View Full Version : Box Keel


Brands01
02-22-2007, 08:54 PM
I'm interested in designing a seabright skiff (as a theoretical design project), similar to outlined by Dave Gerr in Nature of Boats, and Aitkin. The idea of a hull that operates very efficiently at displacement as well as semi-displacement speeds, and is also very seaworhty due to its low COG is of great appeal.

I assume that it is all the flat sections in the underbody that produces the lift required to reach higher-than-displacement speeds.

What factors should I be focussing on to ensure it is capable of operating at the speed I nominate? Is prismatic coefficient most important? (I am refering purely to hull shape as opposed to power and weight).

I know this is a broad question, but any suggestions will be greatly appreciated.

FAST FRED
02-25-2007, 05:31 AM
AM attempting something similar , and believe low weight , DL well under 100 is needed for a good performing boat.

Look in power boat section for plans from Atkin I posted with article.

FF

duluthboats
02-25-2007, 02:09 PM
I have for a long time been fascinated with this hull form. It seems to be a frequent topic on this and other forums especially with the current interest in efficiency. There is a huge variety of box keel boats, from the long-established Sea Bright, the many Aitkin versions including his tunnel hulls, to the contemporary displacement glider. Things to remember are increased efficiency dose not mean increased speed. The DWL is very important, and I think the length to beam ratio is critical. Small changes in the box can mean big changes in displacement so try to get a handle on the required displacement early in the design. The advantages of this hull may be individually small like better use of interior space, the ability to sit upright on a mud flat, the allowance for the drive shaft to be near parallel to the water line, and the prop being protected by the box but they all add up to make this hull very appealing.

Gary :D

FAST FRED
02-25-2007, 04:07 PM
"to the contemporary displacement glider."

The folks in Austria still have a web site , but I think there gone.

The interesting part is these boats were designed by Atkin for simple construction in wood.

Only the "power glider" version was done with modern materials that might allow a good improvement.

Don't know to look at submarine shapes or bows of a modern tanker for a direction to modernize Atkins work.

FF

Brands01
02-25-2007, 04:49 PM
I've been having a good look at the Aitkens website for some time now, and there's no doubt that his boats all work. Its the science of how this particular hull form works that fascinates me, along with the benefits as mentioned by Gary that make it so practical.

FF, I had a read of of the Pocket Cruiser thread, and there is definate interest in the hull form. It seems that the DL ratio of under 100 is required for a slender hull that can reach high speeds without planing. I don't think this is practical for me, and I'm sure not many of Aitken's designs would have had a DL of 100 or less.

Rather, I would be envisaging a higher DL and a hull form that develops a small amount of dynamic lift to allow a speed length ratio of higher than 1.34.

Reading through the pocket cruiser thread highlighted to me that I know nothing of the science of planing. So I plan to research this area some more, then I will return to the drawing board with more knowledge in hand to attack this very interesting project!

How are you going with your plans?

PAR
02-25-2007, 09:18 PM
I've been working occasionally on a 39' Atkins (John), box keel motor yacht, in the last couple of years. It certainly doesn't have the ability to plane, nor an especially low D/L though can be pushed past it's theoretical speed limit with surprisingly small amounts of power. At 11 tons (net and dry) she's no small craft (christened Namaka and one of the designer's favorite boats). With her fresh 50 HP diesel she touches 11 knots, but generally does 8 - 9 in all but dead flat water. The efficiency of the type is reasonably well documented, but I don't expect particularly high speeds from the hull form. They have a point of rapidly diminishing returns and get unstable if pushed beyond this limit, which I've done in over powered skiffs. Another point to ponder is backing maneuverability, which generally sucks in fixed shaft versions of the type.

FAST FRED
02-26-2007, 04:42 AM
"With her fresh 50 HP diesel she touches 11 knots, but generally does 8 - 9 in all but dead flat water. The efficiency of the type is reasonably well documented, but I don't expect particularly high speeds from the hull form."

The speed is directly related to the fineness of the boat and the weight.

I don't think the slim lightweight hulls available today will find a S/L of 2.5 or 3 to be out of range for semidisplacement .
Although a D/L of under 100 was probably beyond Atkin for EZ construction in wood in the 60's.

AS the multi-hullers have found after a LB ratio of at least 5 or 6 , the losses to wave making are far less.

What IS the best shape for the box keel remains a question , that I don't think many tanks have investigated.

I would certainly accept unremarkable backing , for the ability to run aground , or take the ground on a family cruiser..

FF

moTthediesel
02-26-2007, 11:14 AM
I have been thinking along the same lines for some time. What I'm looking for is a "retirement" boat that has enought room for a couple to spend several months aboard, yet is narrow and light enough to be moved and stored on a trailer. It would need to operate at moderate speeds (10 to 12k cruise) and show execellent fuel efficency (say 10nm/gal +).

The box keel hull has much to recommend it for a boat like this. To show this kind of performance with a very small diesel (say around 30 hp) the boat must have a long dwl (I'm toying with around 37'), narrow beam (say 7.5'), and very light weight (5000# or less dry). The design I've been noodling around with has a long raised deck forward with a wheelhouse aft, rather like a Lake Union "Dreamboat" style. One of the advantages of the box keel for this is that it allows a deep, narrow "aisleway" that gives standing headroom through the fore cabin without resorting to outrageous freeboard or a "crazy-crown" deck. It also, of course, puts the engine weight (considerable by proportion in a boat this light) low and allows for a near horizontal prop shaft.

Have you all read the artical in WoodenBoat #189 by the late Robb White? His description of his version of Atkin's "Rescue Minor" was a real eye opener for me. He refers to the tunnel Seabright hull form as two boats in one, and as I think about it, it makes sense. The long narrow box keel provides most of the bouyancy, directional stability, and as it quickly reaches hull speed it's wake fills the tunnel void under the stern of the "upper" hull. This upper hull gives the accomidation room, reserve bouyancy, and it's extra beam adds stablity and maybe a little dynamic lift aft.

His belief that the captured stern wake allowed the boat to "surf on it's own wake" while tantilizing, is perhaps debatable. There can be no debate about the pictures in that artical though, the image of Robb and RM zipping along trimmed flat in just inches of water while making virtualy no wake needs to be seen to be believed.

moT

FAST FRED
02-26-2007, 04:51 PM
Your dimensions are right on, as the proposed boat could fit inside a sealand box , allowing cheap shipping to a new cruising area every season.

