Please explain I/O HP VS OB HP

[BigwayNZ]

I have just purchased a 2009 boat that has not been made since the late 70's. A very popular ski boat back then. Anyway it is being custom built for me now and I have to decide if i want I/O or outboard.

it is a 18 footer and at the moment i have the 70s model of it with a 175hp johnson on it.

Anyway, the main options for reasons i dont care to discuss is that I can choose a Sterndrive 6.2litre Mercruiser 320 hp.

I am trying to read up as much as possible about this motor and it sounds like a fantastic motor. Being new to boating i would like to know not what the difference is etc but would like to know what 320 worth of horsepower in I/O compares to how many HP on a outboard?


I read a review that compared a 130HP OB with a 130hp I/O and it was very simialr, so does that mean a 320hp sterndrive is going to be as powerful as a 320hp outboard would be?

I am willing to bet te answer is no but i can not find any articles coparing such a thing and would like for you to give me an idea of what the HP from the mercruiser would be in outboard HP?


Thanks guys.

 

[John_S]

Re: Please explain I/O HP VS OB HP

HP is HP, if it measured at the same place and method. Todays motors are measured at the prop. Older motors, were commonly measured at the crankshaft. Today, the difference is mainly in the weight and torque curves of different motors.

The 6.2L is allot of motor for an 18'er, but if it is built to handle it (power and speed), it should be fast. The I/O would be hidden and would not interfere with the retro look you appear to be going for. The vintage 175 'rude wouldn't be a slug on a typical 18'er, either. You just have to factor in the maintenance and repair on an old motor. When it comes to replacement, a retro boat with a modern outboard, doesn't cut for some.

 

[Bondo]

Re: Please explain I/O HP VS OB HP

I read a review that compared a 130HP OB with a 130hp I/O and it was very simialr, so does that mean a 320hp sterndrive is going to be as powerful as a 320hp outboard would be?

Ayuh,... What John says,... Hp is Hp,....

 

[180shabah]

Re: Please explain I/O HP VS OB HP

The big difference will be the weight. I don't know what OB's your are looking at but figure a little over 1000lbs for the 6.2Bravo combination.

The OB "should" be faster out of the hole due to the lower weight, but it may not be making enough power down low, so it might not be faster. If you custom builder has the right software, he should be able to give you an idea of how weight and weight distribution will affect the power requirements and speed potential. The final speed numbers will be approximate, but their relationship to each other is what you are interested in.

 

[JustJason]

Re: Please explain I/O HP VS OB HP

like everone else has said... HP is HP.
But.... torque will not be the same.

If you plan on doing a lot of watersports with the boat i'd opt for the IO because it will make more power at lower rpms than an OB would make.

 

[180shabah]

Re: Please explain I/O HP VS OB HP

I forgot the biggest reason to go I/O. I assume that with the amount of money you are spending to recreate a classic hull that you are interested in "cool points". A v8 with thru-hulls just sounds sweet. And if you use mufflers and a crossover it will only annoy those that "need to be annoyed".

 

[BigwayNZ]

Re: Please explain I/O HP VS OB HP

Actually, this is what is going on...

Sidewinder has just spent the last so many years retooling and building moulds etc for the new sidewinder. they have spent a fortune and I am getting one.

I have a classic one with a old johnson 175hp.

Now i am getitng a new one and they suggest I/O 6.2 mercruiser.

So it will not be taking up anymore space than the 1800 with outboard.

So will a 6.2litre hit 60 mph? It does not need to be any faster than that as the hull will not like it over 70mph.

 

[krisnowicki]

Re: Please explain I/O HP VS OB HP

sidewinders are awesome looking boats in my opinion. http://sidewinderboats.com/

If it were me I would do the i/o for the above reasons. +

The engine will generally use less gas per hp than an ob. At least compared to the older obs. And it keeps the sleek lines of the boat with out a big ol ob strapped to the back of it. And with out thru hulls its a much quieter ride so you dont have a ob screaming at 5k rpms and you could have a conversation.

 

[180shabah]

Re: Please explain I/O HP VS OB HP

....So will a 6.2litre hit 60 mph?

You really need to ask the manufacturer, or have they not built any test models yet.

With that said, I don't think 60mph is out of the question. I have an 18' Mariah that is about 2600lbs wet, and it tickles 50mph with a 175horse 4.3LX.

 

[BigwayNZ]

Re: Please explain I/O HP VS OB HP

The manufacturer reackons so but they only have their first few being completed today. They say it will be easy to get to that speed.


