Fuel Consumption Question

[Nomad Willy]

Our trawler engines are 1500lbs. Transmissions and accessories like heat exchangers and engine mounts ect bumps the weight to about 2300lbs. Extra fuel and tanks makes the twins much or considerably heavier.

Keith,
Boat design often has a range of acceptable weights, volumes ect ect. When it comes to weight the placement and amount of weight has a profound effect on how the boat behaves on lumpy seas. I can’t imagine a GB 36 being too light but becoming too heavy could easily happen. Sure the designer took the weights into account but I’m saying it could be at the higher end of the acceptable range. This could be arrived at from weights and their placement. A boat w too much weight at the bow and stern would respond to the helm .. in a word .. terribly or scary w stern seas. A well balanced boat is a joy comparatively speaking.
And I don’t know who establishes the essential ranges of design. Perhaps there are no numerical guides and it’s just a guess done by the NA. Maybe he (or she) has has graduated from the boat design school “Westlawn” .. or some school/academy or whatever.

All the boat builders dropped down to one of the FL’s or Perkins engines. But some were installed further aft than what was done or what usually is done. In most of these matters there’s no black and white .. but a range of grey.

Koliver wrote;
“What is your take on a pair of 80 hp v a single 160 hp, approx same total weight?”

Now this is a genuine what is better a single or a twin question. I would guess two 80hp engines would weigh close to the same. So no weight consideration for me to comment on. I once saw a GB 36 w two JH Yanmars (55hp each) on YW. It sold fast. I was shopping then but the boat was on the east coast. Almost the same hp. For me this would almost be the Ideal trawler. I like it a lot.

Now if the question was about a GB 36 w two JD engines or one Yanmar ?? But two JD 80’s would be a popular choice among TF members.

 

[backinblue]

I don't believe 2 engines weigh the same as a single with the same total HP especially when you include the transmission, running gear, and everything that supports the engines like water and fuel filters, plumbing, exhausts, controls, possibly rudders, etc.

 

[rslifkin]

The twins will probably still weigh a bit more, but less than double. How much difference the weight makes in boat behavior depends on the hull and where the engines sit. Some boats are sensitive to extra weight, others don't seem to care much if the weight is in the right place.

 

[backinblue]

B&B,
Yes as long as it’s a trawler whereas both engines are the same and the singles utilize the same old engine.
That means any comparison will be comparing a 120hp boat to a 240hp boat.

In some ways they can be compared. However if you’re going to compare the two boats need to have the same amount of power. Total power.

I agree Willy and sorry if I wasn't more clear. I was talking about 2x120 HP vs. a single 240 HP. The single would still take up less room and less maintenance, no?

 

[Nomad Willy]

rslifkin wrote;
“Depending on the boat, there can be a noticeable weight difference. It's not necessarily bad, but it's there. In your example of small twins vs big single, things will often be more comparable, as the engines are likely better chosen (rather than 1 good configuration and 1 sub optimal one).”

“not necessarily bad” ... YES it could likely be bad.
You could create a monster w a boat that was designed at the top of the max weight range. TF is loaded w questions about what should I buy or what is best. If you look at the boats weight on “the showroon floor” most are quite to way different years down the road. Many things added to boats are hundreds of lbs or considerable weight. So if a boat is designed heavy ....... ?

So whether or not a boat is too heavy is probably the result of skippers (and/or their wife's) just loading too much stuff onto the boat.
And once in awhile I miss Tad Roberts on this forum. My response as just a skipper is most likely OK but there was a bit of what I’ve said that I’m hazy about. If anybody can help me out w good input please do.

I regard to most of the subject here I actually have a little experience in industry. I worked in the engineering dept. at Uniflite in Bellingham Wa. My job there was to move things around in the boat to make the boat float level. As in removing any significant list. Trimming a boat fore and aft is more important and usually harder to do.

 

[Nomad Willy]

B&B wrote;
“I don't believe 2 engines weigh the same as a single with the same total HP especially when you include the transmission, running gear, and everything that supports the engines like water and fuel filters, plumbing, exhausts, controls, possibly rudders, etc.”

I think they do. When you design a transmission for an 80hp engine it surely dosn’t need to take the forces of a 160hp engine ... only half the forces. And that should be about half the weight and size. Oil filters are probably half the size/volume, motor mounts ect ect. It probably usually would come out one way or the other .. a bit but I’m sure it would be very close.

