Help with calculation for electric catamaran hull: speed, length, with, depth

Ah, I thought 0.56 was the el-mech efficiency. It seems it includes the prop efficiency. So that's a bit more optimistic, if you have 1120w max propulsive power. And then you have a 32cm propeller instead of the listed 19cm, that may make it even better I would guess.

Edit: It means we don't know the brake power but we can set, in Hullcalc, prop efficiency to 0.56, and then 'Effective Power' will represent the propulsive power, and 'Brake Power' becomes electrical power.

 

Hi Jon.
I have some problems with Michlet/Godzilla, it crashes frequently.
But I did start to optimize four boats for minimum drag. They are all limited to 8m max length, 250kg and are optimized at 10 knots or rather 5 m/s. All of them have two hulls. The input file vary only by weight distribution, plus one of them has fixed cross section shape.

The program is free to play with overall width, stagger, length, draft, beam, and three shape parameters: Waterline, cross section and rocker shapes. The three shapes go from rectangular, via elliptical to parabolic.

This was a super interesting program. It is possible to use more than three shape parameters. It can optimize for several, weighted, speeds.

So that looks like about a horsepower (750w) propulsive/1340w electric power for ten knots then.

I figure the optimum overall width is a bit much for you though.

All the best, Sigurd

 

Thank you, Sigurd. Your data looks promising! I have attached a large image to illustrate the shape of the hulls. This might make it easier to visualise what i have in minds. Easy build is important.

 

The drawing has the hull centerlines closer together than efficient cats usually are. The distance between the hulls is an item of interest because of the interaction of the waves forms from one hull to the opposite one. In the worst case, which I suspect this one is, the waves converge at some point that is most unfavorable for the propulsion unit. If the motor was a lot farther aft the prop interference might be mitigated somewhat. Or... if it was mounted toward the bow in tractor or pusher configuration, it might be a more promising layout.

The graphic that I mentioned, in an earlier post, shows that wider centerline distances is the better choice insofar as performance is concerned. As a land transport consideration a narrow layout is more desirable. This is the natural order of things. When messing with boats you can not have it all.

 

Nice drawing. I'll run a couple more optimizations with WOA limits closer to it.

When it comes to what way is easier to build, you'll have to decide - if it is fair, smooth, stiff and light it will be fast. Keep checking out how others have done similar hulls. Flexing panels are no good. So if you do end up building box sections, there should be more frames and or thicker skin than rounded sections.

 

Woa

Here is a run of optimizations. You'll note the only difference on the two left ones is the WOA is not restricted in the bottom one. Doesn't seem to make a lot of difference at the optimization speed of 5 m/s.

Edit: I found this paper with existing rowing hulls' displacement/length ratios, that Leo Lazauskas put in another thread.

 

Tested a 6 meter foam hull today, with design weight 150 kg. This boat was made mostly for fun, to learn - and try out different setups. I know the hulls are too narrow!

Must lower the engine and add a transom in center. Mercury 3.3 hp (did not went well..!) - 7,8 knots (14,4 km/t) with lots of ventilation/cavitation.

Short video Clip here.

 

Dressing them with glass, wouldn't be a very heavy construction method I would think. My 100 litre sailboard at ~6 kg, including hard points, is probably made that way. When I have tried fairing much smaller foam hulls, the boat just got smaller and smaller without getting both fair and symmetric at the same time. Need a bit more practice.
Thanks for the visual documentation
I can send you the offsets from Godzilla, if you end up building something it can model and optimize.

messabout:

Can you perhaps expand a bit on this subject? I'm a bit unsure how the waves would affect the outboard, and thus how to find a good placement.

 

When underway, look at where the wake converges, you don't want the prop in that area.
I think your beam is too narrow for asym. hulls, they make more wake between the hulls.
You could try cutting the foam on the inside of the hulls to make that side straight.
Than will stop the converging wake on the between the hulls.

 

Jim, what do you mean it is too narrow for asym. hulls, and then suggest planing the inside of them? Typo?
Do you have any reason to think the effect of narrow spacing is worse than Godzilla showed a couple posts back?

Maybe someone is able to predict where the 'wake converges' (do you mean by wake, the bow wave, or any of the wave tops probably?), as a guide where not to put the motor?
I am curious as to why it shouldn't be there also.

 

I think Jim means asymmetric in a way that inner topside is curved and outside is straight(er) as this is the typical asymmetric setup.
btw did you check the wave resistance of your godzilla created hull at different speeds? sometimes the resistance can be ok at one speed but terrible if you slow down a bit for example.

 

kerosene, did you look at the last image I posted (post #36)? It answers your question. I.E. you can see what Godzilla thinks is the difference between an 8m max beam (ends up at 3.86m) and a 1.6m max beam (ends up at 1.6m) - the lines cover each other at design speed (5 m/s). It is a bit worse at slower speed, but I have not looked at what happens with optimization at multiple speeds yet. I'm going to do that once I can get my computer working, and when Jon has decided what construction technique he wants to use. I think many of you can be a much better help than I can in that regard.

This is what Jim said:

I don't recall this thread touching on asymmetric hulls, I don't think that is what Jon has in mind, maybe I misremember. But Jim says the cat is too narrow for asymmetric hulls, but then he goes on to suggest asymmetric hulls anyway, in the opposite way that you suggest.

According to Leo Lazauskas (author of Michlet and Godzilla), laterally asymm. hulls can decrease resistance only if the hull spacing is suboptimal to begin with.
From that statement we can deduce that the gain of asymmetry can be no more than the difference in resistance between the narrow and the wide cat, which is shown in my above mentioned graph.
My suggestion is to forget about asymmetric hulls unless you have some way to design them that you think will be better than what Godzilla suggests, because G. can't work with hull asymmetry.
So my stand here is based on risk vs. reward - I have no way to predict the performance of laterally asymmetric hulls, they could be worse or better, and if they are better, they are not much better.

 

Can you explain to me how the wake waves affect the propulsion unit? I can't find anything about it.

If it is bad, why not slide the motor to the side? Nothing converging there.
In my T, which was about 3m wide, the motor was on the aft beam, near the starboard hull. This had three positive effects.
1) I could sit forward on the hull (drier than the net), and steer with a long stick.
2) The hull flattened the water for the motor: seemed nice at least.
3) The motor followed the sea waves better - along with the hull - than it would have done in the middle.

If it would have been in front, I think it would have ventilated often - I think the surface is less in sync with the hulls there. Especially if I'd have put it so far forward as to regain any steering.

 

Hi Sigurd,
Here's a paper that mentiones the issue, asymmetric cats though. There is a "sweet spot" in the wave energy at given speed, where some of the kinetic energy could be converted to thrust.

Jarmo

 

1. Where the wake converges there is air and water going in every direction you can think of, remember he said the motor was surging so the prop was not getting a good bite.Look at that mound of water behind a power boat as an example.
2. Since he has foam hulls, the bow sections on the inside can easily be filled in and extended slightly to reduce or eliminate the bow wave on the in side to cut down or eliminate the wave converging.
3. sorry I meant symmetric hulls that push water to both sides of the each hull.