What cause I/O failure? Horsepower or Torque?

[fabrimacator21]

Assuming that oil changes are performed regularly and everything is mechanically sound what cause breakage? Hp or Tq?

I had a crazy notion of mating a 350HP 355 to my omc 800 but after reading some responses it sounds like a bad Idea. So I'm looking at putting some vortec heads, a cam, and manifold on my stock 305. I need to know what kind of number to shoot for in order to keep the I/O happy(I.E. what cam to run). I like the idea of hot rodding the 305 because the tq is still below a stock 260hp mercruiser 350 (which were regularly bolted to the 800 series I/O from the factory) but the hp will be around 270-300 depending on cam and prop choice. I plan on using trim tabs to help the holeshot(because of the tq numbers) and want to run a 21, 22 or 23P prop for more top speed. I'd like to hit 55+ mph.

 

[TilliamWe]

Re: What cause I/O failure? Horsepower or Torque?

Torque wears them out.
Jumping waves and overrevving the engine as the prop bites back into the water breaks them.

 

[mkast]

Re: What cause I/O failure? Horsepower or Torque?

When the hull is on plane, where is the drive?

 

[MikDee]

Re: What cause I/O failure? Horsepower or Torque?

Assuming that oil changes are performed regularly and everything is mechanically sound what cause breakage? Hp or Tq?

I had a crazy notion of mating a 350HP 355 to my omc 800 but after reading some responses it sounds like a bad Idea. So I'm looking at putting some vortec heads, a cam, and manifold on my stock 305. I need to know what kind of number to shoot for in order to keep the I/O happy(I.E. what cam to run). I like the idea of hot rodding the 305 because the tq is still below a stock 260hp mercruiser 350 (which were regularly bolted to the 800 series I/O from the factory) but the hp will be around 270-300 depending on cam and prop choice. I plan on using trim tabs to help the holeshot(because of the tq numbers) and want to run a 21, 22 or 23P prop for more top speed. I'd like to hit 55+ mph.

What? 55mph, with that Sled?,,,lol,,, I don't think so, no offense meant, but seriously with a trihull there's so much turbulence under the hull, that I doubt it would get above a certain speed, maybe 50, and that's a maybe, even with a lot of horsepower,,, You need a more efficient Vee bottom boat to get the most out of your engine. Did you ever look at how much water your throwing around under that hull at speed? It would just be a foaming fury. I had 3 Wellcraft Airslots, (trihull style), 1- 16' outboard with a 135hp Evinrude, & 2- 18' I/O's - a 165hp Merc. straight 6, and a 188hp Merc. 302 V8, and couldn't get to 45mph with any of them, and those hulls had steps in them

 

[dirtyoldman]

Re: What cause I/O failure? Horsepower or Torque?

No such thing as horse power, only RPM and Torque. Everything else is just math.

This was my thought. Torque is what breaks things, but I think you don't understand the relationship between torque and HP.

Marine drives are usually rated in HP. I'd stick to that or pack an extra paddle.

 

[Bondo]

Re: What cause I/O failure? Horsepower or Torque?

bolted to the 800 series I/O

Ayuh,... Find another boat man,....

A Stringer drive on a Tri-Hull just Ain't an efficent platform to Start with...That's a Fact....

 

[fabrimacator21]

Re: What cause I/O failure? Horsepower or Torque?

I know hp is a math equation but dyno numbers tend to somewhat dissprove that equation... different combinations can make the same hp with different tq at a given rpm and vice versa. I won't even go into deisel numbers...

I'll go out on a limb here and say with 290hp 55 is very doable... You may not think so but as it sits right now I know it does 45 trimmed out. Thats with the 200 hp stocker 305. I plan on vortec heads, cam, and intake to gain another 80-90 hp which I hear will net me another 9-10 mph if propped right. Yes it's a trihull but the rear of the boat is essentially the same as a big V bottom and when trimmed out the "tri" is pretty much out of the water. Just becuase it's a trihull doesn't mean it won't hit 55. If not then o well... I'd be happy anywhere over 50.


Bondo: if I could afford another boat I would but this thing has EVERYTHING I want in a boat. Good stereo, tons of room, nice seats and upholstery, downriggers, etc. Just want a little more top speed. Not a race boat by any means. Just a weekend warrior.

 

[mylesm260]

Re: What cause I/O failure? Horsepower or Torque?

