GPS trolling motor focused design?

[jtrieck]

GPS trolling motors (like the Minn Kota i-Pilot) are game changers for a lot of fishing in smaller waters. If a boat for protected waters was to be designed around on of these motors, would it be materially different than a jon boat? How would a hull be optimized if it was to be "pulled" by a bow mounted motor rather than "pushed" from the transom?

[kbowen]

In my limited experience a flat-bottom boat like a Jon boat gets blown around by the slightest wind, so it might seem that a motor in the back, (with you sitting in the back) would allow the bow to get blown way downwind. in any light boat the critical factor is where you put yourself.

Ken

[Etdbob]

Uh, what is a GPS trolling motor?

I thought a GPS was a navigational instrument. No matter, I know what a trolling motor is -

If a boat for protected waters was to be designed around on of these motors, would it be materially different than a jon boat?

Yes.

A boat designed for such a low powered motor should be a displacement design like a row boat or small sailboat.

That is to say, something that does not drag its stern through the water.

John boats are designed, like all outboard motor boats, to have great buoyancy in the stern to support the weight of the operator and the heavy motor right in the ass-end of the boat, which is the worst place to concentrate weight!

This is a simple flat-bottomed skiff designed for sail and oar, which is to say, very low power.
It is a low resistance hull because the stern is tucked up and does not drag. It would work great with a trolling motor, you'd get more speed and range with it than in a John boat, but you could not concentrate the weight of the battery and operator right in the back.





This is David Beedes Summer Breeze design. To build the motor boat version of this skiff, one does not cut rocker in the sides, the bottom goes dead flat right back to the stern, and the sides do not tuck in at the stern for maximum beam.


This is Jim Michalaks QT skiff, a design that can be built for oar as seen here -








Or as a small motor boat here -









Notice the lack of rocker on the bottom and wide stern in plan view? This is to get flotation to support up to a 5 H.P. motor and operator in the back of this small boat.


Ah, Jim says it better than I can -

The power QT is nearly identical to the rowing QT except that its bottom runs straight aft from the maximum beam, a "straight run" as they say, with no rocker in the stern at all. This is critical in a power boat that will be getting some dynamic lift from its speed over the water (unlike the rowing version whose lift all comes from "displacement" of the surrounding water as the boat sinks to a level where its weight equals the weight of the water it has "displaced"). I think there are two reasons why the straight run is critical for a power boat. One is that as the boat speeds up it tries to climb over its own bow wave and as it does so it lifts its bow. The boat must have a lot of stern volume or else it will bury deeply at the stern and its bow will point to the sky alarmingly. The second reason is that with the typical outboard motor arrangement, most of the boat's weight is concentrated in the stern, the skipper's weight being usually the largest item. Even with no motion a good rowing boat with its fine stern lines can't handle that weight back there - its bow would point skyward and its stern dangerously close to swamping even without starting the motor. The upshot of this is that a good rowing boat can never be a good power boat, and a good power boat can never be a good rowing boat.

How would a hull be optimized if it was to be "pulled" by a bow mounted motor rather than "pushed" from the transom?

That's a good question!

OK, time for me to look up trolling motors.

Actually, trolling motors can push from the stern or pull from the bow. At least, I've certainly seen them setup both ways.

I don't think the low thrust of a trolling motor is a big deal. The weight of the motor is.

A Minn Kota 1368867 with 80 lbs thrust weighs a whopping 80 pounds!

That's way more I had figured!

A gasoline 6 H.P. gas Suzuki weighs only 54 pounds!

An 86 pound thrust Newport trolling motor weighs only 25 pounds. The difference is the battery is not built into the motor head.

Putting the battery in the motor head strikes me as a very bad idea! It puts the weight right where it does most harm in a boat. They sell, because it's simple, just bolt it on.

So anyway, the very best performance with an electric trolling motor would be with a boat designed for low resistance like the rowing version of the QT or Summer Breeze skiffs, with a trolling motor clamped bow or stern, no difference, and the battery stowed in the middle of the boat, at least out of the ends! The boat and crew must balance the boat so that the stern does not drag.

