Hello Janine,

Glad to hear the project is moving along. I look forward to seeing you on the water. I have some thoughts on your mast. The idea of a flag pole with less weight up high is attractive but there are some other things you may want to consider. Aluminum flag poles including the six inch one you are considering seem to be available in 6063 t6 grade. This has a tensile yield strength of about 210 MPa or 30x10cubed psi. Straight aluminum tube is available in 6061 t6, yield strength of about 270 MPa or 39x10cubed psi. It should be available in 24 ft lengths. (You can check with Bangor Steel.) I’m wondering if a lenghth of six inch tube in 6061 t6 with a wall of 1/8 inch rather than a flagpole with a wall of 3/16 in. would give you a lighter mast, about 64 lbs. for 1st 24ft, with ample strength and stiffness.  While the math part of my brain is rusty the estimating part thinks this might be a good trade off at a greatly lower price. Of  course the last three feet could be wood. What do you think Arne?

Anonymous wrote:

Robert,

there are plenty of websites which help one convert from one unit to another. I just wrote on my browser "MPa=" and then this page popped up:

So 1MPa (=Newton/mm2)=145psi

and 1ft-lb=1.356Newton-meter (=1.356/9.807kilopond-meter=0.1383kpm)

As for  the strength of different aluminium alloys, I just went to Google Advanced Search and wrote "6063-t6", and then there was good info, for instance on Wikipedia.

Arne

Hi Arne--Thanks for websites. I personally can do the unit conversions. My point was that for American junk-rig converters the local vendors for poles speak imperial units on a day-to-day basis. So it'd be much easier to discuss engineering properties with vendors in lauguage they use. The default units here are inches, feet, foot-pounds, square feet, etc. As for example:

"Hi Janine,

the mast section you are thinking of looks OK for the sail that Arnie has sketched for you. The bending moment at the deck is approximately 3,000 foot pounds  (270 sq ft at 11 feet above the deck is 2970 ft lbs.). The mast section at 6" and 5/32" can resist a bending stress of about 5891 foot pounds (at 20,000 psi for the strength of the aluminium, figures vary between 18,000 and 29,000 from what I have seen) This gives a safety factor of a little less than 2, which is fine for normal coastal sailing. The 5" diameter mast would have a safety factor of less than 1.5 which I would not recommend.

All the best with the project, David."

Anonymous wrote:

Hi Janine,

I am excited about your Kittiwake 23 conversion!

It is good to see a new JR conversion getting started here in the US. Seeing a Kittiwake with a junk rig will be even better. My first keelboat was a 1966 South Coast 23.  If sailboatdata.com is right then your Kittiwake was built from one of the South Coast 23 molds after it was lengthened by a few inches. I really liked that boat. The only part I did not like was going up to the mast to raise and lower the sails. With a junk rig it will be great.

I am also interested in what you pick for a mast. I am still trying to decide how strong the mast must be for my S2 6.7 that I am converting. I made an excel spreadsheet and followed Arne's calculations and rules from 'Chapter 6b The Hybrid Mast.pdf'.

I used this sheet along with the minimum and maximum of the ranges provided by Arne. My intention was to find the 'strongest' and then the 'lightest' mast that could be used.

I am not sure if it will be helpful -- but I plugged in the numbers for a 6" OD 5/32" 6063-T6 mast and your Kittiwake 23. 

This is a summary of the results (sorry for the mixed units of measurement):

'Heavy and Strong' Mb_safe target: 2730 kpm

'Lightest and less Strong' Mb_safe target: 1441 kpm

6063-T6 Tube with 6" OD and 5/32" wall Mb_yield: 1460 kpm

It seems like the mast you are considering (6") is just about right based on my understanding of Arne's 'coastal cruising and no wild racing' scantlings.

Please post some pictures of your project!

Scott.

Hi Scott--Your mast calc spreadsheet is a good effort. Unfortunately for your target group of US-based aluminum flagpole or lightpole buyers the commercial vendors still specify metal strengths, bending moments, etc. in English units of pounds-per-square-inch, foot-pounds (ex. 6063-T6 yield spec is 21,000 psi). Without knowing the conversion factors from metric to english, conversing with vendors (which I don't recommend...don't tell them what the pole is for....know what you want and just buy it) is nearly impossible.

A parallel spreadsheet calc page in English units would be useful.

robert self

Hi Janine,

the mast section you are thinking of looks OK for the sail that Arnie has sketched for you. The bending moment at the deck is approximately 3,000 foot pounds  (270 sq ft at 11 feet above the deck is 2970 ft lbs.). The mast section at 6" and 5/32" can resist a bending stress of about 5891 foot pounds (at 20,000 psi for the strength of the aluminium, figures vary between 18,000 and 29,000 from what I have seen) This gives a safety factor of a little less than 2, which is fine for normal coastal sailing. The 5" diameter mast would have a safety factor of less than 1.5 which I would not recommend.

All the best with the project, David.

Shemaya,
as you can see, the underwater profile of the Kittiwake is very similar to that of my IF, Ingeborg. On Ingeborg, I made a minor screw-up by initially fitting the CE a bit too far aft. By shifting the sail quite a bit forward, most of the weather helm was trimmed away. What I have (re)learned is that one needs much higher lead on a long-keeled boat with an integral rudder, than with a fin-keel (or cb.) boat with spade rudder.

When suggesting a JR for this boat, I moved the CE forward to just a little behind the Bermuda mast, probably close to the CE of the BR. Positioned with 15% balance with respect to the mast, the sail can be shifted a bit forward or aft to get the rudder balance just about correct.

Frankly, I think the Reddish sail in the shown position will produce similar weather helm when close-hauled as my JR on Ingeborg initially did, or maybe even worse.

Reaching and running.
Another factor I check, is the ratio between the sail’s max chord and the waterline length. With the Reddish sail, this ratio is about 0, 84 (The chord is 84% of the wl). On my suggested JR with B=4m, the chord/wl.-ratio is only 0.73. Even without these numbers in hand, one can easily spot a lot of Reddish-sail protruding aft of the Johanna-style sail, and that will no doubt result in more weather helm when reaching and running before. The result is that one may have to reef to keep the boat on the course, long before it is needed for safety reasons. Neither the Reddish nor the Johanna rig are that well suited for being shifted forward or aft at the boom, so that isn't an option..

As you can see from the last diagram, my suggested sail also gives better room for the sheets. My special anti-twist sheet (‘Johanna-sheeting’) takes up quite some room, but pays back by keeping the whole sail driving, close-hauled, reaching and when running before.

I know, I know, I might be defending my baby a bit, but at least I can’t be blamed for producing armchair wisdom, since I go sailing once or twice a week with that (AR=1.90) JR on Ingeborg. Ingeborg still makes my yaw drop when seeing how well she sails on all points, and how well the rig behaves (not just performs).

Cheers,
Arne

PS: After the first season (or a few trial sails) I would consider adding Bolger-style endplates on the rudder of that Kittywake. Endplates would most probably improve steering downwind. It suddenly dawned to me that I must try that on Ingeborg as well.

PPS: It actually didn’t take me that long to knock up those sailplans. With a stack of master sails available, plus a CAD program, it is mostly a question of  copy-paste and then scaling up or down...