Aluminum "Shoal Sailer"

Hello ! I appreciate being able to join this network. For the last couple of months an idea has been formulating in my mind about designing and building a rugged, trailer-able aluminum sailboat in the 7-8m (23-26ft) range. Based on what I have read on here and elsewhere, the general consensus about using aluminum for smaller boats is "don't"... however, in this case I think it deserves additional consideration.

Some background on me - I am a mechanical engineer working for a Canadian company which specializes in 5-axis milling and tool design / grinding. Day to day my nose is buried in Rhino and Grasshopper software, python, vb, C#, javascript, on and on, generating cnc code and often tedious geometry. I have been in love with sailing since 2013; starting on Lake Superior, working through ISPA levels and also hold a commercial Masters Ltd. < 60t... although I hardly consider myself a real sailor sitting at this computer every day and without a boat!

And now on to the reasoning - I grew up in Winnipeg, MB. Canada, and every summer was spent on Shoal Lake (Ontario), where my family has owned a small summer cabin on an island since the late 1970's. Getting there is a 15NM passage from Clearwater Bay, ON. through Shoal Lake Narrows which we have always done with old 16-18' motorboats, and learning the winding route 'by heart' is considered a coming of age passage. This lake system is just inside of the Canadian Shield. Shorelines and lake bottoms are rock, simply. The odd thing about Shoal Lake in particular is that as far as I can tell it is incompletely / uncharted (and I have inquired directly with the Canadian government department of Fisheries & Oceans, and was told charting 'has not been a priority'). This applies to the lake proper, otherwise there are marked routes and reasonable but old charts for the area to the north. Anyways, I know first hand about dinging props and shearing off outboard bottom-ends, and every year there must be hundreds of incidents related to surprise bottom contact in that area (Kenora, Lake of the Woods, etc.). Aside from a giant lake full of uncharted shoals (name is not a coincidence), many areas are connected by narrow, often shallow passages. Up until now, the idea of having any sailboat of my own in that lake which would also be large enough to comfortably spend a week at a time on was immediately dashed by the logical probability that it would find the bottom too often to be enjoyable. Additionally, any wooden or glass boat would inevitably be abraded by each small contact with the rocky shoreline, or any scuffing against the typically sharp, prismatic boulders common just below the water level along shorelines. The only other thing is that this lake system is, as far as I am concerned, the most beautiful place in the world.

So, how to sail there without watching your boat crumble and chip away? Aside from Lake Superior , my only mention worthy lake-sailing experience is in the Mazurian area of northern Poland (there is no comparison to make there really). The Mazurian Lakes are very pleasant (if not also a bit overcrowded) to sail and sail-camp on. The shorelines and lake bottom are sand, mud, and grassy. Even the rocks where they exist are smooth and round; the stakes are low. The overarching objective of this project is to achieve the Mazury sailing comfort on Shoal Lake/Lake of the Woods, and to make my own boat.
In slightly more detail:

Objectives:
A) Live-able on week-scale.
B) Trailer-able
C) Unlikely to take on water as a result of grounding @ hull speed
D) Unlikely to be rendered inoperable as a result of grounding damage
E) Draft / hull allows close approach / dry access to shallow grade shoreline (15 deg)
F) Meets Transport Canada requirements, minimum rating "D".

Of course there is no intention to strike bottom at full speed; simply it is a possibility. Additionally, there is a need to figure out some ways to promote less than instant deceleration in such a case to decrease the chances/magnitude of injury onboard.

Other than steel, I don't see any readily available material besides aluminum which would reasonably withstand frequent contact with rocks. I don't intend to contemplate or discuss the matter of general hull materials any further.

That sums up the intent and reasoning. My only previous boat building experience was a plywood and fiberglass trimaran that could fit in an elevator/Toyota Auris hatchback. That boat took me three years to build, if only about 100 hours. I realize that without results talk is just talk and hope that by making myself somewhat accountable publicly here any results will occur sooner and to a higher standard than they might otherwise. Thank you to anyone who actually read this!

The next post will move on to more specific considerations and a summary of work done previously.

