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PostPosted: Sat Nov 26, 2011 2:07 pm 
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A friend of ours has been using an electric trolling motor to power his Outback. Recently he moved from a 30lb thrust motor, to a 40, and more lately to a 55lb thrust motor. All provide him with about the same top speed. He’s perplexed, but perhaps shouldn’t be.

The no-load speed of all these motors is the same. And with such a light watercraft, the small load placed on the motors isn’t really enough to require much power. Under way, each one is spinning the same propellor at about the same speed, and the same prop turning at the same number of RPMs, regardless of which motor is turning it, delivers the same forward speed.

So, even though he’s got a more powerful motor than before, he’s not going to go any faster. With the same prop, he’s not able to take advantage of the additional power. It’s just wasted. In order to get more speed, he’s got to find a larger prop or a prop with more pitch, and I don’t think there are any aftermarket trolling motor props. At least I’m not aware of any.

I bring this up only because guys who want to power their kayaks with eletric motors might be doing themselves a disservice to run out and buy the largest motor possible. It might not provide any additional speed and it’s almost sure to pull more amps and thereby reduce your run time. Just something to think about.


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PostPosted: Sat Nov 26, 2011 7:03 pm 
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I just don't understand why people put motors on their kayaks. Defeats the whole purpose.

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PostPosted: Sat Nov 26, 2011 9:37 pm 
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Defeats the purpose?? What's the purpose?? Exercise?
Unless you have tried it you’re just showing your ignorance.
The same was said about the Mirage Drive a few years back.
Unless you can paddle at 5 mph for 6 hours or more, the guy with the motor is going to cover much more ground than you ever could. A trolling motor shines when fishing, by getting the paddle out of your hand.
Tom, it's not the motor, it's the hull design,the motors do run at different speeds, the top speed is mostly determined by the design of the hull matched up with the thrust capacity of the motor to the point of reaching the max motor load or when prop slippage occurs.
The new MK 40C2 runs almost 1 mph faster than the 30 on the same kayak.
The problems with the bigger motors are the large drag profile, they may accelerate faster but the smaller motor will soon catch up and pass the larger motor.
There are a number of different props out there for the MK motors and there are the Kipiwa props from Eastlake foil. We are currently playing with props and slippage, the slippage can be observed if you have a variable speed throttle and a good GPS, you can acutally control the slippage to a point if your aware of it
There is an ongoing thread on this on KFS

Tight lines
Steve


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PostPosted: Sat Nov 26, 2011 10:04 pm 
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Your friend has hit "hull speed" for the Outback, which has a rather well delineated top speed. As has already been demonstrated, he can double the thrust and it will have no significant effect. Running wide open will only drain his battery faster.

If he wants more speed, he needs a different boat. 8)


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PostPosted: Sun Nov 27, 2011 4:44 am 
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The Outback will go faster. There's more to "hull speed" than that I'm afraid. Until you begin to lose directional stability you haven't reached the limit of your hull's maximum capable speed.

If we took a 10HP outboard with sufficient prop the Outback would run faster, a lot faster. And it will run faster with a different prop on his electric motor.

I can get behind him with my boat and push him faster than his current electric motors will. Another guy fooling around got behind him with a small jon boat and 10HP outboard and really got him moving. "Hull Speed" didn't shut him off, and that's just another means of propulsion at the rear of his kayak.

If you can push it, it'll go. The problem is that his current propellor, with it's top RPM and pitch, isn't pushing beyond a certain point. I'm not saying there isn't a limit to how fast a hull can be propelled (there is) but 5MPH isn't anywhere close to how fast the Outback can be pushed, provided you can make use of the available power.


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PostPosted: Sun Nov 27, 2011 8:08 am 
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Tom,
I'm afraid I am going to have to agree and disagree with you at the same time. First with enough power any watercraft can be pushed beyond hull speed and put up on a plane. Your example of a 10 HP gas motor is a good example. It has more than enough power to force a boat (like a Kayak) to get up on top of the bow wave and go beyond "Hull Speed." At the same time, knowing a boat's hull speed is good for guys like me that don't rely on motor power for our propulsion. I know that I can pedal my Oasis to something around 6 MPH and that's it. Even with the help of a sail I'm not going to get beyond hull speed. That doesn't mean that the Oasis can't go beyond that. It just means that I as a human just don't have the power to vertically lift the weight of the boat, myself and any gear so that the boat can get on top of the bow wave.
Most electric trolling motors are not designed with enough power to push any water craft up on a plane. After all, that's not their purpose. However, some of the new electric outboards can. The Torqeedo is a good example. It puts out about the same power as a 2 HP gas motor and has the capacities to overpower the limitations imposed by the bow wave especially with a light weight boat like a kayak. So the real answer to your question becomes; "how much power do I need to put this boat up on a plane?" I think the answer is; "more than that available on most electric trolling motors." I hope this helps. Have a great day!

