Aluminium masts replacing wooden

[Cooper Dan]

Hi All.

Advice needed.

Have just rebuilt my back wooden mast, bottom 2.4 meters was rotten, it's a long story and i will post the rebuild images online and description soon. The results are great !

Have just got to the stage were I am about to put the finishing coats on when I find that there are many 20 meter lengths of 200mmOD, 10mm wall aluminum extruded pipe lying a few hundred meters away that the marina management would like to get rid of.

Questions I have are:

My masts are a heavy meranti, and reasonably stiff.
Would I need to stiffen up the aluminum masts?

Does anyone have the Wharram mast size chart and can tell me whether 200mm ond 10 mm walls are suitable for a 15m mast.

Any past negative experience with aluminium masts?

 

Regards

[kim whitmyre]

What type of aluminum? Here in the states, 6061 is what would be used for marine use. My mast is 5"/127mmOD, 1/8"/3.1mm wall, for a tiki 26.  What is the current diameter of your existing masts? The wall thickness sounds good. On the t26, the Wharram shop says to keep the diameter the same when going from wood to aluminum; this may have something to do with the wingsail sleeve as used on the t26

[Cooper Dan]

Thanks for the reply.

Existing dia is 220mm the aluminum is 200, so is very close.

Also at 10mm wall it should be strong enough.

Will look to see if the pipe has any stamps, markings indication standard and type of alloy tomorrow.

Also just found out the price, which is 250 Malay Ringet per meter(100NZD or 78 AusD) so is affordable.

We use the wingsail as well

[Rogerio Martin]

Tiki 46

dia

194mm

wall wood

32mm

wall aluminium

5 - 6mm

[Axel]

10 mm sounds a bit thick to me. I think Hanneke specs something like 3.5 for Al masts. I have my T38 Al masts outside and could measure them but it's easier for you to search on the Wharram site or on themultihull.com where the topic has been well covered. I suspect they will be very heavy. Compare the weight to your own masts.

 

 

[Rich Young]

I'm not certain about my math, but I think a 15 meter, 200mm OD, 10mm wall thickness tube of aluminum weighs 124 kg.  Calculated volume of a 200mm cylinder, subtracted volume of a 190mm cylinder to get the volume of the tube walls.  Multiplied that by the density of aluminum at 2.7 g/cm and it came out to 124.05kg.  Compare that to your wooden mast to see whether it would represent any weight savings?

[boatsmith]

10mm wall thickness is very thick for an aluminum mast. A  7 1/2" tube with a 1/4" wall thickness weighs .56 lbs per inch. 15 meters = 590.6 inches x .56lbs/inch =330lbs x 1 1/2 for 3/8 wall = 496 lbs  for 15 meters. HEAVY

 

 

[Cooper Dan]

Hi Rich 

thanks for the reply.

I think that a 200mm minus 190mm cylinders give a wall thickness of 5 mm.

And you would be right regards the weight.

Cheers

Daniel

Rich Young said:

I'm not certain about my math, but I think a 15 meter, 200mm OD, 10mm wall thickness tube of aluminum weighs 124 kg.  Calculated volume of a 200mm cylinder, subtracted volume of a 190mm cylinder to get the volume of the tube walls.  Multiplied that by the density of aluminum at 2.7 g/cm and it came out to 124.05kg.  Compare that to your wooden mast to see whether it would represent any weight savings?

[Cooper Dan]

Thank you for the advice.
Have just been redoing the sums, much easier to minus two cylinders :)
Wood m3
40mm wall
 radius 0.110     0.482962  m3
 radius 0.070     0.195579  m3 -
--------------
0.287383 m3
@600kg/m3
172.43 kg

Alu m3
10mm wall
radius 0.100   0.399142  m3
radius 0.095   0.360226  m3 -
--------------
0.075837m3
@2700kg/m3
204kg

The wooden mast has some very heavy bits of metal capping on the top along with internal stiffening every meter and is glassed.
So the weight difference is minimal.
Am still not sure which way I'll go.

[Duncan Clausen]

Rich you are right about your maths - for a 10mm wall you should be subtracting a 180mm diameter cylinder, therefore the aluminium mast would have a mass of 242 kg (16.1 kg/m).

Rich Young said:

I'm not certain about my math, but I think a 15 meter, 200mm OD, 10mm wall thickness tube of aluminum weighs 124 kg.  Calculated volume of a 200mm cylinder, subtracted volume of a 190mm cylinder to get the volume of the tube walls.  Multiplied that by the density of aluminum at 2.7 g/cm and it came out to 124.05kg.  Compare that to your wooden mast to see whether it would represent any weight savings?

[Duncan Clausen]

Dan, what length mast are you planning - by my calculations you are looking at a 12.7m mast. (the internal radius of the ali section is 0.09 not 0.095 mm but you have come out at the correct mass for a 12.7m section!)

[Cooper Dan]

Hi

12.7 meters for the main section, and an extra meter at the top to follow the existing wooden profile. Is a smaller diameter, 150mmOD and will be only 3-5mm walls. I would like to follow existing profile as then I don't have to redo my stay loops at the top

Yes did calc' with 90mm radius, was playing around with sizes and forgot to update.

Am still undecided as the newly rebuilt mast is better then new and the other mast has no problems. But the aluminium is going for its weight price and wooden masts look great close up but are always a constant  maintenance issue here in the tropics.

[Rich Young]

D'oh!  Of course 200-190 would be for a 5mm, not 10mm, wall thickness.  How typical of me - "almost right" but off by about 100%....