Question for all the Smart ones.

[Teleman]

1 minute ago, justgary said:

Except that I can put a prop on the boat that will allow 5000 RPM at WOT.  Not much thrust produced in that case. 

You must be referring to all power lost to friction in the blade but the drive train still sees the same rotational stress.

[MadMan]

6 hours ago, Teleman said:

What running condition causes the most power-train stress and highest torques to pass through the dampening plate and propeller shaft.  Running WOT with a lightly loaded boat or Running a fully loaded boat surfing at 2500 RPM and at 2/3 throttle? Explain why

Something else to think about.  At 2500 rpm full throttle the engine will only be able to consume roughly 1/2 the air as it does at 5000 rpm full throttle. 

So if the manifold pressure is already at 0 at, 2/3 throttle 2500rpm, going to WOT will have no effect at all. It's already pulling in all the air it can.  This is what happens when you have too much prop (dia and/or pitch) and can't get up to surf speed.

This all assumes old school throttle, not "drive by wire".

Edited December 31, 2018 by MadMan
update

[carguy79ta]

My thoughts, it has to do with throttle position, fuel consumption, rpms, and torque curve.

Assuming the boat is propped correctly, whatever condition causes WOT at the peak of the torque curve, (steady state) will place the highest stress on the drive train components. Prolly surfing. Torque breaks things, power causes failure from unbalanced parts spinning too fast w/o proper lubrication and stability.

[Woodski]

It is actually not a single or simple answer.  At high engine speeds, the valve train will be stressed the most as will other elements that are most sensitive to speeds (think the directional change on pistons and effect on the pins).  Under high loads, as Old Jeep mentioned there will be stress on items like the transmission and prop shaft.  If the system suffers from any imbalance, high speeds will aggravate wear.  Generalizing, running at WOT at the maximum engine speed will usually shorten the engine life quickest as that unfriendly 'ka-blamo' sound is more likely to occur earlier than at a lower engine speed.

[Falko]

If we are talking stress on the prop shaft, I would say surfing. Most people run where the engine can make the most torque for a reason.

I wouldn't get involved in WOT and other areas of engine power judgments. Just because an engine CAN make a bunch of power at a given rpm doesn't mean that it is. I *think* the boat engine is probably making almost peak torque when heavily weighted and surfing. But it is making peak hp at top speed, maybe. Now you have to throw in the idea of the prop pitch. Most boats will hit max rpm without forcing the engine to make peak hp due to the deep prop pitch. Kind of like running your truck to redline in 1st gear. That engine isn't stressing at 5500 rpms, just spinnin' really fast.

My thoughts and opinions only.

[Teleman]

1 hour ago, carguy79ta said:

My thoughts, it has to do with throttle position, fuel consumption, rpms, and torque curve.

Assuming the boat is propped correctly, whatever condition causes WOT at the peak of the torque curve, (steady state) will place the highest stress on the drive train components. Prolly surfing. Torque breaks things, power causes failure from unbalanced parts spinning too fast w/o proper lubrication and stability.

Although at surfing conditions the engine is never at WOT (or you wouldn't be surfing)..... So lets say we run the boat to WOT then we load it down with ballast and people until the engine speed is dropped to the point of highest torque, but still at WOT (Maybe around 4200 to 4500 rpm). I'm assuming this would bring about the highest level of stresses?

[carguy79ta]

7 minutes ago, Teleman said:

Although at surfing conditions the engine is never at WOT (or you wouldn't be surfing)..... So lets say we run the boat to WOT then we load it down with ballast and people until the engine speed is dropped to the point of highest torque, but still at WOT (Maybe around 4200 to 4500 rpm). I'm assuming this would bring about the highest level of stresses?

No true on WOT while surfing. My boat with lead is about 7200# ballast. I am at almost WOT. Going 11.2 mph. With enough ballast and correct prop, you CAN be WOT while surfing. I could prolly use a shorter prop so I'm not pushing everything so hard.

[Teleman]

11 minutes ago, Falko said:

Kind of like running your truck to redline in 1st gear. That engine isn't stressing at 5500 rpms, just spinnin' really fast.

My thoughts and opinions only.

