How many fletches on a crossbow arrow and how to accomodate them?

Okay, so lots of older crossbows only featured two fletches which would lay flat against the table of the bow on either side of the arrow shaft. I've seen some where a third fletch was added sticking straight up although I can't imagine how arranging the fletches in a "T" shape would affect flight, if at all. I know in regular archery more fletches typically help stabilize arrow flight more quickly. For fletches are better than three, or three fletches that are taller and longer are better than three that are short and not so tall. I was contemplating how to design my modern hunting crossbow to accommodate arrows with three fletches without getting that odd looking "T" effect. It occurred to me that I might be able to achieve this by cutting a narrow slot down the center of the table so that on fletch can ride in the groove while the other two stick up and out in sort of a "Y" shape if viewed end-on.

Just to make everything run smoothly with minimal drag, I was also thinking of machining this bolt/arrow track out of something like a slippery phenolic material. I don't have fancy CAD skills like a lot of you guys, so you'll have to try to understand what I'm describing. I'd start with a solid block of phenolic, use a router to rout a "U" shaped channel down the middle, then split the block into two parts by running the whole thing through the table saw. I'd be left with two pieces that would each have half of a "U". With both pieces screwed down to the table there would be a gap in the middle in which the fletch would run. My only issue is that the track extending all the way through the prod-end of the table might make the actual prod socket weaker. Also, I need (for hunting regulations) to design the bow around using arrows/bolts no shorter than 14".

The total draw length of the prod will only be 11 1/4" so I was thinking of designing the table in a similar manner to some target crossbows which have a table that extends out farther than the where the prod is mounted. The last factor is that the lock mechanism is going to be based around a roller nut with a relatively simple trigger system probably using no more than two axles. So, should I keep things more simple and just use two fletch arrows/bolts, or try to incorporate more fletches? What do most crossbows use, taking into account that this is going to be a solid steel prod mounted below the table with and otherwise solid wood stock with no aluminum or other materials of that sort?

A groove for fletching is what all modern crossbows have. While this might make the prod socket a little weaker I doubt by very much to matter. You can shoot longer arrows and just let them hang over the end, however it does "look" mismatched and more important for hunting it is prone to getting knocked off while moving through the bush. I like to keep the end of my arrow with in the stirrup as the stirrup acts as a guard, so extending the tracked is a good way to go.

Two fletching arrows work fine even with broadheads, However if built to use three fletchings you have the option of using store bought arrows which I have only seen with three fletchings. 

On my last build I put in a groove for fletching. It is nothing more than a saw cut about 3/16" wide by 5/8" deep. Then slightly rounding the top square edges. Works fine.

Todd

Todd,

Thanks for the insight. I think I'm probably going to go with three-fletch arrows for this build. I like the idea of the extra fletch for stabilizing the arrow flight more quickly than with two fletches. I think it probably would make a slight difference with three over two, especially if you're shooting with heavy broadheads. I hadn't thought about the potential of weakening the prod socket, but I imagine that can be easily taken care of by simply extending the distance from the nock end of the arrow to where the fletches are. Instead of having that distance fairly close like you'd see on a regular arrow for a normal bow, make it an inch or so longer so that the fletches begin past where the roller nut and socket reinforcement end. It might look slightly strange but it shouldn't be an issue with arrows that will be longer than 14 inches. That also brings up another point. I was intending to just make my own arrows using wood shafting available from outfits like 3 Rivers as I've always done for my regular bows.

The last crossbow I made I just created a bunch of cheap wood arrows using 11/32 Port Orford Cedar at the highest spined weight I could get which was I believe about 75-80 lb. For a target SCA crossbow it probably didn't matter, but I'm thinking for an actual hunting crossbow I'm going to need something more stout. If I still went with wood I'm thinking it'd need to be more like 1/2" and a stout hardwood like Ash or Oak. In addition to that, I imagine that the tip end would need to be tapered down to accept large field points and/or broadheads. This bow is going to use a prod that is 150-190 lb draw weight, and I intend to set it up so that the draw weight is more towards the heavy end of that range. I can't see it being heavy enough for something like bison, but I want to be able to take down elk with it if the opportunity arises. Question is, should I just go with aluminum or carbon shafting for my arrows? I'd really like to stick with wood, but if it's going to present too many problems given the draw weight of the bow I'll go with a stronger material.

Wood is good Celt,plenty strong enough,and the heavier the wood(and the bolt) the better.Dirty coyote recently posted a link for obtaining Ipe dowel,the heaviest wood he's found.Bullets kill as much by shock,as by the damage they do.Arrows and bolts kill by causing massive,rapid blood loss rather than shock,so penetration is what you need,especially if you have to go through bone.You want a quick kill,as trying to track a wounded animal through thick bush for any distance when the blood trail is not very good is to be avoided,and it's that much further you have to go to recover the animal.Life is not Hollywood,and they don't often fall over as soon as you shoot them.
                             Hermit.

What about mitering the table 45 deg down from each side of the quarrel grove so the fletches can be  placed more equaly apart and clear the table

Mitering the table certainly works.  But 45 is the wrong degree.  3 fletches are space 120 degrees apart, so you'd angle the able down 60 deg on each side..

As most table saws only tilt to 45 degrees,we have our first problem........lol.
                                                Hermit.

It can be done on a table saw with a clamping jig so that the table of the tiller faces down, tilting the blade to 30deg's and place the fence on the left side of the blade for a right hand tilt blade.

I believe that a track (table) that is shaped to accommodate two bottom fletches will also cause undue wear on the serving/bowstring.

I've been using a two fletch set up on 3/8 oak dowel I pick up from a local woodworking shop. This is about as large as I am willing to go, as it is already taller than the string. With a 160GR Ace two blade broad head I've found the accuracy to be excellent using the Alchem basic plan "medievalled" up a bit. The prod is a 240lbs one from Alchem as well, the speed isn't blindingly fast, but respectable. An example, at 20 yards using the Morrell Yellow Jacket broad head block I can only shoot once per target dot or risk hitting the earlier shaft. I've also found that the heavier the draw weight and bolt weight (without going to absurdities) the easier it is to tune.