efficient 10m displacement powercat (build thread)

Who is??? )

Ok, do you have a graph of your speed v power, that you did way way back. And by power I mean EHP (naked)..that is just the hull alone?

Let's start with that for now.

 

Nice. So tell us more about the "overdrive" props. Did they end up giving you more top speed? Or did your RPM actually come down even further at a given speed?

I seem to remember us having a discussion about electric VS petrol dive compressors with me saying I wanted an electric and you saying you would probably go with petrol. Funny how now you have an electric and I am about to order a petrol Bauer PE. $6500 ouch!

But obviously the plan will be to convert it to electric eventually, but I am not buying any more lead acid batteries so I can't run electric now. Tesla has announced they will halve the price of their lithium batteries in about 6 months. I predict affordability with in a year or so as everyone else follows suit and many buyers create more demand.

What is the brand and rating of your VFD and inverter and how much current does running the compressor draw from your DC side? Do you have your compressor mounted permanently inside? The petrol will be a PITA to operate thanks to having to take it out and listen to a lawnmower for a few hours.

 

this is the output log from michlet,
first column is speed in m/s and the second is resistance in kN for ship total inc interference drag and transom drag approximation.

1.000000, 0.045034
1.678947, 0.142346
2.357895, 0.321980
3.036842, 0.679367
3.715789, 1.102163
4.394737, 1.570487
5.073684, 1.863388
5.752632, 2.075050
6.431579, 2.297151
7.110526, 2.535977
7.789474, 2.792448
8.468421, 3.067005
9.147368, 3.358480
9.826316, 3.666990
10.505263, 3.992948
11.184211, 4.334618
11.863158, 4.693969
12.542105, 5.071025
13.221053, 5.465755
13.900000, 5.873217



attached a pic of the graph also. This is only for water drag. No air drag is added in at this point and neither is appendage drag of the outboard legs. 13.9m/s is the maximum speed we acheived in clam water, no wind.

Dennis;

The compressor VFD is 2.2kw or 3hp - same as the 3 phase motor that turns the PE-100 compressor. Theres dozens on the market, for this size they should run about $300-$400. At full speed of 50hz, it pulls about 10amps on the 240v supply. This would translate into around 200amps on the DC side, plus 10% inverter losses so it would actually be around 220 amps. I have 50amps input from the solar, plus another 80amps from the outboard motors, so the batteries should only be discharging at rate of 220-50-80=90amps per hour. Lithium batteries would do this without breaking a sweat, not sure how long theyre gonna last or how hot the AGM`S i have will get at this rate, time will tell and perhaps i will simply give them a rest between tank fills which only take about 15mins per tank if you leave 50bar in them.

The overdrive props havnt given me any extra speed, ive got the same speed but at a heavier displacement now. the RPMs vs speed stayed almost identical, so the props are obviously slipping a lot less compared with the old 3 bladers considering they are now 20% less pitch... all should translate into better efficiency - which i have already noticed as improved...

 

Thanks Groper. Seems like the new props were worth it. Did you get better hole shot? I assume single engine running would also have become much more efficient. Have you played with that much for trolling etc?

What brand and model is your inverter? And what about the inductor?

I too get over 50A from solar, and have one 40A alt and another 100A. My inverter cant handle 200A for long without shutting down. Its only rated to 1.5KW continuous so will need an upgrade too (especially with the high starting currents). In reality there would also be filling at night sometimes when I have a full boat of divers. My 540ah AGM bank of used telstra and Chinese AGM would not like it even with engine running. (nor would the alt).

Good to know it only takes 15min to refill. I assume 12L tanks? I will be getting the same compressor but the petrol version is slightly faster thanks to higher RPM. Is the noise from the electric disturbing?

 

excellent..and at what displacement is this estimate run at?

What displacement did you run your actual sea trials at and what outboards -power size - have you got?

 

The sea trials were run at this displacement modelled in michlet. I was pretty darn accurate with the weight calculations- confirmed by where she floats on her lines... the outboards are 85 kw per side.

