Replacing a heavy solid AL plate OB mounting plate with something lighter?

Someone has recommended that I use 3/4" thick G10 to replace the aluminum slab. G10 is easily finished in gelcoat or paint, won't crush, and won't corrode. Not expensive, ultra durable and cosmetically acceptable. I'll glue a rubber pad over the finish for the OB clamps.

Judy

 

Small world. I was looking around for ideas on how to build a large, flat, molding table and found the biggest one I've ever seen...



Turns out the site is Farrier Marine showing their All New F-22. I don't know if that's what you have, brand wise, but in their photos they show how they mount the pusher engine. No idea on what lam schedule they have, but it shows the esthetics of it enough to get an idea on how to build it. As for a lot of things, you can build what you think might work, temporarily attach it and try it out. If it works, then take it off and finish it with gelcoat/paint/etc. If it doesn't, modify it and try again. When you're done, then finish it. It looks to be an accessory that's bolted to the side of the hull.

This is the page the photos came from...

http://www.f-boat.com/pages/News4/FM-Factory2013.html

This looks to be a handy section also...

http://www.f-boat.com/pages/contact/index.html

 

Sam,
It is indeed a small world! Thank you for sending that along. You made my day.

I hadn't seen that before. That's exactly the kind of solution I'm looking for. Elegant and functional. Better functionally than the old version. The OB sits lower to the water and lifts higher.

The F22 is a newer design from Ian Farrier, who also designed my 1992 F24. The F22 is 2 feet shorter, several hundred pounds lighter than my F24, and faster. But no matter, my old F24 is just as pretty as the newer ones

 

how is a slab of this G10 going to be cosmetically preferable to the slab of painted aluminium? Makes no sense. You can cut that existing alloy to a more aesthetic shape, the pattern you'd choose to make one from G10 ( I read it is difficult to machine) paint it, and it would not offend the eye at all. And I assume the G10 suits the application, I have no idea.

 

Here's a picture of some of the aluminum I took off the boat last weekend. The paint is peeling and the aluminum shows significant ccorrosion. Once salt water gets under the finish coat, the corrosion accelerates and the finish deteriorates rapidly. Corrosion weakens the extrusion.



G10 is a pre-manufactured panel of fiberglass. You can gelcoat G10 like any fiberglass laminate

G10 suits the application well if you chose the right thickness. G10 laminate is significantly lighter than an equivalently stiff panel of aluminum. I have worked with G10 in the past on other boats. It's not difficult to shape with ordinary woodworking tools. However, it does dull cheap hole cutters and cheap routing bits rather quickly.

 

Thank you again, Sam. That was a great lead. I sent an email to Ian Farrier to ask his permission to copy his design. He replied with an encouragement and gave me the lamination schedule with instructions for reinforcement where the through bolts are located.

Judy

 

Hard to see from the picture.

However, if indeed your plate is a 6000 series alloy then all you need to do it use 5000 series, such as 5083. Since 6000 series, the most common extrusion alloys used, have 3 times as much copper in them and are susceptible to corrosion. Solution, use proper marine grade 5083/5383 alloys.

 

I believe you're right. The prior owner who modified the motor mount would have been wise to use 5083/5382 and to prime and paint it correctly.

Didn't happen that way, though

 

Don't forget to factor in the engine vibration!

 

That newer design also (a bigly also) shields the motor from most direct splashes of water.

I also bigly like the rudder, a removable dagger board. Very easy to replace if destroyed.

Another yuuuuge bigly like..." He replied with an encouragement and gave me the lamination schedule with instructions for reinforcement where the through bolts are located." I'm not sure that happens much.

 

Yup, Sam, I bigly like those also.

The new design shields the motor from water. Ian sent me details of an even more recent design that improves upon the one shown in the picture. It still protects the OB from spray but it accepts a motor mount with vertical adjustment. It'll be stronger and easier to fab than the one shown on his website. The fiberglass part is lighter weight, but adding the adjustable motor mount adds another 15-20 pounds to the whole thing.

Ian Farrier is exceptional for his commitment to the ongoing updating and repair of his older designs. He will answer quick questions via the Farrier owners' forum or you can buy a consultation for a nominal fee on his webiste for personalized advice. That is indeed a yuuuuge, bigly cool thing.

When I surveyed my boat, I had a couple of structural questions because a Previous Owner had done a few mods to the boat and trailer that I questioned. I bought a brief consultation with Ian for a nomnal fee. I sent him pics of the modifications, and he answered me very quickly with advice. Two of the mods were in fact of structural concern; one was of no concern. I factored that into my offer, and got the boat at a good price and the seller paid to fix both of the structural modifications.

The high aspect rudder which lifts in a cassette is pretty cool, but rather pricey. To buy and convert my old 1992 to a 2017 rudder would set me back $3600. The rudder blade is carbon, as is the cassette. The cassette has fasteners along the aft edge that are designed to break away in the event of a grounding, to minimize damage to the blade. In addition to the performance boost with the high aspect blade, there's another benefit. You can partially retract the rudder blade so that you still have steerage in very shallow water.

The old, low aspect blade swing rudder kicks up when you hit somethng (gently). It's either up or down, so you can't use it in shallow water. The old, low aspect blade cavitates at high speeds in the double digits. The new, high aspect rudder doesn't cavitate

 

Not sure what your alloy is?, but concur with AdHoc on the alloy, although 6082 T6 preferably is pretty good amongst the 6000 series alloys. You can of course get it anodised too, about 25-30 microns should be enough and leave it nautural rather than a colour... TBH it has lost almost zero strength if it really is 20mm thick, it would still be strong enough at 2/3rds or less...

Personally even with a composite or wooden core, a 2 or 3mm thick sheet of aluminium (each face) is one of the best long term protectors for taking the motor mounts in my experience. This is pretty much the same for either the hand screw or bolt through type.