Saving some gas

rlkgizz · Post by **rlkgizz** » Fri Jan 28, 2011 7:41 pm

Hello everyone, I have a 1973 31 express with twin 318. I would like to know if running one motor at slow speeds would save on gas. run one motor for one hour then the other motor for an hour to keep hours the same. would the strain on one motor do any harm or use more gas. this will be the 2nd summer owning the boat. I have had lots of small go fast boats and this my first go slow boat. your coments are well needed. thanks

jimbo36 · Post by **jimbo36** » Fri Jan 28, 2011 9:17 pm

The short answer is no.

There are other considerations involved. Tracking is impossible. The boat will be difficult to steer a constant course. You will travel a longer distance from a to b so you will not likley save on fuel. Not to mention the work involved at the helm. Many of us have been in a position where we have lost an engine and have to limp home on one. Not fun.The boat was designed to run with 2 engines including the counter rotater for tracking and control. Running one engine will put strain on the engine. Boat engines work harder than vehicle engines as it is. Jimbo36

prowlersfish · Post by **prowlersfish** » Fri Jan 28, 2011 9:47 pm

Maybe , I know ruining at hull speed or less helps I will have to disagree on straining one engine as long as the speed and rpm are low it will not strain the engines as they are under light load

When trolling I will shut one down unless there a lot of boats around

I do 5 knots 1 engine in gear 6 knots 2 in gear so I know I would save fuel less then 1 gph at 5 knots vs less the 2 gph at 6 knots

if your boat tracks well on 1 then go for it , just don't try to run at or above hut speed ( 7-8 knots ) so you keep the engine load low , and beware you won't handle as well on 1 so use 2 when needed ,

Torcan · Post by **Torcan** » Fri Jan 28, 2011 9:48 pm

Welcome to the forum
Lots of great guys with a ton of knowledge here.

captainmaniac · Post by **captainmaniac** » Fri Jan 28, 2011 9:53 pm

Don't fully agree with Jimbo's answer - you can get some savings but not a lot. Maybe the boat / hull is a factor in it so just bear with me...

The base line : I tend to get about 1.1 mpg overall at normal cruise speeds (2800-3200 rpm, 16-20 knots). That means each engine is - essentially - going about 2.2 miles on its own gallon of gas. If I run at about 1500 rpm (8 knots), I get about 1.3 mpg overall (each engine getting about 2.5-2.6 mpg).

I have ended up on 1 engine on a couple of occasions (not by choice). Pushing the boat at 1500 rpm on the single engine, I got almost 1.5 mpg. So... compared to cruise speed I am going almost 50% further (but it takes a lot longer to get off the water), but comparing at 1500 rpm I am only getting an additional 10 - 20% distance out of it by having one shut down. Not worth the hassles, steering complications, wear and tear, etc.. in my opinion. Also, you don't want to try to rev over 'fast idle' speed when on a single engine as there ain't no way you are going to get up on plane... Running at hull speed is the best you can expect, and the safest thing to do.

I know a guy with a 35 Chris Craft who went with a different solution (he did it for trolling while fishing). He added a third engine! He had a little diesel (something in the 40-60 hp range) installed, the shaft comes out through the aft end of his keel. The prop is a 3-blade feathering style like you see on sailboats, so when running on the 'big' engines the third prop just folds up and minimizes drag. When he shuts the big engines down and cranks up the inboard kicker, he can putt along anywhere up to hull speed while just sipping fuel. Steering isn't the greatest as neither rudder is in the flow of water off of a prop, but I think he just turns on his autopilot and lets it figure things out.

ready123 · Post by **ready123** » Fri Jan 28, 2011 10:03 pm

After doing consumption analysis low speed vs on plane I concluded that running at 3000/3200 rpm all the time was best. Over a season I use much the same gallons of fuel as when I go slower but I get to my spots quicker and log lower hours for the season. A situation I prefer...

