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Gkaler
02-26-2017, 06:22 PM
I am embarking on a boat building project, a Chesapeake Crabbing Skiff, designed by Doug Hylan. I have a question about lofting and building molds. The table of offsets in the plans lists half breadths from the centerline and heights above the baseline, to the outside of the planking. So, in drawing the outlines of the station molds on a sheet of plywood, should I subtract the planking thickness, so the molds are to the dimensions at the inside of the planking?

Jim Ledger
02-26-2017, 06:25 PM
Yes, that's what you do.

Peerie Maa
02-26-2017, 06:40 PM
However, as the boat curves in towards the bow, you need to take off more, as the station cuts the plank at a slight angle. To do it right you need to loft the waterlines as well. As she is nearly flat bottomed you may not need to loft the profile.

jpatrick
02-26-2017, 09:42 PM
I've been lofting for a new build this past week plus a few days of the prior week. (maybe even more but I'm too embarrassed to admit to that because I'm still not finished) I've reached the conclusion that there are no simple answers to lofting.

Jeff

Eric Hvalsoe
02-27-2017, 02:03 PM
I am embarking on a boat building project, a Chesapeake Crabbing Skiff, designed by Doug Hylan. I have a question about lofting and building molds. The table of offsets in the plans lists half breadths from the centerline and heights above the baseline, to the outside of the planking. So, in drawing the outlines of the station molds on a sheet of plywood, should I subtract the planking thickness, so the molds are to the dimensions at the inside of the planking?

In short, this is probably close enough for your purposes, though not strictly accurate.

The curved waterlines are a clue that something more complex is happening towards the ends of the boat. They are a clue that bevel, or angle, will affect the amount of deduction. However, waterline views in half breadth do not give you an accurate view of the deduction in the body plan. To get a clue about accurate deductions with a master bevel board look at gougeon brothers boatbuilding manual. Or take a lofting class from me in the seattle area.
Eric

Gib Etheridge
02-27-2017, 02:21 PM
On that particular hull if you do as you suggested you will be so close that it won't matter. Without drawing it up I would say that using 3/8 for the sides you might be off by about 1/16 at the station closest to the stem, and less on the others.

I know Doug Hylan is a master of his craft, but I don't see why anyone would want to draw up lines to the outside of the planking except when the lines have been taken off of an existing hull. Maybe, for the sake of calculating the very most accurate performance figures for a racing hull, it would make sense.

Gib Etheridge
02-27-2017, 03:16 PM
That's a bit vague, isn't it?

What I mean is that if you just subtract the planking thickness without considering that the planking crosses the station on an angle, which makes it thicker measured perpendicular to the centerline, you will only be off by a negligible amount.

Bob Cleek
02-27-2017, 05:33 PM
That's a bit vague, isn't it?

What I mean is that if you just subtract the planking thickness without considering that the planking crosses the station on an angle, which makes it thicker measured perpendicular to the centerline, you will only be off by a negligible amount.

And, of course, don't simply deduct the thickness of your planking from the waterline measurements on your table of offsets, or you'll be off a bit as well and maybe more than you'd like to ignore. Remember the planking thickness deduction must be measured perpendicular to the section curve. The planking thickness deduction is most commonly done with a pencil compass or fid block with a hole drilled in it to hold a pencil, full scale on the lofting floor.

Gkaler
02-27-2017, 05:45 PM
Okay. Thanks for the responses. I think I now have a pretty good idea what I need to do to draw the station outlines on a sheet of plywood and then build the molds.

I am thinking that I should cut a notch in each station mold for the keelson, as well as notches for the chine logs. Then I can bend the keelson and the chine logs into place and attach the inner stem and the transom to these, and probably also attached the knees between the keelson and stem, and keelson and transom. That will give me a backbone structure and chine logs to attach the plywood planking with epoxy and some screws. I may also need to cut notches for some ribands to help aligning and plumbing the molds. Then the planking will get bent around all this, but not be attached to the molds or ribands, only to the keelson, inner stem, chine logs and transom. Does this sound right?

Peerie Maa
02-27-2017, 06:30 PM
Have a look on here, good photos of the interior in build.

Gib Etheridge
02-27-2017, 07:06 PM
Yes, but you left out fitting the inner rail along with the rest of the framing before planking.

Whenever I can I assemble the frames and use them as the station molds, leaving the sides frames long enough that they can act as legs and fastening them to cross pieces on a ladder frame. Same goes for the stem and transom. That saves both materials and labor.