 

I would say that the millions of successful small aluminum boats in the market prove that theory wrong. The smallest aluminum boat I own is a 10 foot dinghy and it is lighter than comparable fiberglass or rotomolded ones.

 

Part II - Design & Construction Considerations

Okay, so aluminum sailboat, great... now what ? Being somewhat lazy when it comes to fine craftsmanship (I can't sand a surface perfectly smooth to save my life), the decision to use cnc-cut (laser or waterjet) aluminum panels for as much of everything as possible is a foregone conclusion. That means..

1) Sheet aluminum construction panels will have one degree of curvature (may also be twisted slightly), if not flat, and
2) Any curved panels must be developable surfaces (can be unrolled/flattened without creasing/buckling).
3) No complexity limitation to panel outline profiles
4) Nesting parts within actual raw material minimizing waste saves money.
5) Tons of upfront design work to get it all to fit together.
6) Tons of welding (and hopefully not grinding) later to get it actually together. -TIG exterior, and hopefully just MIG stitching of internal reinforcements.
7) Accurate representations of all major hardware necessary for detail design of panels/fitment and hull stability evaluation.
8) Fixtures. Hopefully limited by interlocking alignment built into panel design.
9) Trailer - Maybe this can serve as the construction fixture as well ?
10) Trailer - likely no permanent internal ballast, maybe water-ballast tank(s).

I definitely don't want to need a set of rolls or English Wheel to do anything for this boat, it should snap/slide together like a puzzle.

Previous experience with laser cut stainless panels for a bespoke laser marking enclosure (255 pieces fitting together effortlessly, almost didn't believe it), and home barbecue projects have proven to me without any doubt that the laser cutting process is accurate and dependable; it makes the hand layout and cutting of my earlier tinkering years an absurdity requiring no future consideration.

How big ? -
Trailer-able. Towing dimension limits which do not require special permits according to provincial government websites are:
In Manitoba: 2.6 m wide, 4.15 m high, 12.5 m long, 4,540 kg
In Ontario: 3.7 m wide, 4.26 m high, 12.5 m long, 4,600 kg

Main take-away here is the width limit; shoot to fit inside 2.6m or stay out of Manitoba. The reason I am including it is simply that my family is there and it is a likely destination for building/storage/repair.

Browsing for information relating sailboat beam to length ratios, 3.25 seems generally accepted as reasonable.

So, potentially OAL = 2.6m x 3.25m = 8.45m

Let's back that up a bit to stay under the 2.6m width limit for the basic hull and also round our numbers

OAL = 8m, final.
Beam = 8m / 3.25 = 2.5m, final.

For the height, it's important to consider trailer height, and the base of the lowered mast will probably be the highest point in trailer-mode. Limit is 4.15 and there are no low overhead bridges on the route, but let's leave some wiggle room.

Max height on trailer, level = 4.0 m, final.

For the weight/displacement, I suspect the trailer will weigh more than the boat...

Without any additional research, "How Much Does A Sailboat Weigh? https://www.lifeofsailing.com/post/how-much-does-a-sailboat-weigh" indicates that an 8m (26') sailboat has a dry weight somewhere between 5000 - 7000 lbs.. so... about 6000 lbs then.

Assumed dry-weight = 2700 kg (~600o lbs )

Definitely want to minimize the dry weight. We will re-evaluate later when there is an idea of basic aluminum hull-shell mass.

Easy. Almost done !

 

Great ! Thank you.

 

Re trailerable and shallow (how shallow?) draft, are you thinking of / considering bilge keels and lifting keel designs as well to achieve this?

This 28' sailboat design by Sam Devlin is intended for plywood construction, hence no compound curvature, and a design like this could be adapted fairly easily for aluminium construction.
However her draft is 4'6", which might be too much?
Onyx 28 https://devlinboat.com/onyx-28/

 

Part III - Basic Layout & Design Tools

Since I don't know much about designing boats, and am not particularly interested in combing through every contemporary or historical work on the subject, my plan is to proceed blindly with good intentions, learning what I can on the way.

Work up until now has focused on two areas -
1) Conceptual layouts for the hull.
2) Developing crude hydrostatic analysis tools to try to evaluate whatever hull shapes look interesting.

