Question for you experts

[bjhbjh]

Hi,

(disclaimer: New guy here.)

Ok, I have been looking at the old Mechanics Illustrated plans for a "Hobby Kat" at:

http://www.svensons.com/boat/?p=SailBoats/Hobby_Kat

I see how it all goes together, I think, except that on the page where the ribs are diagrammed out I have some confusion about the "BASELINE".

The assembly jig diagrammed at the bottom of the page shows a line 3" above the jig which is lableled the BASELINE.

Each of the ribs also has a line marked on it marked B.L. which I interpret to stand for baseline.

Now it seems to me that all the baselines on the ribs should all line up along that BASELINE shown in the asssembly jig diagramm.


BUT the ribs 1 and 2 have more than 3" from the B.L. line to the edge that rests against the assembly jig.

HOW do I reconcile all these things?

Advice welcome, thanks and best regards,

Brian H.
Uxbridge Ontario Canada

[Catsailor]

Not sure if I understand your question correctly, but it is entirely possible for the various lofting points to be below the baseline, just as much as above it. It is the reference line to which everything else should relate.

When plotting, it helps to have a sheet (or two) of MDF (etc) large enough to accommodate every frame with the baseline drawn maybe 20% up from the bottom. Then it is just a matter of plotting the offsets - + usually means above, but - means below this line. When all of the lines have been plotted, you essentiall have a front-on view of the hull and can soon see whether all of the points are true - also what curvature is present at the sheer, keel and (if any) chines. This really helps when setting up the jig

Hope this helps

Alan

[bjhbjh]

Not sure if I understand your question correctly

Thanks for the reply, sorry if I was not clear in composing my question. Let me restate my question a bit more simply.

If you look at the Hobby Kat plans sheet where the rib layouts are shown, notice that the diagram for rib 1 & 2 have their B/L's 4.75" from the bottom edge. The rest of the ribs have their B/L's 3" from the bottom edge.

Then look at the assembly jig at the bottom of the same sheet. Notice that the reference line marked baseline is shown to be 3" above the base of the jig.

If I cut out the ribs and set them up on the jig ready for laying on of the sheet stock etc, the B/L for ribs 1 and 2 will be 1 3/4 inches above the baseline. The other ribs B/L's will line up along the reference baseline.

Is the rib 1 & 2 B/L line location an error in the plans ? If so, how can I correct it, if not what is the intended interrelationship between the rib B/L lines and the baseline of the jig view if not to all line up along a straight line when set up in the jig?

Hope this is more clear.

[Ike]

You're looking at it backwards (or upside down). When a designer draws the plans for a boat the first thing he does is draw a baseline. All measurements are then taken from the baseline. As was said. plus measurements are up, minus are down. The boat was intended to be built upside down on a jig. So the base line is actually 20 inches above the floor that the jig sits on. 17 inches to the top of the jig plus 3 inches up to the stem. The extra length on each frame is because that part of the frame sits on the jig and after the boat is built gets cut off. The lengths are different because when cut off they will follow the curve of the sheer line. Then the deck for each hull lies on top the frames.

It's really quite a clever little design and for some reason I think I saw it when It came out.

[bjhbjh]

So the base line is actually 20 inches above the floor that the jig sits on. 17 inches to the top of the jig plus 3 inches up to the stem. The extra length on each frame is because that part of the frame sits on the jig and after the boat is built gets cut off. The lengths are different because when cut off they will follow the curve of the sheer line. Then the deck for each hull lies on top the frames.

Ok, I appreciate all that you are saying but I still feel that I am not getting my question across so let me try again.
When the ribs are set up in the jig, should each piece's B/L line, line up along the jig's baseline or not?

If so then how does one set rib 1 and 2 onto the base of the jig without their B/L lines being 1 3/4 inches above the baseline?

Assuming, as drawn, the baseline is a straight line 3" above the jig base, then either the edge of ribs 1 & 2 rest on the jig base and their B/L lines are 1 3/4" above the baseline OR their B/Ls are at the baseline height and the edge of the piece is somehow inset 1 3/4" into the jig. That extra 1 3/4 inches has to go somewhere !

Oh, and as I see it, only the transom calls for anything to be cut off.

Thanks and best regards,

Brian H.
Uxbridge Ont Canada

[Catsailor]

Lets use the term "frames" not "ribs" so we are using common boatbuilding terminology.

Ike is correct - you need to consider the frame shapes as they will be built -namely upside down. The baseline is only there as a reference point for lofting the lines and once the frames are made, it is the length of each frame and where it screws to the stongback that determines the line of the keel and the sheer.

A suggestion - draw all of the frames to scale on stiff cardboard, cut them out and glue them at the specified spacings onto a couple of balsa rails. You will soon see the shape of the boat emerge and it is a lot cheaper to experiment with cardboard than with real materials

Cheers

Alan

[bjhbjh]

"Frames" 1 and 2 have 4 3/4" between the edge and the B/L.

The Jig has 3" between its base and the 'baseline'.

Which should line up for construction, the edges of the "frames" along the jig base or the B/L's along the 'baseline' ?

That's all I want to know.

Thanks,

Brian H.

[SamSam]

Originally Posted by bjhbjh

"Frames" 1 and 2 have 4 3/4" between the edge and the B/L.

The Jig has 3" between its base and the 'baseline'.

Which should line up for construction, the edges of the "frames" along the jig base or the B/L's along the 'baseline' ?

That's all I want to know.

Thanks,

Brian H.

I see what you're referring to. I'm pretty sure the BL's should line up on the baseline. The extra length on frames 1+2 are needed to accomadate the forward beam which has a crown, whereas the aft beam is flat. I haven't read everything about constructing the snazzy little craft, but I assume the outside ends of the beams on the outside of the frames are "level", that is 3" above the BL when finished and right side up. The inside of the frame (1+2) on the forward beam will be higher (approx. 1 3/4") than the aft beam when finished to fully enclose the curved beam and afterwards the excess on the outside of the frame will be cut off to approximately 3" above the baseline, like frame 3+4. Detail A and section BB on this image might help explain what I'm talking about.

http://www.svensons.com/boat/?f=Sail...bby_Kat_03.jpg

[SamSam]

Also, don't overlook that 5 degree tilt shown in the BB Section when you assemble the beams and hulls.

[bjhbjh]

I'm pretty sure the BL's should line up on the baseline. The extra length on frames 1+2 are needed to accomadate the forward beam which has a crown, whereas the aft beam is flat.

Thanks! I agree. When I build my jig it will have 4 3/4" from base to reference baseline.

[Richard Atkin]

Now I'm definitely not going to build my own boat.

[bjhbjh]

Now I'm definitely not going to build my own boat.

Well I can't imagine why not, I'm game! I think it'll do me good too.



Brian H.