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Buzz73
11-05-2005, 12:15 AM
Is there a way to calculate hull speed when powered by an electric trolling motor? I have an older 27lb-thrust Minn Kota that I'd like to mount onto an 8-foot 150-lb skiff. The reason I ask, is because I intend to use this trolling motor as a cruising motor, not as an intermittent power source for fishing.

Would a higher thrust motor create higher speed? Would something like a Kipawa prop improve top speed significantly? Any clarification would be appreciated. Thanks.

Victor
11-05-2005, 04:53 AM
A cruising motor? Good luck! There's really only one way to find out, IMHO. Bring an extra battery, and make sure it's charged as well as the first one! When they poop out you don't get much warning!

[ 11-05-2005, 05:55 AM: Message edited by: Victor ]

ssor
11-05-2005, 07:14 AM
Wire in a voltmeter and an ampmeter while your there and you can get a pretty good indication of battery life. Oh, one other thing take a pair of oars. :D

Gary E
11-05-2005, 07:39 AM
or a 3 HP Johnson for a backup plan smile.gif

Cruising in a 8 ft boat,,,huh?

you may find that by the time you get batteries in that boat there is little room for anything else...meaning, eat a lot for breakfast cuz there'l be no room for your lunch and a cooler of beer. And ifin ya cant take beer, why go?

ssor
11-05-2005, 09:48 AM
Let's see; 150 pounds of boat, 165 pounds of people(just 1), 110 pounds of batteries, 15 pounds of trolling motor. I'll bet she rides low but she should carry well between oar strokes on the way home. ;)

Buzz73
11-05-2005, 11:28 AM
Thanks for the replies so far... Maybe I should have used a different word other than "cruising". I intend to just putter around a small inland lake for a few hours, no need for a cooler, beer, or other gear. A pair of oars is definitely a good idea.

JimD
11-05-2005, 11:38 AM
27lbs won't push anything too fast. Probably not even enough to maintain headway and keep you on course if the wind picks up, even if the boat is only eight feet. We use a 65# minnkota for our 11 foot utility and 15 foot sailboat and I consider it a minimum of power.

Bruce Hooke
11-05-2005, 06:34 PM
Buzz,

I think maybe we should define hull speed, because it sounds like you do not quite understand what it means. "Hull speed" is the fastest speed a given hull can achieve without getting up onto a plane, and the motor used to power the boat does not come into play when calculating the "hull speed." A major factor in hull speed is the waterline length, and as Jeff's post indicates the hull speed for a given boat can be approximated just based on the waterline length. Other factors like weight and hull shape do also come into play if you want to get technical. Anyway, what this all means is when talking about the motor for a boat, the question is not what "hull speed" would be for that motor on that boat, but rather whether that motor could push the boat at hull speed.

I just thought I would throw this information into the pot to (I hope) clarify things a bit. Some of the people responding to your question probably assumed that you wanted to push your boat at hull speed, whereas I think, based on your follow-up, that you just want to push it at a comfortable speed for puttering around the lake, right?

Buzz73
11-05-2005, 09:53 PM
You're right, Bruce. I don't intend to operate the boat anywhere near maximum hull speed. Probably just a few mph. Thanks for the clarification.

J. Blazy
11-06-2005, 06:23 PM
Originally posted by Buzz73:
Would a higher thrust motor create higher speed? Would something like a Kipawa prop improve top speed significantly? Any clarification would be appreciated. Thanks.Hey Buzz,
Yes a kipawa prop is a good idea to eek out a little bit more speed, but higher thrust, 24V and 36V trolling motors will be best to get higher speed due to higher RPMS, and more torque for larger DIA props.

Aftermarket props are a good idea due to their higher pitch, cuz they replace the standard props that are only 4" pitch, designed only for quick response for bass fishermen, not high speed - very much like always being in 1st gear.

My 14 ft electric hits hull speed at about 75-85% power using a 65lb thrust MK Maxxum (24V) and can go over 14 miles on four batteries at 60% discharge of each. Top speed is near 4 - 5 mph.

This is possible due to the 6-3/4" pitch of the stainless MK E-drive prop I installed coupled with the kort nozzle shrouding the prop to reduce the torque load of the higher pitched prop.

