250 outboard install

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timruger1
Posts: 5
Joined: Mon Dec 05, 2016 4:34 am

250 outboard install

#1

Post by timruger1 » Tue Oct 30, 2018 1:28 am

Did a I/O to outboard conversion on a 24 ft aluminum boat. Now its time to install the outboard

Putting a used 250 hp G2 on the offshore bracket / pod

It was suggested to me that I install a backer board between the motor and the transom and seal it with Sika Flex. I can see some wisdom in this as it will isolated the motor and creat a better surface for the mounting bolts to seal.

My local independant marine mechanic helps me out and is strongly opposed to the idea. His theory is that the material can compress and eventually lead to bolt failure.

I can say that I share his opinion but admittedly I have not noticed this installation on any other aluminum boat here in my area. So Im turning to the experts here for input.

Thanks

kmorin
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Re: 250 outboard install

#2

Post by kmorin » Tue Oct 30, 2018 11:49 am

tim, lots of fallacies in the idea that wood will isolate the engine! the bolts will provide a path to the hull of low if not very low resistance! So the wood may act to insulate some of the mount- that wood will be 'shorted' by the bolts!

Wood is a poor choice- why is the wood suggested? Why a pad? is the transom not correctly built/located/adequate to accept the engine? Mount the engine to the metal- if you add- add aluminum. Wood, unless completely soaked in epoxy, repeatedly and thoroughly will promote surface corrosion in the metal it's bolted too- so why bother?

Maybe the origin of the idea will suggest you use 'rot proof' wood as well? (Heavy sarcasm here) Then the copper impregnated wood will not rot? That would be good- right? Whoever suggested adding wood may have had a good intent but the execution will become a total disaster.

Bolt the engine to the welded aluminum transom; add a bonding strap from engine to the hull (eng. block to transom) and make sure to use weld-on cathodic protection- contact "BoatZincs.com" and get the aluminum alloy, weld on types protection- like the alloys now provided in the outboard legs' protection.

Cannot see any reason (that you've given or discussed) to ever use wood, and if I did- then move from wood to one of the marine sheet plastic materials that are inert and rigid.

Cheers,
Kevin Morin
Kenai, AK
kmorin

timruger1
Posts: 5
Joined: Mon Dec 05, 2016 4:34 am

Re: 250 outboard install

#3

Post by timruger1 » Tue Oct 30, 2018 3:07 pm

I don't recall mentioning wood

I am having difficulty sealing the mounting bolts and it was suggested to use a backer board. They are made of various types of plastic such as king starboard etc.

Interestingly these suggestions appear to be restricted to australian posters as apparently it is common practice there

kmorin
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Re: 250 outboard install

#4

Post by kmorin » Tue Oct 30, 2018 9:53 pm

Sorry Tim, the word "board" is an synonym for wood! You know what I mean ? "how many board feet of that wood do you need?" That is how....
timruger1 wrote:
Tue Oct 30, 2018 3:07 pm
I don't recall mentioning wood

I am having difficulty sealing the mounting bolts and it was suggested to use a backer board. They are made of various types of plastic such as king starboard etc.
Of course, I assumed a "board" to be wood! And I'd have assumed that King Starboard or sheet nylon (or other specific materials) to be what they are too! Notice (?) you might have stated " I'm thinking of using a plastic plate.... " (??)

If you're having trouble sealing the engine mounting studs- then.... you might have asked: "Is there a good way to seal engine mounting bolts into the engine mount?"

And I'd have replied - "Well.... If you put a counter bore or wide chamfer on the engine mount bolt holes as they go through the aluminum transom... and then put an O'ring of about 1/8" cross section (to over-fill the chamfer or counter bore) on the bolts- then lube them with some sealant (?) that will create a seal around the bolts- at the transom when the bolts are drawn up.

(AND) If the outboard engine mount's cast aluminum were not a totally flat surface- therefore would not seal these O'rings into a bedding? : THEN, use large fender washers of the same alloy as the bolts to cover the O'ring chamfers and those will be held in place by the cast engine mount plate and will compress the fender washers over the O'rings and sealant and that will seal.

(AND) if that is not thorough enough for your engine mount (?) then simply repeat the same counter bore or chamfer inside the mount and the fender washer/O'ring and sealant and now the engine bolts will have two seals- one inside and one outside and they will not leak IF the O'ring's cross section is volumetrically greater than the counter bore or chamfer voids.

In case you want to go the extra mile? Use 5200 sealant as bedding and lube for the bolts and there will be no water inside the engine mount.

In order to reduce galvanic corrosion due to dissimilar metals- I'd recommend you use hot dipped galvanized bolts as the zinc to aluminum galvanic differential is a fraction of that of SS- even passivated SS- will eventually "chalk" and corrode the aluminum engine mount plate at the holes' edges - although using the double "packing gland" type of sealing described above will limit the corrosion sites (usually) to the wetted head and shaft outside the transom.

Sorry for my confusion reading your question's terminology- my most common error, in all I do, is to "assume"! Hope to have cleared up my assumption that board meant; wood board not plastic board? [Just a note: the marine, solid plastic sheeting is not compressible; but cellulose is.]

Cheers,
Kevin Morin
Kenai, AK
kmorin

timruger1
Posts: 5
Joined: Mon Dec 05, 2016 4:34 am

Re: 250 outboard install

#5

Post by timruger1 » Wed Oct 31, 2018 5:36 am

That may be worth a try on the outside between the motor and the transom

It won't work on the inside though as the transom is hollow and if the outside leaked it would simply fill the hull with water.

Gotta say I've never seen a boat with galvanized bolts for motor mounts I will have to check into that.

