Comparison testing of an Atkin tunnel-stern v-bottom Seabright skiff and another boat

I was a while ago playing with an idea of making a scale model of a seabright skiff with replaceable inserts in the aft bottom. With differently shaped inserts one could easily try different tunnel shapes.

Then ride the model with electric motor - log speed and amperage from test runs. I thought this would give somewhat reliable data if one form would be better than other. However I lack the skills to create a solid design to start from.

h

 

A change in venue ...

Given the fact that so many people here prefer to continue talking about off-topic issues rather than focus on the testing program I have proposed (and for which this thread was specifically created) I am going to move this discussion elsewhere:

http://groups.yahoo.com/group/ShoalsRunnerTesting/

You're welcome to join this new Yahoo group and contribute your experience, opinions and funds if you wish, but that's where I'll be reporting my building and testing progress from now on ...

 

using electric power certainly makes it easier to monitor total energy used at any particular time but electric has drawbacks for the tests Ken has proposed--mainly excesive up front costs and then you need to throw in a certain amount of heavy batteries and all of a sudden the test boats need to be larger and the upward spiral continues.

With a model the testing would be done from a lead boat , which doesn't have to be fast due to the scaling.

Anything heavy (batterys), or not water proof (recording KW meter) would be in the lead boat.

Then the weights in the model could easily be changed to vary scale weight .

And the power consumption actually accuratly measured.

I too would donate to Tad if he is willing to go thru with this hassle.

FF

 

Come Back!

Ken
I would urge you to reconsider moving the thread.
I understand your frustration at perceived off-topic discussion, but it's all relevant to the cause. Your proposing to do a great deal of work with this testing program. A few extra days and some slightly wayward comments is a small price to payt for getting it right.....

 

Hi Will,

Don't worry, I'm not actually moving this thread -- so we can continue to discuss the same theoretical testing issues, alternate testing scenarios, and other related topics we've been discussing all along in this thread.

My Yahoo group is simply an additional resource, that's all. I need a place where posts remain on-topic and it's not happening here -- but it will in a Yahoo group because they let me delete off-topic messages before they are posted.

If folks here are interested in my progress but don't want to visit the ShoalsRunnerTesting group, I suppose I can post updates here once in a while, at least until I don't have the time any more. For example, just a few minutes ago someone added another $25 to the kitty ...

 

Low energy reference

Today
I made 14kn with this experimental el-boat. 6m Tornado center hull plus tiny outriggers. Total load 340kg including crew. Propeller 14"*13", three blades. Small wave drag indeed.
Power consumption from battery was = 11.3 kW.
Note1:
If Ken's Atkin model has about a similar total load (340kg), then 15hp motor instead of a planned 5hp is recommended, if max target speed really is 15kn.
Note2:
Using an electrical system makes for instance measurement of wattages and revs simple. Revs increase linearly according to motor voltage, and power from battery is amperes * volts there. Easy!
Note3:
A light 5m*1.4m planing boat hull, which makes 21kn with a 15hp gasolineYamaha, never exceeded her planing threshold using this very same electrical power system. Maximum speed was doomed to 8kn. A hull like this planing boat used as the second boat model in Ken's test would be a joke!

redu

 

Redu,
An interesting experimenal vessel you have there. Quite a contrast of different levels of technology and materials. Perhaps you might share some of the details. The outboard conversion is fascinating.

 

Thanks TollyWally!
Yes, low tech used effectively. The skinny Tornado hull is good to avoid wave drag, but it is too skinny to carry all batteries required for long trips. Also tested on a full Tornado platform with two hulls. The speed was a little bit higher than speed of this Tri.
Attached also photos of low tech adapters between 10hp el-motor and Yamaha15hp rig. Gasoline motor crank shaft terminal was used to match the adapter to rig vertical axle.

redu

 

2. Is there something about the Atkin tunnel Seabright skiffs that is inherently more efficient than the more numerous Atkin NONtunnel Seabright skiffs?


So far its all speculation , one reason could be the capture and use of the boundery layer already accelerated from forward.

I have been contemplating an additional possability.

We all know that the prop has tip losses , and thrust is better when captured by a Kort nozzle or some type of ring close to the blade tips.

Well the reverse deadrise would also capture that energy , which perhaps helps pressurize the tunnel and lift and accelerate the boat as it leaves aft.

Maybe a few percent , added to other few percents (almost flat shaft angle) adds up to the claimed performance.

I would be delighted to spring $200 for a SCIENTIFIC test , not a boat race that will give nothing useful to advance or even test the BKRD concept.

I would also love to visit the portfolio of other boats finished , weight , power amd speed, by the prospective builder.

FF

 

Fred, there have been a number of RM completions as of late, and the performance statistics can be ascertained from at least one builder who finished a hull commercially. Talk with Pat over at atkinboatplans, she would have the names and numbers. I do know for a fact that the numbers from the as-plans hulls in sheet ply and frames are not nearly as stunning as the ones that came from Robb. Robb was an unfettered genius, building by eye quite often, and may have stumbled upon not only light displacement, but may have manipulated hullform slightly by using his tulip poplar strip planking techniques. His boat had bona-fide numbers, but I honestly think it was his tunnel shape that was the key. When building the tunnel, he had no hard edges. The BKRD was to plans, but specifically the chines of the boat were round and voluptuous, and sat precisely at the waterline. It is my wild *** assumption that he enjoyed higher performance with his light weight AND the water's desire not to break away from those round chines. I think he had less propensity for aeration than if you were to build a sheet ply RM to the exact same hull and engine weight. Again, Im just a punk kid, but his boat was leaps and bounds more efficient than the ones being turned out now.

