Discussion in 'Engines' started by chesapeake46, Dec 3, 2015.
Now THAT sounds like a fun job!
As the saying goes, all good things must come to an end.....
I only chop in an emergency. Otherwise, very easy going up and very easy coming down. HUGE amount of stress on those twisty things "props" under the boat and that all is felt from the shaft nuts to the bolt holding on the harmonic balancer and everything in between. Just my 2 cents
Are you saying that the propulsion system imposes more stress on the boat when operating at less than full power ?
I believe he is referring to fast acceleration and fast deceleration.
Well, take the case of the boat in his picture - the offshore racer.
You purposely chop the throttles to unload the engine/propulsion drive train (propeller to engine) as you get airborne / ventilate the propeller. Keeping the throttles in one position would create extreme stress cycles on the propulsion drive train as the boat gets airborne and then re-enters the water.
Different boating application than what we are talking about in the thread, I know.
Technically you're not chopping the throttles on a race boat after the prop is already unloaded, you bring them back enough to keep RPM's within the range your running at, to keep the engine from over-revving and engine valves from floating and hitting pistons, and mainly so once the boat and prop come back in the water and the prop(s) bite, you don't really really put a huge load on the drivetrain breaking things (propshaft, lower unit gears, transmission, drive couplers, motor mounts, engine rods, etc. ), when getting airborne and the prop is ventilated the engine/drivetrain is already unloaded.
If you chopped the throttles completely, once the boat landed back in the water and propellor(s) bites, the boat would decelerate so quickly your head would hit the dash and the handling of the boat would be a very unpretty sight, so you give it throttle at the same time the prop starts biting to try to keep the same engine rpm's and prop+engine speed and SOG. Totally different situation but more exaggerated consequences if you're heavy handed on the throttle and slow to bring it back.
But like anything jamming and chopping throttles puts more stress on everything. Compare it to cars. You can take a new Mustang GT and with a blower and simple bolt on's you can pull 650HP out of them. Drive them rather hard on the street (within reason) and you're not going to break anything in the drive train, or elsewhere, even with drag radials (tires). Start taking the same car to the 1/4 mile drag strip every other weekend, doing smokey burnouts and launching the car from a dead stop with full throttle, and decelerating from 130 mph in less than an 1/8 mile, and you're going to start breaking stuff. Whether it be suspension components, transmission, or engine parts (valves, valve springs, head gaskets, etc. etc.)from the engine seeing a lot more load (the tires will hook/grab a lot better at the track) versus spinning on the street more. Basically the harder you abuse something or the more stress you put on it, the more that's going to break, and the more often.
Consider doing the same thing if you suspend the car so the tires are in the water. Just makes a bunch of froth doesn't it?
A marine diesel can only produce torque in relationship to rpm, power is a function of torque and rpm. a fixed pitch propeller absorbs torque in relation to its rpm. If the prop is accelerated faster than it can move water it will cavitate (and when close to the surface it will ventilate) and unload. A diesel engine cannot accelerate quickly enough for the inertia of the drivetrain to suffer a "huge amount of stress." The exhaust smokes black under rapid "throttle" advance because the engine mass air flow cannot keep up with the fuel flow ... the engine cannot accelerate quickly enough to move enough charge air to support proper combustion.
That is a whole world away from an engine that is already revving at high rpm and having its output instantly applied to what amounts to a stationary mass that has a great deal of resistance.
On the car I described the tires would just spin if you gave it full throttle instantly at anything under 90 mph on the street, very similar to props ventilating or cavitating.
That's besides the point, jamming the throttles to full foward when the boat is at hull speed puts more wear and tear on everything as does chopping the throttles compared to the easy does it approach. The engine is seeing much higher load factors trying to accelerate the boat onto plane, besides pouring black smoke out the exhaust, which in and of itself is not good because it contributes to dirtying the aftercooler and turbo vanes. Cavitating the props is not good for them either and I've seen and had to have a dozen set of props fixed or replaced from prop burn resulting from cavitation. Most modern props won't cavitate when you jam the throttles fully foward while at idle or hull speed in foward, they'll just grip. (Talking diesel inboards here).
