Diver Propulsion Vehicles (DPV) - Underwater Sea Scooters

Diver Propulsion Vehicles (DPVs) range from small sea scooters to small wet submarines. For recreational fishermen, hand-held underwater sea scooters are the most important.

sea scooter 2Underwater scooter and scuba diver.

DPV Types

Common types of DPVs are:

- underwater scooters are hand held devices that diver hold with (usually) both hands. They are usually small and can be handled without any specialized equipment.

- manta boards are unpowered boards that are towed by the boat. They have surfaces in the shape of aerofoil that provide downward force when towed. By changing the 'angle of attack' (angle between 'direction of speed' and 'direction of board's longitudinal axis'), diver adjust diving depth. These boards resemble manta ray fish, hence the name of this devices.

- manned torpedoes are cylindrical or fish-shaped DPVs. Divers usually sit on top of it or in hollows inside. They carry one to two divers, can have scuba air tanks mounted inside and rudimentary navigational equipment like gyro and magnet compass, sonar, lights, depth meter etc. Usually, these devices are too heavy to be operated using human strength only.

- wet or even dry subs are practically category on its own. These vehicles carry divers inside the hull and protect them from water stream and propeller wash. They are very complex and expensive devices/vehicles and using them requires extensive training.

Underwater Scooters

For recreational fishermen, underwater scooters are the most important type of DPVs.

sea scooter 3

Two divers pulled by large underwater scooter.

Size of underwater scooters vary from small 'pool toys' to professional underwater scooters. Small scooters weigh few kilograms only and can pull single diver at limited speed and usually have limited endurance (30 minutes maximum, maybe little bit more if not pushed too hard). Larger underwater scooters weigh up to 30-40 kilograms, can be handled with one or two adults on the deck, but when put to the use, they pull two scuba divers with ease.

Price of these devices vary from 100 or so dollars/euros to several thousands (or even more) dollars/euros. Those cheap ones are barely more than pool toys, while expensive ones are professional grade equipment that can withstand severe punishment by frequent use in hostile environment. If you have such expensive equipment (designed to be used by professional divers in extreme environments) and you use it carefully and maintain it as required, it performs flawlessly for long period of time.

sea scooter 1Depth range vary from several meters to 40 meters. 40 meters is limit for safe scuba diving using ordinary air for breathing. Even then, one must use diving tables for decompression, unless dives were rather short (again, for safety reasons, check diving tables). If you are free diver, then use these devices according to your maximum diving depth and be very careful - annually, much more people die diving than by being attacked by various sharks and other marine animals. Better safe than sorry.

Buoyancy is usually neutral to slight positive, model dependent. Slight positive buoyancy ensures that if scooter is left unattended, it will slowly reach the surface. Neutral buoyancy is used mostly for professional underwater scooters - when left unattended, but connected with rope to the diver, it will float freely in the water and will not disturb the diver at work by pulling him up or down.

Also, some models have variably buoyancy and can be used for fast dives and similar - however, personally I would recommend slightly positive buoyancy scooters.

Power output varies from few hundreds watts to few kilowatts. Stronger engine doesn't mean more noise, since expensive models usually have better acoustic insulation, better bearings and better and larger propellers. Smaller underwater scooters often have high-pitch sound due to use of smaller, fast rotating propellers.

Energy source are almost exclusively batteries. Lead-acid batteries are heavy, but very cheap. Non-lead batteries provide more energy for less volume and weight and are commonly used for underwater scooters of all sizes. Most common non-lead batteries are NiCd, NiMH and various lithium batteries.

Endurance again depends on the size of device, battery type and capacity and the way the scooter is used. When scooter is pushed heavily, it drains battery fast, with running times often less than 10-15 minutes for cheap models. On the other hand, big scooters equipped with lithium batteries and large propellers, when used for slow diving around, can last for hours. Most of the these devices have rechargeable batteries that are easily replaced - a good reason to have at least two batteries (or battery packs) per single device.

Little bit of math: for example - let's say that you are considering to buy underwater sea scooter that has:

- maximum power output 1 kW (1000 W),
- motor that operates at 24V,
- two 12Ah, 12V, SLA batteries,
- 20% battery discharge alarm,

and that manufacturer claims that that scooter can pull diver at 6 knots and that it has maximum endurance of 9 miles.

Note: speed estimates are just estimates and they depends on numerous things, but they are nice starting point.

Now, assume that motor is ideal and has no losses - 1 kW of power, at 24V, requires around 42A current. 12Ah SLA battery, with alarm set to 20%, can theoretically provide this current for about 13 minutes. Since it is just theory and taking into account motor losses, battery capacity losses due to high discharge current, operating time for this scooter at maximum power would be around 8-9 minutes. ONLY 8-9 minutes.

But, have in mind that diving speed of 6 knots is really fast and hard to achieve for scuba divers. On the other hand, free divers can achieve high underwater speeds, but their endurance underwater is limited - after reaching surface, most of free divers require some time to recover and this time is usually more than enough for SLA battery to rest a little bit (low pulling speed while on surface) and prolong operating time.

nimh batteryIf you really want high underwater speeds while diving with scuba equipment, you need a really strong scooter with NiMH or lithium battery pack. Good thing is that such batteries are now common even in underwater sea scooters under 100 or so dollars.

