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. Quietboats
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    Quietboats Junior Member

    Please count me in for $100. Very interesting thread and Ken's test proposal is especially enticing. I am planning on building SERGEANT FAUNCE this year using electric and then had my eye on a petro Rescue Minor or Shoals Runner as I have miles of 1 and 2 foot deep water all around. If we are still thinking about what to test, Shoal Runner would get my vote. Let me know where and how to send funds. Tom
     
  2. Tad
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    Tad Boat Designer

    Dave Gerr designed a few boats (built by Covey Island) based on the Atkin tunnel stern form. The performance of these boats, speed and power wise, is not great, but they are shallow draft. Gerr claims he stuck with Atkin's lines and the published sections of his boats appear to be those of Shoals Runner except the chine (on SR) disappears forward on the Gerr hull. But the outboard chine is well immersed aft, not like Rescue Minor where it barely touches the design waterline.

    The first Gerr Boat was Belle Marie, 41' LOA, 39'5" LWL, 12' beam, 19" design draft, 23" actual, weight 19,500 pounds. The designer's claim was 16 knots top and 14 cruise with 170 HP, she achieved 12.2 knots top speed and cruises at 9-10 with a 210 HP Cummins.

    The second boat was Twombly, roughly identical to Belle Marie with the same power.

    The third boat was Kamarea, 50' LOA, 45' LWL, 13'10" beam, 26" draft, weight about 34,000 pounds and power 210 HP Cummins. Speed is reported as 11 knots top and 9-10 cruise.

    The fourth version was the 47' Nancy Lakin, 45'1" LWL, 13'4" beam, 28" draft and weight about 32,500 pounds, with 220 HP she managed 10.8 knots top and cruises at 9-10 knots.

    These four boats do not exhibit anything spectacular in the way of performance. To my mind this is because they are typically moderately heavily loaded cruisers. The performance is as expected given the length, weight, and power.
     
  3. Willallison
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    Willallison Senior Member

    Since we are all agreed that Ken's proposed tests are likely to reveal only a comparison between two specific boats, rather than the efficiency of the skiff in question compared to other hull types, may I put forward another alternate suggestion.
    Tom has already produced an efficient planing hull that was designed to run in the speed range proposed, in the form of the Blue Jacket 24. Why not build a full size skiff of the same length and displacement and compare the two (with his blessing of course!).
    Yes, I know, ideally you would run comparisons side by side, and yes the engines woud be different so would complicate things. But, at least it would be buildable and at the end of it all, you'd end up with a useful vessel (possibly saleable to fund further tests...?)
     
  4. Willallison
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    Willallison Senior Member

    Then again, since we have the reputed mileage figures for Rescue Minor, why not just build a lightweight conventional hull of the same size and displacement and compare that to RM?
     
  5. tom28571
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    tom28571 Senior Member

    Will,

    Thanks for the thought but I don't really think my boats are suitable for this test program. Even though they are can operate efficiently in the same speed range, the Bluejackets are designed from an opposite direction. I took the premise that it should be possible to have a full planing cruising boat to operate efficiently down to a speed-length ratio of 2 or less. The Atkin tunnel hulls and any others that we have considered come from a displacement speed up to a ratio of 2 or more. One is very light for its size and the other is not. One tries to reduce the speed of a planing boat and the other tries to raise the speed of a displacement boat. That is a little crude but essentially correct.

    Robb may have done something different with his RM in that he also went for light weight. Maybe his RM is a different animal from Atkin's but I don't know the particulars. Much of Robb's observations are subjective. Not that he was incorrect but we don't have the data.
     
  6. Gilbert
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    Gilbert Senior Member

    Didn't William Atkin design a boat of about 30 feet that by all appearances was a conventional hard chine v bottomed hull but in fact was a shallow draft tunnel hull that ran about 18 mph or so with relatively low power? I believe I read about it in one of his books that someone loaned me years ago. Sorry I can't recall the name of the book or the boat. But wouldn't this design be quite appropriate for the proposed testing program here, especially as to deciding what boat no. 2 should look like? Someone else will have to come up with the name of the design and the book name because I don't have it. I'm going to do something risky here and say the he claimed it would run in 18 inches of water; this is risky because it may or may not say something about my memory from something I read quite a few years back.
     