Dennis - i have a 3kw continuous inverter, 6kw peak. The inductor (line reactor ) is raided out of old fujitsu inverter air cons. The inductance is 6.5uH as I run 2 in parallel to achieve close to 5% impedance which provided adequate filtering of the harmonics which were tripping my inverter. There is no high start currents. The starting currents are lower than the peak current at full speed as it slowly ramps up. Noise from the electric drive is minimal, its the compressor itself which is noisy. It's much better if I run it at a lower speed tho- which also uses proportionately less current too, just takes longer to fill... 40hz is quite a good compromise between noise and speed to fill...

 

And what is this displacement value??

 

hmmm - upon checking all this, seems ive discovered why, in part, im slower than i expected... i havnt bothered until now as ive been too busy having fun with it

according to the CAD program which i used for the hydrostatics, the displacement should be around 3900kgs at our measured draft. In the michlet modelling, i used 3500kgs... I assumed the crane operator was accurate when we measured 3000kg empty - but it appears that was a bit optimistic as there is no way ive loaded another 900kg in fuel etc since then... Looks like i will have to run michlet again with a 3900kg displacement...

edit - need to account for trim too, grrrrr

 

i have no idea what reasonable sinkage and trim numbers are... i know it does trims a little at speed, its all a bit time consuming to reduce it down and re input all the michlet numbers etc... but this is how it came out with a more accurate displacement; - not alot in it it seems...

1.000000, 0.073302
1.678947, 0.641397
2.357895, 0.921740
3.036842, 1.162869
3.715789, 1.128677
4.394737, 1.570336
5.073684, 1.901296
5.752632, 2.106263
6.431579, 2.301213
7.110526, 2.515761
7.789474, 2.749813
8.468421, 3.037567
9.147368, 3.331443
9.826316, 3.646692
10.505263, 3.963241
11.184211, 4.323132
11.863158, 4.697784
12.542105, 5.113426
13.221053, 5.495769
13.900000, 5.900854

 

Hmmm..QA is not your strong point!!

So the values above are for a displacement of 3.90tonne...is that correct?...and naked (no appendages)?

 

The difference seems trivial compared to the weight gain? I thought resistance was strongly linked to displacement which was out by over 10%?

Thanks again for the info on the compressor. Sorry I meant if the noise of the compressor itself was disturbing even when powered by the electric motor, but you answered that anyway

I will have to keep an eye out for old inverter AC systems for the line reactors. Is your inverter a pure sine wave mosfet style unit?

 

I was trying to find the page where I thought maybe Leo Lazauskas was talking about sinkage and trim. But instead I found this

http://www.cyberiad.net/flotilla/hiper08-almeter.pdf

This is not what I was looking for but it shows sinkage going negative for a few hulls that were studied around Froude number 0.6 - 0.8. I have no idea how these hulls compare to Gropers boat but a "Wigley" hull has a sharp canoe transom, which would surely have more sinkage than a flat transom design.

http://www.cyberiad.net/library/pdf/tsl01a.pdf

 

Groper

See below.



Firstly this is assuming that Michlet is 100% correct!!...ive added 5% appendage drag for the stern drives..as a starter. Also 4% losses from engine to the prop. Ive also assume the efficiency to be around 0.55 max...you wont get more than that with props of this type on outboards..and then slowly reducing as the power/speed is reduced too.

If you can give me the WSA at the 3.90tonne displacement (draft) I can lay over my quick rough and ready estimate...oh the LWL too.

Then once you have this, you can check the power of the engine, and then check your speed and see where it is on the above graph. Plot it...at varying power settings and note the speed.

 

There is too much uncertainty about HP of outboards, the rating regime industry standard allows them a sizeable leeway, up or down, from the advertised HP, which allows makers to have a neatly spaced, nice round numbered range of engines HP, so all calculations are going to be "rubbery". Fuel efficiency goes to hell when you run at full throttle anyway, so the only figures of any real interest are those when backed off.

 

What would be interesting would be a comparison with a lower hp engine set-up, I'd reckon 2 x 70 would still give good performance (speed), but probably extra MPG.