Jason,
In your message of a few days ago you asked:
Chris, you quoted : 1500 watts = 4.5knots = 6 hrs
I would imagine my cat would be similar and 5 Knots would possibly be < 2 KW?
I don't have much depth theory or practise about these things and all I know comes from tinkering with my boat and some reading over the past year or so.
For what its worth, I can make a few observations that could help you form an opinion...
From the experience of Bryan of School's Out he found that one motor is always more efficient than two at any speed. Even though when using one motor the thrust is asymmetrical it still uses less total watts than when using two motors. When using one there would be a slight additional drag because the rudders are slightly angled to compensate for the "skid steer" effect of just using one motor. But seems that once under way this effect is minimal. And in relation to your earlier contemplation about having the ability to shift a motor inboard for this kind of thing, you won't need to worry about it.
My boat has a waterline of 43 ft, and has bulbous bow, which I understand helps with lower drag (and hey, that's what they use on the big oil tankers so it must be a good thing). So I expect its particularly easy to push through the water. If you take a look at this graph on my blog at http://currentsunshine.com/?p=175 all three curves show greater than 2kw for 5knots boatspeed.
Bryan's cat has a waterline of 45ft and at 5 knots boatspeed against a 10 knot apparent wind he has recorded around 3000 watts power requirement for two motors. It displaces between 5 and 6 tonnnes.
For what we're doing we need to scrounge every % of efficiency we can get our hands on. And, as mentioned by others, using outboard legs and adding a pancake motor makes your task that much harder. I'm convinced that the reason the Torqeedo units are so efficient is because they have control of the whole drive train and have matched motor and prop superbly. It gonna be much harder for you to get the right prop to match whatever motor you choose. You'll be limited in prop choice by the spline configuration of the outboard leg you choose. I've been down this path with an Epod motor which came with an appallingly bad choice of prop from the factory and I had to experiment with buying off-the-shelf props that would match the splines. I even had one repitched to experiment with efficiency but in the end abandoned the chase and bought a Torqeedo. The Torqeedo has proven to be around twice as efficient as the Epod.
I hope this helps.
Cheers
Chris
__._,_.___
MARKETPLACE
.
__,_._,___
No comments:
Post a Comment