costs of electrifying a C-Dory 22

T

Tom Hruby

Member
Bob with Thataway has asked me, on another thread, for some cost numbers for the electric conversion I am doing on a C-Dory 22 cruiser. Since this has nothing to do fixing balsa core, I decided to start a new topic.

My initial estimate is that the cruising weight of the 22 ft cruiser will be around 4000 lbs (myself, my wife, and all our gear). Cruising weight and waterline length are the two factors needed for calculating power needed and the speed that can be achieved.

My first expectation is that we will be traveling at "displacement" speeds so we can use the numbers for displacement hulls. Assuming the hull is balanced I estimate the waterline length is about 19 ft. This puts the hull speed at 6.7 mph (5.8 knots). I have adapted and equation from David Gerr's Propeller Handbook and it calculates that one needs 1500 watts/1000 lbs of displacement to reach hulls speed, 800 watts/1000lbs for 80% hull speed, and 200 watts/1000lbs for 50% hull speed. These estimates have been validated in my previous boats. However, to reach planing speeds, which seem to be around 11-12mph, I would need 12 kwatts; calculated using both the displacement equation and the planing equation in Gerr's book.

Given the numbers above, I have decided to power the boat with 6 kW. I decided to go with two 3kW outboard motors (e-propulsion) rather than their one 6kW motor for redundancy even though it is about $2000 more expensive. This is equivalent to a 10-12 hp gasoline motor.

Cost of 2 E-propulsion Navy 3.0 + throttles, chargers, control cables and steering connector = $7000

Cost of batteries - this depends on how far, and how fast, one wants to travel. At present, lithium iron phosphate batteries that I would consider suitable for a boat (IP rating of 66 or higher), and that have a blue tooth connectivity are about $300-$500/kWhour.
At 80% hull speed (5.4 mph) I would need 3.2 kwatts of power which is 590 watthours per mile traveled.

I have purchased four 50V (48V nominal) 100AH batteries for a cost of $6000. This will give me a maximum range of about 33 miles at 5.4 mph or about 45 miles at 5 mph.
Thus, cost of batteries is about $150 per mile traveled for my case.

MY TOTAL COST FOR MOTORS AND BATTERIES IS $13,000. But NMI has reduced the price of my build by replacing the hydraulic steering with cable steering, eliminating the starting battery, fuel tanks, throttles. I have added the option of shore power so I can recharge batteries from the dock.

I hope this answers some of the questions out there! If not I will try to answer further questions.
Tom

P.S. If any is interested I have written a book about electrifying boats (Plug-in Boats: a primer for converting to electricity) that is available at Duckworks Boat Builders Supply https://duckworks.com/ .
 
On my 2008 CD-22 hull speed is 4.3 mph.

The rule of thumb thing for figuring hull speed doesn't really work for planing hulls. For displacement hulls, the hull speed is the speed where the bow wave wavelength is the same as the WL length of the hull. For planing hulls it is usually considered the speed just before the bow starts to lift (you can also see it in the boat wake).
 
Good point. You note that your hull speed is about 4.3 mph because the bow starts to climb the bow wave at that time. I have no problem believing that because the weight distribution is optimized to get the boat on a plane as soon as possible. The sooner the bow starts to climb the bow wave, the sooner it can get on a plane. As the bow climbs the bow wave the actual length of waterline is shortened and the hull speed is reduced.

An outboard powered C-Dory 22 has about 800 lbs of its 3400 or so pounds centered near the transom (~400 lbs for outboard, ~300 lbs for fuel, and 100 lbs for two lead acid batteries). This is almost ¼ of its weight centered at the very end of the boat. I don’t have the exact numbers for the 22, but let’s assume the center of gravity of the remaining 2600 lbs is about 10 ft from the bow. [I am assuming it is closer to the bow because of the location of the cabin]. If I did my math right, the center of gravity with the motors, gas, and batteries is about 16ft from the bow, or 6 ft from the stern. The center of gravity is the point around which the boat will pivot, so it is easy to understand why the bow starts rising when the thrust is applied to the stern, reducing the hull speed. The C-Dories are very light compared to other outboard boats of a similar size so the weight of the motors aft have a much bigger impact on slow speed performance.

For my electric dory I am proposing a different weight distribution; one that will keep the bow down much longer, and thus letting us get to a higher hull speed. The two electric outboards on the transom only weigh 110 lbs. The four batteries I have weigh a total of 360 lbs and they will be centered (initially) about 3ft from the transom in the cockpit. Instead of having 100 lbs of lead acid batteries I will only have a 30 lb LiFePO3 house battery (12V 100AH) in the battery compartment. Thus, I will only have an extra 500 lbs in the aft section and it will be centered at least 2 ft forward of gasoline powered versions. The batteries will not be permanently fixed in the beginning so I can move them around to get the optimum position.
My previous boats were St. Pierre Dories with a very flat bottom that had a significant rocker. The waterline at rest was only about 2 inches above the point where the stem attached to the bottom. However, at no time in my 26 years of cruising did I find the bow climbing over its bow wave. Of course, the center gravity was close to midship. I expect that I will have a similar success with the C-Dory if I arrange the center of gravity to be closer to the middle of the boat. The shape of the C-Dory bow is very similar to that of a Swampscott Dory, a very stable displacement hull.
 
