[Dan Evans]

I have always thought of a long range cruiser as a bathtub looking thing with one small diesel that tops out around 8 kn. So I was a little skeptical when I saw an artical on the 128' Global making these claims:

max speed 30 kn
range at 10 kn is 7,800 nm
range at 12 kn is 6,000 nm
fuel is 8,000 USG with 1,800 aux tanks

Does this sound right to anyone who knows Browards (or perhaps owns one )? I looked around a little at other builders just to double check and it seems like the 128' has an amazing range for being able to push a 25 kn cruising speed. Are all Browards like this or is this just wishfull thinking?

Thanks for all your input in advance!
Dan

[Marmot]

It is some kind of thinking, not sure what kind though.

7800 miles at 10 knots would take 780 hours
9800 gallons of fuel consumed in 780 hours is ~ 12.56 gallons per hour

According to MTU, the 16V2000 M93 engine (2400 hp @ 2450 rpm) consumes 14.2 gallons per hour while delivering 275 horsepower to the prop at 1200 rpm.

This means that according to the specs, that boat can do 10 knots on 1 engine running at just above idle

This doesn't even count the fuel used by the generator(s) but maybe they use an air motor for hotel power.

[Dan Evans]

Originally Posted by Marmot

This doesn't even count the fuel used by the generator(s) but maybe they use an air motor for hotel power.

Oh don't bring that up again! It looks like you have just disproved the range that they claim. I am still curious how far they really can go on a full tank(s) of gas. Every article that I have found for this yacht, as well as Browards website, gives the same info. Hopefully there is someone out there who has cruised on a Broward and watched the fuel burn...

Dan

[Marmot]

"Every article that I have found for this yacht, as well as Browards website, gives the same info."

That is a sad example of "cut and paste" reportage ... nobody questions, nobody researches, they just cut and paste what the press release said and call it a news item or boat review.

[Capt J]

Originally Posted by Dan Evans

I have always thought of a long range cruiser as a bathtub looking thing with one small diesel that tops out around 8 kn. So I was a little skeptical when I saw an artical on the 128' Global making these claims:

max speed 30 kn
range at 10 kn is 7,800 nm
range at 12 kn is 6,000 nm
fuel is 8,000 USG with 1,800 aux tanks

Does this sound right to anyone who knows Browards (or perhaps owns one )? I looked around a little at other builders just to double check and it seems like the 128' has an amazing range for being able to push a 25 kn cruising speed. Are all Browards like this or is this just wishfull thinking?

Thanks for all your input in advance!
Dan

Most all yachts are like this if you run them slow. If you run most any diesel at 1,000 rpm's or even less they use a minimal amount of fuel. So if you run any yacht at hull speed, you should get tremendous range. A 16v2000 makes 2000 hp,yet at idle only burns 2-3gph. I was on a 75' sportfish that burned 150gph at 35 knot cruise with 16v2000's, yet at idle we did 9 knots and burned 6 gph ie. 1.5 MPG.

Generators use very little, we have a 20KW Onan and a 27KW phasor and both of them will use .5 GPH loaded.

[Marmot]

"... we have a 20KW Onan and a 27KW phasor and both of them will use .5 GPH loaded.."

Those are some amazing generators. Half a (US) gallon per hour is what a 20 kW diesel genset should burn between no load and about 25 percent. Loaded to 20 kW it should burn around 1.8 gph.

What rpm are those 16Vs turning at idle to give you 9 knots? Three gph clutched in sounds a bit optimistic to me. I would expect about 3 times that much or around 9 gph per engine clutched in at around 600 rpm.

[Capt J]

Originally Posted by Marmot

"... we have a 20KW Onan and a 27KW phasor and both of them will use .5 GPH loaded.."

Those are some amazing generators. Half a (US) gallon per hour is what a 20 kW diesel genset should burn between no load and about 25 percent. Loaded to 20 kW it should burn around 1.8 gph.

What rpm are those 16Vs turning at idle to give you 9 knots? Three gph clutched in sounds a bit optimistic to me. I would expect about 3 times that much or around 9 gph per engine clutched in at around 600 rpm.

That is what the generators average over running days upon days 24hrs a day all of the time off of their own fuel tank. That is running 2 refrigerators, 3 freezers, 6 a/c's, hot water heater, and everything in between.

The 16v2000's are at dead idle to push the boat 9 knots. It is a custom 75' sportfish with a very efficient hull design considering it tops out at 43 knots with 16v2000's. I think you should do your homework on custom sportfish because most of the large custom sportfish...... ACY, Merritt, Jim Smith etc. 70-85' cruise right at/around 35 knots and they all do about 9 knots at idle wih both engines in gear. And 16v2000's at idle in gear burn about 3 gph each, in an efficient hull design. Have you ever been on a yacht with 16v2000's or several of them? I have run 16v2000's, c32 CATS, c30 cats, c18 cats, 1350 mans, 1100 MANs as well as many others and they all have single digit fuel consumptions under 1,000 rpms.

