how many they've sold but I certainly wish them luck. The high end of the boat market doesn't seem to have suffered as much so maybe it'll catch on; I hope so.

 

[QC]

Re: I/O vs. Outboard HP rating

automotive type motor as an outboard powerhead.

I think that's what bugs. It's like you get the worst of both options. Heavy engine, and hanging off the back. I LOVE that traditional OBs are purpose built blocks for nothing else but their intended application. Kind of amazes me that the unit volume can support that, but it impresses me that it does. Very hi tech machines. Our V8s are kind of low tech in comparison. I also LOVE that two strokes have survived. They really do make sense as a marine power plant as long as they can keep efficiency within reason.

 

[emilsr]

Re: I/O vs. Outboard HP rating

I think that's what bugs. It's like you get the worst of both options. Heavy engine, and hanging off the back. I LOVE that traditional OBs are purpose built blocks for nothing else but their intended application. Kind of amazes me that the unit volume can support that, but it impresses me that it does. Very hi tech machines. Our V8s are kind of low tech in comparison. I also LOVE that two strokes have survived. They really do make sense as a marine power plant as long as they can keep efficiency within reason.

Until someone starts making a 500hp 2 stroke outboard, that's all you've got unless you want to hang triples or quads on the back of your big center console.