But you’re absolutely right, the single engined trawler will have much more room for service .. on the sides.
But we’re talking about two 120hp engines. You may have plenty of room if your engines are half the size ... as they should be.

 

[koliver]

I agree Willy and sorry if I wasn't more clear. I was talking about 2x120 HP vs. a single 240 HP. The single would still take up less room and less maintenance, no?

I have a comparison to come back with.
My Motorhome has a 400 hp Cummins ISL, 6 cy mounted to an Allison automatic transmission, a Jake Brake, but no heat exchanger or sea water pump.
My boat has a pair of 200hp Volvo TAMD41, 6 cyl each, mounted to a pair of Borg Warner Velvet drives, heat exchangers on both engines and transmissions, sea water pumps, strainers, through hulls on both.

Putting aside the land v water differences, these power plants weigh close to the same per hp, use close to the same fuel per hp, cost the same for fuel filters per hp. The Allison Trans is easily twice the weight of the Borg Warners, the exhaust system is way more than twice the size and weight of the boat pair.

I was shocked when, buying oil filters, the best I could do for the bigger engine cost almost to the penny, the same as two for the 1/2 as big engines. The volvo sumps hold 10l, the Cummins 20l.

 

[Hippocampus]

Placement of weight fore and aft effects gyradius and inertia in that plane (longitudinal). But placement of weight or spreading out of weight (twins v single) effects inertia forces in the transverse plane. So in general given the block is the heaviest piece of an engine placing a single on the midline of a boat will give different handling characteristics and motion in a seaway than twins. Twins spread out the weight more in the transverse plan.

 

[OldDan1943]

Hull design, weight, engine (s)
Hold one constant, vary the others.
Gather up the gph and gpm.
Given weight and engine(s), it falls to the hull design.

 

[markpierce]

"Our" boats need several times the horsepower and fuel consumption to exceed the speeds of those boats not exceeding hull speed. I'm the apparent minority happily consuming 1.5 gallons an hour.

 

[backinblue]

Placement of weight fore and aft effects gyradius and inertia in that plane (longitudinal). But placement of weight or spreading out of weight (twins v single) effects inertia forces in the transverse plane. So in general given the block is the heaviest piece of an engine placing a single on the midline of a boat will give different handling characteristics and motion in a seaway than twins. Twins spread out the weight more in the transverse plan.

Makes sense but does one config have better handling characteristics or just different?

 

[backinblue]

I have a comparison to come back with.
My Motorhome has a 400 hp Cummins ISL, 6 cy mounted to an Allison automatic transmission, a Jake Brake, but no heat exchanger or sea water pump.
My boat has a pair of 200hp Volvo TAMD41, 6 cyl each, mounted to a pair of Borg Warner Velvet drives, heat exchangers on both engines and transmissions, sea water pumps, strainers, through hulls on both.

Putting aside the land v water differences, these power plants weigh close to the same per hp, use close to the same fuel per hp, cost the same for fuel filters per hp. The Allison Trans is easily twice the weight of the Borg Warners, the exhaust system is way more than twice the size and weight of the boat pair.

I was shocked when, buying oil filters, the best I could do for the bigger engine cost almost to the penny, the same as two for the 1/2 as big engines. The volvo sumps hold 10l, the Cummins 20l.

All good info and not what I would have guessed, thanks!

 

[Nomad Willy]

Placement of weight fore and aft effects gyradius and inertia in that plane (longitudinal). But placement of weight or spreading out of weight (twins v single) effects inertia forces in the transverse plane. So in general given the block is the heaviest piece of an engine placing a single on the midline of a boat will give different handling characteristics and motion in a seaway than twins. Twins spread out the weight more in the transverse plan.

Good point and kinda like getting a recommendation to not put your ballast on C/L but up near the gunnels. And that’s like comparing a wide flat bottomed boat to a narrow rounded bottomed boat. To roll the boat must move the ballast weight or the water close to the hull.

The first time I heard of putting ballast just under the gunnels was in the Swedish Albin’s operators manual. I really thought that was odd until I thought about it.

So yes the twin engined boat will have better inherent roll dampening .. all other things remaining the same.

