CE Height and J Measurement?

I have recently discovered the Boat Design Forum and am still reading those threads related to my own project. This is my first posting. If my questions have already been discussed I would appreciate the reference
The Project.
Over the past few years I have been rebuilding an (ancient) Mariner 31 ketch (3.5 m draft, long keel, 7 tonne disp. 35% ballast ratio (?), 26 ft w.l. ketch SA 468 sq ft. Perkins 4-107) including the conversion of the rig to a cutter with bowsprit. (More at: www.marineryachts.com/ “Legato”.) I would classify this craft somewhere between a sailing yacht and motorsailer. Even with a cutter rig I am not expecting much of an upwind performance. I hope use this yacht for N. Pacific passage making.
CE Height?
The Solent Cutter foresail plan seems to be the best option (many thanks to Boat Design Forum discussion – Seaworthy Headsails) – hanked self-tacking jib on a foredeck to masthead stay (J-12.6ft), furling genoa on a fixed bowsprit to masthead stay (J-16.25ft, lead 12% approx) and gennaker. At the time of installing the mast step and terminals (no bowsprit yet) I did not consider the height of the CE and the heeling effects. Is there some practical test that can decide this?
The rebuilding included the moving of the fuel tank from under the cockpit floor to a position under a main cabin berth and batteries (much heavier) from the pilot berth level to under the main cabin floor. Also the weight of the cutter rig should be less than the ketch. Am I right in assuming that these alterations would permit a higher CE than the one shown on the ketch’s sail plan? But how much higher?
J measurement?
This heavyweight cutter (7 tonne) will not have much of a SA (if CE and SA are similar to the ketch). Increasing the SA would mean a longer bowsprit because the mainsail triangle cannot take a much larger sail and boom (E-12ft) than that which I plan to use. But would a longer J measurement produce any noticeable benefit for down-wind sailing? Would it be practical for upwind sailing - considering the increased lead?
Comments.
I really would appreciate any comments that will help me think through this project. And then finally I will have to cut the mast and stays to length and fit sails.

 

Welcome to the forum
What comes to your rigging why you don't post drawings of your sailplan(before and new) so someone will draw on them how to find out CE's etc. generally speaking it's quite strange to make a cutter of a ketch, but there shouldn't anyway be too much difference in CE and SA (not an easy task). If you gain some advantage with weight savings aloft try to keep it that way..
BR Teddy

 

Is this the boat your are referring to? This is the first offering from Far East Yachts.

If so, it's preformance is pretty much as you've described, though can be improved. It's SA/D is absurdly low, so a fair bit more area is needed then it came with. In fact, it did if my records are correct as an option in latter years of the model run. You also appear to have her displacement incorrect, I'm showing 5.5 long with 2.2 in ballast for a reasonable 40% ballast ratio.

At 500 sq. ft. of area her SA/D jumps to nearly 15, which is an improvement, but still a little low, unless you're in the trade winds all the time. At 600 sq. ft. her SA/D is a respectable 17.6, which will improve her abilities considerably.

The information you've picked up from that Mariner site is "provided" information from a specific owner and I'm not convinced it's especially accurate.

Your dreams of a cutter should be left for another boat. First the cutter rig can be handy, but the sloop is considerably more efficient. The cutter rig requires the mast be located between station 4 and 5 (closer to 4) and a less powerful mainsail. The sloop usually has it's mast in the first 1/3 of the boat, meaning the cutter's mast on this boat would naturally fall about the middle of the settee table. If moved forward to the aft corner of the head compartment (about the sloop position), then it'll be much less intrusive, but a balance issue will likely arise (appendages are fixed).

On the other hand, you could employ a double headsail rig on a sloop, which is quite different then an actual cutter rig. The only reason to do this is the single headsail size is big enough to warrant dividing them up into smaller sails. That's pushing it on a 31' yacht, but doable.

Rigged as a sloop the mast will need to be somewhere in the opening of the head compartment, likely towards the aft end of this opening. This frees up the V berth, but presents other issues.

I suspect the "new" rig is possible, though my notes about this design suggest you have a host of other issues, many related to the deck and cabin structures.

Considering the huge complication and additional height you need for a sloop or cutter rig, with the additional area necessary for better performance, just increasing the ketch is the logical choice. A taller main with more roach and a taller mizzen with the related larger headsails and mizzen mule will make for a much better balanced boat. The righting arm will remain fairly low (in comparison to a lofty sloop or cutter rig) and the sail sizes will be manageable for a small crew.

As far as windward preformance your keel as well as rig are conspiring against you. The hull shape isn't helping much either. You can only moderately easily modify your rig as the appendages and hull form are fixed. Given this limitation, any expectations of performance improvements would be unjustifiable.

 

Many thanks for the extensive responses to my CE and J enquiries so far. Right now my yacht is deep in snow so there is time to discuss and think about my sail plan.

For TeddyDiver.
Sorry to say I do not know how to post sail plans on this site but there is a sail plan of the Mariner 31 ketch on the Mariner Owners site mentioned; the cutter rig plan has not been decided yet.

