Inboards vs. outboards

[Reese]

Over the weekend I was having a conversation with a good friend of mine who has a big block 20.5 eliminator. He asked me why outboards were faster than most inboards with less power.

The obvious reason is because outboards hang off the back and generally use notched transoms. So why don’t inboard manufacturers simply cut a 15"- 20” notched pad and put the rest of us outboards out of business?

Notched transoms/pads came largely about from the bass boat crowd’s need for speed, how come this hasn’t caught on with the inboard crowd?

[737jetmech]

The power to weight ratio and higher X-dimension has a lot to do with it.
There are some really fast inboards out there also.As far as which is better? I have both and the outboard boat is cheaper to run and faster than my twin big block boat.Thats 152 cubic inches making 260 hp or 908 cubic inches at 660 hp.The hull type and weight of the boats are completely different.As well as the cost to fuel them.:)

[Travis Fulton]

there was a extensive post on this a while back, they said that it takes twice the pwr of an inbord to out run a out board?? i think like 500hp bbc vs a 250hp outboard would be a close comparison! but any ck out that post if you can find it, it had a bunch of info on this! ;)

[Greg Moss]

Half the Horse power of a inboard is used just to push the wieght of the motor.:D Compare the listed weight of a stern drive big block package to the weight of a 300X and you will see the difference is more than twice. Got a frind with a 598 ci Merlin motor in a 26 American Offshore Claims 900 hp, 108 mph on the GPS Another freind with a 25 Motion Triple 260's 118 GPS with less Horse power.

[Steve Zuckerman]

The main reasons for the speed/performance differentials are (as
Greg said) #1 weight, the I/O, jet, vdrive powertrain will generally weight 2 or three times what the 2 stroke does. Weight placement in (or behind) the hull makes a difference too. Further
aft is usually more efficient.
#2 A 2 stroke fires twice as often, and doesn't have the parasitic drag of a cam and valve train.
#3 Usually have more efficient hull and drives. It's hard to beat a
strong 2stroke on a lightweight tunnel or pad vee for efficiency.
It's been my experience that an outboard performance boat should be able to outrun an inboard with up to twice it's horsepower. Beyond X2, it gets dicey. @ X3, you'll usually lose.
Regards,
Steve ;)

[Reese]

from a boat designer.

“Boat manufactures that use outboards know they have a limited amount of power, usually up to a maximum of about 300 hp. Because of this they rely on a variety of hull design aids including notched transoms, padded bottoms, setback, and adjustable motor mounts in order to minimize drag.

Designers have long known about these aids but have not adapted them to inboards for a variety of reasons including cost, complexity, and safety.

The vast majority of customers want a boat that is both safe and comfortable. A boat that uses any kind of hydrodynamic lift is to some degree no longer in contact with the water. Depending on the amount of lift and hull design these boats will eventually become unstable at speed. This is especially true of padded v-bottom designs.

Hi performance drivers compensate for this lack of stability with training and set time. Most customers do not what to learn how to drive a boat, and don’t like the loose feeling you typically get with these high-planning hulls.

This is a major reason why we produce high horsepower tunnel hulls. Our customers need very little seat time to reach top speeds and the hull maintains most of its wetted surface resulting in a stable and safe platform. Tunnels are the safest high speed designs for the novice boater."

[joe-doggs]

Pound for Pound - an aspirated two stroke will always make more horsepower than an aspirated four-stroke. That being said - there are practical realities such as emissions, piston crown temp - ( recall a two stroke enjoys twice the duty-cycle as a four stroke - IE makes more heat ), crank shaft roller bearing loading & lubrication etc. that will mitigate against the two stroke ultimate horsepower & adoption. However, the reason there are not more two-stroke outboard boat designs is the simple fact the inboards are cheaper .. Outboard engines cost on the order of an OEMs completed hull cost - this leaves very little margin for profit in a particular market segment - particularly if you are competing with the mass produced v-6 & v-8 engines - that which enjoy economy's of scale from the automotive & in the case of the 3.0 MERC - the forklift & industrial pump industry. The OEM price for inboards is very favorable to boat builders. Bayliner is still offering entry level packages for 10 grand ! - Find me a decent outboard that will fit that price point - that is why US Marine hung onto FORCE for so long - it was a living fossil - all the non-recurring engineering dollars were beat out of this design over the past 20 years ! However, apparently, the new orphan for Bayliner's outboard is Merc's walleye special - the 125 outboard .. Concerning duty cycle - the Wankel ( pseudo-two stroke ) proves this theory as well - make more horsepower pound for pound than comparible displacement 4 - strokes.. That is why the have to race in "rotary classes" - nothing is competitive with them in other classes ..

