Boxy, simple ~8 m electric powercat

Any extra load just slows it down. I have not done much analysis of performance under various load states. I expect adding a couple of people and gear would reduce speed from 8 to 7kts at 900W on the motor. For single day outings you would start with full batteries so you can afford to run at higher power.

I tested the outboard idea and it is OK but I can do better with my curved shaft idea. These work nicely and have very low drag. Current thoughts are to mount the motor just to the starboard side of the central hull near the central bulkhead. A curved shaft would run through the cabin floor to a lifting strut with thrust capacity mounted off the transom. The prop would be mounted just behind the transom. This allows the prop to be lifted clear of the water for transport. This gets the motor close to the batteries and out of the weather. Also can be accessed easily.

If you have not seen the curved shaft idea the attached photo gives an indication. Again I have not gone into design detail on the shaft so cannot give sizes but I do not expect it to be too difficult. Getting the best material could be a challenge though.

The limitation with the outboard is setting up the gearing to get high thrust. With the curved shaft I can run a 0.5m diameter prop with 6:1 reduction from the motor. This gives me very good thrust capability if required.

Rick W

 

Right. Think I can visualise that. So, main reason for going curved shaft is the higher drag from the drive leg?

Have seen it. Very cool! Particularly the strutless version. As for the shaft, have you looked at carbon fibre, as in e.g. golf club shafts? Seems it is possible to design for various properties of flex and torsional rigidity.

Why can't you do that just as well with a drive leg?

Cheers

 

When you look at the size of box to swing a big prop reliably it is quite large and has a lot more drag than a simple shaft.

Carbon fibre is just about the opposite of what is required for a curved shaft. Ideally the shaft has good flexibility and good strength. CF is too stiff. I am think of doing a trial with fibreglass wrapped spring steel. The glass would be wrapped to give good torsional rigidity but low bending rigidity. A well bonded wrap would overcome the corrosion problem with spring steel. At present I just paint the shaft and this eliminates corrosion in fresh water but any damage to the paint results in rust spots. Glass would be much more durable and would add torque capacity without much extra weight. My preference for spring steel is that I have done quite extensive testing with it and know its limits.

You can put a reduction at the top of the outboard leg but you are still stuck with transmitting the torque through the box carry the prop. This requires quite large gears that have to be pushed through the water. Also you then have two reductions that are each taking power from the motor.

Rick W

 

Attached shows a bit more evolution including the thoughts on the prop and curved shaft.

Rick W

 

Thanx for all the info!

Looks very intriguing. Have you scrapped the idea of a wind turbine? Weight reasons?

Cabin seems spacious and certainly adequate for comfort and function. What about windage/aerodynamic drag? Just got me thinking of the dutch barges where the pilothouse can be lowered to clear bridges, or the Volkswagen Westphalia vans with the harmonica roofs. When cruising along, one might be willing to compromise on the full height cabin. Probably too much complexity, weight and design/shape restrictions though.

Cheers

 

Hi ASM,

1. I will look for that thread.

Edit: Found it http://www.boatdesign.net/forums/bo...heap-build-electro-plywood-cruiser-22408.html

2. Yup, I am also interested in an electric version. Living directly by the water, I would like to move beyond kayaking and rowing, and build some more advanced form of HPB and/or EPB. Specs are still fluid and evolving, but coming pretty much in line with what is being discussed here.

Cheers

 

The wind turbine would pay for its weight. I can build it using a Mars motor with a total estimated weight of 20kg. I have not determined a location for it yet. Also I have not built a turbine yet. It is my next project. I have been sorting out a problem with my second Kelly controller. It will be used in regenerative mode.

I have considered windage. I am working on a Cd of 0.2 but I do not have anything to determine this other than reference to other shapes. The area behind the cabin will contribute most drag.

Rick W

 

What about sidewind sensitivity? With 2 m high sides and the outrigger just over 1 m from the centerline, could there not be quite a momentum pressing one outrigger into the water? Just curious how such issues might affect a lightweight tri-faux design (assuming it must be reasonably carefully balanced laterally to perform optimally).

Cheers

 

The central hull is not inherently stable so at light load the boat will flop a tiny amount onto one of the outriggers. The drag from this is not great because it does not take much force at 1.2m to produce a decent moment and the CoG will not be much off centre.

Like any boat it will need to be trimmed for conditions. A side wind will certainly increase the load on the leeward outer hull as the Cd for this area will be up around 0.8 I expect.

If it can be built for the weight target it will not be a good boat in a gale. It would need a sea anchor off the bow to stop it from being lifted and blown over. My intention is to be in a sheltered anchorage in such conditions.

Rick W

 

Lifting roof

KalleA

lifting roof is not such a bad idea. I have seen this on an aluminium planing vessel, which had 1.4 headroom in 'cruising' mode, and hydraulically driven roof for full headroom while at rest. This also enhance visability during cruising over the top of the roof. I would imagine, a canvas version like you see on campervans or caravans does not wheigh that much... you loose some weight from cutting down the roofheight and add something with the canvas and a manually powered roof much like a sunroof of a car ? THe solar panels need to be lighter though, what about those flex panels ? Yes the are not that good in effieciency as the normal fixed ones, but weight is important here too !

