When the numbers don't add up?

[MushCreek]

I'm building John Brooks Peregrine 18. I'll preface my comments by saying that I'm a retired tool maker who obsesses over minute discrepancies. I have very good measuring tools, and know how to use them. I put my strongback together, and carefully aligned, leveled, squared, and plumbed the molds. I have a hole drilled through each mold, and crosshairs so I could align everything to a string pulled taut through all of the holes. So far, so good. The problem I'm having is that the angle of the transom is off some 2 degrees. It's supposed to be 11 degrees, and the angle where the keelson meets it is supposed to be 15 degrees. Triple/quadruple checking the location of all of the pertinent parts, it comes out to 17 degrees. Luckily, I have not assembled the keelson, knee, and transom yet. Going over all of the data given, in one instance, the angle of the transom works out to 10 degrees, not 11. Even that wouldn't correct the issue, though.

What do you recommend? Go to the numbers, or go to the actual angle that is formed? As I used to say to engineers, you can't bend a triangle. Trigonometry is kind of hard to argue with. I also have a couple spots along the keelson where if I clamp the keelson down to 'the numbers', it creates a noticeable hollow. I can shim that (although it bugs me to). I guess in the old days, builders went by eye and didn't obsess over the numbers. I'm a bit concerned that I either have A) made a mistake somewhere, or B) The drawings are off a bit. There's nothing left to check that I haven't already been over repeatedly for two days. Do I just make the transom 9 degrees, and not tell anyone? Or do I remake the knee to 17 degrees, and keep the transom at 11? Is this the normal run of things in the boat building world?

[Peerie Maa]

Go with the actual angles, after checking again to confirm your accuracy of measurement.
The Draftsman did not have access to your full size tools, he was working at scale.
If you really want to understand, lay down a lofting floor, and loft the profile full size, and see what that tells you.
However, if putting battens around the erected molds proves that they are all fair, you are good to go.

[rbgarr]

Run your question by John Brooks himself: http://www.brooksboatsdesigns.com/co...form/index.php

[hawkeye54]

Did you perhaps have someone printout the plans, which could have introduced some 'distortion' ?




Rick

[Peerie Maa]

I also have a couple spots along the keelson where if I clamp the keelson down to 'the numbers', it creates a noticeable hollow. I can shim that (although it bugs me to).?

I missed this at the first read through. Ether loft the boat and fair the lofting, or accept that the offsets are in accurate, a not uncommon occurrence, and use battens around the molds to check for fairness and shave off or fir out the bumps and hollows.

[jpatrick]

I had, more or less, the same situation when building Brooks' Somes Sound.... the trig didn't match the given dimensions. I'm a civil engineer in a former life, so I feel your pain. Brooks told me to build according to the given dimensions and the full size plans because that's what he builds to. In your case, I'd go with his given angle for the transom rather than what the angle calculates to given his dimensions. In other words: Put your calculator away.

Keep checking things for a fair line.

Jeff

[MushCreek]

The mold drawings are full-size, provided by Brooks. They do have very thick lines, though. I have two angles to contend with- the stated 11 degree angle of the transom, and the angle of the transom knee. One of them is going to be wrong. I guess the shape of the hull is more important, with the knee simply fitting the space. I am going to lay battens over the molds in various spots to see how fair they are. The slots where the keelson lays are all dimensionally correct according to the drawings, so in effect, I did loft it full size, using wood.

[Peerie Maa]

The mold drawings are full-size, provided by Brooks. They do have very thick lines, though. I have two angles to contend with- the stated 11 degree angle of the transom, and the angle of the transom knee. One of them is going to be wrong. I guess the shape of the hull is more important, with the knee simply fitting the space. I am going to lay battens over the molds in various spots to see how fair they are. The slots where the keelson lays are all dimensionally correct according to the drawings, so in effect, I did loft it full size, using wood.

