How do I get mast rake right?

[TC]

What's the best way to measure to place a hole in a thwart, in relation to the mast step, to get the desired mast rake? The boat is a cat-ketch rigged dory, "Sea Swift," a Sam Devlin design. This seems like a dumb question but I can't recall having read anything on the subject. The plans just tell how much rake, not how to get it, and measuring from the blueprints would be useless in terms of getting the required accuracy. I have 2 ideas on how to do it -- one easy but maybe not precise, the other fairly precise but tedious -- but I'd be grateful for some experienced advice.
Thanks!

[Todd Bradshaw]

If the step and thwart are placed according to the plans and the hole in the thwart is bored according to the plan, wouldn't the rake be fairly automatic? I also wouldn't lose too much sleep over getting the rake perfect. Trust me, the sail isn't likely to know the difference unless it's WAY off.

[Ken Hutchins]

A plum bob and some basic geometric calculations.

[waterside]

Good question... getting the rake right is important, to avoid interference between your tiller and sail/spars.

There is no need to break out your trig textbook, though... You can check the rake by measuring the distance from the masthead to the transom. You should be able to precisely pick that measurement off of any reasonably-scaled blueprints.

Once you have that measurement, cut the mast hole through the center of your thwart (don't worry about the exact positioning) and drop in the mast. Next, have someone help you position the mast so that the masthead-to-transom measurement is perfect. Once the mast is in place, you can precisely mark the position where the heel of the mast strikes the bottom of the hull, and know exactly where to place the mast step.

[ssor]

How about rake per foot of mast? A carpenters framing square will put the centers of the holes in their proper places without much sweat.

Ross in Bel Air

[TC]

Thanks for the help! You all confirmed my ideas on how to get it done, and gave me a little extra confidence that it will be precise enough. I appreciate the input!
Tim

[Bob Smalser]

I draw it to scale on graph paper to get the offset at the mast step.

http://pic3.picturetrail.com/VOL12/1104763/7099973/103642436.jpg

Then it's levels, string lines and plumb bobs from there.

[DougWilde]

Tim,

You loft it. You did loft you boat, didn't you?
That will give you placement as well as angle of rake.

How to check the angle of rake?
Go to Lowe's/HomeDepot and buy one of those yellow adjustable angle gizmos, that reads of the angle in degrees. You already have a small spirit level. That will give you a horizontal independent of your thwart.

Mast and mast step for Williwaw (http://dbwilde.home.comcast.net/Williwaw/maststep.html)

shows how I did this on Williwaw. Her foredeck slopes aft quite a bit, probably a lot worse than your thwart. Go down that page about 3/4ths of the way and behold the Mast Rocket, a quick and dirty jig to help you ensure your mast will be plumb side-to-side while rake is as you intended.

Doug Wilde

[Jack Heinlen]

Measure. It should be in the plans. Not difficult, but pay attention! Get it right.

Establish, or reestablish if you haven't already, a centerline. Measure off that. Typically a mast partner in a thwart will be offset by under two inches in a small boat. Sometimes leveling the boat on its waterline and a plumb bob are in order, to get your bearings.

Have confidence and faith, dear son. Take your time and measure. Think, imagine, what accuracy means. The designer, if good, has given you all the information you need.

[Jack Heinlen]

Direct. Devlin tells you how many inches back the center of the partners is, yes?

Level the boat on its waterline. It doesn't have to be perfect, but close. String a tight line at the height of the thwart and level, fore and aft, a centerline. Make a tick the distance from the center of the step to the center of the partners, on the bottom of the boat. Use a plumb bob from the line to that mark. That's the center of your partners.

Leave the line with its new mark. Make the thwart. When it's right, put it in the boat. Look at where the mark on the line is. That's where you drill/cut your hole.

[ 07-08-2005, 10:32 PM: Message edited by: Jack Heinlen ]

[WayGray]

Are you sure that the sail will perfectly balance its center of effort with the centerboard and rudder? When building a sailboat of my own design, I initially put in a temporary mast step; then took the boat out for some test sailing. Moving the mast step fore or aft only a fraction of an inch can move the masthead by much more and can be used to refine the balance of the helm.

[Dan McCosh]

Any error in placing the mast step/thwart would be multiplied by the ratio of the distance between the two and the overall mast height. This is pretty tricky to do accurately. I would put the mast in at the correct rake with one or the other loose, then attach it when the rake is correct.

[Jack Heinlen]

Oh, good Christ. :rolleyes:

This is a simple bit of mechanics, though I understand how it's confusing if you haven't done it before.

Follow my direction. Someone who has done it a hundred times would dispense with most of it, follow his eye, but if you do what I said you will put the partners where the designer intended. It shouldn't take more than a hour.

Whether the designer was correct or not is another matter.

[ 07-08-2005, 11:24 AM: Message edited by: Jack Heinlen ]

[Gary E]

Originally posted by TC:
What's the best way to measure to place a hole in a thwart, in relation to the mast step, to get the desired mast rake? The boat is a cat-ketch rigged dory, "Sea Swift," a Sam Devlin design. This seems like a dumb question but I can't recall having read anything on the subject. The plans just tell how much rake, not how to get it, and measuring from the blueprints would be useless in terms of getting the required accuracy. I have 2 ideas on how to do it -- one easy but maybe not precise, the other fairly precise but tedious -- but I'd be grateful for some experienced advice.
Thanks!First thing I would ask myself is....
...how close do I really need to be
...what if I am wrong and how will I know it's wrong
...what if I am right and how will I know

Now depending on your degree of fussyness,
...If not so fussy,, just doit
...If your real fussy, make an adjustable one to use untill you fuss with it intill you got it the way you want it. Then either make new perment one, or just keep fussin wid it till you get tired of fussin...

[Todd Bradshaw]

Mast rake angle serves the function of adjusting the C.E./C.L.P. relationship as well as just making sure that mechanically you have adequate clearance between the various parts of the boat and rig. The boom dragging across the deck, for example, would usually be undesirable. Remember though that the whole Center-of-Effort/Center-of-Lateral-Plane relationship thing is a design tool for estimation purposes, not a law set in stone. In some ways on small boats it isn't a particularly accurate way to predict actual performance out on the water. Several factors, including the way you have the boat trimmed fore-and-aft, how you generally trim your sails and how much you are willing to hike out, may have a far greater effect on what kind of helm balance you get than whether your mast rake angle perfectly matches the plan. Two sailors with different hiking and sail-trimming styles, maybe one who likes to pinch up, let her heel a fair amount and point as high as possible vs. another who prefers to bear off a few degrees, hike to keep it flat and sail lower and faster may have very different ideas about whether the boat's mast is raked properly.

Certainly, for craftsmanship purposes if nothing else, it's nice to be able to get the mast position as close as possible to what it's supposed to be on the plan, but a couple degrees of "error" in rake angle is still likely to be quite "sail-able". The boat won't be a total loss and for some people it might even sail better. It's hardly worth jumping through hoops and rigging up a batch of lasers aligned with the celestial pole to insure that the mast rake on your dinghy is absolutely perfect because it really isn't that important and even then it won't always be the optimum angle when you're actually out on the water. Use any of the simpler methods posted above, get it done and go sailing. As long as your rake is reasonably close, you shouldn't have any problems. If you can figure out how to jiffy-rig something temporary to test different angles without breaking something, that's fine. On the other hand, you would probably need to leave it set that way for a couple of seasons to determine if it really is the best all-around rake angle. So my suggestion would be to set it, forget it and get on with the multi-season process of learning how to really sail the boat well.