Fuel Economy – MV Simple Life

I spent the morning enjoying my coffee and checking the weather.

Winds are Still Honking

The winds off the coast are still blowing. A quick check on the Marine Zone forecast and I see that the winds have not abated and are driving the seas into the 6′-9′ range. Times like this if it were not for the ICW I’d be trapped in port.

Wifi Encryption – Known Exploits and Keeping things Private

I use the in-slip time to wash the salt off the boat and modify the encryption settings on my router. Somehow I have accidentally set my internal WiFi to WPA/WPA2 and WPA (Version 1) has a known exploit when it comes to TPIK (Temporal Key Integrity Protocol)

My iPhone is smart enough to alert me that the Wifi network I connected to supports WPA which is not secure

It’s 11AM and my time at the dock has run out. I cast my lines off and pull away from the dock with a bit more throttle than usual as the wind is blowing hard on my beam. Over to fuel dock to fuel up before heading south towards my intended destination of Titusville, FL.

New Smyrna Beach

As I pass an anchorage in New Smyrna Beach I pass several anchored sailboats. New Smyrna Beach is in Volusia County. According to the International Shark Attack File maintained by the University of Florida, in 2007, Volusia County had more confirmed sharkbites than any other region in the world.

Then I come upon SV Da Capo. That’s quite a pink hull!

Big Motor Yachts Look Comfy

Soon this large, comfortable-looking, motor yacht was passing me. Hatteras I believe?

Dredge Islands?

As I make my way through Haulover Canal there is a thin strip of land that is lined with small trees. This kind of natural geography seems odd to me but I encounter it often. I have always believed that you see these thin continuous strips of land along-side canals which are frequently dredged to keep a minimum center channel depth. I have always imagined that it must be far cheaper for the dredge to push the dredged material through a short outflow pipe that starts to build a new island alongside the channel. I have no specific knowledge or experience with dredging so If you do, please comment on this.

I watch as the sun sets.

Burn More Diesel or Arrive at Night?

In this moment I start doing some quick calculations and realize that it’ll be dark before I reach Titusville, FL where I plan to anchor. Luckily this section of the ICW is pretty open and easy to navigate at night. I find my anchorage and let out about 70′ of anchor which is plenty for the 6′ of water I’m in. Coming in in the dark you don’t expect many ICW boats to arrive after you. The winds are forecast to be steady in their direction through the night. I won’t swing around and there is plenty of room between me and my neighbors anchored next to me.

Refined Tastes of Yacht Living

I’m tired and figure I should finish off my fanciest bag of wine before hitting the rack 😉

Today’s route looked something like this…

Random Thoughts

This is a new section that I thought I would add since being ADD & extraverted I often have random thoughts bouncing around in my head that I feel a need to talk about. Today’s thought is for any fellow boaters reading my blog.

Diesel Efficiency

Today’s thought was on Diesel efficiency. Before owning my 1st trawler, I owned three sailboats. As a sailboat owner, I never thought about fuel economy. Now diesel fill ups can cost as much as $1000 rather than $100. This, because my trawler style motor yacht by design has large diesel tanks for long-range cruising. It’s tempting to discuss the days that the wind was pushing my sailboat along for free but this is not about sailboats vs. trawlers. Instead, I’d like to talk about trawler diesel engine power and hull shape. I’m not an expert or this subject it’s just something I have some thoughts on.

Slow or Fast?

There are two kinds of trawlers.

Slow full-displacement
Fast semi-displacement

Every boat’s hull displaces it’s weight in water. The more stuff you put into your boat it sinks a little lower in the water. Ancient Greek mathematician and inventor Archimedes first screamed “Eureka! Eureka!” when he discovered this. This is known as Archimedes’ principle.

To push the hull through the water you must push the water in front of the boat down as well as part it to the sides. When it comes to the force required to move a hull forward, hull shape matters. It should seem obvious as you try to push a boat forward quickly that a slender hull with a narrow angled bow and stern will separate the water gently and then put the water back together gently without causing much swirling drag forces at the stern. A hull with a blunt bow and square stern will plow into the water at the bow and have it come crashing back together behind the stern causing eddys or swirling motions that cause the boat to be pulled backwards by drag forces.

Full Displacement Hulls

Slow trawlers are full displacement hulls and their maximum hull speed is easily calculated by this equation and the length of their waterline (the length from when the hull first splits the water to where it recombines it).

v_{{hull}}\approx 1.34\times {\sqrt {L_{{WL}}}}

Full displacement hull shapes tend to be very round or semi-hemispherical and create little drag. They require a smaller force to push them through the water. However this round hull shape overs little resistance to healing (rocking of the boat). So you often find these hulls using ballast or weight added to the bottom of the hull to keep them upright. Since 1933 fin stabilizers such as Wesmar have been used to stabilize boats. The most modern solutions are active gyro stabilizers such as SeaKeeper. Fin stabilizers require power to move them and create drag in the water. Gyro stabilizers require running a diesel generator to keep them spinning. Full displacement may be the most economical hull shape but stabilization often requires diesel burn.

If you have a full displacement hull you only need a Diesel engine with enough HP to push it along at near it’s hull speed. If you put a larger HP diesel in the boat you will destroy your fuel economy and never go faster than hull speed. Full displacement trawlers will give you the best fuel economy due to tiny fuel sipping diesels. Full displacement trawlers with active fin stabilizers and large fuel tanks can cross oceans. Nordhavn is a popular trawler brand that comes to mind in this moment.

