Constant RPM but Speed Varies

[nveater]

I have a Mainship pilot 30 with a Cummins 6BT5.9 engine. My cruising rpm is about 2400 which gives my about 12-13 knots cruising speed. This weekend I went out and I either felt or sensed a change in engine sound at 2400 rpm and speed began to drop to around 10.5 knots average. I thought it might be current or a foul tide. The rpms did not waiver and really neither did the sound of engine. Coming back home today, I experienced the same issues and cruised at about 10.5 knots instead of 12 +- however, the last 45 minutes though speed picked up and we obtained 12.5- 13 SOG that I have come to expect. I never touched the throttle during these speed fluctuations- also tide was against me the last 45 minutes. Any ideas ?? Something with the turbo system ?

Thanks

 

[Barking Sands]

If it was a turbo issue you would have a corresponding rpm change. My guess would be swirling currents, prop fouling or transmission.

Are you absolutely certain rpm did not waiver?

 

[MYTraveler]

If the engine rpm are constant, no loss of speed could be caused by any problem with the engine. If the transmission is not slipping, the loss of speed must be caused by a problem with the prop or shaft or conditions outside the boat. I had a transmission fail (clutch started slipping and burning, contaminating the trans' lube oil, which exacerbated the problem) and it showed similar symptoms. (After replacing that trans, in order to detect the onset of a similar failure in the future, I configured an electronic device to monitor the ratio of shaft rpm to engine rpm -- as long as that stayed constant, there was no slippage.) Once you rule out slippage, focus on the condition of the props and hull.

 

[ScottC]

If the engine rpm are constant, no loss of speed could be caused by any problem with the engine. If the transmission is not slipping, the loss of speed must be caused by a problem with the prop or shaft or conditions outside the boat. I had a transmission fail (clutch started slipping and burning, contaminating the trans' lube oil, which exacerbated the problem) and it showed similar symptoms. (After replacing that trans, in order to detect the onset of a similar failure in the future, I configured an electronic device to monitor the ratio of shaft rpm to engine rpm -- as long as that stayed constant, there was no slippage.) Once you rule out slippage, focus on the condition of the props and hull.

I had the displeasure of experiencing slipping clutch once. It’s just as MYTraveler says, RPMs stay the same, but you lose speed. Let everything cool down and it may seem ”normal” again…at least for awhile. A possibly useful symptom of slipping clutch that I can share is a very strong, acrid odor. So, the next time your speed drops, go below and have a sniff.

 

[tiltrider1]

Not sure what environment you boat in. I can say that when I make a six hour run to the island's one thing is a fact. My engines are turning 1750 RPMs. Now my boat speed is anything but constant. Even though I might be running with the tide, the tide is not consistent. It’s faster in some areas, runs backwards in others and can do any number of strange things. Even wind can affect my speed. Then there is the pitch of my engine sound. It too changes constantly depending on what the wind is doing. Still my RPMs never change. Finally, there is floatsam and jetsam, usually seaweed, you can pick this up on tHe props, the rudders and the stabilizers. It can cost you speed and then just as quickly it can part ways with you and your speed is back.

 

[nveater]

Thanks for the replies. I am in the Chesapeake , and aware of the vagaries of tide, current, wave action etc and realize the effect they can have on speed. I don't believe this is the cause. The transmission idea is intriguing if not scary, besides the burning smell are there other symptoms that I can be on the lookout for ?

 

[High Wire]

Is this something new? Never behaved like this? Or is the boat new to you? Speed changes are normal when the current, wave or wind changes. Its hard to see an opposing current less than 0.5 knots. You have to look hard at a piling or crab pot float to see the eddy from 0.5 or less.
You can send out a transmission oil sample to a lab.

 

[O C Diver]

Could be fouling on the propeller that eventually came off.

If you suspect transmission issues, get transmission oil analysis done. Slipping clutches would most likely show up in the oil analysis as additional wear metals.

Ted

 

[nveater]

OC Diver I did think of something that was on the prop- certainly a possibility. HIGH WIRE- the boat is somewhat new to me , but I have had it since the fall of last year and yea this is the first time this has happened. I am aware of the whole , wind, tide etc have sailed in my boating area for the last 34 years. I don't discount their effect but I don't think this was the cause. But having said that, I will go down to the boat and do another on water experiment another excuse to go boating

 

[Jeff F]

You probably don't have instrumentation to monitor, but fuel flow, EGT and turbo boost are all useful add-ons to indicate how hard the motor is working. If you've got one keep an eye on it.