At slow speeds 10nmpg is easy , most sail boats do it.

With such a vessel the ability would exist , for folks with thicker wallets to cruise at 5nmpg at a substantially improved speed.

The hassle is most cruising engines in the 120hp and up class are 1200lbs , for a reliable industrial conversion , rather than a diesel auto conversion.

That will bring the trailer weight close to 8000 lbs with empty tanks but batteries and cruising gear aboard.

How does one obtain this article , can you post it?



FF

moTthediesel
02-26-2007, 05:24 PM
Fred,
I'd never even thought about that container thing until I saw your post about it somewhere else (Passagemaker board maybe?). I'm not sure I would ever do it, but it sure does open up possibilities --

Here's a link to the Robb White site:

http://www.robbwhite.com/index.html

We was a great writer and a genuine free thinker, his death was a great loss to boat noodlers everywhere.

Here's the picture I was talking about:

http://www.robbwhite.com/i/rescue.minor.800.shallow.g.jpg

If you want that WoodenBoat story, PM me, I'll scan it to you.

moT

fcfc
02-27-2007, 05:13 AM
I have been thinking along the same lines for some time. What I'm looking for is a "retirement" boat that has enought room for a couple to spend several months aboard, yet is narrow and light enough to be moved and stored on a trailer. It would need to operate at moderate speeds (10 to 12k cruise) and show execellent fuel efficency (say 10nm/gal +).

The box keel hull has much to recommend it for a boat like this. To show this kind of performance with a very small diesel (say around 30 hp) the boat must have a long dwl (I'm toying with around 37'), narrow beam (say 7.5'), and very light weight (5000# or less dry).

Have you ran some numbers for this ?

5000# is around 78 ft^3 for volume. A DWL of 37' and a Cp of .64 typical for a semi displacement hull will give you a midshipsection of 3.3 sqft.
With a Bwl of 7.5', If the section is triangular (Cm = .5) , with chine at waterline level, you would had a canoe draft of 0.88' = 10 " .

If you use a sqare keelbox, 1.5' wide, 1.5' draft (1.5 wide external mean internal width barely above 1' keelside thickness and some framing : one feet atwartship for floor, or a very narrow engine).
Section of the keelbox would be 2.25 sqft. remaining surface for the bottom is 3.3-2.25 = 1.05 sqft, span on 3' each side. Again with chine at waterline level and triangular sections, the intersection between the bottom and the keelbox would be 4" below waterline.

If the keelbox is 2' wide, and 1.5' deep, your hull bottom will be barely touching the surfave of the water ...

And what layout do you plan for living months aboard ?
5000# dry will give you at max 2000# payload for crew and tanks.
Where do you plan to put engine, tanks water and fuels, batteries ?
If you put them in the keelbox, you will no longer be able to use its volume for increasing headroom.
Another point a small engine will be 2' net high, If you add bottom thickness, bottom clearance, engine heigth, top clearance, sound insulation thickness, floor/hatch thickness, you will have an overall heigth or nearly 3'. With a 1.5 ' draft, the top of your engine hatch will be 1.5' above waterline. How do you handle it in your layout ?

I have tried things in the O1 forum, just to conclude that it is very hard to make numbers match ... I have not found any usable/practical layout. You either go houseboat/bolger style with high supertructures (and shallow draft) that will limit you to very protected / inland waters, or you have more sea kindly supertructures, but liveability (ie headroom) suffers a lot.

moTthediesel
02-27-2007, 10:26 AM
Have you ran some numbers for this ?

Ok, here's how I figure it --

First, you need to bear in mind that the box section keel we're talking about is a good bit shorter than the rest of the hull. Say for an LOA of 38' the box keel would be about 30' long. Say a maximum beam of this box is 3', that's 90 sqft. If we multiply by a fineness factor of say, .65, we get 59 sqft. Figuring 14" depth we get a volume of 69 cuft, or 4279# fresh. With 12" depth = 59 = 3658# fresh. So yes, the box keel provides most of the bouyancy for the hull, with the wider upper hull only immersed a few inches. I'm no expert, do you see a problem with that?

And what layout do you plan for living months aboard ?
5000# dry will give you at max 2000# payload for crew and tanks.
Where do you plan to put engine, tanks water and fuels, batteries ?
If you put them in the keelbox, you will no longer be able to use its volume for increasing headroom.

Remember, the keelbox is very long, the engine is very small. Engine would be mounted in the wheelhouse area under a motorbox. I don't mind motorboxes at all, had one in my CC SeaSkiff for years, made a good place to put the plate of crackers and cheese! Plenty of room under the wheelhouse sole for tankage. Fuel tanks don't need to be huge for a coastal cruiser that gets this kind of fuel economy anyway.

That still leaves a full 20' length forward under the raised deck for accomidations. Nothing fancy there, but a good full double berth forward, a simple galley, settee/dining, composting head, and a shower. No air conditioning, no mechanical refrigeration, no bells, no whistles.

I'm still just doodeling here, but I don't see anything that can't be done. When I get something polished up, I'll post it here for a complete thrashing ;)

moT

sal's Dad
02-28-2007, 06:07 AM
Have you seen:

Gerr's tunnel-stern cruisers?
Carlson's Alligator? http://www.carlsondesign.com/alligator.html
http://www.westsystem.com/ewmag/20/Building_alligator.html
The Atkinboats yahoo forum?
River Belle? http://www.boat-links.com/Atkinco/Cruisers/RiverBelle.html

Sal's Dad

moTthediesel
02-28-2007, 08:01 PM
I've seen the Gerr boats but I hadn't seen that Alligator, -- both interesting, but the Atkin boat is closer to the kind of minimalist cruiser I've got in mind.
moT

FAST FRED
03-05-2007, 04:55 AM
From the folks on this board , and lots of other reading I think I may have figured out WHY the Atkins boats perform so well.

I think the "box keel" works like the submerged pods on a SWATH boat , floating most of the hull with no big surface wave.

The small under hull reaches "hull speed" and feeds water to the barely submerged prop , so the description "she surfs on her own stern wave" is mostly true .