Semi Displacement Hulls

Semi-displacement hull shapes are designed to push water not just aside but down at a rapid rate. The water being pushed down forces the hull up out of the water. Less hull in the water means less drag and you can exceed the max hull speed for the length of your hull’s waterline. This extra downward push of the water means you need to put more HP behind the diesel. You are trading fuel economy for speed. When the waves are not exceeding large, speed can get you out of the weather and to a safe port sooner. You can also move at speeds below your hull speed and be almost as fuel efficient as a full displacement hull. Semi-displacement trawlers will have sharp angles called “chines” in the hull shape that help to create active resistance to healing. Think of a basket ball with a child’s action figure on top of it floating in your bathtub. Your basketball hull form is semi-hemispherical. The round basketball offers no resistance to heal and will likely just roll over with the first wave. Now place a box with square 90 chines at the edges of it’s hull in the bathtub. That same hull shape resists leaning to either side because to do so would displace less water on the side lifting up and more water on the side being pushed down into the water. Recall from Archimedes’ principle that displacing water creates an upward buoyancy force.

Fuel Burn per HP

A rule of thumb often used is that modern diesels burn about 1GPH for each 20HP produced. This number is a rough estimate of how much of your diesel engine’s power (force) you are using to push your trawler’s hull form through the water at the speed you are going. You can double check this number against your diesel engine’s HP/RPM curve.

Slow or Fast Religion

What type of boat you choose is often like choosing a religion. Once you decide you often find yourself defending your decision with great fervor. Debates rage on but I’m just trying to discuss just the basics of the science behind the debate. Often I hear other boaters with much larger boats with twin diesels compared to my single diesel tell me they get twice the fuel economy as me. Hyperbole or magic? I often state that my Cummins Mercruiser Smart Craft display of fuel burn is 2 GPH at about 1400 RPM or 6.5kts of boat speed. This is a rough estimate of course since there are currents, wind forces, drag due to hull fouling, propeller diameter and pitch, etc. that come into play here.

Cummins Manufacturer Specs on my 2012 QSB 5.9L Diesel’s Fuel Consumption to RPM Curve

Propeller Curve (lower) is based on a typical fixed propeller demand curve using a 2.7 exponent. Propeller Shaft Power is approximately 3% less than rated crankshaft power after typical reverse/reduction gear losses and may vary depending on the type of gear or propulsion system used.

Final Thoughts

There is no perfect boat. Every boat is a compromise. The discussion above is only about the tradeoff between economy and speed. I gave up economy for speed when I went with a semi-displacement trawler. I’m OK with my choice. I understand why others chose full displacement trawlers. Chose your religion wisely!

Stay or Pull Out?

I was in Savannah. It was 6AM. My guests had departed. I had rented the slip till checkout at 11AM. I wanted so bad to spend my morning strolling around Savannah but the currents were already building against the back of the boat. When you are single handed, a strong current like that can pin you to the dock or worse send you smashing into it. Leaving early before the current got too strong was the right choice. As I pulled away from the dock, I reflected for a moment and thought… “I’ll be back”.

As I headed back down the Savannah River I chose a short cut that leads back to the ICW but further south. I had never taken this short cut and it turned out to be a wise move cutting maybe 40 minutes off my day’s leg.

Fuel Conservation and Distance Traveled

From this point on I would plan my routes to only 50NM a day. I would run at only 1400 RPM and burn around 2 gallons of diesel an hour or about $6/hr. Which is much better than the roughly $18/hr I had been running.

Leaving the throttle at this RPM means my speed will change as the currents are either with me 8 kts or against me 4 kts. Taking an average of 6 kts x 10 hours of daylight for these latitudes / season is 60NM in a single day.

Delays! Think about what slows you down.

Bad weather – Wind against you.
Bad luck hitting currents mostly against you.
Shallow areas where you must slow down to prevent running the boat aground at speed.
Late starts means not pulling up the anchor right at sunrise.
Time spent waiting for bridges to open and let you pass under.
No Wake Zones – Near marinas, anchored boats, homes with boats in the water, kayakers, paddle boarders, etc.
Arriving at your intended anchorage and it’s full meaning you must push on to the next anchorage. Sometimes this is over an hour away.

I have found that 50NM / day can at times mean arriving at your intended anchorage in the dark if you are not timely.

South of Savannah

One of the first spots you come to is Thunderbolt, GA. Here I pass the Hinkley Yacht Services and their docks. There are always pretty boats to be seen.

I pass many commercial fisherman.

Catching bait fish with small throwable nets

When I arrived at the Crescent River where I intended to anchor, the last bit of light was fading. I snapped a picture but I think the video captured more of its beauty.

Day Two in the Crescent River

I awoke in the AM and just decided I was not going to leave. I thought… why not spend a quiet day on anchor?

Early this morning a commercial fishing boat gently passed by me at around 4AM.

I was relaxing when I thought did I hear a knock on my hull? I opened the back door to find a fellow cruiser in his dinghy asking me if I wanted an shrimp from the local fish market that he was going to. I replied, thanks but I was not in the mood for shrimp (after all you have to cook those things). We chatted for a while about where he was going (Exumas, Bahamas) and he dashed away in his dinghy.

The tides run about 8 feet in these parts of Georgia. You must be careful when you anchor at high tide that six hours later you won’t swing into a shallow and ground yourself. I had circled my chosen anchor drop location to test the depths out to a radius of my intended anchor rode length + boat length to make sure. My notes about this location marked an old dock hidden beneath the waters so I made sure to anchor away from it. In the morning it was low tide and I saw that the location was not where the previous captain had stated but in fact was very close to where I anchored. I updated the hazard location in the database for future captains that might choose to anchor here.

Old Dock of sorts a distance off the shore and exposed at low tide

I use an anchor alarm app called “Anchor!” on my iPhone and it showed that while I move with the switching current I stayed parallel to the river bank. That is usually the case when you don’t have a strong crosswind.

Winter 03 Leg #16 had looked something like this…

Tag: Fuel Economy

Titusville by Dark