DIY static outboard bracket

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peiserma

Member
I am purchasing a used 175 offshore merc with a 25" shaft. I have a 20" transom. Thinking of making my own bracket using 4 pieces of angle bracket.

Motor weighs about 400 lbs, i think (per NADA) Smokeonthewater recommened 3x4 angle, but should it be 3/8" or is 1/2" safer? I Like overkill. Stainless? Or is Aluminum better? How many bolts should I use to attach one angle to another? 2 or 3? And what size bolt? What grade bolts? How many bolts to attach the bracket to the transom? How many to bolt the motor to the bracket?

Any thoughts appreciated.
 
I've seen that one on several sites. I have never seen a weight rating for it, only a HP rating (150, which the motor exceeds). I would think the weight capacity is important, I personally can't bring myself to trust it until I know.
 
peiserma said:
I've seen that one on several sites. I have never seen a weight rating for it, only a HP rating (150, which the motor exceeds). I would think the weight capacity is important, I personally can't bring myself to trust it until I know.
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Check out the specs on that bracket...what thickness material..alum or SS...how many bolts and exceed those numbers for your 175
 
Here's my 2 cents worth:

Any Stainless angle over 3" x 3" x 3/8" for outside, Inside I would use a full plate 1/4" thk across the transom to disburse the force on the transom. The plate should be a minimum of 4" larger than the distance across and above the outside area of the angle frame dimension. Bolts 5/16" dia minimum 2" down/inside from each end and then spaces about every 8" apart. Grind the extreme ends of the angle where they lay against the transom. All welds should be full penetration for the bracket and ground smooth on the side against the boat. This will not allow any brunt force against the fiberglass.

Do not use aluminum, you would need to beef up the thickness from 5/16 to 1/2" thick to compensate for strength and it would need to be out of billet aluminum.
 
remember also, you need lift, not necessarily set back, although set back requires less lift(rule of thumb is 2 inches set back covers one inch lift), I made a lift only bracket for a 70 by using two pieces of angle aluminum, I used the original mounting holes in the transom to mount the aluminum angle use flat head bolts so they fit flush), and drilled new holes thru the angle and transom for the engine mounting bolts. The top bolts were not supported by the transom, they only went thru the angle, the bottom bolts went thru the angle and hte transom
 
garagenc said:
Inside I would use a full plate 1/4" thk across the transom to disburse the force on the transom. The plate should be a minimum of 4" larger than the distance across and above the outside area of the angle frame dimension.
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Thanks for the detailed reply. I'll have to see what the local scrapyard has for stainless angles. Its not inexpensive.

I assume the inside plate can be aluminum since it's basically funtioning like a giant washer. Would you think that a 6" or 8" square plate for each of the bolts would serve nearly as well, or would you stick with your recommendation of a giant plate? I think an 18" 6061 aluminum plate 1/4" thick can be had for around $40.

The old motor was bolted to the transom with 4 bolts. OK to bolt bracket to transom using just these four?
 
spareparts said:
remember also, you need lift, not necessarily set back, although set back requires less lift(rule of thumb is 2 inches set back covers one inch lift)
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I'm confused by that statement. If the transom is angled down towards the water, wouldn't the setback due to the bracket require more lift, not less?
 
water is "higher" the further back from the transom you go. the deepest hole your boat makes in the water is at the transom.

now your really confused
 
Ah! Now I get it. I wasn't considering the most important part, and that is the boat moving forward on plane... Thanks for clearing that up. Of course that makes it a little tougher to deterimine how high the motor needs to go.

Maybe its better to just buy an adjustable bracket for that reason alone? If i don't get it right the first time, I need two more SS angles.
 


this is a bracket i made for my whaler. it was built for a 15" motor and when i repowered, i wanted a 20" in case i put it on any other boat. granted this is a lot less weight & hp, but you get the idea. the original transom is about where the starboard stops.
 
no we're ALL confused.. lol..

YES the further back the engine gets the HIGHER it must be... not because of the angle of the transom (you are measuring height above the keel not angle) The reason the engine gets higher further back is because the boat displaces water even on plane... the water right at the transom is lower than the water in front of and behind the boat... as soon as the transom leaves a given point in the water it immediately starts to rise towards it's original level so behind the boat is uphill....