I live in Australia and have been offered a smoking deal on one so they can have one over here in Australia. What makes it even cooler is i am also having my old sidewinder restored. So should have a 1979 model and a 2009 model. Both 18's, one stern and one outboard.

 

[Faztbullet]

Re: Please explain I/O HP VS OB HP

The Sidewider has a narrow pad like the 80's Checkmate's. If you can work with factory suggest raising the X dimensions a couple of inches and test. With a increased height. speed and handling should be better due to less drive dragging behind boat. When we special ordered our Checkmates years ago we had transom plate cutout raised 2 inches and stringer height increased. Boats ran faster and handled better (little chine walk) and other owners wondered why our boats we sold was faster than theirs.(HEHEHE)

 

[QC]

Re: Please explain I/O HP VS OB HP

I get 58 MPH with my 23 footer and a 6.2 Merc Bravo 1. That's empty and just me, but a solid 54 MPH always (GPS). Depending on the hull, I think your combo is 65 easy. My boat is fairly stout and with 20 degrees of deadrise not a particularly fast setup.

A couple of quick points to add to others made. HP is indeed HP, in fact HP = Torque x RPM/5252. Period. Shaft or flywheel or flippin' water pump pulley. Soooooo, at the shaft, the RPM is of course shaft RPM, and the Torque is Shaft Torque . . . So, if we have different gear ratios, and/or RPM differences, then there are different torque and RPM combos to deal with . . . We can't really say that an OB has less torque so therefore acceleration will be poor, or water sports performance will be poor. With 2 cycles and 4 cycles and 7000 RPM with a 2/1 ratio or 5200 with a 1.5/1 ratio, or whatever combos we find, there is no simple rule of thumb that will dictate this. However, 145 HP (320 - 175) will easily make up for the added 500 - 700 lbs. The other issues though are trimibility and weight distribution. Same hulls could be vastly different with different power combos as you suggest . . . Lots to consider here.

 

[BigwayNZ]

Re: Please explain I/O HP VS OB HP

The Sidewider has a narrow pad like the 80's Checkmate's. If you can work with factory suggest raising the X dimensions a couple of inches and test. With a increased height. speed and handling should be better due to less drive dragging behind boat. When we special ordered our Checkmates years ago we had transom plate cutout raised 2 inches and stringer height increased. Boats ran faster and handled better (little chine walk) and other owners wondered why our boats we sold was faster than theirs.(HEHEHE)

Appreciate it your suggestion mate but the sidewinders do not have a pad on the bottom. only those that people have done themselves. Also they have already spent a million or so tooling the boat so asking them to make a new mould would be asking a bit too much, especially with the price i am paying for this demo boat.

 

[BigwayNZ]

Re: Please explain I/O HP VS OB HP

I get 58 MPH with my 23 footer and a 6.2 Merc Bravo 1. That's empty and just me, but a solid 54 MPH always (GPS). Depending on the hull, I think your combo is 65 easy. My boat is fairly stout and with 20 degrees of deadrise not a particularly fast setup.

A couple of quick points to add to others made. HP is indeed HP, in fact HP = Torque x RPM/5252. Period. Shaft or flywheel or flippin' water pump pulley. Soooooo, at the shaft, the RPM is of course shaft RPM, and the Torque is Shaft Torque . . . So, if we have different gear ratios, and/or RPM differences, then there are different torque and RPM combos to deal with . . . We can't really say that an OB has less torque so therefore acceleration will be poor, or water sports performance will be poor. With 2 cycles and 4 cycles and 7000 RPM with a 2/1 ratio or 5200 with a 1.5/1 ratio, or whatever combos we find, there is no simple rule of thumb that will dictate this. However, 145 HP (320 - 175) will easily make up for the added 500 - 700 lbs. The other issues though are trimibility and weight distribution. Same hulls could be vastly different with different power combos as you suggest . . . Lots to consider here.

Thanks for your informative reply.

I have decided to go with the I/O seeing i already have one with an OB that is being restored as of today.

Should be fun to line up two boats with the same name that are 30 years apart from each other. I shall post pictures when it is here in a couple of months

 

[BigwayNZ]

Re: Please explain I/O HP VS OB HP

The Sidewider has a narrow pad like the 80's Checkmate's. If you can work with factory suggest raising the X dimensions a couple of inches and test. With a increased height. speed and handling should be better due to less drive dragging behind boat. When we special ordered our Checkmates years ago we had transom plate cutout raised 2 inches and stringer height increased. Boats ran faster and handled better (little chine walk) and other owners wondered why our boats we sold was faster than theirs.(HEHEHE)

just realised your 'X' means prop. i will take that to them