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

Discussion in 'Boat Design' started by kengrome, May 19, 2008.

  1. kengrome
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    kengrome Senior Member

    This thread continues the discussion started here:

    For those who don't want to read the previous thread, we were discussing propulsion efficiency of Atkin tunnel-stern v-bottom Seabright skiffs and we concluded that maybe some comparison testing is in order, specifically to answer this question:

    Are Atkin tunnel-stern Seabright skiffs more or less efficiently
    driven at 0-15 knots than other reportedly efficient hulls?

    Someone suggested that we would need a financial sponsor to conduct such tests, so I said maybe we can fund the tests ourselves via individual contributions. Then I offered to build and test the boats in lieu of making a financial contribution myself, and I proposed a budget of $700.

    The $700 hasn't been funded yet (and I forgot to include fuel needed for testing in my budget) but it took only two pledges to get a commitment for $800, so it seems we are on-track to proceed. I would like to keep the "funding doors" open to additional sponsors for another $100-200 to cover any unforseen expenses, assuming we proceed.


    Special thanks to Fred and Alan for pledging full funding for these tests! If you are still willing to sponsor this effort I will send you private email so you know where to send the funds, and you can send them at any time. While I'm waiting for the funds to arrive we can discuss which boats to build, and any other details we might like to discuss ...

    The boats in contention include Heron, Nibble, Rescue Minor and Shoals Runner. These are the only Atkin tunnel-stern v-bottom Seabright skiffs I'm aware of. I plan to input the selected design into FreeShip so I can generate precision cutting patterns for the hull panels and station molds. I favor Shoals Runnner since it is Billy Atkin's last (and possibly best) effort in this line of boats. The others are possibilities too of course, and I can propose one of my own designs as well -- but I think that we should first decide upon the tunnel-stern v-bottom Seabright skiff to build -- and then use the same waterline length and breadth on the second boat.
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  2. kengrome
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    kengrome Senior Member

    Not necessarily with the same "power system" ... but definitely using two engines with the same fuel consumption at identical power settings. After all, the measured component here is fuel, and we are not supposed to be comparing fuel efficiency, we are supposed to be comparing hull efficiency. In my mind this means we need two engines with identical fuel consumption curves ... and the best way I can think of to create this situation is to use two identical engines.

    So is it wrong to use a horizontal shaft on the Seabright boat and a 10 degree downward angle on the other boat?

    This is my biggest question at the moment. My thinking here is that if the outboard is commonly tilted down at 10 degrees in normal use, then it would also be correct to set the inboard's prop shaft at the same 10 degree angle. After all, if we are going for the "most efficient" conditions on both boats, using an angled shaft on the second boat is precisely the right thing to do if the most efficient outboard tilt is also at an angle.

    If an outboard and an inboard were proven to have the same fuel consumption numbers at the same power settings, I would very much prefer to put an outboard on the second boat and an inboard in the Seabright. The problem with this is that it is a practical impossibility.

    After seeing your argument I agree -- but only if we find a different way to measure hull efficiency since fuel measurements are more likely to point out engine efficiency differences than hull efficiency differences. What other measurement methods are there for poor people like us?
  3. tom28571
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    tom28571 Senior Member

    Ken, I do not tilt my outboard props at a down angle. They are mostly set at horizontal at rest. Of course this will not be horizontal when underway. However, the effective water flow exiting the transom will not be horizontal either. The true angle of the prop will be the result of the angle to the water flow and the angle of the prop to true horizontal. What this is, I don't know but most of us will adjust the prop angle of an outboard to give what we determine the best performance as we subjectively sense it to be. This is different for different conditions.

    Your last two paragraphs point out why it is so difficult to get a handle on objective measurements like these. I do appreciate you willingness to tackle this project though. If we decide on a specific tunnel hull to test, can we then develop a trial horse that is as near the tunnel hull as possible except that it will not have a tunnel? That seems to have real snags too. It might show the value of a tunnel but the trial horse would not likely be as efficient as it could be without those restrictions. At least it satisfies my wish to have as few differences as possible between any two test candidates in any kind of experiment.
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  4. kengrome
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    kengrome Senior Member

    I can design a hull with the same waterline length and breadth dimensions, but I would want to get some input from other folks as to the specific shape of such a hull. I'm sure I could come up with one that is very easily driven though, probably just by copying an existing easily driven hull.

