Project Solar Boat

What do you need a controller for? If you are looking for efficiency and top speed run it at its maximum. An on/off switch should be enough.

 

Agni motor specks

Notice the specks are quite different at 24V than at 12V, so it is necessary to know which voltage will be used:

http://agnimotors.com/95_Series_Performance_Graphs.pdf

Porta

Ps. Gonzo, the website indicates direct switching or through a resistor is possible. Would get you off the line faster from a standing start of zero. I ran troll motors overvolted with series-parallel switches for years without damage or need for a controller.

 

Vic
The boat prop that Bolly make is 15 X 25. With the high pitch it does not need to spin fast to get a decent speed. It is very strong and incredibly stiff for its size.

The amount of power in the sprint is getting beyond what the Bolly will deliver efficiently so there might be something else more suitable.

Rick

 

Vic
This is a very powerful motor with low winding resistance. You would never make a light weight drive train that could handle the DOL torque.

There is also the fact the the 90 minute efficiency run gets a lot more points so that it what needs to be the focus. The controller will drop 1 to 2% and keeps everything under control. It avoids snapping the neck of the pilot as well!

The rules require the boat to have controllable speed and you do not want to achieve that by manual switching.

Rick W

Rick

 

Rick, I agree it wouldn't be practical to use series-parallel switching for this racing situation. I meant to imply it worked well for me in SOME lower power situations.

Looking at the motor curves and the 55# battery limit, what do you think of using 3 -17 or 18 lb. , smaller AGMs in series? http://www.westmarine.com/webapp/wc...1?&cid=chanintel&ci_src=14110944&ci_sku=80161

At least for the drag racing part, the batteries only need to hold maybe 30 volts for probably under 30 seconds, or whatever the record is. If the rules allow, the batteries could be switched out for others with better reserve capacity (if needed) for the 90 minute run.

Porta

 

Vic
The motor and controller look well suited to 24V and this gives a good motor shaft speed for direct connection to the Bolly prop. This saves the losses of more complex transmission or less efficient prop.

For the sprint the current is starting to get high for cables that are not going to be significant in the total weight. Proximity of battery, controller and motor are a consideration in this regard.

They are permitted to use two different battery packs but they must be charged from solar power through the day. No external power. They are limited to 320W of solar.

I would be inclined to give the Bolly prop a go and see how it behaves at high power. If I was racing I would make my own prop. The most suitable marine prop I can find so far weighs 15lb. It is ridiculous for this application.

I have not looked at what a smallish outboard prop might do but it is worthwhile considering. One of these might need to be spun faster and it would make sense to use direct drive rather than gear up - this is where 36V would make sense. It opens up another option.

Rick W

 

Sorry Rick, when I refer to the Da Rules, I am referring to the rules of the Solar Cup Competition that dictate and control all aspects of design. As I read your posts I think you may have even found the website and read them yourself.

We can use no more than 24 volts, I just read there is a mandatory 350A fuse inline btwn the batteries and the motor. That being said, I am redirecting my search for batteries along the lines Porta mentioned for the sprint competition to batteries that can put out 350A to 400A for 30 seconds, but are very light. I would then switch to a higher Amp/hour rated deep cycle battery for the 90 minute endurance race.
(also, the Solar Panels are used only during the endurance competition)

Now I get to illustrate how little I know (again), a propeller 15x25 denotes what by what?
Last of all, is Bolly Prop no longer in business? The website said something about closing their doors.

 

Bob
You should treat the nominated CCA skeptically until you verify it. You also need to consider the power available; not just the current. 350A at 22V is much better than 350A at 16V. You need to test the actual batteries under heavy load conditions. A weight saving with them in a sprint is unlikely to be beneficial.

I scanned the rules briefly. I cannot recall the 24V limit.

I was mainly interested to find out the time frame. You do not have much time to engineer and build a good system. The design needs to be presented and reasons for selection understood. You and students will be working up a steep learning curve and you need to make sure whatever is done is within the rules.

Attached indicates how a curved shaft could be set up for a 10mm spring steel round bar. The prop shown is a little larger than the Bolly boat prop as it is one I had a drawing of.

The shaft support skeg needs sealed ball bearings to work well. The shaft tube needs a sealed ball bearing and lip seal. The shaft connections should be tapr-lok type wedging couplers and sleeves.

One of the issues that might be raised will be the critical speed of the shaft. My response to that is - Have you ever tried to play a stringed instrument underwater! However there is a limitation and that is that you should not take the motor to full rpm when the boat is out of the water. The shaft will flutter madly and may break the strut or shaft tube.

As a final note these competitions aim to encourage leadership and teamwork as well as the technical aspects. Good project management that considers every aspect of the scoring system gets the best results.

