Bracket theory

[strick]

That dont look too bad at all!

strick

[workinpr0gress]

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.

[Blue_Heron]

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

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

[Blackfin26]

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

[Bushwacker]

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

[Finster]

Originally Posted by Blackfin26

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

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

[3rdday]

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

[bly]

Originally Posted by Finster

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

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.

[FishStretcher]

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.

Originally Posted by Blue_Heron

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

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

[strick]

Originally Posted by FishStretcher

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. .

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