On Sails and Hull Shapes

[Dennis Marshall]

Ron Hotchin's thread on the launch of Swan inspired the following questions and I'm wondering if someone here can help me understand some things. Bear with me as I move from the particular to the general:

First, and as a matter of curiosity, can someone who has built or has the plans of Parker's Small Ohio Sharpie give me the respective sail areas of the fore sail and main?

Secondly, and with respect to cat-ketches. Is there a particular standardized proportional relationship (e.g., with respect to area) between the fore sail and the main sail, or is the relationship in area a matter of preference that is determined by certain loosely parameters?

Thirdly, what considerations must be taken into account when determining the amount of sail area for a particular boat? For example, in Atkin's catalog there are boats of roughly similar length and beam but differ in sail area by as much as 50 sq ft or more.

Finally, hull shapes of flat bottom skiffs and sharpies. If one looks at the picture of Ron's "Swan," it will be noticed that the stem rides a smidgin above the water's surface. I know that this is common to most sharpies. Chapelle, I think, indicates that this increases the maneuverability of flat bottom craft. Pictures of some larger Sharpies in Parker's book, however, are shown with immersed stems. I know that there are a variety of skiffs that have a fine entry and do not trim with their stems out of the water. So, in light of this, are we simply confronted with two different schools of thought about the hull form and rocker of flat bottom craft, or is there a method to all of this madness? My inquiring mind wants to know.

Thanks,

Dennis

[ 09-12-2003, 03:49 PM: Message edited by: Dennis Marshall ]

[bainbridgeisland]

I can't answer your question about Parkers' Ohio sharpie.

Standard proportions?

For cat ketch sharpies, there is no all inclusive standard for the proportional size of the forward and aft sails. Particular designs from specific regions often have similar proportions but this is not standard. Scientifically speaking, each point of sail or wind and wave condition would favor a particular proportion

Many different factors come into play when proportioning a rig. Some are purely practical such as sheeting angles, raising the boom high enough to clear heads or placing the mainmast in a particular spot to suit accommodations. Less practical, but still important factors such as good looks come into play as well.

Size of rig?

The amount of sail area depends on who is likely to use the boat, where the boat will be used, the purpose the boat will be used for, stability, anticipated materials for the rig, handling considerations and if an auxiliary engine is fitted.

Immersed stem?

The relationship of the stem to the waterline is not static. For the size boats you are looking at, crew weight can move around to control fore and aft trim. Thus in light winds, bow down trim and some heel to leeward is usually fastest. Whereas, sharpies usually sail faster in a breeze with a little stern down trim and a bit of heel to weather.

I don't think you should get too hung up on weather or not the stem is immersed. By itself it isn't very important. The overall proportion of the boat is far more important. For example a relatively fine bow that overhangs a bit may be the equivalent of a fuller bow with immersed stem. Also remember, after a boat starts to heel, all these proportions must be looked at anew.

Continuing on this idea of overall proportion: A boat with a full bow and relatively fine stern is easy to handle reaching and running but tends to be slow upwind in waves. So, a boat designed for easy handling off wind may have a full bow. But the full bow could be developed with or without an immersed stem. One is not right and the other wrong.

Maneuvoring?

By the way, the rudder and centerboard so overpower the hull on most sharpies that the boats are maneuverable irrespective of rocker.

[ 09-12-2003, 05:36 PM: Message edited by: bainbridgeisland ]

[Dennis Marshall]

Thanks, Bainbridge. Your last point raises another question with regard to the centerboard and its relationship to sail balance. Let's say I have a 42"L by 20"W centerboard -- I don't, this is just an example to help me understand the relationship of two elements -- and wanted to make the board skinnier by about 4" but keep the same length. How would that effect the lateral plane and the balance of the boat? Or, would it?

[bainbridgeisland]

A 20 % reduction of centerboard area would probably alter the balance of the boat. However, you may be able to simply rake the rig aft to accommodate the new center of balance.

How much will the lateral plane move if you reduce the area of the centerboard by 20%? It depends on how far aft and how big the rudder is and how much the hull contributes to lateral plane. The center of lateral plane is just a center of area of the immersed profile of the hull. Find the new center of the profile with the smaller centerboard and compare it with the original center of lateral plane.

Reducing the area of the centerboard by 20% may or may not be a good idea. If the centerboard is too small, the boat becomes more difficult to sail. It may be slow coming out of a tack in light air. It could also be slow if you over steer in moderate conditions. It would be more highly loaded and therefore stall more easily.

Having pointed out the disadvantage of reducing centerboard area too much, most centerboards could be reduced by 20% for use by experienced sailors. Beginners may have problems in some conditions though.