I'll say it again, horse power is nothing more than a math formula, it doesn't really exist as a measurable unit. Torque x RPM / 5252 = Horse Power. (The divisor of 5252 is arbitary and has been changed many times.)

People also make the mistake of thinking the "hole shot" on a boat is what loads the drive, sorry, it just isn't so. Let's say you have an engine producing 200 lb ft of torque at 800 RPMs. You put it in gear and go to WOT to get on plane. That engine is only producing about 30 Hourse Power until the RPMs come up.


Yeah, I like to think that as an engineer I do understand HP and Torque.

yup, he's 100% correct.

Torque is force. A regular man can exert 200 ft/lbs with a 12" ratchet and 600 ft/lbs with a long breaker bar. Compared to engine, these are great numbers, but if the breaker bar isn't moving than you're horsepower output is zero.

If you want to discuss a diesel engine, horsepower is still horsepower.

Let's say you have a gas engine that makes 160 HP.

That gas engine could make 160 HP @ say 5000 rpms. That works out to:

168.064 ft/lbs of torque @ 5000 rpms


Now let's say you have a diesel engine that also makes 160 HP. But the diesel engine does it at 2500 RPMS. that works out to:

336.128 ft/lbs of torque.

Exact same horsepower, but one it making it at half the RPMS.

Let's say the gas engine was in a boat with a 10P prop and the diesel engine was diesel engine was in the boat with a 20P prop.

The boats would have the exact same top speed (ignoring prop factors of course)

There would however be twice as much force on all the components in the diesel drive.

However, if the diesel drive was built to handle the extra forces, it would last multiple times longer than the gas, because of the lower RPMS.

wear occurs at the square of RPM.





In terms of drive-wear, look at it this way.

Torque increases the force on all the bearings and gears. More torque = more pressure.

At some point the force is going to exceed the strength of something, a gear tooth, the housing, a coupler, something.


RPMS increase the rate at which they wear.


The more torque you add, the greater the chances of a sudden catastrophic failure (like breaking a gear tooth)

The more RPMS you run, the quicker you're going to wear the components. So lets say a leg would go 1000 hours before all the bearings and gears need to be changed, if you increase the rpms by 20%, you've reduced it's life by 50% so it would only last 500 hours (rough numbers of course)

More torque also increase wear, because the components are being pushed together with more force.

 

[mkast]

Re: What cause I/O failure? Horsepower or Torque?

You may not think so but as it sits right now I know it does 45 trimmed out.

How are you measuring speed on this boat? GPS?
If in fact, you can pick up another 80 or 90 real horsepower (some people, not you, but some people get a little easy with guessing horsepower) you might actually see 8 or 9 more miles per hour. Remember, every mile per hour you get, the next one needs a tad more horsepower than the last, and so on and so on.
You are right about the tri hull bow being out of the water when the hull is on plane. The aft part of the hull that left is in the water at wide open throttle is called the wetted area. By WOT, on plane, the bow had better be out of the water. How much junk (equipment) are you carrying around in the hull (weight)? How much does the hull weigh dry? This whole thing is about weight, the more you can lose, the faster the hull will go. Is the flotation water logged? Any rotted wood?

 

[fabrimacator21]

Re: What cause I/O failure? Horsepower or Torque?

I got the speed from running nose to nose with my buddies boat while he was reading his gps... and I've gotten quite a few butt dyno conformations from experienced boaters that say it's doing low to mid 40's. It's been a while since I popped my head under it but IIRC it has 1 step in the rear of the hull.


I really couldn't tell you what it weighs since I can't find the model of the boat. I'd guess somewhere between 3000-3500lbs. I normally haul a few people with a cooler, tackle boxes, and the other usuals. It did have about 300 lbs of water in the middle of the boat where the gas tank sits but I had the hole floor up and the only water was around the tank. no water logged foam or wood.... old wood but not water logged.


I am using desktop dyno to come up with the power gain numbers... trying to keep the hp just under 300 to save the I/O.... I could hit 320 depending on how high I spinn it with the vortecs and marine cam but I don't want to spin it over 4800rpm. I'm planning on hot rodding the 305 for torques sake. It won't have the gear crunching tq of a hot 350 but it will make almost identical tq numbers to a mercruiser 260hp 350 with another 20-30hp on the top end... maybe a little more.