According MTU's website 16v2000's burn 13gph at 1200 rpm's, so what do you think they burn at 600 rpm's?

[Marmot]

"According MTU's website 16v2000's burn 13gph at 1200 rpm's, so what do you think they burn at 600 rpm's?"

If it has a prop connnected to it, around 9 gph, look at the propeller curve and see how much horsepower it absorbs at 600 rpm. Just like the generators, it takes a certain amount of fuel just to turn the engine at idle, generating electricity or turning a fixed pitch prop takes more fuel. Look up the no load fuel consumption for a 20 kW Onan. I didn't dispute your claim of the speed of the boat at idle, that is irrelevant.

[Capt J]

Originally Posted by Marmot

"According MTU's website 16v2000's burn 13gph at 1200 rpm's, so what do you think they burn at 600 rpm's?"

If it has a prop connnected to it, around 9 gph, look at the propeller curve and see how much horsepower it absorbs at 600 rpm. Just like the generators, it takes a certain amount of fuel just to turn the engine at idle, generating electricity or turning a fixed pitch prop takes more fuel. Look up the no load fuel consumption for a 20 kW Onan. I didn't dispute your claim of the speed of the boat at idle, that is irrelevant.

The speed of the boat at idle is part of the equation. The faster the speed at idle, the less engine load, and the less fuel needed for the motor to turn the propellor. Load is load. Browards are relatively flat hulls and all of the ones I have run have had really good displacement speeds. I can believe their range figures, considering I do 15k NM in deliveries each and every year and have done a lot of long slow ones where fuel range is an issue on a variety of different vessels.

[K1W1]

Hi,

I think what Marmot has written has been misunderstood.

No matter who built your engine or how much it's top end output end is there will be a fuel figure given in lbs per hp/hr or grams per kwh.

This means that if say at idle you develop 100 kW your per engine consumption will be 100 times the given figure. This is not a linear calculation through the whole rev range because the developed power will curve upwards sharply as the engine reaches the higher speeds.

I know of a displacement yacht that crossed the Atlantic some 24 yrs ago, in an attempt to save fuel it went on one engine- swapping every few days. They tried to run at their normal cruising speed. End result was more fuel burnt than if they had run on both at a lower rpm setting.

A quick guide to the intricacies of pressure charged engines is that the higher the boost pressure you have the higher volume you are perceived too have and therefore the larger amount of fuel that can be burnt in that volume of air. Hence you will develop the most power and burn the most fuel when the boost pressure is the highest.

As for your 20 kW Genset I don't have ONAN Books to hand but have some CAT info a C 2.2 developing 21 Kw at 1800 rpm ( 60 Hz) uses 1.98 US Gal/hr. I don't see how ONAN could have internal combustion engine package that develops the same sort of power on 25% of the fuel.

[Marmot]

"... there will be a fuel figure given in lbs per hp/hr or grams per kwh."

That figure, the brake specific fuel consumption or BSFC curve is usually lowest at the design max continous output power. At low power and at very high power the weight of fuel burned to produce each horsepower is higher. So not only is there a minimum amount of fuel required to run at idle, the engine is less fuel efficient at that power setting.

It doesn't matter how efficient the hull is, it takes power to turn the prop. Look at any propeller curve and that is very obvious. DD doesn't publish the graphs for some reason, but taking a page from K1W1's book, the 2000 hp CAT 3516 burns 8.5 gph at 600 rpm clutched in. Like the man said, I don't see how anybody else can do much better.

[Capt J]

I understand that it takes power and fuel to turn a propellor. But the load factor still determines how much fuel is needed to maintain 600 rpm's. When you put a motor in gear, you can watch the digital fuel displays and they'll show a higher number say 9gph, as the boat gains momentum and reaches the speed it will travel at, it takes less torque to maintain 600 rpm's and the fuel consumption will go down. It is like taking a tractor trailer and running it up a hill loaded at 2100 rpm's, it is going to use a heck of a lot more fuel because you will have considerably much more throttle then if it is on even ground or going down hill at 2100 rpm's. Fuel consumption is determined more by load factor then the rpm's it is running at. Look at a diesels fuel display (consumption) at 1950 rpm's in neutral, compared to if you are underway.

Anyways, I guess Broward Marine simply made up those numbers out of the sky and they aren't based upon anything scientific and you should get an attorney and sue them for false advertising.

[K1W1]

it takes less torque to maintain 600 rpm's and the fuel consumption will go down.

