Strength vs. Overbuilding and Jonesporter Progress Pics

[MG]

Unlike many here on the forum, I do not posses the skills necessary to restore my own boat. In these early stages, I can only sit back, watch and ask lotts of questions. I plan to be more involved in the finish work (i.e. sanding, varnishing, painting). So, here's my latest question:

There are many weak points in the original construction of my boat. One happens to be the thin, flimsy keel (which is now rotten). It was the weakness of the keel which apparently was one of the reasons she was converted from a skeg hull to a built down. It has been recommended that we build the keel thicker, thereby making in stronger. Sounds good to me, but in the past I've been cautioned against overbuilding her. At what point does one cross the line between strengthening and overbuilding? How much is too much? Here are some pics of the keel in progress. Keep in mind, it's just beginning to take shape.

http://www.imagestation.com/picture/sraid20/p1d9edb3d55490ec1dd4de05c22f81cbe/fdc01ae0.jpg http://www.imagestation.com/picture/sraid20/pd171bc95be4293a6c8432917fdb13c70/fdc01c4b.jpg

<a href="http://www.imagestation.com/picture/sraid20/p4e028d8cd9ff521407bcb47f7a898547/fdc01c58.jpg[/IMG" target="_blank">
[img]http://www.imagestation.com/picture/sraid20/p4e028d8cd9ff521407bcb47f7a898547/fdc01c58.jpg</a>

More Pics of my 1930 31' Jonesporter (http://www.imagestation.com/album/?id=4291959797)

[ 05-09-2002, 05:33 PM: Message edited by: MG ]

[Scott Rosen]

Nice boat and nice pictures. I also like the wedding pictures. You and your hubby will make a nice sight cruising in your restored boat.

Personally, I like overbuilt. Exactly what to you mean by overbuilt? Overbuilt compared to what? It's not easy to say exactly where overbuilding becomes too much. I suppose anything that would change the dimensions enough to alter performance is a potential problem. Heavier scantlings could be good as long as it doesn't add so much weight that your engine can no longer drive her. Judging by the photos, you certainly are not going to have problems with a skimpy keel.

[mmd]

Hi, Melissa. Yes, I agree that the original keel structure was too light - it looks absolutely frail. Without doing a bit of historical (hysterical?) research or scantlings calculations, I would venture to say that the keel on a boat of this type should be a minimum of four to five inches thick. If you want to contact me off-forum, we can discuss this in a bit more detail.

[Donn]

How are the rest of us gonna learn anything if you discuss it off-forum?

[Mike Keers]

Old fisherman saying: "Too strong never broke".

[Wayne Jeffers]

Regarding Overbuilding, from a purely academic standpoint - - -

White oak has a mind of its own. Usually its propensity to wander (move) is kept in check by the structure it's built into.

Would it be possible to make a white oak keel so stout that it would warp after installation and take frames/ribs and planking with it? :confused:

Wayne

[Wild Dingo]

Originally posted by donnwest:
How are the rest of us gonna learn anything if you discuss it off-forum?HEAR!!! HEAR!!!! Total agreeance donn!

Sorry mmd old mate... keep it on forum... we gotta learn!!!

Take it easy
Shane

[Wild Dingo]

ooooops forgetfull buggar I am... meant to say...

Well done so far Melissa!! Good question!! Keep asking them!!!... It saves me asking the "dumb" questions :D Just kidding!! ;)

take it easy

[mmd]

OK, you asked for it...

Using Det Norske Veritas (DNV) "Rules for Construction and Certification of Vessels Less Than 15 Meters" (1983), the method of calculating a keel size is as follows:

(note: I can't do subscripts & superscripts on this editor, so I will substitute (number) for subscript and (+number) for superscript)

Given that a keel timber is to have a minimum mass v(r)at 15% moisture content of 640 kg/m(+3)

and

given the scantling correction factor

f(1)=v(r)/v(a)

where v(r) = mass of timber per rules
and v(a) = mass of proposed timber
and the resultant cannot be less than 0.9

the section modulus of the keel timber Z is not to be less than

Z = 7fLOA(+2) cm(+3) for massive timber
Z = 5.6fLOA(+2) cm(+3) for laminated member

where f = scantling correction factor
and LOA is the length overall of the hull

Furthermore, the aspect ratio of moulding/siding is not to be less than 2 and not to exceed 3.

Now that you have ascertained the proper section modulus for your keel, you can calculate the moulded and sided dimensions by derivation of the formula for section modulus for a rectangular shape,

Z=(bd)(+2)/6

where b = sided thickness of the timber
and d = moulded depth of the timber

There are several other sources of scantlings calculations (ABS, Lloys's, Nevins, and Gerr, to name but a few), but another more common method of deriving scantlings for small craft of common form is to collect & quantify the scantlings of other boats of similar type that have withstood the test of time. For small open craft of the Jonesport lobsterboat type we are speaking of here, the Newfoundland Trap Skiff is similar in size and shape, and happily the Newfoundland Fisheries Loan Board compiled just such information and published it as the "NFLB Scantlings Regulations". From this source I find that the recommended siding for a keel for a 31-ft open trap skiff is 5". As most trap skiffs are built of local softwood, if one were to make the keel from suitable hardwood a reduction of siding of approximately 10% would be allowable, making the keel 4-1/2" thick.

