Row/ motor tender conversion

I am converting a 3m tender that can be rowed and motored with a tender to a cruising dinghy. I plan to use a gaff rig with a 3m tall mast and a 2m luff. I plan to use lee boards. with the mast 60cm from bow and a small foresail atatched to a 30cm sprit where should i place the leeoards to reduce weather helm? I am new to boat building so any advice wou be useful on any part of the convresion. Thanks.

 

Welcome to the forum.

Typically you'd start with the CE (center of effort) of the sail, draw a vertical line down to the hull, and place the front of the board(s) behind this line, give or take.

Which make/model/hull type is the tender?

Below article talks about CE of a gaff sail and 'leading' the CE to the CLR (center of lateral resistance).

Duckworks - Projects https://www.duckworksmagazine.com/05/vintage/sbj/3/index.htm

 

I am unsure of the type, make or anything like that. It is an old hull I got for free. Still floats and all. Is decebt for motoring so it is a good hull. Would the foresail bring the ce forward. I would assume so and can I work out the CE for the whole boat then?

 

Give some more thought to the sail rig. A gaff rig is suitable for sailing but is not as convenient as a sprit sail or lug sail. Why would you further complicate the rig by adding a foresail (jib) ? The boat will sail just fine without a jib. A sail area of about 4 square meters should be enough for a boat that size..........A sprit sail with a luff of about 3 meters, with a foot of 2 to 2.25 meters is about average for a simple rig on a boat that size.

Look at the side view of the hull. Take note of the curve of the bottom. If it curves upward near the bow and also curves a bit upward near the stern, then you have a boat that will probably sail decently. If the bottom does not curve slightly upward at the stern, then you have a power boat that will sail but not nearly as well as one with the aft end of the bottom curving upward so that the bottom of the transom will clear the water when underway.

 

There is a nice curve on the bottom as I remember. Thanks. And I was going for a gaff as i alreafy have the mast for that setup. I also got it for free.

 

If your mast is sized for a gaff rig, it can be used for a balanced lug setup, not needing a jib. You just need to locate it as a stayless mast, avoiding setting up stays. Balanced lugs are simple and effective, and easily made. I've made 2 from bed sheet material, sewed at home, and both work well still after a few years of use.
The lee board doesn't need to be fixed in any one position right at the start. If you have it attached by a strong rope, so it hangs over either side, (swapped over when tacking), it can be placed fore and aft on the gunnels to adjust it to find the most balanced position, going by how much tiller offset you need to use to travel in a straight line.
Free stuff often costs the most money to use. Just either get an OB motor and use your dinghy, or get a cheap sailing dinghy and sail, then you will be on the water having fun much quicker and cheaper. Be safe and have fun.

 

Yes it is possible to 'do the math' for the CE, I don't know/have never had to so I cannot directly advise the procedure.


Edit: pic from Sailing Theory and Practice, C.A. Marchaj, 1964, p 195.

If you could post a picture or pictures of the hull and rig in your possession that would help.
Did you get the gaff sail as well?
I can see some merit in not having the foresail and associated rigging in a small boat, ease of setup and use stuff.

 

The boat is stored about a forty minute drive from where I am so I cant get a picture of the actual hull however i have drawn a rough plan based from memory. I will send it on this post.

 

These are the plans.

 

Thanks for the drawing.
How does it look from the top, pointed bow or pram?
Rounded sides or hard chines?

 

It has a pointed front and round sides with a relatively flat bottom.