Another Try at an Electric Catamaran

You may find some interesting insight on this thread:
http://www.boatdesign.net/forums/boat-design/pedal-powered-boats-23345-59.html

There is also some useful information on this thread:
http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-27.html

You could test some of the ideas on your existing boats so you appreciate the benefits.

 

You might want to consider the flex shaft concept coupled with much more efficient props than come with troll motor. Also outrunner motors are considerably more efficient than any off the self troll but do require gearing down. These considerations should more than double your range and speed. See McDenny's posts for data on using RC props, Jeremy's project, and Rick's postings on flex shaft and efficient props on electric boat forum.

Hope this helps.

Porta

 

What am I doing Right!

Hi Guys,
I spent most of the morning reading the Efficient Electric Boat thread. It sems to me that I am trying to achieve a very different compromise than how light can I make the boat and how small I can make the Power Drive. All that I have done in the past is with these easily driven Catamarans of considerable weight and payload, and have achieved very acceptable results. That's why I am confused by all of the Low efficiencies of all the Props you guys have been testing.

As I mentioned in another post, I have used some very simple tests to show efficiency. One case in point is my Tandem Pedalcat. I spent more time riding Solo than with a fellow rider. The fellow rider always made the efficiency better, as you can see in the pictures, because I offset the Cranks at 90 degrees from the other so that when 1 rider was in full power strike, the other was at Top-Dead-Center with little pedal input, which rounded out the crank revolution. It was easy to know when you were there by listening to the gear noise. When riding Solo, there was always a little pulsing, even with Toe-clips on the pedals.

I am wondering why I believe that I was getting such good Propeller efficiency from a standard Inboard type prop which I had a prop shop do a lot of blade thinning on. My prop was a 12" x !2' Pitch, what some boat people considered a Square Prop. My final drive ratio in 6th gear (I used 2 Bicycle chains and a Derailleur for the primary drive) was 1 pedal revolution to 12 turns of the Prop. Being a 12" Pitch prop, the theoretical, no loss movement through the water would be 1 foot per crank revolution. My simple test, that I did hundreds of times to check efficiency, was to find floating objects in the water, like Coconuts, and pass closley along side them, and measure 1 full crank by associating either a start point of straight ahead or straight down, and watch how far the coconut would travel in that 1 revolution. I had marks on the hull at intervals like 8', 9', 10', & 11', etc. I would consistantly get close to 11 feet of forward motion. This, I believe, translates to about 90% Prop Efficiency, or am I getting something wrong here. A control to this test was that I often took the boat out on very windy days of 20 to 25 MPH. On Downwind runs, even with the coconut being pushed by the wind, My windage pushed me even faster to where I approached the 12' mark. Add to this the fact that the hulls were designed for 2 riders, so I had to work harder for the wider hulls. My hulls, by the way, are 18' Waterline and 14" in beam on about 7' centers. Other tests done were timed travels to certain Islands in the Biscayne Bay in Miami. I used this to measure both my increasing fitness, and to judge boat speed. When fighting against those 20 to 25 MPH winds, I made those trips in an average of .6 to .7 of the time with no wind. Mind you that the boat had little windage and my elevated body was resposible for most of it.

Now I did not spend countless hours on formulas, but got good results from time against distance in real experience. We did not have access to GPS de vices 24 years ago, so reading charts and measuring distances from Channel Markers to others in many different areas was what I had to use for Speed over Ground measure. I would do the run both ways to average for current, etc. This does not say anything about power requirements, just Prop Losses. Could it be that at prop RPMs in the 1000 + range, these large blades do not allow much slippage because it is pushing a much lighter load than the Powerboat it was designed to? Remember that my Outdrive is only 1 1/2" in diameter and is faired right into that 1 1/2" hub on the prop, and my down tube is a well faired foil section at about 7/8" wide x about 4" long with a Cavitation Plate above so as to not draw air and cause Cavitation or Slippage. Could there be another side to this? I simply chose the prop for shallow water usage. Please don't make fun of my "Coconut Testing Method", because if there was any variance from test to test, it was only inches...not feet.

So, can anybody explain to me if I am right?

Regards,
John Vitsur

 

My view is that efficiency is really how good the system (boat, power system, propulsion system) is at converting input power to useful output.

Rick, I and others have spent a lot of time trying to improve efficiency of boats that are operating at displacement speed. There are some key facts (driven by laws of physics) that tend to drive us towards similar solutions:

1. Wetted area and waterline length dominate as the most significant form of resistance at displacement speeds. This makes long, thin, light monohulls the most easily driven, and hence efficient, hull form.

