The Case for Going Slow

[SteveK]

Steve

In my 4788 I get very close to 2 NMPG at 7.5 knots. with each engine burning around 1.75 GPH at 1250 RPM.

If I increase the speed to 15 knots my consumption goes to around .6 to .65 NMPG as my fuel burn in the twin 330's goes to around 12 GPH each.

I have two ways of determining fuel burn. The first and most accurate is as you posted filling up the tank after a run at X RPM. The second is that i have flowscans installed. These are not perfect but give a very good measurement as well. I find the flow scans are more accurate at high RPM and engine loading than low speed BTW.

Kevin,
Your boat is the closest for my comparison. Your engines are 330, mine 250
I just filled my tanks to the brim, filled the same over one year ago.
Both engines 154.3 hours running, the GEN, the diesel heater on many occasion and the fuel replaced is 1574 litres or 416.4 US gal. My math 1574/154.3=10.20 litres per hour or 2.69 US gal per hour at an average speed of 7 knots.
That is 1.35 gph per engine which includes GEN and Heater.

Wait, the bolded you said 2nmpg @ 7.5 knots, that should be 7.5/2=3.75 GPH gallons but then you say 1.75 GPH. more confused. The 1.75 seemed like a good comparison.

 

[socalrider]

Good discussion! We also have a boat where the decision was made for us in the design phase. 2x120hp Lehmans don't get our CHB41 up on a plane, so it's either 2nmpg at 8.5kts or some number much less than that and a tremendous racket at 10kts.

One thing missing (maybe) in the discussion: I like the simplicity of my small NA I-6 engines. No turbos, after coolers, pretty cheap to keep going. We're looking at upgrading to something in the 55' range, and some of the engines on the bigger boats are a bit terrifying to contemplate. Some of the big MAN 800hp engines require rebuilds every 7-8 calendar years at a reported cost of $25k/side. @ranger58sb and others, I'm curious if this factors in? An example would be the Hatteras LRC 58 versus their 58' MY or YF. One has the tiny 120-150hp NA inline 4/6-71's, the other might have 8v71s, 12v71s, or the bigger 92-series. There's certainly an advantage to the smaller engines in cost, reliability & engine room access. That's in addition to the marginal gains in efficiency at 7-9kts for the smaller engines.

 

[boomerang]

40 years ago, when I started annually traveling to & from South Florida , it was most always in a fast sport-fisherman with a good friend and sometimes an occasional 14-18 kt motor yacht. We made good time every day but while the trips were most enjoyable, they were also slightly stressful on the tighter ICW when we couldn't be offshore. I remember flying by all of the pretty anchorages and scenic towns and wishing I could slow down and take the time to enjoy the views. I made a promise to myself that in the future I would try a slower boat and the trawler lifestyle.
My record was 3 1/2 days from Delray to the Chesapeake but again, while on the ICW ,I never took my eyes off of the chartbook, chartplotter and depth machine long enough to see anything other than the next marker I was coming up on at a high rate of speed. Now that we own and cruise a slooooow boat , I can honestly say I wouldn't trade our 6.5 knot cruise and fun anchorage each night for a 35 knot boat and dockage in a marina.
The fact that our Lehman burns 20 gallons of fuel a day and the 16-92's in my pal's 63 burnt 900 gallons a day also makes a tiny bit of difference in my preference for going slow.

 

[O C Diver]

Well my question was: do I take the measurement at right angles to the bow and stern or actually following the curve of the boat?

Waterline length is the longest length touching the surface of the water.

Ted

 

[ranger58sb]

Some of the big MAN 800hp engines require rebuilds every 7-8 calendar years at a reported cost of $25k/side. @ranger58sb and others, I'm curious if this factors in? An example would be the Hatteras LRC 58 versus their 58' MY or YF. One has the tiny 120-150hp NA inline 4/6-71's, the other might have 8v71s, 12v71s, or the bigger 92-series. There's certainly an advantage to the smaller engines in cost, reliability & engine room access. That's in addition to the marginal gains in efficiency at 7-9kts for the smaller engines.

I don't remember saying that $$$ for our normal MAN service? We had a big rehab bill up front because our most previous PO provided no evidence of maintenance... and because during survey we also found two exhaust risers needed replacement and both of our electronics monitor screens needed service. IOW, we were bringing the engines back into the service schedule AND repairing stuff that usually doesn't need repair.