The marine mechanic tells me that the 800 series omc I/O is just about maxxed for hp at 260hp. However thats with a 350 not a 305 so the tq is more brutal with the 350. I didn't ask him what he thought about a 300hp 305. I need to make a call tomorrow. I can't imagine another 40hp with less tq would blow the thing up... . As long as I'm not jumping it and overrevving out of water I think it should be fine.



Mylesm260: your right...:redface:

 

[a70eliminator]

Re: What cause I/O failure? Horsepower or Torque?

A regular man can exert 200 ft/lbs with a 12" ratchet and 600 ft/lbs with a long breaker......

That would have to one bad azz dude, I pulled 200ft/lbs on a pinion nut just recently and had to use a 4' cheater bar, I like to think I'm a regular man.

 

[MikDee]

Re: What cause I/O failure? Horsepower or Torque?

A regular man can exert 200 ft/lbs with a 12" ratchet and 600 ft/lbs with a long breaker......

That would have to one bad azz dude, I pulled 200ft/lbs on a pinion nut just recently and had to use a 4' cheater bar, I like to think I'm a regular man.

You're a better man then I am a70, I would have left that crush sleeve just as it is! My cousin who used to be a certified Chevy mechanic said, just change the gears, and get the preload right, don't mess with the crush sleeve, it's not worth it, and will be fine the way it is. I can't say, I've done anything like this yet, but I see a rear end change in my future

Hey fab 21, Check the pic. here, & my signature link for the pix. & info of my Wellcraft Airslots, this hull was designed for speed, & stability, but still couldn't match a Vee hull. Either way, good luck with your quest!
Mike

 

[mkast]

Re: What cause I/O failure? Horsepower or Torque?

Reality check.
You are guessing at the boat's speed and weight.
Remember what I said about some people?
Good luck in your endeavors.

 

[mkast]

Re: What cause I/O failure? Horsepower or Torque?

A regular man can exert 200 ft/lbs with a 12" ratchet and 600 ft/lbs with a long breaker......

The Snap On 1/2 inch torque wrench I use is maxed out at 200 ft/lb, it is a tad longer than 12 inches, more than once I've had to brace my foot against something to get to 150 ft/lbs. on head bolts.
That 600 ft/lb...
The main rotor nut holds the main rotor blades on a UH-1H (Huey) helicopter.
The torque was 650 ft/lbs.
We had to use a torque multiplier. One person holding the socket and multiplier on the main rotor mast. Two people with an eight foot bar standing on a work platform next to the aircraft. Two good sized teenagers busting their butts to get to 650.
Of Course it was called the "Jesus Nut!"

 

[mylesm260]

Re: What cause I/O failure? Horsepower or Torque?

A regular man can exert 200 ft/lbs with a 12" ratchet and 600 ft/lbs with a long breaker......

That would have to one bad azz dude, I pulled 200ft/lbs on a pinion nut just recently and had to use a 4' cheater bar, I like to think I'm a regular man.

I'm a pretty big boy (260 lbs), and I was referring to a situation where you could stand on it, and maybe even brace against something for a little more push.

 

[mylesm260]

Re: What cause I/O failure? Horsepower or Torque?

I got the speed from running nose to nose with my buddies boat while he was reading his gps... and I've gotten quite a few butt dyno conformations from experienced boaters that say it's doing low to mid 40's. It's been a while since I popped my head under it but IIRC it has 1 step in the rear of the hull.


I really couldn't tell you what it weighs since I can't find the model of the boat. I'd guess somewhere between 3000-3500lbs. I normally haul a few people with a cooler, tackle boxes, and the other usuals. It did have about 300 lbs of water in the middle of the boat where the gas tank sits but I had the hole floor up and the only water was around the tank. no water logged foam or wood.... old wood but not water logged.


I am using desktop dyno to come up with the power gain numbers... trying to keep the hp just under 300 to save the I/O.... I could hit 320 depending on how high I spinn it with the vortecs and marine cam but I don't want to spin it over 4800rpm. I'm planning on hot rodding the 305 for torques sake. It won't have the gear crunching tq of a hot 350 but it will make almost identical tq numbers to a mercruiser 260hp 350 with another 20-30hp on the top end... maybe a little more.

The marine mechanic tells me that the 800 series omc I/O is just about maxxed for hp at 260hp. However thats with a 350 not a 305 so the tq is more brutal with the 350. I didn't ask him what he thought about a 300hp 305. I need to make a call tomorrow. I can't imagine another 40hp with less tq would blow the thing up... . As long as I'm not jumping it and overrevving out of water I think it should be fine.



