Big Aluminum Boats in Rough Seas - The Hull Truth

hossdave12 · 02-05-2004, 01:57 AM

Wondering if there's anyone who has experience with a bigger aluminum boat in an area with fairly rough seas that can say how they handle compared to fiberglass boats of the same size or just generally.

The 28' boat in the link below is the type of aluminum I'm interested in hearing about.

http://www.norscomarine.com/boats.html

What are the pros and cons?

Thanks.

trouty · 02-05-2004, 03:37 AM

It all depends is the answer.

There is a point at which alloy actually handles better - and it's a function of weight/design.

So it depends on the design construction of the alloy boat.

At somewhere around the 30 ft mark (28 - 32) usually alloy boats actually start to get heavier than their fibreglass counterparts

Now thats a generalisation - if the frame spacings are far apart (i.e it's designed as a lightweight hull) and of light material and the bottom and siode sheets are likewise thin to save weight then the size at which an alloy boat becomes heavoer than glass increases beyond that 30 ft mark..

If however it's a comercial designed hull with narrow frame spaceings and very heavy (thick) plate designed to withstand the rigiurs of daily berthing against timber jettys to unload catch - and daily refueling etc etc then - it might be a 28 fter that is heavier than an equivalent glass boat.

ou can design a alloy boat to be light fast and fuel efficient - or heavey strong and slower to last forever...

I've seen an alloy charter boat with frame spacings meeting Lloyds of london design specs at 4ft (1200mm) and a similar sized hull build for the Lobster industry with spacings of 400mm (16 inches)...

Irt all depends on what the owner wants out of the vessel how heavily it is built - and it's that weight which makes them oftentimes heavier than an equivalent glass boat...

I've been in a 60 fter in 48 nots winds (after a 67 knots storm the night before) in combined real wave & swell heights of 20 ft...it rode fine - I'd sooner not have been there but sometimes you don't get that choice.

There was a 6o foot glass boat working a paralelle line of lobster pots not more than 200 yards away from us in those conditions...

I only saw him twice during the 4 hours we were there...we just happened to top a wave both at the same time on those two occasions, otherwise all i got was the occasional glimpse of his aerials on top of his fly bridge...

Talking to him on the radio - he wasn't getting it any easier than we were in those conditions - I'd say both boats were making the best of it and in all likelihood there wasn't probably a lot to pick between them,

Then again - both boats were purpose built and designed by local naval architects for owners who've been lobster fishing the wild west coast downunder for two or 3 generations.

You'd expect that they could cope with those conditions, afterall thats what they are designed and built for.

Take a look at this photo of a 48 ft alloy lobsterboat under construction - the flash indoors highlights the oil canning effect allowing you to see the welds of every stringer and frame.

How far apart do you think the frames are?...

450mm? - say 16 inches apart?

With stringers every what?, 8 inches apart?

Do you think those bpoats you showed in the link - have an equivalent spacing for their legth ot 28 feet?

What are the specifications for them?..frames at 3 ft or 4 feet?...stringers at 2 feet?

If the alloy boat isn't heavily built - then - it won't weigh as much as an equivelent length glass boat until it is substantially longer..

If you buy / build a light alloy boat it will be fast & fuel efficient, but likely slam like a dunny door in a sou'wester, comming off waves.

If however - you build it heavey - then it will outride a equivalend length glass boat - all other peramaters being equal.

You get what you pay for in boats - any fool can build a light boat cheap to go fast - it takes real skill to build a good one!

Same boat finnished, see any oil canning now?

In normal day light you won't be able to see the frame spacing widths and stringer spacings due to oil canning - ergo, no one can tel from the photo's you posted whether these are lightweight go fast boats which will slam or heavey commercial hulls which will out perform an equivalent length design glass hull.

Only reflected light from a flash indoors will pick this detail up - so to answer your own question - you really need to see some line drawings, plans & materials specifications to properly answer your question.

Least thats how I see it...they look the part in the phhoto's - but IMHO there are many pretenders to the throne these days - so far the Aussie alloy boat builders 'own' that crown and I doubt we'll see it head stateside anytime soon is my best bet.

Cheers!

02-05-2004, 05:39 AM

I happen to own an aluminum boat, a 32 foot Marinette, and I can tell you--without reservations--that it's the only way to go. Not only are aluminum boats ALWAYS lighter than an equivalent length fiberglass boat--and this includes really large boats--the hull can take a pounding from "objects" without shattering--they simply dent. (Pound the dent out, carefully, and you're all set.) Note that most of the really big, really expensive yachts are made from aluminum.

Two cautions: The boat needs to be made from the correct, marine grade, aluminum. Some post WW II boats were built with war surplus aluminum--the wrong type--and corroded badly, giving alloy boats a bad name. Marinettes are built from 5086 – H116 aluminum, which has survived 7 years of testing, immersed in warm salt water, with minimal loss.

That said, galvanic corrosion can destroy the best aluminum alloy if it's not kept under strict control. I have a galvanic meter on the boat that I test it with, and the "zincs" (magnesium, mainly) get changed regularly.

On the ride in heavy seas, here the Marinette is not so great. It's an old hull design with little deadrise. The plan was to build a boat that can navigate shallow water, which it does well--only a 2 foor draft! That, combined with its fairly light weight (10,500lbs) and 12 foot beam has created a "skateboard" like boat that bobs and weaves in heavy following seas. (It takes waves head on well, however.)

On the plus side it performs well, getting on plane with ease, and getting good gas mileage from its twin, 5.9 liter engines. We love it!

Fastjeff

PS: Here's what these boats look like:

http://fastjeff.tripod.com/

trouty · 02-05-2004, 07:34 AM

Lets look at their 22ft hull...

I count about 10 or 11 frames.

Thats 24 inches frame spacings on a 22ft hull or thereabouts...

Yet a commercial 48 footer has 16 or 18 inch frame spacings?

Wouldn't you expect a 48 fter being 4 times bigger than the 22 footer would have say 4 x 18inches frame spacings, if it were "scaled up" from the 22 ft hull? (i.e at least 3 ft apart?)

Yep - 3 ft frame spacings it would be for that commercial 48 fter to have the same "arrangement" as the 22 ft displayed in comparative scale?. 48ft length / 3ft frame spacings = 16 frames we'd be expecting to see in that hull to be the same type build as the 22 fter...

yet - I reckon theres roughly at least 24 frames in that 48 fter at a rough count using the photo's as a guide..so at a minimum the frame spacings are 2 feet apart...

Basically - what thats saying is the lobster boat is at least twice as heavily built in terms of it's internal structure (and likely that again in it's plating) as the 22 ft vessel from that range your looking at.

Or another way to view it is - theres only half as any frames or it's twice as lightly built as a commercial boat.

That DOESN'T make it automatically a 'bad boat'.
It might meet al it's intended design charateristics...and indeed be a very good boat overall..

What it does mean however - is that, in terms of it's comparative ride based on displacement weight compared to a glass hull - it's going to struggle to compete becuase it's too light.

Yes the upside is better top end and better fuel consumption figures - savings in cost of powering the vessel etc...

Your question related to the comparative ride characteristics and My reply is directed that way - to try and highlight the potential differences in construction between alloy boats and how that factor might translate to ride characteristcs, when comparing alloy to glass for weight!

Cheers!