View Full Version : converting rowing dinghy to sail

07-09-2012, 01:56 PM
Ok so a couple years ago I built a small dinghy from the old popular mechanics plans called the junior .
the boat is built out of plywood with the exterior covered in poly fiberglass .
9ft long by 4 ft wide at the widest point . pointed nose and curved bottom with a center keel , but the keel is just the structural keel and only protrudes about 1/2 inch or so from the bottom of the boat .
she rows very easily , and i am quite happy with her .

recently i have been thinking of making some modifications to her .
1st when i built her it was with an angled transom , which means that my 4hp Johnson does not sit at a good angle so I want to add on a modification/motor mount to the transom to handle the motor better .

2nd a friend of mine wants me to take sailing lessons with her this august , and i was thinking that the plans said the junior could handle a sail . so maybe i could modify her to be sailed as well.
Mounting a sail on the nose of her should be no problem , the stem is very solid , made from 2 pieces of 3/4 ply laminated together with gorilla glue ,
my concern is that from what i have read i will have to add a dagger board or some other deeper keal modification to the keal in order to be able to sail her against the wind .
if i don't do this ,all I will be able to do is sail with the wind and i will have to row back .

Has anyone converted a row boat to a sail boat b4 ? and how deep a keel do i need , or is it a space thing , can i make a longer but shallower keel over a taller short keel ?

i would prefer to do as little cutting through the hull as possible .
one thought i had was to make a keal out of ply wood , and attach it to the keel of the boat with some bolts .

I was thinking of adding a sacrificial keel to her any way along with some runners to protect the bottom .

finally , i would still like to be able to row her when i want , and run her with the motor , how badly will these modifications for sail change how she currently handles ?

i will try to get some pis up in the next few days when i get a chance .

07-09-2012, 04:00 PM
You will need a dagger board or lee board if you want to sail her. Did the plans not have the dimensions and location of the dagger board and position and size of the sail and mast? You have to have those just right or she wont work. Center of effort and all a mystery to me but I can tell you for certain if you do not have that right you wont be happy.

Mad Scientist
07-09-2012, 05:16 PM
I found all the old PM issues on Google Books, but the only 'junior dinghy' was a plywood kit, circa 1947. Is there more to the design's name?

BTW, Popular Science has its own archive: http://www.popsci.com/archives. No luck there, either, but I think there's something wrong with their search function.

I enjoy looking at those old magazines.


07-09-2012, 05:20 PM
Popular Mechanics

07-09-2012, 06:11 PM
One way to determine the center of Effort of an existing small hull is to drag the boat sideways (in water) moving your line fwd or aft until the boat drags evenly. That point is the Center of Lateral Resistence of that hull as built. The center of effort of what ever sail you want will be almost directly (approx. 1-2" fwd of that spot) above the vertical center line of the dagger board/lee board(s) you install which are installed centered on the hulls CoLR. works for me....................eddielee

07-09-2012, 08:57 PM
A cautionary tale. About 50 years ago I converted an 8 foot dinghy into a sailing dingy. I had no concept of the science behind the position of sails to centerboard. I simply eyeballed where the dagger board should go, put a 2x3 mast near the bow, made a rudder, cut a sail down from an old Snipe that I used to own and had me a sailboat. Now it looked pretty good to me and I took it to the lake (Memphremagog, half in US and the rest up into Canada). Very windy day. Wife sitting on the bottom, me aft with the tiller and off we went. The wind (strong) was off shore, we ripped along, wind got stronger, wife got scared, thankfully we had something to bail with. Bottom line, we could not sail into the wind. I could fly if I was heading off shore, hell, I think the little bugger was up on plane, just ripping. We got closer to Canada than I had planned. Every time I tried to bring her about and head back the wind pushed the bow off and for a while I could almost sail 90 degrees to the wind, but never came close to heading on a meaning ful tack. I finally dropped the sail and paddled to an island where a buddy who had been watching the fiasco came and towed us home. Moral of the story, learn all you can before you cut anything. Eddie gave an excellent description of what you need to do but there are other factors and the smaller the boat the more easily they can shift, for example, too much weight (passengers) aft or fwd and the CLR will change and so on. Learn all you can and by all means do it, just be sure to throw in a bucket and a pair of oars;))
Google is a good place to start, look up, lee helm, weather helm, CE, ClR, balanced helm and so on, very interesting stuff.