Not to suggest it'd be inexpensive, but I'm thinking the normal service would be more like $10-15K per engine if you have a MAN service tech do the work. Much of it is user-doable, though, and having a tech do it mostly means the labor hours are what add up.

My take on the Hatts is that the LRCs were meant to go slow and inexpensive (4/6-71NAs), the MYs were meant to cruise at faster speeds (71s), and the YFs were meant to get to fishing grounds and back (8v71s or 92s). Given my preference for a cockpit, I've for years thought the YFs, especially the 58YF, would be the bee's knees, perhaps ideally with the 8v71s. Cruise it like a trawler but with a brief burst of RPM at the end of each run.

-Chris

 

[socalrider]

I don't remember saying that $$$ for our normal MAN service? We had a big rehab bill up front because our most previous PO provided no evidence of maintenance... and because during survey we also found two exhaust risers needed replacement and both of our electronics monitor screens needed service. IOW, we were bringing the engines back into the service schedule AND repairing stuff that usually doesn't need repair.

Not to suggest it'd be inexpensive, but I'm thinking the normal service would be more like $10-15K per engine if you have a MAN service tech do the work. Much of it is user-doable, though, and having a tech do it mostly means the labor hours are what add up.

My take on the Hatts is that the LRCs were meant to go slow and inexpensive (4/6-71NAs), the MYs were meant to cruise at faster speeds (71s), and the YFs were meant to get to fishing grounds and back (8v71s or 92s). Given my preference for a cockpit, I've for years thought the YFs, especially the 58YF, would be the bee's knees, perhaps ideally with the 8v71s. Cruise it like a trawler but with a brief burst of RPM at the end of each run.

-Chris

I hadn't heard that figure from you, it was from another forum I believe. Thanks for the corrective - I'd conclude that there is a parts & maintenance (and space) premium for the larger engines in addition to the marginal efficiency loss at 8-9kts. Overall I can certainly appreciate the case for a boat with the capability to do 17+kts, but I'd also say that if you're never or very rarely going to use that capability, there's a strong case for the smaller engines and a FD hull. Horses for courses!

I'm still irritated by a lot of the SD hulls out there that have engines 3x bigger than what's required for displacement cruising, but still not big enough to get on a plane. So 1.5nmpg at 9kts, or 0.3nmpg at 13-14kts. Lots of the older Hatteras seem to fit this category. Seems like the worst of all possible worlds.

 

[Marco Flamingo]

I'd also say that if you're never or very rarely going to use that capability, there's a strong case for the smaller engines and a FD hull.

And the justification that I keep hearing for the infrequent need for a couple of monstrous engines (allowing 18 knots at 32 gph) is "in case of an emergency." Somebody really thinks that 18 knots is faster than VHF radio waves? Faster than a CG helicopter? If somebody is going to burn 32 gph in an emergency, I'd much rather that it be the helicopter because the helicopter can land right at the hospital. A boat would have to go much faster than 18 knots to launch itself up onto a remote hospital's helipad.

I finally located the little smiley things.

 

[O C Diver]

I'm still irritated by a lot of the SD hulls out there that have engines 3x bigger than what's required for displacement cruising, but still not big enough to get on a plane. So 1.5nmpg at 9kts, or 0.3nmpg at 13-14kts. Lots of the older Hatteras seem to fit this category. Seems like the worst of all possible worlds.

That was the case with Slow Hand my Cherubini Independence 45. Replacing the single engine from 450 HP to 132 HP about cut the fuel consumption in half. Doing much of the work myself and selling the original motor at a good price kept the cost to about $10K. Probably saved 9,000 gallons ($35K to $45K) of fuel.

Ted

 

[mvweebles]

Over-powering boats is nothing new and driven by market demand. Vega produced the Willard 36 from 1961 to 1970. Bill Garden spec'd a 75hp engine which in my opinion is perfect. But customers wanted bigger so a remarkable number were built with much larger engines, even variants of 6.71 DDs of over 200hp. Eventually, the Perkins 6.354 (135 HP) was offered which was a popular upgrade.

Many buyers just want bigger. Heck, even on this list of knowledgeable owners, many people extoll "you can always go slower." True, but few stay within the sweet spot of efficiency - 1.1-1.15 SL.

Peter

 

[ranger58sb]

I'd conclude that there is a parts & maintenance (and space) premium for the larger engines in addition to the marginal efficiency loss at 8-9kts. Overall I can certainly appreciate the case for a boat with the capability to do 17+kts, but I'd also say that if you're never or very rarely going to use that capability, there's a strong case for the smaller engines and a FD hull. Horses for courses!

