Out on my Northshore 27 on the weekend, tidying up in preparation for a sail, I noticed the outboard mounting board had a big crack in it.
I had noticed the outboard looked a bit more angled down than usual the last time I was out, so presumably it's been cracking apart for at least a month.
We bailed on the sailing and spent the afternoon unbolting it for further diagnosis/replacement, getting the outboard safely inside the cockpit.
This bracket, while a cheapy, is from an Australian retailer and has a weight rating of 50kg.
My outboard is a 25kg Mercury 6HP 4st. It's only been installed for 3 months.
Has anyone ever seen this happen before?

With the existing mounting holes and a custom wedge I fabricated to angle the bracket flat on the curved transom, I don't have a lot of options without starting from scratch.

I could easily buy another very similar bracket from a different retailer, but my faith in that plastic part is gone. I'm tempted to get out the plywood and epoxy again to fabricate a new board. (Using similar techniques, the wedge has been extremely sturdy).

The original retailer was very quick to provide a refund (without product return) when I sent them these pics, which makes me suspicious they wanted the issue to go away quickly.

A board made from plywood, of suitable thickness and epoxied inside, painted on the outside will be suitable. The metal parts are a different matter, but if they are holding up, then they are strong and thick enough for the task.
On one occasion I've made the OB board a little shorter, and on another have had to replace a lost spring. Not big problems, and we move on with life and sailing. A plywood OB board might also be less slippery for the OB to grab and hold on to.

Common problem with those plastic pads and people replace with plywood. Lucky you found it before it broke at sea.

Certainly agree - use marine ply and would suggest epoxy resin seal the whole lot especially the edges then epoxy primer undercoat then oil based enamel or polyurethane topcoat. Don't get cheap marine ply it can give you a lot of grief with the end grain delaminating. If I was doing it would put another 316 ss bolt each side 1/2 way between the two already there securing the pad on - same diameter. For all that this one is only $115 with ply pad.

simplydc.com.au/product/outboard-motor-bracket-stainless-steel/

That's a weird looking plastic. Looks more like a recycled material.

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FabulousPhill said..
The metal parts are a different matter, but if they are holding up, then they are strong and thick enough for the task.

Yes, metal parts are holding up, only bit that's bent was from my ripping at it to remove from the boat.Thanks for your "move on" attitude too, glad it didn't turn into a bigger issue.

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Madmouse said..
Common problem with those plastic pads and people replace with plywood. Lucky you found it before it broke at sea.

Fascinating! I couldn't find any other examples of this, but glad I'm not the only one. As so glad I caught it on the mooring.

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r13 said..
Certainly agree - use marine ply and would suggest epoxy resin seal the whole lot especially the edges then epoxy primer undercoat then oil based enamel or polyurethane topcoat. Don't get cheap marine ply it can give you a lot of grief with the end grain delaminating. If I was doing it would put another 316 ss bolt each side 1/2 way between the two already there securing the pad on - same diameter.

Great detail on the ideal construction, especially adding extra bolts. Not sure where to get quality ply from, however. Buying a complete new bracket is tempting, but I'd rather not have to deal with a new footprint and wedge.

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Ramona said..
That's a weird looking plastic. Looks more like a recycled material.

I'm not sure what ripped plastic is meant to look like, but it certainly seems low quality to tear like that.

Thanks for all the replies so far! Looks like building a new board from ply is a great idea. Will ensure I seal and coat it suitably.

I've not had trouble with a similar mounting but I keep it under tarpaulin when we are not sailing, which might help protect it.

You can buy whole sheets and smaller sizes of marine ply at Bunnings.
www.bunnings.com.au/1220-x-610mm-12mm-plywood-hardwood-marine-aa-grade_p0320031
3 or 4 layers of 12mm should do it,. Plywood means that it won't warp like normal timber, because of the layering.

