"Small" boat design help

Is there any information out there on designing and building “small” boats in aluminum? The books I have found consider anything 25 - 50’ to be small. I’m thinking small is 15 to 20’ and I am assuming that there are structural differences between the sizes. What I want to design / build is about the following:
• 18’ LOA
• 7’ to 8’ Beam (I don’t know what is acceptable)
• Semi-V
• 5” draft (Max)
• 2-3’ freeboard

I know hull design is a series of tradeoffs but this boat is going to need to meet the following criteria (as much as possible)
• STABLE at rest
• Able to handle 3-4 foot waves
• 25 to 35 mph
• 50 to 90hp outboard
• 4 to 5 people

It is going to be used on the Great Lakes (big water – for me) and inland lakes and rivers

I’m just trying to find out what the “rules” are for this type of craft.

 

really all you have to do is look at a few. Small boats, theres not much to them . Darcy Kellahan came out with a good book on aluminum boats a few years ago, I don't think it's available anymore though.

 

The first problem you will encounter is, "How am I going to assemble this boat?" A boat as small as you are contemplating will have very thin plating, and thin plating can be a bear to weld. That is why most major manufacturers of aluminum boats of this size either rivet the hull panels or use high-tech adhesives. The man-hours required to TIG weld the boat is cost-prohibitive. Home builders are not under such labour cost restrictions, though, so don't dismiss the idea of an all-welded hull. Just be sure you can do all the TIG work.

Stephen Pollard's book, "Boatbuilding with Aluminum" is a good text to start with on your path to metal boatbuilding expertise.

 

I'm planning on using 3/16 plate for the hull. The upper sections are going to be 1/8 or less. I have Pollards book and have read it several times. I'm really curious if there are standard rules on length vs beam Im wanting to build something around 18' long with a max beam of 8'.

 

No rules per se, but stability and roll period are related to beam, so a wide boat will roll quicker than a narrow one and may prove to be uncomfortable in a seaway. Shell plate that thick will be able to be welded with wire-feed MIG gear, but the 1/8" plate may need TIG. IMHO, that is pretty heavy material for a boat that size; I have a couple of 36-footers built to my design that used the same scantlings.

I don't mean to be an alarmist, nor try to dissuade you from your project, but I'd like to offer the advice that you proceed slowly and thoroughly with this. A poorly designed boat can not only be a disappointment in looks and performance, but it can also be dangerous to its occupants. Proceed carefully - your friends and family are betting their lives that you are well-informed and thorough in your design efforts.

 

proportions of welded alum. planing boats

H20Fwlr, an 8' x 18' LOA will most likely mean you'll choose either a 6' chine or 7' chine (beam) 28" to 36" sides and if you use a chine flat (reverse chine ) then a 6'x20'x.187" 5086 (or 5052 for fresh water and lower horse power) will make a fine design. You'd get a 7' bottom with a 6" chine on either side of the 72" sheet. There are hundreds made each year within these rough dims on the west coast; they're beamy, bulletproof, safe and can be fast if you want to ride ANYTHING fast that is this light.

If you use a 10 to 15 degree V you'll loose some in your inner chine beam but can widen the chine flat without much effort and can run an 18'er with anything larger than a 50hp engine. The more V you add the softer the ride BUT the more roll when you're drifting fishing or anchored in a beam swell. Going around the "how much V is enough or too much; pick a number..." bush will take you a lifetime, and I'm not exagerating- just confessing. After a few hundred skiffs in the general area of your project, although 95% commercial- not recreational boats, I've gone around that design cycle plenty and I'm still going in circles.

At 0.125" and 0.187" sheet you don't have any reason to worry about hull strength or rivets, distortion can be largely reduced by making all panels convex in section and don't let folks talk you into traditional xverse framing and stringers on skiff this small- that triples the necessary labor and doesn't result in a 'better' boat.

Just for your reference the words "standard rules" as referring to boat design is a moving target. There are government rules for safety from stem to stern and what you wear; there are insurance rules for safety; the are classification society rules to make racing of different boats "equal"; there are HIGHWAY rules for hauling objects on trailers (that is most likely the main design element in your max beam); there are standard rules for dozens of aspects of small boats; but design is more governed by "whatever works". You've described a typical welded aluminum runabout skiff that will hold a small I/O package or mount one or more outboards to the transom and haul you on the Lakes fine.

Your draft of 5" is a bit optimistic unless you build with a nearly flat bottom for the after half, which has adherants in the design community- Lund-Starcraft-Hewescraft.... Besides your outboard leg draws 16 to 20" so whats a few more inches of draft?

A new 4 stroke 90 outboard on such a boat will carry you and four 'fat boys' like me (225 dry wt without breakfast) without any trouble and you'd get the speeds you've said you want to run.

