View Full Version : How to design an outboard on an Atkin Seabright Skiff???


SalmonMan
03-30-2007, 02:51 PM
I want to build an Atkin designed Seabright skiff with a V-bottom and a stern tunnel like the Shoals Runner (http://www.boat-links.com/Atkinco/Utilities/ShoalsRunner.html). There are also a few other similar Atkin designs like the Everhope, Heron and Rescue Minor. I would like some feedback on the idea of how to use an outboard instead of an inboard. I realize that with this type of design that the box keel directs water into the inverted-vee stern tunnel and that the inboard prop also pulls that water into the tunnel.

I have several ideas but am unsure as to how they will work.

1. Locate the outboard prop in the same location as the inboard prop using an outboard motor well. This will remove some of the stern tunnel’s surface area and will probably change the dynamics of the water movement into the tunnel.

2. Locate the outboard on the stern with a hydraulic jack plate and experiment with adjusting the outboard with the jack plate to find the best location of the prop. The inverted-vee stern tunnel is similar to the stern of the Hickman Sea Sled and that design had the propeller located aft of the stern.

3. Extend the box keel all the way to the transom and locate the outboard on transom as in #2 above. At this point I’m changing the entire hull design and I’m not sure that I want to experiment that much.

I am open to other ideas on how to get an outboard to work on this type of hull design.

Also are there any other Seabright box-keel designs that are available with outboard power. My searching has not come up with any.

Thank you in advance for your replies!

charmc
03-30-2007, 04:11 PM
I suspect that either #1 or 2 can be made to work. If you print out the order form for the plans, you'll see contact info for the designer. Why not ask him/her the question? I doubt it's the first time it's come up.

Cheers,

Charlie

tom28571
03-30-2007, 04:30 PM
I am dubious of all three solutions. Dave Gerr says that the tunnel is tricky to design and must be tailored to fit the specific boat. My 30 second solution is to fix a sealing plate at the ventilation plate that matches the tunnel surface. When not in use, jack the motor vertically until the prop is above the waterline. This would leave a big hole in the top of the tunnel but that is well above the waterline.

duluthboats
03-30-2007, 04:46 PM
A few years back there was a traditionally built Sea Bright skiff in the launchings section of WoodenBoat. It had a 25hp outboard hung from the transom. That is much different than your idea; I doubt putting an outboard in a tunnel stern would be worth the hassle.

Gary

sal's Dad
03-30-2007, 07:24 PM
None of your three solutions. Tom's idea is close, but I would permanenetly seal (weld) the venitilation flush with the hull botton (tunnel top), and the skeg welded to the shoe, with the prop precisely in the same position as designed. I may still do this, if I can't get my act together on the inboard installation (all this transmission, shaft alignment, stuffing box, bearing, and custom prop stuff has me pretty frustrated right now! and I have a spare lower unit...)

There was a bunch of discussion of this a while back on the AtkinBoats forum.
Charm, you might notice the designer has been dead for 45 years...

Where are you located, SalmonMan?

Sal's Dad

ted655
03-30-2007, 07:38 PM
:confused:
So...., you like the design (which didn't load by the way), but not the propulsion system? If you have no need for the inboard tunnel then why wirk around or in spite of it? Is this a new build or are you converting an existing boat? Build a hull for the OB. What size OB anyway?
Tunnels are for shallow operation. There are other ways to do that using an OB.
1. Jet bottoms. 2 Surface drives. 3. Jack plates
Remember that extended operation with a jack plate requires modifying the water pickup & delivery system and a water pressure guage.
==="When not in use, jack the motor vertically until the prop is above the waterline."==
Any I've seen only lift 6"'. Depending on transom height, this is not enough travel range to clear the waterline AND still provide adequate settings for depth.. This means the hole for a motor well will still have to be big enough for tilt up.
I personally always wanted to build an updated version of a "Dippy", incorporating a tunnel hull. But now we're back to an inboard engine. It's still the ultimate shallow running design.

sal's Dad
03-31-2007, 05:56 AM
http://www.boat-links.com/Atkinco/Utilities/images/ShoalsRunner-2.gif
6" would be plenty to lift the prop and skeg clear. But again, I don't think it would be necessary or desirable. The total draft (like its more primitive sister, Rescue Minor) is only 6" or so, and the prop shaft is very near the waterline, at rest.

It is really very challenging to find the components for a low-powered "high-speed" inboard installation. The designer writes: "The motor specified in the plans is a Universal Atomic Four, 22 hp at 3,000 rpm. The speed of Shoals Runner will be close to 17.5 mph. Do not install greater power than this."

What is a "dippy"? sounds intriguing!
Sal's Dad

FAST FRED
03-31-2007, 06:03 AM
"My 30 second solution is to fix a sealing plate at the ventilation plate that matches the tunnel surface. When not in use, jack the motor vertically until the prop is above the waterline. This would leave a big hole in the top of the tunnel but that is well above the waterline."

That's a reasonable solution, IF the plate is fair.

We intend such a plate with diesel inboard.

