Mast Rake

Posted by John Cherubini II on the Hunter Sailboat Owners forum.

I accidentally made an egregious error in these boards about mast rake and have to clarify that I confused myself. I suggested 10-16 degrees of mast rake on about a 30-footer. This is WAY wrong– I meant 10-16 inches, measured at the masthead (about 5 degrees? –not doing trig here). Tie a plumb-bob to the topping lift, drop it to the deck, and measure how far back it lies from the aft edge of the mast (discard an inch or so for the masthead sheave). If the plumb-bob is bumping the back edge of the mast or even within about 4 inches you are WAY OFF.My dad’s boats are all noted for mast rake. In fact some of them will look downright ‘rakish’ in profile. Our Cherubini 44 cutter had a J (foot of foretriangle) of about 20 feet from the end of the bowsprit and I swear you’d measure that mast rake in yards. It must have been about 6 ft on a 56-ft hoist. It resembled his 31-ft yawl of 1961 which has 1:16 rast rake– 2 ft for a 32-ft hoist. But no Hunter is going to be THAT raked.Mast rake is sort of a cheater way of inducing weather helm. The standard plain-Jane cruising boat will have its centre of effort (sail plan) about 15 percent of the waterline FORWARD of the centre of lateral resistance (underbody). This would suggest that the boat would have a severe lee helm. But the forward momentum as provided by the wind and the airfoil shape of the keel actually works to fool the design of the boat. That momentum sort of carries the boat straight through the forward difference between CE and CLR and it’s like the boat becomes more interested in moving forward than bearing off.The CE is determined from the standing-still rig of the boat, what we should call the designed sail plan. Mast rake is already designed into that. The problem I belive many Hunter owners are having is that they have had their rigs tuned with the mast too vertical. If the mast rake specificaion is included in the forward justification of the CE, tuning the mast too vertical will actually spoil the boat’s intended balance and further exaggerate that 15-percent-of-waterline difference. You will in effect have induced lee helm, sort of as if you’ve let the jib luff or not even put it up at all. There is logically no way you will be able to point well like that. Drawing the masthead aft will return these ill-tuned boats to a weatherish tendency and then all you’ll have to worry about on the wind is keeping tell-tales parallel and jib lead position and leach tension and other fun stuff like that.Actually it is safe for all boats to have more weather helm than lee helm anyway. It is a sort of natural homing instinct that can help in emergencies, heavy weather, anchoring, and finding your bearings. If boats did not have a designed-in tendency to come into the wind you would never be able to come about! -or make your way home.I tend to suggest to people to lower their boom goosenecks and even have the mainsails recut to be 6-8 inches lower than I do to increase jib sizes. We get so rah-rah about headsails being leading edges and airfoils that we forget that the mainsail is the real power plant on most boats. Preserving mast rake will help the main do its real job fairly.JC II

Actually this weekend I was just looking over the sail plan to the ORIGINAL Hunter 54 cruising ketch (did you know there WAS one?) of 1979 and he has got the masts just about vertical (no angle specified for mast rake). So it may have depended on the boat. For myself I would definitely impose some rake on that boat, for looks if nothing else– which by the way ought to be considered typical of my dad. He was a self-taught engineer and really did know his stuff, but he was also a consummate aesthete (definitely RARE in engineering fields). If he did or did not specify mast rake you can bet the primary rationale was for looks– but then he would have known the physics issues it implied and been able to cope with those successfully too.So I apologise if I gave the impression that ALL his boats had mast rake (although I am quite sure the earliest Hunters did).Maybe I’m just a twit.JC

Posted by John Cherubini II on the Hunter Sailboat Owners forum. (10/26/2011)
The most important thing you can do is to keep the forestay TIGHT. Most people with furlers don’t notice that within the aluminum extrusions the stay itself is actually sagging– both from poor tension and from the weight of the extrusions hanging on it. Headsail furlers add plenty of weight! And the drum often makes it hard to get at the turnbuckle, so people neglect it.

(Forum ‘oldies’ like Ed are already rolling their eyes at seeing me go off on one of my oldest rants.)

The other thing that helps is allowing the boat to have its as-designed mast rake. Many people with furlers end up having the stay shortened to accommodate toggles and whatever else and end up with actual forward rake (yes– I have seen it). This absolutely sucks for windward performance.

Back off on the forward lowers, and a little on the uppers. Draw in on the backstay and see if you can make the aft lowers a little loose. Then, assuming level fore-and-aft, dangle the main halyard down to the top of the boom. On a 33 you should have about 16-18″. That would be about 1-1/2% or 2%. (Someone check my trig here.) If your performance to windward sucks, and your forestay is arguably tight, I would say you have less than that now.

Mast rake moves the center of effort a little farther back. This causes a boat to ‘weathervane’ into the wind more easily. Hard on the wind, all boats should carry a little weather helm (hard to tell with most wheel steering these days). Left alone they should slowly round up. This is desired, for safety, and is a feature of a roundish hull shape (boat being shaped like a football at the waterline plane). Modern boats (including Hunters) with their ‘sled’ shapes don’t do as well. They’re harder to hold on course and harder to get to point under sail alone. The more they heel the skitterier they get.

I don’t know why someone would move the forestay forward; it would do the reverse. Maybe it just adds more sail; and with a big long genny the weather helm (here desired, to a degree) is always exaggerated. I may be a purist; but that’s not the only reason I wouldn’t mess with the rig design on one of these boats. They really are at least as good as anything ever produced in such quantity, especially today.