Boat speed vs Boat length in light winds

Daveinet · #1 (**permalink**) 08-21-2010

Would like to understand some things about cruising speed as related to boat length. While I understand hull speed and how that tends to limit the boat speed, does length have anything to do with boat speed when both boats are sailing at considerably less speed than their hull speed?

Here is the context: I was sailing in fairly light winds and probably cruising at ~3 knots. Hull speed is 5 knots, so I am no where near the hull speed limit. I am way over sailed for the boat weight and have nearly the same sail area as boats weighing 3 times as much. 184 sq ft 950 lbs full batton main. Sails were trimmed well - meaning I was not having to spill wind to keep from excessive heeling. Under these conditions, I watched a larger Hunter just pull away from me, seemed like he was going about 3 times as fast.

I always assumed that hull speed was for the most part, a "soft" limit, rather than something that dramatically effects the boat at all speeds?

LakePirate · #2 (**permalink**) 08-21-2010

I believe it has something to do with the size of the motor

BigZ · #3 (**permalink**) 08-21-2010

In lighter winds where hull speed can't be reached I'd think the main factors would be the SA/D ratio as well as hull shape, i.e., how easily the boat is moved through the water. Some boats don't need a lot of wind to get them moving, others will be slugs until a certain wind speed appears.
Given two boats with similar hull characteristics and SA/D values the longer one would probably have polars that show higher speed in light air than the shorter. Maybe because the exit turbulence/drag for the shorter boat represents a larger part of the equation than for the longer boat?

I don't know... it's been rainy most of the day and I'm PUI.

DwayneSpeer · #4 (**permalink**) 08-22-2010

A major factor in light winds is how slick the bottom is. Even a little scum will slow up any boat.

WanderingStar · #5 (**permalink**) 08-22-2010

You're right to think that, in general, hull speed is an upper limit. A boat with less drag should sail faster in light winds. This is one of the reasons that classic designs had overhangs: low drag upright, long waterline heeled. But there are other factors too: prismatic coefficient, appendage drag. But in your case it might be that the Hunter's taller rig was in more breeze. Also the age of the sails counts. One of the great things about sailing is that we never stop learning.

Daveinet · #6 (**permalink**) 08-22-2010

Quote:

Originally Posted by LakePirate

I believe it has something to do with the size of the motor

I did give that a thought LOL.

I could probably stand to get a good coat of wax on the boat. It is heavily oxidized and the surface has no polish, almost rough to the hand. I also have noticed I drag a lot of water behind me, even when the boat is at hull speed. It just seems with the extra amount of sail I'm flying, the boat should move well in light air. SA/D is 29.88, which by any standard is high. All the numbers say the boat should be fast, but I don't feel like it is.