Like this....

But on something like this....

I build the ladder then support it with 2 or 3 short saw horses which have been screwed to the floor. The ladder frame then gets squared, braced and leveled on the horses using sawn shingle shims, then screwed to the horses.

Here's a variation with cross spalls added and legs added to them....

http://i1050.photobucket.com/albums/s412/GibEtheridge/Faering/Faering%20001_zpsujjqn6t8.jpg

Gkaler
02-27-2017, 07:13 PM
Have a look on here, good photos of the interior in build.

Nice looking boat. I'll be happy if mine ends up looking anywhere near as good. I see that you installed floors and ribs. The plans don't call for them, and the boat shown on the video on http://www.offcenterharbor.com, narrated by Doug Hylan, doesn't show any floors and ribs in the boat. So, I wasn't planning on installing any.

Gkaler
02-27-2017, 07:32 PM
Yes, but you left out fitting the inner rail along with the rest of the framing before planking.

Whenever I can I assemble the frames and use them as the station molds, leaving the sides frames long enough that they can act as legs and fastening them to cross pieces on a ladder frame. Same goes for the stem and transom. That saves both materials and labor.

I build the ladder then support it with 2 or 3 short saw horses which have been screwed to the floor. The ladder frame then gets squared, braced and leveled on the horses using sawn shingle shims, then screwed to the horses.

I agree it would be nice to install the inner rail and the frames before planking, but there are two problems with that in this case. First, the plans don't call for floors and frames, and the video of the boat on http://www.offcenterharbor.com shows it without floors and frames. Second, the inner rail is installed with spacer blocks between it and the side planking. They are 5 inches long and spaced about every ten inches along the sheer. So, there's a block, then a space, then a block, etc. I don't see a way to install this while the boat is upside down on the molds. So . . .

Gkaler
02-28-2017, 11:04 AM
If I offended anyone with my previous two posts, I certainly didn't intend to. I am less experienced than just about anyone else here, and I am sure not qualified to say how molds should be set up for building a Chesapeake Crabbing Skiff. I wasn't offering my opinion as to whether or not the boat should be built with floors and frames or without. I was merely pointing out that it seems to me that Doug Hylan designed and built it without them. I believe he is very highly regarded as a designer and builder.

Gkaler
02-28-2017, 11:11 AM
Here is a drawing of the station 5 mold as I believe I should build it. Oops! It says I will exceed my allocation for attachments by 34 KB (KBlank@baileyhouse.org) if I upload the drawing. I don't understand why, as I haven't uploaded any files and it's only 48.6 kb

Gib Etheridge
02-28-2017, 11:21 AM
No offence taken GK.

Gkaler
02-28-2017, 11:35 AM
Let's see if this displays the drawing:

Gib Etheridge
02-28-2017, 11:45 AM
I would spread those supports out for a wider base.

Gkaler
02-28-2017, 12:08 PM
I would spread those supports out for a wider base.
I copied the method of building the strongback from "Building the Office Center Skiff" on http://www.offcenterharbor.com, where they made the strongback 12 inches wide. I was wondering if I should make it wider, although wouldn't it be problematic to make it wider than the boat in the area at and near the bow?

Mike Seibert
02-28-2017, 03:25 PM
Just so I don't forget this . . . I had plans to cut notches in my frames early in the process and was strongly advised to wait until I had everything on the strong back. I am glad that I took that advice . . . It is much, much easier to get the bevels in your notches right when you can see how everything lines up. I used a batten to line up my notches and it took the guesswork out of it and made it much easier to do a nice job of joinery. If your strong back is sturdy, you will be surprised how stiff things and how easy it is to do the notches correctly the first time. So I might do my keel notches at this point but would wait to do the chine and ribband notches until later.

My Lightning has a beam of 6 ft and a transom width of 4 ft. I made the strong back 4 ft wide, and it was a perfect width for most of the boat. I built a 2 ft wide section of strong back to handle the last 3 frames and the stem. It worked OK and seemed like a good idea at the time, but, if I had it to do over again, I would probably make the strong back 4 ft wide all the way to the stem, which is what they did in the Lightning factories back in the wooden Lightning days. So I think the quickest and easiest thing for you to do is to make your strong back the width of the transom from stem to stern.

It took me awhile to get there, but these days I am very big on doing things the easiest and quickest way I can.