I have been on British narrow boats that transit rubbish filled canals , that have a similar system. A simple box with Watetight gaskets , the cover and fairing piece lift out together , and the prop can be cleared of line ., plastic etc.

The cover is just above the WL,so no water enters the boat.

With the Atkin it should be easier as the top of the tunnel, where the prop sits is above the WL at rest.Sure should make lobster pots easy to clear!

Have never been an enthusiast of knife in the teeth and 55Fdeg water.

FF

ted655
03-31-2007, 08:44 AM
http://www.prodriveoutboards.com/
There is also Go Devil, Gator Tail & Mud Buddy
.
Fast fred describes the essence of the Dippy. A inbosrd "box" with a removable top. The small inboard engine /trans is fixed but the shaft is CV jointed & extends down into the water, a prop guard similar to a 3 tine pitchfork is affixed just forward of the propeller. A seperate foldind rudder trails behind. When the shallow bottom or stump is passed over, the spring tempered tines lift the prop & shaft up, into the box. As F Fred says, there is a lever That can be pulled to manually lift the shaft for debri cleaning by removing the cover.. There is also a self locking latch that activates if the shaf up far enough.
Google them. They were/are being used in lakes & rivers up in Canada. As with all things, there is a following of a few keeping them alive & repairing the originals.

charmc
03-31-2007, 02:15 PM
Charm, you might notice the designer has been dead for 45 years...Sal's Dad

Sorry 'bout that. I was focused on the contact info, assumed there would be a designer at the other end.

dick stave
03-31-2007, 07:03 PM
http://www.disappearingpropellerboat.com/history.html eh.

ted655
03-31-2007, 08:16 PM
:D
I won't steal the thread but thanks. I had deleatrd all my bookmarks on them
Imagine what we could build with todays technoligy.
If I ever hit the lottery.....:)

sal's Dad
04-03-2007, 09:00 PM
What do you think? This is the lower unit of a Yamaha F25. The line drawn on the box keel is the approximate design waterline.

dick stave
04-03-2007, 10:04 PM
You are a Trendsetter...

SalmonMan
04-03-2007, 11:50 PM
Thank you to everyone for your reponses and info.

Sal's Dad - What do I think? I think that it looks great! Is that your boat that you are building? I've seen a few other photos of it on another thread on this forum. What plans is it being built off of? Can you tell me more about it? Do you have a website or a gallery with more photos?

Thanks!

sal's Dad
04-04-2007, 05:24 AM
There are more photos of my Rescue Minor on the AtkinBoats Yahoo group. The hull is as close to the plans as I could get in aluminum, but the sheer is about 3" higher in the bow, and the transom has more curve ; overall length is 20' 1/2". The interior is substantially different from the plans.

http://www.boat-links.com/Atkinco/Utilities/images/RescueMinor-2.gif

Mrs. Atkin (the daughter-in-law of the designer) declined to sell me the plans, as she disapproved of my intent to build a rough workboat finish, so I found Motor Boating's Ideal Series #41 "The Plywood Boatbuilder" http://www.boat-links.com/Ideal/ on ebay, and built from that.

I still intend to use inboard diesel power, but have nagging questions about the details - the offset prop shaft, performance with various prop combinations, etc. and figured the outboard is a quick and dirty way to try things out at minimal cost (and to get it in the water ASAP). But there are likely to be three (at least!) problems with this: introduction of exhaust air into the tunnel, sealing the tunnel roof, and getting cooling water into the engine at idle and in reverse.

Sal's Dad

(by the way - would this be considered an inboard, or outboard, by the government types?)

dick stave
04-08-2007, 07:55 PM
Why not have the input shaft from an outboard power head cut down and re splined thus eliminating the leg and positioning the engine at or just above deck level? Atkins specified a 91 cubic inch motor @340 lbs with 10" dia.x 12" pitch prop @ 2000 rpm. Im not sure how much hosepower an old inboard "91" made , but im sure there is similar output power head. A custom made flanged adaptor could be fabricated. Is the prop turning at 2000 rpm critical? That would have to be determined and suitable reduction installed. By the way does anyone know where to see the Tolman sea bright skiff?

dick stave
04-08-2007, 08:06 PM
To answer my own question there are pictures of the Tolman s.b.s. at fishyfish.com . The cutout tells me theyre hanging an outboard off the transom, score another one for Renn...

sal's Dad
04-09-2007, 06:55 PM
I see the Tolman "Alaskan" skiff on fishyfish, but not a "seabright" version -- what am I missing?

As to the cut down outboard: Maybe. But at that point why not go with a diesel inboard? One of the big advantages of the outboard setup is that I have one, about the right size, with a variety of props; just need to lift it off my Bollger Diablo and caulk up the anti-ventilation plate. Or at least that's what I keep telling myself.

I think I asked, facetiously, whether an inboard-mounted outboard (NOT in a well) would be counted by the USCG and other government types as an inboard. But it really could become a problem, meeting "code" on electrical, fuel, and exhaust installations.

