Displacement versus vessel weight

[TheFisher]

I put my design spec's into Dave Gerr's scantling formulas using Lindsey Lord's ultralight sheathed strip-plank construction method.

The displacement at the designed waterline is 6104 lbs.

I then calculated the boat's weight with two people, half the fuel capacity, gear, etc.

The total weight only came out to 4200 lbs.

If I set the displacement equal to that the chines don't enter the water till station 8!!

Any suggestions?

[gonzo]

The displacement and weight are the same. If they don't coincide, there was a calculation error. Design displacement means the weight of the water displaced by the hull. How do you calculate weight and displacement?

[Timm]

If you have accurately estimated the complete weight of your boat, and accurately calculated the displacement of the hull, then you have two choices. You can either re-design the hull so that it has the correct displacement for the completed weight of the boat, or you can build in another method that produces a heavier weight (I won't suggest ballast as this is a high speed planing hull).

To my eye, your weight estimate looks a little low. I usually figure on 4 people at 165 lbs. each on a boat this size. Did you include all the weight for the frames and stringers in your estimate? Also, you should calculate the weight with all tanks full, not half way. You may also want to assume that the final as built weight will be a little heavier than you figure. Boats tend to gain weight over time.

Lastly, this construction method produces a very light structure and you may want to use a more traditional strip method that will give you a little heavier structure, but maybe one that is longer lived. I tend to be somewhat conservative on structures as living on the bleeding edge can get expensive at times!

[TheFisher]

My point exactly Gonzo!

The software calculates the displacement based on the displaced volume when I set the waterline.

I calculated the weight by adding up the hull materials (Lumber, epoxy, fabric, etc), engine, drive, fuel, etc.

The weight should equal the displacement at the designed waterline for the hull to sit on it's lines.

My problem is that the boat is not heavy enough when using Lindsey Lord's ultralight sheathed strip-plank construction method.

My maximum beam at the waterline is 47 inches with a hull draft of 17 inches.

[Timm]

Fisher,

I hope 47" is your max HALF beam at the waterline! Unless you are building a rowing shell, then nevermind . . .

[TheFisher]

Timm,

I calculated the weight with 2 people and half a load of fuel since she will be used that way most of the time. Occasionally it will have a third person, a fourth max, and the a full fuel load.

My first thought was I missed something and went back over it 3 times.

Ballast wasn't even a consideration.

I really don't want to reduce the beam any more than it is.
I could reduce the deadrise a few degrees.

Looks like I need to go back and recalculate using a heavier build method. Sure was nice to see 4200 lbs though!

[TheFisher]

DOH!

Half beam is 47"!

LOL

[gonzo]

How about a compromise between less beam and more weight. Lord's method produces flexible boats. However there are no restriction on the amount of bulkheads. Extra floors and half bulkheads would give you more weight, it would keep it low and also strenghten the bottom. The other method is to put a bigger engine.

[TheFisher]

The first thing I did was reduced the deadrise from 20 degrees to 18 degress. I could reduce the waterline beam a bit more but it is close to 3 1/2 : 1 right now. There will be a single open cockpit floor. I can add a raised casting platform in the bow built on top of the sole. The addition of extra half bulkheads and the casting platform would give me a several hundred pounds extra. Not sure if I want to go with a bigger engine. I already have a 350 (5.7L) for it that's pushing close to 375 SHP. It gives the Alpha drive a workout! Now if I can figure out how to get one of those 6.5L GM diesels to push a that many ponies I might make it a true inboard! Then I'd have to change the casting platform to a small cabin to hold the head since I wont be able to have it in the center console. Then the hull bottom needs to be redesigned. Looks like I won't be able to go with the more flexble design. I was considering it since it seemed that the slamming would not be as hard if some of the force was being absorbed thruough the flexure.

[Joel]

You guys rok! I have 30 facts to add to my display board!

yay!

[tom28571]

Estimated weight based on adding up all the materials that you THINK will be included in the finished boat is, in my personal experience, going to be low. In fact, there seems to be a substantial amount of weight gain in the final stages of building. I like to track measured weight of the boat during construction to get a better handle on it and avoid finding that the waterline is not where you thought it would be.

For large boats this may not be feasible, but there are ways to do it using something as simple as a bathroom scale and a few equilibrium calculations.

A gain of 20-25% over initial calculated value does not supprise me. Some builders just use a fudge factor based on experience. Stuff always gets added and seldom reduced. I'd expect the error to decrease with the size of the boat.