Sent from my moto g play (2021) using Trawler Forum mobile app

 

[SeaDogAK]

Check your engine rpm’s with a handheld tachometer. Could be the tach fluctuating.

 

[Lepke]

If the rpm is steady, and the clutch or trans not slipping, speed changes by something caught in the wheel is common. It doesn't take much debris wrapped around the prop or disturbing the flow in front of the prop to change the efficiency. It's common around kelp beds or maybe someone tossed a diaper.

 

[DrissZougari]

I would definitely send the trans oil for analysis the eliminate slippage. if the Transmission is okay then you mast had something on your running gears and came off at the end of your trip.

 

[SeaDogAK]

I will repeat I’d check the tach before doing anything more expensive. For reasons I haven’t yet figured out, my tachs fluctuate from one day to the next which was causing me to observe different speeds at the same reported RPM. Very similar issue to what you’re reporting. A handheld laser tach is around $20. I’ve confirmed that my problem is with the tachs and not the engine or transmission.

 

[fgarriso]

I had 15-20' of nylon rope wrap around my prop shaft, and lost 2 Knots at the same RPM. Dive down and check the prop shaft.

 

[OldDan1943]

Is this something new? Never behaved like this? Or is the boat new to you? Speed changes are normal when the current, wave or wind changes. Its hard to see an opposing current less than 0.5 knots. You have to look hard at a piling or crab pot float to see the eddy from 0.5 or less.
You can send out a transmission oil sample to a lab.

 

[nveater]

I did check the transmission oil and it looked very pristine- I am planning on changing it this fall and will send out for analysis- also hopefully tomorrow will be going out for a test run

 

[BobMc]

I have a Mainship pilot 30 with a Cummins 6BT5.9 engine. My cruising rpm is about 2400 which gives my about 12-13 knots cruising speed. This weekend I went out and I either felt or sensed a change in engine sound at 2400 rpm and speed began to drop to around 10.5 knots average. I thought it might be current or a foul tide. The rpms did not waiver and really neither did the sound of engine. Coming back home today, I experienced the same issues and cruised at about 10.5 knots instead of 12 +- however, the last 45 minutes though speed picked up and we obtained 12.5- 13 SOG that I have come to expect. I never touched the throttle during these speed fluctuations- also tide was against me the last 45 minutes. Any ideas ?? Something with the turbo system ?

Thanks

Since the drop in speed occurred both going out and returning, any chance that you were going through some shallow water (@ 6-10 feet) at the time. Fluid dynamics in shallow water can seriously limit/reduce your through the water speed.

 

[nveater]

BobMc- I boat in the Chesapeake 6-10 feet of depth is our norm. I do appreciate fluid dynamics, but it has never happened before- I am pretty sure this was something on the wheel, but I will be going out again this week to see what happens !

 

[Phil23]

Experienced a very similar problem several years ago on Ford Dover Leman in US) coupled to a PRM box it turned out to be low oil pressure/content

 

[Shrew]

You're talking about a variation in speed of between 1.5 -2.5 kts at the same RPM. It wouldn't even dawn on me that there could be something other than tide, wind or current influencing my speed. That variation is almost insignificant.

 

[Refill]

Is there a simple way to measure speed through the water with a paddle wheel type sensor? If the speed over ground and and speed through the water stay the same, your done. If not, and since the RPM and the sound did not change, you most likely have slippage in your Transmission.
I installed an Airmar DST810 Smart Multisensor on my boat at the last haul out and it is absolutely amazing how different the speed through the water and speed over ground are most of the time.
this sensor is powered up through the NMEA 2000 back bone but is also readable through the free app on my smart phone and iPad.


https://www.airmar.com/productdescr...IzlVEfIHy8oWz75wGyjSzZ04-23fUdiYaAtuVEALw_wcB

 

[nveater]

So to follow up and close this thread- I appreciate all the responses- I did get a laser tach and checked my tach which was remarkably close. I will be sending out my transmission for analysis this fall when I winterize my boat along with my engine oil. Since this incident, I have taken the boat out several times and am happy to report that I could not duplicate what happened to me earlier. Cruising speed where is right where it should be- so I am concluding that I probably had something around the wheel- there is a lot of floating grass in the upper Chesapeake so maybe that was it or part of an old crab pot line which are also abundant here. appreciate all the thoughts and ideas and will be back soon