Bracket theory
 

From what I can tell, all the (flotation) brackets all have centrally mounted flotation volumes. And no flotation out at the edges.

It seems like at rest, some flotation under the wide swim platform would help lift the outboard out of the water even more. Why isn't this done? Is the issue making the boat roll in a sea that was coming at an angle to the stern?

Or is there some other reason, like practical construction issues, or something else?

Most of us will have trim tabs that are in the way. Also the shape of the hull rises as you move outboard so it would not add a whole lot of flotation. Once in a while you will see folks that have added large flotation/storage boxes on the outer transom edges of their 20 master anglers. The motor is transom mounted....Their trim tabs are mounted inboard to those boxes. Finally it would look ugly having a bracket that looked like that.

strick

Bracket Theory
 

Don Herman told me that Potter set the Hermco bracket tub width to leave room for a retractable swim ladder under one side of the swim platform. However he said very few folks are actually installing such swim ladders (but I did!), so he's thinking about making the tub wider, taking it out to line up with the second step to accommodate the heavy 4-stroke motors.

Bracket Theory
 

Correct Denny!, I did also

Thanks. I think I will weld on some stepped side flotation to my bracket before I install it. Maybe not out to the edge, though. That's why I am drawing up the transom now, so I can fold some more aluminum.

So far this is what I am thinking of. I still have to fab the stepped side boxes out of 5083 or 5086 and weld them in. That's a 20" motor (I think- top of mount, to anti ventilation plate) on a 20 foot hull transom. I may more accurately model the Master Angler transom later.

nice drawing!!

Yeah bitching drawing...

Since we are talking theory? If the second strake is so very near the surface
 

is there much floatation Volume compared to the extra weight of the wider bracket? Or are we going to extend the bracket to the full width of the transom and have a lengthened stepped boat hull instead. We could also bolt a piece of styrofoam onto the top of the trim tabs and maybe add a little floatation to the heavy stainless trim tabs. What about the loss of floatation because some brackets are pitching up away from the plane of the bottom too much?

If I manage to get the height of the bracket right, then the extra volume was worth another 140 lb of flotation, if I remember correctly. On a 6 foot wide bracket with 30" setback, the extra "boxes" leave ~8" of width on each side for trim tabs.

The story is: I bought someone else's custom aluminum bracket and I want to modify it for more flotation. He seems to be a very talented metal fabricator/artist.

The bracket needs a little work with the bridgeport and plasma cutter first, it weighs a bit more than I like at about 135 pounds and is probably overkill, with 1/2" plate in places, 1/4" everywhere else. I think I can add the boxes on the side and get it down around 105-110 lb dry. I don't think I can modify it to be a lot lighter (if at all) than a fiberglass bracket, but it is what I have. Maybe with a design from scratch with some thinner sheet, but I am not doing that. Plus a thinner wall makes it more susceptible to failure when it eventually does corrode.

I think with the steps, it should still be far enough in height from the planing surface of the standard hull to be dry when on plane.

That dont look too bad at all!

strick

Bly I agree with what you said about measuring for submerged volume, you have to sink them deep in the water especially when they have a hard angle up to the motor mounting pad. The only advantage I still see with aluminum brackets is the weight, but its gotten much closer with new materials. I want to say a single chamber 30" Armstrong weighs 45lbs. I want to say a double weighs 65. I could be way off but it rings a bell, I know mine is like a feather so I believe 45lbs.

Very cool drawing. I hope you found the 3D image of the Etec somewhere online. If you developed it yourself, you have WAY too much time on your hands :D

Since we're talking theory, here's another thing to think about. Boat balance at rest depends on the location of the CG relative to the center of buoyancy (CB). Once you're on plane, balance depends on CG location relative to the center of dynamic lift (CL). If you add buoyancy chambers, the added weight will move the CG slightly aft. The extra buoyancy will move the CB slightly aft. But they don't add running surface, so the CL stays the same. So you get the stern up in displacement mode, but you weigh it down when you're on plane.

Now this might not make enough difference to worry about. If you're adding 100 lbs of buoyancy and only 10 lbs of weight, it may be a good idea. But if you're adding 20 lbs. of weight to gain 50 lbs of buoyancy, it may not be worth the effort.
Dave

Informative posts....but just curious. How much positive floatation will a single floatation 30" bracket provide? 100-200lbs of buouancy? Thanks, Steve

Bracket theory & motor height
 

Fish, NICE CAD drawing/solid model!