2. Accelerating a large mass of water to a low velocity is more efficient that accelerating a small mass of water to a high velocity. This makes large swept area (i.e. diameter) propellers turning slowly more efficient than small swept area propellers turning more quickly.

3. High aspect ratio propellers with the smallest practical number of blades are more efficient that either lower aspect ratio or multi-blade propellers. This is related to point 1 above, as a propeller with low wetted area will incur lower losses.

4. Power conversion and transmission losses can absorb considerable amounts of power at the low power levels associated with human powered vehicles. Anything that reduces friction in the drive system will improve overall efficiency.

5. The electrical losses in an electric propulsion system are generally lower than most of the other losses, so it's worth getting the best out of the areas outlined above before spending time and money on enhancing electrical efficiency.

Finally, you mention using two 105Ah batteries to get 30 miles range at 5 1/2 mph. You don't give the displacement or waterline length, but this is a fair bit higher than the sort of energy consumption I'd expect for this size of boat. For example, the sort of boat that Rick, myself and others are developing typically use around 70 to 120 watts to run at this sort of speed. Two 105Ah 12V batteries, assuming that they are lead acid types and only able to deliver 80% of their rated capacity as useful output, should be able to provide about 2000Wh or so. At 100W (the sort of power my two person river launch should need) these batteries would run the boat for about 20 hours, or a range of over 100 miles.

My guess, based on looking at the photos you've posted, is that the multihull design is giving you a higher resistance than optimum, the high air draft is giving you a fair bit of air resistance and the three blade, wide chord, propeller is costing you a lot in terms of wasted power from poor propeller efficiency.

I think you should be aiming to get at least 80 miles range from your battery pack, maybe more. Even sticking with your basic multihull design I think you could probably double the range with a bit of refinement to the propulsion system design.

Jeremy

 

John
Jeremy has provided some useful comment on efficiency.

Your 12" marine prop will give reasonable efficiency but it is not 90%. It is a relatively large diameter for the power level but the blade area is much more than you need. The slip which you determine to be about 10% is probably even high so by your reckoning the efficiency would better than 90%. The slip on most of my props is about 3% but they are not 97% efficient. Slip is only one part of the losses. As Jeremy pointed out you still need to force the blades through the water. The other main loss is induced drag due to water spilling around the blade tips. A marine prop is intended for much higher loading than you offer with pedal power.

It is not difficult to do accurate speed measurement these days. You can get a GPS for less than $100 new. No doubt they are cheaper second hand. Good data is the best basis for assessing modifications.

The main reason I build light pedal boats is to make them easy to carry. I aim for less than 20kg. It also makes them a bit faster. I can cruise for hours at 11kph in the V14 boat with my modest power output of 120W.

Getting back to your electric catamaran, it seems that you want to carry 4 people and the boat will be quite heavy. So say 500kg total displacement. The maximum length is 6m and maximum beam is 2.5m. Also it will be a dory type flat panel hull. What speed do you want to cruise at?

If you confirm the measurements and nominate a speed I will show you what the lowest drag hulls look like.

Whether you form a prop tunnel using a shallow transom does not make a lot of difference to performance but there might be reasons to do it.

 

Thanks Jeremy

You guys have split hairs to the infinite degree with your studies, and I am grateful to be able to absorb what I can. Again, I'm a Seat of the Pants type of guy. What I'd like to do is get all my postings into one place if possible, as having them in 2 locations means that 1/2 of what I have written is here, and the other half in the Multihull thread, and you and others are not getting all of the info.

Again, my efforts over the last 25 years as a hobby started with the Pedal Power Catamaran that I discussed above. My interest now is to enhance my Electric Catamaran with new Hulls that, if you will, can exceed common laws of waterline/displacement because of the narrow Beam/Length Ratio as my present 18' hulls are already doing very well with, and doing it in a very overweight condition.

My specs for the Electricat that I mentioned were realtime use reports, but I have yet to remark on some projected but not proven specs. For instance, my 30 mile range was not theoretical, but instead, actual trials. My estimation of range took it out to 50 miles based on battery energy remaining. Secondly, I did not keep accurate daily logs, as these were fun projects for me, but I was amused at what could be accomplished using Low-Tech available equipment. This is my testament to what I did without building my boats to a strict design weight.