If running a batten over the keelson recess reveals unfairness, your cutting or set up is in error.
Try putting battens round all the rest of the mold surfaces and see if the same molds are high or low as the keelson batten. That will show whether the pattens/cutting out is in error or the set up.
Check the spacing of the molds along the ladder frame as well, that can throw the set-up off.

[Chippie]

[QUOTE=MushCreek;6677726]I'm building John Brooks Peregrine 18. I'll preface my comments by saying that I'm a retired tool maker who obsesses over minute discrepancies. I have very good measuring tools, and know how to use them. I put my strongback together, and carefully aligned, leveled, squared, and plumbed the molds. I have a hole drilled through each mold, and crosshairs so I could align everything to a string pulled taut through all of the holes. So far, so good. The problem I'm having is that the angle of the transom is off some 2 degrees. It's supposed to be 11 degrees, and the angle where the keelson meets it is supposed to be 15 degrees. Triple/quadruple checking the location of all of the pertinent parts, it comes out to 17 degrees. Luckily, I have not assembled the keelson, knee, and transom yet. Going over all of the data given, in one instance, the angle of the transom works out to 10 degrees, not 11. Even that wouldn't correct the issue, though.

What do you recommend? Go to the numbers, or go to the actual angle that is formed? As I used to say to engineers, you can't bend a triangle. Trigonometry is kind of hard to argue with. I also have a couple spots along the keelson where if I clamp the keelson down to 'the numbers', it creates a noticeable hollow. I can shim that (although it bugs me to). I guess in the old days, builders went by eye and didn't obsess over the numbers. I'm a bit concerned that I either have A) made a mistake somewhere, or B) The drawings are off a bit. There's nothing left to check that I haven't already been over repeatedly for two days. Do I just make the transom 9 degrees, and not tell anyone? Or do I remake the knee to 17 degrees, and keep the transom at 11? Is this the normal run of things in the boat building world?[/QUOT

Reminds me of Ben Sebens " There's the plan and ,then there is what really happens"

I served part of my apprenticeship with an old Shipwright whose favourite saying was there's only three measurements in boatbuilding.
"That's right"
"That's near enough.
"Oh my God".

[Chippie]

Run your question by John Brooks himself: http://www.brooksboatsdesigns.com/co...form/index.php

Just looked at picture of her and what huge problem is 2degrees creating?

[MushCreek]

It's the tool maker in me, mostly. I'm used to taking blueprints, making everything to within .0001", and it all goes together. I just want to be sure I haven't made some error somewhere, but I can't find it. As for the keelson; the dimension between the keelson and the center beam of the strongback is right on the drawings. It's very easy to simply measure that distance against the drawings. The center beam is very straight, and the keelson is bending very fairly.

[Peerie Maa]

It's the tool maker in me, mostly. I'm used to taking blueprints, making everything to within .0001", and it all goes together. I just want to be sure I haven't made some error somewhere, but I can't find it. As for the keelson; the dimension between the keelson and the center beam of the strongback is right on the drawings. It's very easy to simply measure that distance against the drawings. The center beam is very straight, and the keelson is bending very fairly.

Which is good. But you state that there are gaps at some molds. So either there is an inaccurate dimension or some molds are not horned in square and plumb on the correct spacing.

If the gap is only about an 1/8th, fir it out and live with it. But do run a batten around all the other planking flats on the molds.

[willin woodworks]

Youre thinking too hard. You want a fair line, not a completed equation.

[Chippie]

[QUOTE=MushCreek;6678132]It's the tool maker in me, mostly. I'm used to taking blueprints, making everything to within .0001", and it all goes together. I just want to be sure I haven't made some error somewhere, but I can't find it. As for the keelson; the dimension between the keelson and the center beam of the strongback is right on the drawings. It's very easy to simply measure that distance against the drawings. The center beam is very straight, and the keelson is bending very fairly.[/QUOTE

I read that to mean that - someone gave you a blueprint and you manufactured the parts from that and assembled them and the all fitted together perfectly?