FYI - I don't know how high you have the motor mounted in your model, but based on my experience, I'm not sure the often quoted formula for setting motor height on a bracket (raise it 1" for each foot of setback) is correct. Distance from bottom of hull to cavitation/AV plate is 3.5" in this picture.
http://i188.photobucket.com/albums/z...plthtclsup.jpg

With motor in this location, the AV plate was UNDERWATER when running on plane. BRP says it should be ABOVE the water! I had Don Herman redrill the motor mount holes as high as possible which allowed me to raise motor another inch. Handling is still fine at this point; can make hard turns at high speed with motor in normal trim position with no blowout or prop ventilation. However the AV plate is still not out of the water, so I think it needs to come up about another inch. For my 30" setback bracket, it therefore looks like the AV plate height from bottom want's to be in the ballpark of 4.5 - 5".

You mentioned a using 20" motor. Most brackets are designed for a 25" motor, to get the power head as far above water as possible. If you go with the V-4 E-Tec as shown, the slightly higher weight of the 25" motor should not be an issue. Denny

I believe the double bracket for a single "30 motor has 1000 lbs of bouancy to the water line.

Flotation
 

I believe 1 cubic foot or of air /space provides roughly 62.4 Lbs of flotation.

I am a big proponent of brackets but 1000 to 2000 lbs of floatation?
 

I even made my own mold off a seidleman sea craft bracket which is for a twin engine and I very much doubt that I could get enough bracket below the water line to even get 300 LBs of floatation at the most. If that much? What it does do is off set the engine weight which is much farther back from most of the real boat floatation. That is all! Now what it does do great is if you remove a V8 inboard and an IO outdrive. Also water logged engine beds. Yes most engines were lag bolted into wood under the glass. I have had these so heavy that I did not know it could be made to get that water logged. Then the transom and regular stringers can be made lighter also, sometimes.

I am doing a 20 ft potter right now that will have a much lighter transom but the bracket will be integral to the boats hull and new added stringers.

I did a little math, and the software did a lot just now.

With the side boxes, it looks like I net 400 lb of flotation if I foam the interior with 2.2 lb/ cu. ft. foam. That is assuming the swim platform edge is touching the water, so the platform is 2" above the water. Another inch lower (higher water) gets about 60 more pounds of flotation.

I have a 20" Yamaha F100 which is 366 lb, plus steering (going hydraulic), plus a 58 lb kicker. Not sure on keeping the kicker, or trying to transom mount it thru the bracket (which is why I am drawing all this up). It is a 30" 6hp Tohatsu SailPro, and I want to see what I can do without making an abomination.

So far, I am having a hard time finding the shafts to extend the motor to 25". Bay Mfg has the casting, but not the shafts.

As I said before, the side boxes are good for about 140 lb net of flotation submerged, as best as I can tell.

I cannot take credit for the CAD model of the motor, that's an internet find. I still need to radius the outer corners of the bracket in real life and in CAD.

Thanks for all the encouraging comments and helpful information. I still need to figure out what height to set this at. I wonder if someone else could share the height or the swim platform from the keel or the chine?

Also I want to make sure I don't make something that floats well, but has poor handling. I already have the fuel tank and batteries in the forward half of the center console. We will see. I may vacuum bag- wet lay up a new fuel tank access hatch to get another 10+ lb out of that. Mine is very resin rich. The new forward seat is going to be a 150 qt cooler. I hope that gets the CG ok.

What makes you think that adding foam to the interior of the bracket will give you more flotation? All it does is add more weight to the bracket. If your bracket is water tight then there is no need for foam!

strick

Dito! No Foam...small amounts of water does find its way on mine. Would hate to have foam in there soaking it up.

Yep, the foam is 20lb. I just worry what happens with a leak. Then you have an additional 100lb on the transom, and the outboard 30" back. That thought is scary out on Buzzards Bay

I don't want to foam it. I might foam it inside polyethylene bags, so I can remove it if it doesn't leak.

Bracket theory
 

Fish, you might want to send a PM to Don Herman with some of your questions. As I mentioned in a previous post, the only reason Bill Potter did NOT make the tank wider on the Hermco bracket is to leave room under the swim platform for a ladder. Don is now looking at a wider-tank option because so many folks are going to the heavy 4 stroke motors. I think he was just going to extend the existing bottom surface and side of the tank all the way out to the second step. I don't think there is any need to follow the contour of the bottom on your bracket; if you eliminated that step, it would give you even more flotation!

