Help Explaining Why Excess Speed Breaks our Boats

Don't throw the babies out with the bathwater. Gather your data, plot it on a graph. You'll see the common thread, THEN act.

 

Hello all,

Thanks very much for your comments, questions and suggestions. They have me thinking along a couple of different lines for both the long-term and short-term.

Take care.

Marty

 

Overview - Speed and Fatigue

To Marty Gingras:

" Read 'n Heed ! "
- it's all about...." Speed 'n Fatigue "
(To be posted soon - where ever your " boat jockeys " have coffee.)

You have just received enough info. to get you, half way through
Struc. Engineering 101.
From people with a combined experience of a 1000 yrs or more....
( Good job, guys, this is one of the best threads I've seen here.)

One of the most frustrating aspects of talking " boat " to clients:
Is; that most people live in a " 2 plus 2 = 4 world ".
i.e.
Twice as much carpet - costs twice as much.
Logical to presume...yes ?
Unless your brother - law, owns the store, that is.....
Whereas:
With boats, the forces involved, go up by at least the sq. - and
often by the cube root !
(3 x 3 = 9 and/or 3 x 3 x 3 = 27)

With costs moving up at the same rate of multiplication.
At least.......
...'Tis why a 36' vessel will cost, at least, four to nine times as
much as an 18' boat. Unfortunatly for the people, whom you toil for:
- Maintanance costs, also tend escalate, in a similer manner.......

Most lay people and sad to say, in my experience - many mariners
- have a hard time getting their heads around these Laws of
Physics stated.
Which includs; the guys that you work for.
(Who most likely went for the lowest bid.)
- And also, the guys who are busting up your boats.

You can tell them all: - I said so.
In fact, I will suggest, you pass on this entire Thread on to
both of these groups.

......................

To add my two bits...

In my opinion; welded alum. is over rated, for small vessel fabrication.
In the real world - it often fails to live up to expectations.

I have seen an entire fleet of identical alum. boats, all being
taken out of service, for extensive, major repairs.
(Including the failures, that you describe.)
- All at the same time.
- Note: - I repeat: All at the same time.
These boats; all designed and engineered by a well established
NA firm ?!

As ted655 says,
"......welds may be suspect."
I will go futher; ALL the welds are suspect. As the only weld you
can realy trust - is one that's been X - Rayed.

water addict; however, goes to the heart of the matter.
When he states,
"......very poor material in fatigue."

Boeing considers alum. is maxed out in only 20 yrs.
(So when I book a flight I'm more concerned with the age of the
aircraft - than how cheap a flight I can get.)
Also note:
Planes are subjected to far fewer, point loadings than boats are.
How long does an aircraft last if it hits the water hard ?
......Just once, in a crash landing ?

More to the point, how many planes are welded together ?
Rivets cause less local stressing - and are far easier to
trouble shoot.

Also, you will see much less electroiysis damage to commercial
aircraft....If any. As the operaters, wisely, keep them as far from
sea water as possible.

Given the above:
20 yrs is way, too optimistic to consider. In my book - for a small
craft's life expectancy. Harshly treated, that has been
constructed in alum.

As the fleet, I mentioned, were showing " catastrophic failures "
- within only a season or two.
So sight un - seen, I'd suggest that your boats will be lucky to
go - even five years with out, serious maintance concerns.

Almost any other material, commonly used in boat building, has
better resistance to fatigue.
Including: Steel, F. Glass and " Old School "...Epoxy / Wood.

I do not envy the tasks that stretch before you.

' Regards.

 

I worked many years for a little sailing club that bought outboards that were way to big. They always said they got more power for safety. I'd say just the opposite -- less power means more safety! Who would get a Corvette for their kid and claim it's safer? Get engines big enough to get the boat on plane, plus a little more for heavy loads, and make sure you have high thrust props (low pitch) for load carrying ability. Unless they are traveling 10+ miles a day, anything over 20 mph is too much. 40 is WAY too much.


~ ALowell

 

"Boeing considers alum. is maxed out in only 20 yrs.
(So when I book a flight I'm more concerned with the age of the
aircraft - than how cheap a flight I can get.)"

There are many many THOUSANDS of commercial aircraft that are far older than 20! years old in fine condition.

The USAF is using B-52's built in the 1950's today , and there is NO plan to replace them before 2040.

Having flown DC-3's built long before I was , I'm sure 20 years is too short a useful commercial life.

How long the new Plastic aircraft will go IS up to consideration.

FF

 

It's not the age, it's the number of loading cycles and load magnitude that counts - as you probably know. B-52's largely sit around doing nothing. A commercial plane that is in almost constant use, gets loaded and unloaded a lot more in a given time frame. I think it was Air Alaska plane operating in Hawaii doing shuttles betweens the islands that had part of the fuselage rip off in flight. Attributed to metal fatigue as the plane did many more than typical take-off / landings because the flight times were so short.

In general AL is not great in fatigue. It can be worked around if careful. Aye there's the rub. Careful.

 

Military aircrews are required to train on the aircraft to maintain proficiency, so B52's fly more than you think.

Military aircraft can undergo an overhaul process, actual more of a remanufacturing, that rerates the airframe to "zero hours" certification, meaning it's considered to be like new. Complete disassembly and exposure of bare metal, magnafluxing, etc., etc. Mega-costly, close to the original cost of the aircraft, so only governments can afford it. There's a somewhat less costly thorough overhaul procedure for commercial aircraft, still pretty expensive, but less than the cost of replacement. Forget it for boats, unless you're Larry Ellison or Paul Allen...

