Jersey Sea Skiff Performance?

To FF's qestion:

I think it could be done but the model needs attention first. The Motorboating article lines I used are muddy, so the model may be out 1/2 inch any old place, especially around the keel/tunnel.

Also, the rescue minor lines don't show as much vertical step to the keel chine, and I left this feature off my model, although it is not a model of RM, but an overscaled Grampus.

I believe these little steps may be important to the shallow water performance, generating vortices on longitudinal axes that will facilitate attached flow around the keel into the propwash and up into the inv. vee tunnel, affecting formation of the wake and reducing squat.

I think you would need a hydrodynamic modeling approach that could accurately include this type of flow, and prop wash interaction with it, if you want to look at a hull that has theses chines. Unlike conventional step chines, they stay immersed at speed.
Algorithms developed from flow around more conventional hulls might not be right for this one. I won't say more as I know very little of this type of software.

If I tried to build such a boat, I would start from RM lines in the .gif posted by fcfc, keeping in mind that it does not show much of a step to the keel chine. In tuning the design, you could "easily" add or subtract sheets of ply to the flat bottom to vary the degree of step, if needed.
I might also build with less hook and add central trim tabs to try to tune things, then wedge in hook as needed, as I think PAR suggested earlier in this thread or the related one.

 

I have ordered the plans of "RIVER BELLE" from Mrs Atkins , as it is one of the last Sea Bright designed hulls.

Although RIVER BELLE has a deck house of huge size that would indeed condemn her to inshore use the lines plan is what I need.

The plan is to redraw the boat , using Herrishoffs STROLLER as a guide for the above water "look" and a slimmed down RIVER BELLE underbody.

I decided that a 1 inch to the ft model will assist in getting a rational interior .

I intend too tow the model alongside a small power boat , and install an electric motor only in the hull.

I should be able to observe any undesirable features as I power the scale model from the towing boat.

IF it works OK , A "Box Boat", probably of foam core or cold molded ply will be built.

Should be a great retirement cruising vessel, with all the ocean passages done by "Big Box Maru"in a shipping container.

Fingers crossed,

FF

 

That's funny Fred, as I spent about an hour last night entering Atkin's River Belle lines into DELFTship as a background image. I'm following the tutorial the best I can, and I've made some progress, but I've got a long way to go yet. What I need is a copy of "DELFTship for Dummies". Anybody know where I can get one

I'm an old dog I guess, but I'm still much more comfortable with clear pine splines and a #6 pencil --

When (!) I'm successful in getting the lines in there, I too plan to modify them above the W.L. -- Like you, I want to get the B.O.A. down to 7'6" or so. While I've seen LFH's "Stroller" and like it very much, I prefer the simplicity and extra interior volume of the raised deck style, so I'm thinking of something with a Blanchard or a Lake Union "Dreamboat" look.

I'd like to strip plank mine using Lindsay Lord's original method, crown/cove white pine (.40/bf here from Amish sawmills) sheathed inside and out with Dynel or Xynole in epoxy. Time comsuming? -- Hell yes! But v. light/strong and allowing for a much more "boaty" shape with lots of flare/flam in the bow and tumblehome in the stern.

The model idea is a good one, but you might think about making it bigger for hydro testing purposes. I don't think that a model 6' long would take twice as long to build as one 3', but the test data that it would yield might be that much more accurate.

moT

 

"but the test data that it would yield might be that much more accurate."

The only TEST DATA I'm going to get will be from my Mark 1 Mod 2 set of Eyeballs , as we drag it alongside.

FF

 

A couple comments:

There is another Atkin design, similar to River Belle, not in the catalogue. Alex Hadden built it a few years ago for a client, "Noble Cab" is cruising the Piscataqua. It was a difficult build, perhaps beyond even very skilled amateur capabilities. Reading between the lines of a recent conversation with Alex, maybe $400,000 to have one built to the same standard.

Before I thought about building or commisioning a big one, I'd ask for a ride.


It is my understanding that tow-testing of a tunnel-stern design will NOT provide meaningful data, the propeller pulling water into the tunnel is integral to its performance. This was discussed, perhaps on the AtkinBoats forum, a while back.

Sal's Dad

 

The Tolman Seabright Skiff is a brand new inverted-vee tunnel-stern Seabright design. This boat employs a bottom that is very similar to Rescue Minor. This new boat is a dramatic departure from all the other Tolman Skiffs (Standard, Widebody and Jumbo) which are plain old v-bottoms their full length with sharper vee forward transitioning to shallower vee aft.

I'm building one of the new Tolman Seabright Skiffs now, but have only just gotten started and my camera is dead so I won't have any pictures to post for a while. I became interested in these boats when I read about Robb White's Rescue Minor. One of his comments intrigues me more than usual:

"I think the dynamics of the hull that control
the wake hold the boat in a tight grip. It feels
like it weighs about 10,000 pounds."

I think this is one of the keys to the unusual performance of these boats. Most planing power boats do not suck their bottoms down onto the sea surface, but it seems like this is exactly what Robb White was describing. If this is what's going on, it is no wonder why he also had this to say about his Rescue Minor:

"The boat was so stable that my wife and I could
hardly alter the running trim by both us, and the
baby, sitting on the same side. She turns about
level and, even then, weight distribution doesn't
seem to affect the trim."

