Jon Nellis
Contributor
[FONT=&]DPV Comparison Demystified.[/FONT]
[FONT=&] Jon Nellis - Logic Dive Gear[/FONT]
[FONT=&]So you know where every little critter lives at your local dive sites (many of which you have, by now, named) and you're getting a little bored seeing the same things. If you had more gas, you could stay down longer and swim out farther, but then there is the deco obligation that goes with that extra bottom time, not to mention the extra tanks you have to carry. It must be time for a scooter.[/FONT] [FONT=&]
So you search the internet and stumble upon the Tahoe Benchmark Test comparing almost two dozen scooters. You see how many pages are in the report and say to yourself "I'll just look at the results and skip all that verbiage" only to see a big table of speeds and distances and thrust forces and watt hours and now you're even more confused as to what all those numbers mean and how does one actually use this plethora of information to compare the different models. You break down and start reading through it, as it explains how they performed each test and why and you think to yourself "hey, these guys are doing a pretty detailed comparison, but they didn't tell me how to use the data to actually pick a good performing scooter." Well let me help explain it a bit.[/FONT] [FONT=&]
The Results Compendium on page 15 is where I'll start. The "Tahoe Standard" green section is the standard diver configuration (drysuit, single tank, backplate and wing). This is where three different divers performed the same test runs along the quarter mile run on each scooter and the results were averaged. The blue section is the "Tahoe Tech" configuration, which was not averaged between the three divers and will be explained further down the page.[/FONT] [FONT=&]
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[FONT=&]Max Speed[/FONT]
[FONT=&](light green columns)[/FONT] [FONT=&]
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Speed[/FONT][FONT=&] - The speeds of three different divers operating at the scooter's maximum speed with a full battery charge over two, 1/4 mile runs each were recorded and averaged. If you are looking for a scooter to race, this is all you really care about, but anyone that has spent any significant time doing over 240 feet per minute (fpm) will tell you that it is not all that comfortable, especially with a single crotch strap. This column correlates pretty closely to the THRUST column and tells the average diver that when they need to get out of a bad situation (surprise ocean current change, regulator failure in the back of a cave, medical emergency, etc...) this is the kind of performance they can expect, but it consumes much more power to cover the same distance at a faster speed, so your range will be decreased the faster you go.[/FONT] [FONT=&]
W (Watts)[/FONT][FONT=&] - The average power required to achieve the maximum speed. By dividing the batteries capacity (expressed in Watt hours) by this number, you get the run time at max speed in hours. Not a particularly useful number for the scooter shopper, as there is no constant for comparison, since every scooter has a different top speed. [/FONT] [FONT=&]
Wh (Watt hours)[/FONT][FONT=&] - This is the quantity of power the batteries delivered at maximum speed before it stopped working or became really slow. You should expect a little more power than this from the batteries under normal operation since the higher discharge rate at max speed results in power loss due to heat generation inside the batteries and also the electrical conversion properties of the batteries. This number is used later to generate the cruise range based off the cruise power.[/FONT] [FONT=&]
Miles[/FONT][FONT=&] - This is the measured distance that each scooter ran at max speed with divers being swapped out every half mile, so as to create an averaging effect. Since the average scooter purchaser would not want to go full speed on the fastest scooters for more than a few minutes, this number does not hold much significance for most purchases unless they plan to do that.[/FONT] [FONT=&]
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[FONT=&]Cruise[/FONT]
[FONT=&](medium green columns )[/FONT] [FONT=&]
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[FONT=&][FONT=&](medium green columns )[/FONT] [FONT=&]
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W (Watts)[/FONT][FONT=&] - Another power measurement, but this time it is highly relevant. This is the amount of power that each scooter uses running at a 150 feet per minute cruise speed. This time there is a direct comparison since all the scooters in the test are now going the same speed. What this tells you is how efficient each scooter is at moving an average diver at a normal operating speed. The lower the Watts number, the more efficient that scooter is at converting electrical power into motion. Since a scooter's range is limited by the size of its battery. A more efficient scooter needs a smaller capacity battery to travel the same distance as a less efficient scooter with a larger, capacity battery. Since most scooters are marketed by their battery size, it can be confusing for a purchaser when they buy a scooter with a large battery and then find out a smaller scooter has more range, because it uses half as much power to accomplish the same thing.[/FONT] [FONT=&]
