LM 18 Tunnel build question

[GrantL]

Hey folks, in the process of building out the tunnel and getting stuck on one part.

The drawings/dimensions for this a little confusing to me so wanted to see if anyone could help.

Basically i am having trouble understanding what measurements to use to cut the side portions (2 total). the part I am struggling with is understanding on the side portions, how to create the proper angle for the tunnel, ie the angle at the apex of the tunnel if placed up side down. I see measurements for total bottom length and height from bottom to apex, however they are not lining up as properly should. I have already cut the two rectangular bottom pieces and just need the two side pieces to form the box.

Secondly, i have built the jig and thought about just using that as a type of mold to place the tunnel pieces in and fit them to the jig.

Any helps or advise is greatly appreciated! Especially the dimension on each side of the side pieces. Thank you!!!

schematic below for reference

[OlivierP]

Hi

Disclaimer - I'm not an expert in the LM18 nor tunnel designs, so everything I say may be wrong.

I just noticed a discrepancy on the drawing you attached between the tunnel envelope length (at the bottom, 33 1/8) and the dimensions of the tunnel top parts at the top (total length 36 5/8). Is there a typo somewhere ? Should the bottom dimension be 36 1/8 ?

If you assume that the top parts are dimensioned properly, you may use their edge as a guide to draw the sides, as you know the apex height and the exit height - that's how I would do it.

Regarding the build, the building notes should provide guidance, but I would not attach the tunnel to the jig. If I understand correctly you assemble the hull completely without the tunnel, then you cut the hole, then you fit the tunnel inside.

[pee wee]

The way I see it, section B has the dimensions of the tunnel already installed in the boat, so it includes the hull and transom plywood. Another clue is that the overall width of the tunnel is given as 16 inches, but the half width transom cutout is 7-3/4". Inside and outside measurements. I think all the information is probably there and accurate, but it is not laid out in such a way as to grasp right away. You are doing right to pause and figure this out before cutting anything.

If there's any consolation, it would be that as long as you're in the ballpark with tunnel size it should function just fine.

[Aripeka Angler]

My XF has a tunnel. It’s not a super critical build from a dimensional standpoint. I temporarily screwed the front and top of the tunnel between the sides and scribed the angle on the side pieces.
By the way, I helped a builder put a tunnel together a few years after my build was completed. We used cove molding to make the inside radius on the tunnel. Much easier than forming the radius with overheated epoxy and wood flour.