Jay Greer
07-09-2012, 09:02 PM
You can build a mast partner near the bow in such a matter as to have it located in the center of the bow thwart. You will then need a step on the keel which can be a block that is well anchored and provides a hole for the mast to step in. Setting up the sheeting and a rudder should not offer much problem. More than likely you will be better off with a lee board as you will not need to cut a hole in the hull. Making a rough lee board out of plywood from a pattern taken from a boat of similar size will provide you with a test lee board. Attaching it to the starboard rail with a couple of C clamps and a block to keep it vertical against the hull, near the water line, will allow you to sail the boat and move it fore and aft to find the best point of balance for attachment. For this kind of experiment you will be better off sailing on a port tack as the board will be better supported against the hull on that tack. Your best bet will be to use Sabot leeboard hardware as it is readily available as are the boards themselves if that size board will fit your needs. However, from your description of the boat, I suspect you will need a deeper board. A lug rig will allow you to store the spars in the hull for the times you will be rowing.

07-09-2012, 09:33 PM
We'll, here's the Nutshell Pram. The rig should look a lot like this when you're done. The main thing to pay attention to is the relative sizes and placement of the daggerbord and rudder, and their relationship to the sail. The best way to ensure success is to set up a way to temporarily clamp in the mast partner and step to make sure you have it right before making it permanent. Daggerboards on these little dinghies often line up right in front of the center thwart and use them for bracing. They don't get in the way when not sailing, but you'll want a tight cover or plug to stop water from spitting out - especially if you clamp on that outboard.


07-10-2012, 08:29 AM
1st...a picture of the boat would be nice...from above or nearly and from the side at about level. This will give us an idea of what to work with. I too would suggest a leeboard as this requires the least modification of the boat. However...you will need to put it right near the widest part of the boat so you can get it parallel with the center line with a minimum amount of fabricating. This will affect where the mast will go depending on the type of sail you will hang off of it. You should aim to get the geometric center of the sail to be over the board...probably near the after edge or just ahead. A low aspect Sprit, Gaff, Standing Lug (as above) or even a smaller Lateen sail will work well and all hang aft of the mast far enough that the CE will be near where you want it to be. You may have to extend your breasthook aft or make a small thwart for the mast partner to get it right. You can also use some mast rake to adjust if need be. Don't forget you need to add a rudder too so plan this when you consider your motor mount.

07-10-2012, 11:33 AM
thanks for the info , looks like this may be a bit bigger project than i originally thought .
here is a copy of the plans i used :
I will try to get some good pics of the boat this weekend . I have to dig it out of the garage .

07-10-2012, 02:46 PM
This one?


07-10-2012, 06:23 PM
I did up a quick (and rough) modeling with sail and appendages. This is how it works out...to use a daggerboard this is about what you would need to set up. The mast is at the back of the forward thwart and the daggerboard is about 4.75 ft forward of the transom. To use a Leeboard you will need to move the mast aft by something like 9 inches or you will have to move the leeboard forward and make some shims so it stays parallel to the centerline and in the same relative position as the Daggerboard. I would recommend only 1 leeboard to avoid having extra work to do. You can leave it on the one side...this has been done many times and doesn't seem to affect sailing worth a darn. You aren't gonna be racing that thing anyway so you won't need the utmost umpteenth percentages of performance. The pictures show the leeboard in the forward position that I mentioned. It also shows the daggerboard location. You could install a temporary thwart for your mast partner...just hang it off the seat riser with a couple of bolts and wing nuts on each side...that way you can shift the mast aft and use the Leeboard in its proper position at the maximum beam.

