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Old 02-07-2020, 10:06   #1
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DIY retrofit electric minnkota saildrive

Hey all,

I just bought a Formosa sea tiger. Included with the sale was a running, but not installed, penta md2b. I have literally zero interest in using this engine for many various reasons, and want to convert to electric propulsion. As far as I can tell, the md2b produces a max of 51.08yadayada ft/lbs of thrust at maximum rpms.

I have a 55ft/lbs rated minnkota trolling motor laying about (I use it as the primary on my little john boat when taking the kids fishing on inland lakes).

I want to remove the existing prop/shaft assemblage, seal it up, and install the minnkota in such a way as so the "pod" of the motor sits pretty much right were the prop & shaft are now, between the rudder and the trailing end of the keel. There are adequate clearances.

I am planning (tentatively) to take the motor/prop "pod" off the shaft, drill a vertical hole large enough for the shaft to pass through from the bottom of the transom locker (lazerette?), stick the top end w/ shaft through the hole, and reattach the motor "pod" in its intended place.

This way I avoid cooling issues, as the motor is intended to be submerged. I also avoid any problems that may arise from the increased drivetrain weight of a direct shaft drive option (utilizing the existing shaft/prop), such as bearing or bushing wear.

The throttle would only be accessible from inside the transom locker, but I can deal with that for now. Later on I could remove the control head and lengthen wiring to a new throttle lever (6-way electrical switch near the wheel perhaps) without removing any previously done work.

Does this idea hit any snags I've not forseen? What does everyone think about this plan? What would be the optimum procedure for sealing the hole where the shaft passes downward (solid fiberglass)?
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Old 02-07-2020, 10:54   #2
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Re: DIY retrofit electric minnkota saildrive

You want to push a 28000# empty boat with a 50# thrust motor thatís not intended to be submerged full time?

Imho, terrible idea.
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Old 02-07-2020, 11:01   #3
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Re: DIY retrofit electric minnkota saildrive

well, the penta was rated at 51.**** #'s, and the motor i'm intending to use is rated at 55#'s.

I hadn't considered the full-time vs. part-time submersion issue, thanks for bringing that up. The housing is solid thermoplastic, fairly thick, all one piece - no seams in the plastic. The only component susceptible to corrosion (obv they all are, but you know what I mean) is the nut in the center of the prop, and it'd be easy enough to ensure it's proper SS.

Can you kindly specify exactly where you'd think sea water immersion problems would manifest? Thanks, I'm not 100% i'm on the right track here and want to have it all de-kink'd as much as possible.
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Old 02-07-2020, 11:06   #4
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Re: DIY retrofit electric minnkota saildrive

It would also be a fairly simple matter to glass the entire pod in place once installed, to further limit potential problems from water ingress, corrosion, or even the chance of "magic bullet" impact damage.
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Old 02-07-2020, 11:15   #5
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Re: DIY retrofit electric minnkota saildrive

The Minnkota 55 lb thrust trolling motor generates about 1 hp. The penta md2p is probably rated around 18 hp.

Cool idea but perhaps not enough horses?

Maybe you could keep the shaft and propeller and couple the trolling motor to this.
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Old 02-07-2020, 11:30   #6
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Re: DIY retrofit electric minnkota saildrive

I thought about going direct drive, but seeing as the motor I have is intended to be submerged I figured this would be the easier route - not having to worry about adding a cooling system.

I was under the impression that the difference in practical application between my little john boat and my fatty Formosa (with of course the same motor) would manifest in (a pronounced lack of) acceleration. On the john boat when I turn the throttle up I reach full(intended)speed almost instantly, but on the Formosa it would take a significantly longer amount of time to achieve the same speed (of course again assuming the same throttle setting, and electrical current input).

If I end up having to buy a larger motor that wouldn't be the end of the world, esp since I haven't removed anything yet (including the penta's prop, shaft, and coupler) but if the torque #'s match up, in theory I don't see a thrust problem, after all - HP is really an imagined measurement made to boost sales - it's torque->gearbox->final drive (or torque->prop dia.-> prop pitch) that actually provide movement.

