Re-plating the bottom & losing the keel

[jgdyer]

31' aluminum power boat ... See attachment ...

Used to be inboard powered, now outboard powered ...

We are rebuilding the bottom, and now we have an opportunity to make some changes ...

So, I was thinking: What if we did away with the keel ...

And install a 1/4" thick girder up the middle on the INSIDE ... Say 8" tall or so ..

And join the new hull plates at the center ... Maybe plate-to-plate ... or plate-to-girder ....

This is, for instance, how Steven Pollard at SpecMar builds his offshore aluminum boats, albeit with a LOT more deadrise than my boat has ...

What I imagined that might do ...

We no longer have to worry about what happens if the keel refuses to stay straight when we weld it in, and the effect that might have on steering ...

The keel is a major hull strength element in the original design, except that it tapers towards the bow, lending almost no strength forward ... We can run the girder as far forward as we like ...

The keel's function was tied to certain aspects of the boat that are no longer in place ...

The keel provided stability and roll-reduction - Now, however, we have two outboard motors with skegs and fat profiles stabilizing the hull .. Admittedly, this is AFT only .. Doesn't anchor the boat forward ...

However, we're replacing the half-pipe at the edge of the bottom with angle, forming a harder chine .. Giving us more bite on turns ... and reducing, perhaps, the boats tendency to roll ...

The other function of the keel was to protect the inboard shafts, props and rudders during a hard grounding ... Also, not an issue with the boat in its current configuration ...

So, I'm wondering if we don't NEED the keel anymore ... And maybe we would be better off without the drag or the construction issues that come with it ...

And, if sea trials demonstrate we need more lateral stability, we could weld some fins on the bottom ...

Alternatively, if we are concerned about stability right now, because the bottom is kind of round and flat ... we could allow the girder to pierce the bottom, like the keel does now, only maybe not as deep, and run it farther forward, with most of it still on the inside, to get the stiffness we want ...

If we do that, our center foil, if you will, isn't 3/8" thick any more, and probably not as deep ... which we could argue has pluses and minuses to offer ...

But, keep in mind, the original keel TAPERS ... It has less and less influence on rolling as it moves from midships forward ... because there is progressively less of it in the water ...

Any thoughts out there on this topic?

[TeddyDiver]

Never mind the rolling, not so much issue with hard bilges, but there's another matter... without keel or skegs it will have a considerable amount of sliding. I can't excatly describe it but trying to turn a boat with such tendency makes it turn sideways... and continue the same course as before. Eventually the forward bilge will "bite" and everything not tied fast will be flying around. In the worst case the boat might capsize... you can imagine..
so that's my 2c warning

[jgdyer]

I agree, and I've been thinking about that ... However, as now built, with keel, the boat has a little too much traction ... It doesn't particularly like to turn ... And the keel isn't really that big ... Only 8" at its deepest near aft ... I was thinking I might let the girder poke through 3" for about 4' just aft of midships ( where the keel is now ) and see what happens ...

More images follow ...

Thanks!

[gonzo]

Well, with a metal boat you can always weld a keel on later. I say, go for it.