  7. Willallison
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    Willallison Senior Member

    Tom - yes I take your point - these were just off-the-cuff thoughts - the ramblings of an Aussie facing the onset of winter;)
    My logic was simple - reagardless of your initial objectives, the BJ24 is an economical boat throughout her speed range. Yes she could be lighter, and almost certainly even more efficient if this had been your sole design objective. But given the constraints already imposed on the tests (standardised engine package, ply construction) and not to mention the almost unattainable weight requirement imposed by the model size, it seemed logical to begin by building a real 'skiff', then test it against some known entities. The BJ24 could be one, but not necessarily the only one....
     
  8. kengrome
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    kengrome Senior Member

    Brian, let's do your catamaran tests later. I want to keep
    Phase 1 testing simple with only two boats so we can move
    quickly and get some results ASAP.

    Tom, thanks for the Handy Billy suggestion, it is probably
    one of only a few designed for maximum efficiency at or
    around 15 knots and actually being built these days.

    Tom and Tad, I will make the boats large enough to eliminate
    the weight issue, maybe 2/3 instead of 1/2 scale which
    allows 30% rather than only 12.5% of the original weight.

    redu, I'm comparing the relative propulsion efficiency of
    two different hulls to learn which one is the more
    efficient, so I think we need to stick with two
    hulls ... :)

    Tad, a 'family of hulls' to test and compare would be ideal,
    so maybe these first two boats will make a good start to
    such a family.

    Will, testing full-sized boats will always be my preference
    for obvious reasons, but the budget might be an order of
    magnitude higher thus impossible to fund without a
    corporate grant.

    Will, I think Bluejacket is a great boat but not one that's
    optimised for only 15 knots. It would be interesting to
    see what Michlet would do with it when constrained by a 15
    knot top speed.

    Tom and Tom, sincere thanks for your votes of confidence via
    your $100 pledges which together cover the first 20% of the
    proposed budget. I'm not sure we will ever see an additional
    $800 but if we do I'm still willing to move forward ...
     
  9. masalai
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    masalai masalai

    Ken, That is OK by me.
    Brian
     
  10. Quietboats
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    Quietboats Junior Member

    Gilbert, are you by chance thining of Dave Gerr's book "The Nature of Boats"? There he frequently references his 42' "Summer Kyle" design which Tad refered to in his post--boats where eventulaly named "Belle Marie" and "Twombly".

    Ken, I was under the impression that $900 had already been pledged before my pledge--but have to admit that I flew through some of the earlier posts in the origional thread--did I miss something? I agree whole heartedly with your argument that tow tests won't give acurate results and am very excited to see your proposed powered tests get off the ground (and onto the water). I have always been a proponent of seat of the pants, rack of the eye, whatever you want to call it, boatbuildng. Maybe that is not the smartest way to run a business but then again I am still in the boat building business after twenty years while many others are not. What I am getting at is I think your proposal is the fastest way to arrive at real data and that your anticipated cost of $1000 seems extremely low and a fantastic deal for anyone remotley thinking about building one of these boats. All we are trying to find out is whether Atikins tunnel design provides a propulsion efficiency benefit as well as a shallow draft benefit--correct? Have you thought about what additional effort or funds would be involved to test Atkin's sideways offset shaft angle compared to a centerline mount arrangement? That would yield interesting data also and should be easier to test with a reduced scale boat.

    Anyhoo--if we need more money I am willing to bump my pledge to $200. Also, would you consider posting your proposal on the Yahoo groups site--seems like a few there would be interested in this as well. Thanks much, Tom
     
  11. kengrome
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    kengrome Senior Member

    Hi Tom,

    TollyWally pledged the first $100 but not for my tests. His pledge was conditional upon using the funds for the CNC production of a foam model for tank tow testing. But Nordic Cat pledged $600 and Fast Fred pledged another $200, both apparently for my 'real world' testing proposal (or so I thought) ... so I started this thread to work out the details. Then later Fred withdrew his offer and and we haven't heard from Nordic Cat since I started this thread so his $600 may also be withdrawn for all I know.

