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Effect of where deepest dish placement is along chine?

Effect of where deepest dish placement is along chine?
April 01, 2017 07:16PM
I can't find the thread, but i remember Sandy and tungsten discussing pushing the deepest part of the dish back further.

I can't remember why they do this.

I'm looking to float skinny water.

Dead center or further back?
Re: Effect of where deepest dish placement is along chine?
April 02, 2017 01:57AM
Where you dish out a chine the most tends to make that part of the bottom wider. It's conceptually like using a laser beam to chop and slice back the bottom of the boat. Because the sides have flare chopping/lopping the bottom of the boat off, so the actual bottom moves higher makes the bottom wider as you slice.

http://montana-riverboats.com/?page=Driftboats/Reducing-Rocker

Where you usually want the boat to be at its widest is below the boat's center of gravity.

Do you want a fly fishing boat with rower in the middle and two passengers spread far apart? Fore and aft?
Then make the dead center of the boat the widest, so it has a symmetrical bottom. So the rower sits right on top of the widest part of the bottom. Dishing out the chine in that same area not only makes the bottom wider there it flattens out the rocker profile.

For white water boats spreading the payload (the passengers) far apart reduces turning speed and it reduces side to side stability. For maximum speed and stability you want all the weight as close to the boat's center of gravity as possible. McKenzie River boats did that by putting two passengers side by side on the front seat, and then by moving the rower and the front seat back, so all three people (rower and side by side passengers) were like a triangle of weight squashed as close to the middle of the boat as possible. But that boat isn't symmetrical. With that boat you want more width under the double weight front seat, so the boat ends up somewhat pear-shaped, or like a Chinese soup spoon. That's the reason for the asymmetrical shape of the McKenzie River Boat.

With that boat the maximum dish in the chine would be forward of dead middle, closer to under the passenger's seat.

How much and exactly where to make the most dish might be (probably would be) mathematically calculable with hot rod hull design software like Rhino. I don't have it. I do boat design by thinking staring tinkering and experimenting instead. I don't always get it right. But I've done pretty well and my hulls have not been conservative and traditional.

At some point the new boat designer has to jump head first off the high diving board. At some point you have to take a chance.



Re: Effect of where deepest dish placement is along chine?
April 02, 2017 02:09PM
Dishing out a chine is conceptually similar to building a boat with an infamous "flat spot" in the middle.
Building a flat spot is like chopping a boat in half and stretching it apart with a rectangular box between, with a dead parallel chine section.

Dishing out a chine flattens out the dished area a little too but not completely. Not necessarily anyway, depending on how deep the dish. The goal is to flatten the rocker profile some but not completely, so the bottom profile still has the widest spot at the deepest part of the rocker profile, which is supposed to be positioned right below the boat's center of gravity. So the boat trims properly with both ends of the boat up and out of the water slightly when fully loaded.

A looooooooooooong and narrow boat with most of the weight way up front and way behind (less weight in the middle) will be hard to turn and side to side tippy too. Moving as much of the weight as close to the center of the boat as possible is the white water ideal.

http://montana-riverboats.com/?page=Driftboats/Reducing-Rocker

The Briggs is a venerable boat close to many hearts. But it has a weird history. Keith Steele and Jerry Briggs made the first Grand Canyon dories by chopping a 16' foot gunwale boat (what ever you call it) in half and stretching it out with a flat box in the middle. The Briggs is long and narrow (the originals were 48" inches wide) with passengers fore and aft.

Cyrus Happy (Ray's River Dories) has made some large (20' foot side panels) Deschutes River Boats that can take up to six passengers in two rows of three side by side passengers. All six passengers sit in front of the rower. All seven seats are positioned so they balance over the middle of the boat. His boats are enormously wide. 69" or so. I measured one with a tape. That puts the weight closer to the center of the boat than a Briggs (or Keith Steele's Susie Two).

I've rowed both the Briggs and a Susie Two and it isn't fun.

I rowed one of those Cyrus Happy boats, albeit alone, with no weight. But still. It was so wide it floated in 2" of unloaded water. Turned on a dime. Was easy to slow down. A Briggs with six passengers would be unrowable.

My decked boat, that I'm working on now is made with 18" foot side panels and it's 66" inches wide. Has lots of side flare but the bottom is dished out so the rocker profile is reasonable. I put the widest part of the bottom and the deepest part of the dished-out chine dead in the middle of the boat because I will also use this boat for fly fishing day trips. How I do white water seating with that boat is still up in the air. I'm thinking about it. And tinkering.