Sal's Dad

dick stave
04-09-2007, 07:24 PM
www.fishyfish.com/renn_tolman/index.html

tom28571
04-10-2007, 08:19 PM
It will be interesting to see how this design performs. With an outboard aft of the tunnel, the prop will likely need to be fully immersed for it to work well. Atkin's tunnels are designed to have the prop shaft at about the static water level. This is quite different from Renn's design. I'm not seeing just how this boat is going to work other than protecting the prop with the keel. Will just have to wait and see.

duluthboats
04-11-2007, 01:03 PM
I also will be very interested in how the Tolman tunnel hull will perform. Not only will the prop be outside of the tunnel, ( at least it looks that way ) the surface that forms the tunnel seems different than the lines on the RM. It could be just the way the picture was taken. Here is a sketch of the RM bottom fairly true to the RM lines.

Gary

sal's Dad
04-11-2007, 01:34 PM
THat's not quite how mine looks!

duluthboats
04-11-2007, 03:49 PM
Sorry this model was faired and rounded but the lines are still close. What Im looking at is where the tunnel surface and the box meet. I have been following your progress also.

Gary

duluthboats
04-11-2007, 04:03 PM
I have been drawing these to long I went back to the original and see that I am the one who is deviating. LOL!!!
Gary

Willallison
04-12-2007, 12:24 AM
Removing the prop from its designed location will surely only have negative effects :(
The concave (or inverted v) sections are somewhat similar to a prop tunnel in that they reduce the amount of lift available on the afterboady. This is normally offset by the thrust from the prop on the aft end of the tunnel which features that rather drastic downturn. Without it the boat is reduced to being similar to a semi-displacement hull - with buttocks that sweep up towards the transom - and a massive trim tab on the back in some sort of attempt to offset the suction (;) Tom)
The obvious question here, is why bother with the complexity of the SB shape?

FAST FRED
04-12-2007, 05:08 AM
In a word THE RIDE,at speed in rough water!

From my research it would seem that the SB hulls are capable of creating a far smoother ride (at the lower SL speeds of 2-4) than any other aft section.

On a launch their ability to press on in an offshore seaway , with out beating the stuffing out of the operators seems to be a difference between these and other hull shapes.

Of course the folks that need more than 20K will go for the deep V and $7000 each seats to keep their spines intact.

For us folks that cant afford 1/4 nmpg and don't need 60-100K for cruising, the ride , and the other attributes , such as beachability would make the hull shape worth the extra time to build.

Weather the keel should float 25% of the boats weight or as much as 75% is a good question, here Efficiency" should include a very small wake , so that 18-20K can be had even in built up areas.

The other question is if the box keel can be more efficient with a more "modern & hydrodynamic" rendition.

The huge load carrying ability would suit a light weight cruiser, as the difference between light ship for coastal cruising , and loaded with full fuel and water for a couple of months in a more remote area , is a good percentage increase of the vessels displacement.

Im not sure any of the above would apply to a boat with an outboard, mounted aft..

FF

sal's Dad
04-12-2007, 08:23 AM
Let's face it. One Atkin design has been launched, a handful of Gerrs, and a Carlson. Robb White's light-weight round-bottom variation supposedly has few (if any) negatives, All the rest have some drawbacks.

Until we get them on the water for sea trials, nobody really knows how the original designs (let alone the variations!!) will perform. All this speculation is a bit premature.

Sal's Dad

tom28571
04-12-2007, 10:01 AM
Let's face it. One Atkin design has been launched, a handful of Gerrs, and a Carlson. Robb White's light-weight round-bottom variation supposedly has few (if any) negatives, All the rest have some drawbacks.

Until we get them on the water for sea trials, nobody really knows how the original designs (let alone the variations!!) will perform. All this speculation is a bit premature.

Sal's Dad

Well Dad, that is why it is called speculation:D If it weren't premature, would it be called speculation? At least 75% of what we write here is probably speculation, hopefully based on other than pure guesswork.

I find this whole realm of the box keel very interesting. I wish someone would build some of the versions with high displacement in a narrow canoe body keel and a low deadrise upper hull. That is, with the keel L/B ratio over 10:1 and 75% of the displacement married to a planing hull bottom that supported only 25% of the displacement.

Would these two entities fight for a divorce or compliment each other to give a boat that ran quietly and efficiently up to, say 18 to 20 kts. Up to this speed, the keel would not be constrained by any "hull speed" restrictions and the upper body would have such low value of bottom loading that the trim angle required to generate adequate dynamic lift for planing would be very low.

I'm not looking at this as a rough water boat but as a cruising vessel that could run in this speed range with small power and fuel use. The advantage over planing boats like my own would be the ability to carry all the room and weight that live aboard trawler cruisers like.

I don't have the time or wherewithall to get into it other than speculation:D .

Oh yes Will, I caught the reference.

sal's Dad
04-12-2007, 11:43 AM
Point taken. But for questions like "How will the Tolman outboard perform" I have my doubts, but so little is known about this shape, we will just have to wait and see.

As to rough water ride, Noble Cab reportedly has a problem with ventilation in a cross sea - air gets under the chine, into the tunnel.

It's just hard for me to understand going very far from the Atkin designs, without knowing in detail how the originals perform.

Sal's Dad

SalmonMan
04-12-2007, 12:30 PM
Thanks to everyone that has responded to this post. I have learned a lot from them.

