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Old 01-05-2006, 11:35 AM
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mox
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Default how much faster outboard vs. inboard

I have a 2520 parker straight 5.7 inboard. notice that outboard with same horse power goes twice as fast,was wondering if this is normal
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Old 01-05-2006, 11:51 AM
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Default RE: how much faster outboard vs. inboard

I think so. there was an artical in Moterboating (I think) that compaired three SeaRay boat - two with twin inboards and one with twin sterndrives. The sterndrive was about 30-50% faster than the inboard with the same power (~2x350hp?) and about 5 mph faster than the inboard with bigger motors (430(?) hp). (I am assuming that sterndrives and outboards should have about the same drag in the water) Add to that, inboards and stern drive motors are HP rated at the block, and outboards are rated at the prop - so if you have a 250 hp motor, and the outboard is a 250 - he has more power at the prop (you have transsmition losses - 5-10% of power, so you only get ~225 to 240 to the prop).

So, yes, I would expect the outboard to be faster. However, 2x faster seems like a lot, from my uneducated view.

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Old 01-05-2006, 12:20 PM
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Default Re: how much faster outboard vs. inboard

Inboards lose speed to outboards and I/O's primarily because of the shaft angle. Outboards and I/O's simply position the propeller(s) at a more efficient angle. They both direct all of the thrust backwards instead of partly downward like an inboard. Which also GENERALLY lifts more hull out of the water for less drag.
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Old 01-05-2006, 12:54 PM
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Default Re: how much faster outboard vs. inboard

Between the shaft angle and weight savings of an OB....I'll bet on the OB's....Mick
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Old 01-05-2006, 01:47 PM
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Default Re: how much faster outboard vs. inboard

Quote:
slickster - 1/5/2006 11:54 AM

Between the shaft angle and weight savings of an OB....I'll bet on the OB's....Mick
I forgot about weight. Loosing 30lbs is about the same as adding 1 Hp, so if your motor weighs in at around 900-1000lbs, and your buddy is running a Verado at 600lbs, the weight savings alone is giving him the equivlent of about 10-15 hp. More for some of the lighter OB's.

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Old 01-05-2006, 04:40 PM
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Default Re: how much faster outboard vs. inboard

Good article regarding drag, hp, and speed using the same hull:

http://www.boatingmag.com/article.as...&page_number=1

Outboards tend to be even slightly faster than outdrives because they can be raised higher without affecting thrust angle creating less drag. Besides and outboard is lighter too.
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Old 01-05-2006, 04:44 PM
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Default Re: how much faster outboard vs. inboard

Power to weight ratio is the reason.
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Old 01-05-2006, 06:28 PM
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Default Re: how much faster outboard vs. inboard

In big seas an inboard has the advantage of a bigger prop with more torque, but when its flat that big prop is a lot of drag. A prop pocketed boat helps reduce drag and helps create a flatter trim angle. Both have advantages and disadvantages.
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Old 01-06-2006, 10:42 AM
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Default Re: how much faster outboard vs. inboard

Outboards are hp rated at the props, inboards are rated at the flywheel.

Inboard transmissions (not V drives) only loose about 3-5% of the engine hp. Outboards loose 7-10% due to the 90 degree angle in the foot (so a 200 hp outboard really has a 220 hp engine).

Prop angle does have an effect on propeller efficiency, but not near as much as people think it does. Propeller Pitch/Diameter ratio plays a much larger role. Outboards have small diameter, high pitch props and have fast shaft speeds. By design, they are built for speed. Inboards in general have higher diameter, lower pitch props that turn slower and are by design more efficient throughout the middle of the rpm range.

The faster the boat goes, the less the prop has a tendancy to slip. so outboards can have a somewhat double advantage in the speed area too.

There are advantages to both. Speed, on top end, is the advantage to outboards. In calm waters the lack of diameter has minimal effect, if any, on the speed throughout the rpm range. Some people switch to 4 blade props on outboards (and do lose some top end) to help "hole shot" or prop slip in less than ideal conditions.

Inboards have the center of gravity advantage in rougher water, have the larger diameter advantage in rougher water, and are generally easier to run rough water boats.

Explained. When a boat is going from wave to wave, as the hull digs into the next wave a large amount of drag is produced. A smaller diameter prop loses its bite (slips) easier. So the outboard power boat is constantly changing speeds and is most likely being compensated by the captain constantly working the throttles to try to maintain a certain speed. An inboard doesn't loose as much bite on the prop. Also, the extra weight fwd and lower generally has an inboard going "through" the waves more so the drag can be more constant. Ever hear people describe an inboard as a "wet" boat?

I have an inboard now and have owned outboards and i/o's. There are nice calm days when I miss my old twin outboard wellcraft (except maybe the fuel bill). But in anything 3' or greater I am running as fast or faster than any other boat out there.

The main reason I chose inboard had to do with running a boat from a tower on a trailerable boat. The low center of gravity really helps make the tower more comfortable (less sway). I also bottom fish and Tuna fish and the open transom is well worth a little extra running time to me.

If all you are worried about is speed - than outboard is the best choice. But there are reasons that make i/o's and inboards better choices. Just depends on what yours is!!

What I find funny is when people start "cost justifying" one powerplant or another...... it's a boat - by definition they are not "cost justifiable". In the end, most people don't use their boat that one power plant is less "costly" than another - In my experience, if you pay more on the front end, you get more on the back end - I think its pretty much a wash financially for most boaters......

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