Thank you all for being an attentive class. Don't forget to put your scrap paper in the garbage and peel the bubblegum off of the underside of your desk. :D

[Donn]

Oh never mind! :eek: :confused:

[Dave Fleming]

Thank You M, it is that sort of info that will be of much help to those here who wish to design or modify their boats.
Now my question smile.gif I have worked on similar vessels 250' 1800 ton Tuna Sieners to both Bureau Veritas and Det Nortske Veritas. Vessels were intended for similar waters and obviously same usage so why were there different classification societies used.
Is it because of different flag nations or...?
No difference in construction from one to another for a total of 7 vessels for USA, Korea, France and New Zealand and there were 4 societies over seeing construction.
ABS,Lloyds. BV and DNV it was puzzling and unfortunately I did not have the opportunity to query the Engineering Dept. on it. To return to MG's boat and that keel deadwood assembly, it is sorta confusing to me to figure out the way those pieces are joined together. Ayup I do very well grasp that the shaft log will run through but just looks funny to my eye.

[ 05-09-2002, 04:48 PM: Message edited by: Dave Fleming ]

[mmd]

Donwest, you have the wrong attitude for this class. Go stand in the corner for ten minutes. tongue.gif

[Donn]

It ain't my attitude, mmd, it's my aptitude. :confused:

[MG]

Originally posted by Dave Fleming:
To return to MG's boat and that keel deadwood assembly, it is sorta confusing to me to figure out the way those pieces are joined together. Ayup I do very well grasp that the shaft log will run through but just looks funny to my eye.Dave,

I can't explain the assembly to you, but if you look closely at the 1st picture, you'll see that is how the original keel was assembled. The new keel remains to be plained, so while it will be thick, it won't be THAT thick. ;)

Any reponses to the reply post regarding the keel warping once it is in place? I hadn't even thought about that! :(

[Dave Fleming]

MG, its the layup of the deadwood pieces that has got me puzzled I am used to more like this one from one of my Imagestation albums.

http://www.imagestation.com/picture/sraid13/pe140a73e870381a5b5954f7853dd9f3c/fdd48991.jpg

[mmd]

Dave F. - the ship owner has the choice of which agency he will class the vessel under for insurance purposes. The vessel is then built to the regulations of, and inspected by, the chosen agency. Most ships are designed so that if they pass muster for one agency, they are usually acceptable by the others, so it becomes an exercise of having the proper inspectors on site and submitting the vessel documentation in the proper format to suit the agency du jour. Regarding MG's funny keel, I suspect that there will be another wedge-shaped deadwood fitted on top of the shaft log prior to fitting the horn timber or faying up to the existing one. The construction method being done is a bit more involved than just stacking deadwoods parallel to the keel, but makes for a more secure shaft tube as the shaft bore does not cross any timber seams.

Donwest - All right; you can return to your seat. If you want help after class, let me know. redface.gif

[Dave Fleming]

Ah Ha, I sees it now....since the keel/deadwood is on the narrow side and boring for the shaft is going take a bit of wood away the area left for fasteners is reduced accordingly.
So interlocking the timbers assists in keeping it all tight.

Oh Teach, can donw come out to play now? He's got the ball in his locker. :D

[Thad]

On the prospective motion of the oak keel: With the structure of the boat tied to the upper edge of the keel that is not likely to change a lot and if it does some it will bring the rest of the hull with it. Possible twisting of the keel timber then will depend on it's grain and the location of the pith in the timber, It would be best if the piece were quarter sawn away from the pith. How green the piece is as well as how much it dries out before splashing will be a factor. There are many ways to do almost everything in boat structures and surprisingly many successful ways. (For example I think the keel, shaftlog, deadwood, stern structure, elegant.) If the piece is green and gets wet soon to cure in place you might do fine -- better than with a rotten backbone for sure.

[Mike Field]

From an engineering perspective, the ideal is to avoid overbuild (which really equates to overkill.) One tries to design every part of the structure to perform its proper function until the end of the structure's design life, and then promptly crumble into dust simultaneously with every other part because the whole structure's been designed the same way. That's the ideal. I've never known it achieved in practice.

The problem (if that's what it becomes) occurs either because no thought was given to the structure's design life in advance, or because materials properties form a step-function, not a smooth curve, and you can't get a material that fits right on your design-line. Assuming that it's not your intention to skimp (which is not always a justifiable assumption,) some parts will almost invariably be overbuilt as a result.

In the case of the Jonesporter, we're presently saying the keel was underbuilt, along with part of the frames, the floors, and so on, because they've all rotted or whatever and they need to be replaced.

But what if the vessel was only designed to last fifty years? Then it's already exceeded its design life by twenty years. And in that case, ALL of it was overbuilt. But some parts were more overbuilt than others.

