Boxy, simple ~8 m electric powercat

Rick
You are absolutely right, I should start somewhere and focus on a hull design/configuration. I was about to make a scla emodel of the faux trimaran half a year back, remember... you made some Delftship flat panel surfaces and I actually got a quote to make the section from CNC plywood on scale for only ~25 EURO, to sacve time hehe and have accurate measures.

Do you think a speed around 15-20 knots is possible with a faux trimaran design, even if this would mean going to 9 m 's or so and stay with the 2.5 m width ? I have a fast hand sketch which I think would look awesome in faux tri and powered by electrics......... but it has speedy looks so just 8 knts would be silly...

Looking forward to you stepping onto the big things !

 

Mat
I posted this video on another thread. It takes a minute or two to download. It shows a Mars motor being run-up on no load off 24V:
http://www.boatdesign.net/forums/at...cal-motor-maths-25hp-8hp-mars_pmsm_run-up.wmv
I am pleased with the way they perform but the phase locked motor controller combination are more than what is needed for a simple boat speed control.

When you actually look at the power required for the faux-tri you could get away with a much smaller and lighter motor with the appropriate gearing. I am working on 5:1 reduction with the Mars motor running an 18 X 32 prop.

I have given up on the idea of the outboard and will use a curved shaft that allows me to easily raise the prop. It will not be a lot different to what I use on the pedal boats - just heavier. The shaft strut will take all the thrust load.

Rick W

 

The main thing for speed is to keep the hull light and central hull narrow. I optimised my hull for 8kts. However the Mars motor will push it up to 14kts off 48V in calm water. The hull would get fuller in the ends and narrower with higher optimised speed. Windage also becomes a bigger factor. At 14kts the wind drag on the 8m boat will cost over 200W.

There are limitations though. You cannot go very fast for very long. Also it will not go fast with VRLA batteries. You are stuck with expensive lithium. Building light requires expensive materials. (My current pedal boat will be the lightest yet and the materials will be the most expensive.)

I will use a particular NA for my detail design. He needs to confirm the weight target and has a knack of making things look better than I can. I admit I am not an artist or an architect. So my current sketch is for proportions above the waterline. The hull shape is firm if the weight target is met.

From a design perspective it is best to start with a weight estimate. Set up a table of all the things you want with a weight estimate. Once you have this I can tell you what your compromise are caused by making certain changes such as length and design speed.

The 8m version has an external area of 55sq.m counting the outriggers. There are no bulkheads or internal furniture in this. I figure panels around 2 to 3kg per sq.m on average will be close to the mark so a hull of about 300kg seems possible. This would be about half the weight of a ply hull.

You can see that adding things like showers and associated large water storage just kill this concept. You then start traveling down the track of the 250HP diesel.

Increasing the length must add weight. I started at 10m but felt I could build lighter if it was shorter. THe prospect of towing a 10m boat is also daunting. Even 8m will be challenging.

Rick W

 

The tri vs cat decision is affected by several factors, and I do not think there is one simple answer, thus they could both work successfully in different ways. Load is one factor, need for lack of wake is another, stability when moving bodies around the deck (hopefully of their own volition).

Rick is right in pointing out the need for fine hull shape - beam-length ratio is critical, almost as much as friction. Case in point - some years ago at the (then) annual solar boat race in Canberra, a few Sydney blokes had a lot of fun mounting a few panels and electric outboard on a small planing hull - they figured the hull looked quick, should go quick! Didn't of course. I think they had fun, but the low powered slender hulls won the day.

But hull speed is still limited by that damned fellow Froud, at displacement speed. If you look at the resistance curves, to get much beyond 10kn in an 8m hull you need an awful lot of energy. SOlar powered boats that want to go beyond hull speed tend to use foils - but then you still need a sudden burst of power to get up, and you cannot carry much weight.

A US uni challenged us in 2001 in Canberra, in a boat with foils. In the sprint they won hands down, but they could not sustain that, and were carrying just one crew in a 6m boat. For the endurance race they removed the foils and stayed in displacement mode. We had crap panels that 1st year so they beat us by a whisker. We got Solar Sailor panels next year, but the yanks wouldn't come back! (Come on guys, take us on!!)

