Hi Robert,

Thanks very much for the flagpole info – I had not seen that company. I keep wondering about the weights on flagpoles. That one shows that 181 pounds. I'm assuming this includes all the hardware and foundation stuff. But also, my flagpole mast on AUKLET was a lot thicker at the top, where it had been tapered. About 1/4 inch, when the butt was 1/8. So maybe in the interest of being lighter weight, the straight tube and wood topmast might be an advantage. The straight tube at 7 inches diameter and 1/8 inch wall thickness is about $625, and $500 with the 20% discount, so not bad… But I don't know the weight, except for that what David said earlier was much, much lighter than the flagpole listing.

Shemaya

Interestingly, it's over $200 cheaper for the 7 inch x 1/8 inch. I wonder if that reflects actually using less aluminum, so it would also be lighter than the 6 inch x 3/16?

I don't think that Janine is thinking to sail directly from Maine to the Bahamas. More like down the coast to Florida, and then across when the weather looks good.

David, did you use a solid wood topmast, or hollow?

Shemaya

Anonymous wrote:

Phil, thank you so much for bringing up the straight tube – it is shockingly less expensive than the flagpoles, which I had not realized. Making a wood topmast has been daunting, but with that difference in price, it starts looking more appealing. The straight tube is under $1000, even for the thicker 3/16 wall, and the flagpole with shipping is over $3000. Conveniently for this boat, 24 feet would do the trick, which is the longest available from an easy supplier.

Hi Shemaya--I note the $1000 cost for a straight tube on which the buyer still has to fabricate a DIY wood topmast. The small sail area of the Kittiwake (and S2 6.7) allow more choices for 6063-T6 flagpoles (if you decide to go this way). Eder Flagpoles out in Wisconsin is one possible vendor.

https://www.ederflag.com

The following choices can be found in Eder's PDF catalog attached.

ECX25 5" x 1/8" x 27' 129 LB ship wt 2.1 SF $728 plus ship

ECXA25 5" x 5/32"x 27' 134 LB  "   " 2.6 SF $833 "   "

EC25 6" x 3/16" x 27' 181 LB  "  " 4.5 SF $1091  "  "

Specs based on following assumptions:

CE of sail 13' above LWL

Sail area: 236 sqft

Reef @ F3 or around 12 knts

Bending moment on mast at deck level at full sail, wind speed 12 knts, 30 deg heel is 2250 foot-pounds

Yield strength of 6063-T6 is 25,000 psi

Perhaps the shipping cost would make these choices un-attractive. If the buyer is patient perhaps waiting for a bulk delivery to the area could save some $$. Furthur these choices,according to the catalog can be supplied in 1 or 2 pieces....2 pieces being cheaper?

Also I'd be checking Ebay. In the past I've seen appropriate aluminum flag poles for sale with free shipping.

PS--See also fiberglass flagpole catalog attached

Hi all,

Thank you so much for putting so much thought into the Kittiwake mast question. I have some trouble getting my head around the calculations, and the units. Is the conversion found here correct?

Janine has talked about sailing the boat to the Bahamas. It seems this would argue for the stronger mast, even though it would mean an increase in weight?

Phil, thank you so much for bringing up the straight tube – it is shockingly less expensive than the flagpoles, which I had not realized. Making a wood topmast has been daunting, but with that difference in price, it starts looking more appealing. The straight tube is under $1000, even for the thicker 3/16 wall, and the flagpole with shipping is over $3000. Conveniently for this boat, 24 feet would do the trick, which is the longest available from an easy supplier.

Bangor Steel in Maine is interesting, though doesn't come out looking so good online. There have been significant issues with their service described on their Facebook page, which they have not bothered to address. But maybe you have had a better time with them in person? Another nice alternative is Online Metals, with whom I have had good experiences in the past. If one watches for the daily discounts one can get up to 20% to 25% off the listed price. Shipping will add to that, but for 12 foot tubing it has been manageable. It comes by truck either way, so hopefully the 24 foot length won't be massively more expensive for delivery.

From Online Metals, 6061-T6 tubing is available in 6 inch diameter at 1/8 inch wall thickness, or at 3/16 inch wall thickness. 5/32 is not an option. It seems that the numbers you have all talked about, with the goal of sailing across to the Bahamas, would argue for the 3/16? Accepting the additional weight in order to have more confidence in the open water and the Gulf Stream? Or would the lighter mast still be okay, taking into account that the boat will *not* be driven hard for racing, instead reefing early and aiming for low stress on all concerned – people as well as structure.

Sure do appreciate the thinking that you all have been sharing on this – I know that Janine does too! Thanks again.

Shemaya

Interesting, David

If 20mph = 8.9m/s (Low F5) and the wind pressure is 1lbs/sqft (= 4.88kp/sqm), then the max moment on my 35sqm on Ingeborg, (force centre sitting 5m up) be:

4.88 x 35 x 5 = 854kpm, calling for a mast strength of about 1700kpm

I only calculate the righting moment of my boats, and then double or triple the resulting moment on the mast. For Ingeborg, that is (guessing her righting arm to be 22% of her beam):

M_{r max} = beam x 0.22 x displacement =2.2m x 0.22 x 2150 = 1032kpm,
calling for a mast strength of 2000-2100kpm

Your and my numbers didn’t end up too far away from each other. Ingeborg’s mast surely appears to live an easy life, but since that mast is only 2.5% of her displacement, I think it is all right. The strong 6082-T6 alloy surely can save some weight.

Now to Janine’s Kittiwake  -  which this thread is supposed to be about:
If I guess her righting arm to be 20% of her beam (lower ballast ratio than my IF), then her righting moment should be:

M_{r max} = beam x 0.20 x displacement = 2.26m x 0.20 x 1678kg = 758kpm,
calling for a mast strength of just above 1500kpm

That should be plenty strong enough.

Arne