Rowboat hull shape

[Rivercreekguy]

Drawing a hull, 10'6 LOA 4'2 Beam,flat bottom lapstrake ply. Lowest point of sheer, slightly abaft of midship. Should lowest point of rocker, greatest beam have some preferred relation? In the fore and aft sense, they don't seem to want to coincide in a pretty hull, 3 parameters, is there some basic caution to keep from a pretty hull that doesn' work so well?

[jehardiman]

Rocker is mostly independent of waterlines. However, I will say this: the further forward the maximum rocker, the harder to keep directional stability and more deadwood needed. Rather than think of placing the rocker, think of where you want the big weights (rower and passangers) and then make sure there is enough displacement under those locations. Plot displacement vs LWL at loadand try to place the maximum section at ~0.55 LWL aft by varying the rocker then place the rowing seat(s) and weights.

[alan white]

Make sure the stern doesn't drag. You will see, if you look at successful skiffs, that the lowest point of rocker flattens and carries about 2/3 back, while the forward volume underwater increases more gradually from the stem, which puts more stern in the water than bow. At that point where the bottom curves up to meet the stern, the bottom rises more steeply than it does forward. This will allow the boat to carry more weight aft.
There should be zero relationship between bottom rocker and sheer, as the aesthetics of the boat are based on the view with the bottom underwater.


A.

[Squidly-Diddly]

Never heard that one before. Any good links to comprehensive boat design lexicon?

[alan white]

Deadwood is any wood part of a keel that serves to solidly fill the area between the ballast and the boat hull, or any wood that solidly builds the keel beyond a simple skeg or simple keel. I wouldn't consider a keel that was made up of a single piece, and which the planking was attached to, to be deadwood. But if that piece extended straight back, and supported the rudder gudgeon (as on a lobster boat), there would be filler pieces above it around the propeller aperture to carry the prop shaft pipe. All this would be considered deadwood.

[alan white]

Originally Posted by Squidly-Diddly

Never heard that one before. Any good links to comprehensive boat design lexicon?

Most boatbuilding manuals have glossaries. I don't know of any links offhand.

[Raggi_Thor]

There are lots of lists on the Internet, one example,
http://www.teakmarinewoodwork.com/nautical_terms.htm

This is just one of the first I found, googling "nautical terms".

I think a low Cp (0.5 to 0.55) and a LCB approx 55% from the stem is a good start for a rowing boat. A lot of boatbuilders (and buyers) optimize for max speed, while we spend most time at low speed.

[Rivercreekguy]

Ok then,what is cp, and lcb? I think I get the 55% abaft the stem part. One other thing, I've reading H. Chappelle's "Yacht Designing and Planning" and may have missed it (alot of it is JUST TOO MUCH) but is there a not too complicated way of knowing if the waterline you draw will be the real WL?Thanks again you guys, not to gush I hope, but I've never been in a forum and it's way cool. I think my wife knows how to work the scanner so I'll try to put some drawing on soon.

[Çemberci]

DEar Mister


Hull Models of DXF files is available on thşs page. http://www.cembercidenizcilik.com/modeldxf/sandal.htm

Oktay Çemberci

[Raggi_Thor]

Cp = Prismatic coefficient.
A parallel shape like a cylinder or a box will have Cp=1
A rowboat should have 0.55 (approx).
This is the ratio between the actual volume (displacement) and the volume you would have if the midship (largest) section was extruded the whole length of the boat. Smaller Cp means finer ends and lower resistance at low speed.

LCB is as you guessed, longitudinal center of buoyance. This is also the center of gravity, CG.

Here is a good start for reading on the subject:
http://www.tedbrewer.com/yachtdesign.html