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

Right off the bat I was pleasantly surprised at her speed, ....

Darwinian Awards, given posthumously, are testament that some
just don't get it.

Aye.
Grandpa used to say, "Different things behave differently.
Applied to people, appreciate good things happen slowly."

 

[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]

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.

Great advice. And that's why it made me immediately nervous yesterday - it was very clear what it could lead to, especially in any kind of following swell, or really even with a moderate wake from anywhere abeam.

I took it out again today, paid careful attention to trimming more nose up, and it was night and day.

I will say that the sweet spot between porpoising (too much up trim) and digging the bow (too much down trim) seems very narrow.

The solution is simple....slow it down a bit and it goes away.

It's interesting, I am definitely getting this impression the more I read here. Even on a flat calm day, it seems like the message is that these hulls just are going to handle and ride much better on a partial plane than a full plane, where they become somewhat unstable, and also start to pound.

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

Right off the bat I was pleasantly surprised at her speed, ....

Darwinian Awards, given posthumously, are testament that some
just don't get it.

Part of getting a feel for something is cautiously, carefully, taking it to its limits. A flat calm day actually seems like the exactly correct time for a boat owner to get a sense for how a new vessel handles at all throttle settings, how she turns, etc etc. Implying that briefly trying out full throttle on an outboard within a vessel's rating, on a flat day, is so absurdly wreckless that I might (and might deserve to) die from it seems.... a little much.

From your build photos it looks like your have a lot of knowledge to share. If you have something besides condescension to share I'm all ears.

 

[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]

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.

Very good benchmark to have.

With that 40 turning 34-3800, are you on a partial plane, with the bow still riding a little high and the transom still partially settled, or are you on a full plane with the bow and transom both up and the boat running close to flat?

 

[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]

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.

Interesting. What size engine are you running?

I've got an older four -stroke 50. With two adults, two young kids and a light cooler, as well as a 6hp kicker hanging off the back, the sweet spot seemed to be about 60-70% throttle, which gave me 17-18 kn through the water on about a three-quarter plane. That was with a sea state just above dead calm - maybe 4-6" gentle wind-waves. In rougher patches I had to immediately back off to 50%.

No electronics installed yet, so can't say what my RPM was.

 

[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]

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.

Interesting. What size engine are you running?

I've got an older four -stroke 50. With two adults, two young kids and a light cooler, as well as a 6hp kicker hanging off the back, the sweet spot seemed to be about 60-70% throttle, which gave me 17-18 kn through the water on about a three-quarter plane. That was with a sea state just above dead calm - maybe 4-6" gentle wind-waves. In rougher patches I had to immediately back off to 50%.

No electronics installed yet, so can't say what my RPM was.

I have a 2005 Honda 4 stroke 50 hp, that runs real good. I can’t remember the specs on my prop off hand, but I had the pitch reduced because I boat at higher elevation a lot. I get a little over 5k rpm at WOT when I’m at moderate elevation, which should indicate a good prop pitch according to what I understand. The boat gets up out of the hole and on plane pretty quick, which I like. I am curious what different props would do for performance though.
I am running a Cabelas aluminum trolling plate, which is supposed to perform like a hydrofoil (permatrim) while up, but I can’t help but wonder if it’s causing drag. It does seem to help with even small adjustments of trim, I can really dial in and it doesn’t porpoise at all.
I can’t complain about the handling, but it seems like other owners of 16’s can get closer to 30mph on smooth water.

 

[Salish_Explorer]

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.

That's actually great feedback. I have bought but not yet installed a doelphin, but after reading your experience and other I think I'm going to send it back and get a permatrim - about the same cost, but the latter sounds like it does a lot more for stability, steering and trim, VS just getting out of the hole faster.

One thing I do wonder about is I have seen a few people with some kind of adjustable permatrim situation, where the fin is hinged and there's an adjustable arm that runs from the back end of the fin up to a mount on the top of the lower unit, so you can adjust the angle that the prop wash is directed. Seems like probably overkill and/or not effective, but this discussion is making me curious about it.

I have a 2005 Honda 4 stroke 50 hp, that runs real good. I can’t remember the specs on my prop off hand, but I had the pitch reduced because I boat at higher elevation a lot. I get a little over 5k rpm at WOT when I’m at moderate elevation, which should indicate a good prop pitch according to what I understand. The boat gets up out of the hole and on plane pretty quick, which I like. I am curious what different props would do for performance though.
I am running a Cabelas aluminum trolling plate, which is supposed to perform like a hydrofoil (permatrim) while up, but I can’t help but wonder if it’s causing drag. It does seem to help with even small adjustments of trim, I can really dial in and it doesn’t porpoise at all.
I can’t complain about the handling, but it seems like other owners of 16’s can get closer to 30mph on smooth water.

From my limited experience I would say you definitely are having some kind of issue. I have basically the same vintage engine, and with the same amount of weight you describe (two adults and a cooler), I can pop up out of the hole and hit high 20's on flat water no problem. In anything but dead calm water the 50 will push the boat right to the edge of being unstable.

At ~60% throttle I'm still planing, running about 17 knots easily. And that doesn't change with weight - I camped last weekend with about 500 pounds of human in the boat and a full camping load, plus a kicker, anchor, and second gas tank- so call it 800 pounds total - and was still hitting those speeds easily.

Pitch is one potential culprit, as is drag. Problems with your mix, timing or compression will also rob an engine of power, but it sounds like you're not having trouble hitting normal RPM's, but rather you're not seeing those RPM's translated to power at the prop - so maybe you can rule those out.[/i]