 

[Art]

"Our" boats need several times the horsepower and fuel consumption to exceed the speeds of those boats not exceeding hull speed. I'm the apparent minority happily consuming 1.5 gallons an hour.

Ya know the old sayings, Mark... "There's many a slip twixt the cup and the lip." ... And ... "Different strokes for different folks."

Here, in regard to boat speed I simply mean to imply: Just because many of us have boats that can get up and plane, be they SD or P hulls - does not mean we all too very often go fast.

For Linda and I doing 6.5 to 7 knots [below our Tolly's 7.58 hull speed] is enjoyable cruising; with small wake. At that speed, our Tolly is relatively efficient at 2 + nmpg [about 3 gal per hr.]. So, although we use 2X the fuel you use that's really not too bad considering that if we want to we can also go 16 to 17 knots at 1 nmpg [which can get expensive - but only occasionally do we go that speed]. When you take overall cost of boating into consideration... fuel cost for you and me is not a very large percentage of yearly expenses.

Let's do a couple fuel burn and cost projections - figuring 300 hours run-time per year at an average of $3.50 per gal:

Just below hull speed - You 1.5 +/- gal per hr. = 450 gals X $3.50 = $1,575

Just below hull speed - Me 3 +\- gal per hr. = 900 gals X 3.50 = $3,150

For the extra cost of $1,575 annual dollars [$131.25 per month] I'm pleased to pay for being able to plane fast when desired.

For the cost savings of $1,575 annual dollars [$131.25 per month] your pleased to save that for being able cruise at below hull speed to your hearts delight.

There's a tradeoff for many things about a specific boat. Each of us who like our boats also like their tradeoffs!

Happy "Boat-Ownership" Daze! - Art

 

[Hippocampus]

But one would expect poorer AVS for twins if placed in the same hull as a single. Pick your poison.

 

[rslifkin]

But one would expect poorer AVS for twins if placed in the same hull as a single. Pick your poison.

Yes, I'd expect a slight difference in AVS.

Of course, how large the difference due to weight distribution between single and twins will be depends on what other equipment gets moved to accommodate. The single may have some stuff moved to the outboard sides with all the empty space, while the twins put it elsewhere.

Thinking about it, this all might explain one of the reasons why my boat rolls so easily at anchor compared to a lot of other similarly sized hard chine hulls. Many of them have fuel tanks outboard of the engines and things like that, while mine doesn't. There's not really any heavy equipment all that far outboard (and nothing at all outboard of the engines). My water and waste tanks are on the centerline, and the fuel tanks are as close to center as they can be (right next to each other under the aft bunk).

 

[SteveK]

.................

For Linda and I doing 6.5 to 7 knots [below our Tolly's 7.58 hull speed] is enjoyable cruising; with small wake. At that speed, our Tolly is relatively efficient at 2 + nmpg [about 3 gal per hr.]. So, although we use 2X the fuel you use that's really not too bad considering that if we want to we can also go 16 to 17 knots at 1 nmpg [which can get expensive - but only occasionally do we go that speed]. When you take overall cost of boating into consideration... fuel cost for you and me is not a very large percentage of yearly expenses.
.............:

In summary, distance travelled
@ 7 knots you travel 2 miles per gal or 6.5 gal for 7 miles
@ 16 knots you travel 1 mile per gal or 7 gallons to travel 7 miles
1/2 gallon difference for distance travelled.
what is wrong with my math?

 

[rslifkin]

In summary, distance travelled
@ 7 knots you travel 2 miles per gal or 6.5 gal for 7 miles
@ 16 knots you travel 1 mile per gal or 7 gallons to travel 7 miles
1/2 gallon difference for distance travelled.
what is wrong with my math?

2 miles per gallon at 7 kts would be 3.5 gallons for 7 miles, not 6.5 gallons.

 

[Art]

In summary, distance travelled
@ 7 knots you travel 2 miles per gal or 6.5 gal for 7 miles [incorrect] Correct # = approximately 3.5 gal for 7 miles
@ 16 knots you travel 1 mile per gal or 7 gallons to travel 7 miles [correct]
1/2 gallon difference for distance travelled.
what is wrong with my math?

Getting 2 + miles per gal at 6.5 or 7 knots = 6.5 or 7 miles traveled divided by 2 +/- miles per gal = 3 +/- gallons to go 6.5 to 7 miles.