My best guess is that while the boat is running its prop is creating underbody suction that keeps the boat securely 'attached' to the surface -- so effectively in fact that it behaves like a much larger or heavier boat. Nevertheless the boat does not squat because of the reverse curve of the after portion of the underbody. Instead the stern actually rises under more power, and this without the need for it to be wide like most outboard powered boats.

I also find this comment in the Grampus/Disturber article very interesting as well:

"The bow wave, at a certain designed speed, is
to a large extent trapped, and caused to run
smoothly into what amounts to twin tunnels, one
each side of the box keel."

I think this is the key to exceptional efficiency when these boats are run at their designed speeds. When a designer can trap and harness the energy of the boat's bow wave -- instead of letting that energy drift away from the boat -- it suggests to me that there is every reason to expect these boats to perform better than the average boat.

Personally I would like to design and build my own line of similar boats, some in the 15-17 foot size range, for even lower power requirements and even more affordability. Not every one wants or needs a 20+ foot boat, and for those who do not I believe a tunnel-stern Seabright skiff might be just the ticket -- especially for those of us who prefer moderate planing speeds.

 

Where can we see the new Tolman seabright skiff? I am very interested to view this new design.

 

Renn Tolman is working on the prototype and I'm building the first non-prototype from Renn's preliminary plans. Renn doesn't want people bugging him about the boat until he is ready to sell the plans, so this may be why no pix are available yet -- and my camera is broken so I cannot take and post pix myself (and I wouldn't without Renn's permission anyways).

In other words, I think this is one of those times when patience is going to be worth the wait.

I'm taking a chance that the boat will actually perform well by building one before Renn has finished and tested his prototype. Nevertheless I trust that William Atkin knew what he was doing when he designed his tunnel hull Seabright skiffs, and I trust Renn Tolman's preliminary plans (although I have found a couple of errors / omissions) ... so we shall see how this boat actually performs when it is finished.

Don't worry, I will post more info and some pix here as soon as I can.

 

"My best guess is that while the boat is running its prop is creating underbody suction that keeps the boat securely 'attached' to the surface -- so effectively in fact that it behaves like a much larger or heavier boat. Nevertheless the boat does not squat because of the reverse curve of the after portion of the underbody. Instead the stern actually rises under more power, and this without the need for it to be wide like most outboard powered boats."

My reasoning for the flat,smooth ride would be almost opposite.

I think the hull rises aft from the water below being pressurized by the prop , lifting the aft area with enough force that any weight on the gunnels is unnoticed .
This could explain the flat level turns , lack of roll from beam waves and super smooth wake these boats leave.

The wake on the Shannon looks great , but the bow seems a very poor selection , blowing spray many feet into the air.
The much quicker Wallys don't have this spray hassle at low (20k ) speeds at all.

FF

 

Sorry Fast Fred, I don't understand your reference to Shannon and Wallys, are these also Seabright skiffs?

Regarding your thoughts on the reasons why Robb White's Rescue Minor performed the way it did, I think your theory and mine may both be true:

The stern rises because of the water being pumped beneath it, that certainly makes sense. What also makes sense (to me anyways) is that there is no air beneath the hull when running because the propeller has forced it all out of the concave underbody of the boat, replacing it with water ...

Now that there is no air and only water under there, the hull cannot be easily moved up and down vertically -- because every upward movement will have to suck more water into the 'cavity' and every downward movement will have to force more water out of the cavity. Both of these actions are far more difficult with uncompressible water vs. easily compressible air, so the hull simply doesn't move up and down very much.

 

The Shannon is a box keel style Shultz "invented".

The Wally is simply an example of a boat that makes very little spray forward , instead of the Shannon that sends huge sheets of water airborne.

FF

 

From Working Watercraft of the World. A bit of info on Seabright Skiffs.

Link: http://farm1.static.flickr.com/250/455875226_c2f8e271c3_b.jpg

Link: http://farm1.static.flickr.com/193/455875230_2dc2a3ca45_b.jpg

Hope this helps the discussion.

Best,

Leo

 

A Tolman Seabright.

Best,

Leo

 

Hi Leo,

I disagree with the writer's conclusion that tank testing of Seabright Skiffs proves that they are 20-25 percent less efficient than other designs, and that their handling will suffer much like any boat that had a beer keg attached beneath the hull.

Personally I believe the engine needs to be running and the prop spinning for the hull bottom to function as it was designed -- which means utilizing the wave that comes off the box keel. When these boats are simply dragged through the water instead of running under their own power, the advantage of the tunnel stern is lost. Too bad the writer didn't understand this.

Thanks for the picture of Renn's Tolman Seabright Skiff prototype in his shop, I couldn't get any of his Seabright photos to load when I visited his page on the fishyfish.com web site so I thought someone removed them. Is that where you got this picture?

Note that the plans do NOT call for a transom cutout, Renn built his that way so he could use an outboard if he had to for some reason -- maybe a backup engine?

 

Yep. FishyFish. Worked just fine in Firefox for me. Dunno about not loading in your browser.

Best,

Leo