Can a 53' use less fuel than a 43'?

[Top Hatt Craig]

Specifically, can a pair of 8-71 TIs in a 53' with 1.5 to 1 gears cruising at 11 knots at 1300 rpm get better fuel economy than a 43' with 6-71Ns turning 2 to 1 gears at 1900 rpm at the same speed? The bigger boat has four more injectors, but they're firing 600 fewer times per minute, seems possible. Our 43' has a freshly cleaned bottom and runs perfectly as far as I can tell, but sure feels like diminishing returns going above 1750 rpm. Thanks for your thoughts.

 

[Boatsb]

Hull speed.

When you exceed it you burn fuel and get limited speed increase.

The hull speed for the 53 is probably a kt more than the 43.

 

[Glory]

Specifically, can a pair of 8-71 TIs in a 53' with 1.5 to 1 gears

1.5x1, A convertible? With 8-71s?

 

[BobWaz]

Back in 2003 we had a 36 Island Gypsy trawler. On our way to New Bern we stopped overnight at Alligator River Marina. There were 2 other boats there. A 42 & 48 Grand Banks. When the 3 of us got together to talk, there was some interesting results. All 3 boats were powered identically with twin 120 hp Lehman diesels. My 36 burned the most fuel with the slowest cruise speed. The 42 burned a little less at a higher cruise speed and the 48 was fastest and burned the least amount. You could not find a better example of hull speed and efficiency to the length of the boat.

 

[Pascal]

11 kts is the wrong cruise for both boats, but even more for the 43 as its hull speed is lower by about 1kts (8kts vs 9kts)

Many boats don't realize how fast fuel burn goes up once past hull speed until they have flow monitors (or keep accurate fuel logs) I am always amazed seeing boats pushing a large bow wake running stern down at 11/12/13kts. It makes no sense whatsoever.

At 11kts the 43 probably burns 2.5 times what it burns at 8kts. At the same speed teh 53 will be a lot less efficient than it would at 9kts but less so due to the longer LWL.

Engine size doesn't make much of a difference, in theory the largerengine with more cylinders and bigger mass will require a little more fuel jsut to turn but that's nothing compared to th hull speed difference. It takes a set amount of fuel to generate a set amount of hp.

Another factor against the larger boat is beam. A larger beam will increase fuel burn at hull speed and could offset the longer waterline length

Couple of example... Years ago I did a delivery on a 54 hatt My (wide beam) and she burned about 20% more than the older narrow beam 53... No meters but that was based on fuel used between top offs. 8V71 vs 8V92

I recently started running an 84 Lazzara after 8 years running a 70' Johnson. The Lazzara is a little newer (2009 vs 2003) , has 1650hp C32 vs 1400hp 3412Es, both electronic, and an extra foot of beam. At hull speed the Lazzara burns 24gph for 11kts vs 20gph / 10kts for the 70. Makes sense.

What doesn't make sense is that on plane the bigger boat burns about 15% less fuel... 70 GPH vs 82gph at 19kts, 80gph vs 90gph at 20kts... What it comes down to is hull design, balance and weigh. The Johnson is an older design whereas the Lazzara is a newer hull that was tweaked for performance and efficiency.

So don't assume anything when it comes to fuel burn except that jsut a knot above hull speed will cost you time the fuel.

 

[bobk]

Here is the data for a 48MY. The data points were taken from a performance trial reported in a boating magazine many years back. The test boat had no stabilizers and was not loaded. "Hull speed" calculated as 8.98 knots. You can see a big difference between 8.3 knots (below hull speed) and 9.1, just a little bit above.