Hi,

This is because the load has gone down from the initial turn of the prop.

Try turning the shaft by hand it will be harder to get going than to maintain the motion.

[Marmot]

Not to be too pedantic but load factor is a ratio. The amount of power absorbed by the propeller is the load.

Since boats don’t go up or down hill on a road, they can only accelerate or decelerate as they are constantly doing in all but dead calm air and water. The propeller absorbs a certain amount of power to turn a given rpm in water. That amount of power is the same whether the prop is driving a barge or a sportsfish, the only difference is the speed the driven hull will obtain from that power. If you look at the propeller curve you will find the amount of power required to turn the prop at 600 rpms. I suspect that number will be around 70 horsepower in this case.

Using the figures supplied by DD for the 16V200-91 (2000 hp @ 2350 rpm) the engine consumes 104 US gallons per minute to produce 2000 hp. That translates to a BSFC of .364 lb/hp-hr or a thermal efficiency of around 38 percent. Not too shabby a set of figures. Back off to a cruise power setting of 898 hp at 1800 rpm the engine burns 44 gallons per hour for a BSFC of .342 lb/hp-hr giving an efficiency of 40 percent which is pretty much the highest in that class of engine. As the power is reduced, the fuel burn goes down to 13.3 gph at 1200 rpm to deliver 268 hp. This works out to a BSFC of .35 lb/hp-hr. As I wrote earlier, the best BSFC is obtained at the normal cruise setting and increases at either extreme of power output. The thermal efficiency at this power has dropped a percent to 39.

Now, if we reduce rpm to 600, the propeller curve will tell us it absorbs somewhere around 70 hp … give or take a few horsepower. Please correct me if you have a curve for that boat, I don’t have one handy.

Here is where it gets interesting. If you idle that engine in gear and it produces 70 hp from burning 9 gph as I suggest it does, the BSFC increases to .84 lb/hp-hr or more than twice as much fuel to produce each horsepower than at 1800 rpm. The thermal efficiency has dropped to a miserable 16 percent. This is not a happy place to run that engine/prop combination. The reason it is so bad is that as the power is reduced the amount of fuel required to just turn the engine over becomes a much larger percentage of the power produced, so just like a car sitting at a stop sign, the mileage goes to zero.

If that engine only burned 3 gph as you suggest, then the BSFC at 600 rpm is way down at .28, far better than the “sweet spot” at 1800. The thermal efficiency at that fuel flow has magically increased all the way up to 49 percent, right up there with the best slow speed two stroke diesels in the world. And those big 2-strokes are the most efficient diesel engines ever made by man.

(And for those who wish to do the math, I based the calculations on 7 lbs per US gallon diesel with a heating value of 18,390 BTUs per pound.)

[Capt J]

You're forgetting one variable in your calculations. Engine efficiency will be much better at slow rpm's 1000 rpms and under because you are not using a considerable amount of the HP to turn the Turbo as well as cool off the increased air charge temperature from the Turbo's compressing the air into the engine (boost). At under 1000 rpm's, you are not even creating enough backpressure to spin the turbo's and the exhaust air is simply flowing freely through them. Turbo's use HP in order to make more HP. It takes a certain amount of HP to spin them and most figures are about 40% of the additional power they create.

A sportfish has a lower load factor at idle and faster speed once it gains momentum then a tugboat would because you also have to factor in that you are using a different reduction in the gear case and thus you are going to have a different shaft speed as well. There is a lot larger speed range between idle and cruise rpm's and the hull is more efficient at lower rpm's (displacement speed) and takes a lot less HP to move it through the water then getting and maintaining planing speed.

According to the digital displays (which were amazingly accurate) on the vessel with 16v2000's I am familiar with. At 1950 rpm's we burned 150gph (both engines) and cruised at 35 knots. At dead idle in gear with 1 engine we did 7.5 knots and 1 engine burned about 5 gph and the 1 engine in idle burned 1 gph. With both engines in gear at idle fuel consumption was 6-7gph for both engines and speed was 9-9.5 knots depending on current and fuel load. Top speed was 43 knots, 2350 rpms, 220 gph. Our most efficient speed above displacement speed was 1200 rpms where we were doing 22 knots and using around 28 gph (total).

Efficient cruising speed totally depends on the hull configuration as well as the engine chosen. Some yachts at 1800rpm's would be at maximum plow and only doing 14 knots, and at 1950 rpm's on plane and doing 22 knots. Needless to say there are a few planing 100-130ft yachts that cruise at 20-24 knots and post impressive range at 10knots and 12 knots...... Westport's are ones that instantly come to mind. Most of these 100-130ft yachts end up travelling at 10-12 knots 97% of the time, even if they can do 20-24 knots.