Yep. All that.

We can go slow and be more economical than when we run fast. That's not as economical as going slow in a boat made to go slow.

I was aiming for a 20 kt boat, got a 30 kt boat. We still run it more often like a trawler than we do on plane (at around 20 kts). Easier, on calm water. Faster with this hull usually works better in chop and confused seas, though, so there's that.

-Chris

 

[chriscritchett]

Well my question was: do I take the measurement at right angles to the bow and stern or actually following the curve of the boat?

Perpendicular. It’s about the length of the wave the hull makes, not how far the water has to travel to get around the hull. Typically measured (straight line) on a drawing.

 

[rslifkin]

I'm still irritated by a lot of the SD hulls out there that have engines 3x bigger than what's required for displacement cruising, but still not big enough to get on a plane. So 1.5nmpg at 9kts, or 0.3nmpg at 13-14kts. Lots of the older Hatteras seem to fit this category. Seems like the worst of all possible worlds.

Over-powering boats is nothing new and driven by market demand. Vega produced the Willard 36 from 1961 to 1970. Bill Garden spec'd a 75hp engine which in my opinion is perfect. But customers wanted bigger so a remarkable number were built with much larger engines, even variants of 6.71 DDs of over 200hp. Eventually, the Perkins 6.354 (135 HP) was offered which was a popular upgrade.

Many buyers just want bigger. Heck, even on this list of knowledgeable owners, many people extoll "you can always go slower." True, but few stay within the sweet spot of efficiency - 1.1-1.15 SL.

Peter

I agree that plenty of people think more power = better. And that leads to some FD boats being overpowered. But in reality, many SD boats are just underpowered planing hulls in my mind. It's one thing to intentionally power it for displacement speeds only, but once you go beyond that, it only makes sense to power it to plane. Powering it to run at plowing speed is a waste (assuming it's a hull that plows and transitions onto plane rather than smoothly running above hull speed). If you're going to give it enough power to get much over hull speed, give it enough to plane.

I also think that there are far more hulls called "SD" than actual SD hulls out there. A typical lobsterboat would be SD in my mind, but many boats that I see called "SD", including my own, are really just slow planing hulls. They may not handle well much north of 20 kts, may not have some of the hull features of faster planing hulls (like lifting strakes), etc. but they very distinctly go through the plowing speed range and then break onto a clean plane, so I consider them a planing hull. Some are just too underpowered to actually get onto plane (or can only just barely do it at WOT).

For our boat with a 33.5' waterline, we typically cruise at 6.5 - 6.7 kts, which is right around 1.15 S/L. The steepening of the power/speed curve beyond that is very noticeable, and I'm sure it'll be more noticeable once I figure out why my new fuel flow meters aren't reading accurately at lower flows. I did make an interesting discovery yesterday that when running on plane (where the flow meters seem to work perfectly), running at 3300 RPM for a bit over 18 kts is slightly more efficient than our more typical planing cruise of 3200 RPM and just about 17 kts. And even powered to run those speeds, I'd say we're borderline underpowered. Not in terms of total power (WOT is good for ~25 kts), but continuous power available, as 3300 is pretty much max continuous based on the RPM we see at WOT, so there's not much between "best fast cruise" and "max continuous".

 

[RickyD]

Good discussion! We also have a boat where the decision was made for us in the design phase. 2x120hp Lehmans don't get our CHB41 up on a plane, so it's either 2nmpg at 8.5kts or some number much less than that and a tremendous racket at 10kts.

One thing missing (maybe) in the discussion: I like the simplicity of my small NA I-6 engines. No turbos, after coolers, pretty cheap to keep going. We're looking at upgrading to something in the 55' range, and some of the engines on the bigger boats are a bit terrifying to contemplate. Some of the big MAN 800hp engines require rebuilds every 7-8 calendar years at a reported cost of $25k/side. @ranger58sb and others, I'm curious if this factors in? An example would be the Hatteras LRC 58 versus their 58' MY or YF. One has the tiny 120-150hp NA inline 4/6-71's, the other might have 8v71s, 12v71s, or the bigger 92-series. There's certainly an advantage to the smaller engines in cost, reliability & engine room access. That's in addition to the marginal gains in efficiency at 7-9kts for the smaller engines.