Typically this can never happen in a boat due to the fixed prop pitch and lack of a transmission. I also think boat engines may not have rev limiting kick in until it exceeds the typical 5200 rpm. So a boat can only go faster by increasing the output HP (which is a function of torque and RPM). Once the engine is at WOT it can go no faster. If we attach a transmission to the engine output and spin the prop faster without increasing the HP  (Transmission will inversely lower the torque while increasing the rpm) the boat will only slow down.

[Teleman]

3 minutes ago, carguy79ta said:

No true on WOT while surfing. My boat with lead is about 7200# ballast. I am at almost WOT. Going 11.2 mph. With enough ballast and correct prop, you CAN be WOT while surfing. I could prolly use a shorter prop so I'm not pushing everything so hard.

So carguy79ta, it sounds like you may have achieved the maximum output stresses. So if you put another, say, 500# in your boat, you could go WOT and still not exceed surf speed? What would your RPMs be at this condition?

[Falko]

36 minutes ago, Teleman said:

Typically this can never happen in a boat due to the fixed prop pitch and lack of a transmission. I also think boat engines may not have rev limiting kick in until it exceeds the typical 5200 rpm. So a boat can only go faster by increasing the output HP (which is a function of torque and RPM). Once the engine is at WOT it can go no faster. If we attach a transmission to the engine output and spin the prop faster without increasing the HP  (Transmission will inversely lower the torque while increasing the rpm) the boat will only slow down.

No, what I mean by that is many install a low end prop for surfing so it would be like running your car in a lower gear. A high pitch prop would be like running a higher gear. So unloaded with a shallow prop would be like running your can in a lower gear, you'd run out of rpms before running out of hp. Which is the same thing you are saying, I think.

[carguy79ta]

@TelemanSpeculating, another 500 or 1000 would be maxed out. At surf speed, with typ. crew of 4 or 5. Any more than that, would prolly not reach surf speed. IIRC typ  rpm is 3500. Maxing it out would be more.   a shorter prop would not require as much throttle. I have a 2829 16x15  .075cup. The surf prop I would use is 2907 16x13. It is definately Winter offseason..

[Teleman]

20 minutes ago, carguy79ta said:

@TelemanSpeculating, another 500 or 1000 would be maxed out. At surf speed, with typ. crew of 4 or 5. Any more than that, would prolly not reach surf speed. IIRC typ  rpm is 3500. Maxing it out would be more.   a shorter prop would not require as much throttle. I have a 2829 16x15  .075cup. The surf prop I would use is 2907 16x13. It is definately Winter offseason..

Would you potentially over rev with a 16x13 prop and no load at WOT?

[shawndoggy]

15 hours ago, Teleman said:

This can't be similar. I can tie my boat to a big dock and floor it at WOT and go no where, or go for a joyride at WOT. Is the stress the same?.... if the engine is at the same RPM in both situations,  is the stress not the same?

Did you steal Mrs. Neusbaum's credit card?

 

 

[formulaben]

21 hours ago, Teleman said:

What running condition causes the most power-train stress and highest torques to pass through the dampening plate and propeller shaft.  Running WOT with a lightly loaded boat or Running a fully loaded boat surfing at 2500 RPM and at 2/3 throttle? Explain why

One variable worthy of consideration is the centrifugal forces acting on the damper plate at 2500 vs 5000 RPM, regardless of engine load.  It is quite considerable...just ask anyone who's lost one.  I think we all can agree that both regimes put the drivetrain in the upper band of it's designed total load/stress. 

But my question is, why are you asking?  Do you have concerns about throwing a shaft or damper plate?

[bamaboy]

@Telemanis @The Hulk I'm absolutely sure of it 

[Pra4sno]

So...wait.... I should stop towing docks?

[goodmatt78]

@D_Turner got it.  I think surf set-ups are very much like cars running race or slick tires when it comes to power train (damper plate) failures.  These new props are so good that most slip is reduced putting all forces on the power train.....just like slicks blowing out rear diffs.

[Andrew63]

Sounds like you guys in the norther hemisphere are suffering from cabin fever, too much time on your hands to overthink things since the lakes are frozen over or too cold.

Just dream about the next season & have another beer

[Cole2001]

Ya what he said^^ Imteresting to read this kinda stuff. 

But we payed big bucks for someone else to do all the thinking.  3 more months until riding season up here, we can do it!