Ken, why not just finish your tunnel stern Tolman, and test it against Tom's numbers from BJ24? Same outboard, toy with the displacement to get comparable to BlueJacket, and see what the numbers say. That seems like a nice first step into your two-boat concept, considering you already have the hull built.

Also, on the Hand-type hulls. I finished my mahog cored composite iteration of a Ninigret/Hand hull (A Mertens LB22 NINA called Cloud Cap). At 1500 lbs and 50 hp, I am having trouble getting 5 gallons burnt in a 3 day weekend of fishing around our inland lake. If I can track down a fuel meter, id love to give numbers to this cause, giving a start point to a collection of hulls running at SL 2-3, weight, hp, and other desired parameters.

E

 

Hello EStaggs,

My testing goal is to learn which of the two hulls is more fuel efficient at 15 knots. The cheapest way I can think of to determine this is to build two smaller boats optimized for 15 knots then test them side by side. I was surprised at how little this would cost when I calculated everything out, and that's why I proposed these tests.

Finishing the Tolman enough to get it on the water will easily cost twice as much as the proposed tests. What's more, the Tolman requires an inboard engine so I cannot simply plunk an outboard on it like Tom can on his boat.

Yes it is possible to modify the Tolman for outboard propulsion, but it needs an outboard well for this, and the well must be sealed so it doesn't pump water up into the hull. This is not an easy task to accomplish unless I epoxy or Sikaflex the engine into the hull more-or-less permanently -- or engineer a special well and engine attachment that mate together and effectively seal the opening so the engine can be installed and removed more conveniently.

I already have a concept for this second type of engine installation, mostly because I know how popular the new 4-stroke outboards are in the USA. High diesel fuel costs make the new outboards even more attractive compared with inboard diesels. But before I get into prototyping a special outboard engine installation for the Tolman, I would like to know whether its hull is more or less fuel efficient than simpler-to-build hulls ... and right now I still don't know the answer to this question.

There is no question that the tunnel-stern boats are practical and worthwhile if shallow draft is of primary importance, because in places where shoal draft is critical a deeper draft boat simply will not serve the purpose no matter how fuel efficient it might be. But if the tunnel-stern boats happen to be more efficient than the non-tunnel-sterns, then it would seem to make sense to build them ALL with tunnel-sterns, wouldn't it?

I've heard similar reports from others that Ninigret / Nina is a very fuel efficient design. This is why I suggested using one of these hulls as the second boat in my proposed comparisons. Your report encourages me even more to design the second boat as closely as possible to this type of hull. Then when both hulls are optimized by Michlet / Godzilla we will likely be starting with two very refined hulls, each with its own unique characteristics.

I still don't know how to use the software to do this optimization myself, so if I haven't learned it when the time comes to start building, I'm hoping I can impose on Rick Willoughby to help me out. Rick is an expert when it comes to using this software to optimize hulls, so I think his help would be a real value here.

 

I guess it seems to me that the penultimate way to put numbers on the board is to build a hull, power it, and see what happens. If you have a half finished tunnel stern seabright laying out in your yard, why not put it in service. Renn's won't be done for a while, you have the manpower, whats a few bucks out of your own pocket? I see what others are saying about boat race v. true testing, and I can understand why people don't want to part with their cash.

I think if you want to get something out of it, self-fund the study. When you complete it, build a standalone website with an attached forum. In order to recoup your costs, sell advertising on that site using google ads or something along those lines. Maybe Pat will want to put an Atkin ad on the site because it brings up many of the designs she sells. This may be a more effective way to garner the cash, as for most of us, its a little hard to take $100 and stick it in an envelope to the Phillipines on a test that may or may not tell anyone anything other than an anecdote.

Best of luck

E

 

Of course, but only if you care about the specific performance of that hull instead of comparing it to another hull.

I don't have the cash to finish it right now.

Renn has installed his propulsion system, his boat could be on the water in June.

It's not a few bucks, it's a few thousand bucks to finish the Tolman.

It is not a race ... and it *IS* a true test, regardless of the disrespectful or denigrating way some other forum member chooses to characterize it.

People always make their own decisions about how to spend their money, I see no problem with this.



Look, here's the bottom line:

My tests will prove which hull is more efficiently driven of the two. Once we know this, we can all make more intelligent decisions about which style of hull to use on our own boats if we want the best possible fuel economy.

That's it. That's all there is to it.

 

Well said Sir...

 

"My tests will prove which hull is more efficiently driven of the two."

Perhaps , but it MAY only prove which engine gives a bit more power at a calibrated RPM , or calibrated fuel burn.

It may also only prove which guess at the "proper" propeller came closest.

Might only prove which stuffing box was tighter.

There is nothing "scientific" to be learned from complete mismatches of gear , with ZERO chance at measuring the engines output.

May be loads of fun , but surely not "scientific".

FF