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PostPosted: Sun Nov 27, 2011 2:03 pm 
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Thanks for the response. I think the key is, as long as the boat can be kept on the water and controlled (directional stability) you haven't reached the maximum "hull speed" of that craft. I have no idea, but I'd almost bet that you can push the Outback to 10 to 15 MPH before you'd lose directional stability. But I'm only making a guess - it may be less or more than that.

But this guy's Outback isn't even beginning to tax those 40lb and 55lb thrust motors - it can't. It has nothing to do with the hull design or profile. With the provided prop, the motors are both spinning as fast as they can against very little resistance. Now if he had a larger prop, or one with a more aggressive pitch, the motors would then provide him with additional speed because they have the power to turn those type props against the load of such a lightweight craft.

It's just like using the standard Mirage Drive fins that come with his Outback - no matter how powerful or athletic the person doing the pedaling is, those fins can only be pushed so fast and will only generate so much thrust. But put on a pair of ST Turbo fins and suddenly the more powerful person will move the Outback faster.


Last edited by Tom Kirkman on Sun Nov 27, 2011 7:09 pm, edited 2 times in total.

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PostPosted: Sun Nov 27, 2011 6:45 pm 
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SteveK wrote:
Defeats the purpose?? What's the purpose?? Exercise?
Unless you have tried it you’re just showing your ignorance.
The same was said about the Mirage Drive a few years back.
Unless you can paddle at 5 mph for 6 hours or more, the guy with the motor is going to cover much more ground than you ever could. A trolling motor shines when fishing, by getting the paddle out of your hand.
Tom, it's not the motor, it's the hull design,the motors do run at different speeds, the top speed is mostly determined by the design of the hull matched up with the thrust capacity of the motor to the point of reaching the max motor load or when prop slippage occurs.
The new MK 40C2 runs almost 1 mph faster than the 30 on the same kayak.
The problems with the bigger motors are the large drag profile, they may accelerate faster but the smaller motor will soon catch up and pass the larger motor.
There are a number of different props out there for the MK motors and there are the Kipiwa props from Eastlake foil. We are currently playing with props and slippage, the slippage can be observed if you have a variable speed throttle and a good GPS, you can acutally control the slippage to a point if your aware of it
There is an ongoing thread on this on KFS

Tight lines
Steve


Steve, I'm afraid you might be one showing your ignorance. To answer your question, yes I have tried it. There isn't much with a kayak that I haven't done. You are obviously new to the sport. Kayaks are meant to be propelled by human means. Adding an electric motor makes it a boat.......plain and simple. And in Florida it must then be registered with FWC. In my opinion, if a mechanical means of propulsion is desired, buy a small boat. I didn't mean to offend you but don't make assumptions about someone's experience.

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PostPosted: Mon Nov 28, 2011 1:14 am 
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Tom:
This is an interesting discussion. There is a web site (http://www.psychosnail.com/boatspeedcalculator.aspx) that has some nice calculators for computing hull speeds. According to the formula the hull speed of the outback is 4.8 knots. However because a kayak is so light it doesn't neccessarily follow the hard stop rules of the 1.34 rule of thumb. By using the Gerr's DL ratio the max hull speed is 7.9 knots (with 350 lbs), which I think is about the point his formula begins to fail. However if you go to Gerr's power required formula's you can see that as you get over the original hull speed (4.8 knots) the power required to propel the boat faster goes up exponentially (which I think is very true). According to his formula it takes .86 hp to go 5 knots, but takes 14 hp to propel the same boat to 7 mph, beyond that his formulas start falling apart.
I have a 2 hp gas motor that I use for emergency on my boats, and with a 4 1/2 inch pitch prop it propels either The TI, Oasis, or Revolution boats up to 4mph (3.5 knots). The boat type and size didn't seem to make much difference, I think this relates to what you described with the electric trolling motors. I think this is a factor of the prop pitch more than horsepower, hull speed, or weight (basically the max hp of the motor is not exceeded) regardless of boat length since we are below the max hull speed on all 3 boats. With a 7 inch pitch prop, the revo max speed was a little over 6 knots (probably hull speed max kicking in). The Oasis went a little over 5 knots (max hull speed is 5.2 knots) with the 7 in pitch prop, and you can feel the motor just starting to struggle as compared to the revo. With my TI running the same motor and 7 inch pitch prop, I can only get 4.4 knots max with the 7 inch prop (max hull speed is 5.8 knots). I am thinking with the TI, I have maxxed out the horsepower of the motor at 4.4 knots. The 2 hp motor just doesn't reach max rpm when on the TI with the 7 inch pitch prop as it did with the 4.5 pitch prop (HP limitation of the motor). These are all real world numbers on my own boats all with the same motor measured with GPS in normal conditions (light winds minimal waves).
Whether the boat is propelled by electric or gas, I think most of the same rules apply, basically I think you are correct that as long as you are not exceeding the max hull speed of the boat or the max horsepower of the motor, its the propellers pitch and efficiency that determines your speed on boats with displacement hulls (kayaks, and most sailboats).

