DIY static outboard bracket
 

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 edited my suggestion on the other thread w more details... ALSO you can buy one ready to bolt on for $100 .. ish

https://www.google.com/shopping/prod...d=0CMkBEPMCMAQ

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.

whoops!.. I was thinkin you were getting a 150.... my bad

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

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?

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.

http://i377.photobucket.com/albums/o...ps969d3412.jpg

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

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.

Yes. The original holes will do just fine for the inside part of the bracket.

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

What fasteners do you use to keep from having dissimilar metals?

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

X2.... Since most manufacturers build in a certain load above the rated safe load, and if money wasn't all that important, I'd tend to do the same thing. Understand this is NOT a recommendation, just sayin what I would probably do.
Much like me running a 18hp Kicker on an EZ-IN kicker bracket for several years even thought the bracket was only rated for 10hp. But then again, I've never considered myself as the brightest candle in the box... so what the heck do I know.... :head:

More to look at.
http://www.wholesalemarine.com/th-ma...extension.html

I think you are going to have trouble if you mix metals unless you can insulate that aluminum plate from the stainless angle, the plate will act like a sacrificial anode to the bracket, especially in saltwater.

he has no need for an aluminum plate.... all he needs is angle and bolts

i think he was going to put some plate on the inside of the transom for backing plate

Buy this hull get a free jack plate, $300. Then flip the hull.

http://augusta.craigslist.org/boa/4694174420.html

Not precisely. I asked how it should be built, and someone suggested to put a plate on the inside.

It's clear from this thread everything should be aluminum, and consensus appears that it isn't strictly needed. The force applied is greatest at the inside top bolts of the transom and the outside bottom bolts due to the whole thing essentially being a lever. The bolts are stainless only because aluminum bols aren't strong enough (if they even exist).

I personally am uncomfortable using angle for an engine this size, I would feel better if it were at least "c" channel. That is just my gut feel, no actual numbers to back that up. But I have looked at pictures of Z-Lock for comparison. Most notably, the hi jacker design (which resembles the angle design) is rated up to 150hp. For larger engines, the design changes. The amount of force needed to move 2000lbs of boat, plus 1000lbs of fuel, people, and gear has to be significant ... Plus it must apply some amount of torque due to the prop.

[EDIT] Just found this design that uses angle brackets here:
Machine Shop Goes up to 300hp, so maybe angle is OK. I am stopping by a metal place Friday to see what is available and what it costs.

You know.... at $143 shipped THIS^ is another good option

If not you could use this design for yours..... only downside is a couple more holes n the transom.

I understand the sentiment, but I am an (electronics) engineer working in the nuclear power industry. My design comfort level starts in the 30% to 50% margin range, and if I can talk people into a more robust solution, I will. Then I add a backup system. Actually, we like four backup systems for anything really critical.

It may work just fine for a long time. I understand perfectly well that T&H designed in margin, but I could never sleep well at night using something beyond rated capacity. Fukushima got hit by almost 10x worse than it was designed for, which has no bearing on this topic, but it does reinforce the idea that safety can never have enough margin.

While I appreciate the viewpoint, I'd never be comfortable with it

I'd use a 1" plate of aluminum! You know, to give myself some margin ... good idea, I'll price that out Friday, too.

1" is BEYOND OVERKILL! They only use 3/4" plate for a dual engine bracket where the engien mounts to it. For what your doing, bite the bullet and go to ebay and buy a jack plate. I seen several manual ones on there for under $200. By the ime you buy a piece of plate, and do everything your talking about, you will be into it for just as much i bet.

While 1" may well be fine 1/2" angle would be stronger

I haven't mastered the art of conveying sarcasm on the internet yet, sorry.

So I took your suggestion and went to ebay and found a Vance JPL 4400 for $174. Never having heard of them, I looked up Vance and found their website and description of the jack plate here:
Vance JPL 4400.

There is a 3" setback version, too (JPL 4300). Both are rated for 225HP. $174 is their normal price! In stock and made in the USA (a plus in my book)

That price makes it pretty hard to justify DIY cost savings.

Buy that thing. Looks to be a high quality piece.

Not to mention an American owned small business. I'd rather give the little guy a few more bucks.

Hard to disagree with that, especially since I worked as a mechanical engineer/facilities mgr for the company that brought you 3 Mile Island and owns Forked River Nuke Facility. But you have to remember that we're talking compressive strength here, and for 6061 extruded aluminum angle it's 45,000. Since most of your forces are going to be compressive (The engine pushing against the angle which in turn pushes against the stern of the boat) you're going to have to go some to convince me that it's not safe to up the size of the engine. It's a long stretch from a nuke facility to an outboard bracket. Just sayin...

And yes, for the money, I would buy that mechanical jack plate. You can see the quality that went into the design and machining of it. Nicely rounded corners, all edges broken, etc. Def a piece of quality workmanship.

Got the new-to-me motor on the boat today. Not rigged yet. With the motor in the highest position, I think its pretty close to where it needs to be. Vance customer service answered my question within an hour or so. Actually Mr. Vance did (small shop). I get 1.5" minimum, plus and additional 4" of adjustable "up". So the jack plate will raise the motor between 1.5" and 5.5" I think it will work just fine.

I am ordering the JPL 4300 when they open up tomorrow. Good price, looks like a good product, responsive customer service, made in USA by a small business. They get my money and a plug on this site.

Will post pics when i get it and also when installed on the boat.

Thank you all so much for your replies to my questions.

Good to hear about the responsive customer service. Worth it's weight in gold these days.

Gonna post their site to the links section for the next guy who needs them.

Here are the promised pictures of the bracket and installed on the boat. I put the motor on the highest holes and set the jackplate 0.5" up. That puts the ventilation plate about 0.5" above the keel. I can move it up another 3.5" if needed.