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Any mathmeticians in here....
http://www.hobie.com/au/en/forums/viewtopic.php?f=11&t=35704		 Page 1 of 1
Author:  CurtnAz [ Sat May 21, 2011 8:40 pm ]
Post subject:  Any mathmeticians in here....
I am like Kevin Skinner when it comes to math... :o) But, I was wondering something, and I'm sure there is some form of equation for it.

Thrust and/or propulsion...

Lets use the Pro Angler....136 lbs. rigged and a passenger who weighs 270 (me) :) How many MPH could you go with a given variable of x #s of thrust from a motor. Say 55 (since that seems to be the common amount). Then with a PA and the capability to have 2 12V batteries (if you wanted) and a 110# motor :) There's no way that I could figure that out, but I'm sure there's a way it can be done :D Just asking because I got this idea :)
Author:  Spinfisherbob [ Sun May 22, 2011 2:52 pm ]
Post subject:  Re: Any mathmeticians in here....
Ok, so whats the idea?

http://en.wikipedia.org/wiki/Hull_speed
http://www.psychosnail.com/InfoHullSpeedLWL.aspx

The more you load the PA the longer the water line will be.... to a point
Author:  LazyLefty [ Sun May 22, 2011 3:10 pm ]
Post subject:  Re: Any mathmeticians in here....
Since you didn't say for certain I'm going on the premise you're talking about trolling motors. From what I know of them they often share the same prop regardless of lbs of thrust. This means that if you can move at the 55# motors top speed the 110# will get you basically nothing extra in the speed department. Most of these motors are made larger to accelerate a heavier boat to that top speed not to actually have a higher top speed.

Now if you can get a higher pitch prop (like the torqueedo electric outboards have) that'll give you that top speed you're looking for and then you'll need the higher thrust motor to spin it.
Author:  Roadrunner [ Sun May 22, 2011 4:42 pm ]
Post subject:  Re: Any mathmeticians in here....
Curt, if you want a specific answer, you'll need to be a little more specific. Is that gross or net thrust? What's the RPM and prop pitch? What do the batteries weigh? Is this right after lunch or before? Do you want sprint, cruise or trolling speed and for how many miles? Should we assume no wind, tide, current or chop? :roll:

If you don't know the answers to these important questions you can use this handy actual performance chart to get a rough idea:
Image
Note: the PA (red line) demonstrates the similarity with the Outback so you can infer that the range line is also similar. Notice the diminishing returns with additional power vs. speed. Weight is not a major factor here so you can bring home lots of fish without consuming much additional power! 8)
Author:  CurtnAz [ Sun May 22, 2011 5:53 pm ]
Post subject:  Re: Any mathmeticians in here....
Wow, I'm hoping some of that was sarcasm....is it before or after lunch **lol** I figured there should be more of a simple answer. Something having to do with the resistance of water, figured in with the weight of the boat, blah blah blah....but now that y'all got me thinking more...there probably isnt no mathematical way to solve it, because you also have the bow pitch, prop pitch (as stated), plus how many blades, etc....guess it was a pretty loaded question :)

Time for some real world :) As for what it's more, just a trolling/electric motor idea. :)
Author:  Roadrunner [ Mon May 23, 2011 9:43 am ]
Post subject:  Re: Any mathmeticians in here....
There definitely are a lot of variables that go into a detailed result (which I would never be able to calculate either) -- lunch is probably not one of them. But if you don't mind a ballpark answer, you can reasonably expect the PA to cruise at about 4 MPH with decent efficiency and sprint a little over 5 MPH (really runs down your battery fast at that speed). Your max. speed is limited by the PA's hull dimensions so a larger motor will add little if any additional speed. A minimum of about 30 lb. thrust should do the job. 8)
Author:  CurtnAz [ Mon May 23, 2011 1:50 pm ]
Post subject:  Re: Any mathmeticians in here....
Thanks RR :)

I guess I'll divulge the reason for this question, whatever it was :)

All this talk of motor driven yaks and such got me thinking to the MinnKota engine mounted trolling motors, which I think would be easy enought to make some type of addaptive bracket to mount on the rear, if not using the same idea as a transom mount. (basically like the OK Torque). All you need is forward/reverse thrust because the rudder will do the turning. So, I even started looking into the used motors, and taking everything off where all that is left is a powerhead, then make a vacuum form over the head for a mould, make a custom bracket, and hook it up to a remote speed control dial. Almost the same exact thing as the MinnKota. But if everything was purchased used, at a fraction of the cost.
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