With the very heavy Minn Kota motor above, you do indeed need a boat designed to carry a heavy motor on the stern, so you are stuck with a standard John boat or something like the motor version of the QT.

I know of no design intended to take a near-100 pound weight on the very bow!

Time for some of the smarter crew to chime in here!

[Woxbox]

The bass fishermen use these motors to pull their boats around lakes all day long. I don't know what battery capacity is needed to do that, but it works on some very heavy fiberglass boats that carry a huge outboard on the stern. So clearly, a far lighter boat is no problem.

Designing a boat from scratch to make the most efficient use of the motor appeals to me, but I would say that if the primary purpose of the boat is to fish, that should come first. And to fish, you do want a stable platform. This means some beam, and carrying that beam into the ends of the boat. So a John boat makes a lot of sense. If you were to build from scratch, yes, put some rocker in it aft so it doesn't drag the transom through the water. Otherwise, a John boat or garvey type hull should be excellent.

As far as controlling flat-bottomed boats in the wind, I suspect this is why the bass boats mount the trolling motor forward. The boat may move crab-wise to the wind being pulled, but the GPS unit will still move the boat where it needs to go.

Phil Bolger specified a daggerboard forward in some of his flat-bottomed boats to improve control in a cross-wind.

I have an 18-foot, 1-ton sailboat that has a flat bottom. I push it with an 80-pound thrust Minn-Kota (It weighs about 40 pounds). If I don't put a board down at least part-way, it doesn't respond to the motor well at all -- it will slide sideways a lot as it starts to turn. It might look odd to have a daggerboard (or a leeboard pivoted on the side) on a fishing boat, but it would go a long way to making the boat behave if being pushed by the motor.

As to optimizing for pull vs. push, I agree that the only consideration is making sure the total weight put in the boat is centered. Remote batteries can be placed in the center of the boat regardless of where the motor is. Other gear can be shifted to get the boat trimmed out properly. I use two lithium batteries wired in series to drive the 24-volt motor. The batteries and their boxes weigh a total of just under 50 pounds. You don't want motor and batteries at opposite ends of the boat, however, as the cables get longer the voltage drops.

[Breakaway]

The GPS motors function as a virtual anchor, holding the boat in position by the bow. On that basis, the boat swings on the motor as if on a rode. A tapered front and wider stern may work better in that case. Such a boat will more naturally windvane.

That said, I wouild not overthink it. Build the boat to fish and things should be fine.

A comment I might make is that without the weight of an engine aft, the bow of a small boat is going to sink quiet a bit with an adult up there. Especially if the bow has some overhang.

Kevin

[Paul Pless]

I suspect this is why the bass boats mount the trolling motor forward.

trolling motors are mounted forward on bass boats because that's where the casting deck is. . .

there are other options, many canadian shield lakes are fished from square sterned canoes or small round bottom boats (the ubiquitous aluminium lund), the don't use an electric trolling motor, rather they run the outboard in reverse at idle or just above steering the boat in reverse using the tiller

[ulav8r]

Many years ago I ran a 10 lb thrust Minn Kota on a 17 ft square stern canoe. It moved along very well. That was years before the availability of GPS, so I have no idea of the actual speed, but it was at least three times faster than we could paddle it.

If you keep the actual boat weight below 250 pounds, I believe no more than 30 lbs of thrust would be required on a properly designed boat. As the boat gets heavier, has more windage, or is operated in strong current, then a more powerful motor might be required.

[Todd Bradshaw]

My 13' Whaler has a simple 50 lb. thrust Minn Kota Endura Max next to the Yamaha 25 on the transom. I doubt that it is any more efficient at displacement speed under electric power than most Jon boats. You're certainly not going to plane under trolling motor power, but it will move and hold its own pretty nicely in most wind. The bow mounted GPS versions are pricey, but pretty slick technology. I'd optimize the boat for fishing, maybe pull the transom up out of the water, add a 50-80 lb. thrust GPS Minn Kota on the bow and just let the boat weathervane a bit at times.

https://www.dropbox.com/s/f9choetyvf...video.mp4?dl=0

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