Donald,
you seem pretty knowledgeable about this ;-)
Any idea how much thrust required to "safely" move this boat through a 4kt current?
Thanks,
Doug
--- In electricboats@yahoogroups.com, KEN <rekkamurd@...> wrote:
>
> I dont have practical experience with that particular MK, just some general practical knowledge-experience with some of em. 24ft wooden catboat, I'm going to guess is maybe a 22ft waterline sq-rt being 4.69, x 1.56 is 7.31, and that x 0.75 being 5.49 mph..
> rough ballpark estimate for 75% of waterline length hull speed, being right about its threshold for good efficiency, faster just gonna gobble down the power a lot more...
> BUT, trolling motors rarely do over 4 to 4.5 mph anyway, Kipawa prop inclusive.
> (yes they're a good bit better performers, anywhere 10-20% more speed or range)
> Â
> things to think about is how much the catboat really weighs, and how much thrust is it really going to need to reach that 4.5 mph, is about where motor and prop rpm is going to run out of legs anyway! they just arent gonna spin or push any faster, even with a hull thats very efficient and slippery, its just how trolling motors are designed.
> (not saying they arent great and convenient for what they do!)
> Â
> in my own little electric toy (and it is that) where a trolling motor is its primary propulsion,
> 55lb thrust 12v MK with a Kipawa prop didnt perform any better than the 36lb thrust MK with a Kipawa prop. the 55lb'r being about 600 watts, the 36 being about 400 watts, I went right back to the 36lb thrust that offered the longer runtime and range. 50% more power at the prop really didnt do a darn thing for it, but hey its only 12' long is everything why.
> beyond 4.1 to 4.2 mph its only gonna drain harder and harder and harder....
> (not the same when you have the higher cieling of a 22ft waterline)
> Â
> so.. what I'm saying is you really might not be needing 160lb worth of thrust, is going to cost more and end up draining the batteries much faster, for being less efficient maybe.
> I cant think of a way to KNOW, other than maybe towing the boat at just 5 mph with a scale on the towing line, to see how many lbs average is needed to actually do the job.
> which is still under 75% of waterline length hull speed, and just 1/2 mph over anything I'd actually expect something trolling motor to ever really do. not knocking the idea of using a trolling motor as an alternate, just being realistic with what I know about em so far.
> if the boat only pulls 60-70 lbs to do 5 mph, then range just about doubled for doing that.
> (if it DID do 5 to 5.5 mph with 80lb thrust and Kipawa prop, thats a pretty darn good roll!)
> Â
> I've had 30# 36# 55# from MK, and 30#Â 50# from MG, is 5 total (no expert either at all)
> and sure notice a bit better performance for mounting a Kipawa prop on em, where near the same kind of full speed of a stock prop is reached with less power to the Kipawa, or a slight increase in amp draw at full power and a bit better top speed, prettymuch right in line with what they claim their product to do. btw theyre all just 12v motors.
> Â
> to the opposite, I've tried MK's weedless wedge prop is a bit larger diameter, less pitch, and a lot more blade surface area for less prop slip.. it was less speed for me with my lightweight little boat, but for something much heavier loading it very well COULD mean better performance and speed than the stock MKÂ prop, lower current draw and less slippage, but topping out at about 3.3 to 3.4 mph, where the stock prop might only reach 3 mph for slipping badly, unable to overcome a load and getting onto its pitch step..
> but for me it was "gee this sux" and right back to the stock prop that I'd increased pitch to a little by heating blades in boiling water and twisting (with LOTS of measurings!). I'd got it to a pretty happy place, very nearly like how the Kipawa does, is 3 blade not 2, so naturally I went to the 3 blade Kipawa is a better stiffer grade-type of plastic, like a carbon filled injection molded nylon, lots tougher and much more rigid.
> Â
> yeah, sometimes I get long-winded, maybe helpful, and maybe not..
> I'd want to know how much or how little thrust is needed at 5 to 5.5 mph before spending,
> even knowing a trolling motor may still fall into being 1 mph slower than that anyhow.
> Â
>
> --- On Mon, 10/29/12, donald bland <dlblandjr@...> wrote:
>
>
> From: donald bland <dlblandjr@...>
> Subject: Re: [Electric Boats] Re: MinnKota RT160EM
> To: electricboats@yahoogroups.com
> Date: Monday, October 29, 2012, 1:19 AM
>
>
>
> Â
>
>
>
>
> motor guide currently offers a 105 # torque, 109 weight, 36 volt, motor for $1250 US dollars,
>
>
>
>
>
>
>
> From: acsarfkram <acsarfkram@...>
> To: electricboats@yahoogroups.com
> Sent: Sun, October 28, 2012 11:32:27 AM
> Subject: [Electric Boats] Re: MinnKota RT160EM
>
> Â
>
> Hi Doug,
>
> That MinKota should work well. My first electric sailboat project was an Ericson 23 with a MotorGuide 109 Great White (36 volt 105# thrust). It gave me about 4 knots at full power. The longest I motored with it was only about 45 minutes. It had decent power for moderate winds (20 knots). You might want to play around with props I ended up with a Kipawa prop that gave me a bit better top end speed.
>
> Mark
> Santa Cruz
>
> --- In electricboats@yahoogroups.com, Doug McQuilken <dougmcq000@> wrote:
> >
> > I would like to add electric auxiliary propulsion for my 24' wood catboat.This is  a really convenient setup with remote control, variable speeds forward & reverse.Does anybody have any practical experience with this motor?Thanks,DougPS - Specs here, http://www.walmart.com/ip/Minn-Kota-RT160EM-Engine-Mount-Saltwater-Motor-160lb-Thrust/15686827
> >
>
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