Another way to say it: At 6.5 to 7 knots our Tolly uses a little over 1/2 gallon per mile. And, at 16 to 17 knots it uses 1 gallon per mile. Soooo... going just over twice as fast it costs +/- twice as much!

Boats are FUN!! Boating is GREAT!!!

 

[backinblue]

Getting 2 + miles per gal at 6.5 or 7 knots = 6.5 or 7 miles traveled divided by 2 +/- miles per gal = 3 +/- gallons to go 6.5 to 7 miles.

Another way to say it: At 6.5 to 7 knots our Tolly uses a little over 1/2 gallon per mile. And, at 16 to 17 knots it uses 1 gallon per mile. Soooo... going just over twice as fast it costs +/- twice as much!

Boats are FUN!! Boating is GREAT!!!

Correct, but don't forget you also get there in half the time! Sometimes that is worth the cost.

 

[Art]

Correct, but don't forget you also get there in half the time! Sometimes that is worth the cost.

Absolutely!

Old saying: Time is Money! - Or was that... Money is time??

 

[rslifkin]

Correct, but don't forget you also get there in half the time! Sometimes that is worth the cost.

Yup. I definitely took advantage of that at one point last year. Departure on a trip got pushed a day due to weather, and the morning of day 2 was still pretty sloppy out. By early afternoon things had calmed down, so we got ready and got moving. Had a 49nm run to make that day, which would be 7.5 hours for us at slow cruise (~6.5 kts) and should be ~40 gallons of gas. Being that it was 3 PM, I pushed the throttles forward, knocked out the run in just under 3 hours (at 16 - 17 kts) and probably burned somewhere between 90 and 100 gallons. But we arrived before dark, were able to get through a lock to our desired stopping point before the lock closed for the night and tied up in time for a nice dinner as the sun was setting.

 

[backinblue]

Yup. I definitely took advantage of that at one point last year. Departure on a trip got pushed a day due to weather, and the morning of day 2 was still pretty sloppy out. By early afternoon things had calmed down, so we got ready and got moving. Had a 49nm run to make that day, which would be 7.5 hours for us at slow cruise (~6.5 kts) and should be ~40 gallons of gas. Being that it was 3 PM, I pushed the throttles forward, knocked out the run in just under 3 hours (at 16 - 17 kts) and probably burned somewhere between 90 and 100 gallons. But we arrived before dark, were able to get through a lock to our desired stopping point before the lock closed for the night and tied up in time for a nice dinner as the sun was setting.

It's great to have that option. I'm sure you long forgot about the extra $$$ you spent, but you remember the dinner!

 

[Art]

Yup. I definitely took advantage of that at one point last year. Departure on a trip got pushed a day due to weather, and the morning of day 2 was still pretty sloppy out. By early afternoon things had calmed down, so we got ready and got moving. Had a 49nm run to make that day, which would be 7.5 hours for us at slow cruise (~6.5 kts) and should be ~40 gallons of gas. Being that it was 3 PM, I pushed the throttles forward, knocked out the run in just under 3 hours (at 16 - 17 kts) and probably burned somewhere between 90 and 100 gallons. But we arrived before dark, were able to get through a lock to our desired stopping point before the lock closed for the night and tied up in time for a nice dinner as the sun was setting.

Your Chris have 454 cid / 350 hp. twins?

Our Tolly has 350 cid / 255 hp. engines.

 

[SteveK]

2 miles per gallon at 7 kts would be 3.5 gallons for 7 miles, not 6.5 gallons.

Thanks, I knew something was not right
corrected
In summary, distance travelled
@ 7 knots you travel 2 miles per gal or 3.5 gal for 7 miles
@ 16 knots you travel 1 mile per gal or 7 gallons to travel 7 miles

That is more like it, using twice the fuel to get there in less than 1/2 the time, that is an option nice to have.

 

[Art]

Thanks, I knew something was not right
corrected
In summary, distance travelled
@ 7 knots you travel 2 miles per gal or 3.5 gal for 7 miles
@ 16 knots you travel 1 mile per gal or 7 gallons to travel 7 miles

That is more like it, using twice the fuel to get there in less than 1/2 the time, that is an option nice to have.

Yeah... that's the ticket!!

 

[rslifkin]

Your Chris have 454 cid / 350 hp. twins?