Google the west & east coast welded builders and study their layouts, scantlings and overall designs. You know these work or they wouldn't be advertising them so if you stay close to the tried and true sold by builders in the marketplace- you'll do fine and your boat will be a sucess.

Cheers,
kmorin

 

mmd - thanks for the words of warning. That is why I am trying to learn as much as I can now. I don't plan on building it until next spring/summer so I will be able to come up with a solid design. I have looked at several production boats (1 alum. and several FG.) I was not pleased with construction quality of the Aluminum boat and FG is not durable enough for some of the areas I take the boat.

kmorin - The performace criteria I have to balance is the ability to go in the shallows and still be able to handle some amount of big water. I know it is a trade-off between the two. I want to use 3/16 so that I can weld it and for the additional durability. A duck boat has a tendancy to spend some time sitting on the bottom.

My big question is centered around mmd comment. "stability and roll period are related to beam, so a wide boat will roll quicker than a narrow one and may prove to be uncomfortable in a seaway" I need the stability to use it as a hunting platform. 28" to 36" sides is in the ball park of what I am looking at with a top section that closes in to about 5' wide to act as a blind. 90% of the time will be spent sitting anchored but I want to be able to run from a storm if need be. (keep in mind I do most of my boating in the winter so the ability to break some ice is also needed.)

Back to the books for more research.

 

The plot quickens

WATERFOWLER, Ok, OK I admit I wasn't tuned to your handle/name before you described your outboard powered blind.

Now that you make those remarks more clear; then don't use a V bottom design - use a warped bottom where the sections aft the amidship section are convex cambered to about 3 or 4" in 72". This provides an after bottom section that gets up well, is a very stable work platform in ANY seaway, abeam, head on or surf, and pull the chine in profile very high at the stem. (Very high is 1/2 the way to the sheer) This will provide some decent V forward as you run in to a chop, & will keep the after sections more 'flat' and parallel for easier planing with less power, these more parellel wetted sections will float more wt for a given inch of waterplane emersion and will allow you to beach the skiff with complete indifference to rocks.

My owners have landed (up to) 4,500 lb of salmon in a 6' breaking surf in the boats I built using the described layout. These skiffs ranged from 18 to 24' LOA; w/ 60" to 96" bottoms (at the chine) most were 3/16" bottoms although 1/4" did creep in sometimes, and the sides were generally 0.125" but sometimes were increased to 0.160" when requested. They have been in use for more than 20 years as set net boats in the Cook Inlet in S.central AK and I used the cambered bottom on all but a few dozen.

They're rock stable in a calm seaway and routinely come back from 130 deg or less roll. (here the outboard's cg is still acting as positive righting moment regardless of the hull's roll) I've been in them picking fish from a net stretched across the gunwales and been standing with both boots flat on the port topside with the starboard gunwale over my head and they come back after the swell passes. Most were powered by 35 and 50hp engines that were largely changed out to 70's and 115's as time went along. They are launched through the surf on a gravel beach into a head sea by running line, pushed by tractors and some by hand. They land from twice to four times their wt on these same beaches and seem to continue to do it now, these two decades. They've hunted moose, deer, bear and those poor little innocent harmless ducks that you are stalking. Although I'm not familar with anyone using them as a portable blind.

Don't look at constant V designs for this application as they'd beach in the shallows many boat lengths from the waterline with the same load on a shallow sloping beach or marsh inlet. The only way to get a 5" draft is to go with a cambered ( or flat) bottom shape in this size skiff.

ON THE other hand, this boat will pound in a head sea at 15+ miles per hour as the running water line will move aft to the convex area which won't offer soft entry at your targeted 35mph. In a following sea you can run with the swell and lay into the trough so you could stay with anything the Lakes would likely provide. I've had a 24'er in the Gulf of AK in some 30's and 40's rollers (100' to 150' period) and its quite recreational to run WITH this size swell. But the same boat into a 4' head sea will pound you to jello unless you slow down.

What you're describing is essentially a Lund riveted boat made of welded materials so that is a fine set of lines to model after, realizing the hard impact you'll experience at higher speeds in head seas. This is the 'price' you'll be paying to sit still in the shallows awaiting those poor innocent little ducks.

Cheers,
kmorin

 

Waterfowler,

You may do best to look at one of the "established" designs for plywood construction. Pretty much any plywood design can be adapted for aluminum. My favorites are by Bolger (look at his "clam skiff" design http://www.instantboats.com/skiff18.htm) but there are many others. Check out Duckworks and http://www.boat-links.com/boatlink.html for lots of choices.

