Why engines are too sensitive to row water circulation?

[Alex F]

Granted that 100% operational cooling system is a must for the engines and I always try to keep mine that way. However, I’m a bit puzzled about performance differences when the row water intake is slightly clogged. Here’s what I mean in details. While using the boat for the past couple of weekends we happened to pickup some sea weeds in the strainers and when this occurs the boat performance changes right away. Here’s what changes:

• There’s a light white smoke coming from exhausts.
• At the same RPMs the boat would loose the speed (about 1-1.5MPH).
• The engine sound slightly changes.
• In general we feel that the boat has a “heavy” ride.
• However, the good indication is that TEMP gauge show the same normal operation temps.

So, when we stop I go to check on the sea strainers and sure enough there’s some build up of sea weeds (it happened twice this past weekend). This is something we’re aware as it happens periodically. However, I think that my engines are too sensitive, because the changes in performance mentioned above take place when only ¼ of sea strainers have sea weeds. So, was hoping to hear some opinions why such a slight change in water intake would affect the engines performance and the boat ride so much? Does anyone else experience the same changes with their boats?

Thanks,
Alex.

 

[tobnpr]

I can't think of a reason for any of the symptoms you cite.
The boat's speed shouldn't be affected unless the weeds were built up on the outside of the scoop intakes causing a lot of drag.

As long as the engine is producing the same rpm's, there's no reason for a change in speed- the prop is still turning the same # of revolutions and moving the boat the same distance through the water/revolution.

Changes in the "sound" would likely be due to reduced raw water flow changing the sound of the exhausts.

 

[Alex F]

I can't think of a reason for any of the symptoms you cite....

IMO, only the presents of light white smoke makes sense. b/c theoretically the water that comes out could now be slightly hotter. But, the temp gauge shows normal temps.

...As long as the engine is producing the same rpm's, there's no reason for a change in speed- the prop is still turning the same # of revolutions and moving the boat the same distance through the water/revolution.....

My thoughts exactly and this is why I'm puzzled. Usually when this happens the scoop strainers don't have so much drag to add.

I'll give you even more confusing example. One time last year when my scoop strainer did get stuff stuck right on them the boat wouldn't even get on plane, that's how bad the performace was affected.

I'm thinking that since my engines are full of electronics, may be there's a set of sensors that create this affect to slow down the engine to prevent possible damages and send some warning signs to the captain? Allthough, no alarm sounds or SC messages are displayed.

 

[comsnark]

My bet is that with clogged intakes, you have increased drag on the outdrive. Just like having a few extra barnecles.

When things are really bad with my boat (say. . late november) I have seen HUGE reductions in max speed due to fouling of the outdrives.

 

[Alex F]

I don't have outdrives, mine is v-drives. I had ensured (i took a dive under the boat before addressing the sea strainers) that the simptoms are the same when the scoop strainers are clean and only sea strainers have 1/4 worth of sea weeds in the baskets (both side pretty much the same).

 

[osd9]

The only way you would loose speed is if you also wrapped a good wad of sea grass round your running gear.

When this happens to me, I usually come off plane, and then reverse for a bit till it clears. One time in the C&D canal, reversing didn't work, and I had to dive in to clear the gear and the strainers.

 

[Alex F]

Dom,

When I see some light white smoke what I do as the first step is stop and shut off the engines to releave the pressure from the sea strainers trying to allow the sea weeds to slide off them. Sometimes I would see that it's working as there's some sea weed floats from under the boat, but it's not so much to create this drag.

Based on my situation from last year I had confirmed that the lose of power occurs when the props are clean. At least this is what I recall from my observation. It's very strange, but the simptoms are consistent for the same reason (lightly clogged sea strainers).

 

[Dave S]

I'll offer a theory on this one and it has to do with the way your ECM controls fuel and air flow based on sensor readings. Much like an MPI engine in a car where everthing involving fuel and air flow is controlled by an ECM, so are your boat motors. Your car is typically peppier in cold weather before it warms up and then as it warms up the car seems more "sluggish" compared to the way it ran when cold. I would imagine that the ECMs on marine engines also take a number of things into account when determining how much fuel to allow, timing advance settings, etc. It could be that the ECM sensed a drop in water pressure because of the sea weed or even an increase in water temperature (regardless what your gauges showed) and slowed the fuel rate down. Like I said this is a guess but maybe a Mercury Tech could tell you if this happens or not.

