Mast Design 101

[chris95040]

Hi everyone,

I have a 45' tall mast that's made of spruce - 4 sides glued together (a "box" construction.) While it seems to be in decent shape, every year I start considering replacing it with an aluminum one. Varnishing it every year, storing it inside every winter, the various bends it has that can't be pulled out by the rigging, and it being roughly twice as heavy as an aluminum one all factor into that desire...

I have the plans for the boat and the designer (Phil Rhodes) also had an aluminum mast available as an option at the time (1964). Its an oval extrusion, roughly 5"x9", 0.125" in wall thickness. One single set of spreaders halfway up. It uses the same standing rigging placement as the spruce mast.

It seems straightforward to simply buy a comparable extrusion from one of the many suppliers on this coast and rig it up the way I want, but lots of people I've talked to about it imply that there have been a lot of important advances in standing rigging and I'd probably benefit from a lighter, stiffer, and stronger rig by, say, incorporating double spreaders.

I don't want to change chainplate locations (3 per side: a fwd intermediate, the cap shroud, and an aft intermediate) and this is another reason why "building what was originally designed" seems preferable.

Long story short, I'm not going to do anything other than build what Rhodes specified without hiring a professional, but I'm trying to decide for myself which of those two options to take (namely, DIY the plan, or hire out for a more modern design.) I'd just like to understand the problem a little better before making that decision...

Maybe someone can explain a little bit about what goes into designing a mast + rigging, or point me to some good reading material. Or maybe a professional can PM me and I can buy your time to just go over some options with me!

I'm an engineer so don't be afraid to respond with math!

[UNCIVILIZED]

Here's a good place to start (with your rigging education) Brion Toss Yacht Riggers, Sailboat Rigging::Search Results
It covers spar & rigging design, in several different, easy to understand sections. Including the math & engineering to go with the how & why behind rig design, albeit using math & descriptions that a semi-technically savvy layman can understand & use.
That, along with a lot more aspects of knots, rigging, etc. As well as how a rig, & all of its components are an integral part of a vessel's design. And it explains how all of the various bits of such are interconnected.

[Terra Nova]

Stick with the original, single spreader design.

[UNCIVILIZED]

PS: I forgot to mention it earlier, but finding a pre-loved tube in that size range usually isn't all that difficult. And they're a LOT cheaper. As well as many times, coming wth the perks of much of the key hardware already installed. Such as sail tracks, spreaders/spreader tangs, sheaves, goosenecks, shroud tangs w. compression tubes, & mastheads.

Sometimes the tube will need to be cropped down to the right length, & perhaps the gooseneck moved, or the masthead modified... or tossed entirely, etc. But given that it can easly take 10x off of the cost of the spar...

[chris95040]

Originally Posted by UNCIVILIZED

Here's a good place to start (with your rigging education) Brion Toss Yacht Riggers, Sailboat Rigging::Search Results
It covers spar & rigging design, in several different, easy to understand sections. Including the math & engineering to go with the how & why behind rig design, albeit using math & descriptions that a semi-technically savvy layman can understand & use.
That, along with a lot more aspects of knots, rigging, etc. As well as how a rig, & all of its components are an integral part of a vessel's design. And it explains how all of the various bits of such are interconnected.

Awesome - just ordered it. Thanks for the tip.

Regarding finding a used mast, that's another avenue where I've seen a lot of options, but again I'm afraid to explore without knowing WTF I'm doing. Hopefully this will help, thanks!!!

[chris95040]

Originally Posted by Terra Nova

Stick with the original, single spreader design.

That's a vote in that camp. Mind elaborating? Just curious if you have any more thoughts on it beyond just "the designer probably knew what he was doing", which is fair...

I wonder if anyone else out there has a roughly 5"x9" oval section, 0.125" thick, single-spreader, deck stepped rig? Just as a sanity check... Sounds pretty "normal" to me but wtf do I know?

[Snowpetrel]

Principles of yacht design by Lars Larsson and Rolf Eliasson has a decent rundown of the Nordic Boat standard for rig design. It's a bit basic, but the best I've found so far. Maybe the latest edition has a bit more technical info. Certainly worth a look.

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[HappyMdRSailor]

Brion's book is an awesome resource!

He makes a couple of references to other books in there too...

