View Full Version : Design basics for efficient Flat bottom rowing boat
02-21-2001, 12:50 AM
Hopefully someone can enlighten me to some basics on properties
needed in a good Flat bottom rowing boat design.
How important is it to keep both stern and Bow out of the water under
full displacement? Bolger nad Michalak seem to like both ends out
although i've seen other designers keep the bow immersed.
If your designing to achieve a specific Prismatic Coefficient,
should you aim for it at full capacity displacement or just with the
hull's weight in the water. The P.C. seems to vary quite a bit due to
Is there a good rule of thumb for rocker that would give me both
maneuverability and good rowing speed?
In Using the Hulls design program, I'm surprised that a boat with transom is
actually faster than a double ender. I thought this would the
Is it considered true that the more rocker a boat has, the more
draft it will take? If the displacment of the boat is supported over
a longer waterline, will it take up less draft?
Is there a specific freeboard height in which to mount the oars for
optimal rowing leverage? Does this vary with the focus on
maneuverabilty or speed?
Thanks in advance.
02-21-2001, 09:09 AM
Lets pull up some chairs to the stove and uncork a bottle. In one sentence. A hull with a straight keel and 0 deadrise angle will be faster than any other. Will it make a good boat? I doubt it. To get a good boat.... Well just let me fill my glass. http://media5.hypernet.com/~dick/ubb/smile.gif
02-21-2001, 02:17 PM
Let me start by saying that I am not a naval architect. Given that, here are my thoughts.
It's probably not important to keep the bow out of the water at MAXIMUM displacement. The reason being max displacement implies a less than optimal displacement to power ratio. It may be important to keep it out of the water at DWL displacement as you may gain some advantage in resistance reduction as the boat surges forward at the beginning of the stroke. I would think this is less important with a longer waterline.
Prismatic coefficient should be calculated for the operating displacement of the boat. On a small boat passenger weight makes a big difference.
The more rocker, the more manuverability = worse tracking. 1 - 2 " front and rear on a 10' - 14' waterline seems to work well for canoes and kayaks. You will want enough rocker to keep the transom out of the water, though.
If you compare a flat bottomed transom boat with a double ended boat and keep the LWL, beam and displacement the same, the double ender will have more draft and wetted surface area, which will slow her down.
For a given displacement, LWL and beam, more rocker means greater draft. If everything else remains the same, increasing the LWL will decrease the draft.
I have heard that the oars should be about 7" above the seat in a dinghy. It would seem that the height would vary some with the beam at the rowing station, height of the rower, etc.
02-22-2001, 01:27 AM
Don, Thanks for all the info. I appreciate it. This is my first design and i'm trying to get it right. Gary, I'm a margarita man myself. I'm mixing up a couple pitchers full so jump in at anytime!
02-22-2001, 12:00 PM
I was hoping this thread would turn into 3-4 pages. I will also have to say I'm not a NA. I do like to play with hull forms, mostly on paper, some as models, and some as full size boats. Most would be driven by paddle or oar.
Prismatic Coefficient, what a mouthful.
(Disp(lb)/64) / Midship Area(sq ft) x WL(ft)
Now that we have that out of the way, lets get rid of it. Boats powered by oar will benefit very little by such number munching.
I start with a shoe box shape. Put a point on the bow and give the forward 1/3 a small amount of rocker. After that you have to adjust for use, looks, and materials. This is where you make all your compromises. This is only my opinion, developed informally from experience. I put the coffee pot on now and want to hear what others think. http://media5.hypernet.com/~dick/ubb/biggrin.gif
02-23-2001, 09:09 AM
What is the intended use, and environmental conditions that we are designing to?
calm water/rough water/beach surf/etc...
02-24-2001, 01:23 AM
Boat will be primarily used for Fly fishing, Plug fishing and drift fishing.
Water conditions are medium flowing rivers, backwaters of Dams, Lake inlets, etc.
Maneuverability is important for positioning flies as is rowing ease for cutting across backwater areas where on occasion, winds can promote standing waves and 2-3 ft. rollers.
With the skiff shape there is a lot more potential drag from the stern than the pointy bow. For those standing waves I would want some flare to the sides. For drift boat maneuverability you do not want the bow too deep.
02-24-2001, 09:15 AM
Still more use questions. Will you be solo, 2 or 3 people? Will you beach it, trailer it, tie it to a dock? Will you use a motor at all or just oars? Will it be stored outside, or indoors.
02-24-2001, 10:52 PM
My preliminary design information indicates that the boat be beachable, handle 3 men + gear, will be trailered to launch site, stored outside although covered when not used. Oars will be used most of the time although i'm looking into a bow mounted elect. trolling motor to get me into lagoons and just make life easier overall. I figure a total displacement of approx. 850LBs should be in the ballpark. Driftboats are OK but row like bricks, especially when its windy.
Besides the excessive windage, the abundant rocker pulls too much draft. I figure i can afford a maximun of 7" of draft fully loaded, for the sites the boat will be used in. I feel i can keep the bow and stern out of the water at full load displacement and still retain good rowing speed by cutting the normal Drift boat's rocker in half.