Kort nozzles are cool, very effective, but a royal PITA to make and fit. Well worth it though. Do a web search on them.

http://www.imagestation.com/picture/sraid139/p0a04839ec4bfe6871799a10e68ae2fff/f702fdb9.jpg

http://www.imagestation.com/picture/sraid144/p8b235bfb5da08e165457d85b3e538087/f6807950.jpg

Cuyahoga Chuck
11-06-2005, 08:39 PM
The "how fast will it go " problem is aggrevated by the 150 lb. hull weight. With two modest sized adults you're way over 300 lb. This small hull supporting all that weight may be fairly deep in the water. Whether it hydodynamically able to get to hull speed with 27 lb. of thrust seems problematic.
And any suggestions that that you load this poor little boat down additionally with a bigger motor and 100 lb. more of batteries is ludicrous.
I spent considerable time in an 8' 65 lb. pram this summer. It's supposed to handle 3 occupants in a pinch but the only way that would work is to have at least one person sit on someone else's shoulders. Alone, I get hull speed (if I can keep it going straight) with 7' oars. But only for short distances.

Charlie

[ 11-06-2005, 09:40 PM: Message edited by: Cuyahoga Chuck ]

Thorne
11-07-2005, 09:26 AM
There are several other threads on similar issues re: small boats and trolling motors. Check them out as I'm no expert, this is just my take on the option.

I was very hopeful that battery & electric motor technology had improved to the point that this would be a viable option for powering my 13' dory skiff, but it looks like it ain't.

Main reason seems to be battery technology -- for an electric motor with any sort of decent thrust (50lbs+) you often need 24 volts, meaning at least two if not 4 batteries. And for any sort of reasonable duration for that thrust, you need BIG batteries -- usually the golfcart ones are recommended.

So for an electric boat that recharges at the dock or via the sun, trolling motors will work and the batteries provide ballast. But for us with smaller boats that are dry-sailed, that means heaving heavy batteries in and out of the boat.

Again, this is just my take on the issue. But at this point it seems that for any boat that can take a small 2hp outboard, that option is still easier on the back and budget in comparison to a trolling motor that might provide 1/4 of the power for 1/10 of the range (by weight).

J. Blazy
11-07-2005, 10:41 AM
I'd slightly agree Thorne. If it weren't for the other major advantages of electric, I'd agree wholeheartedly.
Cost is very close - 65 - 85 lb thrust 24V Trolling motor is still under 500 bucks, but good gel cell or AGM deep cycle batts start at 150 each for 105 Ahr. Wet cells at wholesale club are fifty bucks, but will leak slightly in waves and only last 1/4 of the AGMs lifespan. A good multi-stage charger that won't ruin the batts are over 150 bucks - don't use walmart/parts store chargers.

You gotta actually WANT to go electric. I'd also put in one more MAJOR factor. Reliability.
my electric ALWAYS starts immediately - even after two years of heavy boating (10 - 14 miles per weekend ), and never a threat of being stranded.

You gotta want the ability to have whisper quiet conversations at full throttle.

My wife won't stand for any gas boat due to fumes. If I ever went to gas, I'd have to go to 4 stroke for this issue (+ reliability), but thats 3-4 grand more than I can spend right now.

I absolutely LOVE my electric, but I also understand that one must put a lot work into a craft to minimize the "wussy" stereotype of trolling motor as primary power. I'd be embarrassed if my TM was any more visible than it is. Power is there though - few believe that its just a TM when I zip by @ 4 knots with four people aboard.

Thorne
11-08-2005, 03:16 PM
Don't know about the whole "wussy" thing -- electric is really very cool to most folks, as shown by the interest here.

My problem is that I need my kicker (of whatever flavor, gas or electric) to push the boat and me and my dog/guest into possible 20 knot winds and 6+ knot tides.

Having been towed ignomiously back into harbor several times under these conditions, I've learned not to venture far without serious power of some sort. This is in the SF Bay and Delta area where weather and winds are uncertain at best -- only the tides are predictable.