What I like most about the Atkin Seabright hull is it’s an “excellent model for rough water service”, “for carrying a heavy load without increasing the draft greatly”, “is the ideal form for use in shallow water” and “especially as to unusual speed with modest power”. Those quotes come from the description of the Rescue Minor http://www.boat-links.com/Atkinco/Utilities/RescueMinor.html written by William Atkin. That is what I am looking for in a boat, but I like the simplicity of an outboard motor verses installing an inboard.

Sal’s Dad has offered up an interesting solution with his outboard. I will be very interested in learning how his boat performs when he gets it in the water. Please keep us posted.

I’ve corresponded with Renn Tolman regarding the Tolman Seabright hull and he is planning to run an inboard on his boat. The outboard cutout is for a trolling motor.

So, maybe I’m barking up the wrong tree. What other hull designs offer semi-displacement speeds at a low horsepower requirement, while at the same time offering a seaworthy, rough water capability? I’ve looked at some boat design plans but by no means all of them.

Thanks in advance for your replies.

Willallison
04-12-2007, 07:18 PM
FF - sorry, I meant why bother with the SB's complexity if using an outboard. It's attributes when built as designed appear to be well documented - and worthy of further examination

Tom - The shape you describe has (unless I misunderstand you) already been tried - and with much success. Many displacement catamarans employ it. The only difference being that the 'box keel' is replaced with parabolic or circular sections, that are more efficient in the displacement realm.
I've been tinkering with a monohulled version along similar lines, which I've called the monomaran. In this incarnation, the L/B ratio is around 12:1, which is considered the minimum in order to be really efficient. It requires some stabilisation, which I've added in the form of inflatable tubes.

FAST FRED
04-13-2007, 05:57 AM
"I find this whole realm of the box keel very interesting. I wish someone would build some of the versions with high displacement in a narrow canoe body keel and a low deadrise upper hull. That is, with the keel L/B ratio over 10:1 and 75% of the displacement married to a planing hull bottom that supported only 25% of the displacement.

It might be possible to allow the box keel to have the plaining angle of attack , so the hull above would still be fairly level.

The trim might be changed by venting the aft negative deadrise section with an OTS trim tab set.

While a 10-1 L-B boat would be interesting , the Old Camper and Nicholson pre-war 50 ft launches ran 20K on 100HP, so its a proven concept.

Adding the light weight of todays materials and engines should allow the LB ratio to fatten and still be cheap to push.

We are planning on a 38.6 loa with about a 5.5 ft BWL, 7.6BOA (all to fit into a container), so it wont be as fine as 10-1.

But I'm wondering if the box keel actually was 75% of the displacement , if ONLY the LB ratio of the box keel would predominate?

Lighter weight equals cheaper high speeds, so the weight of the engine and fuel would be the key in displacement , at 1500 lbs for the inboard diesel and 500 miles of fuel at perhaps 4 nmpg , it won't be too light.

Atkin had a pretty simple shape of the keel , today it would be easy to create a "more sophisticated" shape , for lower drag and less resistance.
This would perhaps allow the boat to run aground harder , so might not be great for a designed beachable boat.

Flat bottom on an airfoil shape?


FF

tom28571
04-13-2007, 08:37 AM
Will,

I would most definitely use a minimum surface area keel. The "box" term was just in keeping with our generic usage. I think a semi-circlular bottom with straight sides. The upper hull would be fairly normal as a low deadrise planing form with more warp than usual to allow a sharp entry. I don't think an inflatable upper hull could have or maintain the shape required for such a boat.

Fred,

I think a flat planing surface on the keel might defeat the purpose of the combination as I see it. If the designed displacement of the complete boat would allow it, the L/B ratio of the keel would go higher than 10:1. The idea is to have most of the displacement in the low drag non planing keel while the upper hull would be planing but with such a low bottom loading that it would tansition to plane with no significant "hump" and at a trim angle as low as one degree. If it could not do quite a bit better than 4mpg, I doubt it is worth further effort. My current boat already transitions at at no more than two degrees, so it seems very possible. The closest thing that I have seen is the "Displacement Glider" but that is not there either as they do have a keel shape that I think has too much drag and that funky hull shape.

There are boats that carry all of the displacement in keels that operate fully submerged but they have to be cats or have fly by wire stabilization. Neither of these fit a low cost, moderate speed, very efficient vessel like I am thinking of.

Willallison
04-15-2007, 06:45 PM
Apologies to the original poster of this thread - we seem to have hijacked it somewhat, so I'll be relatively brief.

FF - as Tom suggests, there's no point in building any planing traits into the keel section as it operates in a displacement mode, albeit at much higher speeds than the traditional 1.34 SL, as a result of its high LB.

Tom - Sorry, obviously didn't explain the monomaran very well. The basic hullform is rigid, with an inflatable collar that would ride with only the aft section just above the at speed WL. A pic says a thousand words, but all I have here is a rather shoddy image from 'paint', so maybe it's worth a hundred or so...:D

Willallison
04-15-2007, 06:47 PM
In reality, the 'keel' section would probably carry greater draft than indicated. The blue line represents that 'at speed' WL.

kengrome
04-17-2007, 07:53 AM
What other hull designs offer semi-displacement speeds at a low horsepower requirement, while at the same time offering a seaworthy, rough water capability?

