Cutting mast down due to electrolysis...

May 27, 2024
2
Custom Trimaran Haleiwa
Aloha, I have a custom 25' Trimaran here in Hawaii that I recently purchased to fully refit. Recently, I de-stepped the mast to replace electrical, halyards and rigging, but inside the mast step I found about 1" of electrolysis corrosion in the mast base. Now, the rest of the mast seems to be in good shape, save for the masthead, so I am less interested in replacing then fixing. Is cutting 2" off the bottom of the mast section a good idea? Would the standing rigging change correspond accordingly, 2"? Would it be wiser to fit the mast step with a sort of sleeve that slips over the base and provides support? Any and all suggestions are appreciated! Aloha.
 
Jan 11, 2014
13,141
Sabre 362 113 Fair Haven, NY
The problem you have is not electrolysis, it is galvanic corrosion from 2 different metals and lots of warm salt water.

Sleeving the mast would be an option depending on how badly corroded the mast is. There will be a lot of compression force on the mast base, so there needs to be enough strength in the mast to take the load. A few rivets holding the sleeve in place probably won't do.

If the mast is shortened, the rigging geometry will change and may require replacing the rigging.

Your best option will be to talk to a professional rigger and get their assessment.
 

jssailem

SBO Weather and Forecasting Forum Jim & John
Oct 22, 2014
23,540
CAL 35 Cruiser #21 moored EVERETT WA
  1. I have a custom 25' Trimaran...
  2. inside the mast step I found about 1" of electrolysis corrosion in the mast base...
  3. I am less interested in replacing then fixing...
  4. Is cutting 2" off the bottom of the mast section a good idea? Would the standing rigging change correspond accordingly, 2"? Would it be wiser to fit the mast step with a sort of sleeve that slips over the base and provides support?
You packed a lot into the query. Trying to address them each.
  1. Custom Build. I have no idea about the original design of the boat. The sails, standing, and running rigging are based on the boat's design. Cutting off 2 inches of mast length will likely impact these systems. I have no idea how much, but I am inclined to think it is not significant as the boat sounds like a one-off and is for fun, not class racing.
  2. Corrosion. You say electrolysis-based. You will want to address this, or it will reoccur.
  3. Fix it and get the boat in the water. I can understand that emotion.
  4. Sailboat designs are based on triangles. One inch on the vertical is not equal to one inch on the hypotenuse.
a= mast height
b=mast base to deck fitting
c= length of standing rigging

a=3 and b=4
c=5
or reduce a by 1
a=2 and b=4,
c = 4.47214

1716905811394.png


My rudimentary geometry suggests that the change of 2 inches of the mast base would affect the standing rigging length. This change may be able to be addressed by the fittings on the boat or you may need to cut a smidgen off the standing rigging to be able to put tension in the rig.

You'll need to do the math for your boat...
 
Mar 26, 2011
3,773
Corsair F-24 MK I Deale, MD
Sleeve will probably actually be simpler.

The compression load is not hard to figure out. Typically the combined breaking strength of the cap shroud and forestay will get you close on a trimaran. The working load is less, but that will get you the shear strength you need from the fasteners. 3/16" SS rivets are typically about 1300 pounds in shear, the breaking load you needs is probably about 7,000-10,000 pounds, so a MINIMUM of 8 rivets, but I'm guessing you would use more than that anyway, just for a good fit-up.

The shroud working load on an F-24 is about 1500 pounds, plus 1000 pounds for the forestay, or about 2500 pounds of compression (plus all of the trig corrections, main sheet load ....). Ball park, you will design for at least 4 times that.

No problem. Easier than shortening the rigging, particularly if a furler is involved.
 
May 27, 2024
2
Custom Trimaran Haleiwa
Aloha all, thank you everyone for the support. I got the mast down and into my shop where I could properly take photos and remove rigging. [B]dlochner[/B], you're definitely correct about the galvanic corrosion. The mast has a steal pad riveted onto the base, that must be what's causing the reaction. As to the solution, I found someone who had a similar problem and solved it by doing what [B][SIZE=4]rgranger[/SIZE][/B] suggested. They made a block under the mast step out of fiberglass and G10 polymer, that raised the base the extra level. I think I like this the most, that way my rigging can be remade to the old dimensions and I am removing the corrosion as a whole, instead of letting it grow. Any good leads to finding thick G10 laminates?

Also does anyone recommend any specific solvents or solutions for heavy duty mast oxidation? I am trying to mine back to its original state and tried a few starbrite products without much luck... I was thinking of using a high grit paper on a polishing pad to it it uniformly dull then maybe a scotch brite pad to blend any scratches. Any thoughts? Im used to sanding and polishing fiberglass surfboards... not aluminum. Aloha
 
Jan 19, 2010
1,312
Catalina 34 Casco Bay
Shrouds and sails will be 2" taller/longer. On shrouds if the turnbuckles have lots of room when tuned, you might be ok. No issue with halyards..The sails might need the 2" removed, and the boom might need the gooseneck raises.. The furler ( if applicable ) will need adjusting.

You should not free hand cut this. Find a machine shop that can make a square cut.

As an option, search out marine salvage yards that might have the same size mast WITH it's step..

Good luck
 
Mar 26, 2011
3,773
Corsair F-24 MK I Deale, MD
Aloha all, thank you everyone for the support. I got the mast down and into my shop where I could properly take photos and remove rigging. [B]dlochner[/B], you're definitely correct about the galvanic corrosion. The mast has a steal pad riveted onto the base, that must be what's causing the reaction. As to the solution, I found someone who had a similar problem and solved it by doing what [B][SIZE=4]rgranger[/SIZE][/B] suggested. They made a block under the mast step out of fiberglass and G10 polymer, that raised the base the extra level. I think I like this the most, that way my rigging can be remade to the old dimensions and I am removing the corrosion as a whole, instead of letting it grow. Any good leads to finding thick G10 laminates?

Also does anyone recommend any specific solvents or solutions for heavy duty mast oxidation? I am trying to mine back to its original state and tried a few starbrite products without much luck... I was thinking of using a high grit paper on a polishing pad to it it uniformly dull then maybe a scotch brite pad to blend any scratches. Any thoughts? Im used to sanding and polishing fiberglass surfboards... not aluminum. Aloha
If you place an ~ 3/16" aluminum plate under the mast butt you won't need G10. Glass and high density foam will work once the load is spread. Or Structural grade fiberglass, which is several times cheaper. I would use a scrap of Coosa board with 2 layers of 1708 over it. The internet says you need to use G10 for everything.
 
Jan 19, 2010
12,673
Hobie 16 & Rhodes 22 Skeeter Charleston
I can’t help but imagine that any reputable welding shop wouldn’t have some scrap aluminum lying around and could cut you off 2 inch thick block and sell it to you for under 100 bucks. And if you find that piece of aluminum ask if he can give it to you 2-1/4 inches And have a machinist route out a track that fits the base of your mast…. Then set the base in the track. While you’re talking to the machinist ask the machinist to take a grinder and knock the sharp corners off the block and round it out little bit so that you won’t keep smashing your feet … and it won’t look like Boudreaux done did dat mod.