    I suggested Nina or Ninigret earlier and I think one of them might be the way to go for the second hull, with whatever adjustments might be necessary to match the dimensions of the Seabright hull -- specifically waterline length and breadth since these would be the most important to match.

    I don't think we will ever get rid of all the potential problems, so we will probably have to agree to "get close" if we are to do this at all ... and hopefully we will get "close enough" to get some useful data out of the effort.
  5. fcfc
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    fcfc Senior Member

    Comparing apples to oranges ...

    But it won't satisfy my own wishes.

    If you take same hull, but without the tunnel, it is likely to displace more, so heavier. Which is very unfair from my point of view. Displacement being a major figure for powering.

    And then, if you make it same displacement, you have to remove volume somewhere. Do you decrease Cp, which disfavourably affect performances, or do you decrease deadrise, which will ease planning ?

    Another point, will the conventional hull still be limited to a 10" prop, which is requested for atkins shoal draft, or will you go to a more efficient 12-15" prop ?

    It will be very hard to agree on what is a fair comparison.

    As for me, a fair comparison would take only some key parameters such as lwl, displacement, bwl, shaft speed, speed range, and design an optimized hull for these parameters.

    After all, the atkins hull is said to be optimized. Why a comparison hull should have same parameters such as Cp or LCB, which are likely not the optimal value for a conventional non tunnel hull.
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    FAST FRED Senior Member

    While I'm waiting for the funds to arrive we can discuss which boats to build, and any other details we might like to discuss ...

    Don't hold your breath ,

    Having the machine hulls carved , glassed and powered with an electric motor (if it can be done in Tad's tank ) will give the useful results that a couple of old motors with no way to measure their power output will.
    Recording the watts (VA) consumed during any run is not expensive and has repeatable accuracy.

    This is not a JOBS Program , this is to find the facts of how efficient an Atkin box keel reverse deadrise ABKRD (not a Seabright) hull is in realistic measured comparison with a similar "std" hull.

    ALL the ABKRD were beachable so any prop would need to fit the origional criteria, and the first test boat should be built as close as can be done to the origional. I will donate River Belle plans if there OK.

    IF the scale were simple enough , perhaps other designers could build a hull of their choice , and stick in the same electric motor and have another trial to see if something is even better.

    My suggestion is the work should be done up to a SL of 3 or 3.5, beyond which a std plaining hull should be superior.

    Although Atkin seemed happy with SL 2 or so.

    Perhaps Rick will help with prop construction?

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    FAST FRED Senior Member

    "The more I think about it, the more I think it needs a government grant over several years at MIT to get some decent data. Damn, that is getting pessimistic"

    Perhaps not , there are lots of folks on wooden boat boards and the Tollman folks with similar ideas.

    All we need is a student with access to a pool and the need for an interesting term paper.

    Any SKOOLS out there?

  8. Willallison
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    Willallison Senior Member

    Ken, as Tom has suggested, under normal planing conditions, there is almost never any downward trim on an outboard. Quite the opposite in fact, most boats will run with at least a little upwards trim. Heavily laden craft will often require perhaps one or two degrees on downward trim, but usually only to get up on the plane. I think what you are referring to is the standard 12 degree aft rake of the transom, but this bears no real relation to the direction of thrust.

    As I stated on the other thread and as Tom has reiterated here, the only real way to establish whether one hull is more efficient than another is to optimise the propulsion system to the boat in question. Which I agree, greatly muddies the water when you wind up trying to compare consumption of different powerplants.

    Analytically, Tad is correct in suggesting that one should start with towed models. The data from this would then provide a solid basis for the construction of self-propelled vessels. Clearly the ABKRD (sorry FF, living on the other side of the world, I'm forever confusing which skiff is which) will be at a disadvantage here, because previous tow tests as well as the real thing have indicated that the action of the propellor is fundamental to any efficiency gains inherent in the design. But it should still provide a basis for optimising the hullform.
    Tasmania is blessed with one of the better maritime colleges - complete with test tanks. Sadly I don't have any real contacts there, but I could certainly make some enquiries.....
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  9. Gilbert
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    Gilbert Senior Member

    I doubt that the prop shaft angle on a boat with a S/L ratio of 2 or 3 or even 3.5 will really matter. I am basing this opinion on the results of a local builder who compared boats they built that were the same except for the shaft angle and said they could not tell any difference at all between them running side by side. They did not test any shaft angles more than 30 degrees. And they said that at 30 degrees there was a difference.
    I also don't think that tank tests at maritime colleges are cheap. I hope I'm wrong about this one.
    Last edited: May 20, 2008
  10. kengrome
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    kengrome Senior Member

    Optimizing both hulls for the same speed range would also be necessary. A good hull tested against a bad hull might produce the opposite results of two good hulls, and I don't think anyone wants the results to be skewed by this kind of error/oversight.

