Thinking about a design

[mudman]

I was thinking of a different hull design. A small planing hull (20' to 25')capable of shallow draft, offshore performance, and decent speed. I have seen the stepped hull, but was wondering what would happen if you inverted the stepped hull.

It seems to me that these bass boats have very little boat in the water to acheive speed. So if there were trim tabs placed 2 or 3 or 4 inches lower than the hull of the boat, would the hull ride on only these "trim tabs" and have the rest of the boat out of the water? Simmilar to a hydrofoil.

The hull could be designed as a standard deep V for offshore waves. It would also need to be wide wo that it only drafts 8 inches or so. Has this been tried before? Could it work?

[rasorinc]

I think you will find it difficult to have an 8" draft and still want to run in seas.
I cannot comment on a stepped hull as I have no experience with them.

[tom28571]

Hi mudman,

You have some incompatible ideas there. Shallow draft and deep V are two separate things. Even if you could lift a deep V out of the water at speed, it would be uncontrollable.

Not sure what an inverted step is.

Some of your idea can likely work and may already be used. The underneath trim tab is much like an interceptor. Search google for some of these features and study the answers a bit.

[mudman]

This is simplified, but you get the idea. You may be right that it will not be feasable on a larger vessel, but maybe a smaller craft? It probably has been tried, I have not seen it though. Does anyone know why?

[BWD]

Thanks for putting that idea up.
There are similar applications that have achieved some benefits under certain conditions.
The main one coming to mind is the "dynaplane" hull.
You might want to look it up on the forum or elsewhere.

Basically, the dynaplane (in my limited understanding of it) is a combination of a hydrofoil and a stepped hull, with the hull area immediately in front of the step functioning more like the high pressure face of a supercavitating hydrofoil. My understanding is that it did work, but didn't offer great advantages over other boats, though the concept may have potential the built boats did not match.

The main thing that jumps out about the drawing you put up, is that the step or wedge is way aft of where you would expect the boat's center of gravity. In that position, I think it would only achieve the kind of effect you want at extraordinary speeds.

There is another thing to look up, called an interceptor, that can have similar function.
As far as I know, none of these devices really dramatically changes the way a hull behaves. They just affect trim or increase lift in ways that are useful, including possible modest increases in speed or efficiency.

By and large a bass boat shape with any of these devices will behave as a bass boat, and an offshore shape will behave in its own way.

Sometimes an individual boat may dramatically improve its behaviour when modified, making for interesting stories, but that is usually because there was something wrong with it in the first place!

For example, I had a small outboard skiff that porpoised and wouldn't quite go straight, no matter how it was set up. After adding a fin to the outboard, and powerwashing one side of the hull, now it goes perfectly, with some barnacles on one side! I could claim the outboard fin was a magic invention, or that all hulls should have "speed bumps" or dimples, or something, but that would be false.

The only reason I saw a big improvement, was because I had a big problem, and the hull's behavior needed to be tweaked just enough to make it more comfortable to drive. Still the same little boat, though, takes green water going slow in chop, and pounds going fast!