If electric power was functional for my situation I'd use it in a flash -- no fumes and no noise -- wonderful!

tidmarsh
11-09-2005, 04:54 PM
Originally posted by Buzz73:

Would a higher thrust motor create higher speed? Would something like a Kipawa prop improve top speed significantly? Any clarification would be appreciated. Thanks.I use a 30-lb Motorguide on my 12' catboat. With the stock propeller, max speed was 3 knots according to my tennis-ball knotmeter. I've since replaced the prop with a Kipawa. It feels faster, but I haven't had a chance to measure just how much faster.

Aeronca
11-09-2005, 05:57 PM
Tidmarsh,

How heavy is your catboat and what are you sailing?

Ron Carter
11-09-2005, 06:40 PM
"Electric Propulsion for Boats" by Charles Mathys will tell you more than you can probably stand to know about electric's in small boats. Lots of detailed experiments and comparative data, regarding motor size propeller selection controller design and solar charging. Also great for insomniacs as a sleeping aid. Available from amazon.com. I have no financial interest in either the book or the company.

Andrew
11-10-2005, 11:00 AM
I need a little insight here. I would have guessed weight is only an issue when accelerating. After that resistance i.e. wetted area is the load on the motor

[ 11-10-2005, 12:00 PM: Message edited by: Andrew ]

AngWood
11-10-2005, 01:28 PM
John, how/why will the affordable (okay, cheap) 2-stage chargers ruin the batteries?

With only a few hours under its belt, my newly built 15ft skiff with a 54-lb 12 volt and two 6-volt Trojan T-105s has done great in quiet water. I'm guessing 5-6 knots wide-open, easy to go 3-4 knots. It's a pretty efficient hull--flat bottom, 4.5 foot beam. I don't think I'd take it out in SF bay, though.

gert
11-10-2005, 02:28 PM
Why hasn't WB done an article on Blazy's floating furniature?

Blazy, please post the link, I recall some pretty amasing features of her and some of the newbies here would definatly appreciate.

I use a 30 lb thrust e motor for occasionaly getting my 16 ft faering in or out of a high traffic channel where the ramp is, I wouldn't want to rely on it in a good chop for too long but Carina seems to like the added balast, she turns to weather more readily.

dunbarpm
11-10-2005, 03:48 PM
I have the biggest 12 volt trolling motor available on my bullseye (12 ft LWL - not quite wooden), and it works just fine for what I need, which is a way to get back home on a 'slick cam' afternoon, nothing more. It will push me faster than I can paddle. I use a deep cycle battery and recharge as necessary but keep one of those dash board solar panels on it. Even one battery is pretty heavy, so you have to locate it with some thought.

The motor is absolutely quiet, stows up forward, and starts with a flick of the wrist (unlike the nisson 3.5 on my other boat, nice as it is -- however the 3.5 is no heavier than battery and motor...) ftp://ftp-www.earthlink.net/Rozinante/Bullseye4.JPG http://home.earthlink.net/~dunbarpm/Rozinante/Bullseye4.JPG

Thorne
11-10-2005, 07:06 PM
Andrew -

Remember that we started out discussing this option for an 8' 150lb skiff -- so 40-60+lbs of batteries is a major factor for boat trim and handling.

I brought up the weight issue more for the "hauling the brutally heavy battery boxes in and out of the damn boat" issue -- those of us who daysail need to keep this in mind.

I suspect that to give my 13' boat enough ooomph for a long enough time to get me back to the launchramp or harbor, I'd have to load 4 batteries, effectively doubling the load in the boat and dramatically changing the trim and handling.

[ 11-10-2005, 08:47 PM: Message edited by: Thorne ]

willmarsh3
11-11-2005, 09:39 AM
I got a 55 lb thrust 12v motor (Minn Kota) with 1 12v 27D battery. It pushes my (1600 lbs loaded) Elver out of the marina at about 3 knots and brings her back to the boat ramp without too much difficulty. It was a windy day too. I believe I used maybe 15-20% of the battery capacity that day since it didn't take long to recharge it.
The motor has 5 forward and 2 reverse settings. I ran at 5 forward to get up to speed then backed to 3 to move along. The motor + battery cost $330.
I don't know yet what distance I can go on a windless day but I am curious to find out.
I would judge it quite adequate for the tedious maneuvering around a marina though.
motor setup (http://home.att.net/~willmarsh3/el/elver_improvements.html)

Will.