Pad Design

Maybe you guys can help me…

The boat is question is a 1300 lbs. 20’ v-bottom with no pad, bass boat with a Hi-perf outboard. How would you go about determining the dimensions for a new pad?

Most of the boats in this category are using pads from 10.5”-14” wide by 6’ to 8’ in length (measured from transom to bow), and running anywhere from 75 to more than 100 mph. The lighter faster boats are using the smaller pad size.

The following are my assumptions:

A wider pad will be more efficient compared to a narrow pad at the expense of ride.
A wider pad will also be faster to plane compared to a narrow pad.
If you had a boat perfectly set up for top speed using a 10” wide pad, how much slower would that same boat be using a 14” wide pad. Would the extra drag of the wider pad be very substantial assuming a 4-degree angle of attack?
Finally how would the length of the pad change the dynamics of the boat?

Sorry about the amount of questions…it just very difficult to find any documentation on this subject.

[Nomad]

It will also balance the boat better, give a better running angle (wich can add to the speed and ride comfort). I will ponder on this a little bit more for you this is just some stuff off the top of my head. Good Luck with the boat though.

Does anyone have a formula to find out the approx. surface area which would be in the water given:
-hull speed
-total weight
-deadrise angle

Well guest number two…looks like we may be out of luck in finding a nice little formula to predict top speed and drag.

What has helped me the most is taking boats of similar weight, size and horse power and comparing the performance characteristics of the various sized pads, and using that information to predict my own boats performance. In other words copy someone else’s design…not very original but probably fairly accurate.

I don’t have an engineering background but the formula to predict top speed using these simple factors cannot be that hard…or maybe I’m missing something?

564 Views and two responses?

I thought this board was geared towards boat design…don’t tell me no one here knows how to design a pad!

Come on, even uniformed responses are welcomed...

[Schoonertack]

If you just want a rule of thumb, take the asumed boat weight, the planed horespower, calculate the correct prop diameter, and use the diameter as your pad width. forgot where I saw that one but it seems right to me.

[Jimboat]

Yup. Check out software for catamaran/tunnel hull and modified vee (with pads) design. Also Secrets of Tunnel Boat Design book - it also has good info including formulae for lift, drag, wetted surface, coeff's and velocity and stability prediction.

[KCook]

Getting the size and angle of attack right would be a concern. Seems the pad on some bass boats has contributed to instability like chine walk. Don't know of any formulas. There is always trial-and-error

Kelly Cook

[Schoonertack]

Instability generally would be the result of an overly large pad, and geometry of the aft hull that would sugest camber.

[gonzo]

There is plenty of documentation on hydrodynamics. It seems like you want an exact answer. For that it is necessary for you to give exact dimensions and specs. For example the bottom design, the weight distribution, structural design, expected load and expected sea conditions. Boats are complicated dynamic systems and the information on your boat is not enough for a detailed answer. That is probably why there where so many views but few answers. Can you post the rest of the info?