Tuesday, August 23, 2016

Displacement, volume, curves, differences

This morning I was in the process of looking over an older skiff hull design of mine and comparing a recent design against it. I had all the latest designs half hull sections laid out so I could look around and compare the entire hull difference at a glance.

I thought you all might enjoy seeing this process so I took a few pictures.

All the hull sections laid out on top of the earlier hull design. The wood half model is a narrower design for many reasons. I won't go into that now.

Here you can see how much narrower the wood half hull will be than the previous design which was designed to carry a big load, engine, live bait and such for Guiding. 
If you slide the wood half hull just out from the center line 2 of the grid squares you can see how the hull curves differ. If knowing how the previous hull performed, as in how dry it was, at that section then you can improve on this curve. Hopefully.

One way to add displacement to a hull is to widen it down the middle. In reality you only gain a few pounds of volume doing this. For this 18' hull it would be less than 100 lbs.
You can significantly by just adding a thicker hull skin though so you have to know the surface area of the submerged hull to calculate the differences. This way the hull will stay at its beam measurement and does not get wider ie. beamier which could change the handling characteristics.

The wood half hull is glued up with 3/4" spruce planks. See the glue lines and how they line up with the numbers on the drawings. These are called waterlines. Each 3/4" plank equals 6" in the design.

The blacked in area is what I have designed to be the true waterline at load. In order to know before you build and launch you need to calculate your displacement in advance and know how much your hull and all its gear will weigh. If not you are just hoping for the best after all your work building.
Any skiff builder out there today that can not show you his original design drawings and calculations are using others' hard work by using putty and glop to come up with a facsimile of what's been done before.
My guess is that Jim Gardiner of Egret has drawings, as do Hal Chittum, the Latest HPX models, Tom Gordon and his Element, the Action craft company, and Willy Roberts.
I believe the rest have buckets of putty. 

Showing the differences in bow height and deck width.
To me this is like having cut outs of very pretty women and laying one on top of the other to see how their curves differ... compare. 

Lots of fun if you love boat shapes. 
I'll show you mine if you'll show me yours...

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