Actually I think Roadrunner or someone did a test a while ago mounting two trolling motors on I think an outback. With one motor the boat did around 4 mph, with two motors it did around 4.2 mph but burned through the battery twice as fast, kind of proves our theory about the prop pitch.

I don't think my TI is capable of getting onto a plane. I have had my TI up to 17 mph on several occations ( I have 265 sq ft of sail on my TI). According to the Gerr horsepower calculator (which I don't believe) it says my sails are delivering around 100 hp of driving force. I design and make all my own sails, and figure that my sails are delivering at best 30 hp in 20 knot winds. At those speeds my boat is definately not planing. I used to design and build racing hydroplanes, I built a 3 point hydroplane that was 8 ft long and could get up to around 70 mph, thats planing. I'm pretty sure if I put a 25 horse motor on my TI I doubt it would ever exceed 15 mph (because it's a displacement hull), though it would be fun trying ( LOL). There is of course a difference between sail power and motor power, because with motors you can control prop tilt and add foils to dial in the hull to get it up on plane (even a displacement hull). So I'm not saying I couldn't get my TI up to 40 mph , but why... I would just get a jet ski instead if I just wanted to go fast.

Now as to why put a motor on a kayak in the first place. In my case I simply will not go out into the ocean without my emergency motor period. There have been several occations where my motor has saved my butt. One time I was about 7 miles out and hit a shoal and broke the rudder. I couldn't have possibly sailed upwind into a 15 knot wind to get back to Key West. Another time a storm came up on us and we were fighting 35 mph winds and 4 ft waves ( my TI can't sail upwind in winds over 30 mph). Another time I was 20 miles from my launch site and the wind died. I have around 1200 miles on my TI to date, and have used a total of 3 gallons of gas with the motor to date (about $10 bucks).
All of our Hobies have always been configured with with weighted keels, jibs, spinnakers and mainsail, actually the Revolution with the Hobie sail (22 sq ft) and a 35 sq ft convertable jib/spinnaker was probably the most fun but I never felt comfortable taking it out into open ocean (even with the motor)

Hope this helps
Bob


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PostPosted: Mon Nov 28, 2011 7:30 am 
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I think your comments make a lot of sense. What I'm sure of is that there isn't some magical force that suddenly stops a hull from going any faster - If you got behind an Outback with another boat and pushed it along, you'd never reach a point where it suddenly felt or acted as if someone had applied the "brakes." What you would reach, is a point where the driver of the Outback could no longer keep the boat steered in a straight line, and that would be the limiting factor as to going any faster. The only limit to hull speed is the point where the boat can no longer stay on the water and/or be steered in a straight line. The hull doesn't just stop or refuse to go faster - it begins to veer off-line when that limit is reached.

Most trolling motors are designed for pushing much larger and heavier craft than kayaks. The props designed for those applications are intended for these motors when used on these larger and heavier craft. For that reason there are no props out there that provide sufficient area or pitch to move any kayak much more than about 4.5 MPH.

This is the point of my original post - if you're going to buy a trolling motor for your kayak, buying a larger, more powerful motor isn't necessarily going to move you any faster. Insofar as our kayaks are concerned, thrust isn't the limiting factor - the propellers are.


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PostPosted: Mon Nov 28, 2011 7:18 pm 
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Tom Kirkman wrote:
What I'm sure of is that there isn't some magical force that suddenly stops a hull from going any faster

Once you get a kayak (boat) moving, it takes roughly similar additional power to get it moving faster until the bow wave finally reaches the stern. Then, it takes considerable more power to cause the hull to break free of the bow wave and get up on plane to go faster.

