First non GM block Merc.

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,454
I make more torque with dyno oil than I do with Synthetic, but oil made from natural gas is lighter, so I go faster.
But if you use dyno oil , does it affect where the boat falls off plane? Of course that question assumes a fictional boat.
 

JustJason

Vice Admiral
Joined
Aug 27, 2007
Messages
5,319
The amount of idiocracy here is mind boggling. It's almost like all of the real technical knowledge left with Dons passing and Toms/Rodbolts falling out. (to which iboats mods/admin were stupid for not biting the bullet on that one... Tom wasn't right, but when you have a Yamaha master tech giving you his free time and you don't take advantage of that and look the other way when you have to is just stupid).

There are many reasons why I post for other websites nowadays. The biggest being money. Have fun armchair quarterbacking over stuff you all read in books, but didn't learn on the deck of an actual boat. Individually you guys might know one or two things about engines you actually own. But other than that there is nothing here. Iboats will surely be the encyclopedia of Mercruiser 470 and Ford 460 engines.... but that's about all this place is worth any more.
 
Last edited:

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Jeez Jason, I guess I should've just listened. Got it.
 

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
See ^^^^ You didn't disappoint... :)

But as soon as there is any movement then it is creating horsepower, right?
 

QC

Supreme Mariner
Joined
Mar 22, 2005
Messages
22,783
Cool. Always a fun discussion. Wish Jason thought so...
 

Tail_Gunner

Admiral
Joined
Jan 13, 2006
Messages
6,237
Yes you can.

The boat is a fictional boat and it really doesn't matter. But in real life single 5.0's MPI's are going into boats as big as 30 feet. With 1 or 2 people the motor and boat performs okay. When the boat is fully loaded with the family, or if you have a mid 20's boat with a single 5.0 MPI and tow with it. They are absolute dogs, and in some cases will not plane with stock props when fully loaded to whatever is stamped on the USCG placard on the boat. This is real life, and I run into this weekly because many a people are either cheap, and buy a boat with the smallest base engine. Or they are mislead by salesmen, promising the 5.0 will deliver good performance on a heavier or highly loaded boat, when it will not. This is a problem I deal with a few times a month.

That "fictional" engine I was hitting rick with earlier was the engine in my truck. It makes 381 HP at 5400 rpms but it makes peak torque of 401 lbs/ft at 3600 rpms. So yes you can be at peak torque at a lower rpm. Some engines make a lot more horsepower than they do torque, like sportbike motorcycle engine, or high reving jetski engines. Other engines make more torque than they do horsepower, like my truck engine or most diesels. Some engines are in the middle of those two extremes, but either favoring horsepower or favoring torque.

Earlier you wrote

My engine is a peak torque engine, it develops 401 lbs/foot at 3600 rpms and that is the most amount of torque it makes. According to the calculator, at this same 3600 rpms its only about 275 horsepower at this very moment. But if you keep on reving it you can get 381 horsepower at the top of it's rpm range at 5600.

In general, and especially with the GM engines. Most blocks produce more peak horsepower than they do peak torque. But big blocks (and strokers) give you more peak torque than they do peak horsepower.

You have your analogies about hp vs tq. Mine is simply tq is a measurement of how much work an engine can do starting from a dead stop. Hp is how much work an engine can do starting from a rolling start. TQ is how much pulling power you have from a dead stop. For example from a dead stop. If you attach a 1 foot bar at the center of my crankshaft and attach a weight to that bar, and run the engine. It can lift 399 pounds, it can lift 400lbs, and it can lift 401 pounds. But it can not lift 402 pounds. That is the point where the engine would essentially stall. However, once it goes rolling. The engine can still get past its peak torque spot and still develop "overall power" overall if you can spin it faster. Between the flywheel, the crankshaft, and a few other bits you have essentially 100+ pounds of dead weight all spinning to the left. Spinning weight has inertia. Inertia is stored energy. You can add energy to that existing stored energy by spinning that weight faster and faster. And this is horsepower. Horsepower is adding energy to existing energy, torque is creating energy from nothing. The limitation as to which you can add it is 2 fold. 1 mechanical, how fast can you spin it before the mechanical stress becomes to much and it falls apart. And 2, how much energy can you extract from the fuel being consumed. Straight gas has more power in it than E10, or E85. Nitromethane has more power in it than straight gas.

There are 3 types of boaters

1. Lake boaters that consistently run engines to full throttle, all of the time.

2. Ocean boaters that can not run engines to full throttle all of the time because conditions do not allow it

3. Boaters oriented towards towsports, or heavy loading. And they never approach full throttle because most of the time they run the boat in what would be considered for all intents and purposes an overloaded condition. IE - Filling up a ballast bag and towing a wakeboarder.

Maximum HP only really matters if your #1. If your 2 or 3 it doesn't really apply to you.



xr258.JPG


Jason where would you prop this engine for maxium performance... 20' 2800 lbs boat deep v runabout

Now the same engine......30 feet 5000 lbs.. flat bottom cruiser.

By propping i mean for what max rpm. I think the conversation is centered lab condition's vs actual application's that can be very frustrating and full of variable's....

The 20' boat i would prop for 5200 and be done with it...it would not need the tourqe to keep it up so to speak
Now the 30' boat id be will to bet she falls on her face at 4200 same engine there's is just no usable energy beyond that.
 
