First Boat

Designed Displacement

Tim, just to be clear: you understand that the designed displacement of your boat MUST be the the hull, rig, foils etc PLUS the crew,right?
A Windsurfer's designed dispacement will be something between 180 ad 200 pounds give or take some carbon with a 160 pound crew; a foiler Moth will be in the vicinty of 220lb at it's designed displacement(pre foiling) with a 160 pound crew.Look at the stats on the Johnson site(above) for all up weights of a bunch of dinghy's.
Sorry, it's just not clear to me that you fully understand that- if I'm assuming the worst forgive me...

 

I reworked the boat and the hull by itself displaces 214 pounds without ANYTHING. I am hoping it will come out to around 250-270 pounds just including equipment, no crew. Thanks.

Tim

 

?say what?

I guess I just don't understand what you're trying to achieve. Why would you target a displacement that does not remotely relate to the sailing displacement of the boat? If your boat weighs 250 all up with two 160 pounders it will have a sailing displacement of 570 pounds,right? If you used carbon, kevlar and air foam you might be able to come up with a 14 that weighed say 140 all up then if you add two 160 pounders the sailing displacement is going to be 460 pounds.
As best I can tell it doesn't make any difference what the hull DISPLACES without the crew because it won't be sailed that way.The hull weight is a critical part of the overall sailing weight but you don't determine that by running the hydrostatics for "hull weight".The numbers for the boat only have relevance at a realistic sailing displacement.
Can you enlighten me on your thinking?

 

Tim,
What do you need the RM for? I'm not sure what you need?
As for the weight of the boat...you need to work that out now based on the materials you will use to build it. Then you can go back to your hull design... add the weight of crew and redefine the waterline.
regards,
Rob

 

Doug, are you aware that he's a 14 year old kid? Just checking.

Tim, Doug and Rob are right that the hull weight alone is not what you should use to design the hull shape. You should add the crew weight, along with a typical average amount of cargo if you expect to have any, and then see how the boat behaves under that load. That's what will determine the boat's displacement and performance when you're sailing it.

 

Ageless

Skippy, I don't look at age : I look at enthusiasm, love of sailing and his obvious interest in AND talent for sailboat design.

 

Lorsail: Sorry, it's just not clear to me that you fully understand that- if I'm assuming the worst forgive me...
Lorsail: ?say what?

Lorsail: I don't look at age : I look at enthusiasm, love of sailing and his obvious interest in AND talent for sailboat design.

I'm not sure what your point is there, Doug. It sounded like you were maybe being a little rough on the kid, thinking he was an adult with more years of experience under his belt. It's nice of you to complement Tim on his enthusiasm and talent, but I'm not sure what that has to do with my point. I was just suggesting that he's not a thick-skinned old guy, and it sounded like maybe you weren't aware of that.

 

I hope you guys know that I am not saying this is how I will determine how the boat will act, but I am just stating the hull weight, nothing else. If I offended some people I'm sorry, I know that you to determine how to boat will act in the water I will need to calculate crew weight, and other things, but I was just stating hull weight.

Tim

 

Hull Weight?

Tim,
The weights you have mentioned so far are the the "designed waterline" displacements.
To work out the hull weight you need to now work out how the boat will be built, what will it be made of, you need to know what each and every component weighs and where it fits on the boat.
I set up a simple spreadsheet to record the items, their weight, and their longitudinal centre of gravity (LCG) from the furthermost point of the bow. Some you can work out but many items you will estimate.
You then multiply the weight of each item X it's distance from the bow.
e.g.( plywood transom 10 lb x 13' = 130)
(stainless gooseneck 1.5lb x 5.5' = 8.25)
..call these "moments"
You do this with every item.
You then add up all these moments and divide the total by the total weight of every item that you have.
The number you are left with is the distance from the bow to the LCG.
You could do the same to get the VCG and the TransverseCG but I don't think it is relevant.
If you then compare this to your "designed" hull you will know if it will float level and what it will weight. If you add the weight of crew to your spreadsheet you can then see the effect on the trim (bow up or down) as you change the position of the crew.
That's how I do it, perhaps others have other methods.
Sorry I can't help with SA. I'd just look at comparable boats...
Regards
Rob.

 

So what exactly does designed displacement mean? I am an avid sailor but not sure what this means, because I have always just called it displacement. Thanks for all the info on calculating the longitudal center of gravity.

Tim

 

design displacement is what it will/should weigh (approximately) when sailing.

 

Other people will know more about this than I do, but I believe the bilges should be centered somewhere in the vicinity of the loaded waterline. Otherwise the hull will be stiffer or more tender than you wanted it. The waterline is determined from the total amount of water displaced by the hull, which of course is based on the total weight of boat, crew, and cargo. 200 lb sounds good for a two-person boat. With 300 lbs of crew and some gear, that's a little more than 500 lb total displacement that you should use to find the waterline and shape and position the bilges. It's not just about how the boat sails after it's built, it's about how to shape the hull in the first place. That's why everybody is saying put in 500 lb for the displacement instead of 200.

But 200 lbs is a nice light boat for that size. All you have to do is figure out how to build it.

 

Design displacement is the total amount of water displaced by the hull when the vessel is being operated the way it's designed for. For a displacement hull sailing upright, the weight of the water will always be equal to the total weight of the boat and everything on it, including crew and gear. A keelboat sailing heeled to leeward will displace more water than that because of the downward force of the wind on the sail. A planing hull, on the other hand, will displace less water since it's skimming over the surface.

 

I see what you mean now and I feel stupid for not getting it earlier lol. Thanks for all the help though. I am now working on the spars, sails, and foils in that order. Thanks for all the help.

Tim