Quick and not so dirty

Quick and Dirty

When I first approached this design I took the section lines of the skiff,

and simply rounded the chine corners. Then I adjusted the rounding so that the curves were parallel. I also narrowed the pram bow at the waterline to give a finer entry

I then went through the process of waterlines, buttocks and diagonals to get this,

 

Compare that to this, which is the hull we've developed over the past few weeks

 Here's a comparison of the sections side by side, the red being the lines developed from the skiff by rounding and the black being our skiff, and also superimposed.

On the face of it I can't see much difference and it was very much quicker. Our skiff is a little finer in the bows and the Q&D version has a bit more displacement, that's about it.

Now that that is done I will run some numbers on the two versions to see what difference, if any, there is.

Numbers

BY THE NUMBERS

I ran the numbers on these two versions, here they are,

Version	RMSQ&D	RMS
Displacement	304.89 lbs	266 lbs
Block Coefficient	0.18	0.16
Prismatic Coefficient	0.48	0.53
Area of the waterplane	25.55 sqft	27.78 sqft
Centre of flotation (aft or forward of Station 5)	.4381 ft aft	.4772 ft Aft
Lbs per inch immersion	152.29 lbs	137.49 lbs

So what does that mean exactly?

Well Q&D can carry more weight at the designed waterline by about 40 lbs and it takes more weight to sink it any further into the water.

Skene's sets out that a prismatic coefficient between .49 and .55 is best for sailing vessels, any more than .55 and you have a tub, any less than .49 the vessels is so fine it drags a huge quarter wave. So Q&D is on the fine side and RMS is within the parameters set out by Skene's. However the block coefficient tells us another story, by that coefficient RMS is the finer vessel.

Skene's also sets out that the center of flotation should be between 54 – 59% of the LWL aft of station 0 or in our case, between .48 ft aft of station 5 to 1 ft aft. So Q&D is a bit too far forward and RMS is just about right.

The LWL is 12ft for both these vessels and the beam at the WL is 5ft 4in which is a beam to length ratio of 2.5 which is a little beamy, anything from 3 to 5 is better, but the beamyness in such a small vessel adds to the initial stability.

Next time we'll look at stability.

On the Fiddle

The Fiddly Bits

Adjusting each of the lines so that they are fair takes a fair bit of time because they have to be fair in all three views. With pencil and paper this takes more hours than you can count as you try a new curve and erase the old one only to find that some other line is not fair because of the change you made.

This takes much less time on the computer. Each curve has nodes where they intersect with other lines,

The blue squares are the nodes. Each of these nodes can be moved to adjust the curve. I use this little gadget to determine how far to move the node.

The concentric circles are 1/8^th inch apart and at the centre is a reference point. I put the reference point on the curve I want to adjust,

and move the node an 1/8^th inch at a time until the curve looks right.

Then of course you have to transfer the new intersection to the other views and adjust those curves. After about a half hour of fiddling I got to this.

The lines are all fair to one another and the flow aft is smooth. In the next post I will show you another set of lines for this same boat which took only two hours to do from start to finish and I'll tell you how I did it.