ALSO the keel will be pointing uphill toward the front so straight back (no matter the transom angle which you build around) is downhill....

sooo your engine moving downhill where the water is moving uphill requires more lift....


You do not need any huge washers or plates on the transom... the angle iron will already be spreading forces much better than the outboard brackets... you only need the same washers you would need to mount the engine directly to the transom..... under power the only forward force on the transom is at the very bottom end of the bracket or angle... the top is pulling away from the transom.....


ALSO nobody mentioned this but the bracket will not increase leverage on the transom... in fact it will lower it... the forces are at 3 points... prop shaft, bottom of bracket, and top bolts through transom..... a simple lever with the bottom of the bracket being the fulcrum

the prop is the fat kid on the teeter totter and the transom top is the skinny kid..... the bottom of the bracket is the hinge and the top bolt is you.... by moving the prop up you have moved the fat kid closer to the hinge and made it easier for the skinny kid to lift him



You don't HAVE to get it right the first time... your motor has the adjustability built in... just get your bracket close but not too high and adjust the engine up as needed for max performance
 
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peiserma said:
I assume the inside plate can be aluminum since it's basically funtioning like a giant washer. Would you think that a 6" or 8" square plate for each of the bolts would serve nearly as well, or would you stick with your recommendation of a giant plate? I think an 18" 6061 aluminum plate 1/4" thick can be had for around $40.
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NOoooo If you use Stainless Steel for one side of the bracket and Aluminum for the other you'll have two dissimilar metals and that will result in galvanic corrosion. Whatever metal you make one side of the bracket from you must make the other side of the bracket from also. Personally I would recommend using 1/2" thick Anodized Aluminum for the bracket since your engine is made of Aluminum also, and you want to keep all metals as close to the same as possible.

Peiserma said:
The old motor was bolted to the transom with 4 bolts. OK to bolt bracket to transom using just these four?
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Yes. The original holes will do just fine for the inside part of the bracket.
 
spareparts said:
remember also, you need lift, not necessarily set back, although set back requires less lift(rule of thumb is 2 inches set back covers one inch lift), I made a lift only bracket for a 70 by using two pieces of angle aluminum, I used the original mounting holes in the transom to mount the aluminum angle use flat head bolts so they fit flush), and drilled new holes thru the angle and transom for the engine mounting bolts. The top bolts were not supported by the transom, they only went thru the angle, the bottom bolts went thru the angle and hte transom
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sorry, I did word that screwed up, Smoke explained it better than I did, you get the idea. The further you go back, the higher you can run the engine
 
RidgeRunner said:
What fasteners do you use to keep from having dissimilar metals?
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Unfortunately, that's one of the conundrums of boat building. The bolts of choice are Stainless Steel because of their strength and corrosion resistance. But, they are dissimilar to the Aluminum that the outboard engine is made of. That's where the use of sacrificial zincs comes in. Just remember that the less dissimilar metal there is, the less potential there is for galvanic corrosion.
 
Thanks a bunch, you've given me enough to go on for now. I really appreciate it. I'll see what 1/2" angle costs locally, take some measurements of transom and motor, then sketch up some plans. I will post the sketch for final input and recommendations for the proper height. Will need until next week.

Last question before I hide in the mad scientist cave to sketch up my plans: if i use 3" x 3" angle 1/2" thick, setback will be around 4". Using the 2" setback = 1" of lift rule, that equals 2". So I should plan to raise the motor 3" (recall it is a 20" transom and a 25" motor) in relation to my old one?
 
the math here would net you 7" BUT instead of just assuming you need to measure from the keel to the top of the transom and from the anti vent plate on the motor to the bottom of the transom hooks.......
Understand that you will set only your lowest possible height here so build to put the anti vent plate 1" below the expected desired height... in this case since you expect to need it 2" above the keel you would aim for a minimum of 1" above the keel.... Then you can raise the motor one hole at a time till you get the best performance.


example: if your motor shaft is actually 26" and your transom is actually 19 then you would need 9" -1" for a total of 8"
 
Honestly I have to admit..... while I can't suggest it to anyone else as it is officially a BAD idea I would probably just run the 150 hp pre made plate.... Jus sayin
 
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