crazy idea???

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Hello, I have a 31ft westerly longbow. She was sunk and now is up. I need a longer boat. I do not have the money so I propose to cut shiloh in two at the fullest part under the water line of the hull. at this point the water flows parrallel to the waterline. I then hope to spread her for and aft sections some ten feet and fill this space in with mutible layers of 1/4 inch plywood epoxied to eachother ( I use this because it is easier to curve thinner wood. there will be no attemptat compound curvature. SO, in profile she will have a ten foot flat section. I will glass over this with some number (I am thinking thre now)of layers of epoxy inside and out. I will use common exteior grade playwood and layer it up to a thickness of 1.5 inches to be sure it is strong enough. This should more than compensate for the voids in wood. Exterior 1/4 inch is 10 bucks a sheat, marine is 40, thats why! 1 foot of overlap inside and a scarf on the outside will ensurebonding. Any thoughts would be appreciated. Yes, it is wayyyy cheaper then buying a 41ft boat and it should be faster as its hull length to beam ratio will be thin and it will still be wayyy lighter than comparable boats. She has a 4k lb keel I will reinstall and make lower for a draft of 6 ft or so...toying with idea of retractable. thanks!

 

You'll be making a 132% adjustment to her length and I'd strongly advise you against such an endeavor. I know it seems an easy thing, but it's way beyond the scope of the average backyard guy. Not that it's to hard to accomplish, but the can of worms, you're unaware of opening, will be quite significant, likely ruining the vessel and your hopes of a 41' yacht. This can of beasties isn't the cutting, attaching new low grade plywood and cloth skin, but the balance, stability, longitudinal stiffness and a host of other issues you're unaware of and simply not easily obtainable information.

I'll let everyone else beat you up (be kind folks he's a new guy) for trying to engineer a substantial structure, one that will take you much farther from shore then you can swim back to, with the lowest grade of materials. Generally it's wise to have a strong grasp of the concepts, principles, physics, chemistry and engineering to insure a reasonable expectation of success.

How hard can it be? I bill out at over $200 per hour, take a quick guess.

 

Sounds Great...

Hello...

So long as you are not buying from the police - ah - can I come to your next BBQ...

SH

 

What PAR said.

What you propose will totally screw up the stability and hydrodynamics of the yacht. It can be, and has been, done successfully. I've seen 800ft cruise boats lengthened like this. But it's done by carefully analyzing the current structure, completely redoing all the hydrostatics and dynamics numbers, computing thousands of stresses on the joints and filler module, and then cutting, dismantling, building and finally reuniting the hull with millimetre precision. Done as you propose, you will have a huge weak region amidships that will be most likely to fail in high winds and high waves, perhaps the least desirable place to have your boat go Titanic on you.

Sorry to bash the idea so hard, but I'd rather you be alive and crowded than on the front page of a Coast Guard report.

 

PEOPLE SEEM TO NOT REMEMBER THEIR HISTORY ABOUT JUST WHAT TYPE OF CRAFT PEOPLE WENT SUCCESSFULLY TO SEA IN HUNDREDS OF YEARS AGO. OF COURSE, I WOULD GRADUALLY WORK HER UP TO LARGER AND LARGER SEAS NEAR SHORE AND ACTUALLLY HAVE A LIFE RAFT WITHME... CAPS ONLY FOR CLARITY, NOT SHOUTING I WAS REALLY LOOKING FOR SOME MORE PRACTICAL ADVICE ABOUT THIS (OTHER THAN DONT DO IT, IT IS BEYOND YOU), THANKS tERRY

 

For Instance

WANTING TO RE USE THE 4K LB KEEL THAT NOW DRAWS 4.5 FOOT, IS THERE ANY EASY ( i KNOW THERE ARE MANY EQUATIONS FOR AN EXACT ANALYSIS) "GUESTAMI" FOR HOW MUCH DEEPER SHE NEEDS TO GO TO INCREASE RIGHTING BY "Y" FT POUNDS AT 30 DEGREES LIST?.......CAPS ARE STUCK ON AND i AM AFFRAID TO TURN THEM OFF

