This has been discussed here before. I posted an analysis of the "Sevcon Brushless Sailboat Kit - 8.5KW" from Thunderstuck in post #17601 on March 3, 2011. Here's some of the salient points:
"The picture on the website shows what appears to be a Mars ME0907 single stator motor. The manufacturer rates this motor for 100A continuous for 48V DC. That works out to a continuous motor rating of 4.8kW
The picture shows a Sevcon Millipak controller which is also rated for a 48VDC max power output of 4kW (from the Sevcon webpage). However the Thunderstruck webpage lists a Sevcon Gen-4 275Amp, programmable, regen controller. The only Sevcon Gen-4 controller that mentions 275A in its ratings is the G4827 controller. This controller is rated for 110A continuous (60 minutes)with a maximum nominal voltage of 48VDC. That works out to be almost 5.3kW.
But a drive system can only be driven as hard as its weakest component. In this case, the ME0907 motor will only take 4.8kW continuous current at 48V. And these ratings are in ideal conditions. In a boat, you will probably find that one of the components will overheat at somewhat less than maximum ratings unless measures are taken to maximize the cooling for each of the key components."
So at best, the "Sevcon Brushless Sailboat Kit - 8.5KW" looks like it should be rated as 4.8kW, not 8.5kW.
I would expect that a vendor should be able to provide supporting data for their claimed power ratings in a marine environment. I know that Propulsion Marine states that their 5.5kW drive will operate its rating for an hour with controller and winding temps of less than 100C. In my boat, I've operated above 5kW for 15 minutes with controller and winding temps lower than 75C and backing off to 4kW for less than 10 minutes cooled the motor to 72C. Given my experience, I find James' claims believable. All of the components have manufacturer ratings that are considerably higher, but running slow in a bilge is not an optimum environment.
Fair winds,
Eric
1964 Cheoy Lee Bermuda 30 ketch
Marina del Rey, CA
--- In electricboats@yahoogroups.com, GNHBus@... wrote:
>
> which vendors are making optimistic power claims ?
>
>
> In a message dated 4/28/2011 3:04:01 P.M. Eastern Daylight Time,
> ewdysar@... writes:
>
>
>
>
> Since you're just starting up, I'll limit my answer to some of the very
> first considerations. Decisions like which prop or gear ratio can't be
> answered until you make some other decisions.
>
> The two rules of thumb for selecting the size/power of your electric drive
> for sailing auxillaries are either 1hp electric for every 2.5hp ICE or 1kW
> rating for every ton of boat displacement.
>
> The hp to hp conversion ratio depends on an appropriately sized and rated
> IC engine to beging with. While the Atomic 4 was rated to 30hp, in
> sailboats, the engines rarley produced that much power.
>
> By using your boat's displacement, you can get a much more accurate idea
> of how much power that you'll need. In your case, you should be looking at
> motors that are rated for at least 6.5-7kW in continuous use. You need to be
> wary about some vendor's optimistic power claims of the systems that they
> are selling. For most installations like yours, building (or buying) a
> drive system that can produce your needed power without overheating will be the
> challenge.
>
> As an example, I have a 30" 10,000 pound ketch with a Propulsion Marine
> 5.5kW drive and that system operates at reasonable temperatures under
> sustained loads.
>
> Once you've got an idea of a couple of drives that you are considering,
> bounce your ideas off of the group and we might be able to provide some
> valuable feedback.
>
> Fair winds,
> Eric
> 1964 Cheoy Lee Bermuda 30 ketch
> Propulsion Marine 5.5kW drive, 8kWh Lithium batteries
> Marina del Rey, CA
>
> --- In _electricboats@yahoogroups.com_
> (mailto:electricboats@yahoogroups.com) , merit131 <no_reply@> wrote:
> >
> > Hi Everyone out there in electric boats and those of you, like me, will
> be installing your new electric motor as well. But the question is where to
> start. It's all new to me and like most, I don't know too much about
> electric, but the young age of 63, I'm willing to learn. I live in Savannah,
> Georgia. Just picked up a 1968, 37 Ft Chris Craft, which to me is a very well
> designed sailboat. If it's still around today, you know it was made well.
> Yes it has Atomic four engine, which I would like to remove and install
> electric motor. I've been looking at the kits and chatting with the dealers. It
> all appears rather easy with these kits. But they all say the same thing.
> You need a reduction gear, 2-1, 4-1 ratio , and I have been looking into
> this, but it's not off the shelf, or is it? The prop is another story. I saw
> some w/ the reduction attached, but costly. I'm a former US Marine on
> disability so I can't afford some of these. It looks rather simple to build my
> own. No machin e shop here, but I do have tools and I'm ready.
> >
> > When I talk about dealers or where to get parts or advise, I will try to
> install links for others. I guess this would help us all. It sure saves a
> lot of searching on the net.
> >
> > So here goes, this is what I am dealing with.
> > 1. A 37 Ft fiberglass Chris Craft sailboat (Apache) born in 1968.
> > 2. Draft: 5'-9" Fin Keel
> > 3. Displacement: 13022 lbs, Ballast: 5403 lbs.
> > 4. Water Line 26'3"
> > 5 Beam: 10'- 2-1/3"
> > 6. Prop, Feather type, ( which I think must go)
> > 7. Engine: Atomic Four 30HP
> > 8. Rig type Masthead Sloop.
> >
> > If anyone can help with what they think would be the best electric motor
> for this size sailboat, please feel free to let me know.
> >
> > If you can also suggest a reduction gear ratio and where I can get it,
> or the parts to build it, (Pulleys, bushings, etc)
> >
> > If you could also suggest a prop to make all this work. It will save me
> a lot of time.
> >
> > I want to thank yahoo for this site and all the people helping other
> people like myself. Taking the time out to advise and share their knowledge
> with dummies like me. Thanks again. Jack
> >
>
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