 

- " Foreverum "

Sorry FAST FRED,

the new metal " Foreverum " that the military received from the Alians
has not yet been released to the generel public yet...
Mostly due to the fact that nobody has found a way to drill holes in it.
Or find a saw that can cut into it.......

It is highly unlikely that all the pieces of alum. in an old aircraft,
are original - to the original airframe.

A great advantage to rivets, is that it is Common Practice to replace
suspect panels and/or rivets - with planes.
This is partly, why they " seem " to live so long.

As charmac says,
" ...aircraft...undergo an overhaul process...more of a remanufacturing..."

For very good reasons. As stated by water addict,
"....it's the number of loading cycles..."

Because, when a small US airline was not dilligent in inspecting their alum.
aircraft skin - of one of their planes...
As also stated by water addict,
"....had part of the fuselage ripped off....attributed to metal fatigue..."
A woman lost her life in the incident.

To water addict:
in these forums we often have to speak in wide generalities.
So I agree - loading cycles are key. To predict in years or months is hard.

But exposure TIME - has a significant and very unpredicable contribution.
Not incidental, that the aircraft you refer to, also operated in a
maritine local.

Well I remember, the sad sight of a very wealthy man, staring at the
pin holes spreading across the body of his beutiful Aston Martin G.T.
" Handmade. in aluminium..." he was heard to mutter, almost
drowned out, by the crash of the surf by his sea side cottage.

In summation:
When you combine - salt - water - unknown welding skills - with an
unpredicable, amount of loading cycles - and impact shocks....!

What value would you place on these boats - when Marty is told
to sell them ?

 

Has anyone considered that these are BOATS we're talking about here? They're not multi-million dollar spaceframes, nor are they built light enough to fly. Aluminum boats have been used for hard-working utility craft for decades because they are cheap, light, and stronger than comparable materials. Everything is a compromise and as such, we should remember that FRP and wood have their drawbacks as well.

For example, mainers Down East swear by aluminum boats for bouncing on island rocks during loading because alum. bends but doesn't break. Can you say that for FRP? It depends on the application what material should be used. Comparing utility punts to Boeing 707's is like comparing your wheelbarrow to an Enzo Ferrari.

Let's not get carried away with the academics!

~ ALowell

 

Correct, we did digress a bit with the aircraft discussion. I would say that does apply to aluminum high speed commercial vessels, like hydrofoils and fast ferries.

I too have seen very old aluminum runabouts still operating far beyond 20 years. It comes down to conservative design, good construction and maintenance ..... and sensible operation.

 

One thing they do Down East is layer the underwater portion of the hull with 4-5 layers of fiberglass for reinforcement against rocks. This is easy to do and creates a very long-lived work boat.

If you are going to buy new alum boats for institutional use, I would recommend this approach - especially for riveted boats - but only if weight is less of an issue.

~ ALowell

 

I don't understand. You say that above and that below. They don't seem to be compatible.

"For example, mainers Down East swear by aluminum boats for bouncing on island rocks during loading because alum. bends but doesn't break. Can you say that for FRP?"

 

the big boats in maine are jc boats and bruno stillman and one or two more I cant think of right now ,,,all glassers

 

The question is incomplete...

How long are the boats, this will have an effect as to wheather of not they can span shorter wavelengths.

What is the deadrise? As pointed out about speed compounding the load / shock issues, the greater the deadrise, the lesser the 'impact'

I assume at the speeds you are suggesting that they are planning hul forms?

Alot of it has to do with build quality and choice of materials, as pointed out earlier, are the boats operated inside their design parameters? We build alot of high speed boats, alot of them into the commercial sector, and capable of around 45 to 65 mph. Payload of around 12 to 18 passengers and at rest displacement of 5000 and 8000lbs (depending on the boat, heavy ship), and an expected life of 5 years (another 5 if the maintenance schedule includes something similar to a 'zero hour'. But higher maintenance is required during this period). Further more, we build primarily out of Aluminium.

As pointed out here, alot of jockeys simply don't care about someone elses gear. That is why the boats must be built and maintained with the 'lowest common denominator' in mind, or in other words for the biggest ******** on staff. Believe me, if it can't be wrecked, they'll find a way, so this must be remembered.

You say your boats are breaking, can the maintenance program extend to gradual replacement of the fleet? What parts are failing? Alloy is a hasstle to reweld once it's gone into service, real care must be taken to clean the area well before hand, even then in some cases it can become a 'best effort' attempt. Most steel workers I've seen atempt a repair have botched the job, making the owner believe they have the wrong material for their boats.

Sorry, but you guys are starting to sound like the local cops, more speed bigger mess, (true-ish), but speed kills (not likely). For speed to kill you must first have an accident (the old addage... it wasn't the fall that killed him, just the sudden stop at the bottom). You need to identify what is 'killing' the boat. If speed is a requirement of the job, then you need to soften the impact on the boat in another manner (deaper deadrise, narrower boat, less weight in the bow, etc..)

My 2 cents worth...

 

You are right, however these are not the ones they use for beaching on islands. When they do go ashore, they use alum boats, often towed behind the glass lobster boats for just this occasion.

FRP reinforcement of alum boats gives you the advantages of both materials. If you used FRP alone, it would crack and sink. If you use alum alone you'd have a dinged up boat that would add running drag.

An acquaintance of mine who makes a business of beaching on rocks for puffin tours does exactly what I'm describing. He has spent decades doing this and he knows what works and what doesn't.

~ ALowell