Hey Socalrider I love my Californian 55. Its 33 years old and only the stb engine has been rebuilt due to a serious overheat from a prior owner. No they don't make them anymore but some survive.

 

[ksanders]

Wait, the bolded you said 2nmpg @ 7.5 knots, that should be 7.5/2=3.75 GPH gallons but then you say 1.75 GPH. more confused. The 1.75 seemed like a good comparison.

Good morning Steve

I get between 1.75 and 2.0 NMPG total fuel burn at hull speed.

Sorry for any confusion.

 

[socalrider]

Hey Socalrider I love my Californian 55. Its 33 years old and only the stb engine has been rebuilt due to a serious overheat from a prior owner. No they don't make them anymore but some survive.

Cool boat! I've seen several around here & the islands but never been aboard one. How do you run it, and what kind of fuel burn do you get?

 

[ksanders]

I agree that plenty of people think more power = better. And that leads to some FD boats being overpowered. But in reality, many SD boats are just underpowered planing hulls in my mind. It's one thing to intentionally power it for displacement speeds only, but once you go beyond that, it only makes sense to power it to plane. Powering it to run at plowing speed is a waste (assuming it's a hull that plows and transitions onto plane rather than smoothly running above hull speed). If you're going to give it enough power to get much over hull speed, give it enough to plane.

You are entirely correct.

a SD boat is a planing hull with too small engines to really efficiently plane.
They do generally include a keel , something you do not need on a planing hull boat.
This makes directional control better at hull speeds than a boat optimized for only high speed travel.

My boat is a perfect example. With twin 330's i can cruise at only 15 knots, hardly fast.

But the ability to double my hull speed has proven invaluable, and the ability to travel reasonably efficient at hull speed is important.

right now I am making way at 1250 RPM, and 7.3 knots. I'm in no hurry.

 

[Marco Flamingo]

Powering it to run at plowing speed is a waste (assuming it's a hull that plows and transitions onto plane rather than smoothly running above hull speed). If you're going to give it enough power to get much over hull speed, give it enough to plane.

So that's it. They think that they are planing.

When we came through Active Pass last summer, we timed it so that the current had changed to be flowing with us. On AIS, we could see that a lot of other boats had been waiting at the entrance. We finally entered with a 7-8 knot favorable current and were surprised to see on AIS a vessel entering from the other direction at 5 knots. Pretty soon around the bend comes a big boat "on plane" and throwing out a 5 foot wake that really knocked everybody around. Apparently, if a big boat hogs down but then has enough power to lower the bow a little bit, that is what they consider "on plane." Nope. Maybe it was an emergency for them. It could be that they were late for a beginning navigation seminar.

The boat has such a long name that it couldn't appear in full on our AIS. All we could see was Yacht Knot S. My wife thought the full name was probably Yacht Knot Sea. We use that as a generic name for all similar vessels "on plane."

 

[rslifkin]

I've never understood why so many people run vessels up on plane poorly. Either too slow and plowing, or with the bow trimmed way too high. It's not fast or efficient, so there's very few operational reasons to run like that (and certainly not in calm water). Much better to just speed up more. And when we're passing near someone on plane I'll typically sacrifice a little speed and trim the bow flatter than best speed trim (conditions permitting) to flatten our wake as much as possible (which is already not nearly as bad as the monstrous walls of water some hulls produce).

 

[SteveK]

Good morning Steve

I get between 1.75 and 2.0 NMPG total fuel burn at hull speed.

Sorry for any confusion.

Kevin,
Your boat is the closest for my comparison. Your engines are 330, mine 250
I just filled my tanks to the brim, filled the same over one year ago.
Both engines 154.3 hours running, the GEN, the diesel heater on many occasion and the fuel replaced is 1574 litres or 416.4 US gal. My math 1574/154.3=10.20 litres per hour or 2.69 US gal per hour at an average speed of 7 knots.
That is 1.35 gph per engine which includes GEN and Heater.

I don't understand why everyone is using much more fuel than I am at the same speeds. Is my math wrong?
On a recent cruise I started at 2200 rpm and noted SOG 10Kn, dropped 100 rpm, noted SOG until 1900 RPM and the SOG was 1/2 knot less at 9.5Kn but that was when the engines seemed to sound like they were at a non screaming sound. Currents ignored. Usually try for a cruise speed of 8 which I can get at ~1650RPM. Now have 9-9.5 option