I asked Carl Moesly one time what he thought about brackets, and he said "Why not just make the boat longer?!" That's an interesting concept . . . make the bracket bottom just an extension of the boat bottom, put the trim tabs on the back of the bracket and gain the advantages of a longer waterline length and planing surface! Sort of a "Euro transom addition" rather than a bracket!

As far as foam in the bracket is concerned, ANY water leaks are unacceptable, just like in the boat itself, so focus on building it right and eliminating any potential leaks!

I'll get you some height dimensions off my bracket and send them to you later today. Denny

take a look at the built in bracket on the 27' silverhawk boat.

27 silverhawk? or 23 sea craft with a 30 in fiberglass bracket? or the 23 ft
 

sea craft stretched original hull to 25 and then a 30 inch fiberglass bracket? This will only confuse the original question of an aluminum bracket for a 20 ft sea craft that he wants to widen, Bob Seidlemann did not work with a 20 sea craft when he was alive. It was a 23 ft when sea craft company was closed up before 1995. His main claim to fame on this as he invited me there to explain while doing it was raise the original floor. and nicer and new hatch openings.The original poster is wanting to widen a single aluminum bracket for more floatation to the second lifting strake instead of the first. I believe there will be almost No floatation effect because of how little water it will be in. I have my own fiberglass mold I made from one of those seidlemann brackets for a twin engine sea craft hull shape. I am doing a 20 ft sea craft now and will have to make my bracket smaller because it is way to big. He also wants to fill the aluminum bracket with 2 lb floatation foam? I can not tell you how much and how heavy that so called closed cell foam is and how many boats I have had to remove it from. To the Original Poster If you are that worried. And I was very worried on my fist bracket that I made for my personal 23 ft formula. I would in your situation install a high water alarm in the bracket. Now at the lowest point of the bracket but above the internal flange drill and mount a 1 in diameter cheap nylon through hull with a ball valve. leave it open and run the wire through it to the float switch for the high water alarm. If you ever needed to close the valve rip the wires through and close the valve. If you do get some water in the bracket it will drain into the bilge and be pumped out by your normal bilge pump. Do not fill a bolted on aluminum or fiberglass bracket with floatation foam. Very bad Idea. Becareful about putting to much bracket on a small boat. It may be too much

So I can't do some of the mods to the 30" bracket I wanted with a Bridgeport- it won't fit. So I decided to look at making it into a 24" bracket with a plasma cutter and TIG torch. I modeled it up and it looks like about 430 lb flotation, less 105 for the bracket mass if I include the side tanks.

Below are illustrations of the bracket placement with a 20" Yamaha F100 motor. It might be a little low?

are you planning for trim tabs?

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.

My strategy is that with a rather tall fuel tank in the fore part of the center console with the batteries in the center console, and the fact that I sawed off the console seat and will use a ~100 quart cooler there for a seat- that will pull the CG forward enough to keep from porpoising.

All of this is done, except for the seat/ cooler- that is in progress. There is no under floor fuel tank now, just ~50 gallons of removable foam insert.

All this might not be enough, but I think the CG was a bit forward of optimal this year, even with the kicker (but no bracket). It was very easy to push the bow down forcefully into the water on plane with just motor trim. So my hope is that fore- aft CG will be not require trim tabs. There were a few times I wanted them this year to correct roll.

If you arn't, you should be! Leave room for a set of Volvo QL tabs.

I would not shorten that bracket any, here is a picture of the clearance between my transom and 225 oceanpro, tilted all the way up. Its mounted on the lowest hole right tight to the top of the bracket. I may have an issue if I have to bump it up a hole, I dont want to think about what would happen if the bracket was 3in shorter!

http://i612.photobucket.com/albums/t...b/PB210163.jpg

http://i612.photobucket.com/albums/t...b/PB210162.jpg

I will think about tabs, but I can't justify a 30" bracket on a 20' boat with a 100hp outboard. I modeled up the Yamaha and it has 2-3" clearance when it is tilted fully up. The extra 6" of length on the bracket is just working against me when I am on plane, so I think I will chop it to 24"

http://www.hermco.net/misc_pics/DSC02812.png
You could make a 20" setback bracket and TH Marine makes a 4" offset jack plate that would solve any tilt issues.

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.

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

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'.

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.

Any one know how short I can go with an Optimax 150 hp.
 

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

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:

http://i155.photobucket.com/albums/s...oncc/Brack.jpg

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.

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.

http://www.wellcraftv20.com/gallery/...2/DSCN7016.jpg

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"!

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