Whatever the displacement all the Atkins Sea Bright Skiff hulls have the transom just kiss the surface at rest , as a displacement boat would.

AS the bow rises from the under hull starting to plane the accelerated prop wash in the simple tunnel aft lifts the stern , so she rides mostly flat.

When one boat was overpowered the stern had too much lift , so steering became wierd until lifting strakes were added , to lift the bow.

So far what I have come across has all been good , Atkin , Tollman Skiff, Power Glider all seem to use the concept.

If this is how it works, it sounds like a good cruiser could be made.

Does anyone have BAD things to report about this style hull?

FF

Oyster
03-05-2007, 08:45 AM
I think that this is getting a bit off base. SO I will edit and bow out here and allow the talk to continue about runaboats.

messabout
03-05-2007, 02:50 PM
FF
I agree with you that the box keel does most of the work. Since the box keel has a LB ratio resembling that of a catamaran, one can achieve SL ratios that are much better than conventional displacement types.

I have an ancient book of plans that includes "Surprise", an Atkin design of the Seabright type. The literature claims that it will make 15kn with about 12HP. I am skeptical about that but ....Maybe with a following sea and a tail wind.

I saw Rescue Minor at the Cortez (FL) small craft gathering last year. Robb White said that RM was his tweaked Atkin Surprise. He gave enthusiastic credit to Atkin even though he had made some modifications to the original design. Indeed it would do all the things claimed. It would run in very shallow water as demonstrated by crossing a sandbar at low tide while little dinghys were going aground. RM had a small, about 25HP, Kubota tractor engine that he had converted. The Kubota is a much more competant engine than the tiny little Kermath specified on the original plans. Robb was an innovator who practiced boat building in a practical and frugal way while being inventive. He had cleverly let a section of a copper box into the bottom of the box keel. The copper box was the tank that held fresh water coolant. The copper part exposed to the sea water cooled the fresh water in the engine cooling system. Not an unheard of scheme, but he made it all himself and eliminated the need for a radiator, fan, shroud and all that. Moreover, he had made it of scrap that had been scrounged somewhere or other.

The Seabright type was touted by Atkin and others as an "able boat in a seaway" I remain fascinated by it's combination of simplicity and ability.

According to the historic stuff that I read, the early Seabrights were strictly rowboats. The flat bottom of the box was ideal when beaching the boat, a practice that was necessary for the fisherman who used them. They could stash their catch in the bottom of the box keel such that the weight was kept very low in the boat. That probably had a lot to do with the acclaimed stability of the type. These boats have been used for all sorts of things, including conversion to sailing auxilliaries.

FAST FRED
03-05-2007, 04:09 PM
This is what the builder had to say,His was an 20 hp diesel , of which he uses about half power.

Rescue Minor

May 21, 04

Dear visitor,

It has come to my attention that due to certain liberties I have taken with poetic license in the stories I write for Messing About in Boats (29 Burley St. Wenham, MA 01984... $28 for 24 issues… comes every two weeks!) my credibility has come into question among some people who have not yet learned to believe every word I say. One of the things that continually surface is a hint of disbelief about my series of articles in Messing covering the building and performance of the radical “Rescue Minor” which is my bastardization of a William Atkin shallow draft Seabright skiff style tunnel boat. I took it to the big-deal Cedar Key messabout in an attempt to dispel a little of that skepticism.
Rescue Minor strip planked skiff by Robb White.
Click for larger image. Photo by Cindy Pitt.

Now, I am informed that there are people chatting in various chat rooms who doubt that the boat even exists because they have never seen a picture of it (despite a bunch of photographs printed in MAIB). One comment was, “Well, if it was a real boat, it would have been in “Launchings” in WoodenBoat. Well, I can lay that nonsense to rest right here: Here is the Rescue Minor at Cedar Key. You see… it says “RESCUE MINOR” right there on the transom. That photograph was taken by Cindy Pitt who is a very credible person (and pretty good with a telephoto lens, too). The reason I look so spastic is because I am steering with my hip while I try to unscrew the cap on my water jug with one hand while holding the spring-loaded throttle with my other hand. I think my hat brim has blown down over my eyes, too.

In case you have never read about the boat, I’ll give you the specs: It is twenty feet long and 76” wide. It is strip built out of tulip poplar and is powered by a three cylinder Kubota Diesel tractor engine rated at 20 hp. It will run 20 knots in six inches of water and gets about 28.6 nautical miles per gallon of Diesel fuel running at its most economical speed of 10.5 knots. It is not for sale and I don’t ever intend to build another one… certainly not for anybody else but me, but the plans for the unbastardized plywood version are available from: www.atkinboatplans.com.

Click for larger image. Photo by Jane White.

That's 10.5 knots in a measured 6" of water. At 12.5k the forefoot runs exactly at the surface of the water. At 15.5 it is about an inch and a half immersed. At Atkin's designed speed of 17.5 (statute) the boat needs about 150 pounds in the stern to bring the bow up a little bit. If the engine weighed as much as Atkin's... wouldn't need that. I think he knew what he was doing."

Click for larger image. Photo by Jane White.

That little rooster tail is always there either in deep or shallow water. I think it is a result of the "outflow" from under that complicated hull form re-forming itself back into something less complicated... a dissolution of confluences or something. There is very little disturbance to the sand on the bottom as the boat passes... doesn't even smooth out the ripples.

In case you need to see some more pictures taken by credible people, you can see them at: http://members.ij.net/wctss/wctss/cedar.htm You can see some of the credible people, too.



Robb White, boatbuilder and writer, Thomasville, Georgia

FF

Willallison
03-05-2007, 05:21 PM
The Sea Bright Skiff, and then later the (faster) Jersey Skiff both evolved from a need to launch and operate in very shallow waters. So this was the starting point for the design. Their efficiency is extraordinary - but - and there's always a but:D - this is probably as much a result of their relatively narrow wl beam and very light weight. Similar levels of economy could probably be achieved with a less complicated hullform than the sea bright if the shallow water thing wasn't so much of a consideration.
In you case FF, the ability toi simply 'drag' it into a container, wher it would sit, without modification, is perhaps enough of a seller. But remember, you're talking about a cruising boat, not a stripped down overgrown dinghy that's used to throw out the occaisional crab trap, so weight will become an issue in trying to achieve those higher speeds with such low fuel consumption.

kengrome
03-06-2007, 04:12 AM
Does anyone have BAD things to report about this style hull?

