Elements of Boat Strength

Hi everyone,
I've seen others on this forum refer to this book as a resource and have some questions as to its use for my project.

I am planning to disassemble a 17' center console down to the hull skin and start over. My intent is to use the scantling rules in Elements of Boat Strength to determine scantlings and laminate schedules for my stringers, floors, deck, etc.

I understand that all these are dependent on the hull skin laminate thickness. If the hull laminate is thinner than what is called for in the scantling rules, the rest of the scantlings will do me no good. Conversely, if the skin is thicker than the rules call for, I should be ok.

I plan to use epoxy resin for all of the rebuild because of its superior peel strength. Am I on the right track or headed for trouble?

Thanks,
Dave

 

For what it's worth, "Elements" will be of little real value on your hull, though much useful information can be gained in reading it. The reason it will not help you much is it's intent is to provide healthy scantlings for substantial yachts, work vessels and commercial craft. Most manufactured "center console" boats are built to a different set of standards, which typically show up as light in Dave Geer's scantlings tables. The size of the boat is another problem. The resulting low scantling number would place your boat off the chart or very close to the lower limits, making it difficult to use the formulas and tables. There are allowances, built into his scantling rules to accommodate boats like yours, but I think you may find the information difficult to interpret, without some prior experience in repairs or construction. A vast majority of production built pleasure craft will fall below the scantlings recommendations found in "Elements" for a number of reasons, but cost is a big one. "Good enough" is the norm (in production boats) rather then the "more then good enough" approach "Elements" takes.

What year, make and model do you have? How would you like it equipped? What changes do you envision for this boat? The service you intend it to perform (location and duties)? With these questions (and others) answered, you then can start with scaling up some structural elements for you vessel.

You can use epoxy if you want, but it isn't necessary unless you plan for a lot of wood in the new structure.

Where are you in central Florida? I'm in Eustis, about 40 miles north of Orlando.

 

tja

I agree with Par. Don't over think this as a lot of other people do and make a fairly simple project deficult. If the boat is ten years or older and in decent shape, what makes you think that you need to redesign the hole boat. If you don't think the hull is strong enough then grind the whole inside and layup one layer of 1708 biaxle and lay in a stringer system simular to what was in before. Costomise what you want to the interior and have fun with your boat. I personally think that epoxy is over kill in most boats. Polyester has worked quite well for forty years. Good luck. Tom.

 

#1 do not get oversold on resins! Unless you like spending money. If everything is clean dry and sanded with 36 grit a vinyl ester will do just fine.

#2 Why not just do some core work to eliminate a portion of the internal structure and provide a stiffer boat?

 

Thanks for the replies.

The boat is a 17' Seasquirt. It was made in 1985. The transom and deck hatches have gone soft. I could make repairs to those parts, but would rather make a project of it. I basically want to build a new boat without all the work of building and fairing the outside of the hull.

I eventually want to build a boat in the 18'-20' class and I'm looking at this as a practice project. I've done some glass work and built one small boat (a pirogue). I'm still in the planning stages of this one, basically finding out how much I don't know.

I'm not really happy with the layout of the boat as it is. I want to reconfigure it with large casting decks fore and aft, wide gunwales, and a self bailing cockpit like a flats skiff.

It will normally be used near shore in salt water. I want it to be able to stand up to rough seas should I be caught in a storm. It's currently rated for 75hp and I want to beef it up to handle 90 or possibly 115. I'd like to get 40-45 knots out of it on smooth water.

Par, I'm not sure I understand what you're saying about "Elements". I understand that my boat is at the low end for his scantlings formulas, but if I were to use Gerr's scantlings, would the structure be too beefy, or too weak? I'd rather it were stronger than a production hull as long as it doesn't get too heavy. That being said, if "Elements" is not the right resource for figuring the structure of a small powerboat like this, is there another book you can recommend that is as well written as " Elements"?

I live in the woods about 15 miles west of Gainesville. I'm building a 24'x 50' barn that will house my workshop (I'm afraid I've outgrown the garage). After the barn is done, I'm going to build another pirogue with foam core instead of plywood. The Seasquirt project will come after that.

Dave

 

Dave,

It might be worth your money to buy a set of plans that's similar to your boat from a proven designer. Then you can 'reverse engineer' the scantlings to come up with what you need on your boat. The plans will also give you guidance on sequence of events and techniques. And if you decide, after further digging, that your boat is a lost cause, you always have plans for a new boat. I've had good luck with Devlin boat plans personally.

Regarding epoxy, if you're laminating over wood, or over old fiberglass, I wouldn't use anything alse. For the little boat you're doing, the cost differential for a cheaper material just doesn't amount to enough to justify the risk.

 

I have a question pertainong to scantlings. Hull thickness of say an 18.5' x 7' 2" beam flats boat. If you check Skene's it works out to 7/8 and trying to design a light weight boat at 7/8 hull thickness would be tough. I shoot for 3/8's, is this to light? Cold molded ply over fir stringers with 4 bulkheads, no frames. Thank you for your thoughts.

 

3/8 seems a bit light depending on the stringer spacing. 1/2 should be adequate. 7/8 is overkill.