KMorin has a lot of good advice, especially for big water, but my impression is that you primarily need a flatwater boat, while being ready to occasionally get caught out on a big lake. If you need to regularly make fast runs in big water, you need a very different design from a flats boat. But a well designed/built skiff can handle a reasonable chop, as long as you don't need to pound through it.

For extreme shallow draft, consider a jet outboard on a flat bottom - or an inboard like Atkins' "Rescue Minor".

Sal's Dad

 

design conversions ply to sheet metal

Sal's Dad, I agree that someone who has already built in welded aluminum a few skiffs could adapt plywood designs -by taking-off the lines, (using your eye or a tape measure) the arrangement by general observation and other details from pictures-say.

But what would be difficult is to assign welded aluminum DESIGN features- not to adapt scantlings. For example, almost all wood and espcially plywood designs have hull joints which are silly in metal. A 14' grarvey in plywood would have a solid 1" x 2" "chine log" or chine strip to hold fasteners, add glue surface and maybe to help stiffen the joint.

A 1" x 2" aluminum bar wouldn't be too easy to bend, weld or even handle, so what rules would the INexperieced builder apply to convert design elements of structure to aluminum? Does the metal boat even need a member at the chine in this capacity? If the element (chine log) were retained how should it be scaled?

Multiplying this decision by the number of 'parts' and their inter-relation in a skiff makes the task more complex than helpful unless you've already built. Gettting started as, H2Ofwler wants to do, is a bit of "a rock and a hard place" since no one would have the informed position to make the decisions without having done similar or other work. How then, does the new builder break into this seamingly solid circle?

I agree that the shapes, overall arrangements and other very helpful examples can be guided by wooden, especially PLY-wooden, boats designed and built by others. I also agree that some of these projects might be the best design for dozens of applications for which their designers conceived them. What is still up in the air is how a beginning builder could wisely convert or adapt the aspects of these boats into a welded metal version.

I don't have an easy answer, it took a fair amount of time and more mistakes than we have time to recount for me to learn what was really needed in various welded boats. I guess that's why I encourage Scott, to look at other welded designs as his guide, more so than differing materials; adapting between materials is hard to do in your first skiff.

On the other hand, he'd probably error on the heavy, strong, stiff and bulletproof side if he were in doubt and that might cost some gasoline overtime; but the skiff would be stout!

Cheers,
kmorin

 

No disagreement, just a different perspective. My experience has been with "stitch-and-glue" ply techniques, which seem to translate pretty well into aluminum. Just look for a plan with curved, rather than flat panels. Flat panels need a lot more stiffening than the equivalent ply.

Of course the construction details differ, but there are so many good plywood designs for this application. Most of the aluminum designs I am aware of are west-coast inlet-runners or whitewater jetboats. Very different animals.

At the very least, find a few tried-and true designs for your specific requirements - in WHATEVER material. Then figure out if/how the 'right' hull can be built in your prefered material.

At any rate, it's a great project, and I hope to read more about it.

Sal's Dad

 

thanks for the inputs. I have a lot of looking and reading to do before I cut the first piece of metal. I have had people tell me to avoid the hassle and just buy a deep V and modify it.
That just does not sound like as much fun as this is. Of course "fun" is in the eye of the beholder.

 

where metal and wood meet-door skins

Waterfowler and Sal's Dad, there is an interesting point at which both materials (aluminum and plywood) meet- that is the "door skin" (Home Depot; 1/8"-3/16" luan plywood in 3'x 7' panels for 7.oo $ US).

This is the best (read; throw-away level cost to try a design) modeling material out there. I make final hull models of 0.060" aluminum as a rule, but the technique is non-trivial for the beginnner since TIG tacking 0.060" takes a bit of practice. What is trivial is making models of luan plywood with a hot glue gun! Tons of fun and informative beyond the investment.

Making a plate model isn't all that hard to do, it does require some fundamentals like laying down your lines, drafting and layout- but beyond that its time well spent in terms of what will become a 'design' of your own.

Plate models, sometimes called builders models, are nothing more than the body plan sections, sometimes called stations, cut of door skin or other 'card' material and lined up in scale on a half keel plane. Using a "surface band", or tick strip, you can take off the plate dimensions and scale it up to full sized in a simple single step. This can be recreational if you're interested in the process and is completely informative of the shapes that will form from flat sheets into hulls.

I've used door skins, like all builders to make plate templates, plate models, and other helpful low cost patterns for metal boats. The process is simple, reliable and will generate accurate hull sheets by simply fairing the offsets using full sized battens.

Deciding how to stiffen panels, gauging convexity and other more complex design issues is a bit more involved; but getting the shape of the bottom, sides, reverse chines, and transom can be done with modeling and DOOR SKINS.

Cheers,
kmorin