For PAR
The photo posted is the Mariner 31. The empty rig-less yacht when lifted clocked 6.5 tonne (6.4 long). Also the wetted surface of 240 sq ft, using the former w.l., would mean a disp of 6.9 tonne (6.8 long). There were several builders of this yacht with differing production standards.
As mentioned I have already installed the mast step (on deck just forward of the dog house), mid-station 4-5, leaving sufficient space for a boom (E-12ft) to clear the backstay. The internal post, standing on a reinforced bridge over the keel, is also the forward end cabin table support. With a possible 2.4 ft bowsprit the lead would be about 12% - could it be more? I am wondering about a possible J measurement.
The Solent Rig I want to adopt has double mast-headed stays, furling genoa (and gennaker) on the bowsprit. I suppose that is not the classic cutter sail plan – more of a light wind sloop, heavy wind cutter.
My Comments.
I opted for the single mast plan because a ketch is not the most efficient rig on a small yacht and also the mizzen and boom blocked up the cockpit. But even with a similar SA as the ketch a more efficient rig is preferable - a logical choice or not?
If I could start all over again and simply boost the ketch SA by raising mast heights, adding roach etc. (and raising the CE) I would still be wondering how high the CE should be and if there is any method or test to decide this.

 

With the taller sloop rig, you'll have higher loads that need to be transmitted to the hull. I'm not sure how you decided on mast placement without a full work up of the proposed rig, and it's placement over the CLP, but a sloop rig will want 15% or more lead, depending on several factors, which also would have been part of the work up. With the sloop, you might get a degree or two further into the wind, but don't get too excited about windward ability with that hull and appendage configuration.

The boat is grossly under canvased, so a slightly more efficient rig, with a burdened hull form and it's low aspect appendages, means it'll still suck to windward (sorry).

If your wetted surface calculation is correct then 600 sq. ft. of sail area is right in the ballpark for reasonable preformance. Naturally, as a sloop you'll be raising the CE quite a bit, so righting leverage should be calculated, so you know how big to spec things. This isn't one of those things you want to guess at for several reasons, most of which you probably know.

 

For PAR,
Thanks for your observations. I started this project with very limited knowledge of boat design. Even so I did not think that a 600 sq ft SA was practical.
The mast step position was decided, as mentioned, on space for the boom available, access to the hull so that I could mold-in chain plate bulkheads and a mast step reinforcement without tearing up the toilet compartment and a 14% lead approx (100% fore triangle) using a 4 ft bowsprit. The centre of the mast is 2.4 ft forward of the CLP.
According to Chapelle the lead applies to sail area used on a close haul and has a wider range for long keelers. On my vessel it is approx 11% (100% jib fore triangle), and, of course, increases as main and working jib are reduced - genoa furled.
The purpose of the conversion was not primarily to improve pointing; there are other points of sailing on which the ketch is less efficient. Also it simplifies handling and removes the mizzen from the cockpit.
I am still hoping to hear about some method of deciding the CE height on this particular hull and also the pros and cons of longer bowsprits for a mainly downwind sailer.

 

The CE height is a simple geometric computation. You could do it at your kitchen table with a scale drawing of the rig, a ruled straight edge and a pen.

Chapelle's observations on lead and other "centers" relationships are over a half a century old now, actually pushing 3/4's of a century, which sucks, seeing as he was a friend and making me feel my age. You'll find modern ketch proportions are typically a little longer in lead. I try to hit 12% - 13% on normally proportioned ketches. The same is true of a sloop, especially a good one (efficient, healthy roach, etc.). How did you find the CLP? How much rudder area did you include in this calculation?

 

CE Height

for PAR.

Thanks again for your interest. The CLP estimate used in deciding mast placement (several years ago) excluded the rudder. It was based on a sail plan that was probably drawn by the designer. It was surely a poor estimate now that I have discovered that this plan and the actual yacht do not match. Draft on the plan is 3.6ft but the actual yacht's draft is 4.25ft., no doubt the consequence, as mentioned, of increased displacement. Also the actual w.l. is lower that the w.l shown on the drawing according to the appendages at bow and stern. Beam, w.l. and oal do not match.
Thanks for reminding me of how to find the CE on a drawing. Can you suggest a method of deciding on a CE height on this particular yacht, (after its various modifications) that will not cause excessive heeling - before I set up the rig. (ref: 4th para of original enquiry).

 

Typically, most would include 1/2 the rudder area on an appendage configuration such as your, during the CLP calculation. Taking this into consideration, you might expect the lead to increase of your current estimation by as much as a few percent.

There really isn't a "deciding on CE height" decision to make. Your new sail plan, assuming not excessive will place the CE where it falls, then your leverage calculations. If results of these "redesigned" aspects don't fall within your liking for some reason, then you have options, typically a lower aspect ratio.

I'm not sure what the PPI is on that yacht, but 9" or so of additional draft is an awful lot of stowed gear. Empty the boat of everything that isn't nailed down, every locker, cabinet and storage area should be empty, no ground tackle, chain, extra dock lines, nothing but a bare boat. See how much your draft has decreased.

Nothing personal Zeno, but you do seem to be in over your head on this and require profession assistance, for the calculations and major rig decisions.