Scooters, how FAR do you GO?

[rjack321]

Here's a snippet of what we had planned with a visual reference for 3/4 of the trip. Our dive plan was to stay at depth the entire trip to avoid boat traffic (with 6' SMB's in case of an emergency). We were also thinking of using cave line from the break wall toward the wreck and using a gps deployed to the surface long enough to get a fix and reel it back down to fine tune our location. The scooters we're thinking of taking are Hollis H-160's which the most critical part would be to get us off the break wall and to the wreck (2000') and back.

For the record, I have two lifetime memberships on a charterboat that I could hit this wreck on any weekend. The point of the dive is to have fun doing something no one else has done, and maybe get others interested if it proves to be a routine type dive with some semi-permanent guideline laid.
View attachment 165598

You aren't the first person who's decided to scooter a boat dive because they are bored and looking for new challenges...

I think you should work up to this though. If you need line to figure out which way is home from that you have major issues. If you can't hit the target without a floating GPS, then you need more practice running a consistent course.

 

[ScubaInChicago]

You aren't the first person who's decided to scooter a boat dive because they are bored and looking for new challenges...

I think you should work up to this though. If you need line to figure out which way is home from that you have major issues. If you can't hit the target without a floating GPS, then you need more practice running a consistent course.

Following a compass heading while on the trigger and figuring out distance traveled based on time isn't the part that's bothersome. The GPS is more a tool to spot check around the half way point to make sure current or a tenth of a degree isn't throwing us off the mark and again near the target. Viz is usually less than 20' most of the time and could see how easy it would be to over/under shoot the wreck by a 100'+. It would afford us a safe way to double check and not need to run a search pattern around the suspected target.

The line off the wall wouldn't be there for the sole purpose of returning back so much as being a semi-permanent lazy reference for return to/from trips by us or others. Outside of personal gratification, I would like to inspire more interest in scooter diving (not necessarily this dive) in the area if possible. I can count on one hand the number of people who own a scooter in Chicago.

 

[corvettejoe]

We do a lot of shore diving in South Florida. Scooters work great to get us way out to the further shallow reefs. We usually go 1/2-3/4 mile out, then swim around and do our thing, then scooter back. I wouldn't go any further on my scooter than that though (Oceanic Mako with some kind of miracle battery that lasts almost 2hrs on max thrust). My buddies have SS scooters.

 

[fdog]

We routinely do scooter runs of 1 to 1.3 miles out here on the MoCal coast.

So, to set the stage, you'll need some Google Earth data. First, download the Point Lobos Overlay.
Then, to place over this, here are typical dives for us:

Twin Peaks circuit (1.1 miles, Google Earth path)

Montana (1.3 miles, Google Earth Path)

---

For those that just want a summary without getting a headache from thinking:

• Typically we do these dives with Cuda 650's or bigger, like a Magnus or a Cuda Fury. The 650 is the lower range of what's acceptable, as is a UV-26.
• You'll need to tow a scooter if you're diving a Sierra, Gavin short or UV-18.

---

Warning! Nerd content ahead.

Planning
The first part is determining if your scooter has the range to execute the dive. To do this you'll need to know:

• Your true underwater speed
• Your true capacity (either in Watt/hours, or, minutes of burn)

Underwater speed
We have pestered a lot of divers, asking if we can add a data recorder to their scooter while they go on a dive. After several years of data harvesting, one thing is clear:

Scooter drivers universally overestimate their underwater speed.

As a number, we find that technical divers - doubles & stages - typically run 135 fpm on a Cuda in gear 3. If on a Gavin, 120 fpm. (These numbers are generated from our surveyed track)
The reasons for this over-estimation are varied, ranging from a long history of manufacturers overstating performance, to a certain machisimo - but, the results are the same. Divers are always slower than they think they are.

From there, if you know burn time or Watt-hours, it's simple math to produce anticipated range. This tells you if your scooter has the legs to do the mission.




Reserve
We've done a lot of testing involving instrumented scooters on our track, in a towing situation. The results were surprising and not what we'd expected.

