Help me design the "Poorqeedo", another efficient electric boat

The prop efficiency drops off quite a lot with such low pitch to diameter. There is much more blade area being pushed through the water than needed.

For the loads you need a 14" prop is probably on the limit. I doubt that you can get 4" pitch in these. If you can then the attached gives an idea of the efficiency.

You can make efficient reductions with belts. I have driven one of my pedal boats at 6kts with a motor 40mm diameter and 40mm long with a 12:1 belt reduction.

 

The two linked videos will give you an idea of how a little motor goes with a gear reduction:
http://www.boatdesign.net/gallery/data/500/V11JE_10kph.wmv
In the end of the clip the boat was doing around 6kts. The force to achieve this on this hull is about 40N.

This one shows a bit more of the prop and motor:
http://www.boatdesign.net/gallery/data/500/V11JE_Drive.wmv
The motor is a little black Turnigy outrunner. These little motors are not suited for long-term reliable operation but they are quite efficient. There is a two stage belt reduction giving 12:1 overall. The did rob some power but they can be set up better to have very low losses. A reduction of 4:1 should cost no more than 2%.

Something to be aware of is that using thrust to provide steering moment is not particular effective compared with a rudder once moving. Hence the main advantages of two motors are that you have some installed redundancy and ability to turn tightly at zero speed.

A single motor the size of an Etek could be directly coupled to a suitable prop. These with a good controller will give better than 80% overall efficiency from battery to motor shaft.

 

Keeping draft to a minimum reduces steering force, but it is most effective when reduced at the stems which requires rocker; deep draft midships has little effect on steering effort. Rounded chines also help.

A long, flat bottomed, unrockered boat with hard chines is going to be harder to steer than a shorter one with rocker. That describes the canoe I built a couple of years back which has only a little rocker and is a bear to turn, although it is a delight in a straight line. Increasing flare at the stems would help reduce steering effort somewhat but will reduce waterline length.

A low speed boat may not generate enough rudder force to turn satisfactorily when approaching a dock or avoiding a swimmer, and will not have an outboard. However, a steering nozzle or Kitchen rudder might help considerably, and could be retro-fitted if the steering is too ungainly.

Basically I am trying to alert M-boy to trade-offs that might prove disadvantageous in his quest for speed and low drag.

 

I'm definitely not going to be making my own props any time soon. If you haven't noticed I'm going to be applying the KISS philosophy quite liberally with this boat =)

The motors are brushed, but their rated rpm is 2630 rpm at 100v, not 7200. I'll be running them at 2000 or less at 72v.

 

I recently built a stationary bicycle that displays your pedaling effort in horsepower. It only reads up to 1 hp, but I've found I can produce 1hp for a few seconds at a time. I took it to the Maker Faire where I met a BMX rider who claimed could produce 1.5 HP on a Dyno (needless to say he had little trouble hitting the 1 HP mark on my machine).

So for sprints I imagine you could put 500w into an APC prop for a couple of minutes. I read somewhere Lance Armstrong has produced 500w for 1/2 hour during the tour de france, but he's not exactly human.

 

Jeremy

Yaw, is a simple concept. A vessel running flat calm water with no disturbances, experiences no yaw. When the vessel is running into head seas, also experiences no yaw. When the vessel is running in seas/waves which are oblique, ie. at some angle to the vessel, this shall introduce tranverse forces on the hull, which in turn introduce moments tending to produce a ‘yaw motion’. Thus, yaw only occurs in oblique seas; assuming the vessel to be in equilibrium. (Or after any other external influence that causes the vessel to alter its course/path, see below)

You’re now talking about directional stability. (If the vessel is directionally “unstable”, this too can induce a yaw motion).
There are 4 categories of directional stability.
A vessel on a straight course experiences a disturbance, the effects are:
1) The final path/course after the disturbance remains on a straight course, but not in its original direction.
2) The final path/course after the disturbance, after some ‘oscillations’, retains not only the straight line course of the original, but also its direction. But that actual path may be different, ie parallel.
3) This is similar to 2), but does not oscillate, just passes ‘smoothly’ to the final path/course, again parallel, to the original path.
4) This is where the final path/course, after the disturbance, is the same as the original, as is the transverse direction. (In other words exactly the same, as if no disturbance).

Category 1, is termed straight line stability, and case 4 is terms positional motion stability.

Thus, the directional stability, the moment acting on the vessel and appendages, after a disturbance, must oppose any yaw caused by the said disturbance, ie the resultant force must act aft of the centre of lateral resistance. (or some call neutral point).

As a general guide, vessels with large skegs aft (and rudders) with well rounded forefoot will tend to be more directionally stable than a vessel with these characteristics. Similarly, long slender vessels are more likely to be more directionally stable than short, tubby forms.

As a rough guide to the neutral point, it is approximately 1/6th of the Lwl, from the bow. Which is why stern rudders are better than bow rudders, owing to its greater moment.

So, in profile view, the underwater shape is important. More ‘area’ aft, versus fwd, is better.

As a side issue, I wont bother with all the long equations of motion, however, in the “moments” and those specifically for the ‘rate of rotation’, Nr, this is negative when at slower speeds, when at higher speeds, becomes positive.

 

Super nice looking catamaran Motivator!