No sketches, straight to Grasshopper/Rhino...

Previously I had made a calculation tool for evaluating my little trimaran design. Basically all it did was slice the hulls with a water plane, calculate their displacements and the centroids, and reported back the shift to the center of gravity required to achieve that posture.
The objective was just to make sure I could sit near the back of the boat without it going under.. (and it didn't!)

Anyways, more recently when I started thinking about this new boat, the interest in the subject returned. In particular, looking on the internet at copies of ISO 12217, the tedious 'hand-calculation' methods look like a major pita.. I think it will be easier overall to develop the calculations from scratch than deal with any hand-methods or cryptic spreadsheet tabulations.

Basically the goal of the calculations from what I can tell is to make sure the vessel will not sink or readily capsize, and if it did capsize what conditions are be necessary to effect that. Strength and stiffness considerations are obvious.

Not saying I am absolutely sure I can achieve all that, simply that is what I am going to try to do... for fun basically.

So started playing with Grasshopper python scripts for evaluating a crudely modeled 'panel' hull:


The hull uses three developable panels per side, just a guess at how many would be enough to have a reasonably simple hull (Adding one more panel to the set adds something in the ballpark of 32 m of watertight tig welding to the outside alone! Mustn't get carried away..)

The first thing I noticed is that the scale difference between my 2m long trimaran and this new concept made my crappy older buoyancy calculator lock up solid. Now, Rhino has a really neat "Hydrostatics" command which does indeed go a great job of calculating displacement etc. given a model and a waterline elevation.. but it takes awhile to do the calculations, and I don't think it is available as a python scripting component. I decided to try something else, just replace the hull with a point cloud representation.


Hull represented by about 94K points, spaced ~ 5cm grid,

To make the points, the hull was sliced into station sections every 5cm, those were sliced laterally into lines 5cm apart, and then those lines divided to have a maximum spacing of 5cm.
This can be quickly applied to any volume.. in terms of hydrostatic calculations, the 5 cm spacing amounts to a displacement resolution of about 125 g (approximate because the spacing is not strictly maintained, it is only a maximum limit).
The actual mass per point was calculated by dividing the high-resolution volume by the number of points contained within it.



In terms of hydrostatic static calculations, the process is very simple -

Displacement = (Number of points below water plane) x (mass per point)
Center of Buoyancy, CB = The average coordinates of the displacement point set.

Again, the accuracy of this approximation is probably within a few kilos, and ~10cm^3.

The next step was to come up with some sort of way to evaluate the CB at various displacements and postures.

At first a direct 'real-time' calculation was attempted, however my programming skills were not up to the task and it took too long to make it pleasant to play with.
Next, better idea was to crunch the data for every orientation and displacement combination ahead of time, then simply simulate the results reading from a file.
With poor programming maintained, it took 1.5h for my python script to create a small JSON file with the necessary data. The resulting data was then able to be visualized without any lag.


Simulation of result data using XY slider to control pitch and roll. Displacement set to 3200kg in the image above.
Displacement was calculated in the range of 700-3700 kg in steps of 500kg.
Roll and pitch were calculated for each displacement, +/- 20 deg for pitch, 0-90 deg for roll in increments of 5 deg.
I'm pretty sure for non-zero combinations of pitch and roll, yaw is effectively represented when resolving back to a fixed CG, or is otherwise arbitrary.
In order to create a smooth simulation, the data was set up as 3d surfaces, which offer limitless resolution with the assumption of linear behaviour interpolating between them.
I hope that this can be used to create stability plots, as well as determine effects of CG location/shifts.

I cannot overstate how much I appreciate Rhino and Grasshopper in terms of providing a relatively simple but powerful calculation/simulation platform.

Next up, starting to model some real hull concepts and determining a general layout for lift devices!

 

Yes, shallow like about 30-40cm with everything up... haven't got that far yet but leaning towards twin boards and rudders, retractable.
When dropped, the boards and rudders will need to be able to take a hard hit and come up quick, no solid locks etc.
Will get into all that soon! That Onyx 28 looks great, but yeah, a bit too much under the waterline for this theater...
Thank you for your comment, link, and interest!