Last edited:

JustJason

Vice Admiral
Joined
Aug 27, 2007
Messages
5,319
Jason where would you prop this engine for maxium performance... 20' 2800 lbs boat deep v runabout

Now the same engine......30 feet 5000 lbs.. flat bottom cruiser.

There are alot of assumptions here. But With either application I would prop it to 4600 rpms. On the chart it looks like peak hp is 4400, and on gas engines if you can do it ideally you prop it for just a smidge higher than peak hp. And then of course either boat here would have it's most economical cruise at 3600 to 4k.

Keep in mind there are no real benefits for spinning an engine any faster than what is needed. With fuel quality always a concern, (and the detonation that comes with it) and the wear and tear that comes the faster you spin any engine, you don't want to spin it any faster than you have to. And because this engine makes peak hp at 4400 with the proper gear ratio and proper propeller, there is no reason why the boat would be faster at 5200 than it would be at 4600.
 

HT32BSX115

Supreme Mariner
Joined
Dec 8, 2005
Messages
10,083
There are alot of assumptions here. But With either application I would prop it to 4600 rpms. On the chart it looks like peak hp is 4400, and on gas engines if you can do it ideally you prop it for just a smidge higher than peak hp. And then of course either boat here would have it's most economical cruise at 3600 to 4k.

Keep in mind there are no real benefits for spinning an engine any faster than what is needed. With fuel quality always a concern, (and the detonation that comes with it) and the wear and tear that comes the faster you spin any engine, you don't want to spin it any faster than you have to. And because this engine makes peak hp at 4400 with the proper gear ratio and proper propeller, there is no reason why the boat would be faster at 5200 than it would be at 4600.
[emphasis mine]

Just to be clear, you're saying with the above engine specs:

If one selects a prop pitch that allows the engine to turn a MAX RPM (@WOT) of 4600,

And THEN selected a different (lower) pitch that allowed a higher WOT RPM of 5200,

The prop that allowed a higher RPM would NOT produce a higher (boat) speed..........?? [I think I agree with you on that!]

Based on the above graph, the Power output is essentially "flat" from 4000-5200. and the torque reduction is linear (straight-line) from 4000-5200. (the reason the HP curve is flat from 4000 to 5200)

So in effect, you could actually select ANY pitch to produce ANY WOT RPM between 4000 and 5200 and not get a significant difference in top speed.

However, selecting a prop that gives you a WOT (engine speed) of 4000 RPM would likely result in a MUCH poorer hole-shot performance compared to a prop that allowed 4600 (or 5200) RPM at WOT.... (which is why most manufacturers suggest "propping" engines [OB IB, or I/O] towards the top RPM in the range of recommended WOT)
 
Last edited:

JustJason

Vice Admiral
Joined
Aug 27, 2007
Messages
5,319
rick said:
The prop that allowed a higher RPM would NOT produce a higher (boat) speed..........?? [I think I agree with you on that!]

The thing is it may, or it may not. Propping as you probably know and have heard is part art, part science. With to many variables. It might, but it also might not.

When it comes to anybody it's their motor, and your going to do what your going to do. Wether you agree with me or not is up to you. I can tell you Evinrude tells you to prop for absolute peak horsepower and not higher, which on this graph if i'm reading it right is 4400. Mercury instructors say prop for just over peak hp. Yamaha tells you to prop towards the high end of the RPM window, without telling you where peak HP is in that window. And the offshore racers, the big Donzis, their train of thought is to prop about 1000 rpm over peak torque, which on this fictional engine looks to be about 4k rpms, which would make the proper prop whatever the engine needs to hit 5k. And if this was a diesel engine, we would prop for just over peak torque, a few hundred over peak torque.

There are a couple of ways to think about it, as it is part art. That all being said there are a couple of undeniable facts

1. The faster you spin any engine the faster you wear and tear

2. The faster you spin any engine the greater your chances for detonation go up, and most people don't know it until it's to late.

3. The faster you spin any engine the chances of it leaning out go up because most folks don't take into account injector flow rates, injector pulse times.... but more importantly... keeping all those parts clean. If you live in a northern state were you winterize. Injectors will flow as they are supposed to for the first couple of years. But after a couple of years of sitting all winter as well, they are not going to flow the same. Lean means mean and the engine is going to run the best the leaner it gets... until it melts a hole in a piston

4. This fictional engine looks to be a small block GM. In general, 5k rpms is kind of a magic number. Pushrod engines don't like extended run times above 5k. Small blocks also come with high volume low pressure oil pumps, right around the 5k mark is really when it should have a high pressure oil pump on it. And if this is a 2 bolt main... forget it.

If you want reliable... prop this one at 4600 tops.... If you want high performance.... do what you want. Just don't cry about it if the engine has a meltdown at some point, or you get 400 hours on it and your front and rear main oil seals are leaking on you. A good general rule of thumb is you don't spin anything faster than you need to. If you want to test the theory, take your lawn mower or snow blower engines and crank those up so they rev just 300 to 400 rpms above whatever the governor is set to... and see how long that lasts before you hotrod a boat engine. Don't get me wrong, the engines will run awesome, and cut grass/blow snow like no tomorrow... But you won't get any more than 1 or 2 seasons out of it before the rings are smoked, the bearings are scored or spun, and it leaks oil everywhere.
 
Last edited:
Top