DIY Boat­ing

Tarting up your tinny

Boating NZ - - Contents - BY NORMAN HOLTZHAUSEN

The rea­sons aren’t hard to find. Durable and long-last­ing, an alu­minium hull is im­mune to dents and scratches from sinkers and fish­ing gear. It’s eas­ily cleaned by a sim­ple hose-down and a scrub, is un­af­fected by UV ra­di­a­tion or any­thing the el­e­ments can throw at it.

It may not look as smart as a gleam­ing gel-coated fi­bre­glass hull, and cer­tainly can be nois­ier and less warm if not in­su­lated prop­erly, but for many own­ers the prac­ti­cal­ity wins out over ap­pear­ance.

An­other ben­e­fit of the tin­nie is that it can be cus­tomised to suit a buyer’s pref­er­ence. Un­like moulded fi­bre­glass, where changes to lay­out re­quire a whole new mould, an alu­minium boat can have el­e­ments added or changed at any point dur­ing the ini­tial man­u­fac­ture or later on. Items welded on at a later date can be just as strong as the orig­i­nal con­struc­tion.

In many re­spects an alu­minium hull is pretty in­de­struc­tible, and there are nu­mer­ous ex­am­ples of 30-year-old tin­nies still go­ing strong. But they do hide one dirty lit­tle se­cret which can turn an oth­er­wise bul­let­proof work of art into a crum­bling pile of white pow­der – cor­ro­sion. If left unchecked this will qui­etly eat away at your hull un­til it causes ma­jor dam­age. There are three types that af­flict recre­ational boats.

The most com­mon form is at­mo­spheric cor­ro­sion, which is ba­si­cally the metal re­act­ing with oxy­gen in the air. Alu­minium starts to ox­i­dise al­most as soon as it is made. That’s why a bright, shiny alu­minium sheet turns a dull grey a few months af­ter leav­ing the fac­tory. Luck­ily, this usu­ally causes few prob­lems be­cause the metal ac­tu­ally pro­tects it­self through this very process of ox­i­di­s­a­tion.

A thin layer of alu­minium ox­ide (also called alu­mina) rapidly forms on the sur­face, just a few mi­crons thick. This ox­ide layer is ex­tremely tough, is bonded tightly to the metal and doesn’t flake off. The alu­mina it­self is un­re­ac­tive and acts as a nat­u­ral bar­rier to salt, acids and chem­i­cals, and will pro­tect the metal from any fur­ther cor­ro­sion once it’s ox­i­dised.

Note: Pol­ish­ing the sur­face of your un­coated alu­minium boat is a bad idea – it will cor­rode faster if you keep buff­ing the coat­ing to keep that shiny ‘new’ look. And this nat­u­ral coat­ing is also why a 30-year-old al­loy boat may be just as solid as a brand-new boat.

But two other types of cor­ro­sion do cause ma­jor prob­lems, and these need to be guarded against. The first is crevice cor­ro­sion – where a gap or crack al­lows salt wa­ter to be­come trapped. This can oc­cur along a seam or weld, in joints, where fit­tings are in­stalled, coat­ings have been ap­plied, and un­der car­pets and deck­ing.

Crevice cor­ro­sion is dif­fer­ent from gal­vanic cor­ro­sion (see be­low) be­cause only one metal type is in­volved – the alu­minium it­self. The struc­ture trap­ping the wa­ter can be wood, plas­tic or an­other part of the same hull, but it is most com­monly seen as bub­bles form­ing un­der poorly-ap­plied paint.

In this sit­u­a­tion the ini­tial oxy­gen in the trapped wa­ter is con­sumed as alu­minium turns into alu­mina. More dam­ag­ing though is that the re­main­ing wa­ter then be­comes more acidic, and with­out the pres­ence of oxy­gen this re­acts with the metal to form alu­minium salts.

These form part of the white pow­der you see along such cracks and crevices. The best way to pre­vent this form of cor­ro­sion is to rig­or­ously wash the boat with fresh wa­ter to en­sure salt wa­ter isn’t trapped in cor­ners or crevices.

By far the big­gest cause of prob­lems is gal­vanic cor­ro­sion, which oc­curs when two dif­fer­ent met­als are in con­tact with an elec­trolyte. And salt wa­ter is a very good elec­trolyte. The met­als need not even be very dif­fer­ent – even slightly dif­fer­ent grades of alu­minium are suf­fi­cient to set up a gal­vanic re­ac­tion.

Alu­minium, re­mem­ber, is usu­ally an al­loy con­tain­ing traces of mag­ne­sium, chromium, cop­per, zinc, tin, man­ganese, iron and sil­i­con.

Gal­vanic cor­ro­sion can oc­cur any­where and can be rapid even if the other metal is very sta­ble. It is al­ways the less ‘no­ble’ metal (on the gal­vanic ta­ble, which ranks the re­ac­tive­ness of dif­fer­ent metal al­loys) that cor­rodes first.

There are doc­u­mented cases where a dropped ball sinker made of lead has sat un­seen in the bilge of an al­loy boat un­til it even­tu­ally cor­roded a per­fectly round hole right through the hull. Zinc is the only metal lower on the gal­vanic ta­ble (than alu­minium) that’s com­monly used around boats, which is why zinc is usu­ally used for an­odes on en­gine blocks and gear cases.

So what does this mean for main­te­nance or up­grad­ing your boat? The first thing is to reg­u­larly check for places where cor­ro­sion could set in. Un­for­tu­nately, a poor­lyprimed paint job on an alu­minium hull can, through crevice cor­ro­sion, ac­tu­ally cause more prob­lems than leav­ing it un­painted. If wa­ter is able creep in un­der the edge of the paint, you will see bub­bles start to form and, af­ter a dis­ap­point­ingly short pe­riod of time the paint can loosen and start to flake off.