    Well ... if this is the requirement for a valid comparative efficiency test, it looks like I'm not the one to be doing such tests.

    A propulsion test in a tank, using an electric motor, is a very different scenario from what anyone other than Fred has suggested so far, and I believe it deserves more consideration -- if enough money can be obtained to actually get such a complicated and costly testing program accomplished. The problem is the lack of money ...

    Just think about all the issues that would have to be resolved because of the need to compare an inboard with an outboard, even in a simple tank-based propulsion test using an easily measured electric motor:

    Where are you going to get a tiny little outboard electric motor to use on the second boat? Will it be custom made for these tests, and at what cost? Outboards have gearing and 1:1 Seabright inboards do not, or if they have a gearbox they don't need it and thus can be "optimized" by not using one. So what about the inefficiencies in the outboard's gearbox, how will this be taken into consideration?

    Who will make the propeller, and will it be a realistic scaled down likeness of a standard prop that comes with a full-sized outboard, or will it be a custom made, super-efficient prop which is optimized for maximum efficiency -- but that no one in the real world will ever have on their boats because of the extreme cost? And if the second boat gets a super-efficient custom prop, the Seabright gets to have one too, right?

    How do you decide the outboard's tilt angle? Will you run an extensive set of preliminary tests to figure out what's the best angle for each speed setting then adjust to this angle during later testing? And if you're going to do this on the second boat, are you also going to do it in the Seabright boat -- just to be certain that the prop angles on both boats are truly optimized? After all, just because we 'assume' that a zero degree shaft angle in the Seabright hull is the best does not necessarily make it so.

    Then there are still questions about the actual hull shape to be used in the second model ... and related questions about whether or not to use an original Atkin hull or to use "something similar" for the Seabright model:

    Do you use a computer to optimize a simple hull shape for 15 knots and build that model hull as the second hull? And if you're going to use a computer to optimize the second hull, shouldn't you also use a computer to optimize the Seabright hull as well? After all, Atkin never had computers available to optimize his hulls so it is likely that they really are not optimized at all, regardless of the claims we hear to the contrary.

    I think what all this comes down to is that there needs to be a clear explanation and agreement of exactly how much "optimization" is going to be required -- or allowed -- and where the line is drawn between more or less optimization in each feature of the propulsion systems and the hulls.

    Easy questions to answer with everyone in agreement, right?


    Basically I thought we could avoid optimizing the propulsion systems by equalizing them. Then all we would have to do is optimize the hulls which can be done easily in computer software, build them, and then compare their performance in side-by-side real world tests.

    The fact is, if you put a bigger slower spinning prop on the Seabright skiff you will increase its fuel efficiency over the stock propeller too -- just like when you put a geared down outboard and a bigger prop on the other boat. In other words, it seems that by allowing any difference in power systems you are automatically favoring the more efficient system and therefore the hull in which it is installed ... aren't you?

    I was actually hoping (against my wife's better judgement it seems) that I could proceed with my proposal and it would be a good cheap way to quickly learn which hull takes more power to push at various speeds. But she thinks it's a huge waste of shop time and manpower, not to mention hundreds of hours of my own personal time, so she is against it -- and with good reason it seems.

    Nevertheless I've made my offer and I'm going to stick to it. If the funds become available I will proceed. Until them I will simply take Fred's good advice and "not hold my breath" ... :)
  11. redu
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    redu Junior Member

    angled axle drag

    Thanks Gilbert!
    Would it be possible to learn more of these local builder tests? What is essential, are prop diameter and rev/min (pitch). In theory, wide diameter prop and low revs are bad. Lots of more angled axle drag.

    These Atkin tunnel boats are a Myth? Mythbusters requested!

