High GPH at normal rpm or low rpm at normal fuel consumption

Hey all, I’ve become rather needy again I’m afraid. New problem this time and it has me stumped.

I used the boat for 5 days straight 1 1/2 months ago and it ran great. Took it out this morning for a weekend jaunt and ran into some odd behavior. 43’ trawler style single engine semi-displacement boat.

After a 20 minute idle leaving the harbor the engine temps were normal (175F) and didn’t notice anything unusual. Not that there wasn’t but nothing I noticed.

After leaving the harbor and gradually increasing the throttle to my normal 1400 cruise RPM I noticed that my GPH was much higher than normal. The boat felt somewhat “sluggish” (but it is a big, slow boat anyway) and the GPH when I got it to 1400 was 5.6 GPH. Under ideal condition such as today, 1400 rpm would give about 2.1 GPH fuel burn.

The engine just didn’t “sound right”. Hard to describe other than that, but it sounded as if it was working harder than normal. When I tried increasing to 1/2 to 3/4 throttle I could feel an unusual vibration in the boat so backed it down right away and decided to return home.

2.0 GPH was only giving 1190 rpm. 1.3 GPH at 1000 rpm, and .5 GPH at 600 rpm idle.

No smoke.

Gear temp was reading 105F

Another odd thing was that my Raymarine Autopilot gave a “Current Limit” error and would shut down shortly after I would engage the AP. If left on standby it functioned fine without any error (ie heading, and rudder position were working). I’ve never encountered that error before but after a real quick check it appears that error occurs if the AP drive unit is drawing too much current.

The only thing I have done with the boat since the last trip was to change the alternator output from the House bank to the Thruster/Windlass bank. The Start battery remains being charged by an echo charger off the House just as it was before. The Start battery was at ~14.0 volts while the engine was running.

Too cold for me to dive on the prop and rudder (PNW) but one thought was that possibly I could have picked up something in the running gear?

I’m looking for other possible reasons so I have a clue as where I can look for a professional to diagnose and fix it.

My first thought would be fouled running gear causing higher engine load. Especially if the boat was sitting without use for ~6 weeks.

I’m thinking something on the running gear as well. To cold to dive but maybe you could get some indication by turning the shaft by hand-if you’ve done this before to have reference.

Yeah, there is always a first time. I may see if a GoPro on a stick might be able to give me a view. I would LOVE that to be the problem.

Mac2 said:

I’m thinking something on the running gear as well. To cold to dive but maybe you could get some indication by turning the shaft by hand-if you’ve done this before to have reference.

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I have while it is on the hard, but never in the water.

Maybe someone around you has a waterproof camera you can drop down to get a quick look

I had a wobble and vibration at 1400 rpm and above earlier this summer. I had a clump of barnacles on my prop for the first time ever. Removed them and everything was back to normal.

I don't have much, but maybe a line wrap around your prop shaft? Try rotating the prop shaft by hand. Should turn easily of course. I'm not going to recommend it be done in the slip but bring-up the RPMs in forward. Try again in reverse. Maybe back/forth a couple times (standard pause-shift between forward/reverse). See if you have any debris that breaks loose.

Sorry to hear this - nice boat.

Peter

Had a similar problem. Used the boat one weekend the next weekend the port engine was using much more fuel. Weird right. Went swimming and had a wacked blade on the prop. Hammered it out and all good. Yes i fixed it at spring haul out. I must have wacked something in the marina while docking.

OK, it is surprisingly difficult to accurately aim a GoPro attached to an extended Boat Hook at a prop and shaft that you can’t see. Anyway, after many trials I was able to see that there isn’t anything wrapped around the shaft, the rudder looks fine, but I’ve never seen so much growth on my prop. The Diver cleaned the bottom 2 months ago and noted hard growth on the prop and shaft, but it sure doesn’t look like he actually cleaned it. However, it has been warm and with the warmer water temps we get more growth.

Anyway, this is the best screen shot I was able to get of the Prop. Is this enough growth to cause that large of a decrease in power? I suppose if the growth was asymmetrical it could also cause some vibration at the higher rpm?

Bingo! Congratulations on figuring out the problem.

Yeah, the growth has more surface area in the water than your prop does.

Dave, you started off describing what in the end you found, barnacles fouling the running gear.
But you mention AP shutting off, current limit, this sounds like low voltage on the house bank. Separate from the growth on the hull. What is now charging the house bank? The drain to start trickle charger and other sources are overloading the house bank and probably bringing to less than 11 volts.

