Lateral Hunting/Wandering on plane

Seeing this is a boat that is quite common, it should be a commonly reported problem with them. The plans provider would have heard about it, or it should have been mentioned on their forum. Unless there is something peculiar about the set-up here, there has to be a clue in what was said about the non-standard skeg making it worse.

 

I think the most significant detail is that the behavior only occurs in extremely calm water. Sounds like lamenar flow transitioning to turbulent flow on one side or the other. Since the transition point is infinitesimally dependant on intial conditions, it will inevitably occur asymmetrically. In even slightly turbulent water fully laminar flow can't occur at all, so the phenomenon disappears.

 

Thanks all.
The family is normally much further aft (except when tending to cleats, stowing the anchor, or trying to prove to their brother that they did NOT steal his root beer).
I agree that a lower prop position might ameliorate some of the issue, I'm just unwilling to do it because I'd have to fill in the bolt holes with epoxy (again) and cut the transom down as an experiment. The boat runs very well in 95% of conditions, this particular problem seemed interesting and I kept racking my brain thinking about it, so I thought I'd consult the oracle(s) here. I can envision how the transverse line would cause laminar flow separation, just as a means of temporarily proving or disproving the asymmetric detachment theory.

 

Ok, this is my view regarding the course instability in flat water.

 

Baeckmo, OP says above.
Your diagram doesn't show what he describes.

 

...and exactly how do you pinpoint the location of the pivoting point irl then?

 

If it's a flow transition issue, or a reattachment issue, the rope is revealing either way. It seems small vortex generators just where the hull meets the water forward during planing in flat conditions should prevent it.

 

You don't need to pinpoint the exact location.
There is a huge difference between bow pivoting and stern pivoting.
Never mind, just try and help the OP.

 

I'm with baeckmo here. I just want to add that as long as this is only a problem when it is glassy smooth on the surface, you don't need to fix anything. This typically happens when you enter a sheltered creek from a bay, or run in the lee of a bank near shore. This is why power trim and steering wheels are nice in small skiffs. It is very common for best speed and best economy to be at the limit of directional stability. If you can hit 18 knots in a 14 foot boat with 20 hp, then you can pitch the bow up to get less of a slidey feel, but at lower fuel economy. Or you can just accept that this is sort of normal and carry on.

 

OK, so if I understand baeckmo correctly, he is saying that water is attaching to the SIDES of the boat, aft, asymmetrically, depending on the initial turn direction, that should be easy enough to confirm visually, I think that should be the next step, investigate that.

 

I have this same thing happen in my 16' Lund skiff with a 50hp engine. The tiller seems to be almost loose, but it is not.

The boat is trying to decide between two options, like water passing just to the left or just to the right of skegs or bottom strakes. Once it changes it is steady, or you can play it back and forth some.

I have never noticed this on other boats, but I think you would not sense it in remote steering as @philSweet mentions.

For the sake of discussion, what would happen if you put a 1/2" metal rod right down say like 6" in the middle of the hull? Would it behave more or less in this fashion? I think more or horrible even. As you move the tiller left or right, the water would part oddly there for a moment until you run straighter...it is a thought experiment is all..

 

I think he is 100% correct. But the cause may be any number of things.

 

Logically, you would observe what is actually happening, by using a couple of children, one each side, looking over the sides aft, to report whether the sides are staying dry, in gentle turns anyway.

 

If this reattachment thing is the explanation, it should be worse on flat bottomed skiffs, and especially those with parallel sides aft. I will await the report from the observers !