    If there's one thing I've learned in life, it is that talk is cheap and it is much harder to come up with the money than to make a pledge ... so I don't want to assume that anyone's pledges are valid at this point. That's why I am going on the assumption that only Tom Lathrop's $100 pledge and your $100 pledge are more or less certain at this time. After all, both of you made your pledges *AFTER* I re-posted my proposal -- in which I made a point to ask everyone to make unconditional pledges this time, and only if they were certain they were going to send the money when the time comes.

    I agree ... and I'm probably a fool for volunteering my personal time in this project because I can see it sucking up a whole lot more hours than I first anticipated. My guess is that I will end up attracting a bunch of flack for not doing every little thing the way others expect me to do them too. This is only one of several reasons why my wife keeps harping at me to "get out of this before you're sorry you got yourself into it". But I can handle a lot of crap if I truly believe I'm on track to do something useful -- and that's what I believe -- so I guess that's why I'm still interested in getting myself into this mess. My father always said I'm too stubborn for my own good. Maybe he was right.

    Yes, basically that's it. My goal here is simply to learn whether or not the Atkin tunnel-stern hull is more or less efficient than a simpler hull shape optimised for the same 15 knot speeds while using the exact same propulsion system.

    No, I haven't thought about this or any other testing yet. If we cannot get this simple program off the ground I cannot imagine trying to encourage another such effort. Look at all the time this one is taking as it is! I must have spent hours debating this stuff in this and the last thread alone, and we still haven't chosen the second hull because everyone still wants to talk about other things rather than focus on what needs to be done for me to proceed.

    Don't get me wrong, I enjoy the banter and the debates with all the intelligent guys who drop in to chat about such things! On the other hand it ends up becoming very time consuming and distracting rather than productive in terms of getting decisions made so we can proceed. Fortunately we cannot proceed without the money anyways, so participating in all these sideline discussions is not really hurting anything at this stage of the game.

    I'll tell you what. If YOU come up with a testing scenario for comparing straight vs. sideways angled prop shaft installations, I'll take a look at it and give you a price if I think it will produce the results you're seeking ... and I won't even consider it if I'm not convinced that there's a very good chance of getting some useful results. Then if YOU come up with the money I'll run the tests, and I will send YOU (and only you) the results since you're the one paying for them ... :)

    Thanks for the offer to raise your pledge, Tom. Let's just see if we can get to the target budget with other people's pledges before we ask you to raise yours even higher.

    Regarding my mentioning this in some of the Yahoo boating groups, I'd rather you or others do this if you don't mind, since it looks too much like self-promotion if I do it myself.
     
  12. tom28571
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    tom28571 Senior Member

    Ken,

    I think that limiting the scope of the trial as you have suggested may be the best reasonable way to get some practical answers to the long standing discussion of enhanced efficiency of the Atkin hull. Having been involved in evaluation programs of many kinds, I know how difficult it is to get answers that cannot be challenged and I don't expect these to be either. I do think it is worth the small moneys that we are talking about here. The main contribution is yours of time and energy.

    I would hope Fred and the others can come back on board. It ain't perfect but perhaps we will be able to tell if the results mean anything or not. That is way ahead of the blue sky banter that has gone on for so long. I have been one of the skeptical ones but would not mind being proven wrong.

    I don't do any computer programs for my designs but it would be interesting to see the results of such predictions compared to actual results. I am currently building what I think will be an accurate device to look at fuel use vs speed and load. Hope to get that done this summer.

    Tom of Marshallberg, I don't know if we have ever met but we must have crossed paths many times. Were you in Beaufort for the last wooden boat show? I was stuck under the big tent as a judge in the boatbuilding contest and did not get around very much.