For your information, my electricat with the batteries weighed close to 450 lbs., the Hulls are 18' on the waterline with a beam width of 14", a draft of about 7", and I usually went out on fishing trips with 2 persons (Average 380 lbs.) and equipment adding another 50+ lbs. (fishing gear, nets, 5 gal bait bucket, 80 qt. cooler with 20 lbs. of ice, Yes we caught a lot of fish too, etc) The boat also sported a very lavish slatted Teak Decking that offered up about 48 SF of Platform (not counting the decks of the hulls). So I am pushing a nearly 1000 lb craft with a 45lb thrust Trolling motor at 5 1/2 MPH and potentially going 50 miles with it. Not High-Tech...but not bad in my opinion. I am happy with the results that I have, and have enjoyed hundreds of trips in this boat without going beyond the range of this boat. I also like the fact that it costs about $0.25 cents to charge back up.

I am interested in better Props and Batteries along with the Hull improvements. In the next design, I intend to up the power to 2 X 107lbs. thrust (4.75 times the power) if needed. My question remains, how fast can this take me with new 20' hulls and the added weight of more batteries. Maybe I should back off all of this power and try just 107lbs. thrust and less weight.

I will try to assemble some specifications to my various projects and post them. Right now, if I could only post sizable pictures here, you would get my point. Apparently my scanner is saving my photos at about 24mb, and I can't figure how to get them small enough for this site to allow them to load without them being too small for evaluation. Again, help here would be nice.

Thanks again,
John

 

John
The attached file has cat hulls for flat panel construction close to the lowest drag you will get for 6kts displacing total of 400kg. The hulls will draft 180mm when loaded. The portion I have shown above the surface is not important for the drag calculation I have done.

The power on the hulls to achieve 6klts is 290W. The actual battery draw will be higher of course depending on the props, motors and controls. With the very best design you could expect about 400W. With three 105Ah batteries at 80% discharge you could get 45nm range in calm conditions.

To do 8kts would require 780W on the hulls. 4kts would require 95W on the hulls.

Of course if you reduce the weight then you will go further and the lowest drag hull shape will be a little different.

 

You probably have the scanner setting for bmp images. These are very large files. You should be able to select the format. jpg is the most efficient.

If you already have the images then you can import into powerpoint, crop any of the white edges then <Save As> jpg files. This will producer a folder with all the photos in jpg format.

 

Thanks again Rick

I'l try to get to the office tomorrow and re-scan some photos.
John

 

Hi Rick,
Thanks for the DXF Hull suggestion. If I am measuring properly, is the widest part of this hull at the .54 rule back from the onset of the waterline? That seems to have always been the rule for the Midship section of a sailing vessel from what I recall. Has anyone played with this form and found a better location? In one of my hulls, I moved it back to about the .60 position and it seemed fine for a narrow hull. Any comments here?
John

 

If you look through the poorqueede thread you will see I gave some analysis for the different boat configurations, hull shape and hull constraints for same speed and almost identical displacement to what you are after.

You will see that the lowest drag is a monohull of 9.7m. Going to a cat configuration have no length constraint incurs a 39% increase in drag. Applying a length constraint of 6m is less significant. Applying a shape constraint that allows flat panel construction has almost negligible impact on drag. I expect small changes in the location of the widest beam will not cause much increase in drag.

What I normally do is determine what is the lowest drag shape and configuration as I have done here with the 9.7m monohull. Then I look at how various factors increase the drag. I determine what is significant and what is not to get a starting shape as I have provided here. I then work on that shape to fair it out and to suit how I am going to build it. After I have something close to what I will build I go and recheck the drag to make sure my fiddling has not increased it much.

 

Thanks again Rick,
I have been following the thread on Poorquedo. My questions still remain as all I see suggested are Double-ended hull forms (Like what I did years ago). Are there any other ways to go, like with Wide & Full above WL transoms, or Fantail transoms like on Whitehalls, or partially submerged transoms, etc.? Also, if you would be so kind, could you run your analysis on the Hull lines that I posted earlier (The lower double ended shape with the Waterline Chine). I would like to know where my thinking is taking me...for better or for worse. I believe that my total displacement is still within the boundaries of the Hull that I have drawn, unless of course, I can use less power and conserve battery weight.

I'll post the hull lines again here.
John

 

Can you provide a 3D iges file in metric otherwise I have to spend time converting your 2D to 3D.

 

John
If it is going to take you more than a few hours to do the 3D then I can do it. It will take me about an hour to get it close to what you have drawn.

I have to travel for a few days next week so that will keep me busy then.

 

Hi Rick,
I did not want for you to have to spend that much time on it. If you have something that comes close to my Length, Width and Draft already calculated, then that would be enough to put me in the Ball Park. If it were my final design and I would not be just wasting your time, well then I'd be happy to hire your services for this. Thank you, though, for your consideration.
John