I take my hat off to you Sir if that is correct.

PS.
Then again whatever you were making may not have the complexity of shape as a boat.
Although I did bring up a picture of a Peregrine 18 wouldn't have thought that alarming to anyone's?

[Chippie]

PPS.
" Although I did bring up a picture of a Perigrine 18 wouldn't have thought that alarming to anyone."

If anyone was alarmed I apolo --Oh - Bollocks.

[Jim Ledger]

Ya gotta loosen up a little. It might go against all the learned habits that have served you well for years, but trust your battens, not the plans. Those plans are probably not as accurate as the ones you're used to and they depend on the builder to interpret the shape. Two degrees of transom rake just doesn't matter visually, it can't be detected visually. If it looks good and the battens lay fair on it, go with it.

[MushCreek]

I found the answer. I drew it up full-size on a piece of cardboard I had, and it does work out to 17 degrees total, instead of 15. 11 degrees for the transom rake, and another 6 degrees from the slope of the keelson. That's going by measuring the full-size patterns. I checked it both with trigonometry, and laying a protractor on the drawing I did. The little detail drawing of the transom knee is wrong. As for the keelson, I'll shim it by eye. I'm going to lay battens over the molds in various places to make sure no other problems are lurking. I'm rather OCD about things being 'right'. You can imagine how long it took me to build my house!

[Jim Ledger]

The force is strong in this one.

[Canoeyawl]

There is also the problem that none of the joins are straight lines, they are curves...

I am a machinist that occasionally makes tools and fixtures and my first boat wouldn't work at all until I lofted it entirely on barnacle board and corrected the lines. A 1/16" in the offsets is generally considered pretty good in boatbuilding, but in my world that .060" is usually scrap!
Full size lines printed on paper are a different size every day...
I first started out using a scribe knife (like a cabinet maker) to make my cut lines until a real boatbuilder explained that could be a stress riser and or a place for rot to originate. He explained how to work with either side of a pencil line, and how to denote that specific working side as you drew it. That will get you within .003" ish which is close enough for a glue line

[willin woodworks]

When I was an apprentice I worked for an old carpenter who never measured closer than an 1/8th. The 1/16ths were "Leave the line" or "Take the line".....

[Jim Ledger]

Another "when I was" story...

When I was working building aluminum boats we marked the aluminum plate with a magic marker, outside the line. The inner edge of the line was where you cut.

[Canoeyawl]

Another "when I was" story...

When I was working building aluminum boats we marked the aluminum plate with a magic marker, outside the line. The inner edge of the line was where you cut.

Yup!
That works with a pencil, and accuracy becomes a matter of how well you can see (or is that how well you can look?)
Which is when I learned to really appreciate cheap magnifier reading glasses! Long before I needed them to read, except maybe Chapelle's wee drawings and notes in Small Sailing Craft...

I learned to make a "cut line" by jogging the pencil (or sharpie) every so often to note which side of the line to stay away from
Like this, it takes less than a second when tracing a batten
836C9552-6C59-4DF5-A2F7-102A151BBB10.jpg

[Jim Ledger]

That's it, Jake. A marker actually makes an excellent line for cutting in many cases. The edge of the line is very sharp and and the line very visible, an important quality in a low-light situation. It's the only way that I know to mark sheet aluminum for cutting, in a fabricating shop setting, anyway. I'm sure machinists have their own methods, more suited to the work.

[jpatrick]

MushCreek: I checked the plans (yes... I have a set. never built the boat) Brooks states right on the lines drawing that the angle of the transom is to be 11º. So that is that. Any other angle that you may have measured or calculated isn't intended by Brooks.

When I built my Somes Sound I attached battens at each plank lap prior to planking. This was to check his line-off. I made some minor adjustments so that each would be fair. I will bet money that when Brooks built the boat, the battens he attached at the locations noted on his plans were fair. So his set-up was a tad different from mine. This is the way of boatbuilding. Each boat is the same... but different. We all must accept this.