Oh...and don't expect to sit on the after thwart... you're butt will probably be on the floor so install some floorboards to keep it out of the bilge.




07-11-2012, 10:51 AM
yup that's my boat :) thanks for those drawings , that will help alot . i did some research on leeboards the other day , and i think that is the direction I will go in , good call on the floor boards as well , I was thinking of putting some in any way so this is a perfect excuse to do it . I did not add the skeg shown in the plans when i build her , as a row boat i didn't need it , and i had planned to use her to fish for bass and chain pickrel close to shore so wanted as shallow a draft as possible . she has always handled well without it , in fact it is quite a maneuverable little boat which i like . It was a decision i was conflicted about when i built her but I figured i could add it later if i needed to when i put on the sacrificial keel . I don't use her much now that i have a canooe which is easier to load and unload for fishing , so if i have to add the skeg it is not a big deal if it will help with sailing .

07-11-2012, 11:34 AM
yup that's my boat :) thanks for those drawings , that will help alot . i did some research on leeboards the other day , and i think that is the direction I will go in.

From the pictures shown, you can build a leeboard that slots down over the gunnel, and not have to alter the hull. Here's an example, on Bruce King's Salt Bay Skiff:



There might be other details of King's design you can use.

Recently, I built a mount for a trolling motor to fit my skiff, which has a raked transom and rope steering.


Or you could attach a simple fixed mount to one side of the rudder. On a boat that small, I'd reckon sail vs. motor use as an either/or choice.

07-11-2012, 12:38 PM
Actually I think you'll be better off without the skeg. That puts the CLR a little bit more forward which helps in the board and mast placement.

07-11-2012, 04:17 PM
Probably a silly question but if you were to determine the CLR by using the method of pulling the boat towards you with a rope attached amidships at various points until it comes without the bow or stern pulling on or falling away would you do it with or without the rudder (presumably in the dead ahead position). I would imagine that would move the CLR aft of what you would get without the rudder.

Todd Bradshaw
07-11-2012, 04:54 PM
That's not a silly question and the answer seems to depend on who you talk to. Some people say to include the rudder in the CLP calculations, others say to leave it out. I guess they figure that a rudder which is streaming properly with just a little bit of helm isn't causing much drag or lateral resistance. I have no clue what they teach in yacht design schools. The good news in this case is that the boat is so small that fore and aft trim can most likely be adjusted by a huge margin by simply shifting your weight and seating position a bit.

This hull is so fat that I would probably give the sailplan's CE about 10% of waterline length lead over the CLP (that's lead as in "leed" not "lead" as in a heavy soft gray metal). Stubby, wider hulls tend to round up more than skinny hulls when pressed and heeled (which this one might do to a limited extent) and moving the CE forward a little bit can help prevent excess weather helm due to this.

Mad Scientist
07-11-2012, 07:28 PM
That's not a silly question and the answer seems to depend on who you talk to. Some people say to include the rudder in the CLP calculations, others say to leave it out...

...and still others say to include half of the rudder.

In a boat of this size, the weight and location of people onboard might make precise calculations somewhat meaningless, perhaps?


07-11-2012, 07:52 PM
This might be interesting

07-12-2012, 07:46 AM
Here's more
A lot of science and a little magic apparently needed to get it right, just a bit of weather helm seems to be the best option

07-12-2012, 09:37 AM
I put the CE above the CLR in the static profile because when you get in the boat it will trim bow up somewhat and you will end up with a shift in the CLR aft. The mast will rake back a bit more but the CE won't change nearly as much as the CLR and you will gain your "Lead" from that. After that...it is seat of the pants adjusting...you adjust where your butt is to affect the balance of the boat. Your movable ballast is such a great factor in such a small boat that you don't have to be too exacting in following the rules of thumb. For a 200 lb-er...every inch you move is 16.67 ft-lbs of leverage +/-... nothing to sneeze at.