Could be the same growth fouling the rudder and making it hard to turn.

SteveK said:

Dave, you started off describing what in the end you found, barnacles fouling the running gear.
But you mention AP shutting off, current limit, this sounds like low voltage on the house bank. Separate from the growth on the hull. What is now charging the house bank? The drain to start trickle charger and other sources are overloading the house bank and probably bringing to less than 11 volts.

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When the AP gave the error, that was my first thought. However, it isn't low current condition that causes the error. It occurs (apparently) when the autopilot drive unit is pulling too high a current. This can happen if there is a problem with the steering system, or if there is an issue with some of the electrical connections or with the hydraulics. (All this was complete news to me until this morning). So not a low battery current, but too high a demand on the part of the drive unit.... maybe...

Delta Riverat said:

Could be the same growth fouling the rudder and making it hard to turn.

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That would be really nice. FWIW, I went back and checked on the invoice from the diver. The bottom was cleaned July 16th. This is what he reported;
"There was hard growth on the prop, shaft, and trim tabs and algae accumulation on
the thrusters, thru-hulls, rudder and waterline."
This was what the prop looked like before and after he cleaned it in July.
Before


After


Today:


Big change in 2 months.

dhays said:

When the AP gave the error, that was my first thought. However, it isn't low current condition that causes the error. It occurs (apparently) when the autopilot drive unit is pulling too high a current. This can happen if there is a problem with the steering system, or if there is an issue with some of the electrical connections or with the hydraulics. (All this was complete news to me until this morning). So not a low battery current, but too high a demand on the part of the drive unit.... maybe...

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OK, let me rephrase. Your AP is asking for more than the batteries can give and draw down the voltage below what will run the AP. Once the AP shuts down any battery will display a higher voltage. Unless you have a graph you will not see that momentary drop.

SteveK said:

OK, let me rephrase. Your AP is asking for more than the batteries can give and draw down the voltage below what will run the AP. Once the AP shuts down any battery will display a higher voltage. Unless you have a graph you will not see that momentary drop.

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No, I'm just not describing it well enough. This is from a 2018 reply by a Cummins Support Moderator.

"Autopilot control head reports Current Limit
The reported message is indicating that the autopilot course computer is sensing too much current draw from the drive unit. Causes for this condition may include one or more of the following:"

He then lists a bunch of possible reasons one being "- a problem with the vessel's steering system causing the drive unit to draw excessive current ..." I am hoping that maybe growth on the rudder is creating higher load on the drive unit forcing it to use more power by drawing more current.

Low battery voltage or current was not one of the reasons he listed for the current limit fault. However, like you, my first assumption was that the battery wasn't supplying enough. Often what is obvious to me turns out to be entirely wrong.

I have been in Olympia for 15 years and this year is the first time I have had hard growth on a prop. Glad it was just that. I was worried about my vibration/wobble too. See you out there

Kaniksu said:

I have been in Olympia for 15 years and this year is the first time I have had hard growth on a prop. Glad it was just that. I was worried about my vibration/wobble too. See you out there

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Yeah, I am really surprised. Never seen that much on my prop and never in under 2 months time. We are going to have to used to the changes wrought by warmer temps.

Mac2 said:

I’m thinking something on the running gear as well. To cold to dive but maybe you could get some indication by turning the shaft by hand-if you’ve done this before to have reference.

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what cold sea temperature is now 25°C you are in wrong place or you don't have adequate equipment. i buy for winter sea 18-19°C ,66F

Scuba Diving Wetsuits 6,5mm https://www.decathlon.co.uk/p/men-s-diving-wetsuit-5-mm-neoprene-scd-500-black-and-blue/_/R-p-306905?mc=8544637&c=asphalt blue_denim blue is always good have on boat.​

also after clean propeller I put marine white grease like this Gruebb Ramonol White Marine Grease 500g | Bosun Bobs

for the propeller best paint is Hempel silicone one. i don't know do you have this in USA.

more said:

what cold sea temperature is now 25°C you are in wrong place or you don't have adequate equipment. i buy for winter sea 18-19°C ,66F

Scuba Diving Wetsuits 6,5mm https://www.decathlon.co.uk/p/men-s-diving-wetsuit-5-mm-neoprene-scd-500-black-and-blue/_/R-p-306905?mc=8544637&c=asphalt blue_denim blue is always good have on boat.​

also after clean propeller I put marine white grease like this Gruebb Ramonol White Marine Grease 500g | Bosun Bobs

for the propeller best paint is Hempel silicone one. i don't know do you have this in USA.

