The Offshore Voyaging Reference Site

Impact Resistance—How Hull Materials Respond to Impacts

yacht crash on the rocks

I have a confession to make: I’ve run boats aground on several occasions. Granted, it’s always been at dead-idle speed when poking up shallow rivers. And the damage, thankfully, has so far been limited to a few nicked propeller blades and shredded rubber hubs. But, despite all precautions, it happens. If we were to rank groundings and collisions on the scale of skippers’ worst fears, I suspect they’d fall somewhere between an engine room fire and the pirates’ scene from Captain Ron.

Hitting something in a well-built boat is usually survivable, but is still a scary and potentially life-threatening situation. In this chapter, we will take a look at how some common hull materials respond to impacts.


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More Articles From Online Book: How To Buy a Cruising Boat:

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  2. Is It a Need or a Want?
  3. Buying a Boat—A Different Way To Think About Price
  4. Buying a Cruising Boat—Five Tips for The Half-Assed Option
  5. Are Refits Worth It?
  6. Buying a Boat—Never Say Never
  7. Selecting The Right Hull Form
  8. Five Ways That Bad Boats Happen
  9. How Weight Affects Boat Performance and Motion Comfort
  10. Easily Driven Boats Are Better
  11. 12 Tips To Avoid Ruining Our Easily Driven Sailboat
  12. Learn From The Designers
  13. You May Need a Bigger Boat Than You Think
  14. Sail Area: Overlap, Multihulls, And Racing Rules
  15. 8 Tips For a Great Cruising Boat Interior Arrangement
  16. Of Cockpits, Wheelhouses And Engine Rooms
  17. Offshore Sailboat Keel Types
  18. Cockpits—Part 1, Safe and Seamanlike
  19. Cockpits—Part 2, Visibility and Ergonomics
  20. Offshore Sailboat Winches, Selection and Positioning
  21. Choosing a Cruising Boat—Shelter
  22. Choosing A Cruising Boat—Shade and Ventilation
  23. Pitfalls to Avoid When Buying a New Voyaging Boat
  24. Cyclical Loading: Why Offshore Sailing Is So Hard On A Boat
  25. Cycle Loading—8 Tips for Boat and Gear Purchases
  26. Characteristics of Boat Building Materials
  27. Impact Resistance—How Hull Materials Respond to Impacts
  28. Impact Resistance—Two Collision Scenarios
  29. Hull Materials, Which Is Best?
  30. The Five Things We Need to Check When Buying a Boat
  31. Six Warnings About Buying Fibreglass Boats
  32. Buying a Fibreglass Boat—Hiring a Surveyor and Managing the Survey
  33. What We Need to Know About Moisture Meters and Wet Fibreglass Laminate
  34. US$30,000 Starter Cruiser—Part 1, How We Shopped For Our First Cruising Sailboat
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Wally Moran

I thought we were talking about cruising boats here, so your comment: “It is for this reason that aluminum is the material of choice in the uncharted, rock-strewn lakes of Northern Ontario” leaves me a bit bewildered.
You see, I happen to come from Northern Ontario – and I can assure you, the cruising boats I see in the 30,000 Islands, Georgian Bay or in the North Channel of Lake Huron, are just about 100% fiberglass. Just like the cruising boats on the Hudson River and all the way down the US east coast in fact.
Now if you’re talking small boats, yes, there are lots of aluminum boats – 12 – 20 foot recreational fishing boats, and they are everywhere. That’s because about 90% plus of the boats made for this market are made out of aluminum due to the economies of scale and the fact they can be built cheaply, like the one in your photo – not because they are tougher. I tow a 12 foot aluminum as a dinghy, because of its durability and because I cruise some pretty remote places.
France, which has NO rock strewn lakes with cruising boats on it, is known for aluminum cruising boats however. For whatever reason, Europeans and especially the French, have accepted aluminum as a material for cruising boats.

Wally Moran

No edit function, so I’ll add to my post here – the problem with aluminum is that it is very costly, especially in comparison to fiberglass. In part, that’s due to the availability of fg boats – there’s simply an awful lot of them.
Additionally, while accidents do happen, the likelihood of being holed is fairly low – and can certainly be made less possible with proper training and care while cruising. Does that nullify the advantages of aluminum? No, of course not, but essentially, the availability of aluminum cruising boats along with their cost, puts them out of consideration for most voyagers.

Wally Moran

I grew up in Sudbury, worked as far north as Wawa region – so I’m familiar with the areas you’re talking about. Generally, locals are familiar enough with where they boat to avoid a lot of problems even on the unmarked lakes. A lot of the damage one sees on the bottom of these aluminum boats is from beaching them, or dragging them ashore across a rock bottom. Which, of course, makes aluminum a good choice.