    Cheers, Tom of Oriental
     
  13. FAST FRED
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    FAST FRED Senior Member

    The latest issue of Pro Boat Builder has a great article on hull resistance,and it would seem all the concern is really weather the Atkin can lower "the hump' between simple (and cheap low power) displacement speed and what we would all consider "plaining" .

    I prefer SL to the Froude numbers as the distance between is larger , so easier to contemplate.

    SL 1 is really cheap SL 1.34 is the fat heavy boats usual limit on hull speed.

    SL 3 or 3,5 is what all would see as plaining.

    The area of SL 2 or 2,5 is where perhaps the Atkins BKRD reign , although real performance there can be had simply by lightning and narrowing any boat.

    In the past there have been boats that required LESS power , and gained in transport efficiency after getting up on the plane.

    So it is of real interest IF the BKRD can be efficent in the hump range, or if it would be cheaper to just go faster.

    Although a sorta similar to sort a similar boat off will have some (very limited) value in finding efficeincy results , I'm not sure it will give any reliable information that could be used for future designs.

    The inability to know power used , will make it a simple boat race rather than anything near scientific.

    I for one would like to see the gallery of previously built boats to know weather the builder has ever done boats before.

    FF
     
  14. Quietboats
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    Quietboats Junior Member

    Fred -- 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. He will aready have weight verse boat size issues with one human and the gas motor. Maybe the simple answer is to pour in a measured amount of fuel into each tank and operate each test hull at a certain speed until all fuel runs out and measure the distance traveled. Do this at a variety of speeds and you will develop an accurate energy efficiency analysis.

    Tom of Oriental -- I am not sure if we have met yet. I have not gone to the Beaufort boat show for many years but perhaps need to start again--all reports are it was a blast--we certainly had fun eight,ten years ago. Oriental is on my list to visit again soon and I will certainly look you up. Please do the same the next time you get down east.
     

  15. TollyWally
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    TollyWally Senior Member

    With the exception of some slightly off topic comments regarding West Coast fishing boats I have refrained from this thread. My own desires were modestly in opposition to the direction that this second proposal regarding Seabright skiffs was going. As generous as Ken’s proposal was I did not feel it would necessarily provide very much insight. Yet, be that as it may, I did not feel it my place to try and diminish the efforts of Ken and others to proceed with this experiment. It appears that the chances of the model/motor experiment actually happening are dwindling.

    In light of this I venture a few observations. It seems to me there are several key questions we are trying to answer.

    1. Is there something about the Seabright skiffs underwater hull form that is inherently more efficient than similar sized craft in the relatively narrow performance envelope of 14 to 20 mph.?

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

    3. Is it possible that a perhaps significant portion of the reported success of the Atkin boats both tunnel and nontunnel comes from superb optimization of motor, prop, and hull characteristics?

    I am not a naval architect, but it seems to me at the heart of any discussion regarding hulls and effiency, the idea of resistance must be addressed. I would like to put forward the suggestion that we consider the first proposed experiment, towed testing. While we will be hampered by a necessarily small number of data points we may gain some insight into question number 1. If question number 1 can be answered in the affirmative then other experiments would seem more likely to have a chance of success.

    I have been quite interested in these Atkin boats for many years, as any glance at my profile and posts will show. I was the first to pledge money in the interest of satisfying my curiosity regarding these boats. If Tad is still willing, I am too. I hesitate to go further without some input from Tad, but what the hell, in for a dime in for a dollar. I will send Tad a signed check for $100.00 to hold, after all talk is cheap. Furthermore I am willing to purchase the plans for Naiad to further this venture. The blurb about this boat at http://www.atkinboatplans.com/ mentions adding 2000# of ballast. In my mind this removes some of the chance that lightweight construction alone might be responsible for the performance claims.

    I don't think it likely we'll get all our questions answered but I feel that the proposed tank testing my offer us the firmest foundation upon which to build. In my eagerness I may have inadvertently stepped on some toes. I offer my apologies to both Tad and Ken if either feel I am imposing on them or raining on thier parade. Sometimes it is easier to ask for forgiveness than ask for permission. :) Does that make me a shallow person?
     
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