Click to expand...

Thank you @more . Good suggestion.

I am just a cold water wimp. The surface water temps in Puget Sound are warmest in August, with an average of about 14.5°C. The temperature drops significantly starting about 10m below the surface, but for diving on a boat hull the temp will likely be within 1-2°C of the surface temps in my small, shallow harbor. Yesterday surface temps were supposedly 14°C. That is about 57°F which is still pretty chilly. To spend much time in the water, takes a warm suit. The professional divers in my area that are doing bottom cleaning, are wearing very heavy suits even in the warm months.

I wouldn’t think of diving for an extended period in puget sound without a dry suit.
“More”: thanks for the suggestions. I was wondering if you could add anti fouling to the prop while in the water. I just got done hauling and put Petit running gear anti fouling on my prop. I couldn’t justify paying for propspeed-Im in freshwater. I noticed more options for anti fouling for props. Maybe I should start a new thread to compare thw different beands. I’m also thinking we should limit the survey group to boats going under 10 knots-to keep it relevant for trawlers and fair for comparisons.

When we had our boat in So Cal we had to have the bottom done every month and at times that wasn’t enough. So you guys in the PNW are lucky that you can go that long between cleaning.

Comodave said:

When we had our boat in So Cal we had to have the bottom done every month and at times that wasn’t enough. So you guys in the PNW are lucky that you can go that long between cleaning.

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lucky you for the location .i be rich on your location .

invest Drive-in Boatwash

​

Where we are now I don’t have to clean the bottom at all during the season, but the season is half as long…

Comodave said:

When we had our boat in So Cal we had to have the bottom done every month and at times that wasn’t enough. So you guys in the PNW are lucky that you can go that long between cleaning.

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We are seeing more growth than in years past. As our summer temps have increased significantly, with less cloud cover, we are getting more growth on the south side of the boat than we used to ever get. I'm in a small harbor with max depths of about 30' MLLW. So it warms up a bit more than some other areas of the Sound. I've had a boat in this marina for 12 years and we never had such significant growth in such a short time. If it continues, we may need to change from the quarter bottom cleaning to maybe 5 or 6 during the year. adding an extra 1 or two from June-September.

After cleaning running gear in the water with no plan to immediately haul or run the boat hard, hand the diver a rag full of lanoline (sheep grease) and have it smeared on the bare metal. It is biologically "clean" and will last quite a while. Been there and done it on my twin engine GB42 in Florida waters.

My problem in 2018 was less severe, but found a thorough hand cleaning of the exhaust system helped a great deal. Too many hours at less than 20% of effort allowed for too much carbon sludge to gather in the exhaust system. Mechanic cleaned the system, told me to run a little harder. A Steve Zimmerman article in PassageMaker a few years later talked about "% of effort" needed to avoid exhaust system sludge build up and other issues. I searched for that on line a few years ago and found it for a re-read (is 74 too young for memory slippage?). Now I run a little harder and spend about 15 minutes on most cruises at 50% of effort or more. If my youthful memory is right, percent of effort is percent of Wide Open Throttle GPH at a given speed. Hull speed for my 10,000 pound vessel at 6.7kt is less than 10% of effort - 200 h.p. 4.1L turbo charged Volvo TAMD41P-A (great engine). I run 7.2kt-7.8kt most often, 8.2kt for about 25% of effort, higher for cleaning out exhaust a bit on the homeward leg. Maybe checking out your exhaust system is worth the time while you're "messing around" in your boat.
Good luck!~

It is usually not a seriously big deal to disconnect the rudder tillers from the system giving you the opportunity to see if there is serious resistance there. I found out that my stbd rudder took a six-foot-long 2X4 to work it back and forth while the port rudder could be swung with two fingers on the tiller. The cable steering on that old GB was so efficient that I had not noticed a problem until I happened to swing the wheel back and forth while in the slip with no machinery running. I heard a deep groan from back aft. That's when I disconnected the rudders and found the problem. I ended up building a new stainless steel rudder table to replace the original galvanized one and used a big-ass stainless steel collar ball bearing to replace a "permanently greased" bronze flat bearing. Smooth as silk after that.