For suitable quality marine ply see Bruynzeel but you only need a small bit - they might sell offcuts of 18mm ply which you can epoxy glue two pieces of together to get to 36mm which could be needed - how wide is the throat of your outboard clamping system can it take 36mm? See their cautions on edge sealing.

www.bruynzeel.com.au/products/plywoods/marine-plywood/

www.bruynzeel.com.au/cut-to-size/

The plastic used in the original pad is probably too low a strength grade, and also the forward facing face is really badly designed for the large bending moment that the outboard will inherently apply to it, creating high tensile stresses on the forward facing face just above the upper bolts and very rapidly leading to the failure. So it is obviously way down on static and fatigue strength and the failure has occurred. Maybe the designers used a generative design program to come up with that design geometry - congratulating themselves on the material saved per pad but ignoring static and fatigue strength and hence durability issues.

parametricmonkey.com/2021/07/27/generative-design-fail/

The brunynzeel prices are very competitive.
It depends on what you have access to, I would use form ply off cut as its a sorta consumable item as the OB clamps chew it a bit

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Woylie said..
I've not had trouble with a similar mounting but I keep it under tarpaulin when we are not sailing, which might help protect it.

You raise an interesting question; has UV exposure been a contributing factor to the failure?

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FabulousPhill said..
You can buy whole sheets and smaller sizes of marine ply at Bunnings.
www.bunnings.com.au/1220-x-610mm-12mm-plywood-hardwood-marine-aa-grade_p0320031
3 or 4 layers of 12mm should do it,. Plywood means that it won't warp like normal timber, because of the layering.

I've got some "Marine Plywood" from Bunnings, but seen mixed reviews about its quality. I wouldn't want a new board to deteriorate again because I cheaped out on materials. Having said that, this is the suff I used for the wedge, but I soaked it in epoxy, fibreglass and painted with Gelcoat.

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r13 said..
For suitable quality marine ply see Bruynzeel but you only need a small bit - they might sell offcuts of 18mm ply which you can epoxy glue two pieces of together to get to 36mm which could be needed - how wide is the throat of your outboard clamping system can it take 36mm? See their cautions on edge sealing.

Bruynzeel look like a great supplier. I might enquire about getting an offcut.
The OB can accomodate up to 52.4mm under the clamp according to the service manual. That plastic board is 43mm thick. Would want to go as thick as possible to avoid failure, I presume.

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r13 said..
The plastic used in the original pad is probably too low a strength grade, and also the forward facing face is really badly designed for the large bending moment that the outboard will inherently apply to it, creating high tensile stresses on the forward facing face just above the upper bolts and very rapidly leading to the failure. So it is obviously way down on static and fatigue strength and the failure has occurred. Maybe the designers used a generative design program to come up with that design geometry - congratulating themselves on the material saved per pad but ignoring static and fatigue strength and hence durability issues.

Taking a closer look, it really does look like a terrible design. The geometry would only be suitable for compressive forces in plastic, from my understanding. All the strength for the perpendicular load is in the 7mm of solid plastic. Wouldn't surprise me at all if you're on the mark with generative design and cost cutting.

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I've had an offer from a fellow yachtsman and builder by trade to help construct a ply board. I'm currently working on some sketches.

I'll be buying a new bracket soon as the Ali supports on my current one have corroded beyond repair. I'm hoping to get the same model so I don't have to drill extra holes in the transom. The current version comes with a plastic pad. Very interesting that the plastic has broken on yours. I'll be checking my new one carefully.

I did make up a new pad for my old one before I discovered the corrosion issues, so maybe I'll ditch the plastic straight away and reuse my ply pad. I made it out of Bunnings marine ply epoxies together and coated in more epoxy and a clear coat. Here's a pic of the new vs the old ply pad.

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Microbe said..
I'll be buying a new bracket soon as the Ali supports on my current one have corroded beyond repair. I'm hoping to get the same model so I don't have to drill extra holes in the transom. The current version comes with a plastic pad. Very interesting that the plastic has broken on yours. I'll be checking my new one carefully.

I did make up a new pad for my old one before I discovered the corrosion issues, so maybe I'll ditch the plastic straight away and reuse my ply pad. I made it out of Bunnings marine ply epoxies together and coated in more epoxy and a clear coat.

Where did you get your bracket from?
I replaced a few bolts on mine to try change the lift mechanism (long story) and accidentally mixed them. All that weren't 304 or 316 SS have completely rusted. Cleaning that up today, the bracket itself is totally corrosion free. Definitely pay attention to the pad, sounds like it's not unheard of for the plastic ones to fail.
Nice work on the ply. Glad to see that Bunnings stuff can hold up (or at least not crack like bad plastic). Sounding like wood is the way to go. Certainly don't chuck it.