I'm almost finished reworking a ten year old 16' gunwale open boat. Two more days. Then I'll get back to work on the 18
foot gunwale decked boat..........with the 66" inch wide bottom.



Re: Effect of where deepest dish placement is along chine?
April 02, 2017 02:59PM
In other threads people have asked: "If a boat is super wide won't that put the oar handles up too high?"

No. Not necessarily. Longer oars and a higher rower's seat fix it right up. You can keep the passengers down low if you're worried about high center of gravity. A slightly higher rower's seat is way more than offset by the added stability of a wider boat. Longer oars balance nicely on a wider boat too, and the longer the oar the lower the handles. Longer oars boost the horse power too.

Another advantage of a rower's seat a few inches higher is visibility. It's more than handy when you can see over the tops of the passenger heads in front of you--for fishing and for picking a white water line.



Re: Effect of where deepest dish placement is along chine?
April 02, 2017 03:41PM
Wow! Lots of great info Sandy. All my panels are done. Time to dish and put them on frames.

I've been winging it. Using the BB as inspiration and for comparison. I think mine will end up looking kinds like a small adipose flow skiff.

My CAD help fell through so I'm just going for it.
Re: Effect of where deepest dish placement is along chine?
April 02, 2017 04:13PM
Please please please -- as James Brown once said -- keep us updated with photos. Cell phone pics are more than good enuf
Re: Effect of where deepest dish placement is along chine?
April 02, 2017 08:25PM
Yes what Tungsten said is true for the most part. But there are always exceptions.
My little one man boat has so much side flare I had to dish the chine considerably, even though it has square ends. After all that dishing it still has a lot of rocker. I like the radical side flare. I can get that little boat suddenly sideways to fast water and not much happens.

I'm going to take the one man Dayak on a 5-day Desolation Canyon Green River trip in early May. There is a good amount of snow this year and it's getting warm early, so it should be pretty close to peak high water. Maybe a week too soon but you have to go when the permit says. It should be fun. The Dayak is only 9' feet long but it has a 54" inch bottom. Pretty wide for such a small boat. I can drop the oars, stand up and walk around on top. Without falling in.






Jason's Skiff


Jason's Skiff is a very good shallow water boat. People love that boat. At a boat show on the Yellowstone about ten years ago I watched Jason's wife (can't remember her name) row one of those skiffs back upstream. I was impressed.




Re: Effect of where deepest dish placement is along chine?
April 02, 2017 08:48PM
The pic below is the Joe Hutch rapid on the Green. In Desolation Canyon in May. I'll be there next month. Can't wait.

That's Jeremy and Kai rowing Larry Hedrick's stretched out Honky Dory. This is an HD with a straight line chine. It keeps the rocker within bounds by not having much side flare.

I made the HD in 1986. What I'm thinking (and building) now is still similar, about this much length (18' foot gunwale) but wider and this time with lots of side flare. To get that side flare you have to dish the chine no matter what. Else you get ridiculous rocker.






Re: Effect of where deepest dish placement is along chine?
April 02, 2017 08:57PM
So compared to the BB, I flared the sides a bit more, widened the transoms, and I'm gonna chop 4" of the bottom. Trying to post see pics, but having some trouble. I'll see what i can do.
Re: Effect of where deepest dish placement is along chine?
April 02, 2017 08:59PM
Here's a Peter Mac Pram. I really like this one. You can't get dimensions for this boat. I used to correspond with Peter but we lost touch. I haven't heard from him in years. He once considered manufacturing this boat but decided to lose money some other way.

Although there are no offsets for this boat it wouldn't be too hard to imitate with some plywood test panels and a set of adjustable ribs. This pram has a straight line chine (no dishing). I think so anyway. It keeps the rocker under control with almost straight up and down sides (very little side flare) and, as Tungsten pointed out, wide square ends.

I'm guessing it has a 12' foot to 13' foot gunwale. The bottom width is anybody's guess but I have a hunch it's 48" inches wide.






Re: Effect of where deepest dish placement is along chine?
April 02, 2017 09:28PM
Sounds like you are right on track. If you have trouble uploading photos you can send them to me as email attachments and I'll post them

sandy DOT pittenrigh AT g m a i l DOT COM



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