Cruising On One Engine Only

dane507

New Member
Apr 14, 2008
65
North Carolina
Boat Info
330 Sundancer 1997, Sea Eagle 810 dinghy w/6HP Johnson
Engines
454s with V-drives
Hello All,

I was unable to find an answer to this question by searching the Forum.

If you are trying to save fuel while cruising slowly (8 MPH @1500-1800 RPM) down the ICW, is there any harm or disadvantage to travelling on one engine at a time? Does it really save fuel?

Thanks from a Rookie...
 
I don't know your boat but most boats I've been on create one UGLY wake at those rpm's. The wake indicates bad mpg. I usually idle along around 1100 rpm or less. At idle speeds you probably could run on one engine, but anything more than that and you might as well fire up #2. I'd love to see some real world numbers on it from someone with a fuel flow meter and twin engines to see the results.
 
Some of the shaft seals require the engine to be running to circulate water through the seal (my last my last three boats were like this) - therefore if you only are running one engine, water is not hitting the seal on the other spinning shaft (I am thinking that you don't have I/O's - this is a mute point if you have them rather than inboards).
 
Some of the shaft seals require the engine to be running to circulate water through the seal (my last my last three boats were like this) - therefore if you only are running one engine, water is not hitting the seal on the other spinning shaft (I am thinking that you don't have I/O's - this is a mute point if you have them rather than inboards).
On my boat I have experienced running on one engine and I thought I had burned out the other dripless shaft seal. What a coincidence, just by chance the collet which keeps pressure on the seal loosed up and was allowing water to come in. A SeaRay dealer recommend I installed a cross over water feed kit. Either engine running will supply water to both shaft seals. Without this crossover kit you can definately damage the seal. I was hesitant but I had it done seven years ago. No problem.
 
I don't know your boat but most boats I've been on create one UGLY wake at those rpm's.

At 8mph in a 330, you are traveling at hull speed; if you try to go any faster, the boat tries to get up on plane, and will plow until it does. Yes your one running engine might operate at a more efficient rpm range to propel it that fast than two engines running at 1500-1800 rpm, but you will lose some of that efficiency to the extra drag caused by the asymmetric thrust of the single running prop; assuming you are running your starboard engine (is that the one on your 330 that runs the power steering pump?), you will have to apply some right rudder to go straight, increasing drag. Also, the non-powered prop will cause more drag than if it were under power. If you still want to try it, get the crossover water feed kit installed and test your actual fuel burn with both engines running and with one one. Even if you decide against cruising under one engine, it's nice to know you can if you need to.
 
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If you are trying to save fuel while cruising slowly (8 MPH @1500-1800 RPM) down the ICW, is there any harm or disadvantage to travelling on one engine at a time? Does it really save fuel?

I can't imagine that running with an engine out would be very efficient given the drag from the freewheeling propeller. It's not like you're running a full-feathering hybrid prop like on a sail drive.
 
A friends 330 must have been propped differently. My experience was that it's hull speed was around 1,200 rpm. Any faster than that and the wake started growing. That was an inboard as well. Of course, he still drove in that 1,500 rpm range and it drove me crazy. His excuse was that he was saving fuel since it wasn't on plane. We had a long discussion and I THINK he understood hull speed and that a large wake was reducing his mpg, but he's still a go fast kind of guy and always will.
 
A friends 330 must have been propped differently. My experience was that it's hull speed was around 1,200 rpm. Any faster than that and the wake started growing. That was an inboard as well. Of course, he still drove in that 1,500 rpm range and it drove me crazy. His excuse was that he was saving fuel since it wasn't on plane. We had a long discussion and I THINK he understood hull speed and that a large wake was reducing his mpg, but he's still a go fast kind of guy and always will.

Regarding hull speed, my 37' Silverton (flybridge convertible), for example, has a hull speed of 8 kts., which occurs at 1800 RPM. The economy is better at idle and decreases some as you approach 1800 RPM, but still isn't too bad until you get over 2000 RPM. After that RPM, the curve on the RPM vs. MPG graph drops a little faster. Yes, it starts to throw a wake over 1400 RPM.

I used a GPS and took data points to determine these numbers, and used Crusader-supplied engine data for the fuel burn data used in my calculations. To make things even more interesting, I ran numbers for Upbound and Downbound on the river (2.5 mph current):

smpergal.jpg
 
My 340 does have a cross-over water feed line. If your boat does not, and you lose and engine, tie off the shaft so it can't rotate. My boat gains about 50% in mpg with one shut down at RPM's between 1250 and 1500 (best at about 6-7 mph).

Someone once posted the question about the transmission oil pump, but, as far as I can tell, if the transmission is turning, there is oil circulating, but it's worth further investigation.
 
