Horsepower Conversion Explained

[mrsswordfisherman]

Below is an excellent piece of work written by member GreyNurse. Thank you Martin for the contribution

Horsepower conversion explained ......or not

If you are planning to, or have bought a kayak for fishing, it’s inevitable that at some stage you will want to purchase an electric motor. Most people are familiar with outboards being rated in horsepower, but are not so sure what a 24 or 40 lb thrust electric motor will give them by way of performance.
I’ve seen a few posts now where people ask to convert horsepower (HP) to pounds of thrust (lb thrust) in relation to comparing outboard motors on boats to electric motors on kayaks. The question often comes up under the topic heading “Which motor” or “Which battery” or something of that nature. I know. I posted a similar question when I was in the process of deciding on a battery for my motor.

Converting HP to lbs thrust is difficult and a somewhat pointless exercise. The reason being that HP is the power available (from an internal combustion engine), as opposed to lb thrust which is the power supplied (typically from a battery driven motor). Many things affect a simple conversion, such as battery voltage, the yak’s hull shape, the hull displacement, which directly related to how much non buoyant weight is in the yak, and the force against which the yak is opposed (read speed, wind and current). Even propeller size and pitch can affect the figures. However, manufacturers try to anticipate an average for each size of motor they make, so you can get the best motor for the size of your boat or kayak. Most, if not all battery powered motors for kayaks are designed to run off a 12volt battery.
For those who must know, the best that I could estimate is that a 12v 24lb thrust motor running off a fully charged battery, pushing a 3 meter long, 80 cm wide SOT kayak with approximately 100kilos of weight (includes the weight of the yak, pilot and gear), is roughly equivalent to about 1/5 to 1/6 HP. That’s not as bad as it sounds. But don’t expect more than 7 kph top speed in still water and still air, which is about the same as a Hobie peddle drive. The difference, of course, is that you’ll have a bit more energy at the end of the day using a motor.

Battery Performance

In reality, what it’s all about is how much run time you can expect from your motor to cover the maximum distance. This will depend mainly on the capacity of the battery. If the motor draws 20amps at top speed, then a 60ah battery should last for 3 hrs in a perfect world. But as I’ve alluded to above, factors such as shape, weight, wind and tide can slow you down. The more those factors slow you down, the more time you need to get from A to B, therefore the less distance you can travel before the battery is depleted.
The type of battery is important as well. People new to using a motor will think a car battery is required. While it will power the motor, it’s not the best choice, as car batteries are intended for fast current draws when starting the vehicle’s engine, then being continually charged by the alternator. They are also often a wet cell (flooded lead acid) design, meaning that they can leak acid. The better choice is a deep cycle gel cell, or AGM (Absorbent Glass Mat) battery. These batteries use a jellied glass matt to absorb and hold the acid. They are designed for continuous current draw and charge much more quickly than flooded lead acid batteries. They are lighter than the flooded lead acid car batteries and can be orientated at odd angles to suit your particular installation without fear of acid spillage.
The one caveat with these batteries is that they do not tolerate being repeatedly completely run down. This practice will dramatically shorten the overall life of the battery. They will last years, however, if you can recharge them before much more than 30% current drain. As a guide, below are the cycle performance figures for both types of batteries.

Depth of Discharge Starter Battery Deep-cycle Battery

100% 12–15 cycles 150–200 cycles

50% 100–120 cycles 400–500 cycles

30% 130–150 cycles 1,000 and more cycles

Extending Battery Usage
So what can you do to improve your lot and maximise your time on the water under power?

