Interpretation of hydrostatics for rowboat

Discussion in 'Boat Design' started by BayBoater, Jan 19, 2017.

  1. BayBoater
    Joined: May 2016
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    Location: East Coast Canada

    BayBoater Junior Member

    This boat called "Coskata" was designed by Pete Culler as a recreational rowboat based on a lifesaving surfboat.


    I traced the lines from a little scan of the linesplan into Freeship Plus 3.4 and stayed as true to the original as possible. However, I drew Norwegian stem horns because I like the look. The lines are very similar to the Shetland boats and it looks like it is neither a raceboat nor a tub and can handle a good bit of rough water for its size. In one linesplan it is tilted ~1 degree bow like in the original linesplan; in the second it is level. I assume it is meant to be rowed trimmed slightly down at the stern.

    CullerCoskata_Linesplan tilted.jpg

    CullerCoskata_Linesplan level.jpg

    This boat was originally 16'8" long and designed for 3 people; in the attached linesplans it is scaled it down to ~4.5m (~15') long, 128 cm (50") wide and 40 cm (16") deep for a single smallish person. I'm working on designing something similar for stitch and glue plywood construction but first I want to fully understand the scaled down original as much as I can without being in it. I'm guessing that this is close to the right size and shape for me but want to be sure.

    I simulated two displacement states,125kg (~275 lbs) for a person, oars etc. and the boat built light of plywood, and 165 kg (363 lbs) for the boat constructed in traditional lapstrake and the same person and equipment. The attached linesplan is for the heavy version that sits 1.7 cm (~3/4") deeper in the water.

    Can you help me make sense of the hydrostatics numbers output from Freeship? I would like to know which ones of them have any importance to a rowboat in the real world, and what sort of performance elements cannot be predicted from the lines and the numbers alone. I have some specific questions below the numbers but am interested in other related comments as well. I've glanced at some design articles on the internet but things tend to get too mathematical for me really quickly and it is often hard to figure out whether there is any relevance to rowboats because most of it is about larger vessels.

    The first of the numbers is for the lightly loaded version, the second for the heavier version (left blank if same). I used the point of largest cross sectional area in the water as the "midships" because a post on this forum recommended this for calculating prismatic coefficient. The numbers below are for the tilted boat.

    I prefer metric but can repost the numbers for anyone who would like them in imperial units.

    Length over all : 4.750 m
    Beam over all : 1.283 m
    Design draft : 0.111 m; 0.128 m
    Midship location : -0.450 m
    Water density : 1.000 t/m^3
    Appendage coefficient : 1.0000
    Volume properties:
    Displaced volume : 0.126 m^3; 0.165 m^3
    Displacement : 0.126 tonnes; 0.165 tonnes
    Total length of submerged body : 3.946 m; 3.996 m
    Total beam of submerged body : 0.943 m; 1.025 m
    Block coefficient : 0.3037; 0.3142
    Prismatic coefficient : 0.5474; 0.5523
    Vert. prismatic coefficient : 0.5203; 0.5306
    Wetted surface area : 2.602 m^2; 2.898 m^2
    Longitudinal center of buoyancy : -0.231 m; -0.206 m
    Longitudinal center of buoyancy : -4.732 %; -4.136 %
    Transverse center of buoyancy : 0.000 m;
    Vertical center of buoyancy : 0.072 m; 0.083 m
    Midship properties:
    Midship section area : 0.058 m^2; 0.075 m^2
    Midship coefficient : 0.5547; 0.5688
    Waterplane properties:
    Length on waterline : 3.946 m; 3.996 m
    Beam on waterline : 0.943 m; 1.025 m
    Waterplane area : 2.170 m^2; 2.425 m^2
    Waterplane coefficient : 0.5837; 0.5921 Waterplane center of floatation : -0.138 m; -0.120 m
    Y coordinate of DWL area CoG : 0.000 m
    Half entrance angle of DWL : 13.830 degr; 15.360 degr
    Transverse moment of inertia : 0.104 m^4; 0.139 m^4
    Longitudinal moment of inertia : 1.429 m^4; 1.657 m^4
    Initial stability:
    Vertical of transverse metacenter : 0.895 m; 0.921 m
    Transverse metacentric radius : 0.824 m; 0.838 m
    Longitudinal transverse metacenter : 11.427 m; 10.108 m
    Longitudinal metacentric radius : 11.355 m; 10.025 m
    Lateral plane:
    Lateral area : 0.512 m^2; 0.580 m^2
    Longitudinal center of effort : -0.161 m; -0.147 m
    Vertical center of effort : 0.042 m; 0.051 m
    Hull characteristics above waterline:
    Lateral wind area : 2.831 m^2; 2.764 m^2
    Z coordinate of wind area CoG : 0.442 m; 0.450 m
    X coordinate of wind area CoG : 0.123 m; 0.127 m
    Distance from wind area CoG to D : 0.382 m; 0.373 m
    Distance from bow (FP) to wind area CoG : 1.600 m; 1.596 m

    How stable is this boat? I have a prototype rowboat that I built out of coroplast signboard and but I didn't follow a plan and haven't measured the boat, so I cannot mock it up in Freeship right now. It is 4.25m (14') long and only 96cm (3') wide, I'm guessing the waterline beam is ~61 cm (2'). To get in I have to brace it like a kayak or I will swamp it. This scaled down Coskata, could one jump into it while pushing off without tipping it? I have done it in a canoe with similar waterline beam but a flatter bottom.

    Is there sufficient initial stability to stand in this boat in calm water? In waves ~30 cm (1') trough to crest? Or is the best to be hoped for in a seaway to be able to crawl around a bit? I had the seat shift in my prototype and was terrified when I had to lift my butt that little bit required to push it back.