You need to give thought to project management. My suggestions are:
1. Appoint a project manager - someone with leadership skills, respected by the team members, able to delegate and include, ability to get others enthused about the project etc.
2. A quality manager - someone assigned to know the rules inside out and exactly how the scoring works.
3. A project planner - making sure the times can be met and people are prepared.
4. Technical area specialists such as boat, motor, batteries, solar panels, tansmission/propulsion.
5. A PR specialist - someone with the "gift of the gab" but also good at organising presentation material, organising presentations and getting others experienced presenting to diverse audiences. Being a good photographer or appoint a specialist photographer as project recorder.
6. Other roles as issues emerge that need a lot of attention.

It is primarily a learning exercise with a carrot and tangible result. It is certainly good to have the fastest and most efficient boat but also nice to win overall.

Rick W

 

I think you had better make a catamaran.
So you can get more Solar board and keep small drag.
http://www.solarnavigator.net/images/catamaran_model_into_wind.jpg

 

Rick, your outlook on this project and the importance you place on organization and division of the duties is something I am stressing with the kids. Some were a bit skeptical about being involved, as working with tools was not a strength or interest. However, when I explained positions were needed from project manager to media production and art work for the boat more kids found a talent they could bring to the team. We build the hull this saturday and getting to work on the rest right away. I inherited this project last year as a hull with a coat of epoxy that never set and we completed it in just over 2 months.
NOT fun and NOT to be repeated.
So back to Das Boot
How far below the boat is the tip of the propeller in this design? Is the prop you are suggesting 15" in diameter?
You recommend sealed shaft bearings. The shaft bearings I have seen and used with our aluminum shaft from last year are made of heavy plastic or teflon that are cooled and lubricated by the water.
As far as the batteries, we will be getting new batteries this year so They should be putting out pretty much peak volts and amps. With that considered, IMO saving 20 lbs takes us back to back to the basics of power and weight/drag.
With respect to design from motor to propeller, you would run the spring steel from the motor shaft through the stuffing box and shaft log to the strut? It would be able to handle this 12° bend from shaft log to HZ strut for the propeller? When you say shaft support skeg is this what I am calling the propeller strut?
How much more would the spring steel weigh v an aluminum shaft?
Here is a basic motor to shaft log design like the one we used and a strut / prop / rudder design so you can see what we are working with.
Thanks Again
Bob

 

SEE ABOVE.

Porta

 

Almost forgot, Jake Free is your man on props in the USA:

http://www.freeenterprises.net/HPBoats.specs.html

I think he still does custom builds.

Porta

 

Bob
That prop may actually be better than a Bolly for the high speed but the Bolly will definitely be more efficient for the efficiency run.

If you give me the pitch and diameter of the prop you have I will do some performance comparisons with the Bolly boat prop. It will give you an idea if further development is worth the trouble.

The efficiency of a propeller is related to the diameter and the aspect of the blades. The larger the area the blades sweep the less slip. The higher the aspect of the blades, the less induced drag.

The reason prop driven planes have long slender blades is to achieve high efficiency. In water, cavitation comes into play and it is the fundamental reason that drives the shape of water propeller blades. It shows its ugly presence as speed increases.

In your low speed case cavitation is not going to be a problem with a Bolly prop. However in the high power case it will be operating with cavitation. Given that you get more points for the efficiency part of it this is probably the best area to focus on.

Going for efficiency with a high aspect blades requires the prop to be aligned with flow to avoid high shaft loads and vibration. It is not so critical with a prop having wide, low aspect blades as your marine prop has.

The Bolly prop is 390mm in diameter and only needs to have about 20mm clearance to the hull. If it is too close to the surface it will ventillate when you accelerate. I have shown it deeper than necessary. The shaft strut does not need to be very heavy.

A spring steel curved shaft only needs to be 10mm in diameter to handle the torque. Having a very thin shaft like this and a thin strut reduces drag.

This video shows a boat I designed using one of my props running on a 1/4" spring steel shaft:
http://www.youtube.com/user/adventuresofgreg#p/u/12/OTITPVv_Rac
The prop is completely unsupported at the outboard end. It just hangs down unless rotating as you can guage from this photo:
http://www.boatdesign.net/forums/at...d1247576346-prop-shaft-systems-picture-14.png

Rick W

 

Vic/Bob
I just had a look at the Bolly site and they are in administration - bummer.

I do have a Bolly boat prop but I am not inclined to part with it.

There may be a distributor in the USA still holding them.

The APC prop will be OK for the efficiency run. Will just cavitate and eventually break if you try to push 6kW into it.

I can do a design for fabricating but you need someone with good welding skills. Attached shows one of my better efforts.

Rick

 

Here's some other ideas from a fast HPB:

http://www.openwatercycling.com/images2/20040219/INDEX.HTM

V