[67King]

To an engine, inertial forces exceed compression forces.  They kind of have to, otherwise single cylinder engines wouldn't run.  Interesting bit of trivia for you.  NASCAR engines are limited by rules to a specific displacement and compression ratio.  They are designed so that the pistons will flex at both TDC and BDC, increasing both displacement and CR, at the cost of durability.  That's a high RPM scenario.  RPM will be harder on the cranktrain, valvetrain, etc.  However, an engine is MUCH more prone to detonation at low RPM WOT.  Detonation is BAD, as it leads to hot spots, which leads to pre-ignition.  PI will kill an engine with a quickness.  So, there is an "it depends" element.  If you have a robust knock system and run good fuel, you have a degree of robustness that will help.  But if not very robust, then there is a big risk. 

Basically, I'd say that you are going to wear out an engine running higher RPM, but you are more likely to blow up an engine by running lower RPM WOT.

All that said, people use the term "WOT" improperly at times.  You don't cruise at WOT.  As alluded to earlier, running fast is not running WOT.  WOT only occurs (unloaded) for a few seconds at a time.  But in a boat, I guess we can run 40-45 or so at WOT.  The engine really doesn't care how loaded it is.  The transmission cares what kind of torque it sees from the engine.  You are probably making more torque at 5500 RPM than you are at 2500RPM, but I still think that in the real world, surfing (i.e. loaded up at higher throttle and lower RPM) is going to have the engine making more torque than cruising at really high speeds.

[MadMan]

I believe most of the damper plate failures are isolated to around 2004 when Indmar had a quality issue with their bell housings being out of alignment.  This caused the damper plates to flex and fail.  RPM or torque wasn't the cause.

[kerpluxal]

13 minutes ago, MadMan said:

I believe most of the damper plate failures are isolated to around 2004 when Indmar had a quality issue with their bell housings being out of alignment.  This caused the damper plates to flex and fail.  RPM or torque wasn't the cause.

From my understanding this statement is correct. This was an issue with all brands that have an indmar engine. Friend's MasterCraft just blew the dampening plate and he upgraded bell housing as well, even though it was not damaged. 

[Teleman]

This all started with a discussion a friend of mine and me were having over the replacement of his dampening plate. He said that surfing at about 2/3 throttle with a heavy boat would compress the springs more than running it at WOT while barefooting. I disagreed with his concept due to the fact that the throttle was the limiting output feature and when in WOT mode, the limits were all balanced. While slightly above the maximum RPM to produce the highest torque, the maximum HP output is then balanced with all the friction, slip, and viscosity lossed. So after more boring winter waste of time thoughts, I have now re-thought the scenario with these conclusions: maximum dampening plate stress occurs when the engine is in WOT and the loaded to the point the RPMs drop the the maximum torque level. While max HP will still be at the max RPM in WOT. The plate sees the biggest torque stress (assuming it is aligned properly ) under the max torque RPM while under WOT.

[Woodski]

@Teleman:  Also not necessarily a simple answer, (bad) harmonics will shorten damper plate spring life.  We may not know at what operating condition those exist.  

[tccombs]

1.   Weight  and drag on a boat are big  factors.   the  heavier a boat  and the configuration  will induce  more drag  load on the boat. 

2.  Imagine we  start with WOT   lets say  engine is turning its  max..at 5000 RPM . regardless of whatever speed that might be on the boat .    we'll say 45 MPH  for this example.  

3  now  lets say you add 1000lbs to the boat .  The boat has more drag  now  because  it displaces more water.    so now the  boat is still at 5000 rpms..  but now the speed has dropped  a bit   to say  40 MPH.      so the prop is spinning at the same rate for less of a result  which means increased drag.. therefore more engine stress.  it is doing the same work for less result.. 

4  continue  adding  weight  and  eventually  now it drops to 35MPH  and the engine  can no longer keep the prop spinning anymore at 5000RPM   it now  slows to 4800 . 

5 continue on with even more weight and at some point you will no longer be able to plane anymore at max power. 

 

 

so with that in mind...  my definition  of "max stress" ... would be   "the point where you are using the most power  to achieving the slowest  speed."     

in other words     that point would have to be when you tie your boat so it cant move..  and floor it . 

 But for any practical  real life condition , then  I think that must be  " Surf configuration" for most of our boats