Making a bird's mouth mast

[gonzo]

Do you have the original designed dimensions or are you trying to calculate from scratch?

[tsugak]

Well, the original mast that came with the boat was aluminum so even though a wooden mast was built later there are no real specs to calculate from, that I know of.

[PAR]

A solid mast of Douglas fir will weigh about 110 to 120 pounds of those dimensions, so I'm not sure what your mast is made of, but it's a lot heavier then necessary by a huge margin. The rigging could easily account for some of the extra weight, what are the general dimensions to the wire and fittings? 1.25" stave thickness is about right for a 6 1/2" mast. This is less then 20% wall thickness and just about right. Are you sure you weighed the mast properly, because being nearly twice as heavy as a solid spar doesn't sound even close to right.

[Chuck Losness]

I don't understand why you just don't determine the righting moment and do the calculations. It will take less time to do that then to go back and forth wondering about what to do. Then you will know and not be guessing as to how much material that you can take off your existing mast and still maintain a proper safety margin. The calculations are not difficult. There are only two equations. The equations are Bending stress = (RM*Od/2)/I and I = .0491(Od^4-Id^4). RM is the righting moment times 12. You have to multiply the righting moment by 12 to convert it from foot pounds to inch pounds. .0491 is pi/64. I just simplified the equation for you. Od is the outside diameter of the mast. Id is the inside diameter. I is the moment of inertia. You can do a simple spreadsheet to do the calculations.
If I was doing this and wanting to use the existing mast I would keep Id the same in the equation and change Od until the bending stress was 5000/3=1,667 or less. This would give you a safety factor of 3. Once you know what the Od will be you can go back to the duckworks site and check on the strength table posted there to see if the wall thickness is still adequate. If the wall is too thin, then you will have to either live with a heavier than necessary mast or make a new mast.
How many staves does your current mast have? Is it 8 or is it some other number? Just curious.
Good luck.
Chuck

[tsugak]

Well, I don't know what to say about the weight. I have a hanging scale in my business that we use to weigh odd shaped objects. Its quite accurate and I weighed my self on it just to be sure. We set the mast on saw horses, found the balance point and then hung it from the hanging scale. There's really not all that much rigging. Even if we called it all 30 lbs that still makes the mast 160 lbs. I know for a fact that its made of douglas fir. Mystery to me. It takes three of us to lift it.

[Chuck Losness]

Did some more research and punched some numbers. First I rechecked the tensile strength of Douglas Fir and found it to be 7,700 psi according to the Gougeon brothers book on boat construction. I also did some searching on the internet and found a boat on Dudley Dix's web site similar in size, displacement and sail area to your boat. Dudley listed the righting moment as 3211 lbs. So I used this number for the righting moment in the equations.
For your existing mast I calculated a bending stress at 1668 psi, safety factor of 4.6. If you decrease the diameter to 6 inches, the stave thickness would be 1 inch and the bending stress would be 2264 psi, safety factor 3.4. If you decreased the mast diameter to 5 3/4 inches, the stave thickness would be 7/8 inch and the bending stress would be 2696 psi, safety factor 2.8. If you decreased the diameter to 5 1/2 inches, the stave thickness would be 3/4 inch and the bending stress would be 3275 psi, safety factor 2.3.
All of this assumes a righting moment of 3211 lbs. Your boat's actual righting moment will be different and depending on the amount of the difference, there could be large changes in these numbers.
I am not a naval architect nor involved with mast design. I just crunched a bunch of numbers for you to give you an idea of what you are looking at. You really need to do an inclination test and calculate the righting moment for your boat. Then everything will fall neatly into place.
Good luck with your project.
Chuck

[PAR]

3.9" at the base? You really think I can go that small? I was figuring 5" at the base and 3 1/2" at the head. 30 lbs sure sounds great but I would be happy with anything under 70 lbs

Yes, with a head stay and shrouds, this will easily handle 240 sq. ft. of gaff cat rig. If you elect to go with the 5" base/3.5" head it'll be in the 50 pound range and could handle 400+ sq. ft. of area. Again assumes a 20% wall thickness and Douglas fir.

I don't know what's going on with the weight either, but Douglas fir doesn't weigh that much. Are you sure it's hollow? How far inside can you see? Can you insert something into the heel drain and find out? Douglas fir is only about 31 pounds a cubic foot. There aren't that many cubic feet in that mast of yours.

No disrespect Chuck, but your numbers are meaningless, even without knowing his boat. If it's a typical cat, which I'll assume is traditionally built, it'll be at least 25% beamer then anything Dudley has drawn and likely a few times heavier as well. Dix as a fine reputation for light weight hull forms, which this very likely isn't, making his designs for comparison quite unreasonable in this case.

[Chuck Losness]

Par,
No problems from me. The boat from dix's web site was his Cape Cutter 19 which is a gaff rigged cutter fairly close in size, displacement and sail area. I just threw the numbers out there so he could see what happens when you decrease the size of an existing mast and how it impacts the mast. We are all really just pissing in the wind until he determines the righting moment of his boat. From my limited understanding of mast design, everything hinges on righting moment and until you have that, nobody can figure out whether he can lighten his existing mast or has to build a new one.
Chuck

[Waddie]

Par,
I couldn't find any contact info for you on this forum. I would like to buy a set of plans for the 23' daysailer you designed sometime around 2007 and posted a picture of here on this forum. So please contact me if you read this.
thanks,
Waddie
jnkessler51@hotmail.com

[PAR]

Waddie, if you click on my name a list of options appears, including my email. Just drop me a line when you get a chance. Which 23' daysailor are you talking about?

[Eric Sponberg]

Tsugak,

I would suggest you buy a copy of Blondie Hasler's and Jock McLeod's book, "Practical Junk Rig". You can get it from Tiller Publishing at www.tillerbooks.com. This book describes how to design and build a hollow section wood mast in a manner very similar to how the professionals do it. You can also talk to Pleasant Bay Boat and Spar in Orleans, MA. They specialize in wood small boat building and mast building. Their website is www.pleasantbayboatandspar.com. They can give you advice, and they can build your mast for you.

Good luck, I hope that helps.

Eric

[tsugak]

Par, Its sounds like I'll be more than fine if I go for a 5" diameter, and at 50 lbs or so I'll be very happy with the weight. I think 5" is aesthetically please for the size of the boat as well. I know the last 4 or 5 feet of the existing mast is solid but the rest is hollow and 1 1/4" thick, this I checked. I cant imagine the hardware, some rope and one wire cable could weigh 90 lbs or so. I'll weigh it again once it is stripped down.

Eric, thanks for the advise. I will get the book you suggested as well as contacting the boat and spar builder you recommended. I'll run Par's recommendations by them as well to get their take.

chuck, I appreciate all your input about the righting moment but its way too technical for me. I'm just looking for someone to give me the dimensional specs I need to actually start building the mast. I'll report back after I talk with the spar builders that Eric recommended.