making a mast from one boat fit another/ moving spreaders?

Greetings All,

This is my first post hear but I've read the forum for a while and enjoyed it.

I am trying to do some "backyard engineering" and make a mast that I have fit another boat. ...that said, I have a mechanical engineer helping, a welding engineer as well as some very high quality welders able to assist.

A little background. I have an almost new sparcraft mast that came from a boat we lost in a hurricane. She sank straight down in the slip and the mast was not damaged.

The mast came from an Beneteau First 42 and we are attempting to put it on an old CCA era boat. It is the same weight/foot and close dimensionally to the mast that came from the CCA Era boat.

We are shortening the mast by approximatley 2.5 feet bringing it to an overall length of 55'5". This is about a 4.5 foot increase in rig height over the mast that came off the CCA boat. The boat was plenty stiff before but underpowered and actually suffered a bit from Lee helm.

The plan is to build this mast into a slight fractional rig by taking advantage of the full 4.5 feet for the mainsail and about 2 feet for the headsail.

Changing the masthead and adding a stay fitting for the headstay is no problem and we have that worked out. We are also tapering the top 2.5 or maybe 3 feet of the mast section.

Now here is my question and problem. The mast that we are using from the Beneteau was originally a masthead double spreader rig. The spreader placement will not work on the CCA boat with the new configuration. We are left with holes where the old spreader bars once were. The mast is plenty stiff and the moments of inertia are overkill for what the boat calls for so a single spreader rig is all that is necessary. Although it is dimensionally similar and close in lb/ft with what came off. ...actually a tad bit lighter.

Is it feesible to patch the holes left from where the spreader bars were and if so would it be best to rivet on either a cover plate or some kind of internal doubler or go for a full penetration welded on patch?

The section is 3/16" inch thick and the welders are very good with aluminium. My concern with welding is the loss of the heat treatment in the weld effected area. But it would also eliminate any cuts in the section at these places.

We have considering milling an internal doubler so that it can be flush with the cut out and then riveted in place.

Does anyone have any thoughts? ...other than drop $12,000-$35,000 on a new mast.

Thanks in advance.

 

Generally, anything you do a tubular column structure will weaken it - so welding, patching and riveting more on will probably do more damage than just leaving the holes where they are. The less you do, the better. Adding stiff spots, things that will affect bending and columnar stability are not recommended. Mast flex needs to be smooth.

I've done mast replacement in dinghies re-purposing used masts for new and test designs, and other than removing obvious areas of damage and trouble, I've just mounted spreaders and fittings where they need to be, using metallic duct sealing tape over holes from previous uses.

I've sleeved masts to provide more length/strength in specific areas - but have contained the extensions to the lowest 10% of the mast, below the gooseneck. Sleeving is done to high stress areas like gooseneck/vang locations in Proctor D sections used in 505s.

If one of your "team" is a mechanical engineer it really helps - knowing the theory and math can help evaluate your mods and the projected use.

Just an opinion, but it sounds like you've done good homework so far, and given the expected results, getting an engineering opinion may be a cheap ounce of prevention, preventing pounds of expensive cure.

--
CutOnce

 

On a mast like that, even as a masthead rig, the taper would be at least 8 to 10 feet. You will not want to only taper the top 3 feet.

You better make sure your welder has a nice strongback (a big steel I beam will work) that the back side of the mast can be held against while you weld in the taper. If you don't do this yoiu are going to get a funky taper.

You also better be sure to cut in a relief cut at the bottom of the taper so when you band the two pieces together for welding you will not make a hard spot at the base of the taper.

Since you say bars I'll assume your mast uses though bar spreader bases. When you pull the bar out it will leave a hole maybe a third of the way across the side of the section, and probably more than 3/4 of an inch high. You don't want to just leave that open.

I wouldn't weld it up. As you mentioned, welding will anneal the metal.

I would make up cover plates that can be mechanically attached externally. I would make them diamond shaped to avoid hard spots, although this isn't so important athwartships. I would consider welding a short chunk of the through bar to the inside of the cover plate, so it is inserted to fill the existing hole, to add strength on the compression side. This is probably overkill, but why not?

I don't think this is a very good idea.

 

I like the idea of a bigger taper I am just concerned with attaching the headstay and terminating the shrouds in the welded area (the taper).

Agreed

Not a bad Idea

not sure why you think this. if we mechanically fasten a cover plate albeit an internal cover plate what is the difference? Doing it the way I have mentioned would seem to make a cleaner (flush) patch.

 

It is done all the time. Standard stuff.

Sorry, I misunderstood your use of the term Doubler. To me that means a full sleeve, not a cover plate.

With the plate attached externally it is stronger (mass farther from the centroid). With an internal plate I worry about the fit at the corners becoming a trap or knife for internal halyards. I've seen that problem a couple of times with splice sleeves.

It is also easier to inspect and service the external plates.

But an internal plate would probably look better and the issues I worry about are not very likely.

 

Thanks to both of you for your responses to my post.

it will be a few weeks before we can get started. -waiting on a new mast head to be fabricated.

I will let y'all know when we get it done.

-might have a few more questions before this since this is the first time I've done something like this.

 

Interesting how the advice that gets attention is the advice that supports the opinions and planned work of the original poster. Nowhere in the comments received is there evidence of science or research backing up the ideas presented - just cover over this and weld that.

I'm perfectly willing to accept opinions that are technically valid, but much of what has been said is cosmetic and not necessarily technically valid. Tapered masts "look" right, but cutting, tapering and heating the top of a mast for aesthetics, while you may be weakening it isn't perhaps the best course of action. Adding rivets and cover plates are basically "makeup" - the added holes and patches will not affect columnar strength positively at all.

Granted, flush cover plates and adding a taper to the mast top will make things look and perhaps sell better, but unless there is engineering backup to the work it is just cosmetics. I'm sure the original naval architect's engineering people specified the mast and rig design quite well in it's original configuration - and choices were made in concession to cost and intended use.

I certainly know the value of appearance in the yachting world - pretty girls get to dance more often. Don't confuse form with function though.

--
CutOnce

 

hmmm,

Well I did say Thank You didn't I?

I think I understand everything you've said. If you are telling me not to taper the mast then we will simply have to disagree on this one. I do not think that any spar manufacucturer heat treats after cutting and welding to taper a mast. We will to this to industry standards and it will be done with good engineering.

As for the patches or cover plates, that is still being looked into and my P.E. friend that is helping will model this to determine the best course of action. ...incidentally I do not think the solution will be metallic duct tape.

 

Wow, you are a loud know-nothing, are't you? Have you ever been in a shop that builds spars of the type being discussed in this thread?

I wonder what "technically valid" means? You use the term twice, so it must be very important.

So all those masts in every marina that have a taper are somehow not "technically valid"?

I guess you've never hear of things like internal stiffeners, running internally the entire length of masts, held on by structural rivets? Those seem to add to the moments quite nicely. I guess your plan, to leave a hole roughly one third of the sidewall across and about an inch high, is much better for "columnar strength". Funny, I've seen a lot of well engineered spars that use just such cover plates over access ports. Of course they used screws and not rivets, and they sure were not for aesthetics.

 

I know LeFiell did do some spars that were heat treated after fabrication. They have a very large oven for military applications. But overall you are correct, it is not commonly done.

 

Thanks for that bit of information.

 

heat treated or stress relieved?

 

LeFiell used to advertise their ability to heat treat post-welding. The interesting thing was the oven they showed mast sections being treated in was vertical.