[sho305]

Hey Reese, found this on a search and maybe it has been addressed elsewhere but...you posted the question:
So why don’t inboard manufacturers simply cut a 15"- 20” notched pad and put the rest of us outboards out of business?

Isn't that exactly what Fountain did with the pad? I rode in one and it did exactly that, the bow would stay up nice and it went faster. It rode hard too. In a 36' it rode harder than a 28' vee. I think another problem is when you double+ the notch in the pad you are losing a lot of running surface. It is not as good with the waves then by effectively shortening the hull, at least comfort-wise. They have won races with it aside from any quality/business issues, but it has not taken over yet.

I still have a hard time with the cost of OBs. 2 strokes are so simple and so few parts yet they can hardly compete with comparatively giant I/O powerplants. I thought others would enter the market and drive prices down, but nope. How can you buy a 100+mph sled for $8k, or a hyabusa for 8.5k, etc., and what size OB is $8k? I know, bitchen don't help none.:rolleyes:

[supergenius74]

I agree about the outboard price thing. It doesn't make sense why they cost so much, maybe they have to pay huge taxes because they are 2-strokes or something. But the parts are usually expensive too, I paid 300 bucks for a new stator for my OB, what does a new alternator cost for a v8 ? 1/2, 1/3rd maybe? If OB's were cheaper, I'm sure they would be the most common engine on boats, they are simple to repair, lighter, more powerfull for their size and easy to switch too. I doubt I will ever own an inboard, but I doubt I will ever own a new outboard either. What does a new 2.5 cost? 18 grand? jeese for 18 grand you could buy a nice car, whole boat, or even a heck of a harley. Must be the R&D cost or something.

[Techno]

Besides what has already been mentioned is the Hull weight built for OBs or IOs. That adds more to the already heavy IO.

Also something very very important is an OB can change it's X dimension easily. With a jackplate, while underway. It can adapt to load. And it can be exactly tuned with proper setback.
The IO can't do any of these. Once you guess at the setup your pretty much stuck with it. Imagine rigging an OB like that!
Bolt it to the transom and thats all you get to do.

To fully apreciate this Mercury has a jet drive system that uses an OB power head (inboard jet). Take that 2 cylce as the IO power source. The power and weight of the whole rig are about equal now. You still can't tune the set up though.

[Rickracer]

...check out the site in my sig. You will definitely save a bunch of moolah. :cool:

[Blizz]

While i'm an outboard person myself there are plenty of fast inboards outthere, whether it be big or small. I would hate to be screamin down the water at 90 95 and get past by a 45ft inboard..And don't forget about the old 60's and early 70's era true inboards or vee drives ie sangers and such.. But i've seen guys spend as much on a set oh heads for a big block as most guys will spend on an entire outboard setup.. Just my 2 cents. Ken

[INXS]

I/Os are nice but on allot of boats you're talking allot more weight and allot of money for any serious speed! I had a 87 Checkmate Enchanter with a 260 I/O. I sold it to someone who had to put in allot $$$$ of extras to pick up about 8 MPH, it lasted a year with the outdrive blowing and then the engine blowing!

INXS

[sho305]

Remember like someone posted we are talking speed. The light OB and padded hull will get you no time on Lake Michigan here without a kidney belt. Here there are piles of deep I/Os so they can run the big lake. When it somes to nicer water, it has to be this cost issue making I/Os popular. Also when I look at new boat sales stuff or boat rags, I see few OB hulls from the mass makers. At the same time someone posted a boat association site that listed open bow hulls as the most popular? They are the hardest to find around here if you want one under say 15 years old and in the 17-20' range. Didn't get that, maybe just this area or I read it wrong.

One thing that would help is an assisted steering system. Can't believe nobody has a common affordable electric like in some cars for OBs. Only need it for low speed and towing, it would help OBs IMO.

Once helped a guy put a backyard 400 sbc in a 19' I/O, much like a checkmate looking hull. With a big 305 degree cam and pretty wild idle it took a bit to get on plane with 4, but ran low-mid 70s. It had a little notched pad, and it wiggled around just like an OB over 60mph. The speedo was close to other larger boats with nice gaffrig speedos. It ran around 45 with the stock 305ci Merc. One can do that pretty cheap, but more than that will cost a lot. Better to use a marine cam though, and I don't know how long his drive lasted. Good thing he was only running smooth lakes.