Bracket theory

[hermco]

You could make a 20" setback bracket and TH Marine makes a 4" offset jack plate that would solve any tilt issues.

[FishStretcher]

I did some stress analysis on a 24" bracket with (importantly) a center rib in the flotation volume.

The center rib helps a lot.

The loading is for 5 Gs. (not 10) Plus full thrust from a 100 hp outboard. Something sort of like the loading you might see landing after skipping off the top of a wave.

The deformed shape is magnified 50 fold so you can see what happens.

A rib that goes from the real transom to about halfway between the lower two outboard mounting holes seems to be an important structural feature.

Yield strength for this aluminum is ~180 MPa, but repeated stresses above 50 MPa would be a bad thing, in very general terms. I expect the loading from the outboard holes to be much lower, as the stress is distributed along the casting, not just inside the bolt holes.

[Bushwacker]

Originally Posted by FishStretcher

I did some stress analysis on a 24" bracket with (importantly) a center rib in the flotation volume.

The center rib helps a lot. . . .

Nice analysis! FYI, Don's bracket has a center rib much like you've shown but it appears to be the full height of the tub. Most structural problems begin at stress concentrations, and there are some concentrated stresses on Don's bracket (as indicated by gel coat cracks) right at the 90 degree concave corners of the swim platform where it extends back to the motor mounting surface. Gel coat is so brittle that it's just cosmetic and I'm not concerned about it, but he's looking at adding a 45 degree gusset there to spread out the loads in that area and eliminate those cracks. Most aluminum is much more ductile than gel coat, so you shouldn't have a problem, but if you're concerned about stresses, that's one area I'd pay close attention to and make sure you have large radii and no sharp edges or other stress risers in that general area. Denny

[workinpr0gress]

FishStretcher what is the est. weight of your model. I like the center brace, hard to believe it would be necessary but it is a nice touch.

hermco I love the short bracket & jackplate, my plan when my transom goes is to shorten my Armstrong to 18", and widen it a few inches give or take. Then its getting hydraulic jackplate. Btw, I'm curious what would you estimated the weight of one of your typical brackets at 18" that you would hang on a 20'.

[FishStretcher]

The computer says ~110 lb, depending on how much welding wire is in it. That's for a 24" setback and a 3/16" center rib and transom interface. (The part that bolts to the real transom. the rest is 1/4" or 1/2" on the O/B mount plate.) I extended the center 3" rib higher and it didn't add much in terms of performance. But I suspect that if it went all the way to the top (and thru the swim platform) it might help. Maybe I can get a FEA run with that feature in some lunchtime this week. But I think I am narrowing in on a solution- additional returns in stiffness/strength are costly in terms of material or mass.

If this were fiberglass and as thin as this construction is in aluminum (they aren't made that way- they are usually lots thicker) then yellow and green colors should be avoided. But the stresses that hit the bolted joint from the bracket to the boat transom are at a plywood/fiberglass to aluminum joint, so repeated pounding might be problematic. Especially if there isn't a lot of gap filling/ stress spreading material in the joint, like 3M 5200 compound might do. Without that, you might see compressive failure/loosening/crush damage at that joint?

The more I think about it, fiberglass is the way to go, if you can build it right and get the 2 halves to share the load well. I just don't have that construction technique figured out yet. And I own an aluminum bracket that I am waiting to install.

[bly]

I roughly measured one of these big boys in a boat yard that was tilted on a normal transom and It looked like it needs 28 inches? I went to the show room where the engine is sitting and it is huge! I got a spec sheet from the dealer and the height measurements do not help with knowing where it farthest point forward is tilted up and forward

[thehermit]

That’s a pretty cool analysis you got going here.

Like Bushwacker said...the stress points on the 2 piece glass bracket seem to be slightly different. On mine it is here:



I have a lot of glass there but i do get cracks in the paint. This bracket has a plywood core...and over time I expect more flex there. I would like to see what Hermco is thinking for a gusset.

[Bigshrimpin]

Originally Posted by FishStretcher

Not planning for them, no. There is ~8.25 inches outside of each flotation box to the edge of the swim platform. So if I had to, I might be able to squeeze some in.

Fish - You need tabs if you are going to put a bracket on the 20. Have run with them and without them . . . I am telling you you need them Lose those boxes on the side of the bracket and leave room for tabs.

25" motor on a bracket is fine. You can see how high the motor sits out of the water. I had a 26" setback on my seafari bracket and I had 2 - 3" of room to tilt up a merc v6 cowling. Hope this helps.

[Bushwacker]

Originally Posted by thehermit

That’s a pretty cool analysis you got going here.

Like Bushwacker said...the stress points on the 2 piece glass bracket seem to be slightly different. On mine it is here:



I have a lot of glass there but i do get cracks in the paint. This bracket has a plywood core...and over time I expect more flex there. I would like to see what Hermco is thinking for a gusset.

The hairline cracks in my bracket are exactly where you drew that circle, in the center of the radius, running vertically up and over into the horizontal radius between the top surface of swim platform and forward face of the engine mount area. The cracks have been there for a long time and haven't grown, so I don't expect any problems from them.

I believe the gusset Don is thinking about would extend the swim platform to fill in the area in the circle. He would add material that runs from the aft face of the engine mount surface to the aft face of the swim platform, intersecting it a 45 degree angle instead of the current 90 degree angle. I believe this would spread out the load and eliminate the stress concentration that typically occurs in a 90 degree corner.

On a related note, when we wanted to know where to put strain gages on an experimental part at Pratt & Whitney 20 years ago, we often didn't have the fancy 3D models because they were very expensive back then. Our lab guys would spray the part with something called "Stress Coat", which was nothing but a very brittle lacquer. We would then apply a load to the part, and where the stress coat cracked is where we would lay the strain gages! That stuff was so sensitive that you couldn't even take a coated part out of the air conditioned lab because just a small temperature change would crack it!

I view gel coat as simply a slightly less sensitive version of "Stress Coat"!

[workinpr0gress]

Bly, do they give a trim/tilt range in deg.?