Here's a little one I've been designing in the evenings while building my Tolman Seabright Skiff during the days. It's 12 feet long, 4 feet wide, 24 inches high at the bow, 18 inches high at the transom, 15.5 inches high at the lowest point along the gunwales, and should weigh 250 pounds or less including the engine.

I plan to build it using mostly 1/4 inch plywood (maybe 3/8 inch on the box keel bottom panel) and power it with a cheap 6.5 HP industrial gas engine. The transmission will be a simple v-belt system with idler pulley for forward and neutral, and no reverse except via oars or pole or paddle of course.

My concept here is a "Personal Seabright Skiff" ideally suited for one person to use as a recreational / fishing / hunting / coastal explorer, a boat with great fuel efficiency and enough seakeeping ability that the owner won't feel threatened if he gets caught out in the rough stuff. I also want this boat to be able to sneak into just about any shallow backwater areas a person can find, and to run at top speed in less than a foot of water, and to beach it anywhere there is a beach available -- all without the need for a jet drive and with virtually no risk of damage to the prop or drive train or rudder.

At 6 inches of draft it has 9.5 inches of freeboard and its displacement is 500 pounds, 250 for the boat and 250 for a big person and some fuel and gear. A nice feature of this little boat is that it still has 6.5 inches of freeboard at a total displacement of 980 pounds -- which means it can actually haul 3-4 full-sized adults around safely when the seas are calm enough.

Note that I colored the sides light teal, the garboard/tunnel sections orange, and the flat box keel bottom white. I also used white on some of the pix to show the submerged portion of the boat. The profile shows 500 pounds displacement (typical running load with one person) and the underbody pix show both 250 and 500 pounds displacement to illustrate the waterline difference between an empty boat and "running mode".

DaveH1
05-06-2007, 11:33 AM
Ken,

I think this design speaks to a needed niche.

Could reverse be added to the drive train by means of another couple pulleys and belt(s)? Reverse seems to be to important a feature to live without in remote areas. Getting hung on a rock in water too deep to easily push off or finding a way through a duck marsh with lots of dead ends would be examples of where I would want reverse gear.

From a hunter's perspective, I suspect there might be a fair amount of waveslap noise generated by the tunnel portions when slipping along at displacement speeds or when still in the water. Maybe not....

I like the idea of a tunnel-protected propellor as an alternative to a jet, and the low expense and user-friendly maintenance of the simple drive train.

Do you have a speed estimate for this design?

thanks,

dave

dick stave
05-06-2007, 03:09 PM
It occurs that such an elaborate bottom design would be better suited to fiberglass construction (and Im no fan of glass boats) for production. Once a plug and mold were built ,they could be produced inexpensively for a mass market. If you wanted to think big ,a stamped aluminum bottom with proprietary chine / sheer extrusions built in a production jig.

tananaBrian
05-06-2007, 03:56 PM
Dave Gerr is likely referring to the fact that you lose buoyancy when you cut sections out of the bottom of a boat, e.g. a tunnel. As with all designs, you have to carefully iterate between CG, CB, and hull form design until you achieve the best-compromise answer. If you change the tunnel dimensions, then it will have an impact on the boat's waterline and lift characteristics when on plane. Get the waterline right and the lift will generally take care of itself on a boat like this.

In any case, the boat referred to is a very interesting and good design and it would be neat to see it converted to an outboard version ...most likely a smallish (25 hp) outboard in a well and no high expectations for being the first guy on the fishing hole ...but what's wrong with 17-18 knots? Most sea conditions limit you to that anyway and this boat will save a lot of fuel for you so you can go twice for everybody else's once.

It's very funny and coincidental that I've been beating up on these same questions, trying to come up with an idea that will result in high mileage, reasonable speed, very shallow draft, and good load capacity boat design ...for Interior Alaska's rivers. Most folks around here use shallow V hulls with very very modest (1" deep) tunnels and jet-conversion outboards. These things suck a lot of fuel but are tough aluminum and can slide right over gravel bars. If a guy can beat the efficiency equation, which generally means switching to a prop, and can get up gravel bar-ridden braided Alaskan streams, then he'll be onto something. Suddenly your river hunting range will outstrip the other guys' ... a good thing! I hope to get a design together this summer sometime and I'll be the guy that uses it :)

Brian



I am dubious of all three solutions. Dave Gerr says that the tunnel is tricky to design and must be tailored to fit the specific boat. My 30 second solution is to fix a sealing plate at the ventilation plate that matches the tunnel surface. When not in use, jack the motor vertically until the prop is above the waterline. This would leave a big hole in the top of the tunnel but that is well above the waterline.