Also the garbage truck reference was totally an incorrect comparison. Yes they go from idle to mid throttle and back to idle a lot, but they are in 1st gear at a very low gear ratio when they're doing this, so the engine is not seeing the load factors like a yacht does with a fixed gear ratio and fixed propellor that is designed to see WOT rpm. If the garbage truck did the idle to mid/high throttle to idle deal in it's top gear from a dead stop, then you would see them have a lot of engine issues. Also most garbage trucks these days are automatics that have a stall converter that allows slippage so the engine isn't so loaded from a dead stop.
There is no way that a yacht does not see added stress doing the jamming the throttles/chopping the throttles on a regular basis compared to the easy does it approach. How much depends on the yacht and configuration, but it definately adds wear and tear.
And that is precisely the point ... the garbage truck engine sees maximum torque and horsepower swing between idle and maximum loading every few seconds. A yacht will not develop full power unless running at full rpm and it wont get that until the boat is moving as fast as it can. Look at a prop curve.
There are many good reasons not to play dozer boat with a yacht but none of the myths and pseudo tech presented in this thread are among them.
Nope, simply saying that due to the stress's imposed on a boats power system that for me personally, I am as easy with throttles and clutches as I can be. Nothing more, nothing less.
You're missing the point. The garbage truck sees MUCH less load between idle, throttling up, slowing down, because it is in such a low gear. The yacht engine has a HEAVY load which sees an even higher load factor when you jam the throttles foward.
I keep checking this thread to for new info. None here. Summary, nobody recommends chopping the throttle on a regular basis. Maybe its time for this thread to fade away?
It probably would have if it had not pushed it back to the top.
Please keep in mind the title of the thread is "Chopping the throttles & turbos" and is was based on the myth that rapid power changes will damage the turbocharger. The thread was derailed by an attempt to divert attention from some very obvious misinformation about turbos, misunderstanding of the difference between diesel and spark ignition engines, gas turbine operations, and the injection of a great deal of boatyard mythology.
I won't bother reiterating all the myths and pseudo tech stuff presented as facts but a reread of post #35 might be in order.
I suggest reading the studies linked below. Both of them provide clear data on the power cycling of garbage trucks.
SIMULTANEOUS OPTIMIZATION OF SUPERVISORY CONTROL AND GEAR SHIFT LOGIC FOR
A PARALLEL HYDRAULIC HYBRID REFUSE TRUCK USING STOCHASTIC DYNAMIC
Inventory and Prediction of Heavy-Duty Diesel Vehicle Emissions http://tinyurl.com/hk9z2fk
(Page 28, figure 4.16 ...torque vs time)
This is a good time to point out that a terrestrial engine/drive train uses a transmission to allow the engine to operate at its best power/rpm/torque point. From a standing start the engine is accelerated to the point it is capable of producing enough torque to break the vehicle free and begin accelerating before power is applied to the drive train. First gear is used to allow the engine to reach the high rpm necessary to produce the torque/hp required for breakaway. It is not used to reduce the power needed.
A boat doesn't need the clutch because it operates in a fluid medium that allows drive shaft engagement at zero vehicle speed and idle power. The boat engine is not heavily loaded until it reaches the speed and torque required to fully load the propeller. This is not going to happen instantly despite the mythology presented in this thread. A boat propulsion system operating astern will not produce or absorb the same amount of power and drivetrain loading as normal operation in the ahead direction.
The diesel engine on a boat driving a fixed pitch propeller is not operating at high rpm when the speed control is rapidly pushed forward, the mass of the shaft and propeller is a very small proportion of vehicle weight, unlike a heavily loaded truck. The exhaust produced by the boat engine is what turns the turbocharger, until engine load increases to a certain point, the turbocharger is not heavily loaded. This is obvious when manifold pressure is displayed. A turbocompressor is not a positive displacement device, its rotation is not stopped when the speed lever is retarded, and its oil supply is not effected by load. It is designed to function with the amount of lube oil delivered by the engine across the range of rpm it operates. Modern diesels with electronic control limit the rate of acceleration, jamming the "throttle" from idle or neutral to full power is not the equivalent of "popping the clutch" on a road vehicle.