Mast Rake 40-50 ft Catamarans

[Gboy]

Hi all..

I would like some opinions (not too many) on mast rake, related to sail plan and hull shape as much as possible.

My CAT (Conser 47) is a more performance CAT with narrow hulls, 14,000 Ibs deplacement and wing mast..so not your regular cruising CAT.
Have 625/329 sqft sail plan, mast is 20 ft from bow on this 47 ft CAT..

Just replaced the composite rigging and have a 3 ft rake on a 58 ft mast (actual mast length). I read everything from 1-4 deg rake..

Presently I have 3 deg, which is a 3 ft rake, measured at mast base.

As one may expect good upwind performance...but seem to have more weather helm.

Is it possible to calculate the recommended rake angle, given some factors/measurements ??

Or is this a sail and play type of correction ?

If anyone really know their stuff, I can supply all info required....



Thanks for advice

[caradow]

so......if you have too much weather helm just keep reducing it until the helm feels right.
at the point you are at why all the complicated measurements?
in my 40 years of racing I always thought boats were tuned best by feel.
even in one design classes the sailmaker numbers only get you in the ballpark.
sounds like a fun boat
good luck

[Southern Star]

I agree totally with caradow - as you move the truck of the mast forward, you will reduce weather helm. There are so many variables on any boat that effect weather/lee helm, it really is a matter of trial and error.

Brad

[salticrak]

The mast rake for my wharram tiki 26 is 4 deg. Naturally it is not a performance boat but does have narrow hulls.

I am not sure but i think the rake is the same for the larger Wharrams.

[Gboy]

Thanks for the initial feed back..I agree it has to be want feels right in the end.

This is my first real CAT (only charters previous)..

I also realize there are so many variables in the boat design details (and even hull condition, etc..) that can effect any such advice or initial considerations..

Just trying to get a general experienced view point. One problem I have is the hydraulic steering design, that gives very little feel. I can only really go by rudder in dictator ...

The initial discussion came from slower speeds than expected with this CAT, that has history of achieving near apparent wind speed (in certain conditions of course)..and if the rudders were adding too much drag.

I have to state as well, I am new to sailing (2 yrs)...but learning fast.

Can I asume that anything between 1 - 4 degrees would be normal range and I just need to find where works the best for current boat design and condition.

Thanks again.

[caradow]

yes those numbers are fine
just dial out the excess rudder by getting rid of some rake.
hydraulic steering is problematic in that sense but if you have a rudder indicator then that will work.
I personally am not a fan of hydraulic steering for that reason although I had it on my 50 foot ketch for over 12 years and it never failed me but sort of "sailing blind" when it comes to feel.
again I would not get too hung up on numbers other than reducing sail by numbers and not feel!
a lot of factors/forces will be affecting your boat at any one time.

[colemj]

Before doing any rig work, check to make sure the rudders are aligned properly. Hydraulic steering can creep over time and one rudder could be misaligned to the other.

Mark

[SVNeko]

Switch 51 calls for 4 degrees. You can check by doing the math on the triangle a halyard makes dropped to the deck.

[beiland]

Originally Posted by Gboy

Hi all..

I would like some opinions (not too many) on mast rake, related to sail plan and hull shape as much as possible.

My CAT (Conser 47) is a more performance CAT with narrow hulls, 14,000 Ibs deplacement and wing mast..so not your regular cruising CAT.
Have 625/329 sqft sail plan, mast is 20 ft from bow on this 47 ft CAT..

Just replaced the composite rigging and have a 3 ft rake on a 58 ft mast (actual mast length). I read everything from 1-4 deg rake..

Presently I have 3 deg, which is a 3 ft rake, measured at mast base.

As one may expect good upwind performance...but seem to have more weather helm.

Is it possible to calculate the recommended rake angle, given some factors/measurements ??

Or is this a sail and play type of correction ?

If anyone really know their stuff, I can supply all info required....
Thanks for advice

Happen across this subject thread while looking for something else.

Wondering if you still have the boat, and if you ever solved your problem?

[Gboy]

Yes, but don't have much of a problem, just trying to optimize and learn..

Thankd


Sent from my iPhone using Cruisers Sailing Forum

[Stumble]

I don't know of any way to calculate rig settings. There is the designers original plan, and then racers get ahold of the boat and throw the book out of the window. Just as an example, the original J-35 tuning guide called for 2 degrees of rake and no prebend. This was right up until a boat named Big-J showed up at worlds with 0 rake and 16" of prebend and almost lapped the fleet.

I would think that 1-4 degrees is a pretty conservative place to start. My A-Cat for instance starts at 6.5 degrees prebend, and rakes back to 9 degrees as the wind picks up.

[AD28]

Originally Posted by Stumble

My A-Cat for instance starts at 6.5 degrees prebend, and rakes back to 9 degrees as the wind picks up.

Are you talking about prebend, which is mast bend or curve within the mast itself, or rake, which is mast angle, or the mast's relationship as a whole to the deck/level of the boat? Two different things.

[thinwater]

All about rudder angle and the relative size of keel and rudders.

High rake is relatively common on beach cats, particularly Hobbie 16s, as the sailors learned that the best foils were the rudders (no dagger board). Thus, if they rake the mast, it keeps the bows up reaching and places lateral load on the rudders, which they can handle at speed as they are relatively deep. Even better on the Prindle 16/18.

However, this exaggerated rake does NOT apply to most larger cats. The rudders are small by comparison, the keel/dagger is better, and the lee bow does not dive on a reach.

Because cats have little helm feel, it is useful to know how far off center on the wheel corresponds to 4 degrees of rudder; lets you know when the balance is off.

[Stumble]

Originally Posted by AD28

Are you talking about prebend, which is mast bend or curve within the mast itself, or rake, which is mast angle, or the mast's relationship as a whole to the deck/level of the boat? Two different things.

Sorry I should have been more clear. My neutral rake setting is 6.5 degrees for 8-10kn of breeze. As the wind lightens the mast is raked forward to about 5.5 degrees at 0-3kn. In heavy air I pull the mast back to 9 degrees at 20+kn.

The boat has almost zero prebend at rest, rig tensions aren't high enough to induce much. But as the mast rotates it indices side bend, so where a normal rig is bend backwards on the A-Cats the rig bends to the side.

[AD28]

Originally Posted by Stumble

But as the mast rotates it indices side bend, so where a normal rig is bend backwards on the A-Cats the rig bends to the side.

Actually, if you care for some history, the original 'normal' is what we used to call 'bendy' rigs (what you have) ergo the mast had no prebend/tightened diamond wires, but was flexy enough with loose diamonds such that when we'd (over)rotate the rig the mast would bend on the minor axis. Back in the old 18 Square days, there was quite a contention between 'bendy' and 'prebent'.

The bendy rigs lose a bit of sail efficiency, in that the bent mast creates a bit of a shadow into the luff of the main. The advantage of prebend is that one doesn't have to over-rotate, in that the mast camber is induced in line with the foil shape.

I was under the impression that the A guys these days are all messing with diamonds that tighten on the fly to induce bend for ambient conditions... but then my high-tech was the 18 Square