dick stave
05-06-2007, 05:22 PM
I agree an outboard in a well would be the simplest solution for power.The anti cavitation plate could be half of a gasketed sealing surface with motor down, and jacking vertically with a rack and pinion device or a foot operated lever as the old disappearing propeller boats had. For better prop protection, one of those encapsulated (ducted fan ) outboards the U.S. military uses could potentially be employed ("USED OUTBOARDS.COM") sells them at surplus prices. The lower unit sealing issue could be solved with a rubber bellows surrounding the leg and sealed at bottom with aforementioned mating gaskets. There is a greater demand in areas where weeds are encountered, as in the pacific northwest rivers and in alaska the jet drive will always rule.The 4 stroke jets are good on gas and nothing can run more shallow...

tananaBrian
05-06-2007, 07:08 PM
To be honest though, I hate putting outboards in a well. There are too many issues that can arise, especially if trying to minimize the size of the well so it mates with a tunnel properly. Water sloshes up the well, exhaust can creep in and stink, lids cause motors to snuff out if exhause IS creeping in, props are hard to service if you get wound up in someone's line or in river gunk. I've never owned or operated a boat with an outboard in a well, but these are complaints that I've heard from friends. Maybe today's clean running 4-strokes would make a difference.

Noting that the most efficient planing hull possible is going to be the good ol' flat bottomed boat since it doesn't waste energy pushing water out to the sides in trade for a softer ride. This is a fair trade for water that isn't too rough, e.g. rivers and bays.

If someone forced me into a corner today and made me draft up an efficient design for shallow water use, here's what I'd do:

- Use a TLDI (type) 2-stroke or modern 4-stroke for the lowest gallons per hour possible. I'd probably select the TLDI 2-stroke since 4-stroke motors can sometimes be higher-momentum motors and you damage more than just a sheer pin (or prop key) if you hit something.

- Go with a smaller-diameter larger-pitch stainless steel prop (maybe a 4-blade? But probably a standard 3-blade cupped prop.)

- Flat-bottomed boat with a minor double-chine and modest half-round type tunnel, the boat designed from the start for the tunnel so a) the waterline is appropriate, and b) planing works properly and in a stable manner.

- Hydraulic jack plate and shallow-water nosecone on the outboard

- Consider a prop guard on the motor

Going in reverse in shallow rocky, gravel bar, or weedy conditions is always a pain, especially with a prop guard, but there isn't any perfect answer for the shallows. Most 'extreme shallow' boat designs really don't work in the shallowest of conditions, other than jets and jet conversions ...but that's the trade-off. Work in the shallowest of all conditions and burn too much fuel (jets), or work in not-quite-as-shallow conditions and save a little on fuel (prop). I think that if you design the boat to be light, then getting out and dragging it now and then isn't the worst situation (not dragging on dry ground, but with no people in it ...floating across the shallows with a bit of scraping along the way instead... else give up.)

Good ol' boys down south with their bass boats use jack plates and shallow water nose cones on flat bottomed boats ...experience tells all when it comes to what works and what doesn't.

Brian

dick stave
05-06-2007, 07:31 PM
Were getting into a regional discussion here.The guy with the nitro wouldnt last the day where I run,and I would be pulling weeds out of the foot all day where he does. There are always going to be V8 jet sleds in the river and fountains with blown big blocks in the lakes. Moreover, these guys dont give a s--t about what the fuel costs are. That being said, I admire an effort to resurrect an old design and give it another try. I dont think it will gain mass appeal as there is no greatest all around boat and it may be perceived as a gimmick. Still its a pretty cool boat for the home builder.

kengrome
05-07-2007, 01:54 AM
Ken, I think this design speaks to a needed niche.

I hope you're right, I like it myself so I hope it performs well.

Could reverse be added to the drive train by means of another couple pulleys and belt(s)?

Yes it could, although it would obviously cost more.

Reverse seems to be too important a feature to live without in remote areas. Getting hung on a rock in water too deep to easily push off or finding a way through a duck marsh with lots of dead ends would be examples of where I would want reverse gear.

I figured that most people would prefer to save the cost of reverse gear and just use the oars to row backwards (or push off other things) until they could turn around. Then again I understand the kind of use you're thinking about, and reverse would be very nice for your use of this kind of boat.

From a hunter's perspective, I suspect there might be a fair amount of waveslap noise generated by the tunnel portions when slipping along at displacement speeds or when still in the water. Maybe not....

Maybe when at rest, but I think I should build one and report on this first before guessing at it. From what I have learned about these boats, the tunnels fill with water almost immediately when the prop starts turning, even at the slowest prop speeds, so I don't think there will be any wave slap when you're moving -- at least not under the hull.

I like the idea of a tunnel-protected propellor as an alternative to a jet, and the low expense and user-friendly maintenance of the simple drive train.

Me too, that's one reason why I'm so interested in this particular hull style. Other reasons include very shallow draft, very fuel efficient, inexpensive inboard engines that any lawnmower shop can fix for you cheaply, etc.

Do you have a speed estimate for this design?

It will probably do 15+ MPH on 5-7 HP but this is just a guess. One thing William Atkin told his builders repeatedly is to not overpower these boats. I think it was Robb White who said the handling gets squirrelly if you try to run the boat too fast.

kengrome
05-07-2007, 02:19 AM
It occurs that such an elaborate bottom design would be better suited to fiberglass ...

