Speargun ballistics

[redline]

Hi All,

Hoping to exploit some of the knowledge on this board in the matter of setting up and tuning spear guns. Pls forgive the possibly basic questions, but I'm relatively inexperienced...

First question: Is there an optimum spear shaft length relative to the gun length? What are the benefits and trade-offs in longer or shorter shafts?

My Cressi carbon is 130cm long and its 6.5mm spear is 140cm long.
One of my freediver guns is 117cm long and it 7mm spear is 152cm long.
My other freediver is 130cm long and its 7mm spear is 170cm long.

I would think that for maneuverability reasons the spear on these guns should project about 10cm beyond the gun muzzle (as with the Cressi). The two freediver spears project a whole 35cm beyond the muzzles! I would like to shorten these both by roughly 25cm, but am wondering what effect this will have on the ballistics?

I am presuming a flatter spear trajectory, possibly slightly better range, but less impact energy due to the lower mass of the spear.

Any advice/comments?

redline

 

[Pastor]

The reason for euro guns having a largish shaft overhang is because typically the shafts are only supported at the muzzle and at the trigger. The lack of central support lets the shaft sag, so the overhanging tip is used to counteract this sag. There isn't a single correct length for overhang but it should be roughly 1/3 - 1/2 the distance between muzzle and trigger. Deviating to far from that will result in either the shaft over the barrel sagging or bowing upwards, Obviously a shaft that is bending as the power is released into it will wobble more than one that was straight as the trigger was pulled. This is why you often see guns with rails sporting shorter shafts than similar looking standard euros.

 

[redline]

Hi Pastor,

Thanks for the reply. What you say makes a lot of sense, but in this case both are rail guns, so the shaft is supported all the way between trigger and muzzle, so I'm fairly sure there's no bowing (at least visual).

redline

 

[Pastor]

Then the idea of the rail is to support the shaft during the time it is being powered i.e. taking on energy. It is as you know at this time when bending/wobbling is induced. The rail will stop the shaft bending down which in turn reduces the upwards bending. The shaft length can in this case be anything up to about 1/2 the rail length.

A basic rule of thumb is that the longer the shaft the more accurate it should be in an ideal world. However there are many factors which will have an affect on accuracy, momentum and speed of the shaft. Some like shorter/fatter (stiffer) while some will like long thin shafts on the same gun. The goal is to eliminate shaft whip/wobble in flight, hence foxfish's wonderful enclosed track composite gun that uses a thin shaft (fast shaft speed needing less power used for smaller fish :king)

Shaft trajectory is dependant on its speed and aqua-dynamics

 

[foxfish]

For many years we (my brother OMD) believed that a longer spear was a big asset when hunting fish at close range. This simple & basic logic was based around the spear tip being closer to the fish! In fact this still applies when hunting the shallow weed beds that are so common where we live.
First thing we did to a new gun was fit a longer spear.

 

[redline]

Lol - hard to argue with that logic foxfish
My reasons for wanting to shorten the spear are:
(1) Better handling. The guns are both nose heavy.
(2) Shorter spear = lighter spear = quicker spear. Without having to reduce dia and increase flex.
(3) I love to tinker

redline

 

[spaghetti]

Let's put it this way: all the established companies who make and sell spearguns in Europe use the same barrell/shaft "ratio". That is: shaft=barrell (full band stretch) + 40 cm (so that a shaft for a 100cm eurogun is 140 cm, for a 110 gun it's 150, for a 75 it's 115 et cetera). They also give a tolerance of 10cm shorter than the standard if guns have a full length rail (100cm euro railgun: 130cm shaft)
The fact that ALL companies apply the same ratio, makes me think that this is the best, the most well balanced. You may think this is petty of me, trusting what big companies say, without a thought of my own. But they (the companies like Omer, Cressi, Beuchat, Sporasub, Picasso et cetera)invest lots of money and reputation, so I assume they know what they're doing when they sell guns kitted with a shaft length=barrell (full band stretch) + 40cm.
Redline: since you're an engineer who speaks italian, you will enjoy reading the article "balistica comparata delle armi subacquee, parte 1 e 2", that you'll find on the Maori website. Go to Maorisub - Entra, click on "articoli" on the menu on the left of the homepage and there you find it. Only italian version, unfortunately.