Miles[/FONT][FONT=&] - This is the calculated distance that each scooter would travel. It is calculated by dividing the battery's capacity, in Watt hours, obtained during the Max Speed portion of the test and dividing it by the 150fpm cruise power. This results in a run time, that when multiplied by 150fpm results in the calculated cruise range. This column shows the effect of scooter efficiency. This range is what you can expect under normal operation and is a better comparison than the Max Speed range since it is a direct comparison between all the scooters in the test operating at 150 fpm.[/FONT] [FONT=&]
Thrust[/FONT] [FONT=&](Redish column)[/FONT] [FONT=&]This is the static force that each scooter was capable of creating in a test pool. As mentioned earlier, this correlates to max speed and for the average diver, tells them how much force will be exerted by their crotch strap at full power. A semi-useful metric for when you need to tow another diver or two with dead scooters.[/FONT] [FONT=&]
Tahoe Tech[/FONT] [FONT=&](blue columns)[/FONT] [FONT=&]This section consists of single runs after the test divers exchanged their single tank for a set of doubles and an aluminum 80 stage bottle slung on the left side. The Tech configuration data is not averaged across three different divers. The results shown are from single diver runs and since there were noticeable and consistent differences in each diver's drag, the results do not offer a good comparison for purchasing decisions. These differences can be seen in the bar graphs on page 22. If the generally slower (higher drag) test diver performed the tech configuration test, it resulted in a slower result that was not averaged out, like the single tank test. For this reason, I don't recommend basing any comparative purchasing decisions on the Tech configuration results. They are useful data point to show the differences in speed and range from a lower to higher drag configuration, which should be factored into the purchaser's decision, as to how much of a reduction in range from the single tank cruise configuration they might expect.[/FONT] [FONT=&]
Weight[/FONT] [FONT=&](Gray column)[/FONT] [FONT=&]If anyone needs further explanation as to what this column is, they should take up badminton. The only question here is, how much weight do you feel like carrying around out of the water, since they are all neutrally buoyant in the water. Surf entries can be difficult with a large scooter and boat operators don't like to strain their backs pulling a heavy scooter out of the water. More weight is usually appreciated when traveling long distances, since it helps the scooter track straighter because the large mass is harder to change direction. The smaller and lighter scooters are more maneuverable, making them easier to control when navigating kelp or tight winding caves.[/FONT] [FONT=&]
Exceptions[/FONT] [FONT=&]There are a few exceptions in the result that are explained in the verbiage, such as scooters that could not operate at the prescribed test speed and tests that ended prematurely. Additionally there are design limitations of some scooters in the test that resulted in less than optimal results. Particularly the use of variable pitch propellers, since they become increasingly inefficient as they are de-pitched, but when you are trying to match your dive buddy's speed you may be forced to run at this less efficient setting and your range will suffer because of this type of speed control design.[/FONT] [FONT=&]
Other things to consider.[/FONT] [FONT=&]Now that the performance aspect is under control, there are some other things you need to consider when operating a scooter that might affect you purchasing decision. [/FONT] [FONT=&]
Speed matching[/FONT][FONT=&] - Each diver typically has a different amount of drag, so two scooters operating at the same thrust will result in different speeds. If you plan on solo diving all the time, then the issues of speed control is not as big a factor, other than convenience. When scooter diving with a buddy or two without a fully variable speed scooter, it can require extra effort to match speeds on scooters that have a few preset speeds and even harder if you have to stop and adjust the propeller pitch. On scooters with a few preset speeds, this can be addressed by routinely speeding up and slowing down to stay near your buddy if you cannot match their speed exactly. This is not difficult to do, but can be a bit annoying, especially if you accidentally change to the wrong speed. On a scooter with a single speed motor and a variable pitch propeller, it is more difficult if you are falling behind, since now you have to stop, thereby increasing your separation, and adjust the pitch to try and catch up. In low visibility, you could lose your buddy very quickly. Most people try to mitigate this by taking time at the beginning of the dive to adjust each scooter so they go the same speed and then only go that one speed throughout the entire dive. The problem with that scenario is that if the diver in back has an equipment issue, he can't catch up to his buddy quickly to let him know, because he now has to stop, adjust the propeller and then chase down his buddy, who is now further away. Some people also set their scooters a little faster and then create extra drag with their fins or body to match their buddy's speed, but doing this is a bit of a waste of the battery's charge. There is also bit of a learning curve while getting your muscle memory used to diving a scooter with a few preset speeds or variable pitch propeller. [/FONT] [FONT=&]Additionally, when recording video underwater it is much more fluid following something with fully variable speed instead of having the noticeable transitions of switching between a few distinct speeds. Stopping to adjust your speed is really not an option when filming moving sealife or other divers.[/FONT] [FONT=&]
