Bracket Position

[dsherod]

I was looking at some of the "bracket" posts on the site and there's one thing that keeps bothering me. I consistently see the brackets, with respect to the hull is higher leaving about 5 inches from the riding surface of the hull to the riding surface of the bracket.

This, at planing speeds, will create a low pressure area below the bracket and forward of the motors causing the reverse of lift and driving the hull down in the water slightly. This can be overcome by running trim tabs down to bring the aft part of the hull up and the bow down, but at a penalty of increased drag.

It also destabilizes the laminar flow of water over the hull just prior to it entering the props which I can't imagine would have a positive performance result.

Since this is consistent with all of the bracket installations I have seen, I must be missing something. I know many of you have done the bracket thing. Can any of you explain why they are done this way?

[FELLOW-SHIP]

Boy Oh Boy that’s a tuff one I think I will refer this question to Spidercrack our resident hydraulic engineer he would know this stuff.



Ha Spider what do you think????


FellowShip

_______________________________________________

My motto: Just for the Grins

[other tom]

Bracket Theory 101.....The water exits the hull at the original transom line. The bracket bottom is entirely dry at speed, and is not a running surface. (it may get wet, but a properly designed installation will NOT result in the bottom of the bracket becoming a running surface). I have seen several rigs that resulted in the far aft edge of the bracket "touching down" so to speak....but that is not the way the gurus would design it. Reducing the length of the running surface decreases drag (less wetted surface) but at lower speeds, the buoyancy of the bracket (that is now submerged) helps offset the distance aft the engine is now placed (compared to the transom mount scenario). So, you have a longer boat at displacement speed, a bigger boat (engine out of the cockpit), and a faster boat (less drag from hull bottom, and less lower unit drag from having the prop work in cleaner, higher water.) I didn't pay too much attention in all those naval arch and fluid dynamics classes, but I did stay at a Holiday Inn........
once or twice.

Remember, the original "bracket" was simply a framework "cage" that was intended to move some big huge racing Black Max Mercuries up and aft....

[dsherod]

OK, that makes sense now. Thanks for the explanation. So, the best designs would have the aft end of the bracket just off the water at planing speeds and the motors in the water just enough to not cavitate. Sounds very efficient.

I'm assuming the cantilever angle of the bracket is primarily to counteract the fulcrum while trailering.

[hermco]

I'm assuming the cantilever angle of the bracket is primarily to counteract the fulcrum while trailering.

Huh?

[dsherod]

I'll have to draw a mental picture in the absence of a real one.

Standing from the side of the vessel looking at the bracket. Draw a line parallel to the keel, and another from "false keel" of the bracket. Most designs I see would create an inside angle between these two lines of about 30 degrees.

This angle would cause the "moment" of the motors extended past the transom to deflect some of the force into the strongest part of the transom where the transom meets the hull acting as a brace when flotation is not supporting the weight of the motor(s).

If true, I would conclude, that the shorter the set back and/or the greater this angle the more weight can be supported during trailering.

[hermco]

Huh.

[Bigshrimpin]

I'll have to draw a mental picture in the absence of a real one.

Standing from the side of the vessel looking at the bracket. Draw a line parallel to the keel, and another from "false keel" of the bracket. Most designs I see would create an inside angle between these two lines of about 30 degrees.

This angle would cause the "moment" of the motors extended past the transom to deflect some of the force into the strongest part of the transom where the transom meets the hull acting as a brace when flotation is not supporting the weight of the motor(s).

If true, I would conclude, that the shorter the set back and/or the greater this angle the more weight can be supported during trailering.

I think this is a non issue. 400lbs of motor on a 26" back bracket is not that much force. It's not a lever prying on one small spot . . . the load is distributed out on a pretty large surface area and slightly cushioned with a gasket of 3m 5200.

[doug7488]

From Physics 101 - yes, you are correct in your reasoning.

[hermco]

I would assume the reason for the angle is to minimize the ammount of material used. The first brackets were just welded pipe.