If that doesn't work I think I may have the bolt holes bored out and an aluminum bushing welded in.


Apparently the plastic backing plate is a common thing in Australia

Odd

5: Fit a backing plate

Install a suitable backing plate between the engine and the hull. It doesn’t need to be huge and a 6 to 10mm polyethylene bit will do. It aids insulation from non-aligned metals on aluminium and spreads the load like a big washer, and can help reduce some vibrations.

Thanks for the help

kmorin
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Re: 250 outboard install

#6

Post by kmorin » Thu Nov 01, 2018 2:32 am

timruger1 wrote:
Wed Oct 31, 2018 5:36 am
That may be worth a try on the outside between the motor and the transom
It won't work on the inside though as the transom is hollow and if the outside leaked it would simply fill the hull with water.
Tim if a transom, bulkhead or other mount is doubled/hollow/honeycombed... IT MUST BE welded bushed. That is not really even debatable. If a bulkhead/transom is built of a couple of plates- the bolt holes MUST be lined with TIGged in pipes/bars drilled to size/bushings that hold the bolts in holes stabilized at both surfaces.

To build with any less standard is poor practice, at best. Other descriptions would involved derogatory descriptions of your Mother, whom I am confident is a wonderful, gracious and upright paragon of womanhood. So Please.... do not besmirch your Mom's good name by doing substandard work!

If a leak path into the hull can be one simple bolt seal- and that could result in flooding? Please rethink this poor decision- it is inadequate and the result of ill-thoughtout design decisions.

The method I mention, and the bolts used are common practice in many commercial fishing boats in Alaskan waters- not exclusively as we too have our share of woefully inadequate builders.

Re- galvanized bolts- please ask yourself: "Will i be purchasing SS/304 anodes for my hull?" If not?
Then... why would you use that alloy for wetted fasteners?
Further to the point: "Will I be purchasing zinc anodes for my boat?" Perhaps you'd given this some thought? And what is a galvanized bolt hot dip coated with? would that be ..... zinc? So what is the galvanic potential from a zinc bolt to the aluminum engine mount? that is; ..... what is the galvanic potential when compared to (even) passivated 304 SS?

Why do so many installers insist on making a battery of the install bolts and the welded hull? I truly don't know but then we almost all call outboard engines: "motors" even if they're not. They're internal combustion engines- and don't run on AC or DC electrical power... but we call them motors? So from a group of folks who call engines- motors? I'm not surprised to hear that to seal an ENGINE MOUNT BOLT- you should put a plastic plate on the transom.... instead of just O'Ringing the bolt!!

[I am the site's self-appointed MillScale-Gnat-ZEE so...) And why doesn't everyone remove mill scale? Instead 95% (rough number I can't support) of all new welded aluminum boats refuse to acid etch the mill scale off their boats. And that same 95% have flowers and corrosion cells starting in 20 months! So some standard practices are questionable- but still wide spread.

Find anyone who insists on SS fasteners and try to get them to look at the galvanic chart and then get them to justify their alloy choices- of course- they will begin saying things about your Mother in perhaps sailor like language! But in the end- they can't defend their choice of alloys because physics doesn't support their choices- but the same people make the same choices over and over......facts, physics and chemistry be dammmmmmed!

Cheers,
Kevin Morin
Kenai, AK
kmorin

timruger1
Posts: 5
Joined: Mon Dec 05, 2016 4:34 am

Re: 250 outboard install

#7

Post by timruger1 » Thu Nov 01, 2018 5:06 am

I stopped by Pacific Boatload yesterday and looked at their new duckworths. I didn't see any that were bushed and the ones with motors had stainless steel bolts.

So it appears to be a quite common practice from the boat manufacturers.

That being said I think pulling the motor and having my local shop bore the holes and weld in bushings is the way to go. Perhaps more expensive but certainly more beneficial.

Are you statin you believe using a plastic backer board does or does not have merit?

kmorin
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Re: 250 outboard install

#8

Post by kmorin » Fri Nov 02, 2018 4:36 pm

timruger1 wrote:
Thu Nov 01, 2018 5:06 am
So it appears to be a quite common practice from the boat manufacturers.
yep, those guys do what they want, and repair shops see "used" boats prematurely devalued; penalizing owners for these many erroneous decisions. Its been happening for a long time..... Most manufacturers skip : etching (leave that shiny mill scale on!!), attention to galvanic potential (using SS to "shine"!!), and double siding weld seams using chine or sheer extrusions!!

[Just to name a very few instances of poor building practices that are widely practiced.]

But visit a repair shop- ask what they "see". Like I've mentioned- widely practiced building errors "don't make 'em right."

plastic plates may seal bolts (but you could use an O'ring for a few pennies) , may act as some level of isolation (ONLY IF they put full isolation kits on the bolts; nylon washers, goop sealed sleeves, and re-torqued as the isolation washers collapse under full torque), and rigid plastic has no vibration dampening properties compared to any other rigid material (vibration dampening only occurs when the durometer reading is below some given point- ie. not rigid materiel: see inboard engine mounts for a review of this concept.)

I am simply observing: plastic plates/board could be used if desired, but they are not the simplest way to
#1 isolate leak paths into the hull from engine mount bolt holes.
#2 help in any manner with vibration of the engine transferred to the welded metal transom/engine mount
#3 isolate By Themselves the engine from the boat's DC NEG potential to water.
#4 avoid crevice corrosion of a plastic plate sized corrosion area of the outer transom.

hope this helps clarify my earlier remarks?

Cheers,
Kevin Morin
Kenai, AK
kmorin

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