Bracket theory

[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