 

[redline]

Spaghetti: Thanks for the reply. My thoughts were exactly as yours, but I noticed that a lot of guns (at least the ones I've seen here!) do not follow such a simple ratio or proportion! Of my own guns, the spear overhang varies from +10% to +40% of barrel length, hence my question, as it surprised me there didn't seem to be a standard ratio.

Anyway, in the interest of science I have decided to start cutting down one of my spear shafts. I will do so progressively 5cm per time to see if there is any noticeable difference.

Cheers,
redline

PS - Those articles on the Maorisub site are great! Its a shame they are not in english! Along with those on the Dapiran site, I have lots of interesting bed time reading, much to the annoyance of my long suffering girlfriend!

 

[Old Man Dave]

Trouble with speargun performance is that there are so many variables. My euro 90cm gun with a longer 140cm spear was deadly accurate with a single 16mm band but hopelessly inaccurate with a 20mm band. I assume that the non tracked gun and 6.5mm spear was not distorted by the power of the 16mm rubber but was distorted by the 20mm band. Maybe a shorter standard spear would have worked but who knows?

What I like so much about my Omer XXV Gold 90 is the fact that the short 6.3mm spear in the tracked carbon stock is not distorted by the super powerful 18mm band. This must be so as the gun is famously accurate. My efforts to produce such a set up has been woefully pathetic which just goes to show that manufacturers do go to long lengths to R & D their products to produce a successful design.

Still it is fun to try your own thing.

Dave

 

[Pastor]

I remember reading here somewhere that someone had watched a slow motion sequence of a speargun being fired. It apparently showed the shaft bowing upwards in the middle by about 6" (15cm), I can't verify that but I have seen one of an arrow leaving a bow and that was similar.

 

[ulysses]

It appears worth mentioning that reducing shaft length (as compared to the traditional ratios 75/115, 90/130, 100/140, etc.) does not only impact ballistics but also aiming. For example, on a closed muzzle 90cm classic eurogun with a 115cm shaft (instead of 130cm) you will have difficulties seeing the tip of the shaft and, hence, aligning your shaft with the target is greatly impaired.

Cheers

P.S. This is obviously not an issue, if the gun in question features an open muzzle and circular bands. That is why, for example, Pathos/Effesub or Seac Sub (X-fire) arbaletes are sold with shorter than standard shafts.

 

[redline]

Ulysses,

Gun is closed barrel with circular bands. In the end on my 117cm gun I replaced the 152cm shaft with a 140cm one. I tested it this weekend, and the gun "feels" a lot better: balance has improved, its easier to wield. In terms of ballistics I haven't been able to test penetration, but I paid a lot of attention to the articles that Spaghetti pointed out, and spent most of my weekend testing bands stretch vs. load and going through the calculations. I then selected the "appropriate" band material and length to suit the spear weight, and in shooting the shaft acceleration is much smoother and the spear has a flatter trajectory.

The aiming issue which you pointed out: True, the shorter the shaft, the more impeded you are with sighting down the spear tip. It is noticeable with the 140cm shaft. However I don't ever really consciously aim that way. I normally just point the gun at the target, without traditionally sighting it as one would at a target, so I feel that in my case I won't notice the difference in practice.

Cheers,

redline

 

[redline]

Just out of curiosity, has anyone tried mixing band diameters? Eg 1x16mm and 1x18mm at the same time? The total energy stored would clearly be greater than 2x16mm but I'm wondering if the bands "contraction" speed is the same (ie independent of diameter). I'm guessing that the band contraction speed is a function of the rubber composition so it is possible to mix band diameters.

Any opinions?

cheers,

redline

 

[Bill]

If one band contracted faster, it would have to do all the work. Not possible. I've mixed diameters and lengths many times. Didn't notice any difference except for the power.

 

[redline]

Hi Bill,

My point exactly: thats was the only reason I could think of for not mixing bands, as the slower band would effectively be redundant.

Not that I need more power than 2x 16mm, but out of curiosity I will try 1x 18mm with a 1x 16mm to try and gauge the effect.

Cheers for the feedback!

redline