Bobk

[TABLE="width: 320"]
[TR]
[TD="width: 64"]RPM[/TD]
[TD="width: 64"]Knots[/TD]
[TD="width: 64"]NMPG[/TD]
[TD="width: 64"]GPH[/TD]
[TD="width: 64"]run angle[/TD]
[/TR]
[TR]
[TD="align: right"]1000[/TD]
[TD="align: right"]8.3[/TD]
[TD="class: xl63, align: right"]1.48[/TD]
[TD="align: right"]5.6[/TD]
[TD="align: right"]1[/TD]
[/TR]
[TR]
[TD="align: right"]1250[/TD]
[TD="align: right"]9.1[/TD]
[TD="class: xl63, align: right"]0.99[/TD]
[TD="align: right"]9.2[/TD]
[TD="align: right"]2[/TD]
[/TR]
[TR]
[TD="align: right"]1500[/TD]
[TD="align: right"]9.5[/TD]
[TD="class: xl63, align: right"]0.55[/TD]
[TD="align: right"]17.2[/TD]
[TD="align: right"]5[/TD]
[/TR]
[TR]
[TD="align: right"]1750[/TD]
[TD="align: right"]10.2[/TD]
[TD="class: xl63, align: right"]0.38[/TD]
[TD="align: right"]26.8[/TD]
[TD="align: right"]6[/TD]
[/TR]
[TR]
[TD="align: right"]2000[/TD]
[TD="align: right"]15.8[/TD]
[TD="class: xl63, align: right"]0.49[/TD]
[TD="align: right"]32.1[/TD]
[TD="align: right"]5.5[/TD]
[/TR]
[TR]
[TD="align: right"]2250[/TD]
[TD="align: right"]18.8[/TD]
[TD="class: xl63, align: right"]0.43[/TD]
[TD="align: right"]44[/TD]
[TD="align: right"]5.5[/TD]
[/TR]
[/TABLE]

 

[Freeebird]

Here is the data for a 48MY. The data points were taken from a performance trial reported in a boating magazine many years back. The test boat had no stabilizers and was not loaded. "Hull speed" calculated as 8.98 knots. You can see a big difference between 8.3 knots (below hull speed) and 9.1, just a little bit above.

Bobk

[TABLE="width: 320"]
[TR]
[TD="width: 64"]RPM[/TD]
[TD="width: 64"]Knots[/TD]
[TD="width: 64"]NMPG[/TD]
[TD="width: 64"]GPH[/TD]
[TD="width: 64"]run angle[/TD]
[/TR]
[TR]
[TD="align: right"]1000[/TD]
[TD="align: right"]8.3[/TD]
[TD="class: xl63, align: right"]1.48[/TD]
[TD="align: right"]5.6[/TD]
[TD="align: right"]1[/TD]
[/TR]
[TR]
[TD="align: right"]1250[/TD]
[TD="align: right"]9.1[/TD]
[TD="class: xl63, align: right"]0.99[/TD]
[TD="align: right"]9.2[/TD]
[TD="align: right"]2[/TD]
[/TR]
[TR]
[TD="align: right"]1500[/TD]
[TD="align: right"]9.5[/TD]
[TD="class: xl63, align: right"]0.55[/TD]
[TD="align: right"]17.2[/TD]
[TD="align: right"]5[/TD]
[/TR]
[TR]
[TD="align: right"]1750[/TD]
[TD="align: right"]10.2[/TD]
[TD="class: xl63, align: right"]0.38[/TD]
[TD="align: right"]26.8[/TD]
[TD="align: right"]6[/TD]
[/TR]
[TR]
[TD="align: right"]2000[/TD]
[TD="align: right"]15.8[/TD]
[TD="class: xl63, align: right"]0.49[/TD]
[TD="align: right"]32.1[/TD]
[TD="align: right"]5.5[/TD]
[/TR]
[TR]
[TD="align: right"]2250[/TD]
[TD="align: right"]18.8[/TD]
[TD="class: xl63, align: right"]0.43[/TD]
[TD="align: right"]44[/TD]
[TD="align: right"]5.5[/TD]
[/TR]
[/TABLE]

Seems to me the big change in fuel consumption was based on the disproportionate increase in RPM vs speed. Hull and prop efficiency has to play into this vs just a mathematical calculation of hull speed. No doubt hull speed is higher on longer boats, and with identical power, it would make sense they would be more efficient.

Try paddling a shorter kayak vs a longer one, for example. You'll get more speed with the longer one.

 

[SEVEN]

Here is the data for a 48MY. The data points were taken from a performance trial reported in a boating magazine many years back. The test boat had no stabilizers and was not loaded. "Hull speed" calculated as 8.98 knots. You can see a big difference between 8.3 knots (below hull speed) and 9.1, just a little bit above.