I also played round with a scale model of an air injection hydroplane. The idea was to have a dead flat hydro hull with the step at about 25% way back from the bow. The flat hull from there back has skirts down each side to hold the air in, which is be pumped in to greatly reduce friction, all while carrying a load. The model worked, carrying its own battery, which was encouraging, but dynamic stability could not be tested.

For a PRACTICAL boat - that can carry passengers and some load - you need to satisfy Archimedes as well as Froud. This then brings in friction as well as hull form. A true tri with floats that that just skim or dip to provide dynamic stability will be faster than a cat of the same displacement. This i sthe solar race boat that we have.

But if your displacement is high enough to force all three hulls into the water, then I maintain a cat is faster. You cannot use sailing multis to inform this argument though - these tris should always lift the windward float. The other advantage of cats is the dynamic stability as a work/fishing platform. A faux tri has a lot of advantages in managing a seaway, but is that the kind of environment you want to operate the boat in?

So many questions, so many options, so little time!

Hey Rick - build a solar racer to the International Rule and come to Penrith Lakes in November - we want the competition!

 

I have one more Murray Marathon to attempt and then I will get serious with solar. Where is the International Rule? I need to see the rules. Some rules push designs in silly directions. When I was younger I thought sailing boats built to the IOR were the bees knees but it was a stupid rule for sailing pleasure craft. I admire designers like Joe Adams who had little time for the rule and just designed for performance and value.

By the way the 8m faux-tri I sketched will do 10kts with 1.5kW in calm conditions. So you do not need a huge amount of power if you can keep displacement low and the hull fine.

The idea is really a monohull with training wheels. I use the same principle on my pedal boats. I can push these 100kg boats to 10kts with 500W at the pedals and they are a mere 7.2m long not 8m. The art is setting the outriggers to just skim the surface so they provide some initial stability but offer little to no drag. The smoother you pedal the less rock and roll the boat undergoes.

Rick W

 

Rick,

Just curious, how does the cargo capacity on the 8 metre faux-tri look?

The reason I ask is that your project is a similar size and a closely related use profile to my next boat project ( http://www.boatdesign.net/forums/projects-proposals/playing-around-10-m-trailer-cruiser-17597.html ), the main difference being that I'd like to go somewhat faster and will need to carry a tonne, maybe 1.5 tonnes payload.

I've come to the conclusion that a power cat in this size range just doesn't work out if you want any kind of interior- the hulls are too narrow, and you end up with either a jazzed-up pontoon boat or a planing tunnel-hull. But for low-speed, efficient running like Rick, ASM and I seem to prefer, I get the impression that a folding-tri or faux-tri could carry a decent cabin, and still fit on a trailer.

8 m is not that bad to tow if you have a truck/van that can handle it, good trailer brakes, and a decent set of mirrors. I actually find 30' trailers somewhat easier to handle than 12-15' ones.

 

Mat
The 8m one as sketched is no longer a trimaran after 1.3t total displacement. At that stage the drag will rise rapidly.

If you want the capacity to carry over a tonne at some speed you would need to design for it. I expect with a length constraint of say 10m the cat might win out providing you could build a bridge structure that did not weigh a large proportion of the displacement.

The concept would need complete rework. I have been able to get the predicted performance by lowering the displacement. I started at 1.2t but I have seen what can be done with carbon fibre. Getting down to 500kg will only be possible by going to expensive material like carbon fibre for the hull and lithium for the batteries. I am proposing an 8m long by 2.4m wide voluminous hull with a weight of 300kg before fitout - no simple task. Going bigger to get load carrying makes everything bigger, heavier and it will cost heaps if electric powered.

If you are happy with IC outboard then it might be practical.

If you take 2T and 10m as the design condition Godzilla produces a hull that requires 21kW to do 20kts - I am a little skeptical of Michlet predictions at high speed on relatively short hulls because sinkage is potentially an issue - need Flotilla?. If it was going to have large variation in load the outriggers need to be shaped accordingly so the boat does not flop side-to-side when unloaded.