Hans

Hi Matt,
as a steelboat owner I’m well aware that she has certain disadvantages when compared to the same boat in aluminium, the main and only worth mentioning for me being the weight issue. But I always thought I had an advantage when it comes to impact resistance, and I’m a bit shocked about this statement of yours: “Mild steel is somewhat more fracture-prone in impacts than marine aluminum, …” Isn’t it the other way round? Steel has a considerably wider range of flexibility than aluminium, so that it can absorb more energy by deforming itself before being fractured. I just assured myself that this is still the case at http://www.kastenmarine.com/alumVSsteel.htm

Hans

which of course I don’t. Thank you for the clarification. Otherwise it would have robbed me of my main argument in favour of steel. Its hard enough to drag around all that weight of a steel hull only 33ft. long, but being without argument is harder I suppose.
But tell you the truth: even the weight doesn’t bother me much. And the rust issue is almost non existing, provided there is good construction, proper insulation, a good paint system and some vigilance involved. It surely is as you say: Every material has its strenghts and weaknesses and , John, I didn’t accuse Matt of putting one material above the other. Thanks for sharing your knowledge.

John Harries

Hi Hans and Wally,

I’m sure Matt will answer your questions and concerns, but I just wanted to step in here and point out that Matt has never said that one material is better than another.

In fact, in his last post in this Online Book his base hypothesis was that all the common boatbuilding materials have advantages and disadvantages and that any attempt to pick a “best” material is futile.

Matt is simply sharing his engineering training to explain what the trade offs of each material are.

Trond

Hi this is interesting, i hava a quistion. What is the best material for a cruising boat if i want too go realy north and ice can bee suspekted (alminium?)
Trond

Erik Snel

Matt, of abrasion is a concern for using composites in ice-infested area’s, could this not be solved by using an outer layer of kevlar in the composite?

Regards,

Erik

John Harries

Hi Trond,

I can jump in here. Having spent years in the north, much of it around ice, my thinking would be that aluminium (as long as only the bottom is painted) wins over steel for a high latitude sailboat that might push a bit of loose ice around but won’t be ice breaking, due to the fact that it does not corrode when the paint is abraded and that maintenance of an aluminium boat is much cheaper and easier than steel.

(It is interesting to note that both Skip Novak and Hamish Laird went with aluminium for their later boats after years of maintaining the first Pelagic that is built of steel.)

If you are going to break ice, then steel is the winner due to the abrasion resistance (as Matt points out), but very few yachts (none?) actually break ice, if for no other reason than that activity takes tremendous weight and size.

Erik de Jong

My personal preference for operating in ice infested waters is steel. If you were to build a vessel officially ice-classed and certified, aluminium would not even be allowed, and in my opinion for a good reason.
Both Mild Steel and 5000 series Alu will get bridle when exposed to sub zero temperatures, but for steel, there are special alloys available to make it “sub zero proof”, something that is much harder to achieve with aluminium.

Opposite to what John mentioned, I do not feel that steel is more expensive to maintain IF the proper paint was applied when the boat was built. There are special paints for steel boats in ice waters, and is generally sold under the name “ice-breaker paint”. I have sailed on Steel boats in icy waters since 1989, and own my own, specially designed and built, steel “polar boat” since 2008.

I personally would not consider any other boats but steel for serious cruising in the (Ant-)Arctic, especially if you consider spending some off-seasons there as well. Although this opinion is contradicted by a lot of highly respected sailors, including the people mentioned by John, and John himself.

John Harries

Hi Erik,

All good points. Probably what I should have added to my comment is that its a close run thing between steel and aluminium. For me, it would be aluminium, for you steel.

I do stick by my assertion that an unpainted aluminium boat is the easiest and cheapest to take care of. Painting metal boats is very labour intensive, as I know all too well having lived with a painted aluminium boat for 22 years. And even with the best paint system in the world, you will be repainting every ten years or so.

I would also question the brittle from the cold assertion in that the water will be around zero until it turns to ice, and as far as I know aluminium does not experience any substantial brittleness at that temp. Matt?

John Harries

Hi All,

For those that are thinking about the steel or aluminium decision, you may wish to read Colin’s excellent Online Book Why Not Steel, which clearly details what it takes to properly paint a steel boat.

For me at least, its a daunting prospect, particularly when you take into account that a paint failure on an aluminium or fiberglass boat is a cosmetic issue, on a steel boat it is, at least potentially, a structural issue—the stakes are high when painting steel.

Trond

Hi John, and thanks too all of the answers. I hope that i not will need too break ice, but too sail in to small ice that can bee poosh away, whith no damish on the boat, is what i want.
In my mind a sailing cruiser abaut 40 – 45ft, whith engine power only 50-75 hp is no icebreaker.
Do you, or anybody else have a tips for the best boat? Rigt now am i looking at some of the france boats. And i like too sail as fast as possible whith a cruiser (the normal thing).

John Harries

Hi Trond,

If I were looking today for a boat to make the voyages we have made over the last 20 years the Boreal 47 would be at the top of my list.

richard s.

there was an article in ocean navigator magazine at least 20 years ago that recounted the experience of a typical blue-water fg sloop about halfway between bermuda and virgin gorda barreling along one night and suddenly crashing into something unseen that brought everything to a screeching grinding halt as if hitting a brick wall…the crew had barely enough time to escape with their liferaft before their main life raft (sloop) sank…they never saw any trace of whatever they hit…the skipper’s later research revealed there was a navy sub in that area at that time but never could pin down whether or not that was the object possibly cruising close to the surface…sobering article…talk about hitting something hard ! doubtful any sloop could have survived that

John Harries

Yikes Matt, do we Canadians—well, I’m almost one—really want to let on about that one. If for no other reason than it’s an admission of how badly we got took by the Brits! 🙂