Mast location advice sought

[Zaynab]

Hi,
I checked Larsson's Yacht Design and Gerr's books to find information about where to put a mast in the fore and aft direction, but did not find it. Some boats have the mast quite more forward than others (e.g. BOC-racers having the mast almost in the middle). I am looking for a source of information, possible including (rule-of-thumb) hydrostatistics, concerning mast location.

[SailDesign]

Your problem is that you are only looking at mast location. TIt is the centre of area of the sail plan that is important, coupled with the centre of lateral resistance (keel and rudder location and size).
Skene's Elements of Yacht Design has some guide-lines, as do great many others. If you want waaaay too much info on the subject, look in any of Marchaj's books ("Aero-Hydrodynamics of Sailing" or "Sailing Theory and Practice")

[gonzo]

One of the details to take into consideration is the angle of heel. As the boat heels over, the center of effort of the sail and lateral plan separate. The weather helm increases with the separation of CE. This means that a hull with large initial stability needs less distance between the mast and the keel than one with low initial stability. Another considration is the hull shape. Narrow aft sections make the boat point up when heeled; wide sterns do the opposite.

[giramonti]

Gonzo makes two very good points: The more a boat heels the greater the "rounding up" moment; the finer the stern the sooner a boat will round up given equal heel angles with a wide sterned hull. Also, he mentions that a boat with stiffer initial stability will heel less, so the CE of the sails will be closer to the CLP of the underbody. Well, that's a good start, but the problem is: where does your boat fall within these parameters?

In our office we use a parent hull that has worked well in ULDB's for many years, a Herresshoff derivative. We have a pretty complex spreadsheet that takes into account many factors. We don't use the CE vs. CLP at 1/4chd that Eliason uses, though we check our numbers against it. For this hull type, we know exactly the lead we need. So we always place the Mast to solve for this number. If we want to move the LCG aft, generally the mast will move back. There was mention of the Open 60's having their masts at about station 5. The LCG of an Open 60 that I worked on was at .62*DWL, so to get the right balance, the mast was put at just aft of .49*DWL.

So, now that you are confused, here's my advice: if you are working on a design that will not be pushed or raced, use the Larsson & Eliason rule of thumb, it will work given that the mast is at about station 3.8 +- .35 . If you are designing a boat that will race, make ocean passages, or both, then sit down and analyze the force vectors on all appendages and sails and solve for the angle of heel you want, at which wind and boat speed, before the boat rounds up. Then use that much lead, dictating where the mast goes.

[gonzo]

I agree my comments were very generic. Every design takes careful consideration. It is always possible to look at examples of the same type of boat to estimate sailing behaviour. Of course, with experimental designs, long calculations and tank testing are the only options. I believe that conservative cruising designs are better. After all, speed is secondary to safety and confort.

[Stephen Ditmore]

Starting with the mast step at sta. 4 was the convention at Sparkman & Stephens for the short time I was there. While this was originally for masthead rigs, it often works for fractional rigs as well. The exact keel placement can be based on Larsson & other sources once you've decided upon the rig dimensions.

For my detailed response see
http://boatdesign.net/forums/showthr...highlight=mast

[Zaynab]

Thank you very much for the replies, they have been very helpful.
Follow-up question: how do I find the center of effort of an aggressive roach main. I guess it is not a good idea to disregard the luff in this case, because it can make up 20-30% of the mainsail area.

[Stephen Ditmore]

It should be possible to do that using Simpson's rule. I've heard both opinions concerning whether it's really necessary. Keep in mind that most published info on lead is based on a CE that ignores roach, headsail overlap, and the fact that you don't actually sail with the sails trimmed flat on centerline. In a best case scenario sailmaker, sparmaker, and designer would all be in communication and would develop a consistant methodology since things like the fullness of the mainsail and the ability to flatten or reef it as the wind increases matter.

Team NZ showed that it's useful to be able to adjust the masthead forward downwind by reducing rake or even raking the mast forward on a run or broad reach. Even if the balance could be nailed perfectly there might be reason to make mast rake adjustable (and to keep the rudder large).

[Laz]

SailDesign's simplified answer (I like it this way) suggests that you could do it the other way. Plant the mast, and then figure out the location and shape of the keel to match.

Or do you have to design the hull and appendage first because of all the hydrodynamic issues, and then place the mast as dictated by the hull parameters?

-Laz

[Stephen Ditmore]

I'm not saying mast at sta. 4 is the last word. It's one possible starting point that should produce a result consistant with past practice. If putting a masthead rig on a light displacement boat with a near vertical bow I suspect I'd end up putting the mast aft of sta. 4. In any case, I agree with you that the final keel location should be decided after the rig dimensions and location have been established if ballast placement and structural considerations allow.

[gonzo]

Now that you mention ballast. With fin keels the positioning of the ballast is restricted to a small area fore and aft. You need to consider the displacement curve of the hull to position the keel. It will, in turn, determine the position of the mast.

[Stephen Ditmore]

The design spiral keeps turning, and turning, and turning, and...

[Tim Dunn]

http://www.boatus.com/goodoldboat/Helm_balance.htm --
Ted Brewer makes it as easy as possible.

[Ward]

Boats that have the mast close to the bow of the boat usually only have one sail, which usually comes pretty far back, maybe all the way to the stern. Boats with the mast in the middle usually have 2 or more sails, but the sail area still runs about the same length down the boat. I'm by no means an expert at this, but it seems to me that you could really put the mast on any spot down the centerline of the boat, as long as you have the right sail shape to put the center of effort where it needs to be. Now how you figure out where the center of effort point needs to be is beyond me, but im still learning

[jimthom]

I was wondering per the previous post if the position of the mast in relation to the leading edge of the keel was sacrosanct, or indeed if there was any significant reason at all to establish this relation.

I am planning to move the mast aft from station 4 to station 5(approximate), but keep the CE of the sail plan where it is by altering the sail plan to a cutter rig. The leading edge of the keel is fixed around station 4.

Of course I will have a qualifed person design my new cutter rig, but I was wondering in theory if this is a practical objective.

Thank you,

jimthom