So by replacing the keel and other parts now, Melissa is providing a new design life of X years for the boat, either intentionally or by default. The question is, how long do you now want the boat to last, Melissa? If the planking's going to fail before the new design lifespan is reached, then clearly the planking too will need to be renewed somewhere along the way (with obvious inferences for programming and budgetting.) Alternatively, if you only want the boat to last until the present planking gives up and then scrap it (maybe five years perhaps?) then you're overdesigning the new keel. You could be saving time, effort, and money by using cheaper material, lighter scantlings, whatever, so that the keel gives up the ghost when the planking does.
____________

(Disclaimer for the literal-minded -- I am NOT advocating that Melissa only design the new keel to last for five years. I am merely pointing up what "overbuilt" and "underbuilt" mean. I think the concept of design life is a useful one in this context.)

[ 05-11-2002, 08:06 AM: Message edited by: Mike Field ]

[MG]

Thanks to everyone for their insightful posts. While I may not posess the skill I'm glad to be gaining the knowledge.

Mike, dare I say I hope my boat will be around for another 72 years (am I crazy?)! Here's another question, how long can well cared for cedar planking last? My boat's planking is still in very strong condition. I hope I won't have to replace it any time soon.

[mmd]

The much-talked-about "Elly" has pine planking that is around 140 years old, so there's hope for your planks, too. Aside from bumps or poor maintenance, the usual place for planks to go bad is where they are fastened, especially at the ends. Keep an eye on these places for loose bungs, discoloration, swelling, or splits. It's not a big deal to replace a plank or two, and the likelihood of having to replace all the planking at once is somewhat remote. She should have plenty of life left in her yet, and you've given her both a new lease on life and a firm foundation upon which to do maintenance by putting a new keel under her. Just remember that wooden boats last a long time through regular small-scale maintenance rather than infrequent major surgery.

[ 05-10-2002, 01:18 PM: Message edited by: mmd ]

[MG]

Thanks mmd. It's a shame to see how she deteriorated so fast from her last, neglectful owner. I intend to not let anything get ahead of me. She's far to precious.

[Figment]

those keel timber photos reminded me of a question that's been on the rearmost burner of my mind:

Just how DO you bore a hole for a propshaft through that much wood????

[mmd]

Ya weld a little bitty drill bit on a lo-o-ng steel rod and procede very carefully. :D

In all seriousness, there are a number of methods such as the semi-facetious method just mentiond, drilling a small pilot hole and using a boring bar to ream out the hole, building the shaft log in two halves and routing each half-hole on the bench, etc. Most builders have their favourite tool & technique. I believe there was a thread on this topic a while ago, you might want to check the archives.

[Dave Fleming]

Deep hole drilling or How to keep it straight.

Usually it works like this: for small craft the shaft
wood can be made in two pieces if wide enough. The top and bottom are
given a saw cut dead on center and two other cuts just far enough
inboard from the outside.
Splines of a soft wood will be inserted into the two outboard cuts.
The center one is for use as a guide,
The pieces are assembled with the spines set in some sort of long
lasting flexible goo or red lead putty.
The shaft wood is bolted to the structure and a regular metal twist
bit with its point ground for wood cutting is used to bore into that
center saw cut for the full depth of the bit say 4". Then a 'barefoot
ships auger' is used to finish the job. With the saw cuts being dead
on and the twist bit following the saw cuts and the barefoot auger
following the pilot hole of the twist bit it bores a nice clean dead
on hole. In small craft with a solid shaft log it is similar but done
with a jig using a batten to establish the line of boring on the side of
the wood and with careful eyeballing the pilot hole is drilled and then
to the full depth as above.
On big craft a carefully aligned jig usually 1/3 again as long as the
hole to be bored is set up using lines drawn on the outside of the
keel/deadwood assembly. When all is lined up the procedure
is begun. First that all important pilot hole is drilled. That
determines the accuracy of what follows. Using barefoot augers of the
same diameter with successively longer welded shaft extensions the
hole is drilled through the wood from outside in. Then to get the
final size bore for the shaft liner a 'boring bar' similar to what a
machinist would use in a lathe, is used to expand the hole to proper
dimension.
It is a tiresome job deep boring with the barefoot augers. They have
to be withdrawn regularly to clear the chips that build up in the hole
behind them or else that bit can snap deep in the hole and, 'OH Boy,
Katie bar the door!', for there will be hell to pay to fix that mess.

[Wild Dingo]

Melissa... I gotta say Thanks for posting this topic!!

We have not one educator not two educators but 3 educators!! CRIKEY! Is this a great place of what?!!

That sounds like hell on the muscles Dave! particularily if that bit breaks deep down in there... ouch!

mmd you lost me way back about when you said... "Okay you asked for it" :eek:

Move over Don!! :rolleyes:

Take it easy
Shane

[MG]

Don, Shane, Three's company. Maybe there's a remedial class for us. I'm hanging out w/ my new best friend (my boatbuilding friend)this weekend and will try to sap him of his knowledge. :D

[ 05-11-2002, 11:13 AM: Message edited by: MG ]