Technical Information from Lars Strom (The Professor)
Dear all

Matteo suggested that this information be put on the Forum

Trim screws

In Rod Stephens famous Inspection Check List under the Hull Section he mentions amongst others "Reference marks necessary to control trim and flotation".

On Swans these reference marks are installed in the form of small screw heads on the centerline at bow and stern near the waterline. Unfortunately many yards and owners do not know the purpose of these screws, and assume they are some sort of mistake, and prefer to remove them.

With the help of these screws the accurate trim and flotation position can be determined, and for example the displacement calculated provided the lines drawing or the corresponding hydrostatics are available.

The trim screws on Swans are located 12 inches = 305 mm above the theoretical design water line shown on the drawings. By measuring the vertical distance from the screws to the water surface in calm weather it can be determined how the yacht floats in reference to the design water line.

Swans generally float a little deeper than shown on the drawings, meaning that the trim screw distances are less than 305 mm. If the aft trim screw distance is smaller than forward the yacht has stern trim, if the opposite is true she has bow trim.

If the screws have been removed it is very difficult to determine their correct position, the best way is to find a sistership having them and copy the location.

Perhaps the Association members having trim screws on their yachts would be willing to share the positions, it is suggested that two measurements are taken for each screw:
- forward from the lower end of the stemhead fitting along the stem, and from the uppermost boot top edge
- aft from the lower tip of the transom along the hull, and from the uppermost boot top edge.

Boot top (Some people refer to waterlines)

The boot top has a big influence on the aesthetics of a yacht, and enhances the overall appearance if properly shaped. S&S has a standard scheme for boot top width and shape, related to the waterline length, It should be noted that the S&S boot top is not a straight line but reflects the sheerline curvature, having more height forward than aft, and being a little narrower in the middle. When the boot top is divided into two stripes the top and bottom edges still conform with the S&S concept.

In connection with painting of the topsides it may happen that the boot top position and shape are lost, and the original scheme can not any more be traced.
Also in this case sistership owners could be helpful and provide information, but it needs to be pointed out that it is a demanding task to mark out a boot top with even curvature on a curved topside. It can not be shaped by eye alone, assistance from a water hose or levelling instrument is needed.

The appended pictures show the trim screws and divided boot top on a Swan 44. The position of the boot top in relation to the aft trim screw confirms that the boot top is a bit lower aft. By coincidence the forward trim screw in this case is right at the upper edge of the boot top, but this changes with the size of the yacht. For bigger yachts the screw will be below the upper edge, for smaller ones above it.

Thanks to the Owner and Matteo for providing the pictures

Lars

Lars,

This is really good information. I was just looking at these screws the other day and couldn't figure out what their exact purpose was.

Thanks for sharing your wealth of knowledge on these lovely boats with us and keep it coming!

Regards,

Nathan (s/v Inspiration 38/090)

Dear Lars,
Referring to your interesting technical information about trim screws, in my 44 the forward screw is exactly at 12" from the waterline but the aft one is at around 7".

I would like to have your opinion about why Pippus would be with the bow a little deeper in the water.
Can be the anchor and 50m chain weight that probably originally weren't considered ?
Can be the bow thrust weight?

Or simply because We did the check measurement during the initial survey before purchasing the boat, of course with the boat out of water. So our reference of the distance of the trim screw was the dirty of the vegetative sea line and not the real waterline and being that boat moored for long time the bow was pushed in the water due to the bow mooring rope?

Moreover considering not only the aesthetic point of view but by the technical point of view what can be negative that kind of trim ?

Thanks for your help as usual.

quite a few of the 44's are facing the same problem, anyway Pippus previous owner got rid of a few hundred kilos of lead which was located on the aft locker. perhaps you could try to get it back and see if things change

Fair winds

matteo (38/067 Only You)

Dear Alberto,
maybe I got something confused but if the forward screw is 12" from the waterline and the aft screw is only 7", doesn't that mean the boat is too low in the stern rather than in the bow? About 5" lower than the bow?
The mooring ropes used in the Med pull the bow down visibly, so I guess the vegetation line will follow ... but that too would shorten the distance to the trim screw in the bow and rather enhance the distance in the stern, although I think the mooring effect is much less visible here. So, if you measured the distance from a "distorted" waterline, it will even add to the effect that the bow seems to be higher out of the water than the stern.
Put aside that a trim and even boat is the most desirable, of course, I would say: lucky you. Having the stern lower will give you a slightly dryer deck and a tendency for the boat to drop of the wind rather than luff into the wind. As the luffing tendency (weather helm) increases with an increased heeling, you will need less rudder angle to compensate. I am moving the batteries in the boat aft for that reason, having the feel that our 48 is a bit too low in the bow.
And as we don't have the trim screws, it is just a feeling so far. Lucky you.
Best wishes,
Martin (Age of Swan, 48/039)