DLR (slenderness) and LOA/LWL ratio

A bit stuck on this one, let me see if I can describe the scenario:

A parametric study of boats of the type that I am designing indicate a DLR around 235. With minimums on the very largest boats studied around 175 and maximums on the smaller boats around 270.

Most of these boats have reasonable overhangs, with LOA/LWL ratios of 1.1-1.3. Most around 1.2. The boat that I am designing however, has no overhangs at all. She is completely square ended, like an open 60, a LOA/LWL ratio of 1.

So the question is, if I maintain the DLR of 235, does a boat with a long waterline but no overhangs then become quite heavy? This doesn't seem quite right. Conversly, just taking a boat that has a DLR of 235 with overhangs and carrying her waterline length out to her ends without changing her displacement, resulting in a much lower DLR doesn't seem quite right either.


Any feedback would be apreciated.

Thanks,
Sam

 

DL ratio

Sorry Sam. Thems the rules.
I had a design I was working on myself where I shortened the waterline by just 5% and the DL shot way up from 171 to 200. Since use of the DL assumes like hull designs for like numbers, it should probably be used as a general guide line anyway for displacement purposes. Besides, It seems most designers look to make the DL as LOW as possible. I wouldn't worry about your lower DL numbers if I were you. Especially after considering where they are comming from (length of waterline rather than length of boat).

Bob

 

Ok, perhaps some specifics will spark a discussion on this topic:

The boat I am designing is of the french "deriveur integral" type, or internal ballast. Like the Alubat or the Kirie Feelings. This sail boat basically has a centerboard, no keel, but a huge chunk of lead inside the hull on either side of the CB trunk.

A parametric study of the two manufacturers mentioned above indicates that the displacement / length ratio should be around 235. What I want to do is design a boat that is 28' LOD and DWL. Following are some numbers:


ALUBAT OVNI 28
LOD 28.38'
DWL 23.29'
DLR 245
DISPLACEMENT 6614LBS

Sam’s 28 Version 1 (Strict DLR From Parametric)
LOD 28'
DWL 28'
DLR 235
DISPLACEMENT 12,096 LBS

SAMS 28 VERSION 2
(SORT OF AN OVNI WITH WATERLINE LENGTH CARRIED OUT TO ENDS)
LOD 28'
DWL 28'
DLR 129
DISPLACEMENT 6600 LBS


How much should I really be worried about this? If I keep up with the VCG and make sure the rig / righting moment is properly sized (Dellenbaugh angle / WPC) does it really matter how light or heavy the boat is? This is my first 'real' design, and there is sort of a fair possibility that this boat might get built, so I am a bit loathe to go way out of the design parameters of existing boats. On the other hand I am already doing that by mandating a LOA/DWL ratio of 1.

Sam

 

DL ratio2

Perhaps it would be more instructive to compare the LOA's of your design and the other one using the same formula. You will find they are very close. The main use of the DL ratio is to determine how fast the boat may be able to go without planing. The higher the DL the closer to 1.34 x sqrt (in knots) WL the boats maximum speed will be. Boats with low DLs tend to have higher displacement speeds.
A lot depends on how you extend the WL. The two ways that I can think of are 1.) adding a skeg and cutwater (with reverse curves in the waterlines), and 2.) fairing the entire hull to the longer WL with a nice even taper. In the second case, you may get a quicker pitching motion than the original (along with a possibly greater speed potential). And in the first case slightly more wetted area and longer turning radias.

Bob