 

WELL GEE, DID YOU THINK I WOULD GO OUT IN 30 KNOTS FIRST TIME OUT WITH NO LIFERAFT, NO EPERB ETC??? IF SO, I DESERVE TO NOT BE AROUND TO MUDDY THE GENE POOL ALL CAPS FOR CLARITY, NOT YELLING

 

My intent wasn't to get you pissed, but more to an understanding that there is a great deal of information you will need to absorb, so you'll have a reasonable level of success. This isn't the best place to attain this information (on a wholesale level), not to mention most will need a few years of schooling to acquire it and I'd expect you'll have a similar route, if you elect not to use a designer or NA.

The ideas in your original post (exterior plywood, lowering the existing ballast, etc.) suggested a less then reasonable knowledge of the forces involved.

Your boat can be lengthened, to 41', but much needs be accomplished, before the first sheet of 1/4" ply (good idea to laminate thin ply into thick) gets cut. It's very probable you'll need to add ballast, which will require a redesign of the sail plan, plus a number of other issues, such as appendage concerns, hard points, localized stresses,etc., all quite normal for such a major revision to this hull configuration.

In direct answer to your questions, yes it can be done, there isn't any calculus necessary in the redo of this craft and the math skills can be accomplished by any high school grad that had good grades, the 1/4" box store ply isn't near as strong, but more importantly is very prone to rot and usually is of poor construction, those NA's you "queried" should be consulted about your plans and ideas, a heavily built, stiff midship section will cause the ends to snap off the boat (honest), loading has to be matched, the additional displacement (of the added midship section) will require a substantial ballast increase, appendage reinforcement if not redesign and any significant drop in the resulting CG will typically require a new rig be designed and installed, generally it's not wise to "guesstimate" the stability calculations in ocean going vessels, containers that fall overboard do have hydrodynamic qualities (hydrostatic too) I'm a bit of a history buff and am well aware of the ships that went to sea a hundred or more years ago, they were very well engineered structures and surprisingly clever to boot, using formulas and calculations for thousands of years, the ALL CAPS doesn't help the clarity, much like this run on sentence . . .

If my data base is correct (it usually is) you have a Laurent Giles penned cruiser. LOD 31', LWL 24', beam 9' 6", draft 4' 6", 442 sq. ft. sail area, 7,600 lbs. displacement, 3,600 ballast, 245.4 D/L, 18.3 SA/D The SA/D indicates she's a good light air boat and the mid 200's D/L shows she's a pretty easy going craft, in spite of the weather going south on her. A solid cruiser.

With the healthy displacement, short waterline and generous overhangs it's understandably a little cramped by current standards, which could squeeze in an aft cabin in that length, though wouldn't be near as pretty as your yacht.

Lets see what happens if we use just a little math on her and add 10 feet to her waterline length at 54% of her LWL, the point of maximum beam on the rail, which is different then the bottom of the keel and the LWL, but hey, it's just for fun. She now has a 34' LWL, LOD 41' and everything else is the same. The SA/D drops into the mid 12's and clearly you'll need a new sail plan, the once comfortable mid 200's D/L is now mid 100's making the motion much more violent then before and the comfort aspect of this yacht is gone, she's nearly a racer, but will fair quite poorly against one. This is factoring a very modest 5,000 lbs. additional displacement. In reality, you'll be gaining in the area of 175 to 250 cubic of extra immersed volume. If you multiply that number by the weight of a cubic foot of sea water, you'll get a pretty quick picture how much we're talking about. Frankly, you'll be quite lucky if you can bring this yacht in (with the proposed 10' addition) at double it's current displacement. Nothing on this boat is designed to withstand the tremendous increases in loading this would bring on. I didn't need to do the math to understand this, but the quick demonstration for you, I felt was necessary.