In the grand scheme of things I find that the fuel cost tends to be the lowest $ I spend, so I like to pay attention to the other stuff.

jddens · Post by **jddens** » Fri Jan 28, 2011 11:02 pm

I regularly run on one engine in my 30 foot Fly Bridge Sea Raider. I have one engine with low oil pressure so mainly use both to get in and out of slip, then run on one. I get about 8 knots at 2200 rpm, basically hull speed. Boat tracks great and I sip fuel. I haven't done all the analysis but sure seems I'm saving fuel.....Wife likes to plane so sometimes we do the 18 knot cruise too.......18 knots is about 3100 on both motors and the fuel consumption increases dramatically.................won't hurt anything to run on one, just keep it slow...................

rlkgizz · Post by **rlkgizz** » Fri Jan 28, 2011 11:32 pm

thanks guys for the response,good points from all. a lot of times my wife and leave the dock and in five min. were on lake erie with no place in mind to go, just being there is great. a three hour cruse to west sister and back. no where to go - no where to be but there. thats why i wondered about the one motor thing saving gas. motor only have 300 hours, my as well use them.thanks again,,

Struts and Rudders · Post by **Struts and Rudders** » Sat Jan 29, 2011 5:25 am

welcome to the forum

rbcool · Post by **rbcool** » Sat Jan 29, 2011 6:57 am

When you guys run on 1 engine you aren't completely turning off the other engine are you???? You still need cooling water for the tranny in nuetral since that prop is still turning as you go through the water. Am I right here??

Ron

farfrompuken · Post by **farfrompuken** » Sat Jan 29, 2011 7:03 am

i RAN AGROUND IN LATE JULY AND COULDN'T RUN ON 2MOTORS ..DIDN'T WANT TO MISS THE REST OF THE SUMMER BY PULLING HER OUT TO FIX THE BENT SHAFT AND PROP I HAD SO I RAN THE NEXT 8 WEEKS FROM MY DOCK TO OUR FAVORITE HANG OUT ON ONE ENGINE. ... TOOK ABOUT 45 MINUITES LONGER BUT I SAVED A TON ON GAS......PROBABLY SAVED HALF WHAT I WOULD HAVE USED.

summer storm · Post by **summer storm** » Sat Jan 29, 2011 9:56 am

Moving up the coast from Florida to NJ I blew my starboard engine near the sassafras river, I decided to push on to Point Pleasant NJ on just the port and fix it there. I think I made about 9 knots on the two day trip. Felt very slow but I seem to remember a big saving on fuel. This could also be because I was cruising at 18 kts before. Steering wasn't a problem underway but docking needed some planning.

I really can't recall for sure but I think I locked the starboard shaft with a line to keep it from spinning.

wowzer52 · Post by **wowzer52** » Sat Jan 29, 2011 1:37 pm

If I run on one engine for any length of time I'll clamp a pare of visegrips on the shaft or some other means of locking and stopping the shaft from spinning but if I'm only going slow for a short distance I will let it spin.

alexander38 · Post by **alexander38** » Sat Jan 29, 2011 2:40 pm

Lost the use of the port engine on a long weekend trip , had to run for a few hours like that in chop know thank you. I've got 2 screws and I'll run them , if time and gas are the issue I'll run below 8.5. I can get 5 to 6 knots by just bumping up the rpms a bit and keep my steerage feeling good. But when it comes down to it, its your choice. But if I did it I'd switch it up to keep the off line tranny cool. Some say you don't need to but I would cya doesn't cost . Imo.

jimbo36 · Post by **jimbo36** » Sat Jan 29, 2011 5:23 pm

I am a bit suprised by the answers here.

Sure, I get the one engine trolling thing but there is a big differance running a boat at hull speed, with high torque diesels, then there is pushing a 31' Trojan with older small block 318's, with the rudders cranked to maintain a straight course, and the drag associated with that, no additional strain?? jddens has to run his one engine @ 2200 RPM to make 8 kns, with his rudders cranked over, no additional strain on that engine?? Will you save a bit on fuel? Probably, if you can keep the boat srtaight. But at what cost? If you want to run a boat on one engine buy a boat with one engine, you know, mounted in the middle of the boat instead of off to one side. IMHO, jimbo36