Help with establshing the cg of JAE21

[John MacLean]

Can anyone help me get this right. I built my rigger from the public domain plans (FE) that a buddy sent me over the winter. It seems that the plans have evolved since this particular release because the boom tube position is located further back than is pictured in the FE instructions posted on the Zippkits website as is the bulkhead. At any rate, with batteries and everything in, the cg is right at the rear boom tube which is located immediately behind the trailing edge of the turn fin.

I had it in the water yesterday for the second time and although it is difficult to be sure from a distance it seems to be running wet. My buddy who shot me the plans thought that the centre of gravity should be located another couple of inches further back from where it is right now. Not happy about having to relocate motor and esc but I want it running properly so I'll just do whatever it takes short of rebuilding the tub from scratch. If anyone has a fast electric version and can shoot me the dimension for the cg from the transom I would really appreciate it. I realize that my boat might be a little different because of different electronic but I would use the info as a starting point to locate my cg based on he plans I have.

The attached pics show the location of the rear boom tube and the bulkhead.

Thanks for looking.

 

[Martin Hamilton]

The cg on the JAE 21 fe is around 60% / 40%, so cg is 40% from rear of the ski.

 

[John MacLean]

Thanks. I just finished removing the motor mount from the tub, opening up that rear bulkhead and shortening my stuffing tube so I can move the motor closer to the transom. I'm still not clear on that 40%. Is it 40% of the tub's length or 40% of the length of the tub from the back of the ski to the trailing edge of the sponson? That was the kind of information that was lacking in the plans and since this is only my second build I am feeling a little lost.

 

[Jim Caldwell]

WEIGHT= 60% on front sponsons, 40% on rear ski.

 

[RodneyPierce]

Take the overall boat length from sponson tip to ski end. Then take 40% of that measurement.

Or do like Jim said above and use a scale. Block the front sponsons up to the same height as the scale so the boat is at ride height when your measuring the weight. Then get 40% of the weight on the rear ski

 

[dwilfong]

When setting the tail weight on any rigger the easy way is to balance on the front riding surfaces and put the rudder on a scale. Make sure the boat is level when doing this.

If shooting for 40% on the tail total weight x.4 will give you the number to shoot for.

this way you can move things around in the boat with it on the scale. make for a easy time setting it up.

David

 

[John MacLean]

Thanks for all the input, guys. I have a much better idea of how to accomplish the job now. I've done a preliminary check using the scale which confirmed that the motor indeed had to be moved back further. I will probably situate the ESC behind it as well. That will allow me a little room to move the batteries around if needed. Wished I had this right from the get go. I really don't like do-overs. But I guess that is somethimes part of the hobby.

 

[Chilli]

John, On the FE kit the shaft end of the motor ends up about 6-1/2 inches from the transom and the rear of the battery can go as far back as 11-1/2 inches. The rear boom tube was moved up so the battery could be placed behind it moving the CG back.

 

[David Santistevan]

That actually looks like a JAE 21 tub instead of the FE tub. The FE tub is much taller and narrow. The motor appears to be in the right place but the battery needs to move forward (just glancing at it).

D

 

[gil sonsino]

John take a look at my pic and see where is the main difference with your boat.As your rear boom is placed differently I think you may misplaced some parts.It is a must respect the 60/40% CG rule on this boat otherwise you will have running issues.

 

[John MacLean]

John, On the FE kit the shaft end of the motor ends up about 6-1/2 inches from the transom and the rear of the battery can go as far back as 11-1/2 inches. The rear boom tube was moved up so the battery could be placed behind it moving the CG back.

Yep. . . That's pretty much exactly where my motor is sitting now that I've done the 60/40 thing. The boom tube location is giving me a little grief right now. One of my 2s packs can slide underneath it very neatly but the other will have to sit on top of it. I'm thinking of making some kind of shelf for it to sit on while keeping the profile low enough for the cowl to fit back down on it. I have a single 4s pack on the way and I don't know if I'll be able to use it at all in this setup. If the back of my battery can be 11, 1/2 inches from the transom that gives me about an inch and a half closer to the motor that I can move each of the 2s packs if neccessary.

 

[John MacLean]

John take a look at my pic and see where is the main difference with your boat.As your rear boom is placed differently I think you may misplaced some parts.It is a must respect the 60/40% CG rule on this boat otherwise you will have running issues.

Thanks, Gil. you mentioned from the outset that you though my motor might be too far forward. The tub and sponsons were built according to the plans I received and I didn't really notice the difference in placement until I went on the Zippkits website to look at their assembly instructions. I've since concluded that David's (Begginnerboater) observations above were correct. This tub, even though the plans are titled as JAE21 FE, were probably originally either nitro or gas. Being new to building and to riggers I didn't really have anything to compare it with.

Live and learn. I take comfort when I look at your build thinking that if you can get that 6s setup a 21 tub I should be able to get my little old 4s setup in without too much grief. My hat is off to you, sir.

 

[Joe Petro]

Actually, the Zippkits 21FE and the plans (21GE) are two different animals.

The 21FE was modeled after the 21G2 Nitro as we wanted to use the same sponsons on both.

The 21FE and 21G2 have different tubs, but everything else is the same.

Joe

 

[John MacLean]

Thanks for the input, Joe. What is interesting is that this JAE21 was built from plans that had JAE21 FE across the top of the prints showing the suggested location for motor and batteries however when I checked the file name of the PDF file it was named JAE21 GE. I guess that explains some of the issues I've had with it but only having seen a nitro JAE12 before I built this boat convinced I was building the current FE version. I noticed a couple of posts over on Rumrunner racing where the GE was built as a very formidable FE.