Our Tolly has 350 cid / 255 hp. engines.

Yeah, mine are the 454 cid / 340hp Mercruisers. They're very thirsty, but otherwise great engines. I've seen this pair run a 12 hour day with a couple of consecutive 10 hour days after it without a single hiccup, so I trust them to do the job as long as I can feed them enough fuel.

 

[Art]

Yeah, mine are the 454 cid / 340hp Mercruisers. They're very thirsty, but otherwise great engines. I've seen this pair run a 12 hour day with a couple of consecutive 10 hour days after it without a single hiccup, so I trust them to do the job as long as I can feed them enough fuel.

I love both the 454 and 350 GM engines.

Besides the twin 350 cids in our Tolly and one in our ski boat... we have a 454 tbi in our 1996 RV. And, a camed 350/325 hp. in my 1T, 4WD, 4" lift, long bed fleet side, single cab, 4 spd / stick transfer, locking hub, limited slip, 1985 Chevy Cheyenne.

Then of course there is always one of my favorites - 430 cid, 400 hp. [was 360 factory but upon rebuild I added 40 +/- hp.] in my 1967 Buick Wildcat.

Another engine that is impressing me VERY much is my wife's - 3.5 L V6, 365 hp., EcoBoost, twin turbine... in her AWD, 6 seater SUV, 2014 Lincoln MKT. That critter's got some really impressive torque power. From what I can tell... that engine has no problem lasting into the 300K + mile range with good maintenance. Same vehicle and engine that is used by limo companies. I've seen a few for sale with over 400K miles and still running good. Ours has only 87K miles - Will outlast us at the rate we drive it! Nicest SUV I've seen. Trick stuff inside. Even a built in cooler mid seat in the middle seating area and two buckets in the rear seat area that electrically fold flat or face either forward for travel, or, to the rear for travel or relaxing while watching the grand kids play soccer!

 

[Hippocampus]

Find the numbers given as interesting. Once helped move a very large boat the wrong way from Sag Harbor to Sea Island. Although just a kid was real interested in how the captain thought about things. This was in the days of loran and RDFs. He had a loose leaf filled with graph paper. In it were graphs of mpg against rpm at different angles to wave train. Separate graph for different wave heights. I thought that was a lot of work and maybe he did it just to fill his days when moving the boat around. He explained the owner paid him a flat fee with a bump if he got to the next destination on time and a penalty for each day he was late. So he in effect paid out of his flat fee for fuel but took a big hit if he didn’t show up on time.
Reason I post this is to parce out when giving a gph or mph it’s very imprecise unless you also consider seastate and direction of travel. Find the addendum at the end of Beebe’s book informative. In the promotional literature or selling listings for the ones that are series production there’s information about range +/or mph. But the actual mpg listed in Beebe’s is much lower. Generator time, fins or fish make a difference but think most of that decrease is from seastate.

 

[backinblue]

Find the numbers given as interesting. Once helped move a very large boat the wrong way from Sag Harbor to Sea Island. Although just a kid was real interested in how the captain thought about things. This was in the days of loran and RDFs. He had a loose leaf filled with graph paper. In it were graphs of mpg against rpm at different angles to wave train. Separate graph for different wave heights. I thought that was a lot of work and maybe he did it just to fill his days when moving the boat around. He explained the owner paid him a flat fee with a bump if he got to the next destination on time and a penalty for each day he was late. So he in effect paid out of his flat fee for fuel but took a big hit if he didn’t show up on time.
Reason I post this is to parce out when giving a gph or mph it’s very imprecise unless you also consider seastate and direction of travel. Find the addendum at the end of Beebe’s book informative. In the promotional literature or selling listings for the ones that are series production there’s information about range +/or mph. But the actual mpg listed in Beebe’s is much lower. Generator time, fins or fish make a difference but think most of that decrease is from seastate.

Thanks Hippo, good stuff. For me at least, I don't obsess about exact numbers because it doesn't really matter to me, unlike the capt in your example. I've never even done a real study on my boat but I have a feel on how much I am using. My boat is smaller than most here, single engine, and pretty efficient. If I am on a cruise for a few days, etc, I go as fast as I want based on how soon I want to get there. I rarely travel slow enough to maximize efficiency. In the end, as others have said, my fuel spend is a small % of my boating budget.