Mylesm260: your right...:redface:

Look, the reality is this:

You're drive and HP: You can take pretty much anything mechanical on the planet and push it past it's design limits. IS your drive going to completely fly apart at 301 HP if it was rated for 300? no. If you want to run 30-40% more power through you're drive than stock, go for it, it will work, it just won't last nearly as long, and if something is close to falling, it's much much more likely that it will fail a whole lot sooner with the added power. You're talking something that was designed to last X hours with X power, and changing the numbers around. so now it will last 0.5 X hours with 1.25 X power.


Top speed and HP:

Think of a boat as drag car that's cruising around with it's parachute out all the time.

Some boats have bigger parachutes on them than others, from what people are saying, your particular boat has a relatively large parachute on it.

The thing about parachutes is, the force required to push them faster goes up more and more with every additional MPH. Meaning that the first MPH is the easiest.

+1 mph = +10 HP
+2 MPH = +22 HP
+3 MPH = +36 HP
+4 MPH = +50 HP

(all rough estimates, of course)

and so on and so forth. So for all you're effort of making more power, you're REALLY not going to get anywhere in terms of MPH.


go to you tube and type in "scream and fly". There are guys with 250 HP outboards doing 95 MPH. 250 HP is a reasonable amount of HP, lots of stock IB/OB's came stock with more power than that. but the reason their boats are so much faster is simply their hull and their weight.

 

[fabrimacator21]

Re: What cause I/O failure? Horsepower or Torque?

You're a better man then I am a70, I would have left that crush sleeve just as it is! My cousin who used to be a certified Chevy mechanic said, just change the gears, and get the preload right, don't mess with the crush sleeve, it's not worth it, and will be fine the way it is. I can't say, I've done anything like this yet, but I see a rear end change in my future

Hey fab 21, Check the pic. here, & my signature link for the pix. & info of my Wellcraft Airslots, this hull was designed for speed, & stability, but still couldn't match a Vee hull. Either way, good luck with your quest!
Mike

wow that thing is funky looking... sort of like a cat tri-hull. I like it though. Mine isn't stepped like that... it's a smaller step and it's almost all the way at the rear. I'll try to get some pics of it.

Could you get the sides(steps) up out of the water at WOT?

Reality check.
You are guessing at the boat's speed and weight.
Remember what I said about some people?
Good luck in your endeavors.

Yup you were talking about power which I have pretty much dead on, thanks to desktop dyno... as far as guessing goes on the weight I'm being pretty "safe" on those estimates. The 19' version of the closest model I could find was 3500lbs.... mine is only 17'. What do you expect me to do when I don't know what model the boat is?:redface: As far as speed... it's pretty close to 45mph. I'm gonna try and borrow his gps this weekend though to get a real reading.

I'm a ways off from actually going through with the motor work as I have to sell my truck and a motor before I will have the money. Still have alot of research to do also... still not sure if it'll be worth it. I need to find out what my top speed is for SURE then go from there.

 

[mkast]

Re: What cause I/O failure? Horsepower or Torque?

Yup you were talking about power which I have pretty much dead on, thanks to desktop dyno... if it'll be worth it.

Here's another reality check.
If you have access to the Mercruiser manual, look up the specifications to your 305. Run the numbers on desktop dyno. I ran the numbers for a stock MIE 454, didn't even come close to Mercruiser's advertised horsepower 330.

 

[fabrimacator21]

Re: What cause I/O failure? Horsepower or Torque?

Well, I'm not gonna push the issue... I've gotten a few different responses from marine mechanics, most said you might be able to get away with it but I don't want to risk it.

Thanks for the help guys.


Also can anyone tell me why I would need a marine water pump? The marine pumps are 200 bucks while the heavy duty automotive pumps are 50 bucks. I know the marine pumps use a stainless impeller but that doesn't seem necessary in fresh water. This thing won't see salt water so could I just use an automotive pump?

 

[mkast]

Re: What cause I/O failure? Horsepower or Torque?

Also can anyone tell me why I would need a marine water pump? The marine pumps are 200 bucks while the heavy duty automotive pumps are 50 bucks. I know the marine pumps use a stainless impeller but that doesn't seem necessary in fresh water. This thing won't see salt water so could I just use an automotive pump?

Did you ever think about a water pump rebuilding kit?