Another Try at an Electric Catamaran

John
The attached linesplan shows what I have modelled for your hull.

Two of these set 1.8m centreline spacing will require 550W at 6kts before allowing for any propulsion losses.

Dropping to 5kts power goes down to 294W.

 

Thank you so much, Rick, for doing this for me. Now that you have, where do you see the greatest deficiency in the design? Also, will there be much difference having these hulls at 2 meters apart as they will be used on my present bridge deck?

I have read a number of posts here, yours included, talking about lbs. thrust conversion. Has this been brought to any simple statement yet. In some other forums, I've heard it expressed as about 1 amp @12 VDC per Lb. of thrust. Using this, I was hoping to see what adding excessive power to the hulls will give me. My last boat made 5 knots using a 45 lb thrust stock 24 VDC Trolling Motor, and this would assume 540 watts, or 22.5 amps @ 24 volts. If I intend to use dual 107 lb motors that calculate to 2568 watts, what could be expected in speed, and would this power go too quickly to waste. The boat will be operated at these conditions only for short 10 - 15 minute trips, assuming I can go maybe 3 miles plus in this time period. In other words, how drastically does the Wave Drag go up in Hulls like these with 15 to 1 Length to Beam Ratio? The fattened Rear Chine in the design is there hoping to keep the hull from Squatting, and with the very long entry, I hope to further minimize the creation of the Bow Wave.

I have read about, and heard from a friend of mine, Bill Roberts, (who is a well known designer and races his very fast Catamarans) that they apply another factor to the square root WL principles as they pertain to Extremely Narrow hulls for different Lengh/Beam ratios, allowing for a substantial push of the Hull Speed. Do you have anything on that?

AnywaY, Thank you again for all of your help, and from you others out there.
John

 

Unless you want to have a tug-of-war with you trolling motor the thrust does not mean much. What counts is the power. With a large diameter low pitch prop you can generate a lot more thrust.

If you work through the Efficient Electric Boat thread you will find some comparisons of performance with different props on trolling motors.

I have attached the power curves for the various hulls that have been discussed here so far.

Slender hulls do not hit the wall like wide hulls but the power still rises rapidly with speed.

If you have lots of power you can get dynamic lift from most hulls. From what I have analysed a transom tends to pull the stern down. This increases the bow up trim and helps getting onto the plane. However with a 500kg slender boat that will take a lot of power - more than you will get from a couple of trolling motors. If you made a light planing hull fitted with lithium batteries you might get on the plane with a total of 4kW. I have planed at over 20mph with a 6HP outboard but the boat was very light and only 6ft long by 3ft wide.

 

Thanks once again for all of this. I'm certainly getting a better understanding of what to design to. Your JV6 hull sure has the good specs, and I am considering adding my reserve bouyancy, and re-drawing it for your review.

I am still trying to keep prop diameter small for my shallow water cacability. I did have a thought, though, while while utilizing a Faired-in to Hull Trolling motor. What do you think of, Say, using two 10" diameter airplane props on the same shaft, but having one aft of the other by some (to be determined) distance, and at 90 degrees to the front prop. Possibly the two props would be able to operate in a separate swept rotational area without interfering with each other based on their pitch??? Just a thought!
John

 

See notes above, sorry about caps.

Porta

 

The blade flexing on a 10" model plane prop will likely be significant. A 4-bladed prop or two 2-bladed halve the blade load so less flexing.

The prop slip is related to diameter. There is diminishing returns to increasing diameter but the bigger the mor efficient until you get to very large diameter.

If you go through the pedal boat thread you will see there have been a few efforts with folding props. These avoid some of the isuues of operating in shallow water using large diameter prop.

 

Thanks Rick and Porta,
From what I have gathered so far with the speed that I want to push these hulls, with my 36 volt power source, I believe a 10" Pitch prop is in order. I would probably have to go the Dual Prop mode, or try the Torqeedo prop that has 10" of pitch, but the 12" diameter is more than I want. I intend to run them on the Twin 107lb. or the Twin 85lb Trolling Motors. if you do not believe I need the extra power, then I'll stay with the 85lb ones. I will still want the 6 Batteries for the range, though I'm not sure how to hook them up. I looked at the videos on the Torqeedo site, and they had some of the boats moving surprisingly fast, though I know there is extreme inefficiency. Rick, in your theory, is it possible to push the JV6 hulls to 12 MPH with the 36 volt system using the Dual 10" Props? That figure was off of your chart. Short blasts at that speed would be a great plus for my boat. Again, any help here is much appreciated, as I want to get closer to my final design and start building. The only feature that I would like to modify on the JV6 hulls is to leave the full draft all the way back to the stern (or slightly beyond) in order to Fair-in the 4" Diameter Trolling motor body.

Porta,
Did you experiment with not only smaller diameter props, but different pitches that could maybe work harmoniously from Low output to High, or is this simply out of the question?

Regards,
John

 

The attached shows the prop condition for two motors on the JV6 hull at 12mph (statute). This is in calm conditions with no allowance for windage and appendage drag. You could expect about 10% higher power by the time these are allowed for.

The thrust rating of the motors only has a meaning if you are building a tug. You need to know the motor power if you want to determine if it is adequate. The smaller ones will probably do the job if the larger ones are over 2kW.

As you can see the thrust is 145N. This is a lot to expect from plastic model plane props. Ganging 2 to get a 4-bladed prop will reduce efficiency. You end up with more blade area than needed.

You could do all the set up with the model plane props and then make your own prop if you find the strength is not suitable for what you want to do.

