skeg question for 18" crackerbox style

[Jesse J]

I have a "my own design" scratch built that needs some skeg action. It is very flat bottom (I call it the "ski boat") and it resembles the crackerbox style. I can post some pics if necessary, but my question is to those who have set up their crackerboxes with a skeg. I saw Marine Specialties double skegs and think they pretty cool - just a bit big? smallest is 1 3/8" deep. I will make my own as a pair of these would be over $20 w/shipping.

How deep/long would you recommend a double skeg for a 18" crackerbox? It is set up with a surface drive and offset rudder. It has a keel, but still spins out way too easy on the turns.

I will get some pics in a bit to help give the idea.

thanks
J

[wakeboardfusion]

Jesse, On my 27 inch sk daddle 2 build it calls for a 1 3/8" skeg. That leads me to believe that it would be too big for a 18" Using some cross multiplying, it would lead me to believe that scaled down to a 18" hull from a 27 you would need a .91 inch one. An inch long might work good. I saved money by making mine from aluminum sheet from the LHS

[Jesse J]

That is what I was looking for, I knew there were some data out there.

so, where do the plans place the skeg? right at the back corner, or ??

thanks,
J

[wakeboardfusion]

i put it right in the middle of the boat under the motor mount
Not my boat, but same location

http://img42.imageshack.us/img42/2794/img2072gl.jpg

It looks like your boat isnt as flat as the sk daddle or most crackers. I need some of those rails because mine runs way too wet, is it just a square dowel or is it cut into a triangle?

[FE Wannabe]

Jessie,

Wakeboard is correct, from what I have heard you want to mount the skeg along the keel line at the COG. Since your keel flattens out from your COG as you head toward the transom you use a skeg to help hold the COG as a fulcrum point to turn around.
I like to use the analogy of trying to turn a shopping cart that has 4 wheels that can turn. Try to turn the shopping cart as it moves forward with one finger. The cart will just continue in the same direction and just rotate around its COG. This is like a skegless flatbottom hull with a rudder on the transom.
You need a fixed point or points to rotate around to get good steering.
Hopefully this makes some sense.
There maybe some other physics going on here as well, but I think this is fairly accurate.
Have fun fine tuning it.
Brad

[Jesse J]

i put it right in the middle of the boat under the motor mount
Not my boat, but same location

http://img42.imageshack.us/img42/2794/img2072gl.jpg

It looks like your boat isnt as flat as the sk daddle or most crackers. I need some of those rails because mine runs way too wet, is it just a square dowel or is it cut into a triangle?

To do it again, I would use this:

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXDM75&P=7

Actually it is 45deg angle balsa - I might have cut it in half to make it smaller, but I love this styrene stock. Our architecture section of the bookstore and LHS carry a wide selection of 45deg or 60deg - I like this more.

So I already have a skeg... I was thinking about the fins under the back corners of the hull - not too unlike turn fins on the monos...

[wakeboardfusion]

are you looking for placement tips on installing something like this one just one side to help turning?
http://www.offshoreelectrics.com/pro...rod=ose-506b70

[Jesse J]

could be... I thought it might work to put them under the hull, but maybe it be better to add something off the transom. Will have to try and see.

[Fluid]

Flat bottomed hulls like the Crackerbox will not turn well without a fin on the keel at the CG. In fact, they will flip over very easily without such a fin. I remember a race in Seattle in the mid 90s when Dick Crowe (CBox Nat Champ at the time) was running, and he flipped in the first two heats....very uncharacteristic. After flipping in the final heat, Dick retrieved his boat and was walking back to the pits when a young kid asked him about his boat. Dick being Dick started telling the kid all about his CBox. When he said that you need a turn fin on the bottom he flipped the boat over - there was no fin! Apparently it had broken off in the first heat and Dick had not noticed it. No wonder he never finished.

I had good luck with my CBox running a sheet metal fin at the CG about 1-1/4" deep and 1" wide. I seldom flipped mine in race heats. IMO it is almost impossible to have too much fin, so if in doubt err on the large side.


.

[Jesse J]

So does my "keel" look a little too big? Should I break it off and just have the Cbox skeg?