Tom Hruby":2lk93oad said:
The shape of the C-Dory bow is very similar to that of a Swampscott Dory, a very stable displacement hull.
Click to expand...

Tom, I met a gentleman that built his own Swampscott Dory based on the design from 1900. He also had a Kitchen Rudder fabricated for it which is very unique. The topic thread can be seen here - http://www.c-brats.com/viewtopic.php?t= ... 0f1b3a45dd


If you scrool down through the comments I have included several links to the boat including a U tube video of him showing how usefull the Kitchen Rudder is manouvering in close quarters. He powered it with a 3hp inboard turning a constant 800rpm. It is a very beautiful 19' boat.

Rob

Pictures here -http://www.c-brats.com/modules.php?set_albumName=Swampscott-Dorie-circa-1900&op=modload&name=gallery&file=index&include=view_album.php
 
Thanks Tom,

On our ~46 foot Length Water Line, 65,000lb full displacement sailboat a 4.5 hp dinghy motor would push the boat at 2.5 knots, and a 25 hp dinghy motor the boat would run at a little over 5 knots. The 90 hp with 24" diameter prop would push at 9 knots /WOT, when the boat was digging a deep hole in the water--no climbing over its bow wave. Our average speed was just a little over 6 knots burning about 1 gallon an hour (or about 20 HP of available engine power.--24" prop had a lot to do with more efficient propulsion than with the small dingy motor props. We built a 38 foot/30 LWL with an 18 HP diesel driving a 16" diameter controllable pitch prop would cruise easily at about 6 knots--not a lot over that at WOT. Fuel burn about 1 gallon per hour. I also look at my experience using the Torqeedo 1103 as a kicker--it would push a CDory 22 at about 3 knots WOT.

On our ~46 foot Length Water Line full displacement sailboat a 4.5 hp dinghy motor would push the boat at 2.5 knots, and a 25 hp dinghy motor the boat would run at a little over 5 knots. The 90 hp with 24" diameter prop would push at 9 knots /WOT, when the boat was digging a deep hole in the water--no climbing over its bow wave. We built a 38 foot/30 LWL with an 18 HP diesel driving a 16" controlable pitch prop would cruise easily at about 6 knots--not a lot over that at WOT.

David Gerr NA in his excellent Propeller handbook points out that the formula for "hull speed" is rarely the 1.34 x Sq. Root of LWL although it is widely used. Here is an article related to David Gerr and hull speed

My point is that the 1.34 x Sq Rt. LWL is not a universal number and it will be considerably different influenced by many factors: including weight, waterline beam to Length ratio, hull form, etc. For example planing hull vs full displacement design. I suspect that your weight for the C Dory is a little low. It will be interesting to see what it is if you have the inclination to weigh it.
 
Given the importance of operating at hull speed, would the Tomcat, with twin near-displacement hulls be a better candidate for electrifying, albeit with somewhat larger motors?
 
I agree with Thataway that the hull speed equation is only an approximation. I have used Dave Gerr's book a lot and found that is equations and numbers are approximations, BUT most of the results when actual measurement are made with real boats are about +- 15% of the calculated values. I have not done the statistics, but it looks like +- 15% is close to one standard deviation from the mean and represent about 2/3 of all data points. Given that every boat will be unique we need some way to make estimates that we can use in planning.

Using Dave's equation for power and speed I have found that the data I collected for my St. Pierre Dory matches the power curve calculated from the equations almost perfectly (I will try to post the graph in my photo album).

The weight estimate I have been using is that provided in the C-Dory website (3400 lbs with motor and gas but not trailer). My motors and batteries will reduce that amount by about 300 lbs so that leaves 900 lbs for myself, my wife, and our gear when I use 4000 lbs as the basic travel weight.
 
Oops,
I also need to answer the question from Easy Rider.
Almost any boat can be electrified - they are even now building electric Washington State Ferries. The critical factor in deciding is how you plan to use your boat and how much are you willing to spend. If you are willing to travel at slower speeds (5-8 kn) then an electric conversion can be cost effective. The power requirements are based mostly on weight. By adapting Dave Gerr's equation I calculate that it takes 1500 watts to power a boat at hull speed for every 1000 lbs of weight but only 800 watts to reach 80% hull speed.

There is a lot of progress being made in electric outboards and boats. At the Seattle Boat Show there were a couple of electric boats that can travel at 30 knots for 100 miles but they are expensive. Outboards up to 100 kwatts are now also available.

Sam Devlin has built a 50' solar powered electric catamaran, so catamarans can be coverted.
 
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