There are some photos on the linked thread of hand fabricated props:
http://www.boatdesign.net/forums/props/seagull-prop-cat-racer-29910.html#post333047

Getting the basics right like diameter, pitch and blade area are more important than absolute perfection with the fabrication.

 

I reverse engineered the prop arrangement. I knew I would need short bursts of very high power for what I was doing but did not want to use two motors and associated weight and complexity. So I found the most efficient prop at 24V and the most efficient prop at 12v for the same motor. I could not get one prop and PWM at 24v to match the efficiency of this setup.


To have one prop go from low to high and stay at the highest efficiency would require a CVT type gearing and variable pitch and variable length of prop blade.

Porta

Porta,
Did you experiment with not only smaller diameter props, but different pitches that could maybe work harmoniously from Low output to High, or is this simply out of the question?

Regards,
John[/QUOTE]

 

If you need momentary shallow water capability, then a folding prop is the way to go. If you need extended shallow water capability, you will have to go with smaller props and higher rpm on the motor. I have the capability to lift the flex shaft with a small prop so that only half of it is immersed, reducing draft required. When run in this raised, surface piercing mode, the power and speed are reduced to a great degree.

 

Hi Guys,
After re-evaluating my hull project, I find that I am going to have to design the Hulls to displace 500kg total unless I can go 24 VDC and pull 2 batteries saving 55kg or so. What I really need to find out is what kind of power Trolling motors can put out, given a reasonably efficient Prop. The reason that I like the trolling motor concept is that the appendage drag will be little, with the motors being faired into the hulls, and in the design, it will be a simple un-screwing a nut off of a short steering tube above the waterline and pulling the wires from a terminal strip, to remove the motor for replacement or repair. Plus, you get Motor, Shaft support, Thrust Bearing, and Motor Cooling all in one. Has anyone played with improving the performance of these motors like Re-winding the armatures or fabricating better brushes to push the Voltage up? Also, does anyone actually know what the actual RPM of these motors really are? I am getting absolutely Zero help from Motorguide or Minn Kota Engineers as to RPM. Motorguide just gave me the size fuse to use.

Minn Kota did state the following numbers, though:
70 lb Thrust @ 24 volts X 42 Amps
80 lb Thrust @ 24 volts X 56 Amps
101 lb Thrust @ 36 volts X 46 Amps

Can these numbers give us anything else but the wattage draw?

So my dilemna now is determining the true available power from 2 trolling motors in order to continue with my design.

Thanks again to all for your inputs.
Regards,
John

 

See post #252 and a couple of pages surrounding. http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-17.html

Troll motors are designed for thrust ie. moving heavy bass boats at slow speeds. They were very cheap PM rear bushing type when I last disassembled one, several years ago. Outrunner motors with electronic commutation would be state of the art and reasonable in price if you want to cut battery weight. Li-ion batteries would be about 25% the weight of lead but are expensive. The 12v trolls run at around 1200 rpm, higher voltages usually more.

The more unblocked, undisturbed water in front of a prop, the better. So the front of a troll blocks off water and reduces efficiency. Doesn't matter in their intended use, since they are not designed for speed. I don't know about fairing them into hulls, possibly may drop efficiency. Perhaps the hull could be designed to somehow improve the streamflow to the props. Maybe Rick will chime in on this aspect.

Hope this helps.

Porta

 

Thanks Porta,
I'm not sure that a Faired in Trolling Motor will be much different than having the same efficient prop spinning behind the closing point of the Hull. Thanks for pointing me to #252 thread...I picked up a lot from that and some other leads from there. Now I am thinking of going more efficient and spending the extra effort to go with the Outrunners if there are some large enough to push me up to my hopeful 10 MPH+ speeds. I could go with a Straight Shaft Inboard concept with my Hull and have the motor up several feet from the stern, and possibly use a Rotary Ceramic Shaft Seal from a Pool type pump to seal out the water. My concern then is to keep the motor dry and properly cooled. The hulls would need to be airtight and I might have to look into Water-cooling the motors.

Unlike the Poorqeedo project, I am fortunately not limited to funds, but more now to time, as my business keeps me very busy. That's why I just wanted to know what a Trolling motor could do before moving to the next more complicated platform. I have a full Machine Shop with CNC mills and Heli-arc equipment, so I can get the job done. I'm going to be looking further into these Higher Tech options.
John

 

One good thing with these outboard electric motors is that they reduce a lot the mechanical losses. If you believe you can make a perfect shaftline arrangement, that's fine, but it seems hard to compete with the outboard solution.
The motor just in front of the screw is not a bad thing. The center part of a screw is inefficient anyway.

Concerning the post 252, the electronic control is important only if you don't want to use the full power of your motor, and I don't think it is anyones goal...

 

Thank you Eric for your concurring response.
I have held steadfast to the benefits of the Trolling Motor Base as a viable power source. I agree that the center area of any prop is simply Screwing it's way thru the water with little to do with Prop efficiency. This is why I feel that a Faired hull into the Trolling Motor eliminates the common consensus that even if these motors are for Thrust purposes only, their power can be utilized to some degree in a simplistic manner. My last boat had all of this appendage of a trolling motor body pushing through the water at about 5 knots, whereas I want the 2 new motors integral with the hulls, as this is ultimately the way that I go. I am presently trying to follow the path of the most efficient possible drive with respect to a practical propeller solution as I have been guided here by those helping me with my pursuit. Please keep the ideas coming.
John