It loses a lot of efficiency when heavily loaded, so it is probably not suited to carrying heavy loads any more efficiently than other boats.

Their efficiency is extraordinary but this is probably as much a result of their relatively narrow wl beam and very light weight.

I think so too. I'm sure the bottom helps, but maybe not as much as some people think.

FAST FRED
03-06-2007, 05:27 AM
"It loses a lot of efficiency when heavily loaded, so it is probably not suited to carrying heavy loads any more efficiently than other boats."


Actually the lore is they did really well during prohibition carrying a large number of heavy cases of booze ashore in ocean conditions .
Dave Gerr remarks of their ability to carry a heavy fish load too.

Heavy is of course relative , but with 2000 lbs given to the weight of a true industrial marineized diesel , with tranny , short shaft and 22 inch prop , that still leaves 6000lbs for the hull, and installed gear and batts.

At 2,5 lbs per sq ft for a foam core hull , she should be able to meet 30K structural load requirements , and not be "too" heavy , figuring 8000lb trailer weight , 10,000 with food fuel & water , ready to cruise.

FF

fcfc
03-06-2007, 05:47 AM
Before speaking of miracle, what is the displacement of the Rescue Minor ?

And at what loading where the performance test done ? Boat empty, one people aboard ?

On the "other" woodenboat forum, seems that NO Rescue Minor was ever built. There is not a single completed photo.

That a bit short to claim some "real" efficiency.

Robb White built a look alike, the only one with completed photos, but seems it is a significatively modified design. From the ones on the other forum who are trying to build the original Rescue Minor, seems that the hullform is not developable.

Seems also that Gerr Marine designed also some bigger look alike, but the performance are not that impressive. 11 Kts with 210 hp. The 23' draft and the fact that a 42 ft boat is beachable is more interesting.

There is also the camano 28 which is more or less box keeled. The point is to have a rather light boat, and to have deep enough volume to hide the engine under a floor. I fear the performance come from the weigth, and the keelbox is just an attempt to fair a bit an ugly bossing under the hull to put the engine in, not to interfer with the layout. IMHO, It performs better than a normal boat heavy enough to have space for the engine under the floor, but it perform less than a normal same weigth boat. But you would have an engine box taking 3/4 of the space of the salon.

I think also a bunch of navy all countries have studied a lot semi displacement hulls for patrol craft, and the keel box have never been a solution.

duluthboats
03-06-2007, 01:25 PM
This is starting to get interesting. Remember what I said previously there are many types of box keel designs. Rescue Minor is a light weight planning tunnel hull. Atkin’s Surprise and Gerr’s DR Northwest Cruiser are both displacement/semi-planning hulls. It’s a big category, lets not mix them all together.
Gary :D

nordvindcrew
03-06-2007, 04:14 PM
I don't know about power boats, but I row an 18' long Jersey Skiff with a box keel. The beam at the gunnels is almost 5', but at the waterline it is just over 3'. It has a very fine entry and is a true double ender on the waterline. With two at the oars we can do 5 knots for long periods of time. Obvoiusly, an easily driven hull type. I wouldn't hesitate to build a larger version for power.

moTthediesel
03-06-2007, 08:02 PM
I would agree that in a wider and heavier boat like Gerr's "Summer Kyle" you would not likely see a great improvement in fuel efficency over a more conventional hull. The box/tunnel hull was used there to gain shallow draft, and for that purpose it was successful.

Naval patrol craft are not likely candidates for this type of hull for a number of reasons. Firstly, most boats of this type would be designed to capable of opperating at speeds higher than the 10 to 20 knot zone where the box/tunnel would be at it's best. Also shallow draft would seldom be a primary design goal for this kind of vessel, and lastly, in my experiance the military seldom has a primary interest in fuel efficiency anyway!

A more interesting question would be this: Suppose you built two powerboat hulls of identical length, beam, and displacement, say 30' X 8' X 4000#. One hull is a conventional "canoe" form, the other a box/tunnel hull. Which would have lower drag characteristics in the speed range we are talking about?

The canoe form would have less wetted surface, but at these moderate speeds that might be less important than wave form drag. Would the box/tunnel hull, with most of it's immersed volume in a much narrower B/L form, have significantly less of this type of drag? It seems it might, but I don't know, what do you all think?

moT

Willallison
03-06-2007, 09:56 PM
As Gary suggests, this is getting interesting....

Setting aside the box keel for a moment, common sense would suggest that the way the buttocks sweep up towards the transom, and the fact that they are not straight, we are not looking at an ideal hullshape for higher speeds. Now there is also a relatively abrupt downturn aft of the propeller, which would offset the tendency for the boat to bury its stern as speeds rise. Indeed this is so pronounced that as speed increases beyond a certain point, the boat starts to bury its bow sections.
These two 'opposing forces' certainly suggest to me that a more efficient shape could be arrived at. BUT, as I said earlier the Sea Bright Skiff evolved out of a need to travel in shallow water and to launch and retrieve off the beach. To have this ability AND run reasonably economically, then the SB Skiff takes a lot of beating

PAR
03-07-2007, 12:39 AM
A possible consideration maybe to install adjustable tabs to add or remove the amount of hook aft of the prop. The 39' Atkins I mentioned earlier, can pull a 1.91 speed/length on her 50 HP diesel, with a 11 ton vessel. She's also an economical boat to run, which goes to serve, she fits well within her designed target speeds. This tab idea would still develop a diminishing set of returns after a point, but higher speed operation may be possible with the same HP.

If you've had much time with these hulls, you'll note the wake is different and flat. It has a bit of a "wine glass" shape just aft of the transom in the separation zone. I've often theorized that this was a function of the reversed deadrise, pressurizing the top of the "tunnel" area, focusing directly aft, rather then having it splay naturally along more conventionally upturned deadrise, releasing off the chines. I also suspect this increases prop efficiency to some degree.

I do know that Atkins refined this hull shape over many decades, feeling that he had taken it as far as he could by the late 50's. Judging by Namaka's performance, he'd had his home work well done, before he penned her up.