Hey... ya never know what might work?For Sale: Mast and rig - Cruisers & Sailing Forums

There's a couple guys here that have more knowledge than most of us combined

[HappyMdRSailor]

Originally Posted by Snowpetrel

Principles of yacht design by Lars Larsson and Rolf Eliasson has a decent rundown of the Nordic Boat standard for rig design. It's a bit basic, but the best I've found so far. Maybe the latest edition has a bit more technical info. Certainly worth a look.

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

This was the other book I couldn't remember the name... good one Snow!

Worth a look here....

http://stores.ebay.com/donsmarine-20...fsub=376940219

[chris95040]

(By the way, in case anyone is paying super close attention, its a 3/16" wall thickness that's specified, not 1/8".)

[HappyMdRSailor]

Originally Posted by chris95040

(By the way, in case anyone is paying super close attention, its a 3/16" wall thickness that's specified, not 1/8".)

Chris... Don't get stuck on wall thickness... This is only a minor component... What you're looking for is Ixx/Iyy, lb/ft

This will become clearer after you start reading!

I'd be happy to help if you're anxious...

PS
I know of this guy who has a super fat stick on a 28 Newport... But it seems to work for him...

(golleee imma stinker....)

[zeehag]

varnishing yearly is a dock queens luxury.
mine is painted and hasnt been repainted for 10 years.
yes it is in need.
the rough look now lets me be able to inspect the glue lines readily and feel the integrity of the wood. i plan on stepping and maintaining once i arrive in mazatlan, hopefully this year, as there is a location in which this is possible for lesss than many pay.
i like my spruce masts and booms, yes, hollow box with reinforcement blocks handily located, as they are repairable easily.
they do not suffer corrrosion nor electrolysis nor lignting attraction.
aluminum ones cost minimum 15000 usd new EACH plus booms, and are more difficult to repair. spruce is relatively rot resistant, and makes excellent masts. i have 2 of em. love em. just paint them if you dislike varnishing so much.

ps did i say that replacement of one boom made of wood here is less than 300 usd TOTAL, as opposed to the many thousands of dollars required for replacement of aluminum booms, each??

[estarzinger]

Originally Posted by chris95040

I have the plans for the boat and the designer (Phil Rhodes) also had an aluminum mast available as an option at the time (1964). Its an oval extrusion, roughly 5"x9", 0.125" in wall thickness. One single set of spreaders halfway up. It uses the same standing rigging placement as the spruce mast.

lots of people I've talked to about it imply that there have been a lot of important advances in standing rigging

another vote for sticking with the designed rig.

Neither aluminum tube nor stainless wire has changed/improved much since 1964. And the designers certainly knew about double spreaders back then.


You could get a more 'sophisticated' (aluminum) rig, but it will mostly have the same trade-offs that the designer already decided against (more complicated tuning, more parts and expense and not all that much noticeable improvement for your boat).

THE improvement in rigs since those days is carbon fiber . . . but $$$.

[Stumble]

You will never beat a used aluminium rig for price, but if you are replacing with new I would at least explore carbon. The last two masts I have priced (on much smaller boats admittedly) the carbon came out to be just a little more than an aluminium tube. If I remember correctly it worked out to be about a 10% price premium. Even ten years ago the prices were almost double.

This is why even new budget conscious small boats are being offered with only carbon rigs. The performance difference is noticeable, and the price difference is shrinking fast.

[chris95040]

Originally Posted by Stumble

You will never beat a used aluminium rig for price, but if you are replacing with new I would at least explore carbon. The last two masts I have priced (on much smaller boats admittedly) the carbon came out to be just a little more than an aluminium tube. If I remember correctly it worked out to be about a 10% price premium. Even ten years ago the prices were almost double.

This is why even new budget conscious small boats are being offered with only carbon rigs. The performance difference is noticeable, and the price difference is shrinking fast.

I got a rough estimate for an aluminum mast at $10K USD. That's not including boom, rigging wire, etc.

I got a similar estimate for carbon, for $30K USD.

My thought was "wow, carbon is a lot more expensive. Forget that."

Now I'm thinking, I may have just gotten an insane quote. I wonder if you could point me to some suppliers? I love the idea of a nice lightweight carbon rig, even if it would be a bit strange on a CCA-era cruiser. Theres a guy in my boatyard with a 50 foot, triple spreader carbon mast that he can lift by himself. Really neat.