Do you really need 1kW output sea scooter, or even stronger? If you are not considering pulling 2-3 fully equipped scuba divers, then you don't need such a powerful scooter. Also have in mind that most professional diving scooters rarely have power output of more than 500W. Why? Well, to double the diving speed, one has to increase output power by factor 8 (2 to the power of 3 equals 8; 23=8). In real life this factor vary between 5 - 10, sometimes more. In order to double the speed of 500W scooter, one would need around 3-5kW. Endurance would be really to short, unless really expensive batteries are used.

So, professional sea scooters are focused on reliability, ease of use under difficult conditions, have much longer endurance (for example, 400-500W maximum power, at least 2 gears AND high capacity, high voltage NiMh or lithium batteries), etc. but can be heavy, really heavy and expensive.

Safety is very important. First of all, propellers are protected with metal or plastic mesh so that not even small objects (small fish, rocks, fingers ...) can enter propeller area. Also, diver is usually positioned behind and above the scooter so that propeller wash flows under the diver. It is also very important to note that DPVs enable diver to change its depth rapidly and this can be very dangerous. If you are free diving with such devices and you start to feel bad, ascend immediately. Not sure what is going on? Let DPV go (most of them have slight positive buoyancy!), exhale few bubbles and follow those bubbles to the surface...

Some models have features like battery alarm, depth alarm and failure alarm:

- battery alarm create enough noise to be heard even when sea scooter is under full power. These alarms are used to warn divers that battery is low - they are usually set to beep at 10-20% of battery left. This way, diver can safely ascend and reach boat or shore, to replace batteries or take a new scooter.

- depth alarms warn when sea scooter reaches predetermined depth. For example, some models that have operating depth of 40m, have this alarm set to, for example 35m. Some free divers set this alarm to depths of 15 or 20m - just to be sure.

- failure alarm warn on general failures like seal breach, motor or battery overheating and similar.

Note that not all models have these features - be sure to read the manuals before you decide on buying any equipment, not only sea scooters.

Automatic shutdown is a must-have feature of any DPV regardless of its price. If sea scooter doesn't have it, don't buy it - who knows what is else missing.

Sea Scooters Under 100 Dollars - Cheap DPVs

Today, there are cheap DPVs, even under 100 or so dollars, that it is simply a shame to have one of those on boat.

Although most of these devices are hardly more than a toy, they still can be very useful in many situations.

When buying such devices, there are few things to consider:

- power output - these are low power devices (rarely more than 250-300W) and their pulling power is limited. However, they are used for pulling single diver, or more often, a single swimmer to the limited distance.

- battery type - most of these devices use lead acid batteries, some more expensive models (price range usually little bit higher than 100 dollars) have NiMH batteries. Lithium batteries are rarely found on cheap DPVs, but if you are ready to pay 300 dollars or more, you can find very usable devices.

- motor types - mostly brushed DC motors, some models have PWM (Pulse Width Modulation) controllers with brushed or brushless motors. However, most of the cheap sea scooters have 2-3 speed settings.

- materials - in order to cut on the price, cheap materials are used. This limits durability of such devices - remember, most of these sea scooters are little bit more than ordinary pool toy!

- diving depth - rarely more than 20m, even such depths can limit durability.

- buoyancy - usually, these devices have positive buoyancy, in fact, some of them floats so well, that they can be used almost as life jacket :)

Cheap DPVs or not? Well, I have found myself many times in situations to be anchored close to the shore (100m, for example), with lines in the water and I wanted to check if there are locally available baits or chum - mussels, worms, prawns etc. - in the shallows, near the shore. It is amazing how 100$ toy can come handy in such situations :)

If you plan on using DPVs regularly, for swimming and diving, if you can, consider more expensive models - just my 2c. But if you have little kids and you want them to learn how to use these devices, get them some of these 'toys' and if they broke it, no harm done (at least not much!). And if you happen to like playing around with DPVs, you will sooner or later buy some sturdy and reliable, more expensive, model that will last for years to come.

DPV Usage

Beside pure leisure and recreation, sport fishermen can use DPVs for various purposes.

First of all, DPVs help a diver to cover larger area, regardless if diver is equipped with scuba equipment or not. This can help in finding the fish, fish positions, fish traps, lost anchors and another equipment and similar objects that can't be always located with GPS and sonars. Finding and collecting live bait in the form of sea shells and various crabs is sometimes much easier using underwater scooters. For example, imagine collecting a row of 20-25 small prawn traps as a free diver - that could be physically challenging and can lead to time loss, while fish is awaiting near by :)

Also, when fishing for example, from shore, when using baits that can be harmed by casting, it is sometimes better to put on a wet suit and swim 100 or so meters from the shore while pulling the line and being sure that bait is OK - this is much easier to do with underwater scooter than just by swimming. Or you can use small RC fishing boat.

Anyway, underwater scooters are optional equipment not often found on fishing boats, but they can come handy. After all, if fish is not interested in your baits, cast your baits far away and jump into the water with DPV and have fun :)