Hi Dick,

I'm no fan of fiberglass either, and before I built the hull of this Tolman Seabright Skiff:

http://www.bagacayboatworks.com/images/seabright02/

... I was thinking that a fiberglass bottom would make everything a whole lot easier. But once I built this bottom in plywood I realized that it is only the first build that would create problems. The second and third and all future builds will be much easier, especially when a custom jig is used that makes the panel-twisting task easier. Now that I rebuilt my jig specifically to make twisting this panel easier, I don't even think about using a glass bottom.

kengrome
05-07-2007, 02:33 AM
It's very funny and coincidental that I've been beating up on these same questions, trying to come up with an idea that will result in high mileage, reasonable speed, very shallow draft, and good load capacity boat design ...for Interior Alaska's rivers.

I think it is also coincidental that Renn Tolman lives in Alaska and has designed his new Tolman Seabright Skiff for precisely the purpose you've described here! This is what he wrote about his new boat design in his building plans:

There's a lot of thin water in Alaska -- tide flats and rivers, and a skiff that draws only 6-7 inches has tremendous attraction for a hunter and fisherman like me. And as a lot of boaters know, there's a lot of other places on earth with shallow water as well. So maybe the Seabright skiff's time has come -- again.

sal's Dad
07-09-2007, 09:27 PM
Some photos... I'll be off-line for most of the summer, though :cool:

http://i122.photobucket.com/albums/o248/sals_dad/Bingeyafloat.jpg

http://i122.photobucket.com/albums/o248/sals_dad/CareenedB.jpg

http://i122.photobucket.com/albums/o248/sals_dad/about8kts.jpg

http://i122.photobucket.com/albums/o248/sals_dad/BowseatB.jpg

Sal's Dad

artemis
07-09-2007, 10:25 PM
Hope you read this before you go offline for the summer.

Read your comments on the Atkins forum. Couple of thoughts:
1. Without people and much dunnage she should float as drawn in the Atkin plans - at the waterline. She obviously doesn't which makes me think that location of the outboard motor needs to be offset with some weight forward of same. The motor Atkin recommended was around 360# as I remember. To float properly to her lines she should weigh about 850# so balllast her according.
2. That will probably take care of the problem you mentioned on that site of the bow digging in and very sensitive handling at speeds over 12 mph.

Ron

sal's Dad
07-10-2007, 06:38 AM
First, sorry for the cryptic postings, but there are separate discussions going on here, at the woodenboat forum
( http://www.woodenboatvb.com/vbulletin/upload/showthread.php?t=52985&goto=newpost )
and AtkinBoats group ( http://groups.yahoo.com/group/AtkinBoats/ ). There is a full album of construction photos on the Atkin group.



Yes, clearly she is squatting quite a lot - it is even more pronounced with the bigger motor. I added some weight in bulkheads, tanks, and lockers, but thought it was roughly distributed fore and aft. Shifting weight forward only increases the extent to which the bow digs in at speed.

Bare hull is about 550 lbs, so figure 1000 with motor (150) skipper (200) and gear/lookout.

I'll be working on it . . .

brian eiland
08-20-2013, 10:42 PM
Let's face it. One Atkin design has been launched, a handful of Gerrs, and a Carlson. Robb White's light-weight round-bottom variation supposedly has few (if any) negatives, All the rest have some drawbacks.

Until we get them on the water for sea trials, nobody really knows how the original designs (let alone the variations!!) will perform. All this speculation is a bit premature.

Sal's Dad

Well Dad, that is why it is called speculation:D If it weren't premature, would it be called speculation? At least 75% of what we write here is probably speculation, hopefully based on other than pure guesswork.

I find this whole realm of the box keel very interesting. I wish someone would build some of the versions with high displacement in a narrow canoe body keel and a low deadrise upper hull. That is, with the keel L/B ratio over 10:1 and 75% of the displacement married to a planing hull bottom that supported only 25% of the displacement.

Would these two entities fight for a divorce or compliment each other to give a boat that ran quietly and efficiently up to, say 18 to 20 kts. Up to this speed, the keel would not be constrained by any "hull speed" restrictions and the upper body would have such low value of bottom loading that the trim angle required to generate adequate dynamic lift for planing would be very low.

I'm not looking at this as a rough water boat but as a cruising vessel that could run in this speed range with small power and fuel use. The advantage over planing boats like my own would be the ability to carry all the room and weight that live aboard trawler cruisers like.

I don't have the time or wherewithall to get into it other than speculation:D .

So I'm just reading thru this subject thread and noted the date 2007. What has been learned about these hull shapes since then?? Are there bigger examples?

...a sailboater trying to learn about powerboat shapes :confused:

brian eiland
08-20-2013, 10:53 PM
"I find this whole realm of the box keel very interesting. I wish someone would build some of the versions with high displacement in a narrow canoe body keel and a low deadrise upper hull. That is, with the keel L/B ratio over 10:1 and 75% of the displacement married to a planing hull bottom that supported only 25% of the displacement.

It might be possible to allow the box keel to have the plaining angle of attack , so the hull above would still be fairly level.