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PostPosted: Mon Nov 28, 2011 8:00 pm 
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As long as you can keep supplying additional (or necessary) power, the hull will continue to go faster until that point where it will no longer travel straight. As long as it will travel or can be steered in a straight line, it will not simply refuse to go faster provided you supply enough additional power.


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PostPosted: Tue Nov 29, 2011 1:39 am 
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Tom Kirkman wrote:
The Outback will go faster. There's more to "hull speed" than that I'm afraid.
Using parenthesis around the term "hull speed" was apparently mis-understood by some.

Like aircraft, every boat has a unique performance envelope of sorts, wherein maximum efficiency, maximum speed, yaw, pitch and roll stabilities can be plotted for various power and CG applications. Of course, nobody here has disputed the notion that hull speed is not an absolute; perhaps a more accurate term would be "wave trap" speed.

How a hull behaves at that speed depends not only on hull length, as is commonly used to calculate hull speed, but also, displacement, length/width ratio, bow and stern shape and several other lesser factors. Most significantly, the traditional 1.34 x sq. root of the water line length formula was derived experimentally by William Froude for heavy displacement conventionally shaped hulls such as military and cargo ships, not planing hulls or kayaks which are usually classified as semi-displacement hulls, among others.

The Hobie Outback has a comparatively "hard" hull speed where the amplitude of the wave trap is greater then many kayaks such as the Adventure. Regardless of prop pitch, the thrust requirement increases rapidly approaching and exceeding this hull speed. For instance, the Hobie eVolve electric motor uses 90 watts to obtain 4.0 MPH, 200 W for 5.0 MPH and tops out at 345 W and 5.4 MPH. I can briefly double the thrust with Turbofins to gain another .1 MPH for a top speed of 5.5 MPH as the boat remains effectively trapped between its bow and stern waves, unable to climb out under ordinary circumstances. We usually acknowledge that Turbos are not speed limited by prop pitch anywhere near that speed. The idea here is that you don't get a lot of bang for your buck beyond about 4.5 MPH with this boat.

That's not to say that the speed couldn't be increased. If one needed to prove a point by mounting a large outboard, they could certainly go faster if they didn't sink or capsize the boat first. Wake rides, where the boat gets a little boost similar to Tom's 10 HP boat push example, can take the boat beyond 8 MPH. But under realistic normal operating conditions, the Outback won't go significantly faster; any small increase in speed will come at an enormous energy cost and greatly reduced range, probably not what Tom's friend had in mind. 8)


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PostPosted: Sun Dec 04, 2011 5:17 pm 
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Tom Kirkman makes alot more sense than the rest of you.... "hull speed" hocus pocus deals with the sailing sector...at least that's the only circles I hear it being discussed.
At least as to the first post. Tom talking all that "control" business just clouds the water because the principle that was brough up in the first post tries to equate hull length to top end speed. and that's Sailing. Why do "yacht races have class sizes based on hull length? Because of the hull length to top end speed formula.

From my perspective Tom's got it completely correct.
"My Perspective" being someone that has played with some propeller pumps...
The props on the different trolling motors are reaching maximum rpm's ( I wonder if they have the same rpm's or stronger motors run a little faster than another) and burning thorough the water and not getting a better bite.
Simple enough to test would be to take the bigger electric trolling motors run them with the standard props, and then put beefier props on them and see what they do.
If you give a bigger motor the same work to do then all it can do is do the work the prop gives em... if i were to build a prop pump and put a bigger motor that ran the same RPM'd on a pump with a small prop I wouldn't pump more water.


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PostPosted: Sun Dec 04, 2011 6:23 pm 
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frisbee wrote:
Tom Kirkman makes alot more sense than the rest of you.... "hull speed" hocus pocus deals with the sailing sector...at least that's the only circles I hear it being discussed.
Hull speed has nothing to do with method of propulsion (prop, pedal, paddle, sail or fan). Maybe this will help -- it deals specifically with kayaks (for a quick overview skip down to the charts -- speed, drag and HP required):
http://www.keelhauler.org/khcc/seakayak.htm

Any boat still has the same thrust requirement if a propeller replaces a paddle or sail. If an electric motor is the designated method of propulsion, prop speed, pitch and HP are are combinations that have to be solved in order to provide the required thrust to reach a given speed.


Last edited by Roadrunner on Sun Dec 04, 2011 7:27 pm, edited 1 time in total.

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