FAST FRED
03-07-2007, 05:29 AM
From the Nature of Boats , Gerr

"Perhaps the finest thing about Sea Bright skiffs is that they are good at nearly everything, this claim is made by many hulls , but - unfortunately- is seldom so.
SBS are easily driven at low , moderate semi plaining and even low end plaining speeds. They carry large loads very well.
A 30ft SBS pound net boat (one of the larger SBS) could carry nearly 15 TONS , while a 40 fter could safely haul as much as 25 Tons , even through the surf!
The SBS shoal draft and beachability make them ideal gunkholers , yet their stability , buoyancy and easy motion make them excellent rough weather boats. In fact a SBS cabin cruiser made the passage from NJ to Bermuda as early as 1928."

Rather a remarkable statement by a well known NA.

FF

fcfc
03-07-2007, 06:34 AM
A more interesting question would be this: Suppose you built two powerboat hulls of identical length, beam, and displacement, say 30' X 8' X 4000#. One hull is a conventional "canoe" form, the other a box/tunnel hull. Which would have lower drag characteristics in the speed range we are talking about?


It is more complex than that. When you look at performance prediction analysis, you find that not only Lwl,displacement and Bwl have an influence, but also Cp, Tc, LCB, LCF, At, Aw among other, not including wetted Surface.

So, you must design identical Lwl, Bwl, disp, but also Cp, Tc, LCB,LCF, At,Aw. This is a time consuming task. And it is not even garanteed that you can design different hullform with the same parameters.

And if you have the same parameters, most resistance prediction analysis will tell you your hull with the bigger wetted surface have the biggest resistance.


And even, are you sure you have to compare boats with identical hydrodynamic parameters ? If I take the camano example, the fair comparison for me would not be same hydrodynamic parameters, but same layout : ie engine under the floor, and floor at same heigth above water. Now, the displacement (main resistance parameter with Lwl) would not be the same, the keel box one will be ligther and will have the less resistance. Now if you remove the engine placement constraint, ie a outboard, or accept an engine box, the ligth one without keelbox will be the best performer.

All the keel variation also seems to concern only powerboats with single inboard engine and straigth shaft. No sailboats, no twin engines, no outboards.

If you look at small cruising powerboats with inboards, around the size and weigth you quoted, engine placement is a sh.t. Logan 33 put it under the front v berth. Andreayle 33 put it under a front cockpit. Czarina 30 put 2 small under side settle. C Ranger tug 25 but bossings in the hull. Camano used kind of keelbox.
The bunch of others simply keep an enginebox protuding in the cockpit, go the heavy way to have enough space to put it in the bilge, or give up going outboard.

kengrome
03-07-2007, 08:24 AM
I do know that Atkins refined this hull shape over many decades, feeling that he had taken it as far as he could by the late 50's.

I don't know how many more Seabright skiffs William Atkin designed after Rescue Minor, but he seems to think this particular boat was his best in the Seabright line:

" ... one can rest assured that because I have perfected the
design by slow development over many years and by the
production of many boats of this particular model Rescue
Minor will astonish people who may be under the impression
that an extremely shallow draft boat cannot be fast and at
the same time seaworthy and able."

Here's the page with the rest of his write-up:

http://www.boat-links.com/Atkinco/Utilities/RescueMinor.html

By the way, someone said he thinks none of these boats have ever been built. I suspect that this is only true in recent times. Here's another quote suggesting that several have been built, perhaps 40-50 years ago:

"I am glad to be able to say that the owners or users of these
shallow draft boats have written me that the performance of
each has more than exceeded my predictions, especially as
to unusual speed with modest power, and the ability to travel
in as little as six inches of water either forward or backward
and when fitted with proper weedless propeller to work through
grass, mud and snags of serious character."

I'm not saying I believe everything William Atkin writes, but if it is true this boat certainly deserves a closer look. Robb White certainly thought so, and he documented some very good performance in his modified Rescue Minor:

http://www.robbwhite.com/rescue.minor.html

If the classical styling of the original Rescue Minor is not your cup of tea, maybe a modern Tolman Seabright Skiff would be more appealing. It has the same bottom as Rescue Minor so its performance will be the same more or less, but the topsides looks like a modern oceangoing skiff.

I'm going to build Tolman Seabright Skiffs on a per-order basis in my shop in the Philippines. Renn Tolman will be selling plans to home builders too when he gets them finished. I like the original styling of Rescue Minor, but I like the Tolman Seabright even better ... :)

PAR
03-07-2007, 07:31 PM
John continued the development of his father's efforts, on these hull forms into the late 50's that I know off. Several were larger boats, but most were small sheltered waters craft. Many of these hull forms aren't available on the Atkins and Company web site, including the one I mentioned. Without discussion with Billy or John, the extent of further development is speculation and rehashing of previously covered work, though there are sources available, such as Jay Benford who sat directly behind Billy's drafting table in the early 60's during his tenure on the design team. I haven't spoken with Mr. Benford in a couple of decades, but his recollections and input could be quite enlightening.

Willallison
03-07-2007, 09:55 PM
Yes - as you say, we are all really just speculating - it may indeed be that this is the hullform of the century and we've (nearly) all just been overlooking it!

FF - It stands to reason that the SBS would have good load carrying capability. I wonder though, how its economy is affected by the additional weight.....

Oyster
03-07-2007, 10:28 PM
There are two hulls being built as we speak, that I know of, but none are in the water. One is in Wash. state and one is in Maine, both with plywood.

FAST FRED
03-08-2007, 04:53 AM
Although "only" 20K -25K or so is desired as top speed , I have been reading Lindsy Lords book on plaining hulls.

I notice that lots of plaining hulls do negative work due to hull suction.

With the prop filling the aft cavity , I would imagine these R-M style hulls do not suffer from this suction drag at speed at all.

Also Lord thought many plaining boats were far too light on their waterplane for good efficiency. While he preferred .35 lb ratio the light skinny hull of the RM should be as loadable at speed as the military vessels he designed for.

Unhappily many tables bottom at 10 ft beam , and water plane areas way higher than a boat the size of the Sea Bright I'm dreaming of.

However the Surge tables (for a good sea ride ) in Gerrs book say that the hull should at least be comfortable in ocean work.

FF

sal's Dad
03-08-2007, 06:55 AM
There are two hulls being built as we speak, that I know of, but none are in the water. One is in Wash. state and one is in Maine, both with plywood.Three, I think - one in Massachusetts/Maine, in aluminum.

