Planing speed and pad bottoms

Is there a general rule of thumb regarding at what speed a pad bottom starts to show a benefit? I realize it probably changes with weight, beam and length, deadrise.
And do the benefits of a pad bottom decrease the more the deadrise decreases?
I’m not building a boat but I’ve wondered why pad bottoms are not used more in small-medium planing hulls.
Are there major drawbacks to pad bottoms?
Thanks

 

Are you refering to an area of the hull with an undercut at the aft end and a flat section? Those are usually to be able to run a propeller higher and have less draft. If it is about the edge of the keel having a flat surface, that promotes earlier planing with less power. However, at higher speeds the hull will be unstable because it will be running on a narrow flat surface. At what speed it is beneficial or detrimental depends on many variables that have to be considered globally within the design.

 

I was wondering about the keel having a flat running surface. The instability at higher speeds is interesting. I guess if you’re trying to really fast it’s a trade off.
But would a pad bottom benefit a boat with a top speed in the 30-50 mph range?
And would a pad bottoms benefit diminish as a given deadrise became flatter?

 

It is too broad of a question. A boat design is global. It may benefit some, be indifferent in some, and be detrimental to others.

 

67-LS1 - The main benefit of a vee-pad is the higher lift generated by a low-deadrise (flat) planing surface, as compared to the higher deadrise (deep-vee) surfaces. The benefits of a vee-pad depend on the hull design, and in particular, the amount of vee deadrise. Higher deadrise vee hulls can benefit more from a pad than lower deadrise vee hulls.

Optimization of vee-pad dimensions are based on unique boat weight, hull configuration, power and speed requirements. Vee-pad characteristics can then be established, including: pad width, pad length, pad shape, pad deadrise, pad height, pad angle, notch. Some boats can benefit from a vee-pad while others won't see any improvement. Vee-pad hulls are also sensitive to power and weight.

There is a corresponding sacrifice of some softness in ride, this modified vee-type hull design allows for potentially increased speeds.

Here's an article on vee-pad optimization. Send me a PM or email and i'll send you a copy of this article on "Vee Pad Sizing"

 

Good question! In general, a bottom pad really starts to show its advantages around 60 mph, especially on light shells. But yes, the weaker the deadrise, the less the pad brings, and it can make driving more unstable at low speeds. Maybe that's why we don't see it so much on medium hulls.

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