Bobk

[TABLE="width: 320"]
[TR]
[TD="width: 64"]RPM[/TD]
[TD="width: 64"]Knots[/TD]
[TD="width: 64"]NMPG[/TD]
[TD="width: 64"]GPH[/TD]
[TD="width: 64"]run angle[/TD]
[/TR]
[TR]
[TD="align: right"]1000[/TD]
[TD="align: right"]8.3[/TD]
[TD="class: xl63, align: right"]1.48[/TD]
[TD="align: right"]5.6[/TD]
[TD="align: right"]1[/TD]
[/TR]
[TR]
[TD="align: right"]1250[/TD]
[TD="align: right"]9.1[/TD]
[TD="class: xl63, align: right"]0.99[/TD]
[TD="align: right"]9.2[/TD]
[TD="align: right"]2[/TD]
[/TR]
[TR]
[TD="align: right"]1500[/TD]
[TD="align: right"]9.5[/TD]
[TD="class: xl63, align: right"]0.55[/TD]
[TD="align: right"]17.2[/TD]
[TD="align: right"]5[/TD]
[/TR]
[TR]
[TD="align: right"]1750[/TD]
[TD="align: right"]10.2[/TD]
[TD="class: xl63, align: right"]0.38[/TD]
[TD="align: right"]26.8[/TD]
[TD="align: right"]6[/TD]
[/TR]
[TR]
[TD="align: right"]2000[/TD]
[TD="align: right"]15.8[/TD]
[TD="class: xl63, align: right"]0.49[/TD]
[TD="align: right"]32.1[/TD]
[TD="align: right"]5.5[/TD]
[/TR]
[TR]
[TD="align: right"]2250[/TD]
[TD="align: right"]18.8[/TD]
[TD="class: xl63, align: right"]0.43[/TD]
[TD="align: right"]44[/TD]
[TD="align: right"]5.5[/TD]
[/TR]
[/TABLE]

What an eye opener. Thanks

 

[SKYCHENEY]

Where are you going to find a 53 with 1.5 to 1 gears? I thought all of them had 2 to 1 gears.

 

[Pascal]

Here is the data for a 48MY. The data points were taken from a performance trial reported in a boating magazine many years back. The test boat had no stabilizers and was not loaded. "Hull speed" calculated as 8.98 knots. You can see a big difference between 8.3 knots (below hull speed) and 9.1, just a little bit above.

Bobk

[TABLE="width: 320"]
[TR]
[TD="width: 64"]RPM[/TD]
[TD="width: 64"]Knots[/TD]
[TD="width: 64"]NMPG[/TD]
[TD="width: 64"]GPH[/TD]
[TD="width: 64"]run angle[/TD]
[/TR]
[TR]
[TD="align: right"]1000[/TD]
[TD="align: right"]8.3[/TD]
[TD="class: xl63, align: right"]1.48[/TD]
[TD="align: right"]5.6[/TD]
[TD="align: right"]1[/TD]
[/TR]
[TR]
[TD="align: right"]1250[/TD]
[TD="align: right"]9.1[/TD]
[TD="class: xl63, align: right"]0.99[/TD]
[TD="align: right"]9.2[/TD]
[TD="align: right"]2[/TD]
[/TR]
[TR]
[TD="align: right"]1500[/TD]
[TD="align: right"]9.5[/TD]
[TD="class: xl63, align: right"]0.55[/TD]
[TD="align: right"]17.2[/TD]
[TD="align: right"]5[/TD]
[/TR]
[TR]
[TD="align: right"]1750[/TD]
[TD="align: right"]10.2[/TD]
[TD="class: xl63, align: right"]0.38[/TD]
[TD="align: right"]26.8[/TD]
[TD="align: right"]6[/TD]
[/TR]
[TR]
[TD="align: right"]2000[/TD]
[TD="align: right"]15.8[/TD]
[TD="class: xl63, align: right"]0.49[/TD]
[TD="align: right"]32.1[/TD]
[TD="align: right"]5.5[/TD]
[/TR]
[TR]
[TD="align: right"]2250[/TD]
[TD="align: right"]18.8[/TD]
[TD="class: xl63, align: right"]0.43[/TD]
[TD="align: right"]44[/TD]
[TD="align: right"]5.5[/TD]
[/TR]
[/TABLE]

These numbers don't surprise me. Again the difference between just below hull speed and just above is huge so the jump to 17 GPH at 9.5 is probably pretty accurate That 1/2 knot of speed will cost you dearly...