I've drawn in Dory style flair to (1)make it stable in rougher conditions and (2) allow room for removable compartments to be added for storage. Any ideas or comments? .....
user 1 250 #
user 2 200 #
user 3 150 #
gear 1 50 #
gear 2 40 #
gear 3 30 #
battery 50 #
motor 50 #
oars 20 #
anchor 20 #
rode 20 #
PFD 10 #
beer 30 #
I'd suggest 1200 pounds is a better target, for 7" draft you're looking at things in the 18' - 20' class, which are going to be around 300 pounds.
02-25-2001, 03:16 PM
Actually here's the breakdown.
Hull, 14.75' x 6'-0, okumme, spruce etc.=168LBS total
3 fishermen @ 180 ea.=540LBS total
Gear for 3 men including PFD's=110LBS total
(I don't drink on the water and my fishing companions are in reasonbly good condition therefore there is a considerable reduction in your figure. Also as i stated in my previous postings i was considering a electric trolling motor. At this time it is not calculated in the total Displacement figure.
My initial design with the previously mentioned hull measurements draws 6.8" of draft w/ 1063LBS of disp. This still allows the bow and stern to remain out of the water. Therefore if i do add the weight of a motor and battery @ 65LBS total weight, it still fits within the confines of the design's limits with a 10% fudge factor left over.
02-25-2001, 09:01 PM
I reread your first and last post. It seems you have a good idea what you want. And Don's answer is correct. My mind was elsewhere and I wanted to turn this into a discussion on basic design. I best do that on a separate thread. http://media5.hypernet.com/~dick/ubb/smile.gif
[This message has been edited by G. Schollmeier (edited 02-25-2001).]
02-25-2001, 10:44 PM
Actually both you and Don have pretty much hit the mark. While i understand the concept of the different formulas, most design publications on the market do not give a lot of time to flat bottom rowing boats. While it may look like a simple design on paper, several highly esteemed designers have made the statement that a successful flat bottom skiff is very difficult to do properly.
My interests lay in determining if the generic design formulas are appropriate in designing a small fishing boat and what modifications to the boat i can make to enhance the design's performance.
I have drawn up 3 different hull shapes and the only real deviations are in LWL and freeboard, and flair angle. It is interesting to watch how the P.C changes as you make adjustments to the design. This may not be a good barometer of a successful design, although the lines do look cleaner & fairer if you follow the "Pro's" guidelines. Also it is not altogether clear, the importance of locating the center of bouyancy...There seems to be different views on this subject.
Most important to me are design elements that are not given in the Design publications. Items like compromises with rocker, freeboat and experimenting with LWL by varying the bow & transom angles. These variables are due to the intended use of the boat. Experience seems to be the best guideline to achieving this and that's why I posted initially.
Thanks to everyone who has posted.
02-26-2001, 02:42 PM
I should just quit, but heres $.02 more. This is the opinion of an amateur. Narrow the beam to about 54-60 inches. Get your volume back by increasing the length. It will handle much better. Should still be stable for casting. And build a model, even if it is just paper. It always helps me to hold it in my hand. LOL, what good is an opinion if you don't share it. http://media5.hypernet.com/~dick/ubb/wink.gif Keep us posted, boats of this size are my daydreams.
"Reasonably good condition"? Well, that's all well and good, but it doesn't take pounds away; the day I ran a three mile run in 14:27 I weighted 242 pounds and was in fine shape.
Small can be light, but good condition means muscle -- which is denser than fat.
Later I became a running fool and got down to about 220, when I was running 25 miles a day. I figure I was about 2% body fat then, looking at some old snaps. We've (my siblings) all got big bones; our average is probably 280, including the 180 that my sister always complains about (she's 5'13", barefoot.)
175 is a USCG average for "person"; I suspect that they're including women and children in that average.
For the same volume, longer and slimmer is better than wider and shorter. More like a canoe than a jon boat. (Why are they called that?)
I've often thought that something like a pontoon boat, only equipped with oars, would make an idea rowing boat, and can't figure out why I've never seen one. Something like two racing shells with a very light deck between them.
02-26-2001, 03:59 PM
FWIW, Karl Stambaugh in "Good Skiffs" considers a 15' LWL ideal for a rowing flat bottom skiff. At 16' he says weight begins to become a consideration [presumably for a single rower]. This is a good book on the subject. It doesn't contain a lot of formulas, but does have a lot of good info based on practical expeience.
[This message has been edited by Don Maurer (edited 02-27-2001).]
02-26-2001, 05:42 PM
Holy cow htom I don't even drive 25 miles a day.
02-27-2001, 01:34 AM
Don, Your the second person today who has mentioned Stambaugh's book to me. Thanks for the tip, I'll check out Amazon for details.
Gary, I agree with you about the slimmer boat theory although i'm trying to utilize 2 sheets of Okumme for each side plus 2 for the bottom. 14.75' is as much boat as i'm going to get from this. I did cut the chine width down to 44-45" which is approx. 4-6" less than a comparable Drift boat hull. This along with the reduced rocker should make for a more efficient rower. I picked up some Scale aircraft ply today to build a model of my final design. Your absolutly right, there's something about holding a completed design in your hands and rotating it to view the boat from all angles. I'm going to plug in some more calculations before i'm done and I'll let you know what I end up with.
Why not one sheet for the bottom, scarfed to 23" x 15'8", and then use five sheets, two and a half on each side?
casem, I did say I was a running fool. I should have throttled back on it and kept going, rather than finally stopping.
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