The form did get developed a bit further, with "Shoals Runner", a lovely, curvy, sleek shape.

fcfc wrote:On the "other" woodenboat forum, seems that NO Rescue Minor was ever built. There is not a single completed photo. This is an assertion I made, on the WoodenBoat forum, in the hope that somebody could document another (I have heard rumors, but have been unable to track one down). When I say the hullform is "not developable", it is based on my experience trying to get very large sheet materials to bend to shape; it is clear to me that some CAD work to tweak the design would be a big help.

This summer, there will be some hard data, with a heavyish RM running on 18HP, with a wide variety of loads. Robb White reported that a full load of roofing materials didn't substantially effect the performance. My wife will be sorely disappointed if it takes more than 45 minutes for a grocery run (11 miles).
Sal's Dad

Willallison
03-08-2007, 05:38 PM
Although "only" 20K -25K or so is desired as top speed , I have been reading Lindsy Lords book on plaining hulls.

I notice that lots of plaining hulls do negative work due to hull suction.

With the prop filling the aft cavity , I would imagine these R-M style hulls do not suffer from this suction drag at speed at all.

Also Lord thought many plaining boats were far too light on their waterplane for good efficiency. While he preferred .35 lb ratio the light skinny hull of the RM should be as loadable at speed as the military vessels he designed for.

Unhappily many tables bottom at 10 ft beam , and water plane areas way higher than a boat the size of the Sea Bright I'm dreaming of.

However the Surge tables (for a good sea ride ) in Gerrs book say that the hull should at least be comfortable in ocean work.

FF

FF - you have to remember that Lord's book was written rather a long time ago now. Much of the thinking has changed - suction being one of them. Imagine drilling a hole in the bottom of your boat, just fwd of the transom. Does water run out - or shoot up in the air? It's the latter, of course. The pressure at the back end of the boat is indeed lower than at the front - but it's still in an upward direction.

Oyster
03-08-2007, 08:39 PM
I see several are discussing speed. Well folks, speed and shallow draft doesn't always go hand in hand. The angle of the boat that is required to get a normal boat on a high speed and planing requires some addtional angle and water. A conventional planing hull does not get on plane without the need for some additional draft , what it takes for even a tunnel hull to run on top or on cruise. Many of the flats boat require you to place it in a huge turn creating a swell to get the boat on plane . Even with those hulls which are marketed to really shallow water folks, still require hydraulic lifts to raise the running gear.

I think all this must be kept in perspective. A tunnel hull, is not the fastest hull, on plane with the same hp of any given propulsion. RW seemed to be very comfortable running the boat at a nice leasurely pace which gave the boat the ability to reach a decent speed with the reduced hp, which in turn keep the angle of the hull to a minimum. This also gave him the feature we have come to know and thats RW running along the beach.. BUt his hull did not jump up on plane. If you actually look at his hull, and if I recall correctly, even though he had a huge tumblehome, the bottom was left fairly broad to the transom, giving the boat some additional lift. In a lot of tunnel hulls, to make up for the loss of planing surface, we let the bottom run full from midship aft to the transoms.

Willallison
03-08-2007, 09:45 PM
Oyster - The SB skiff appear to differ from most boats in the way it behaves regarding angle of incidence. As you suggest, hulls of NORMAL form require an angle of attack in order to get on the plane. This angle is greatest at 'semi-displacement' speeds and reduces as the boat goes faster. The SB skiff, however already has this angle of incidence built into the bottom, in the form of the downturn at the aft end of the tunnel. Indeed, this hullform reaches a point where the upward pressure on the tunnel is so great that the boat is in danger of burying its bows.

tom28571
03-08-2007, 10:00 PM
Will, that argument which is also erronously used by Dave Gerr to prove that suction does not exist under the aft part of a hull is not a proof at all. All it proves is that the NET pressure is positive, not that there are no negative partial pressures generated.

It is well established that aft rocker in the hull bottom results in a stern down attitude. Any fore and aft convex shape in the aft hull bottom must create negative pressure unless Bernuli's theorem is invalid, which it most certainly is not. Whether it is called negative pressure or suction is irrelevant. Lord's use of the term suction may be unfortunate but he was still correct in his analysis of the events.

In addition to hull shape, most aft appendages such as skegs, struts and prop shafts do create at least some of ths offending shape and do cause suction, or negative pressure if that term offends.

Willallison
03-08-2007, 10:36 PM
Tom - I accept your statements up to a point, but at the end of the day, the nett pressure acting on the aft sections of a hull are indeed in an upward direction. I would suggest that the main reason rocker results in greater angles of trim is because as the buttock lines sweep up toward the transom, so too must the angle of incidence increase in order to present an angle of attack to the waterflow.
It's not that Bernoulli isn't there - it's just that Newton is stronger!

I didn't know that Dave uses that as proof of upward pressure - he must be a smart fella!:D Actually it's my bastardisation of of what I can remember from the Westlawn text (written long before Dave came on the scene). I'll dig it out again and see what additional detail is given...

fcfc
03-09-2007, 03:16 AM
If someone could explain me the differences between theses two photos:
http://www.euroshipservices.nl/afbeeldingen/770t-01.jpg
and
http://www.danae-yacht.dk/motorbaade/Camano_31/MO4-161.jpg

One is Camano 28 keelform and the other is the skeg of a traditional looking steel tug (sloep 770).

NB I am interested by keel box as a way to put the engine off the layout. I am not too interested by tunnel hulls or ultra shoal draft.

FAST FRED
03-09-2007, 05:43 AM
Although Lord was written long ago he does point out that the Sea Sled , which is a form of inverted V does not suffer from the negative work aft of many flat bottomed plaining vessels.

Lords complaint about the Sea Sled is it has too much lift forward , and is a very hard ride.

Perhaps the R-M form combines the normal bow sections , with roughly the sea sled stern to get around the hard ride?

A remarkable concept.

Since the drive of the prop stream is responsible for much of the lift aft, would towing a model work at all?

I plan on building a 1 inch to the ft. model to be sure the proposed interior will function as desired.

EZ enough to waterproof the model to tow , but not if an operating engine is required for lift.