 

[gbharrington]

As the owner of a 43 that runs exclusively at hull speed, I can tell you that true hull speed is closer to 6.5/7 knots than 8.

 

[krush]

If you want to go slow, go long. Otherwise, go light and get on plane!

 

[Top Hatt Craig]

Where are you going to find a 53 with 1.5 to 1 gears? I thought all of them had 2 to 1 gears.

That is unknown to me, thanks for bringing it up. Anyone know?Regards, Craig

 

[saltshaker]

If you want to go slow, go long. Otherwise, go light and get on plane!

Or go big, fast and burn some fuel! My boat's not exactly fast but it sure does feel better and is a lot more fun running 23kts than 10kts.

 

[BOATER BOB]

I have a 53 SF with 250 hp turbo perkins and at 9kts it burns 5 gal hour with 3.75 to 1 trans turning 30x30 props or I can run 16mph and burn 18gph but why bother? So yes less than 43.Also I have a 200 hr range or 18 hundred miles.
Bob

 

[gpellissier]

Concur with everyone else.

Hull speed is roughly 1.34 x (waterline length) ^ 1/2. The speed to get on plane is roughly 2 x (waterline length) ^ 1/2. This is waterline length, not length overall, for example the 48 MY mentioned previously has a waterline length of about 45 ft. Running between the hull speed and being on plane burns quite a bit more fuel.

Numbers mostly work...for my 44 Tri-Cabin, my hull speed is between 8-8.5 knots, and I can plane at about 12-13 knots with 8-71Ns. I'll burn about 7 gph at 8 knots, but increasng to 11 knots over doubles that.

I'm somewhat envious of the 48 MY mentioned previously, as at 1000 rpm I only get about 6-6.5 knots with a clean bottom...but then again I only burn about 4 gph.

 

[Pascal]

I be always known there was a huge penalty when running between hull and planning speed sominneber really plotted exact numbers. Until this morning.

Different boat obviously but the physics are the same and will apply pretty much to any planning hull. Actually I think that with the heavier and less efficient older hatt hulls the difference is even worst.

1000rpm 21gph 10.5kts 28% load
1100rpm 29gph 10.9kts 34% load
1200rpm 36gph 11.2kts 36% load
1300rpm 45gph 12.1kts 39% load
1400rpm 54gph 13.5kts 41% load
1500rpm 56gph 18.3kts 39% load

So, for 1.6kts extra speed (1300rpm) we burn over twice the fuel than we do at hull speed (1000rpm). Makes no sense whatsoever Then jumping from 1400 to just 1500 get the boat on plane and the speed shoot up for the same fuel and load....This is a 2009 Lazzara 84 with 1650hp C32s (normal cruise 25kts, top 31kts)

I m really curious to see the numbers when I finally put the C series cummins in my 53. They re mechanicals but I will put flow meters to get precise numbers.

 

[scottinsydney]

This is a very interesting discussion.My own boat started life as a 1984 61MY. It was later extended in the cockpit to 70'.Following its purchase in 2015 we had no idea what the real fuel consumption would be so on the first leg of the delivery voyage of 43 hours the boat averaged 8.5 knots at 875rpms. When refuelled we were amazed to calculate that the combined hourly consumption was just 32 litres or 8 US gallons per hour.I wonder if having the props well forward of the new transom may also have improved its efficiency as well as the increased length?

 

[krush]

Here's a trick....a strap on waterline extension....make it inflatable (and ballasted)! Used only for slow speed crusing.

 

[Top Hatt Craig]

Thanks all, excellent inputs. This weekend I experimented with what I learned here and the one thing that I initially had a hard time with is how engine speed influences perceived boat speed. Now past that we're saving fuel while cruising at only a little less speed.