Handling odditties - what are your strategies?

[Salish_Explorer]

Finally got the CD16 back from the mechanic yesterday and took it out for its first run. A beatuiful day, nearly glass-calm water most of the way.

Right off the bat I was pleasantly surprised at her speed, but also experienced a couple of oddities, including what felt like an unpredictability and lack of self-correction on a plane: The boat would sometimes drop a rail and then just stay there.

To elaborate: On plane, every so often something would cause the boat not to turn, but just to heel and stay heeled [/i]while running in a straight line[/i]. Sometimes it was an odd set of waves, other times it was just one of us shifting our weight, but either way a rail would dig and then just stay dug, with the boat running at about a 20-30-degree list.

If I gave a steering input, especially a slight correction to the outside (ie port list - starboard helm input), the boat would flatten out, but if I didn't the boat would stay listed.

My brother was with me, and we grew up running speedboats and dhingies. So we aren't experts but I'd say we have a strong sense of how a boat 'should feel,' and this was extremely unnerving to both of us. The first couple times he shifted his weight so far uphill so fast that he basically climbed into my lap, and later referred to it as a 'brown shorts moment.'

Again, absolutely flat-calm day. Reducing power and letting the stern settle ever-so-slightly seemed to neutralize it.

Is this a normal effect of the flat bottom? Is there any way to minimize it?

Two other oddities: In my experience v-hulled powerboats bank/heel into turns extremely predictably. By comparison, the Dory would sometimes heel into a turn, but other times felt like it was drifting/sliding sideways. The overall unpredictability was nerve-wracking, and the side-slide especially was actually quite scary, as it felt like there was the real possibility of a roll if the outside rail dug.

Also, I noticed a fair bit of flex in the cockpit sole whenever we hit a chop on plane - like somebody shaking the wayer off a taught tarp.

Very interested in thoughts from others on any/all.

[Salish_Explorer]

After doing a bit of research (Which some would say I should have done before I posted (but hey)....

It sounds like what I experienced was "non-oscillatory dynamic roll instability."

Basically, especially with a hard-chined boat, a variety of things can cause enough down-force on the bow that the hull will roll sideways on a plane and end up stabilizing on one of its chines as if it were a V-hull - so rather than being 'pushed' back onto a flat plane (self-righting), it stabilizes on a listed plane. The solution is to shift center of gravity aft and/or trim up to create down-force on the stern.

Great write-up on it here https://www.boatdesign.net/threads/planing-on-the-chine.37851/ - described what I experienced to an absolute"T".


I think when I dropped her in, I just trimmed the motor all the way down without thinking about it. We're taking her out for a picnic run to either Blake or Deer tomorrow, and I'll defintiely be paying a lot more attention to the trim this time!

[ssobol]

In my 22 I have experienced the flat drift in turns. This only occurs when the water is very flat and the boat is going very fast. In this condition, the flat hull is skimming across the water sort of like a skipping rock. The only thing that is really holding the boat in the water is the motor leg. This is not really a safe condition because of the chance of a rapid upset. should you hit something or need to maneuver suddenly.

[T.R. Bauer]

Flat bottom boats have low (ish) speed limits. I won't give a hard fast rule with it because there are a bunch of variables....but it is mostly due to a ratio between the surface of the hull, weight, lift, and the coefficient of friction at a given speed. Anyway...whatever...nobody cares....lol...If you think a C-Dory drifts on the turns, try a flat bottom jet skiff! There are a real handful. The solution is simple....slow it down a bit and it goes away.

[Foggy]

Sort of elemental, Watson.

Whether it's your first tri-cycle, your first bike, boat, car, motorcycle,
water skis, etc the rule is: First learn control, then speed.

Not

[thataway]

You described the root of your problem. You had the bow trimmed down too far, Experiment with trim. Also you boat may do better with trim tabs.

I find the most of the C Dorys at times do better with both trim tabs and Permatrim.

If you trim severely bow down (motor all of the way down), you can evoke an unattended broach.

[Salish_Explorer]

[JDrew]

Our 16 cruiser is fine with just perma- trim, and being a cruiser, we cruise. Had her out this weekend in 2 footers up and down-sea, no worries either way, down- sea takes a little more attention to the helm but nothing major. We cruise with the 40 hp at slower on plane speeds, 3400-3800 rpm usually. Rarely go above 4100 with 2 people, just handles better, and it’s not a v- hull so it might take some time to learn the different feel.

[Salish_Explorer]

[chieftele]

And here I wish my 16 would go faster. With 2 people and an ice chest I’m lucky to get 20 mph, and it never feels sqirrely.

[Salish_Explorer]

[Hunkydory]

When we first purchased our CD22 in 2003 it had 40hp twin Honda’s & trim tabs, but no Permatrims. On a tight turn the prop on the outside of the turn would ventilate. I could prevent this by trimming the motors too far down, which would cause the bow to dig in & create problems as described by others. Adding Permatrims corrected the ventilation & steering problems. With the upgrade now to twin Honda 60’s, the Permatrims are way oversized, as they are made to fit Honda motors from 60hp to 150hp, so I have twice the permatrim surface as would be on a single Honda 115. Emergency tight turns can be made at high speed with no lack of control, especially if boat is properly trimmed. I’ve been in fairly rough seas & heavily loaded & kept complete steering control, even when surfing large close spaced waves.

Don’t know how my experience would relate to your CD16 other than hard to go wrong with either or both Permatrims & trim tabs as proper trimming is key to get the most out of these flatter bottomed boats.

[chieftele]

[Salish_Explorer]