FF

tom28571
03-09-2007, 08:04 AM
I didn't know that Dave uses that as proof of upward pressure - he must be a smart fella!:D ..

Will,

Actually I was amazed that Dave Gerr misinterpreted the results of his test of bottom pressure and chose to claim that Lord was wrong.:confused: This sort of thing occurs throughout engineering studies. That is, the summation of several factors to make up a final result. It is called the superpersition theorem and it allows the solution of some complex situations that are impossible to solve when considered as a whole.

To deny that negative pressure exists under the aft bottom of some boats because of longitudinal convexity is to deny that lift results from the convexity of the upper surface of an airplane wing. These ideas are identical and Bernulli lives.

The total pressure in Gerr's experiment was positive as shown by water squirting upward, but it was the summation of all the pressures, both negative and positive, if both existed on that particular hull. The opposite of aft rocker (negative pressure) is a hook (positive pressure) on an aft hull bottom and the results are also opposite.

I hope this I am not belaboring the point but it does seem important to me to see the distinction .:)

tom28571
03-09-2007, 08:18 AM
Since the drive of the prop stream is responsible for much of the lift aft, would towing a model work at all?

I plan on building a 1 inch to the ft. model to be sure the proposed interior will function as desired.

EZ enough to waterproof the model to tow , but not if an operating engine is required for lift.

FF

I suspect you are correct Fred. The prop first raises the water (along with rebound from the forward hull) and then directs it downward on the aft curved bottom.

I have had this problem with all jet propulsion systems. That is, lifting the water upwards from the inlet must result in a down force (from Newton) which effectively increases the displacement and reduces the efficiency of the jet system. The tunnel stern is not the same as a jet system but does show some of the same factors.

moTthediesel
03-09-2007, 10:03 AM
Since the drive of the prop stream is responsible for much of the lift aft, would towing a model work at all?

I plan on building a 1 inch to the ft. model to be sure the proposed interior will function as desired.

EZ enough to waterproof the model to tow , but not if an operating engine is required for lift.

FF

I had the same thought -- so why not buy a cheap RC model boat, remove the controls and drive and mount it in your hull? Far from perfect from a controled conditions standpoint; - true. But using the same powerplant in different or modified hulls might result in some worth while information.

Or if not, at least it would give you a good excuse to play with toy boats :D

moT

BWD
03-09-2007, 11:50 AM
I think the interesting thing about the RM type hull is the potential to use the prop stream to exaggerate/simulate the stern transverse wave system, and recover that energy with the aft hull, so I would agree towing couldn't model it well....

Perhaps a hull truncated at the prop, with a really big trim tab, could broaden the range of such a system. Or as was already said, just take out the hook and add off the shelf tabs. It may be too much like chasing a conventional OB/trim tab solution, but I think there is potential in a more integrated system....

Which brings another question, anyone know how the tolman skiff with seabright bottom performs? I have seen only one picture on the web, and the transom appears to be cut for OB. Very curious how (if?) it will work with OB trim.....

fcfc
03-10-2007, 02:32 PM
AT 12 kts, the next crest is (12/1.34)² ft behind your bow = 80 ft. I do not see how any boat smaller than 40 ft can regain any energy from the wave system. But I may be wrong.

messabout
03-10-2007, 03:44 PM
MOT, FF;
Building models for experimental purposes, is one of my numerous character flaws. I have an attic full of them. Some of them actually worked as hoped. Some of them did not. In either case they were/are fun.

FF I suggest that you build a model to a much larger scale than 1" to the foot. Here's why. Say that your full sized boat is to be 25 ft. LOA. The all up weight will be 3000 pounds. (These are only arbitrary numbers for illustration) Then your 1' scale model will be constrained to a maximum weight of 1.74 pounds. That is hard to do. What with servos, motor, reciever, and batteries the boat will be overweight and similitude will be lost. With a 1/8 scale model the prospects are better. The 3000 pound simulation can weigh 5.86 pounds. That is more realistic for the working model. 2" scale is even better. and model weight can be 13.8 pounds which gives you some latitude for shifting weight around in the model boat to simulate the real thing. What is more, the larger model is far easier to build to acceptable scale tolerances. Also, knarled fingers work more easily with larger parts. By the time you get to a model of 50" length, you can use door skins or 3 mm Okoume for many of the parts including planking. The larger the model the more nearly it can deal with the wrinkled water on the pond.

As a matter of fact I wish this whole damned subject had not attracted my attention. Now I will be forced to think seriously about building an RM model.

sal's Dad
03-10-2007, 05:36 PM
A 1/4 or 1/3 scale model can make a nifty bookshelf, too! Just cut every other station out of a nice 1/2" ply. But it is hard to get those underbody curves, even with wetted luaun doorskins.

(see photos of rougher models in the Yahoo AtkinBoats group files).

FAST FRED
03-11-2007, 05:48 AM
2 in to the ft would be even easier to use to see if the hoped for interior and design features would fit into.

With such an unusual hull shape would the std computer performance stuff work?
I'm now just using a simple nomograph found in Lord's book , and a modern similar one from Pro Boat Builder to estimate weight vs speed .

Although I plan on building a full sized junk mockup ( cardboard and damaged ply) to work on the first 12 ft of hull.

With a very narrow beam forward and my personal requirement to NOT have to climb out of bed over the pillow (prefer narrow side isle on one side) the drawings sez it will fit , but....

"You don't BUY beer , its only rented"

FF

messabout
03-12-2007, 03:39 PM
FF;
The Propeller Handbook by Dave Gerr has some good stuff about weight, power, speed. You'll want to select a prop sooner or later. Very good stuff about that too. Get the book if you do not already have it.

Sorry Brands01, we seemed to have gone riding off in several directions. We did not intentionally hijack your thread.

An old 1945 edition of "Boats you can build" has an Atkin SB that is both powered and rigged with a sizable sail. The text claims that it sails well including windward work. Sure enough the keel turns down a bit aft of the prop. In this iteration of the SB the bottom of the transom is rounded up toward the chines. Presumably in deference to heel under sail.

FAST FRED
03-12-2007, 04:15 PM
I'm wondering if a hair brained idea might help speed.

As the box keel could as easily be built of metal, the surface could work as area for a keel cooling setup.

IF the cooling water was run thru piping fixed to conduct very well would the "waste heat" warm the water in contact with the appendage enough to lower the waters viscosity?