I would like to know myself so when I get a chance I'll talk to one of our Master Technicians at our dealership to see what they say.

Dave

 

[osd9]

RPMs is RPMs.

 

[Alex F]

Dom,

I hear you but I think that due to the fact that I have MPI engines Dave's theory is what I'm suspecting for now and hope to learn more about. I'm sure that I'll pickup more weeds in the near future, so I'll take more detailed readings from the SC gauges. I think that water PSI reading should be different between clean vs. slightly clogged sea strainer operation. So, I'll have to take two readings, 1 - at my normal cruising 3600RPMs and 2-when I pick up some weeds.

I'll post if I have anything new.

Dave, it would be great if you can share your findings from the merc technician.

Thanks,
Alex.

 

[JV II]

Could you be due for some new impellers? If you are missing some vanes or pieces of them, your cooling efficiency and water flow would be amplified with blockage of flow in the strainer. You've got a 2004 boat. How many times have the impellers been changed in the last 6 years?

 

[Alex F]

JV,

My impellers were replaced before last season (fall of 2008). I assume they were original 4 years old impellers. When I took them out they were in excellent shape. I have all the confidence that my impellers are in great shape, b/c when the strainers are perfectly clean the boat runs 100% to the specs (nice and quite). It's an ineteresting thought, but the exact the same behavior I had observed last year when the impellers were new. So, I don't think this is the reason.

 

[dpmulvey]

My bet is a change recognized by the ECM.

 

[tobnpr]

Changes brought about by the electronics (fuel/air mixture, timing, etc.) still shouldn't have an effect on speed- fuel economy, yes; but not speed.

Regardless of fuel burn, if the engine's turning "X" rpms- the prop is turning "X" times the gear ratio. The prop has no clue as to how much fuel is being burned to get it to trun those revolutions- it's turning and moving the boat through the water...

Anyway, that's my story and I'm sticking to it...

 

[Alex F]

Changes brought about by the electronics (fuel/air mixture, timing, etc.) still shouldn't have an effect on speed- fuel economy, yes....

Speaking of fuel economy. My usual number 0.7-0.8 and sometimes 0.9MPG. When I pick up some weeds the numbers drop to 0.6-0.7MPG and don't get better than that until I clean my strainers. It's very strange feeling that the boat just rides very heavy (like I have 15 people on board).

 

[Dave S]

All the more reason to suspect the ECM is altering the fuel curve if fuel mileage is also affected.

We can all speculate till the cows come home but if I were you I would place a call to Mercury to see what the issue could be.

Dave

 

[osd9]

OK....Tobnpr and I seem to be the only ones in the ' RPMs is RPMs ' arena.

Let me state my opinion a different way:

If the motor can reach 3600 RPMs after running over the eelgrass then it is obvious that the ECM is allowing the motor to reach 3600 RPMs.

The fact that the boat is not going the same SPEED has nothing to do with the RPMs that the motor is reaching......the ONLY thing that could be different is DRAG...either drag from a dirty bottom, drag from fouled running gear or drag from fouled PROPS....IMHO....

 

[Alex F]

Dom,

May be I dind't make myself clear, but I actually in agreement with you guys that if a boat does 24.5MPH@3600RPMs when all is fine it should run at the same speed even with clogged sea strainers as long as it's reaching the same 3600RPMs. Since the difference is only 1-1.5MPH we can blame it on possible minor weather/sees changes and of course possible slight drag. Just because I don't see anythging when I dive under the boat that doesn't mean that the eelgrass wasn't there and didn't get loose while I was anchoring. I don't rule out this posibility.

Over all the years I've been boating I had only freeky experience that had no logical explanation. One day my 240DA all of a sadden started sounding a bit strange while cruising and at full thruttle was only going 35MPH (usually she did 44-45mph at WOT). Drive was clear and RPMS were there. Local MM ran her few times and for no particular reason she returned to normal. No work was done to fix anything. The test conditions were similar and before taking her to MM I ran her with the wind to see if a little extra push will make a difference. She ran fine since then until the last day I owned her.

So, going back to the current scenario I'd say that you're right.

 

[SCORPIO]

Have you taken the direction of the tidal current into consideration when you make these observations? I don't know how much tide you get in your part of NJ, but in DE we get some wicked tidal currents, especially in restricted bodies of water.

 

[Four Suns]

I'm in the "RPM's is RPM's" camp... if your props keep spinning the same RPMs, how in the world is it slowing down other than increased drag?