Mast rake and stem to stern level

[yelkenli]

Setting the mast rake raises a few questions. First, for a Catalina 27 standard rig, the manual says no rake. Being human, there are some threads out there recommending auction house values of rake aft.

Second question might explain the different values. And that is 'how do you know you are level stem to stern?' The boat is sitting in the water. I can walk around and change that. Some threads claim the stern drops when under sail, but I don't plan to set my rake while I am under sail and at most only informs what rake value you want, not achieving it at the dock.

The simplest is to understand if the boat is level when I plan to stand on it to check the rake, and maybe have my movable ballast make it level.

Some recommend a mechanic's level on a berth or settee or a bulkhead. Some claim the designer intended that, but the builder might have gotten it wrong. This seems the most practical though.

Some recommend looking at the waterline, but I don't have a good shot of that, and not sure I would trust the boat is not sitting low at the stern or stem.

Let me know how you deal with what could be arbitrary if not measured.

 

[dlochner]

The easiest way to measure rake is to suspend a plumb bob (or heavy wrench) from the mast head with the main halyard into a bucket of water. The water helps to dampen the motion of the plumb bob as it swings from the boat's movement.

It is difficult to get a repeatable and reliable rake measurement, for all of the reasons you identified. Look for the average or tendency of the rake. If the manual says no rake, then plumb bob should fall right at the mast step. If it is an inch or two behind the mast, I wouldn't worry about it. A level could be placed on the mast track, that will indicate if the mast is perpendicular to the water. Place the level on the deck by the mast. With those two reading, it should be possible to estimate how close to perpendicular the mast is to the waterline.

For most of us this is more art than science. For the folks tuning IMOCA 60s, it is more science than art.

 

[DArcy]

The correct mast rake won't change due to the stern squatting. Rake moves the center of effort of the sail plan fore and aft. The longitudinal distance between the sail plan center of effort and the underwater center of lateral resistance (due to keel and rudder mostly) is what will cause weather helm. If you find you have too much weather helm rake should move forward. Not enough weather helm, move rake aft. When the wind picks up the boat heals over which moves the center of effort outboard from center of lateral resistance increasing the moment arm which also increases weather helm. Sail flat, sail fast.

There are enough other variables affecting weather helm on a cruising boat which might override rake, mostly sail condition and trim. If your helm isn't balanced try adjusting halyard tension, backstay tension, outhaul, vang, sheet position. If your helm is balanced, enjoy the sail.

 

[colemj]

Get one of these: https://www.amazon.com/SHAHE-Magnet...6257034&sprefix=digital+level,aps,278&sr=8-26

Place it on the base of the mast step and note its value, then place on the mast and adjust the rake until it matches.

Mark

 

[jssailem]

Get one of these

I note that the digital level uses a magnet to attach itself. That might be an issue on an aluminum mast.

When establishing a mast rake, I have always used the main halyard with a weight on the end. As has been stated, the boat is constantly moving. Level becomes a relative term. Luckily there is no need for precision here. Close is adequate.

When you get it established, you can sail the boat and decide. If you think you need a little more or less. Make small adjustments to reach your best results.

 

[PaulK]

To set the rake on my 505 dinghy we attach a measuring tape to the main halyard and see how far it is from the bottom center of the transom. The sailmaker's tuning guide tells us the length it should be. If a Catalina 27 isn't supposed to have any rake you could find a profile drawing of the boat (try here: CATALINA 27 - sailboatdata) and use the scale to see what the distance should be from the head of the mainsail to the transom. Attach your tape to your halyard and set your mast so the distance to your transom is what the drawing tells you it should be. Voilà! No levels, no angles, no variations depending on how the boat is loaded. Easily repeatable any time with the boat floating, heeling, grounded, trailered or hauled out. If you decide later that you want more or less rake, shorten or lengthen the target measurement and write it down somewhere so you don't forget. Very simple.

 

[jssailem]

Using Paul's link, the mast appears vertical to the water line (at 90º) but raked when compared to the top of the cabin. Note the rig is a mast head sloop.

This is an image of the design drawings. I superimposed the black line on the mast.

 

[pgandw]

To set the rake on my 505 dinghy we attach a measuring tape to the main halyard and see how far it is from the bottom center of the transom. The sailmaker's tuning guide tells us the length it should be. If a Catalina 27 isn't supposed to have any rake you could find a profile drawing of the boat (try here: CATALINA 27 - sailboatdata) and use the scale to see what the distance should be from the head of the mainsail to the transom. Attach your tape to your halyard and set your mast so the distance to your transom is what the drawing tells you it should be. Voilà! No levels, no angles, no variations depending on how the boat is loaded. Easily repeatable any time with the boat floating, heeling, grounded, trailered or hauled out. If you decide later that you want more or less rake, shorten or lengthen the target measurement and write it down somewhere so you don't forget. Very simple.

I do exactly the same on my Mariner. Tape a 35ft tape measure to the main halyard, haul it up to the top, and get the top center of the transom less than 28ft (recommended rake by class association). While I have the tape hoisted I also measure to the chainplates to get mast head centered athwartships. Then attach the Loos gauge and set the shroud and forestay tension. On a Mariner, backstay should be very light tension. Much quicker and easier when launching from a trailer than any hand and mast sight tuning. Far more repeatable than a plumb bob. Note you could lose a pre-measured or pre-marked low stretch light line instead of a tape measure - just something you can pull tight enough if there is wind blowing.

Fred W
Stuart Mariner 19 (25ft mast) #4133 Sweet P

 

[colemj]

I note that the digital level uses a magnet to attach itself. That might be an issue on an aluminum mast.

While it has a magnet on one of its sides, the magnet isn't needed for it to work - it is just a feature. Otherwise it is like any level where you lay it flat on the surface you are measuring. I find I use it for a lot of things other than setting the mast rake every 10yrs. Since nothing is level or square on a boat, the small digital level is useful for taking relative measurements.

On a moving boat, a level is more accurate than a swinging weight, but the boat shouldn't be moving while doing this measurement anyway. I disagree that precision isn't needed, but that is likely boat-dependent. 1* definitely makes a difference on our boat.

Mark