I'm looking for "free" speed from the engine waste heat?

FF

Brands01
03-12-2007, 05:20 PM
Hi Messabout - no worries about hijacking the thread - Its far more interesting now than when it began!

sal's Dad
03-12-2007, 09:25 PM
As the box keel could as easily be built of metal, the surface could work as area for a keel cooling setup.
Yes, that's the plan! I wouldn't expect a very modest change in temperature to have much effect on efficiency, though.

FAST FRED
03-13-2007, 06:32 AM
Yes, that's the plan! I wouldn't expect a very modest change in temperature to have much effect on efficiency, though.

The big boys (PT Boats)found a prop that is fine in 70 deg water may need 2 inches of diameter taken off for freezing water ops.

Of course 40deg F is a huge difference , but an engine running 3gph will still be able to dump 100,000/hr btu into the keel surface which might allow a speed improvement?

FF

FAST FRED
03-13-2007, 12:11 PM
The folks at Shannon ,,

http://www.shannonyachts.com/38faq.html

have a hull structure that mimics the Atkins to a large extent although the keel is better faired , and the area aft of the prop does not perform lifting the stern as much.

Same claims tho for excellent sea keeping , and low fuel burn at speed.

FF

BWD
03-13-2007, 10:29 PM
fcfc wrote:
"AT 12 kts, the next crest is (12/1.34)² ft behind your bow = 80 ft. I do not see how any boat smaller than 40 ft can regain any energy from the wave system. But I may be wrong."

Well, my simple thinking is:
The crest of the following transverse wave is there because a volume of water has been rising since it left the transom (or before...).
Propelling the boat gives the water the energy to form the wake. If part of the wake is a wave, each part of the wave is as much a part of the wave as the rest. I can't see how this would exclude water under the boat, although it may take longer equations to describe.
Although I am unschooled, maybe have the wrong idea.... I am a naif in hydrodynamics.

This is a neat design because it challenges rules of thumb, so I have not tried to think of rules of thumb in understanding it. I see no free lunch in the designs, just unusually sophisticated optimization for a small boat.

My trim tab idea is quite crude, just an idea for cheap variable geometry, perhaps allowing a boat to retain the shoal performance and some other qualities of Atkins skiffs, but act like a typical planing form at higher speeds, at some cost in efficiency, obvious questions with LCG, etc. Probably PARs idea is better -simpler and off the shelf, recyclable.
My apologies if the trim tab idea seems an affront to the charm of the Atkins hull shape, which appeals because it is just that, and needs no fancy hydraulics or materials.

BWD
03-13-2007, 11:24 PM
After looking briefly at the page for the "SRD 38"
I will venture to add,
vortices ain't all bad.

FAST FRED
03-18-2007, 05:14 AM
On most plaining boats the trim tabs lever the hull Down to a more efficient running angle.

With the Sea Bright design the tabs could work very differently. The aft end of the boat curves down , forcing the back to lift at speed. The Motor Boating Article described a problem with too much lift from doubling the engine power.

Atkin's cure was a simple set of wide chines forward that lifted the bow to stock running trim.

It is possible that if the aft curve WAS extended trim tabs , going faster would only require there retraction , to lessen the aft lift , and regain efficient trim.

It might be possible to use the trim tabs (getting thinner ) to lower the bow to the cheapest/fastest running angle at any speed.

FF

tom28571
03-18-2007, 09:22 AM
It is possible that if the aft curve WAS extended trim tabs , going faster would only require there retraction , to lessen the aft lift , and regain efficient trim.

It might be possible to use the trim tabs (getting thinner ) to lower the bow to the cheapest/fastest running angle at any speed.

FF

Sounds like an excellent idea Fred. However, the worst things about the bow down attitude of too much power on the Atkin hull are handling problems and safety like bow steering and foundering in turns or waves at speed. Wetness is a nuisance and we should remember that these boats were designed to give best performance at low planing speed and are not ideal for high power and high speed no matter what modifications are made.

Another thought is that adding lifting strakes on the bow also requires some energy to counteract the aft lifting force of the tunnel downturn. Both of these reduce the efficiency of the boat below that of a boat designed for the higher speed in the first place. Your tab mod might give better overall satisfaction throughout the full speed range though since it could be trimmed for each speed.

xarax
08-09-2009, 10:07 AM
In some designs the box keel s midsection is boxy/rectangular, with a flat bottom, ( to increase the total surface of the semi-planing hull ???), and in some others it is U shaped, with a round bottom, ( to increase the total volume of the semi-displacement hull ???). Should the box keel s longitudinal axis remain horizontal while the boat is moving forward ?

FAST FRED
08-10-2009, 06:23 AM
"Should the box keel s longitudinal axis remain horizontal while the boat is moving forward ?"

In other words would a "Water Ski" style bottom be efficient at creating lift , and further reduce power required at speed?

Good question as these boats seem best below SL 3 and cruise even slower , but there IS dynamic lift there.

Some thinking is that 3/4 or so of the displacement should be in the box keel, which would give a good area to generate lift.

The newest navy tri seems to be a box keel with training wheels (amas), to create a wide platform for choppers to fly off.
Wonder if it would be faster with a bit of main hull also creating lift at 60K, and no training wheels.?

FF

xarax
08-10-2009, 12:32 PM
There are many elements in a box keel design. Lets point out some of them;

1. Distribution of volume between the box keel and the rest of the hull.
2. Length of the box keel. Should it run over the whole length of the hull or not ?
3. Form of the box keel. Should the whole hull remind a slender catamaran hull placed underneath a planing hull ? Or two superimposed planing hulls, the beamier one placed over the other ?
4. Underwater nose and tail shapes of the box keel. Bulbous, pointed, vertical,etc.
5. L/B ratios of the box keel and of the whole hull.

The whole concept seems very interesting, but I can not find any systematic evaluation even of the few existing variations , ( and no systematic series of course...) Why is that so ? I believe that the main reason is the limitations of the existing computational tools, that are not so advanced to handle such complex hull forms...The low CG of these hulls might be an advantage even in the case of future faster motorsailers, with powerful heavy engines, batteries, fuel and water tanks, ( even some of the lead ballast in the case of a swing keel boat like the Southerlys) placed into the box keel.

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