  12. Willallison
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    Willallison Senior Member

    Gilbert - what kind of boat are you referring to? I can imagine a scenario where the lift created by an inclined shaft might offset the loss of thrust, though off-the-cuff I would think that this should only be the case in a relatively inefficent hull (perhaps heavy, deep-v...).
    As far as paying for tank testing at the Maritime College, I was thinking more along the lines of suggesting it'd be a great project for their students to undertake;)

    Ken - don't mistake my apparent negativity as a dismissal of your proposal - but it would be shame to go to the considerable trouble of building and testing two "models" that don't represent best case scenarios.
    I absolutley agree that, carried out correctly, your suggestion would go some way to confirming the relative efficiencies of the skiff in question and an otherwise similar shaft driven vessel. But is that what you want to know?

    It won't answer this question...

  13. kengrome
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    kengrome Senior Member

    Will, don't worry about the negativity, I'm not afraid of it. Then again I'm not sure I agree that the "best case scenarios" need to exist before the effort becomes worthwhile. Maybe "good enough" will work here -- you know, like a workboat finish vs. a yacht finish? The workboat isn't pretty but it gets the job done, and there's a lot of good to be said for workboats even if they will never satisfy the champagne and caviar crowd.


    Actually I think this would be a very good start at this point in time. In fact this may be the only comparative study that ever gets done on these boats for all we know. So if it tells us something we didn't know before, then maybe it really would be worth the effort ... ?

    You know, most of the boats I'm designing are inboard-powered because using a diesel inboards gives them great range ... and in countries where diesel is still cheaper than gas it makes them hugely more economical too. But I also designed a tunnel-stern v-bottom skiff that takes an outboard instead of an inboard -- because if I want to sell it to Americans I have to give them an outboard option.

    I would think nothing of using two outboards in these tests if someone were to provide them. But then everyone will complain that the Seabright is not properly optimized ... and in one respect this may be true, but only because the outboard leg would produce more resistance than the inboard shaft tube. Nevertheless, I woudn't let this stop me from trying to learn something from the comparison, even if things aren't exactly perfect.

    Here's a thought ...

    I could do the inboard comparison now, then just save the boats until we can get enough additional money to install outboards in both of them -- then re-test them again with outboard propulsion systems. The bigger outboard prop may extend below the hull bottom in the Seabright boat, but so what? At least we get to test inboards and outboards in the same exact boats, with no more than minor modifications in between.

    Just imagine what might happen if we learn that in
    both tests the same boat is the efficiency winner!

    The results might not turn out this way of course, but that's what testing is for -- to learn something new -- even if we learn something other than what we were hoping to learn.

    I'm sure we can learn other things too ... the difference in Seabright handling with different propulsion systems for example, or the behavior and misbehavior of the two different hulls in side-by-side comparisons when turning at high speed, running into a chop, launching, beaching, capsizing (I hope not!) and more.

    There's lots to learn, this could be just the beginning ...
  14. ASM
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    ASM Senior Member


    for what its worth as a non naval architect but more as a customer, I would be interested in fuel consumption vs livable space vs price tag of drive train (thus boat). So this means the two vessels need more or less to have the same usable space (and weight allowance) and the best (most efficient)propulsion for that boat at certain cruising speed settings, no matter what drive train this will take. The reported handling could be an additional plus, though would not be the drive to make a certain hull shape (provided one does not create an impossible to handle craft).

    But again, see me as a customer, without the need for ultimate speed, but efficient cruising at a reasonable speed.

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    FAST FRED Senior Member

    "But again, see me as a customer, without the need for ultimate speed, but efficient cruising at a reasonable speed."

    That too is where I'm at looking for good (or GREAT!) fuel consumption at 10K to 18K on a 38ft lwl.

    There HAVE been some of these ABKRD built over the years , I have seen one at least on a wood boat site.

    It is unknown weather the builder stayed with Atkin , but also unknown is the results of the build.

    Perhaps we could find a couple of these boats , get their particulars and talk the owners to allow an actual performance measurement .

    Running fuel with a measured can , and a gps would allow at least similar boat comparisons.

    Wouldnt be the same as same boat comparisons with actual energy use, but.

    With identical models the boats could be yoke towed behind a small boat , and the yoke would show the results as equal VA ,( power) is fed to each at the same time.

    Two motors and props would raise the model cost , but at a small scale the motors are not too expensive.

    Best of all it would be really EZ to add weight to each to find performance difference from the added weight.

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