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  #1  
Old 06-27-2010
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DIY sliding outboard mount

Hi all.

I DIY'd and installed a outboard mount on my boat a few weeks ago and used it for the first time this weekend. I posted details on the Columbia Yacht Owners Yahoo group, and thought SialNet members might find it useful and/or interesting. No pictures of the OB powering the boat - I couldn't persuade my wife to ride in the dinghy taking photos.

BTW, all the metal is marine grade SS.

--Tim.
S/V Wind Gypsy
1978 Columbia 8.3

SLIDE BARS


The slide bars are pretty basic.
- 2 1/2" x 2 1/2 1/4" angle
- 30" x 1" x 0.12" wall square tubing
- all drilled holes are 3/8"I drilled the bars and
angles holes for bolting the bars to the angles,
and for bolting the angles through the transom.

The bars are bolted to the angles with the 1" bolts, the nuts are *inside* the tubing. This allows the block to slide up and down the bars unobstructed.

Two stop bolts (2") were placed about 3" from the lower end of each bar, one stop bolt was placed about 3" from the top of one bar.


SLIDING BLOCK


The OB bracket bolts through the sliding block, so all holes in the block are 5/16" - the size of the OB bracket mounting bolts.

The sliding block is made from:
- 2 pieces of solid teak
- 11" w, 8"h, 1" t
- 9" w, 8"h, 1" t
- 2 pieces of 1/2" HDPE (cutting board plastic)
- 3" w, 8" h, 1/2" t
- 2 pieces of flat steel
- 11" w, 2 1/2" h, 1/4" t

The parts of the block are arranged so that, when bolted together, the ends form a U that 'wraps' around 3 sides of the square tubing. From the bottom (side closest to the transom) up, the order of assembly is:
- flat steel,
- HDPE (acting as frictionless slide pads)
- small teak board
- large teak board

The teak boards were glued together to make a single, solid block. The pieces are held together by the OB bracket mounting bolts.

The 'ease of slide' can be adjusted by tightening/loosing the mounting bolt nuts.

The transom on my boat is angled back about 5deg towards the bow, so slide friction is mostly against the inside of the tubes. This meant I only needed HDPE friction l slide pads against 1 side of the tubing, but if I were to install pads against all 3 sides of the tubing, I would have routered a groove on each surface that touches the tubing. The groove would be centered on the surface, about 1/2" wide, 7" long, 1/8" deep. I would then cut a 1/8" thick piece of HDPE to fit the groove - when installed it should stay in place without glue, screws, etc.

OB BRACKET OPERATION
Lowering:
Push the sliding block down to its lowest position. Lower the engine using the OB bracket arm adjustment.

Raising:
Raise OB bracket arm. Pull sliding block up to highest position. Secure sliding block.




IMPROVEMENTS
In no particular order:
- HDPE slide pads on all surfaces touching the tubing.
- blocks and lines for raising/lowering engine
- three pairs of angles, one pair in middle, one pair
at top, one pair at end (this wasn't possible on my
boat because of the previously installed thru-hulls)

USABILITY/EXPERIENCE
It certainly does the intended job! Between the slide and the OB bracket, the engine can be raised and lowered about 34" so it can be slide down well below the water line and raised completely out of the water when not in use.

Leaning over the transom to operate the OB isn't ideal, but it's not difficult either.

Pulling the entire assembly up the bars is possible, but it does take *A lot* of effort - it really does need blocks and lines to make raising the engine easier.

I would have liked to install a pair of angles right at the end of the tubing - with the slide block all the way at the end of the tubes the OB weight causes the tubes to flex slightly. My (very very good) mechanic pointed out that the tubing is stronger than the OB mount and a little flexing wasn't an issue ..... but I'd still like to have everything 100% rock-solid.
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Old 06-27-2010
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Very nice job!
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Old 06-28-2010
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Good job.marc
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Old 12-23-2010
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Would it be worth mounting a lifting eye on the top of the teak block in order to use a block of your stern rail to raise and lower the mount? Also I was wondering if there was any thought given to adding a bar between the top and bottom of the angle iron for a little extra strength?

I know this is an old post, but my inboard just crapped out, and this looks like an awesome alternative to an expensive repower.
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Old 12-23-2010
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Greenman -
Quote:
Would it be worth mounting a lifting eye on the top of the teak block in order to use a block of your stern rail to raise and lower the mount?
A couple of blocks and some line for lifting on my 'repairs and upgrades' list for this winter! The sliding setup has worked really well this season, but there were a few ocassions when leaning over the transom, pulling on an engine while the boat pitched and rolled, was 'exciting'


Quote:
Also I was wondering if there was any thought given to adding a bar between the top and bottom of the angle iron for a little extra strength?
Never thought of that. It'd certainly make everything more rigid, but IMO it's not necessary.

If I was doing it over, the changes would be:
* Make the Nida-PTC cored transom thinner. A 2" thinck transom is total overkill!!
* Use starboard (or similar) to mount/slide on the bars instead of wood because the wood tends to swell slightly by the end of the day (even though it's sealed with penetrating epoxy and several surface coats of epoxy).

All in all, this was a cheap, simple and effective solution to mounting the OB and getting the prop out of the water while under sail.

BTW, the 316L Stainless was from OnlineMetals.com

--Tim.
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Old 12-23-2010
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I wouldn't recommend using starboard, it isn't very strong and would probably not withstand the loads involved.
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So it would be best to stick with the HDPE slide pads?

What about aluminum construction? Or would there be too much issue with dissimilar metals etc (stainless fasteners)?
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The HDPE slide pads are an excellent idea, and well worth using regardless of how you make the system.

I'd stick with stainless steel for the rails and such. Aluminum depends on an oxide coating to prevent further corrosion damage, and the sliding motor mounts would keep damaging the oxide coating, leading to increased corrosion of the aluminum rails. Also, stainless steel is stronger, so the rails can be smaller. Finally, stainless steel has better fatigue characteristics than does aluminum.

Quote:
Originally Posted by Greenman View Post
So it would be best to stick with the HDPE slide pads?

What about aluminum construction? Or would there be too much issue with dissimilar metals etc (stainless fasteners)?
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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