Getting cold easier[/FONT][FONT=&] - Since you are not kicking for propulsion and there is increased water flow over you, it is common to get colder on dives than you had previously been, when swimming. More insulation or a heating vest are the most common solutions.[/FONT] [FONT=&]
Air consumption[/FONT][FONT=&] - As an upside to getting cold from not kicking, you will also use less breathing gas, since you are not kicking. It's like having larger tanks without the extra weight, but the deco obligations do not change. For planning purposes, especially in an overhead environment, you will still need to know your swimming SAC rate, should the scooter fail and you now have to swim out.[/FONT] [FONT=&]
Care and feeding[/FONT][FONT=&] - Just like cleaning your gear, having regs serviced and filling your tanks, scooters require a bit of attention as well. Unless you only dive in fresh water, are able to recharge your scooter without opening it and it does not use shaft seals, you will need to do some kind of maintenance. Salt water divers need to rinse their scooters well when finished diving to prevent the buildup of salt crystals. These sharp crystals can damage o-ring seals and shaft seals. If the saltwater is allowed to remain in contact with metals, it can result in corrosion which can lead to flooding if it occurs on a sealing surface. Scooters made with plastic housing do not have to worry about corrosion, but they are more susceptible to accidental damage of the sealing surfaces, since they are softer, which can result in flooding. Additionally, rotating shaft seals do not last forever and need to be replaced regularly. Many manufacturers recommend replacing the seals annually. Whenever the scooter is opened to recharge the batteries, the seals and sealing surfaces need to be cleaned and carefully inspected for damage or debris and re-greased before reassembly. [/FONT] [FONT=&]If the scooter is not used for an extended period, certain batteries such as sealed lead-acid (SLA) and Nickel metal hydride (NiMH) need to be recharged occasionally due to their self draining characteristics. Additionally, these types of batteries must be allowed to vent hydrogen to the atmosphere, so they should not be recharged inside a closed scooter. Lithium based batteries can be stored for extended periods and recharged inside a closed scooter, but are generally more difficult to transport due to the government regulations. Most Lithium scooter batteries cannot be taken on a passenger airplane, since their capacity is beyond the allowable limits, which varies for different countries. [/FONT] [FONT=&]
Reliability and support[/FONT][FONT=&] - Some people want a scooter that's easy to work on, from a manufacturer that has awesome product support and easily obtainable replacement parts, while others want a reliable product that doesn't require any significant maintenance in the first place. With scooters, most people prefer reliability since support is rarely available half way through your dive when you break a propeller blade, shear drive pin or wrap some line up in the propeller and strip the teeth off the plastic clutch plate. Nobody likes getting towed back or worse, having to abandon a flooded scooter because they thought they could get a few more dive before they needed to change their shaft seal. Most of the scooters today in the +$2000 price range are pretty reliable, but a few have weak points or require more maintenance attention. This is something to investigate for the specific unit you are interested in before making a decision.[/FONT] [FONT=&]
Cost[/FONT][FONT=&] - Well, as anyone would expect, that Cadillac with the adaptive suspension, 300hp, heated leather seats and collision avoidance system is going to cost a lot more than the 4 cylinder, manual transmission import without power windows. Unlike being able to tell much of a difference between a $300 regulator set and a $600 regulator set when you are first starting out, you will be able to tell the difference between a $3000 scooter and a $6000 scooter, but that is a sizeable difference in price that definitely factors into most people's buying decision. It does however correlate to a significant difference in performance, features and maintenance requirements. Your best bet is to take a few for a test drive and make a decision based on the test data and your brief experience with each model, since whatever feels most natural or intuitive initially, will usually be the easiest to master in a shorter time.
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