    Could you sit in this boat in 61 cm (2") whitecapping waves with your oars in at rest without tipping or would it take constant attention and effort by weight shift or oars to keep it upright?

    What do the "Vertical of transverse metacenter" or "Transverse metacentric radius" numbers mean in real life? I know higher is more stable and a heavier boat is more stable but the numbers differ only slightly. Would there be some "cutoff" number for standing in the boat or are there so many other factors that these numbers are meaningless? I have reasonable balance and am no more fearful of the water than average boater.

    Is there any effect of the extra weight on stability that isn't captured in these numbers? Does Freeship let you specify the location of the centre of gravity?

    About windage, is the lateral wind area sufficient to make this boat hard to manage in say a 15 to 20 knot wind? Ten knots? I doubt I'd set out in more than 10 but will probably end up "caught out" in more or run into it as I round a point. How much lateral plane is required to counteract this? Is there particular ratio between the two to aim for does it also depend a lot on the shape of the underwater surface? I have drawn a full length keel about 3 cm (1.25") deep and it is included in the hydrostatics analysis.

    Is the light version (more wind area and less lateral plane) so light that it needs to be ballasted to the heavy version to be able to handle in wind?

    I have looked at the resistance tables and graphs (kaper) and I wonder if there is a way to get Freeship to calculate it at the level of immersion that I specify. The resistance numbers from the "light" and "heavy" versions come out the same, and the draft and waterline beam used in the calculations is less than in either the light or heavy version. Or should I get the Michelet program or something else?

    Is there a ballpark figure of how much resistance a smaller person of average fitness can overcome when rowing at a relaxed pace for an hour or more? How much in a sprint of maybe a couple of minutes? Is there any way to actually measure that either on the water or in a mockup in my shed?

    How would it behave compared to a Rangely boat, small St. Lawrence River skiff or peaopod of similar waterline lengths?

    Is the prismatic coefficient good, or would lower or higher be better for flat water? Is lower or higher cp better in waves? With my plywood designs I get a value around 0.58 to 0.6 and no hollow waterline at the entry. This boat's cp actually goes up to 0.6 when tilted level and the hollow is much less.

    How do I interpret the block coefficient and Vertical prismatic coefficient?
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    How stable is it ? As a general principle, anything that is fine at both ends will be much tippier than a transom-sterned boat, if you wonder about that, make some simple small models and put them in the bath-tub. You don't gain anything worth talking about in terms of ease of rowing either, by either a fine bow or being double-ended. The more rectangular the plan, the better, in small boats, imo. May look less ship-like, but they are not direct analogues.
  3. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    Hi BayBoater,

    would it be helpful for you, to get the hydrostatic values and graphs on this site, if it was calculating your hull?

    Button "English" top right, Buttons "i" for information. (Righting arm curve tells you about stability and the with "mass to relocate" you are able to move the skipper (or something else) araound and look at the resulting trim and heel.)
  4. BayBoater
    Joined: May 2016
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    Location: East Coast Canada

    BayBoater Junior Member

    Vielen Dank Guenter!

    Ich verstehe Deutsch aber nach ueber 35 Jahren in Kanada ist English meine bessere Sprache.

    It would totally be helpful if I could input the hull into the calculator on that website you liked to and shift the centre of gravity around, maybe even compare the full sized version of the boat to my reduced version, and then to the simplified plywood versions. I would also like to test out (both graphically and in real life) what the effect of adding sponsons of different diameter would be on the stability.

    It would of course be even more useful if I could input several other boat types for comparison, including some that I actually have been in. Empirical data would help to calibrate everything, so I see this as a long term project. This summer I will try to get into more boats where I know the lines. Hopefully I will finish up and test out my current build project, which is much slacker-bilged and close in shape to a Norwegian Faering, but very small at 4 m long and 1.25 m wide (overall, not waterline).

    I'm especially interested in comparing slack vs. hard bilge and different amounts of deadrise. In the plywood building method I want to design for, boats with a slacker bilge and more deadrise are easier to build (bends less tight, less plywood torture and less wastage of plywood).
  5. Heimfried
    Joined: Apr 2015
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    Location: Berlin, Germany

    Heimfried Senior Member

    To deal with your hull (or hulls later on) a data file with cartesian coordinates of the hull would be necessary.

    A few weeks ago I wrote a tool which transforms a STL-file (exportet from freeship) into the required data format. Since this was only tested with one single file, I am not sure, if it will work.

    If you send or post a stl-file of your hull, I could try if it works again.
  6. Heimfried
    Joined: Apr 2015
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    Location: Berlin, Germany

    Heimfried Senior Member

    Have a look at the behaviour of a simple, light boat (30 kg):

    (If you switch between different hulls to compare, it will be necessary to click the "restart" button after starting the new hull. Otherwise the calculator is crunching the old numbers.)

    The VCG is set to 200 mm (zG) above base plane, which will be reasonable for this empty boat. If you draw the righting arm curve (wait after each step until the answer is complete) it shows the boat is stable until the water comes in (heel angle about 47 deg.).

    Click "position of rest" and then input 0.1 deg heel, then "calculate heel". The initial (= upright position) transversal metacentric height (GM) is nearly 2,5 m (last row right). Input 47 deg. heel, calculate: the apparent metacentre decreases to 0,32 m.

    "position of rest", now let the skipper enter the boat, standing upright: input mass to load: 80 kg, vertical dist. to CG: 950 mm; heel 0.1 deg. the metacentric height is less than 0,16 m. Look at the righting arm curve (start with "new", otherwise the old one is used).

  7. flo-mo
    Joined: Jul 2010
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    Location: Vienna, Austria

    flo-mo Junior Member

    Last edited: Jan 29, 2017
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