Box Keel

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.

 

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.

 

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

 

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 - 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.

 

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.

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

 

"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

 

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.

 

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

 

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.

 

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

 

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

 

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.

 

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

 

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.

 

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 ...