isplacement-to-planing dynamic conversion?

[DaveH1]

Hi everybody!

I have learned much about boat design issues from lurking on this forum for the past several months, and I appreciate the high signal-to-noise ratio found here. Quality exchange of information between like-minded folks is one of the higher uses of the Internet, so everyone here should give themselves a pat on the back. Thanks very much.

Numerous threads here have helped me gain more insight into the possibility of designing a modernized duck-hunting scull boat (here's a concise description of a scull boat and the hunting technique: http://www.whiskeyriverboatshop.com/sculldugery.htm ). The recent thread regarding water ballast chambers was one, and the discussion of box keels had relevance to my concept.

One of my design questions is this. Would be possible to design a boat that would be displacement-mode and efficient when used under human power, but planing-mode when motoring between hunting spots. The design driver here is the need to efficiently yet comfortably move the boat by muscle power. Using a sculling oar all day while laying on your back is rough on the forearm. Hence a maybe 30" beam and a length that would achieve the 8:1 to 10:1 L/W ratio I understand give the best displacement speed...

H-P mode could be via a traditional through-transom sculling oar or by Hobie's Mirage-Drive foot-powered flipper system ( http://www.hobiecat.com/kayaking/miragedrive.html ). When motoring, a small outboard would be used. (But if money were no object, the low-vertical-profile 40 HP 2-stroke developed for the Powerski could be ideal for this application. Powerski's corporate site URL currently points to a placeholder site-I hope they haven't gone under-but here is a link to a distributor's site showing the motor: http://www.northeastpowerski.com/techengine.html) )

I imagine a hydrofoil arrangement might accomplish this design goal, but any excess underwater protrusions would be counter to a duck-hunter's need to maneuver in shallows...

Therefore, my current concept is to have triangular inflatable bags or pods of Hypalon, polyurethane, or similar as integral components of the hull. This is what I am seeking your input on. The bags would be positioned on the aft lateral portions of a long, slender hull, strongly attached, in a design and manner that would allow the bags to provide lateral planing surface from a point near the stern to maybe 1/3 of the way forward along the hull length. The bags would confer a hull shape, size, and strength adequate to plane a hull of 100 lbs., 200 lbs. of hunter, 60 lbs. of motor, and another 75 of battery, gear, and miscellaneous.

When the hunting spot is reached, bags would be deflated via valves, folded in to the sides of the boat, cinched tightly to minimize drag (perhaps even tucking into recesses in exterior of hull).

Deflation of the bags would reduce buoyancy, allowing boat to settle lower in the water and minimizing its profile, a required design attribute for this style of waterfowl hunting.

When hunter is ready to move to another spot, reinflation of planing pods would be accomplished by onboard 12V air pump.

With such a light one-passenger craft, I imagine that longitudinal COG could be shifted during and after the attempt to get on plane by the hunter moving their weight fore and aft as needed.

Does anyone know of any research done along these lines, or have opinions or suggestions on the concept of what I guess could be named displacement-to-planing dynamic conversion?

Thanks much!

dave

[DaveH1]

Oops. I just read the planing velocity thread and now have a better grasp of something which I may have vaguely intuited before reading it. Sorry, my bad.

So let me now ask if the following workaround is possible.

Assuming a beam of 30", a hull length of 20 feet would be the minimum required to achieve an 8:1 W/L ratio. I need to get the hull's intersection with the waterline shorter and wider in order to make it planeable, right?

Potential workaround:

To give more movable payload to shift around, let's make it a tandem hunting boat. One hunter lays towards bow of boat, one towards the stern.

Let's configure these inflatable pods (more accurate description than bags) as follows: Tall enough that they would effectively become the new sides of the boat, reaching maybe 6, 8, or 10" above the existing gunnels of the hard-sided hull. Wide and deep as they need to be to give us planeable dimensions and adequate stability.

And maybe this is where teh water-ballast chamber could come in handy. In the longitudinal center of the boat, we would have as much as 4' of hull interior to devote to a ballast chamber, if needed. 20' total length minus two six-foot hunters minus two-foot hull tapers at either end leaves 4' to play with. (and I realize that 2' tapers would not be the best for hydrodynamic efficiency, but let's roll with that for now...)

While hunting in displacement mode, the chamber could be filled with water, allowing the hull to be as low-profile as possible. When ready to make a down-lake run, pump the chamber dry (gaining an inch or two of freeboard as we shift to planing-hull mode), then inflate teh planing pods.

At that point, both hunters would move near to the stern and sit on their integral collapsible seats. The combination of weight shift and emptied mid-ships ballast chamber now tilts the bow far enough up into the air that the hull length at waterline is only maybe 15'? And the planing pods could extend from the stern forward to whatever point is necessary to create adequate freeboard at all points along the hull...

If you're still with me, does this seem feasible?

I ain't figgered out how to handle the reality of the transom becoming submerged as the COG moves to the stern. A raisable jack plate with a Hypalon skirt??!!! Ay carumba! I'm creating a monster..... But it's still fun thinking about it!

Input?

thanks,

dave

[DaveH1]

Hmmmm. Resounding silence on this one. Time to stir the pot.

Hypothesis No. 1: Boat designers have evolved to a point that discussing anything involving taking the life of another creature is beneath them.

Looking forward to some input,

dave

[DaveH1]

As Longliner45 stated in a recent post: "...,,sometimes cool stuff comes from wild ideas,,,,,..."

I'm puzzled that this thread has not sparked anyone's design muse.

One addition to my concept: the planing pods would probably require integral structural members. It would be best if they would not impede the folding-away of the pods when shifting to displacement mode.

Thank you for any input you may choose to impart,

dave

[gonzo]

Build a flat bottom double ended narrow hull. It will plane and be easy to drive at displacement speeds.

[tom28571]

The displacement part of the design is not too hard to work up. The planing part is more involved. To start with, you need to know what the load is going to be and then estimate how much can be allowed for the boat. In planing, weight is all important. A long flat bottom like Gonzo advizes is a starting point although I would prefer a transom stern for the outboard rather than a double ender. I am not a fan of flat bottoms either but some form of flat with a multi chine with shaped bow entry like many of Atkins designs should be good.

It is usually a mistake to start with a vision of the boat design before knowing just what the boat is supposed to carry and what the boat is supposed to do.

I think you may have the need for sponsons backwards as a long hull can be made to plane, but such a hull is not very stable at rest, especially if the occupants are moving around or shooting things. Permanent sponsons above the waterline might work better.

After you have the allowed load and estimate of boat weight, figure a bottom waterplane area that will give a loading of about 40#, or preferably less, per square foot of area. See how this affects the size and weight of the boat and go around this loop a few times.

There is much more to learn but maybe this can get you thinking in a positive direction.

[PI Design]

If you have a 20'*30" hull which is reasonably light, you won't really need to plane. A hull of those proportions shouldn't suffer from a drag hump, much like a catamaran hull.

But the pod idea sounds fun, if a little complicated...

[jeemboNC]

A fun thread intellectually, but I agree with Gonzo - the potential benefits of all this claptrap over a simple lightweight skiff would be marginal. Wrong market for all this technology. K.I.S.S. seems to apply here.

[kach22i]

Simple is best.

Although if you want to get crazy that can be fun too.

Link:
BS meter - Jet Wings for PWC