 

The side projection is not great for windage on the 8m boat but I have the vision of easily surviving a complete roll. A lifting top would be something for protected waters but it does offer some advantage for windage. What I have sketched only has full headroom over the central hull.

I am still working on getting lightweight panels but I expect there will be a price premium. The Sunpower cells are not ridiculously heavy. They each weigh 14kg so 4 comes in at 56kg. It will affect stability but also add some roll inertia. The batteries and motor are low in the hull.

I do not have much weight allowance to play with things like hydraulic lid lifters.

Rick W

 

I figure that a carbon structure w non-operable lexan windows (and possibly a simple hinged sunroof/hatch) would be pretty light. It could slide up-and-down with spring assistance to counterbalance the weight, requiring only manual pushing-pulling on the roof to operate it - with the right spring rates, this should require very little force. It ought to be possible to lock it in place in both positions in such a way that it still remains watertight and contributes well to overall hull stiffness and rigidity.

The solar panels are indeed quite heavy. Even the Solara (which I believe are similar to the Sunpower) panels work out to 8 kg per 100 rated Watts. The film panels seem to be MUCH less efficient per area unit. Actually, why is that?

I was considering the potential benefits of having detachable panels, i.e. you would have the option to store them in the bottom of the boat (whatever the naval term for that might be... keel perhaps?) when cruising, achieving a lower COG and opt to just charge when anchored. Along those lines, one could also consider having more panel capacity than will fit on the roof, like a "solar awning" on a light, foldable frame. I.e. as long as one has enough battery (and wind turbine) capacity to cover an acceptable range, one could compensate the solar charge loss during cruising with more capacity at anchor (which for many people would be of longer duration anyway). This might also offer more design freedom for the superstructure, with potential aerodynamic benefits.

Cheers

 

Another idea I was playing with was something like a typical yacht with a long low cabin with about 5ft of headroom and an aft cockpit with a windscreen and folding canopy. It would be more like a day boat with sleeping accommodation. The cabin top would be about 500mm lower than what I have sketched and gives a sleeker appearance. The solar panels would be on the cabin top.

I went away from this because I preferred the idea of piloting from inside a cabin. Full head room also appeals to me.

Rick W

 

To me too. Your solution with the lowered central walkway seems absolutely fine, and almost predicated by the hull shape. Very efficient space use.

It seems that your concept for the Faux-Tri ("FT") is very close to meeting all my requirements for a small, light and highly economic EPB for cruising the Baltic coastline and archipelagos. I wonder if you could shed some light on the potential consequences from where we might differ in spec:

1. Live load. With a range from 100 kg to 400 kg. From one person (me at 110 kg) to two people + two big dogs + luggage/provisions = approx. 400 kg. I assume this is a higher load and a greater variance in load than what you are designing for. Still, the cabin space of your design seems absolutely fine for our needs.

2. Seaworthyness/-kindlyness. My main captaining experience is driving onto a car ferry, and the missus is justifiably concerned about going anywhere with me in a little boat. Planning to avoid bad weather, but a bit of extra comfort for an unforeseen worst-case situation could certainly be traded against ultimate performance/economy.

3. Dimensions. Trailerability is always a plus, but not a necessity since we have direct water access. We could easily go up to 10-12 m, and I guess the added weight from this would be reasonably low, both in absolute terms and with respect to COG, if it were pure hull/bouyancy space w no changes in cabin space? The other criteria above probably mean a slightly heavier boat anyway. I would plan to go the carbon/lithium route, as you have set out.

Cheers

 

If you have some real interest then take the time to work out what you would want to carry. Things like shower and in-built fridge start to add weight and are usually basic requirements for women. I am past all this and like the comfort of 4+-star hotels. The boat is just a very relaxed means of getting places. If needed I can bunk up but it would be roughing it. I do not mind sleeping with a layer of salt on my body and actually prefer to do the washing up in a bucket in the well than wasting fresh water in a sink.

Once the weight starts to increase it makes sense to increase length if you still want efficient operation up around 8kts. The alternative is to accept a lower speed.

If you want to continue along this line then maybe start your own thread setting out the requirements with a total weight estimate of all the "luxury" items and live load.

Adding extra live load and increasing length will mean a heavier build and there is a point where carbon fibre becomes wasteful in terms of cost. If you have the room you add another solar panel to handle the extra weight rather than trimming weight by using expensive materials. My hull estimate of 300kg will not be easily achieved.

The costs begin to climb but if it is for family and friends with a long term view then it might be worth it. I only got back into boating a few years ago after my sons no longer took up my time travelling to sporting venues and their other pursuits. They were not particularly keen on boating. My wife did not like the light weight racer after the deep keeler so my last yacht became an expensive garden gnome that I had to mow the grass around.

Rick W