• The purchase of the kayak is a highly individual decision based on a whole range of factors I won’t go into here. But obviously, shape is a consideration. Fortunately, kayaks by their very nature of purpose are designed to be quite hydrodynamic. Just some are more so than others. I will say no more than that. The choice is up to you.
• Be realistic about the ground you want to cover. Make a plan before you go out and don’t be distracted by that interesting bit of water across the other side of the waterway that you didn’t plan on exploring that trip. It’ll still be there for next time.
• Launch as near to your chosen destination as possible. Kayaks have the advantage of not needing a boat ramp to launch and retrieve. Any bit of beach with a nearby place to park the car is a distinct advantage over having to launch a boat at the often overcrowded ramp.
• Use the tides and wind to assist you. Try to launch downstream on an incoming tide and upstream on an outgoing tide, making the return journey after the tide has turned. The wind is not so predictable, but checking the weather forecasts may help you decide the day you go out. It all comes back to making that plan. Sometimes you may get stuck with a slight headwind regardless. That’s OK if the wind is light. However, if it’s too windy (15-25kmh) it may be better to leave your outing for another day. Unfortunately that’s one of the downsides of kayaking.
• Use a slower speed setting on the motor. If you have the tide in your favour, you can drop the speed setting and enjoy the scenery along the way, pulling fewer amps from the battery and extending the battery life for that trip.
• Paddle as much as your fitness and willingness will allow before turning to the motor. Even when under power you can still paddle if your yak tracks well. Every little bit saves amps.
• Travel as light as possible. For a start, factor in the weight of the yak as well as weighing everything, including the motor and yourself. Now, do you really need a heavier 60ah battery when you could manage with a lighter, say, 26ah battery? Leave the anchor at home, or buy a plastic anchor. Only take as much fishing gear as you absolutely need. Be ruthless. Your yak has a rated load capacity which was probably stated in the specs. You don’t want to exceed that in any case. But don’t skimp on safety. There is equipment you must have on board. It’s worth sacrificing some weight for the assurance of a planned return.
• This may seem petty and too obvious, but remember to tighten the bung if you have one. Dragging tens of kilos of water around in the hull will not do the battery any good to say the least. Not to mention getting to shore before sinking, then having to unload everything and draining the hull, just to be able to continue. Talk about a buzz kill!
• Recharge your battery religiously after each outing. Use an “intelligent” charger, rather than a car battery charger. Intelligent chargers automatically monitor battery condition and, if left on after the battery is fully charged, will “top up” the battery’s charge as required, then drop back into a monitor and stand by mode.

Having applied these tactics to my own kayaking experiences, I’ve managed to almost halve the drain on my battery, doubling my time on the water under power as a result. My 26ah battery that powers my 24lb thrust motor lasts me hours, despite the advice I received from well meaning people telling me I “must” have at least a 60ah battery. As long as I charge it when I get home, I’ve never had the battery run flat on me. If you do decide to use a larger capacity battery and employ the above points, there’s no reason you couldn’t make a whole day of it.

Happy yakking

GreyNurse

[ambrose607]

What good advice. I've just come across this post as i am in the process of pimping the family kayak. Nursie you will remember that I sent you a PM asking you about 12v motors for kayaks. Thanks fo your help. This ppost has all my other questions answered on batteries and chargers. Great informative post and interesting read. Well done.

[GreyNurse]

My pleasure Ambrose. This is what the site is all about. Sharing information. And I don't mind being corrected if I get something wrong. As accurate as I try to be, I'm still just a broken down old alter boy.

[batfishing]

Very well written.

I would like to add something with batteries. Common questions with batteries, are Gel, conventional, start, deep cycle etc...

Whichever way you decide to go with a battery, be very aware there can be quite big differences between 'Brands' of battery of the same specification.

Personal experience antidote here.

We imported a variable message sign on a trailer or from a company in China to evaluate. One of the first things we noticed was the batteries unable to give us 4 continuous days of running.

What ensued was, adding an extra solar panel, replacing the solar panel controller to an Australian unit that was reliable and maximised the available amps. Re-wiring from the solar panels to the batteries completely as the wiring was undersized and overheating.

Checking the batteries and finding them to be below acceptable condition.

We replaced with the same size and specification but a quality brand.

As we removed the old batteries and replaced the new, we noticed a weight difference as we lifted them. So we weighed them, the difference was 7 kg per battery.

That weight difference is lead, which explained the poor performance of the batteries.

Sent from my iPad using Tapatalk

Edited September 9, 2015 by Ian Bat (Roadrunner Parts)

[Foghorn]

I am thinking of getting a kayak, thanks that is great advice.