Intuitively, we expected that towing a diver would double the drag. The numbers do not support this; it is actually 2.2 times the drag. In hindsight, we saw from video that most divers end up with mismatched buoyancy as they are towed, due to a lack of visibility by diver #2. This subtly drags the pair into a position that presents greater surface area.

The real speed results (on a Cuda 650 @ gear 3) are 83.7 fpm when towing with doubles and stages.

It is possible to click up the gears on a Cuda or Magnus to maintain velocity. This costs in Watts, though. We needed to click up to gear 6 to 7 to maintain velocity. This took power consumption from 321 Watts (gear 3) to 709 Watts.

So, in a towing situation, you have two choices:

• Keep the scooter at the same "gear". You will go slower, which means more time, which in turn increases the battery draw over the same distance.
• Go faster. This increases battery draw over the same distance and time.

After trying stuff out on the track, we found using 0.8 to generate a safe range with a reserve was inadequate. You would be forced to swim ~ 1/4 of your return distance.

If you examine the typical rule of thirds as it applies to battery power, here you'll see that by accepting the slower towing speed, the scooter will complete the run. However, your gas burn will be ~40% greater over the towing distance - because of the slower speed - which will eat up quite a bit of your rock bottom reserve. Thus, you are vulnerable to cascading failures, and need to plan for additional (bottom or deco) gas.

Conversely, if you have a scooter that has gears, such as a Cuda or Magnus, you can go faster. If you follow the rule of thirds, though, your scooter will die 10% of the return distance away from the exit and make you swim. Dang.

---

Planning Summary

What we do...you're free to make your own decisions...


We look at the scooters in our team, and:

A) There are (any) single-speed scooters on the team: Plan the battery at thirds, and add 25% to rock bottom and deco gas.

B) All the scooters are variable speed: Plan the battery for a reserve of 1/(team). For example, if you had 2 divers in your team, you'd reserve half the battery ( 1/2 ) for towing. This means you'd plan for an out-distance of 1/4, and a return distance of 1/4.

C) If you scooters don't have the legs to perform (A) or (B), tow a scooter to make up the "missing distance".


Hope this helps. It's all based on the observed numbers we saw on the track.


All the best, James

 

[ScubaInChicago]

Thanks James, very helpful information. On the drag numbers, were those worst case #'s or did they get better with towed diver maintaining trim, buoyancy, and while staying in the slip stream?

On the fpm, we laid out a 100' spool and did 10 test runs and found 150 fpm was our running average on 2nd speed/prop pitch 7. I'm going to pick up a Watt's-up meter/Eagle Tree to put inline to accurately figure out usable burn times.

 

[fdog]

The drag numbers came from two opposite runs on the track (which is surveyed to be 1/4 mile long) with a 2 minute recovery pause at the far end. One of our test divers is a professional surveyor; the track measures 1325.5 feet with an accuracy of 2 inches.

2 minute pause: a test diver watches the clock during sprint testing of a Gavin Short body.


The value of 2.2 x the drag was generated by two 1/4 mile runs, so it is the average. The technique used for this towing test was that that is most commonly used, holding the crotch strap with the scooter stowed between the legs, similar to this:

Divers practice towing using the "crotch strap" method.


There is a method that can produce significantly lower drag numbers - approximately 1.9x the drag of the solo diver. This method uses a grip on the thighs, and the towed diver "sticks their forehead in the butt crack", as seen here:

Towing using the "leg grip" method


Although far more efficient, this method is quite tiring. Our test divers were only able to tow for one lap this way.

So, as far as towing goes, in technical gear, the best our test divers could do was 1.9x over a short distance. Consequently we use 2.2x as our planning number, assuming we'll be tired and cold and generally pissed off.

---

You'd referenced your observed speed over 100' of line.

We, too, started our first cursory tests this way. We found the speed numbers generated ended up being ...optimistic...would be a good way to describe it. The way we found out they were optimistic was that we'd work out a very accurate dive plan, and end up swimming home. What the...?

So, we gradually increased the length of our testing runs. First 200', then 400', then 800'. We finally settled on 1/4 mile, as the data we were generating seemed to match the real world performance of the scooters.

The reason behind this has to do with the voltage of the battery. As you start a test run, the battery has had a rest period, where its voltage recovers somewhat. This generates higher speeds in the beginning of a test run.