Jeff

[Chippie]

So if you stray slightly over the line you mark (half the thickness of the mark say) it's scrap?

Jeez!

[MushCreek]

A lot of this is new to me. I've built dozens of boats over the years, but most of them I just cut plywood sides approximately to shape, bent them around a single midships mold frame, and fastened them to the stem and transom. What happened in between was based on the bending characteristics of the plywood, chine logs, and rails. This boat is much more complicated, so I'm going to obsess over it.

Yes, the plans call for a transom angle of 11 degrees. The numbers given (he doesn't provide an actual angle) for the transom knee don't work out. It would be off by about 3/16". That's a heavy pencil line.

[Peerie Maa]

A lot of this is new to me. I've built dozens of boats over the years, but most of them I just cut plywood sides approximately to shape, bent them around a single midships mold frame, and fastened them to the stem and transom. What happened in between was based on the bending characteristics of the plywood, chine logs, and rails. This boat is much more complicated, so I'm going to obsess over it.

Yes, the plans call for a transom angle of 11 degrees. The numbers given (he doesn't provide an actual angle) for the transom knee don't work out. It would be off by about 3/16". That's a heavy pencil line.

When you have checked the fairness of the entire set up, with a batten, and it is correct, start assembling. As it goes together, leave some green on the components to be shaved off as you fit them together. Ensure that you maintain symmetry and build the boat.

[Chippie]

Just put a comment on the "put down Thread" on the Bilge.

That may be appertaining to this thread.

[Jim Ledger]

5 of us drank regularly and we were having a a go at one another as to which of us was the most accurate Trade.
This stranger obviously overheard us and decided to parcipitate his offer was "I'm a Fitter and we work thous's"

My Father who was Cabinet Maker said "I work Right".
The Fitter giving a derisive smile said " What better than a thou?"
Pop said.
"I'll take a shaving off, you measure the Bugger."
,

That's a good one, Chippie.


But to that point, Chippie, there was a day last winter when I was doing just that, adjusting a plane while measuring the thickness of the shaving with a digital caliper. This was in order to know how many strokes of the plane were needed to remove a certain amount of material, in order to close a gap. The gap in question having been measured with a feeler gauge. It worked reasonably well, and when I had the gap down to five thou I said good enough.

I try lots of crazy things, absent someone to just show you, it's the only way to find out.

[Canoeyawl]

I thought I was the only one crazy enough to measure plane shavings
(To know how many plane strokes equal what thickness? i.e. 10 strokes = 1/32")
It works pretty well though

edit to add: sometimes you can find a used "paper micrometer" for a couple of bucks

4AB63A4C-0A63-48D2-8BC3-9B0E13917F51.jpg

[Jim Ledger]

Well, not so much the individual shaving, Jake, but, say a stack of three so it averages out. Turns out a three thousandth shaving is pretty coarse, depending on your definition of coarse, of course.

[Peerie Maa]

Well, not so much the individual shaving, Jake, but, say a stack of three so it averages out. Turns out a three thousandth shaving is pretty coarse, depending on your definition of coarse, of course.

You aiming for this, Jim?

[Jim Ledger]

That's just making shavings for their own sake, Nick, a rather pointless endeavor, useful only for exhibition.

There's an apparent obsession among tool aficionado's with the micro-thin shaving. This obsession that completely ignores the more common use of the plane to remove a quantity of wood quickly, a use that requires a certain strength of arm, shoulder and back. Fairing up a planked boat comes to mind in this respect. Why use fifty strokes to effect what accomplished with five.

Jim

[Peerie Maa]

That's just making shavings for their own sake, Nick, a rather pointless endeavor,
Jim

DSC04011.jpg
Not quite as thin as the demo, but thin enough.

[Jim Ledger]

How precious is that, then!!!


Calendars for the gift shop?