Coheorn bed, need opinions

[Scota4570]

My coehorn project is nearing completion. The tube is soda can size ID, the OD is 5 1/2; and it is 20" long. The trunion is 2" and on the bottom of the tube.

I made by bed by laminating oak (per Buckstix) to make a block 20" x 13" x 6". Foolishly I re engineered it without thinking ahead. I moved the trunion groove from center to 1/3 from one end and 2/3 from the other. The idea was to have the tube not stick out over the end so much and bring the center of gravity to the middle.

In hindsite I am a bit worried that recoil might make it topple over backwards. If you sketch it on paper you will see the recoil is centered on an area of the bottom of the bed about 3/4 the way back. At very least it will stick in the ground more than it would it I had centered it. that would cause lots of fussing with the earth or sandbed between shots.

OK, Should I be worried? Should I inlet the bed so that the tube is sticking out over the short end? That will certainly help by making the the recoil go in the center of the bed. It would look goofie and aestieics matter on this one. I have spent a bunch of time on this project and don't want to mickey mouse anything now.

 

[andrellj]

Is the carriage near completion too or can it be changed to a mortar sleigh? Check out member resources for Canadian cannon company and check out his sleigh carriage. something longer like that could reduce the chance of it backflipping.

 

[GunneyG]

On a US Coehorn, the tube is offset slightly from center already (the opposite direction from yours.) Since you really don't need maximum range, inlet so the tube is at a 60° angle instead of 45°. It may look a little weird but so will the offset tube.

Or you could turn it around and lnlet in the proper place and fill the mistake with epoxy filler. That will look a lot less weird than either of the above options and the paint will cover it anyway.

 

[TheDoubleD]

You can also shorten the barrel. It's to long now. The rule of thumb for the bore bearing the ball is 2 to 2 1/2 calibers.

What you have now is not a mortar but a high angle howitzer.

 

[coehornboy]

I would think that you could fill it in w/wood or filler, then re-inlet the trunion hole. If you paint the base, as most military ones were, it'll never show.

The photo below is of my coehorn mortar, the same as in my avatar. The inlet for the trunion is in the middle...I used the coehorn plans available from Dixie Gun Works. It is front-heavy when picked up, but doesn't move when fired, even if I use 1200 gr. of BP.

Good luck, and post pics. :v

 

[TheDoubleD]

1200 grs??? What is the diameter of your powder chamber???? That much powder calls for a powder chamber 1.3 inch in diameter. What bore diameter is you mortar?

 

[coehornboy]

The bore is soda can diameter (approx 2.6 inches...the powder chamber was made (my error :cursing: ) to the original size (scaled to barrel diameter), so it is about 1.75" by 5".

 

[TheDoubleD]

Is there any chance you can do some re machining of this tube?

Weigh out 3.5 ozs of FG Blackpowder.(Max load for a 1.75 inch bore) Put it in a tube that 1.75 inches in diameter and measure the height. Deepen the bore on your tube to reduce the depth of the powder chamber the height from your measurements. Then cut the barrel so the 2.6 diameter ID cylinder bore is 6 inches long.

 

[coehornboy]

Not to hijack this thread....but the outer dimensions of the mortar are for a 3/4 scale replica of the original...the bore is not 3/4 scale due to the ease of obtaining soda cans as projectiles. I thought about lengthening the bore more, but with the trunion welded in place, it would be a bear to chuck up in the lathe, etc.

I only fire a big charge to "show off," and ususally stick with 175 to 300 grains, depending on the range of the target, humidity, etc. Accurate?...I have been placing in the top three every year at our west coast cannon shoot, from between 8 to 12 shooters, for the past 7 years.

Now back to the original topic, I believe that the block-style mortar base helps with accuracy and consistancy of shots, as the added weight and amount of base-to-dirt contact will keep the mortar from moving. Many of the guys with "sled" type bases or metal frames to hold their barrels have to constantly reposition their mortars after every shot when they jump or move about (or sink into the dirt). When I have a shot that is in line of the target but falls short or long, I just need to adjust my powder charge...one less thing to worry about. :v

 

[TheDoubleD]

I missed the fact that we are talking aobut two different mortars here. Kinda like mixing apples and oranges. Scota4570 tube needs shortened.

Then on top of that I got my math wrong...for the wrong tube.

My apologies.

 

[Scota4570]

"Scota4570 tube needs shortened"

The actual bore is 9" long and 2.6" inside diameter. The tube in longer due to the breech section. It is turned, finished, polished and lacquered. It is polished bright steel. It is truely beautiful ( if I do say so), I wish I could post a photo.

I am not going to cut it off. I have about $150 in materials alone in this thing. I did not set out to make a scale model, I made a replica. I think it would look odd to see a soda can peeking out of the muzzle. I can not imagine any sort of effeciency or accuracy out of such a rig. Old formulas aside, why would it matter if the bore is a little "long"?

 

[TheDoubleD]

What muzzleloading mortar are you replicating?

Send me an email and attach your pictures I'll post them for you. Or send them back to you formatted so you can post them.

 

[andrellj]

Pressure builds up more in a longer barrel but yours is beefy enough to withstand it.