Rick W

 

Need Flotilla- yes, I've been asking Leo about that periodically, but last I checked (a couple of months ago) he wasn't ready to put it on the market yet. From what he's told us on here, it should be a valuable tool for faster multis (and more exotic forms) and I for one am eager to give it a try.

ASM's original post seemed to be directed at "boxy" craft, which I take to mean "developable from sheet goods". The Rick W faux-tri is, I gather, developable- it would be interesting to see how much of a drag penalty, if any, is encountered by using a developable shape in this case. Somehow I suspect it's not nearly as big an increase as one might think at first glance.

 

I wonder if he will sell it? Maybe I can get a deal if I promote it. Michlet/Godzilla is brilliant.

I have done the following numbers.

1. The optimum 600kg craft for 8kts requires 579W and is 12m long.
2. With length constrained to 10m the optimum 600kg for 8kts requires 623W.
3. With length constrained to 8m the power goes up to 754W.
4. Reasoning that 8m can be built lighter than 10m, a 500kg boat for 8kts requires 633W.
5. The hard chine version I have drawn requires 641W at 8kts. So the penalty for a hard chine is 8W or 1.2%. (Note I do not force a complete hard chine in Godzilla.) I constrain the section shape but not to slab sides. I then produce the hard chine in Delftship with the hull sides off vertical. I re-check the result in Michlet.

My new pedal boat is hard chined for flat panel construction. It suffers a similar penalty but it can be built lighter with flat panel and this more than offsets the hard chine. The result is about 1.5% less power for the same speed.

Some people get really bent when you suggest a hard chine does not cost much in power because everyone knows that a half cylinder will give greatest volume for wetted surface but this is only part of the equation. When waves are included in the analysis the narrower hull resulting from a hard chine gains and is not far behind the optimum.

Rick W

 

"..Some people get really bent when you suggest a hard chine does not cost much in power because everyone knows that a half cylinder will give greatest volume for wetted surface but this is only part of the equation. When waves are included in the analysis the narrower hull resulting from a hard chine gains and is not far behind the optimum..."

As with everything in naval architecture it is all about understanding the application from the theory.

In this case, Froude number plays the key in understanding into which hull shape for what speed etc, as also noted above.

 

Rick

The International Solar rule is generally understood to be...

- let's start another thread - this thread has another intent. And maybe it will flush out a few more interested parties.

bad dog

 

Rick,

very interesting stuff! Is there a dedicated faux-tri thread that I have missed? If not, maybe one could be started?

You also had an interesting concept for a long-and-narrow "Coastal Cruiser" with bolt-on bow and stern, is the Faux-Tri the preferred solution?

Cheers!

 

Hi KalleA,

The faux tri was a result from a mono/tri idea started in a thread called::
Cheap to run, cheap to build electro plywood cruiser ? which I started.... it was back in august 2008, I do not know how to put a link to that thread in here.. sorry. Are you interested in a electric version as well ?

 

The faux-tri provides a better package for a trailerable size boat. You can have a very slender hull that has a trailerable maximum width superstructure. ASM set the idea in my head in the thread he noted and the actual underwater parts are similar to the hulls I use on my pedal boats.

The long slender hull needs a certain minimum width to be stable and to provide useful accommodation. With these constraints it requires more power than the faux-tri to achieve my target of 8kts. The faux-tri as sketched will get 14kts with 5kW while the long slender but stable single hull only gets 10kts with 5kW.

The comparisons are not direct because I have assumed considerable weight reduction by going for more exotic materials and components. At this stage I do not even know if the weight target is achievable.

Rick W

 

I gather from the earlier posts that you presently assume a hull weight of 300 kg and a fitted out weight of 500 kg incl. one person.

How will you deal with a higher live load, say a 2nd passenger and some luggage? Could the outriggers be height adjustable, or might this add unwanted weight and complexity? Variable ballast seems obviously counterproductive to the whole light weight concept. It strikes me that allowing for a wideish range of live loads becomes quite difficult with an optimised ultra-light boat.

The Coastal Cruiser had a removable/serviceable drive leg/prop, I believe. This seemed like a good way to handle shallow waters and beaching - lift up the drive leg and go with the wind turbine as an air prop. Presumably, something similar will be difficult to do with a curved shaft?

Cheers