This doesn't mean you can't do it, you can, but you'll be building a new boat, probably with over twice the volume of the old one and none of her wonderful qualities will shine back through, except the cosmetic ones.

 

I will not address the design principals. I will however address the exterior plywood. First off you assume fiberglass is "waterproof". In fact blisters are water finding it's way in through solid fiberglass. There was a test posted recently were the person used different coatings on a piece of wood like west system epoxy and submerged it for several days. He then weighed the wood and noted the increase in weight. every coating showed some increase in weight meaning that water was getting in. The voids in exterior plywood will cause it to rot quickly.
An observation: all the lenghtening jobs I have seen have been at the stern and occasionally the nose as well. I have not seen a boat your size lengthen in the middle.

 

marine ply and water ballast

one of the good things about going to the 3 ft lengthening is that I can afford to use super duper ply! I was under the impression that marine ply had actually been treeted against rot though ...
another thought about the water/ballast. this may not be the optimal way of keeping her at her waterline but almost all things come with tradeoffs...in this case, it is that , once grounded, I can lighten my boat by some1500 lbs and get her off easier!....every cloud has its silver lining! If I undertake this, I shall make a websight and post url here..... it is a choice between this project and a cat which I am goingto make a post about now. I appreciate all the input! thanks much! And I am going to duplicate this experiment covering wood with 4 layers of 8 oz and epoxy (not West though, they are part of the united nations plot to take over the free world) and submerging it to check wieght. Great idea!!! I must see it to believe it....probably a marine plywood manufactorer said they did this just to sell more marine ply!...not that I am cynical or anything Have a HAPPY day!!
Terry

 

You seem to have a pretty nice boat now. If I were in your position I'd sell her and use the $$ to buy a 40-footer with the qualities you want. One in need of repairs can probably be had fairly cheap, and you could then repair and outfit it to your needs. From what PAR was kind enough to calculate earlier, it's pretty clear that stretching your current yacht would make her tricky to handle and uncomfortable to ride in. You'd be miles (and dollars) ahead to start with a hull that already has the dimensions you want.

 

Since you started this thread with an interrogative, you are soliciting opinions, so don't be surprised if people respond with opinions you don't want.

I'd also restate what others have said- this is a bad idea. Big difference going from 31 to 40ish feet. Loads will be more than double. IF - you analyze and engineer the structure for the new design, compute stability, etc., then have at it. If you are pasting in a plywood midsection at the zone of the boat that is most highly loaded without crunching the numbers, you are asking for trouble.

Feel free to yell back at me with the caps lock on - for clarity of course.

 

From a costing point of veiw, sounds like a lot effort. You might be better off building a similar design from scratch. Will use be using the same sails?

Lot depends on how much you're attached to your current hull. It can be done. Just have to figure out the cost/benefit ratio.

 

I did have caps on for clarity as it was easier for me to respond within the quoted material...if no caps mine would be betwixt his and no one would know which was which.Hope it did not offend! I really am not the type to yell You maybe missed my post of yesterday where I said that I downscaled to a 3 ft section (see post for reasons). I like guessing! lol The hull I have could not be readily sold for much and a bigger boat is at least 9 thousand bucks. I know I can do this for les than that! For instance, I use galvanised wire for all stnding rig and thimbles and U bolts. All gotten from a power line support company. I rigged the whole deal for less than 200 bucks! 4 staylocks would eat that much! Galvanized wire is 10 % stronger than stainless! Electrical: I use regular old speaker wire of the appropriate gauge and solder all conections, then I use sealant from conector to insulation. I can wire a boat way cheaper than this and as a live aboard I have not had a failure yet! There are many ways to go cheap! The system is agaiinst it though and so people are educated (indoctrinated) that I shall surely die a hideous death for afronting the gods of greed! Have a happy day! (this was the first time I had posted anything anywhere and have found out how not to intersperse my repley, sorry again for the caps
God bless, Terry