Small boat design

Hello,

I am designing a small sail boat. The idea is that I am also learning.
The boat will be 15'4" x 6'8" and I want to be a very good boat in terms of stability and easy of sail. I looked at some boats - John Welsford SCAMP 11'11", Goat Island Skiff 15'6", 16' cabin catboat, Class Global 580 19', and try to morph mine from those. The sail is exactly GIS 105 sf balanced lug sail. Construction with douglas fir and 1/2" bottom and 3/8" marine ply. The boat it may take up to 4 people but generally one or two so have to be easy single handed. Centerboard will be compete out of metal - not sure of design yet.

I want to go on with the drawings.
But, I have to understand more about initial position of the boat in the water, are my frames vertical or not? Is the general design ok ? Is the mast position goof for the lug sail? How I calculate the centerboard position? Wat if I want to change the sail type? - I want to be as simple as possible.
What you guys do not like about this? Thank you in advance.

 

Welcome to the forum.
What you should do next is calculate the fully loaded weight, and its CoG, and check that the boat, in equilibrium position, floats as you wish. To do this you must calculate the hydrostatic values.

 

The center of gravity of an unballasted boat of this size is dominated by the position of the occupants. The weight and location of the occupants must be included in any calculations.

 

Why would whether the frames are vertical when the boat is floating be important?

 

The cockpit will force all the crew to sit aft, so it need more volume at the stern. Also, the sail area is a lot forward of the CE of the daggerboard.

 

Yes sir, very well said. Even in a very large ship, with a lot of ballast, the weight of the occupants must be taken into account in the maximum load condition. So, yes, I agree with you, on this small, unballasted boat, you also have to count the weight of the occupants when calculating the fully loaded weight. On the other hand, when calculating, for example, the weight of the lightship, on this and any other ship, the weight of the ocxcupants is not taken into account. That is (on this I do not agree with you), the weight of the occupants "must not be included in any calculation".

 

Thank everybody for advice.

Now I am doing some calculations as it stands, for the purpose of understanding, even if I have to limit the nr of persons (or to limit the weight of people) before I change the design.

I calculate the weight of the empty boat as it is now being
-400lb with a wood daggerboard
-5/8 metal daggerboard about 100lb - not sure how practical is to have a different box to lower it - I do not like the pin under the waterline, ... have to find a design with a higher pin.
-with the 500lb waterline is 6" from bottom
-with 900lb boat (like 3 persons position uniform along the boat) 11" waterline

Already I found that the wood mast (hollow 3" square with 1/2" wall) is about 18 lb but it takes the center of gravity much higher that I like. Based on this I may have to look at a carbon mast or aluminum to see what can be done.
I also thought that a full metal daggerboard will help bring the center of gravity much lower - but not much, have to recalculate the center of gravity with it 2' forward.

@gonzo
Now I remember reading somewhere that the front of the daggerboard has to be close to the center of sail - so that is one change to be done.

In terms of ballast, I have to think about some - internal - like triangular shaped lead, concrete, or bags of stones, to see if make sense under the floor - on the sides of the keel. Also part of the bottom of the wood keel (like 3/4" or 1") can be change with lead.

@TANSL
I like to continue to do some more calculation - like to find the center of bouyancy, metacenter and GZ arm under different heels. Now I am not sure if all this calculation have to be done with people. I have to understand how location of people influence stability. After this I will look again at the overall design.

 

It is up to you to decide what regulations your boat must comply with. Given its size, the most appropriate thing would be to check that it complies with the ISO standards for small boats. These rules define, among other things, the weights that must be taken into account in each load condition and give some stability criteria that must be met depending on the design category of the boat. In the case of a sailing boat, light in weight and with a large sail area, it is certain that you will need to place fixed ballast, as low as possible. You will probably need to fit a keel, quite wide, filled with lead. You will also need a relatively high freeboard to achieve a high downflooding angle. These two parameters, the lowest possible position of the CoG and very high freeboard will be decisive for your boat. Also think about the possibility of placing spaces in the double bottom and/or sides as a buoyancy reserve.
If you need help with all those calculations, feel free to ask. Here are fellow members who will gladly help you.

 

Thank you @TANSL. I am already reading from Construction Standards for Small Vessel (2010) - Government of Canada website, do not know much about iso. Of course I have to read a lot more. Makes perfectly sense what you are saying. I already read Stability Guide for Smaller Vessels - Danish Fishermen's Occupational Health Service. I found out that building a boat is science and art, I like that. And before I want to start building it I want to sail it in may head. All this calculation help me find out more staff.
I am really grateful about you guys help.

 

...sailboat designer 2023:
asking on the internet how to go about it!
(some boats out there look exactly like that was their design procedure...)

 

Where should he ask, in the local newspaper?