The trim might be changed by venting the aft negative deadrise section with an OTS trim tab set.

While a 10-1 L-B boat would be interesting , the Old Camper and Nicholson pre-war 50 ft launches ran 20K on 100HP, so its a proven concept.

Adding the light weight of todays materials and engines should allow the LB ratio to fatten and still be cheap to push.

We are planning on a 38.6 loa with about a 5.5 ft BWL, 7.6BOA (all to fit into a container), so it wont be as fine as 10-1.

FF
Did you ever get around to this experiment?
Brian

brian eiland
08-20-2013, 10:59 PM
Here's a little one I've been designing in the evenings while building my Tolman Seabright Skiff during the days. It's 12 feet long, 4 feet wide, 24 inches high at the bow, 18 inches high at the transom, 15.5 inches high at the lowest point along the gunwales, and should weigh 250 pounds or less including the engine.

I plan to build it using mostly 1/4 inch plywood (maybe 3/8 inch on the box keel bottom panel) and power it with a cheap 6.5 HP industrial gas engine. The transmission will be a simple v-belt system with idler pulley for forward and neutral, and no reverse except via oars or pole or paddle of course.

My concept here is a "Personal Seabright Skiff" ideally suited for one person to use as a recreational / fishing / hunting / coastal explorer, a boat with great fuel efficiency and enough seakeeping ability that the owner won't feel threatened if he gets caught out in the rough stuff. I also want this boat to be able to sneak into just about any shallow backwater areas a person can find, and to run at top speed in less than a foot of water, and to beach it anywhere there is a beach available -- all without the need for a jet drive and with virtually no risk of damage to the prop or drive train or rudder.

At 6 inches of draft it has 9.5 inches of freeboard and its displacement is 500 pounds, 250 for the boat and 250 for a big person and some fuel and gear. A nice feature of this little boat is that it still has 6.5 inches of freeboard at a total displacement of 980 pounds -- which means it can actually haul 3-4 full-sized adults around safely when the seas are calm enough.

Note that I colored the sides light teal, the garboard/tunnel sections orange, and the flat box keel bottom white. I also used white on some of the pix to show the submerged portion of the boat. The profile shows 500 pounds displacement (typical running load with one person) and the underbody pix show both 250 and 500 pounds displacement to illustrate the waterline difference between an empty boat and "running mode".

Did you build this??

brian eiland
08-20-2013, 11:09 PM
Hi Dick,

I'm no fan of fiberglass either, and before I built the hull of this Tolman Seabright Skiff:

http://www.bagacayboatworks.com/images/seabright02/

... I was thinking that a fiberglass bottom would make everything a whole lot easier. But once I built this bottom in plywood I realized that it is only the first build that would create problems. The second and third and all future builds will be much easier, especially when a custom jig is used that makes the panel-twisting task easier. Now that I rebuilt my jig specifically to make twisting this panel easier, I don't even think about using a glass bottom.

Interesting, then a metal hull might not be so difficult,...a frameless type construction with an external jig to hold the plating?
http://www.trawlerforum.com/forums/showpost.php?p=172896&postcount=18

brian eiland
08-20-2013, 11:33 PM
...from another subject thread

mydauphin

It is more than just looking at hull shape and saying...hmm..flat bottom, with a Vee, must be a planning boat...or wow, boat is fast so why is it not called a planning boat.

Any "body" moving through the water, ie at the air-sea interface, will create pressure variations around the 'body'...these pressures variations manifest themselves as waves. These waves are a measure of energy and hence drag.

The shape of a hull can affect this pressure distribution considerably. In a nut shell to cut a long story short, the stern experiences suction pressure fields. Speed then also begins to play a part....ie trim/squat, the faster one goes.

For a "normal" boat, the faster one tries to go, the more trim and the greater the power required for little gain. The back is sucked down and dragging creating a lot of wash/waves. Too much energy is being used in making waves. The reason is the hull shape and its length displacement ratio. This is seen in the resistance curve by humps, and the main prismatic hump. A hull must over come this main hump to go faster, ie make "lesser waves" or better still no waves. Long thin hulls, hydrodynamically, behave differently to short fatter ones.

However, just making the aft section flat with/out a vee, doesn't mean that is all that is required just to go fast, or get over the "hump" in the resistance curve. That is just hull shape.

Making the hull longer and thinner, reduces the 'near vertical' curve in resistance of a 'normal' hull form...ie its length displacement ratio. Not only does the resistance curve become less step, but also the main hump is much less pronounced. The longer and thinner one makes it (that is light for its total length), the curve slowly approaches a smooth curve and almost no discernable pristamtic 'hump'. This is why fast ferries, for example, go fast, have a high froude number, but are not planning. They are long and thin.

The length displacement ratio (ie long and thin) allows the hull to be driven faster than would normally be the situation. The wave making reisatnce gets less and less, the longer and thinner the hull becomes.

The down side is, one of these long thin hulls is generally far too unstable to be used effectively. Solution, put two side by side, ...a catamaran. Utilising the benefits of the long thin hydrancimc effects, but providing a stable platform for use in almost any application safely.

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