Paint doesn’t stick very well to alu­minium ox­ide, so paint­ing an older boat in­volves sand­ing the alu­mina layer off down to fresh metal. Then, be­fore it has a chance to ox­i­dize again you need to ap­ply a high-qual­ity etch primer that is spe­cially for­mu­lated for alu­minium.

This is then fol­lowed by the ap­pro­pri­ate num­ber of sur­face coats of your cho­sen paint sys­tem. New boats gen­er­ally get an acid wash be­fore paint­ing to achieve the same ef­fect, but this is hard to do on an older boat with­out strip­ping it back to a bare hull be­fore the acid bath.

When it comes to fit­ting ac­ces­sories you ob­vi­ously want to iso­late the dif­fer­ent met­als, so they do not ac­tu­ally come into con­tact with each other. One way is us­ing a suit­able non-metal­lic wa­ter­proof layer like a piece of rub­ber or plas­tic, and most stain­less rod hold­ers come with just such an in­su­lat­ing pad for fit­ting un­der the flange. If not, a thick piece of rub­ber can be cut to the same shape as the fit­ting and lo­cated be­tween it and the hull.

The bolts fix­ing any ac­ces­sory in place are more prob­lem­atic, be­cause it is hard to get some­thing to go around the thread and un­der the washer, bolt head

By far the big­gest cause of prob­lems is gal­vanic cor­ro­sion, which oc­curs when two dif­fer­ent met­als are in con­tact with an elec­trolyte.

and nut. Never use stain­less steel screws with alu­minium – ev­ery­where the thread of the screw ‘bites’ into the alu­minium will quickly cor­rode, and within months the screw will sim­ply pull out.

The best op­tion is a bolt right through the sur­face with a match­ing stain­less washer and nut on the other side. This is also eas­ier to in­su­late from touch­ing the metal, and the solution here is a spe­cialised in­su­lat­ing sealer. There are two prod­ucts avail­able from most chan­dlers:

Du­ralac is a hard-set­ting sealer de­vel­oped by the US aero­space in­dus­try, where un­seen cor­ro­sion could have cat­a­strophic con­se­quences. A bright yel­low (and quite fluid) sealant, it is lib­er­ally ap­plied ev­ery­where that the two dis­sim­i­lar ma­te­ri­als come into con­tact, be­fore they are fas­tened tight. The yel­low comes from its ac­tive in­gre­di­ent bar­ium chro­mate, and it is best ap­plied with a brush.

When left undis­turbed it lasts for decades and is def­i­nitely the best op­tion for a per­ma­nent solution. It is also best for ar­eas which may be reg­u­larly splashed with salt wa­ter, such as on the deck or tran­som. Note the ac­tive in­gre­di­ent is a haz­ardous com­pound so read the safety in­struc­tions when ap­ply­ing.

Tef-gel is the al­ter­na­tive prod­uct, but un­like Du­ralac it does not har­den. De­rived from a Te­flon gel, it is a bet­ter solution for bolts that need to be un­done reg­u­larly for main­te­nance. For ex­am­ple, the bolts hold­ing a stain­less an­chor winch to an alu­minium deck would best be pro­tected with Tef-gel, as the winch may need to be re­moved ev­ery cou­ple of years for clean­ing and lu­bri­ca­tion.

Since it is a gel, this prod­uct can be eroded by reg­u­lar wash­ing or dunk­ing, so not suited for fit­tings on the board­ing plat­form, for ex­am­ple. It pays also to re­mem­ber that Tel­fon is an ef­fec­tive lu­bri­cant, so rig­ging bolts and struc­turally-crit­i­cal con­nec­tions should not have Tel-gel ap­plied un­less you are pre­pared to check reg­u­larly that they are not work­ing them­selves loose.

One fi­nal trick when fit­ting ac­ces­sories is to try and at­tach ob­vi­ously want to iso­late the dif­fer­ent met­als, so they don’t ac­tu­ally come into con­tact with each other.

them some­where that is not go­ing to be crit­i­cal to the in­tegrity of the hull. Some boat man­u­fac­tur­ers, like Ev­ery­man Boats, weld ad­di­tional plates to the tran­som for mount­ing trans­duc­ers, wa­ter pick­ups, trim tabs and the like.

This means that the bolts hold­ing these items are not go­ing through the hull it­self, mak­ing the ac­ces­sories eas­ier to at­tach and caus­ing fewer is­sues in the long run. If you need to in­stall a num­ber of small metal items you might con­sider fit­ting a longer strip of wood or plas­tic, with suit­able sealant on the bolts, and can then more eas­ily screw the smaller items to the wood with­out wor­ry­ing about cor­ro­sion.

A lit­tle bit of fore­thought and the proper prepa­ra­tion can make all the dif­fer­ence. It can be heart­break­ing to spend thou­sands of dol­lars and con­sid­er­able ef­fort on up­grad­ing your boat, then a few years later re­alise it is self-de­struc­t­ing be­cause you ne­glected a small but crit­i­cal pre­ven­ta­tive step.

BE­LOW Left to weather nat­u­rally, alu­minium is cov­ered by an ox­i­dised layer – it pro­vides pro­tec­tion.

ABOVE No­tice the use of a com­pound to iso­late dif­fer­ent met­als from one an­other.

OP­PO­SITE An­other vic­tim of poor sur­face prepa­ra­tion.

BE­LOW Two prod­ucts that can be used to in­hibit cor­ro­sion.

LEFT Some builders at­tach sep­a­rate brack­ets for mount­ing trans­duc­ers.ABOVE Typ­i­cal blis­ter­ing re­sult­ing from poor sur­face prepa­ra­tion; if you want to paint your ves­sel, make sure the sur­face is bare alu­minium.

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