John Meachen
02-28-2017, 04:50 PM
Lofting always seems to cause a lot of concern.Maybe the designers that use 3D CAD should advertise that their designs don't need lofting as its such a simple matter to offset the skin thickness to arrive at the true shape of the moulds.

Gkaler
02-28-2017, 05:42 PM
Just so I don't forget this . . . I had plans to cut notches in my frames early in the process and was strongly advised to wait until I had everything on the strong back. I am glad that I took that advice . . . It is much, much easier to get the bevels in your notches right when you can see how everything lines up. I used a batten to line up my notches and it took the guesswork out of it and made it much easier to do a nice job of joinery. If your strong back is sturdy, you will be surprised how stiff things and how easy it is to do the notches correctly the first time. So I might do my keel notches at this point but would wait to do the chine and ribband notches until later.

My Lightning has a beam of 6 ft and a transom width of 4 ft. I made the strong back 4 ft wide, and it was a perfect width for most of the boat. I built a 2 ft wide section of strong back to handle the last 3 frames and the stem. It worked OK and seemed like a good idea at the time, but, if I had it to do over again, I would probably make the strong back 4 ft wide all the way to the stem, which is what they did in the Lightning factories back in the wooden Lightning days. So I think the quickest and easiest thing for you to do is to make your strong back the width of the transom from stem to stern.

It took me awhile to get there, but these days I am very big on doing things the easiest and quickest way I can.

Thanks for the advice. I am thinking that I will do as you suggested and make the strongback the width of the transom.

Also, I have been rethinking how long to make the strongback. The boat is 15 feet 4 inches, and I was planning to make the strongback 16 feet long, but I am revising that plan. The intersection of the planking and the stem at the sheer is 34-1/2 inches above the design baseline. So, I plan to make the construction baseline (the top of the strongback) 34 inches above the design baseline (34-1/2 inches less the 1/2" bottom planking thickness). That means the sheer intersection of the planking and the stem would be right at the top of the strongback. If I subtract 1-3/4 inches (inner stem thickness) from the length from the inner stem to the transom, I think the strongback should be that long. Then I can leave the inner stem long and screw it to the front edge of the strongback. At the stern, the top of the transom is 29 inches above the design baseline. If I cut it to size and then screw on an extension to its top edge (screwing it on where the sternpost is eventually going to be attached), the extension would then project downward below the strongback and can be screwed to its back edge. Does this make sense?

Peerie Maa
02-28-2017, 05:59 PM
^ Leave room to work between the top of the side plank (the sheerline) and the top of the strongback.

Gkaler
02-28-2017, 06:33 PM
^ Leave room to work between the top of the side plank (the sheerline) and the top of the strongback.

Not to be argumentative, but Doug Hylan recommends a construction baseline (top of the strongback) of 34 inches above the design baseline. That would put the sheer intersection with the stem right on the top of the strongback, but the distance up to the sheer line would increase to 3 inches at station 1 and gradually to about 12 inches at station 5, remaining around that distance until about 10 inches at station 8 and about 8 inches at the transom intersection with the sheer. Is there a reason for wanting much more room than that between the strongback and the sheerline? I would think the whole mold-strongback structure is sturdier the less the height above the strongback to the bulk of the molds. No?

Gib Etheridge
02-28-2017, 06:42 PM
What Nick said, for sure.

Just having one post supporting the transom will leave it pretty wobbly. Two legs will be better, you can fill or plug the screw holes.

Making the ladder frame the width of the transom for the full length has always worked for me. Sometimes I'll screw a triangle of plywood on top of the ladder on each side of the stem. It makes for good lateral bracing and is a good and handy place to put tools, paint cans etc.

There are as many ways to do a strongback as there are building situations. Once it's level, square and secure you can snap a centerline down the middle and base everything from that.

Make it high enough that you can get under it to clean up squeezeout before it hardens.

Putting it on short sawhorses screwed to the floor has worked the best for me, and when I flip the hull I already have a couple of horses right there to set it on while I do the interior, decks and whatever else needs doing.

Once you have the hull partly built (the whole unit tied together) you can remove some of the vertical braces to the legs and have more room to maneuver underneath, but only if you keep that in mind while screwing the braces to the legs so that you have easy access to the screws.

W Grabow
02-28-2017, 09:16 PM
I don't see why anyone would want to draw up lines to the outside of the planking except when the lines have been taken off of an existing hull.

Really. Offsets to the inside of planking make so much more sense. More accurate; easier to use; requiring no modification for different plank thickness.