If you want to increase the life span of the ply plate perhaps screw some aluminium plate on where the OB clamp & screws do the work,

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lyuzashi said..

Microbe said..
I'll be buying a new bracket soon as the Ali supports on my current one have corroded beyond repair. I'm hoping to get the same model so I don't have to drill extra holes in the transom. The current version comes with a plastic pad. Very interesting that the plastic has broken on yours. I'll be checking my new one carefully.

I did make up a new pad for my old one before I discovered the corrosion issues, so maybe I'll ditch the plastic straight away and reuse my ply pad. I made it out of Bunnings marine ply epoxies together and coated in more epoxy and a clear coat.

Where did you get your bracket from?
I replaced a few bolts on mine to try change the lift mechanism (long story) and accidentally mixed them. All that weren't 304 or 316 SS have completely rusted. Cleaning that up today, the bracket itself is totally corrosion free. Definitely pay attention to the pad, sounds like it's not unheard of for the plastic ones to fail.
Nice work on the ply. Glad to see that Bunnings stuff can hold up (or at least not crack like bad plastic). Sounding like wood is the way to go. Certainly don't chuck it.

Not sure where you are but Mr Stainless In Five Dock Sydney has good stocks of 316 stainless fasteners. Stainless Central has also, including the high strength Bumax grade 8.8 316 stainless fasteners if needed.

www.stainlesscentral.com.au

This is the rear view of the plastic pad on the new bracket I bought today from RTM. It looks sturdy enough so I'll leave it on for now and keep the ply pad in storage in case I need it later.

Finished building a new mounting board! I took some extra time to plan a couple of customisations from the standard board to fix other issues with my outboard setup.

Angle
To get the outboard to sit right in the water, I found I needed to use the very last position for the tilt pin, which prevented the reverse lock from working. My original installation set up the mounting board to be exactly right angle to the waterline. The service manual describes an ideal transom angle to be 12deg.
I've incorporated this angle into a cut on the new plywood board. Hopefully this doesn't weaken it.

Height
When at the correct angle, my outboard just sat deep enough in the water, but would bounce out in anything other than calm waves. I've reduced the distance from the bracket to the top of the clamp to as small as possible. Still tempted to re-drill holes in my transom but will see how this fares.

I've incorporated some ideas from the replies here.
* Used marine ply, epoxied two 24mm boards together, added another layer of 9mm on exposed side edges and 4mm board over face that I cut through at an angle to seal the inner layers
* Coated entire piece in polyurethane, epoxy on exposed edge in mounting holes (plus some butyl to seal the bolts)
* Brand new, completely 316 SS hardware (from Stainless Central - great service!)
* 0.9mm 316 Stainless plate on clamping face to provide extra strength over angle and prevent gouging from o/b clamps
* Didn't do, but still considering adding an extra bolt each side 1/2 way on the bracket

Just waiting on the rain to subside to go out and install it. In the mean time, working on a beefier backing plate to prevent the transom flexing - though haven't seen any signs of cracking when I removed it all.

Top job right there better than a bought one.

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r13 said..
Top job right there better than a bought one.

Cheers! Thanks for all your tips

First class job, and great follow-up post, my only very slight concern and I think would be a good idea to keep an eye on, would be the SS plate, I think the clamps are better off slightly gouging into the laminated timber as that would also provides a slight push-back on the clamps, keeping them tight, you'll need to watch that the engine vibration doesn't cause the clamps to loosen.

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BlueMoon said..
First class job, and great follow-up post, my only very slight concern and I think would be a good idea to keep an eye on, would be the SS plate, I think the clamps are better off slightly gouging into the laminated timber as that would also provides a slight push-back on the clamps, keeping them tight, you'll need to watch that the engine vibration doesn't cause the clamps to loosen.

Cheers! Yes I also have the same concern. The screws are positioned so the clamp ends will bump against them if it starts to slip. Along with a safety cable, I hope there's enough backup to keep it safe from disaster. If it does loosen enough I can always remove the plate.
I wonder if one of those outboard locks which slip over the clamp handles would keep it tight? Nowhere for it to unscrew.

And now installed! Sits nice and low in the water. A southerly started to hit just as we were finishing up (got drenched by waves in the dinghy back) and the prop never jumped out of the water while testing the position on mooring.

Will report back if I see the new mounting board ever fail.