Thanks again and please do continue to poke holes in my theories, without said poking I wouldn't be able to refine theory beyond initial conception.
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Old 02-07-2020, 11:37   #7
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Re: DIY retrofit electric minnkota saildrive

I have seen something related to this but it wasn't a main propulsion. The boat I saw used two opposing MinKota motors tied together acting as stern thrusters. These were mounted on a pivot arm that you could clear them of the water when not in use and pivot down when used as thrusters, I thought it to be quite ingenious and probably saved thousands of $$$.
Perhaps you could also design it to pivot clear, like an outboard. It would be grossly underpowered but if you sail primarily and just used it to navigate in and out of a slip, it might work. There are plenty of engine-less boats out there that sail up to a slip. I wouldn't want to cruise this way personally but people do.
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Old 02-07-2020, 11:49   #8
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Re: DIY retrofit electric minnkota saildrive

Make a temporary bracket and clamp that motor to the boat before you do anything and see how it pushes you
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Old 02-07-2020, 12:08   #9
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Re: DIY retrofit electric minnkota saildrive

Temporary bracket to test real-world performance is probably the best idea I've heard all day, it's amazing that with all the math and schematics flying around inside my noggin that I miss ideas like this, good call bud I absolutely will do.

As far as a pivot head to negate full time submersion, the motor is actually already equipped with such a system - so maybe if it works out on a temporary bracket that's the route i'll go.

Like so many others considering electric conversions I have no illusions about range or top speed, just enough to get in and out of the slip without a liability claim is what i'm after. In fact, "stern-thruster" is the perfect term for what I have in mind. Especially since the entire motor system is light enough to pick up with one hand and stow down below once the sails are up. I could even mount a bracket on either side of the transom so I don't obfuscate the gold filigree name/home port plaques.

Thanks again all, and please do feel free to keep the ideas coming
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Old 02-07-2020, 12:10   #10
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Re: DIY retrofit electric minnkota saildrive

Quote:
Originally Posted by motion30 View Post
Make a temporary bracket and clamp that motor to the boat before you do anything and see how it pushes you
Great idea. Also look at the size and shape difference between the 2 props. And it's torque that makes the big difference between diesel and electric.
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Old 02-07-2020, 12:12   #11
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Re: DIY retrofit electric minnkota saildrive

1) Are you talking about Formosa sea tiger 41? 28000lb sailboat? 55 lb-f trolling motor is too small for that! I am using 85 lb-f trolling motor on 3000 lb sailboat (10 times less) and it is barely meet my requirement for speed.
2) Why to create something that will create constant drag while you have ability to mount the trolling motor behind stern and pull it out when it is not in use?
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Old 02-07-2020, 12:19   #12
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Re: DIY retrofit electric minnkota saildrive

You are confusing ft/lb of torque with pounds of thrust.

The HP equivalent of our motors:

36lb Thrust (All Series) = 0.48HP
40lb Thrust (All Series) = 0.55HP
46lb Thrust (All Series) = 0.64HP
55lb Thrust (All Series) = 0.84HP
62lb Thrust (All Series) = 0.93HP
86lb Thrust (All Series) = 1.50HP

Trolling Motor Thrust vs Horsepower - Trolling Motor Thrust
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Old 02-07-2020, 12:28   #13
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Re: DIY retrofit electric minnkota saildrive

oh man,

Your right I was confusing thrust w/ torque - this is why I posted I knew I must be missing something! Clearly I've not had enough wine this morning haha.

As far as the constant drag issue, I agree completely - it'd be extremely simple to put it on and off a bracket, or to fabricate a pivoting bracket, or even to utilize the existing pivot system (though i'd worry about the control head getting too wet too often) whereas my originally posted plan would require a fair amount of fiberglass work to create an inferior system.

Thank you all so very much, given my mis-interpretation between the penta's torque and the minnkotas thrust I'm back to the drawing board with some much handier concepts to play with! Dudes, seriously, Thank you quite kindly!
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Old 02-07-2020, 12:44   #14
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Re: DIY retrofit electric minnkota saildrive

If you decide to go with a diesel, stay FAR FAR away from the green monster down below. I have an MD11C (African Queen 2 banger) in our little Formosa and if I could get it out without destroying the boat -it was apparently installed before the deck was put on- I would probably install a new Beta/Kubota.
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Old 02-07-2020, 14:23   #15
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Re: DIY retrofit electric minnkota saildrive

Definitely not going diesel, but I found a company in santa Barbra CA (thunderstruck-ev.com) that sells EV kits specifically for sailboats!

Verrryyyyy nice not to have to worry about which type of controller goes to which type of motor goes to which type of throttle switch needs which type of cables etc etc etc etc

it's one click, $1500usd, 18hp replacement. probably has an illustrated "lego kit" style installation instructions too.

I'm about to order one, I'll make a thread when I get it covering the conversion process, Thanks again!
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