Notes from our recent trip

We got back last weekend from a two week trip out on the boat.  While the trip was a pleasure cruise, and we still only have the stock props on the motors, we still found the electric propulsion very useful a number of times.

On our way across the Puget Sound to the Bluewater Yachts rendezvous in Port Orchard, we had just exited the locks and headed SW at the Shilshole buoy.  Somehow the fuel line accidentally got kicked loose from the fuel tank and the main engine died.   In the course of trying to restart I ended up flooding the motor.   Knowing I needed to let it sit for 10 minutes and with the wind blowing us toward the shallows of West Point, I dropped the two rudders and cranked up the electric motors.  We cruised quietly along down to the West Point buoy where we were then able to restart the main motor and resume our 13 knot cruise to Port Orchard.

I found the electric propulsion quite handy throughout the trip when we wanted a little more push with the sails up but didn’t want to go to the trouble of lowering and starting the main engine.  A week or so later we were out for an afternoon sail with some friends in very light air.   There were a number of times I used the electric motors to supplement the sails when we needed to point higher, or in some way maneuver to avoid traffic in a way that would have been difficult under sail alone.

Coming back into the locks with just one of my sons aboard as crew, I found the electric motor very useful in supplementing the main motor.   Initially we were directed to the right side wall of the small locks.  As we got closed the lockmaster decided to squeeze us in the middle between two boats already on the walls.  There was only about 8’6″ of space between the two boats. We actually had to compress the fenders as we slid in.   Rather than try and creep into this small gap with lots of shifting of the main engine I simply used the electric motors to silently guide us cleanly into this tight spot.  Having the twin screws enhanced the handling of the boat as well.

Once I get the different props made for the electric system so we get 6 knots instead of 3.5 it is going to be an even great addition to the boat.

Some Rough Prop Calculations

We now have construction of a new set of props underway.  These should address the upper speed limit we encountered testing the prototype out on the water.

The logic goes like this.

The motors we have installed top out at 1200 rpm.  The props we have on the motors have a 4″ pitch, great for low speed grunt, but not top end speed.

• 1,200 x 4″ = 4,800″ per minute
• 4,800″ per minute x 60 minutes = 288,000″ per hour
• 288,000″ per hour / 12″ = 24,000′ per hour
• 24,000′ per hour / 5280′ per statute mile = 4.54 statute miles per hour
• 4.54 statute miles per hour / 1.15 = 3.95 Nautical miles per hour

With theses props on these motors we could expect to top out at 3.9 knots in the perfect world with no losses for slip, etc.  Given that we maintained just over a 3 knot average speed over the hour test going both with and into a strong wind, this proves out our gut instinct out on the water that our performance was limited by the low pitch props.

To prove this further we are engineering props with a 8″ pitch which should have a theoretical top speed of around 8 knots.  With all the real world factors such as slip allowed for this should get us to our desired top end of around 6 knots which is just under the boats calculated hull speed of 6.45 knots.

A Dramatic Efficiency Increase

After crunching some numbers from our first trip on the water, the efficiency increase for “Allegro” is quite amazing.

We start by rounding down to 3 mph for the average cruising speed achieved with our inefficient stock props.  Cruising at this speed required 60 amps of 12v power.  The 12v generator we have paired with this system produces 80 amps of power from it’s Honda 2.8 HP GX100 motor, more than enough to power both motors.  Producing this amount of power it runs for 4 hours on .2 gallons of fuel.  At 3 mph for 4 hours we would cover 12 miles on .2 gallons of fuel.  On one gallon of fuel we can run for 20 hours covering 60 miles.  With the 24 gallons on board we have a range of 1,440 miles.

This is a staggering increase in efficiency over the current gas outboard.  At 3 mph, just above idle speed it gets just 8 miles per gallon for a range of 192 miles.

As we add our custom designed higher pitch feathering props to the prototype we expect to raise top end speed closer to 5 knots yet still stay under the 80 amps produced by the 12v generator.  This will increase our range even further.

Success On The Water

We spent this afternoon on the water testing the first prototype RudderProps! We left the dock around 1:30pm under gas power and made our way across the east channel to Mercer Island. There is a 10 knot wind out of the NNW so we are looking for as much shelter as possible along the east side of the island.

Once in the lee, we tilted the Tohatsu out of the water and lowered the two rudder props. They have off the shelf fixed props on for the first test. Starting with the starboard RudderProp we brought the electric direct drive motor up to full speed. At 2.9 mph we were using just over 30 amps of power. Adding the port RudderProp brought the speed up to 3.7 mph and just under 60 amps of load. Speeds with and into the wind varied by 1/2 a knot. The motors had no problem moving our high windage boat into the stiff wind. Once we fit our custom props, we expect to easily hit the 5.75 mph target speed. The 60 amp load is well within the 80 amp capacity of our 12v on board generator and will allow us tremendous extended range in hybrid mode.

As we cruised around the lake we must have been a curious sight. Our mast was down in it’s trailering position, no sails were flying, yet our outboard was tilted up clear of the water and we were silently cruising around on this gorgeous sunny day. On board, the only sound was a very faint hum as the RudderProp motors went about their business. The loudest sound on board was the gentle splash of the water at the transom as the boat sliced through the lake. Now under sail or power, cruising aboard Allegro is a peaceful tranquil experience.

In the hour we cruised around the lake we covered 3.68 miles under electric power. The motors performed great in both forward and reverse. Docking was easy even in the strong cross wind with the two widely spaced motors.

The next task is to build the custom feathering props that allow us to use a higher pitch blade for higher top end speed on our high torque electric motors.

Spinning Props

Electric motors are turning aboard “Allegro” under boat power alone. The motor mounting on modified rudders is complete and they are back on the boat. All the internal wiring for the motor controllers is done and a dry run produced oh so quiet sounds. Next up, Lake Washington!

Winter Weather

For the first time since last December temperatures are below freezing in Western Washington. This has delayed our launch and testing of the prototype boat, but we expect to be on the water next weekend.

Prototype One

It’s motor install day.  I’ve cut off the tips of my rudders and after running the motor power cables up inside them I am fabricating the mounting brackets.

The first prototype will use lower powered 12v motors as that fits all the battery bank, charging, and monitoring equipment currently installed on the boat. Subsequent ones will be 24v or 36v.

During phase 1, we will use stock off the shelf props which have a much lower pitch than is optimum.

Phase 2 will add our custom designed higher pitch feathering props which will give us a performance increase as well as feather when sailing.