I noticed the '86 license plate tag, sounds like the pedal cat is coming up on it's 25th anniversary. Congratulations!

Even though we're both working on electric catamarans, it sounds like we have very different projects. I'm guessing you're interested in exploring the efficiency/performance envelope of electric catamarans, while i'm more interested in balancing efficiency with utility and ease of construction. Hopefully some of this information will overlap =)

 

Thanks again for running the prop analyses.

Looks like I'll lose about 12.5% efficiency by switching to a 14x4 prop with direct drive even with belt efficiency considered. In an earlier post I thought the cogged belt would be 90% efficient, but looking at gates website it's 97% (essentially the same as the 98% you stated) I think I'll just suck it up and build the reduction drive and use my 16x16 props.

APC does make large props with 4" pitch, but they're "slow flyer" type with fat blades and they don't look very robust. The 6 inch pitch props come in sizes up to 17 inches:

http://www.apcprop.com/ProductDetails.asp?ProductCode=LP17060

 

Do you have better drawings of this hull? Perhaps something I can put into the hulls program, make the hull a little taller and get a dxf drawing of the flattened panels?

I could also recreate the hull from the offsets.

 

A 3D dxf file is attached.

I like the flared shape for a number of reasons but the drag will not be much different to a rectangular section with the same waterplane. The development of the side panels is simpler if you have a square section.

The gradual curve is easier to build fair than straight side with tight turn to bow and stern sections.

I would also add a bit more rocker to the bow starting further aft than what is there now. This gives a bit of lift in the bow and makes the bottom panel stiffer.

If you modify the shape it does not take very long to recheck the drag on the final shape if you can provide an iges file.

 

I can get a bit over 600W on a dyno. This is not possible for me on a pedal boat because there is no flywheel to carry through the dead spot. I can hit just over 18kph for a few seconds, which requires around 500W. Heavyweight Olympic rowers can get up to about 600W for the 5 minutes it takes to do 2000m.

For long range cruising/racing I design for 120 to 130W.

The 16X16 APC props should work OK with your modest loads.

Think about the aerodynamics of the bridge deck. With good hydrodynamic hull shape the windage ends up a greater proportion of the total drag. 10 to 15kts winds are quite common so a big slab fronted bridge deck will have a lot of drag.

 

Efficiencies that high require carefully optimized test conditions and are most unlikely to be experienced in actual use. Such claims are in the same category as gas mileage claims from auto manufacturers, which can only be reproduced with sky-high tire pressures and special driving techniques. I suspect the testers have separated the losses in the gearbox bearings and lubricant viscosity so only the mesh losses are being reported.

A real, single stage spur gear or chain and sprocket in good condition, carefully run-in, clean, well lubricated and lightly loaded might have an efficiency of about 90%, measured shaft to shaft, a bit lower for a bevel gear.

 

Common Goals

Hi Mental,
Thanks for the compliment!! The truth is the I, too, am trying to come up with a somewhat efficient, yet practical Electric Catamaran. As you could tell from my Pedal Cat pics, this was no Light-weight craft, but instead a rugged and capable boat. You also noted my use of what everyone considers a very non-efficient propeller....still this boat performed quite well. I have attached some pics (If it works this time) of my last Electric Cat using the hulls from the Pedal cat, that again demonstrates a functional boat. I'm just trying to get help with an optimum hull shape with a drive system that will still allow me to navigate shallow waters.

I'm going to restore the bridge deck of this boat and fabricate the new hulls and power system in this second attempt.

I also included a picture from a later project I called Stealth Cat.
Regards,
John

 

Ad Hoc,

Thanks, but I was thinking of the yaw moment input from the steering system and it's effect on turning the hull.

Ancient Kayaker,

Measured losses on my 5M HTD belt reduction show that, with the correct belt tension and choice of pulleys, losses tend to be relatively non-linear with regard to power transmitted, rather than a % of transmitted power. There is a fixed loss due to belt flex and tooth friction that is fairly modest (typical measured values on my own system show a typical no-load, maximum rpm, power absorption of around 3 to 4 watts). There is also a small incremental power loss with increasing torque, but even at torque loadings close to the maximum for this size of belt these losses are smaller than the fixed loss.

At the maximum power that my system is capable of (around 280 watts) my measured belt power loss is just under 5 watts. This gives a transmission efficiency at maximum power of slightly better than 98%. At cruise power of around 100 watts the transmission efficiency is a little worse, due to the fixed loss element of about 4 watts, giving an efficiency figure of 96%.

I know of quite a few people who have used HTD-type belt drives on EVs and experienced similar levels of transmission efficiency, so this isn't a one-off fluke. Overall I think that the claim by the belt manufacturers is a fairly reasonable estimate of real-world efficiency.

A clean, well-oiled chain is slightly more efficient than a toothed belt, according to all of the loss and efficiency measurements that the cycling community have undertaken over the years, although dirt, wear and lack of lubrication will quickly cause a chain drive to become less efficient, whereas a belt will tend to stay close to optimum for longer, with no maintenance.

Jeremy

 

So I built a prototype, making a few modifications to what we've been talking about.

Only problem is I cant hit the 5 kts we were shooting for. Anyone know what I'm doing wrong?