James makes some very good points and as I have said before on this forum,
"Don't let your thumb get in the way of good engineering".
Every system is different and circumstances can vary greatly from one conversion to another. Boat design, intended use, future resale, safety, potential liability, skill set of the operator, and economic resources are all factors to be considered in these nautical engineering endeavors.
Some considerations will weigh more or less for different applications and circumstances however there is never anything wrong with considering SAFETY FIRST. Unfortunately, although people prefer black and white choices, there is a lot of gray area.
Capt. Carter Quillen, PE
www.shipofimagination.com
On Friday, March 21, 2014 7:44 PM, Chris Hudson <clh5_98@yahoo.com> wrote:
James,
My error, I actually have 2 X 10 KW systems.
Chris
Sent from myPhone
Sent from myPhone
James,Are you talking nameplate KW or the KW that is actually possible for some duration? As you have spoken earlier about this difference in advertised power versus actual sustained power, I wanted to make sure. Is the 1 KW/ton rule based on actual, sustained KW, or the peak KW that most mfg. use to rate their systems? I also assume use of the normal ton, not the long ton. I have a "10 KW" system on my 41' 27,000 lb. sailboat and top out around 5.6 knots in calm water.Regards,Chris
Sent from myPhoneMost of the vessels being converted in this group are up to to 35 feet long. We have little data on vessels longer than that.We converted a 38 foot Shannon with displacement of 18,000 lbs which achieves 6 knots at 10 KW.Usually we consider 1 KW per ton would achieve 90% of hull speed. In the case of the Shannon, we are at 1.1 KW per ton and achieve 6 knots - 82% of hull speed.As vessels grow in length, the also grow in width, height, depth, windage etc and the power requirement also grows.My estimate is 1 KW per ton achieves around 6 knots with larger vessels, and not 90% of hull speed.If we are to get 90% of hull speed for larger boats, the 1 KW per ton grows to 2 KW per ton and beyond.My thinking is recommendation for vessels should be 1 KW per ton up to 40 feet, 1.25 KW per ton up to 45 feet, 1.5 KW per ton up to 50 feet or something in this neighborhood. Additionally, larger vessels should be backed up by a generator, because their power requirements will quickly go through battery capacities.I recognize that boats sailed our oceans for millennia with no auxiliary propulsion at all. I've also rounded Cape Hatteras with the Gulf Stream on one side and the Cape on the other, the big diesel at full power, a sliver of sail up, winds of 50 knots on the bow and making only 2 knots. It took us 10 hours to make 20 miles and we certainly wished we had more power at that time. That was heavy blue water sailing at its finest. Boats should always be built for how they are going to be used but we can't always predict that. Even fair weather sailors can be caught on a lee shore in a sudden blow.Underpowering a vessel is what I would call adventurous. We need to remember that electric propulsion will likely outlast the current owner and the new owner may have different cruising in mind for the vessel. If the new owner does not understand the vessel is underpowered, he could be put in jeopardy. Perhaps a vessel should have a warning label if it is powered at less than 1 KW per ton to keep people out of harms way.Any data on larger boats being powered by Electric propulsion would be of great interest to me.Thanks,JamesJames Lambden
The Electric Propeller Company
625C East Haley Street,
Santa Barbara, CA
93103
805 455 8444
james@electroprop.com
www.electroprop.comOn Mar 19, 2014, at 6:55 AM, Carter Quillen wrote:Julian,I think the key to what anyone's power requirements are is how and where they plan to use their vessel. If you're operating strictly on calm water like lakes and canals, the power requirements are much lower because you can rely on gaining momentum and only need to overcome the hydrodynamic drag of your hull.However, if you plan on being in any kind of wave action such as open sounds or bays, your power requirements increase and if you're going in the ocean, they become even greater. I think .5 kW/ton or even a little less is fine for calm water operation and will even allow you good control in moderate seas but you might be going slower than you'd like. However if you're going to be in open water a lot, the 1 kW/ton is a better standard.Your point about the prop is a very good one.Carter
On Wednesday, March 19, 2014 9:26 AM, Julian Webb <julian.proto@gmail.com> wrote:
i agreei used the "you need 1/3 the electric power to match any i.c." so as my 13 ton boat had 2 x 53 hp diesels i installed 2 x 15 hp electric motors. i had the props designed to maximise efficiency/speed etc at river/canal speed of 5-10 knots, as i thought it would be a shame to go to all that trouble and expense and ruin it all with lousy props (just like buying a great sound system and finishing it off with lousy speakers).
i would guess i could do with 1 x 15 hp motor most of the time. i have mine on 1/2 speed (1/4 power load near enough) and as i'm not after the fast life it's easily enough.
i have seen some barges and boats 30+ meters long with 1 x 75 -90 hp motors, and they're commercial units carrying freight.
once you get a displacement hull moving ( or any really without getting into an argument over planing/making waves isn't more speed etc) a bit like a train, it takes way less to keep them moving than it did to get it there.
i have really only used full power twice, to get out of the way of some dick head who was out of control. as they were fibreglass and i'm steel maybe i shouldn't have haha.
good luckOn 14 March 2014 08:01, Hannu Venermo <gcode.fi@gmail.com> wrote:
Hear hear !
The old rule was much less Hp per ton, half the figure mentioned.
As you have managed, succesfully, at 0.2 kW/ton, I personally believe
the old rule of (between 1/2 to 1 Hp per ton) is much more appropriate.
Thus somewhere between 300-740 Watts/ton, or about 500W, is likely a
good option.
(10m) Sailboats with 12 Hp auxiliaries manage perfectly well.
Thats about equivalent to a 3-4 kW electric solution.
On 13/03/2014 16:41, Carter Quillen wrote:
> I have been pushing a 20 ton boat with a 7 kiloWatt peak motor,(.35
> kW/ton) that I almost always operate at 4 kiloWatts, (.2 kW/ton) for
> almost a 1000 miles now. It is fair to say it's a bit slower than most
> people would want but it works for me and I've always been able to
> maintain control of my boat, albeit in slow motion sometimes, even in
> some pretty adverse conditions.
--
-hanermo (cnc designs)
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