So we instrumented a popular scooter with a recording speedometer. Here's what we observed over the 1/4 mile run:

Speed decay as seen over a 1/4 mile run. Speeds in the first several hundred feet are roughly 15 fpm faster than sustained speeds.


It's this variability in speeds that makes us use two 1/4 mile runs for the Benchmark testing. The results more closely match real-world performance during a dive. If you wanted to produce high numbers simply for "bragging rights", then of course, you'd use the shortest run possible.

The speed decay isn't limited to the first 1/4 mile. We find that virtually every scooter exhibits a first fast 1/4 mile; then, a plateau of fairly consistent speeds; and finally, the last 2 laps are significantly slower.

Lap speed decay seen over the course of an Enduro test run. The scooter is running at maximum speed throughout.

So, I'd be wary of speed results over very short measurements.



Speaking of our technical speeds, our typical dive configuration is with a couple of stages, as seen here:

Scootering with two stages on the side, and a leashed stage. Typical speed is 135 fpm on a Cuda at gear 3.


Since the folks we scooter with are typically carrying 2 stages on the side, they seem to match this speed as well. Reducing bottles brings speed up incrementally.

Unfortunately there is entirely too much ego tied up in how fast of a scooter driver a diver is (or isn't). Personally I prefer to plan on keeping the team together in the easiest fashion, and planning for the most commonly seen speed accomplishes that.

---

Absolutely I would encourage getting a data recorder. If you have a choice between the Watts Up and the Eagle Tree, get the later, there is so much more data available for trending and troubleshooting.

Two of the Eagle Tree data recorders used in the Tahoe Benchmark

---

I'm not sure what scooters you're using, since you referenced 2nd speed @ pitch 7. For example, if this was a SS Viper, speed 2 would not be congruent with the Dive-X or Suex scooters. <shrugs> No big deal, your speed and range is what it is (within the measurement caveats discussed above). Good scooter drivers will settle on a plan that works well, even with a mixed team of scooter types.

Last thing: our testing has shown that using motor speed control is far more efficient than using prop pitch. If your scooter has variable speed, and you can avoid using prop pitch, you'll pick up ~6% more efficiency.


Kinda long winded, my apologies. I hate making "just do it" statements, and I'd prefer to explain what's behind the curtain, so to speak.


All the best, James

 

[LiteHedded]

i think your towing efficiency would be much improved with a grip on the crotch strap but with that diver getting in high and tight with his elbows over the thighs of the towing diver. and having the tower be flatter in the water.
it works quite well and I've been towed this way for quite a distance with no fatigue. it looks like the second pic but the grip is more secure and would work much better sharing gas

 

[PfcAJ]

So for towing procedures, the crotch strap method above isn't correct (within the context of DIR).

The diver needs to hold on with both hands, and pull himself in tight, head down. The tautness of the crotch strap locks the diver's hands in place. Elbows kind of stick out to the sides and the lead diver's lower legs hook under the upper arms of the rear diver. This is somewhat similar to what's going on in pic 2, but with the diver holding the strap vs the thighs.

If the rear diver pulls a hand out and squeezes the front guy's leg, front guy stops and communication takes place (otherwise the rear guy is probably just adjusting his wing or drysuit, no need to stop for that). Don't leave a working scooter to go into a towing position, either (thing OOG). Propulsion is wildly important, especially if deep and/or far.

It also helps a lot if the rear diver is a tiny bit positive. The extra drag really comes from the rear diver dragging the butt down of the front diver. You can see what I'm talking about in the pictures.

 

[fdog]

FWIW the crotch strap method tested was double-handed. I have some photos but for the life of me can't find them right now.

The tow started out as you describe, but, somewhere around the 8 minute mark the stages would shift and make it tough to keep the 2 divers together as a unit. This would make the assembly draggy. Happened both ways.

Conversely, the wrap-around leg grip made for wider spaced stability and squeezed the stages into submission. There's less wetted area, so there's less drag, you just can't do it for long.

<shrugs> So that's where the numbers come from.


All the best, James

 

[LiteHedded]

nah dog
it works fine with multiple stages. I promise you