 

Boat plans are one of the relatively cheap items in boat construction. You should ask yourself if you want a boat or to learn naval architecture. Simulations are great to make presentations, but are rarely accurate unless you have the experience to discard wrong results.

 

You lost me totally there with post #6.

But I can understand what you are aiming for when you say in post #7 "Yes, shallow like about 30-40cm with everything up... haven't got that far yet but leaning towards twin boards and rudders, retractable."
Now we are getting somewhere.
Forget all those fancy simulations - find a boat similar to what your Statement of Requirements is looking for, and use this as a reference to start with.
Basically you want a shoal draft, multi chine aluminium boat with a lifting centreboard (not a daggerboard) and a lifting rudder.
I can't see the point of having twin centreboards - one will do the job quite well. Twin lifting rudders could be useful though if you have a very wide transom.
I am thinking that you probably want to build a smaller version of an Ovni - have a look at their site here, and see what you think.
OVNI Range | Alubat Shipyard https://www.alubat.com/ovni-range

Here is a photo of the Ovni 395 sitting happily on the sand - they mention that her draft in this condition is 60 cm, so if this hull form is scaled down proportionately from almost 40' to 28', you should be able to achieve a draft in the region of 40 cm.

 

A boat that fits pretty much into your SOR. It is Not a sexy boat. It is a marvelously practical boat that thrives in shallow waters, sails well, easy build, and several other attributes that caused me to appreciate its practicality. I charted one of these many moons ago. It was to mess around in The Bay of Florida. That is a very large and very shallow body of water that few conventional sailboats dare to go.. It is Bolgers' Black Skimmer. Don't laugh or demean that boat until you have tried one.

 

I love Bolger designs - I think that one either loves or hates them, there is not much middle ground!
The Black Skimmer looks neat - the design could no doubt be adapted fairly easily for aluminium.
Bolger Black Skimmer http://www.ace.net.au/schooner/bskimmer.htm

Ross Lillistone Wooden Boats: April 2011 https://rosslillistonewoodenboat.blogspot.com/2011/04/

 

I read your SOR and what I see is that you need a Al chined 25 Jollenkreuzer. Such a thing does not exist, since this boats were round chine and steel, but you could modify one. Here is an old article with some drawings (in german but google translate is your friend): FREUNDESKREIS KLASSISCHE YACHTEN https://www.fky.org/yacht_des_tages/blitz.htm

 

I think a major design goal should be to get the most effective lateral resistance for the least sailing draft.

For this reason, I suggest you go with a vertical side with just one chine. This way, as the boat heels, the Lee chine digs in and helps keep the boat from sliding sideways.

You would also be able to use the windward center board to help, which would reduce your sailing draft considerably.

If you want flared sides, I suggest you consider using chine-runners, which could be merely the bottom plating extending past the side plating, along the middle third of the hull, a decimeter or so.

Now, the bottom could be kept flat side to side and maybe even nearly dead flat lengthwise along the middle third of the hull. This would make installing the twin center boards so much easier. Another advantage of going this way would be that, if the boat strikes the bottom while sailing, merely letting the sails luff will free her.

The bottom could be made much thicker than the sides, and could do some ballast duty as well.

Just a few ideas.

 

In the 70's , some good sailboats in alu were produced in your targeted dimension :
** Jean-Marie Finot designed La Galiote for the sailing school Les Glenans, I sailed it as skipper and it was an excellent and performing sailboat for those days.
In aluminium 5086 , multi chines with developable panels,
Loa 8,5 m , Boa 2,9 m , draft 1,4m (fixed keel), weight 2,7 t, ballast 1,0 t
Galiote http://www.finot.com/bateaux/ancienbt/galiote/galiote.htm
** Another good design by Philippe Briand in 1974 was the Ovni 25 :
Loa 7,60 m , Boa 2,65 m. , draft 1,5 m (fixed keel) weight 1,7 t. ballast 0,7 t
https://sailboatdata.com/sailboat/ovni-25

** For welding reasons, I recommend you to use alu thickness of at least 4 mm.

 

.... and here is the linesplan of the Ovni 25, can be helpful :
http://ovniclub.free.fr/notices-techniques/ovni-25.pdf