There is no free lunch... On top of the increased drag from either an unpowered prop or stationary prop and deflected rudders, you probably have one hydraulic pump for your steering so you'll only be able to run a particular engine and not either/or.

Also, I'm not convinced you save money running a gas boat designed for plane speeds at displacement speeds. Let's say you want to travel 1000 miles down the ICW. You are putting 3-4 times the hours on the engine and running it at a low RPM for extended periods. At the end of the day, you are going to have to account for increased oil changes (and other issues that may arise) that are based on the hour meter. The big kick will be how does this affect depreciation as well. Is the savings you get by running a gas engine in a boat at displacement speed going to counter the fact one or two engines have 3-4 times the hours on them? I really doubt it.... Yeah... tell people "the high hours are at displacement speed" when you sell it and their eyes will glass over. It's like telling someone the reason a car has 300,000 miles is because it was on the highway... no one cares. They'll go buy the boat with lower hours.

Once you factor in the additional depreciation and the increased frequency of maintenance, I bet you are better off running the boat as it was designed... The biggest cost on your boat is depreciation (not fuel) and, unfortunately, running a boat around at displacement speeds to save fuel cost increases the depreciation costs on a gas boat. People will say "that's not real money"... All money is real in my world.

For example... let's say there are 20 1997 330 DA's on the market when you go to sell yours. They are all $50K and have 500 hours on their gas engines. Your boat, however, has 1500 hours on the gas engines. Cost of new engines is about $40K installed... What's your boat worth? Zero.

My 2 cents
 
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Our 330 hull speeds around 1200 RPMs. If yours is carburated pay attention to the 2nd 2 barrels kicking in. At that point you are defeating any gas savings.

We will hull speed for 20-30 mins at times but get her up on plane after that.
 
Now we've got a diesel guy answering gas questions (joking - here's the damn smile : )

My 340 doesn't have a power steering pump, that I know of. You can use either engine for single engine ops. I agree with everything else that Gary said, but the issue for me is not whether to cruise on plane or cruise slow, the issue is when I have to or want to cruise slow, do I use one or two engines.

I have used only one, but I'm not convinced it's a good idea yet. I alternate which one is used to keep them within a couple of minutes of each other.
 
My 340 does have a cross-over water feed line. If your boat does not, and you lose and engine, tie off the shaft so it can't rotate. My boat gains about 50% in mpg with one shut down at RPM's between 1250 and 1500 (best at about 6-7 mph).

Someone once posted the question about the transmission oil pump, but, as far as I can tell, if the transmission is turning, there is oil circulating, but it's worth further investigation.

John,

I haven't received my boat yet, but did all 2005 340's have the cross-over water feed line for the shaft seals?
 
We run on one engine all the time to get out to Lake Michigan. It's slow no wake the whole trip. I'm not sure what this does for mpg but it takes about an hour and we do it all the time. We keep our engine hours down by alternating engines. Many fisherman troll on one engine to get a better speed from running their baits. Most I know alternate engines as well.
 
Now we've got a diesel guy answering gas questions (joking - here's the damn smile : )

Hey... This wasn't in the gas boat section and I have owned gas boats.

My 340 doesn't have a power steering pump, that I know of. You can use either engine for single engine ops.

Umm... You sure about that? If you have V-Drives with rudders, I'm pretty sure you have hydraulic steering... and I'll bet you a beer you only have one hydraulic pump. That means one of the engines will shut your steering down if it's not running.
 
Our 330 hull speeds around 1200 RPMs. If yours is carburated pay attention to the 2nd 2 barrels kicking in. At that point you are defeating any gas savings.

We will hull speed for 20-30 mins at times but get her up on plane after that.

I'm fairly certain twin engine boats such as yours are not designed to be run at planning speeds on one engine. This is more of a get home feature if you lose an engine or transmission. We have a friend who ran a formula 42 pc home from 60 miles north of our port at 7 knots the whole way. Cummins told him to take it easy. If you are running the secondaries on your engine you are not taking it easy and setting yourself up for issues. One that comes to mind is a broken shaft.
 
I stand corrected... (I did it myself). John's boat does not have a hydraulic pump on the engine. It uses hydraulics but is off the turning of the steering wheel only... PM me your address and I'll send you a beer.

Man... what a wiener of a boat. ;-)
 
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Here is an hour meter question. On the old combines we used to drive the "hour" meter was gear driven. If you were idling, it would take almost a full hour to turn the meter 1/10 of a notch. At full rpms, an hour equalied an hour of running time or a little less. Anybody ever timed the hour meter at idle vs. at cruise? Just curious.....
 
I always thought the keys turned the electric engine hour clocks on----even if the engines were not running.
 

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