 

Built the same boat 15.5 loa x 7'2" with 1/2 bottom and 3/8 topsides and ended up with a 1100 lb hull. 4 layers 1/8 ply bottom 3 on topsides. I am in the process of streching the boat to 18.5' and thinking of only laminating 3 layers on the bottom, of course there is a 5 oz. dynel over the bottom which would carry it to 1/2". by the way I used Kevlar on the 15.5'er never ever do that again.
Although the 15.5'ter runs 38 knts fully loaded that is quit good i felt, better yet a 70 uses very little fuel

 

"Elements" is a wonderful tool, for those inclined to use it. The book shouldn't be thought off as a quick guide to the engineering involved in yacht design, though it does have a number of clever formulas, charts and graphs.

The biggest problem, in an undertaking like the one you have in mind, is weight control and placement. This is the number one difficulty designers struggle with in small craft. Small boats are very weight sensitive, it's location and allocation are vital for a successful outcome. This requires a reasonable understanding of the concepts involved, which "Elements" doesn't provide.

I'm not sure how your SeaSquirt is constructed (chopper, hand lay up, etc.) nor what part of the elements of your boat are structural members, but you can rest assured over building or over burdening this boat will kill its' performance very easily.

Understanding how your boat is built will go a long way toward redesigning the accommodations (insides) and deck layout. Pop the deck cap off the boat. This task alone may make you wish you'd just fixed the transom and built a few new hatch covers, but once it's off, the liner is next. Odds are it's tabbed in several locations and will have to be cut off. Don't toss any thing away yet, you'll need to weigh it so you can have an Idea of how much structure you've removed and (more importantly) how much you've got to play with in building a new one.

Use your head and think about how things may have to go back together. I'm referring to skins (like on the transom) that can be reused after the internal core has been restored. These can usually be cut away cleanly and bonded back in place so that it looks (with a little paint) like nothing has happened. You'll find there may be several areas that you might want to keep, like molded in storage, glove boxes, dash board, engine controls pod, etc.

Once the boat is stripped of it's hardware, deck cap and liner, you can have a good look at the structural grid, which may also need repairs. You can use similar techniques to repair the support system, or one of several newer methods and materials. This is where you shouldn't get carried away. The structural support system accepts the loads, imposed by the sea, crew, stores and supplies, plus propulsion. Unless you are quite comfortable designing a new arrangement for the anticipated loading (performing the required math), then leave well enough alone.

A comment about raising the sole (the floor), don't raise it much. By much I mean a few inches (read 3 or 4 max) as the balance of your little boat can be tossed off severely enough to cause a capsize in the right sea conditions. If 3 to 4 inches doesn't get you a self draining cockpit, live with it and buy good, big pumps. Ditto on adding a bunch of additional deck space high up at rail level.

A common misconception about over building is that it will be stronger, but in fact it's likely you'll make it heavier and weaker (point loading or localized stresses) in a pound for pound comparison. Do you have any pictures of the boat and can you post some sketches of what you intend to do?

Wdnboatbuilder, just to be a dick and using "Elements" your SN will be about .25 to.28 (had to take a guess at midship depth on a "flats" boat that size) which makes your planking thickness around 3/8" (light) or 7/16". I'm not sure why you got a 7/8" planking thickness figure from Skene's, unless you were using solid lumber formulas.


This is the problem with these types of books. Would you remove a balky gall bladder after reading through a copy of "Grays Anatomy"?

 

Thanks for the input, PAR. I understand the importance of maintaining the center of bouyancy and center of gravity so the boat's stability is not adversely affected. I'll find a way to weigh the hull and find it's CG before I start any disassembly.

As I said earlier, I'm still in the early research stage of the project. I expect to get some professional advice on the project before I attempt the rebuild. So far I'm just kicking around ideas. Not ready to go for that gallbladder, yet.

Dave

 

To make the light weight required a sandwiched construction structure/hull shell should be the first thing considered down here the flats boats have a low free board to keep the wind influence minimal which means we do not have a great freedom with just blindly adding structure.

The boats back in the 80’s weighted around 550 pounds with tank full liner and two bait wells before nuts and bolts that was a boat 18’ by 8’ around 53 mph with a Yamaha 70 of course that is heavy by today’s standards! Oh it was self-bailing also. Vacuum bagged Airex core !

 

Steve

I am about to replace the cockpit sole on the sporty. We are going to rip out the old teak and plywood and redo the supports and then go for a new surface. What core do you think gives me the highest strength considering we are only going to be about 5/8 thick in total covering a 12 X 10 area with four openings. One for the genny, one for the lazarette and two mullet boxes. Ray:?:

 

thanks for the info PAR It was a while ago I had figured and was more than likley solid lumber formula's. I would go back and refigure but I have loaned the book to a builder/ designer who has past on and the family can't find the book now. It is very tough to find books on the subject so i usually go with the gut and what I have worked on in the past. So was the 3/8-7/16" on the 18.5'ter or the 15.5'ter? DWL moved 3' it's 6'6" on the chines at the transom and warped bottom. any info is only a learning tool from some one who knows more than I which is probly most of you. A 1100lb hull on a 15 foot boat is a bit much even though it makes for a very nice ride and only draws 9" for it's weight but would like to take about 2-300 lbs out of her in a 18.5"ter. so instead of a 1/2" botton and a layer of kevlar to a 3/8" bottom with a layer of Dynel It could be possible

 

http://www.spaceagesynthetics.com/

They have some nice glass reinforced material for an industrial strength cockpit sole! Composites1 handles that material also you have the advantage of being able to bolt anything anywhere.