Spar strength

I'm sure everyone will reassure me that a hollow mast is stronger than a solid one for similar reasons that a Grp sandwich creates an I beam and produces strength. BUT Using the formulas from Howard Chapple's book I was surprized that a 22 foot mast 3" mast would break at around 1718 lbs load and a hollow mast of 25% wall thickness will break at 5401lbs. These numbers have not been reduced by the 2.5 safety factor to get their designed loads. It was difficult to get accuracy but do you think I have miscalculated?. mik
Sorry I magnified the page and recalculated 1583 and 3713lbs, still seems to large an increase in strength any thoughts?.

 

Solid is is stronger than hollow for the same exterior dimension, but add a slight increase in size and you can have a mast that is just as strong or stronger AND lighter. This is true for bending loads. Compression load is based on cross section area. Buckling loads are also improved by increased sizing of a hollow mast.

Play with your numbers by adding incremental increases in mast sizing. Typically, a 10% increase in exterior dimension will give you the strength of a solid mast, but at significant savings in weight.

 

Thank you LP. I hesitate to tell anyone they are wrong, so here is what I know. The formula I was using was (D^2 - d^2/4)^.5 instead of ((D^2 - d^2)^.5)/4 When the whole calculation is done this gives me 1519lbs for solid and 1856lbs for a hollow 3" mast. I have a formula for sizing junk masts which when the calculation is done (it is for a hollow mast) you have to reduce mast size by 10% for a solid mast. In truth I don't know exactly what type of load I am calculating (buckling I think). If you reduce 1856 * 0.9 = 1670 which is quite close to 1519. your input proved I WAS doing something wrong. Thanks again mik

 

When you calculate the mast bending due to loads perpendicular to its length, you must use the moment of inertia relative to the axis X or Y, depending on the direction of the loads. But if you are calculating the mast buckling due to compression, you must use the polar moment of inertia. Therefore, before starting calculations it is very important to analyze what is the case under study.
The mast will work in compression, tension, or both cases only depending on the design you make.
All formulas are good if used correctly. You can not use a formula to a different case to that for which it was designed. If your rig is similar to a junk, it is possible that the formulas you use will be useful. Otherwise, you should not use them.

 

Yes and I was still doing it wrong, I can only blame my eyesight pressing wrong calculator buttons and believing what it tells me. Double checking the calculations I get about 945lbs for a 3" hollow mast WHICH MAKES YOU VERY RIGHT. It also leaves me wondering about the junk mast formula. It seems to prove Chappele's formula for solid masts is better. I could use Chapples formula to get a solid mast diameter then work out the load it can handle then work out the equivalent hollow mast diameter to suit that load. A littel knowledge is a dangerous thing thank for keeping me on the straight and narrow mik

 

Tansl My copy of chapples book is on kindle which is part of the reason for my mistakes. I appreciate your replies to my posts which may often seem strange a disjointed. I am working toward a boat design but I have given up on being able to draw it. I am now in the process of calculating everything I can with the intention of getting someone to draw it for me from details I intend to specify.mik

 

Hi Mik, I do not understand if you have felt annoyed by my words. If so, I beg your pardon. That was not my intention at all.
I'd like to help you in your project and, if you go telling me what you need, I will see to what extent I can help. You will need to define a SOR and either by hand sketches or by photographs, indicate how you want your boat.

 

I never get annoyed with people who are trying to help me. My project has evolved from trying to design a Catamaran to rival Richard Woods designs to a Trimaran. I decided what I was designing was a beefed up Romany with a different sail plan. So I thought a cruising trimaran would be better or at least different. Most trimarans seem to have their floats designed so that when there is no heel there floats only just reach the water. In this attitude a Telstar 28 has a significant part of its floats in the water. What I am aiming for is a trimaran with dismountable/retractable floats with the same Tc as the hull with 0 degrees heel. The underbody shape is chined in the manner of Par's Rocky. What I hope would make it unique is the hulls having a metal framework covered in ply 3.5mm steel is what Gerr scantlings require for a shell plate so the frame would be 3.5mm steel incorporating a steel tabernacle joined to the sheer with steel straps. Plywood with GRP inside and out. My spar question on this thread leads me to believe that 3" solid spruce spars would join the floats. I hope this makes sense to you. My biggest problem is I cant draw. mik

 

It would be helpful if you could draw on paper the idea of your three helmets. Similarly, the SOR of your boat, among other things, the main dimensions. If you have a boat of reference, get a photo or picture of it.
As a general idea, without knowing anything about your boat, I like most metal pipes as the connecting elements of the floats.

 

SOR for "TURBULENCE 2"

Main hull LOA =39ft, LWL =38ft BOA =6ft 5ins BWL =6ft Tc =20ins
Beam with floats extended = 23ft
Floats LOA =35ft LWL =34ft BOA =2ft 11" BWL =2ft 9ins Tc =20ins freeboard for hull and floats 12%.
When the floats are drawn inboard the overall beam would be about 12ft 3ins.
39^0.66 + 1 = 12.22ft which is what you would expect a monohull beam to be. I have not yet done enough analysis for a displacement, hoping for about 4 tons. Rig could be Bermudan or Gaff. I also like the Idea of pipe beams but can't guess at their loading. I thought wooden beams would save weight and add some bouyancy to counteract the metal frames. The rig would be a bit under powered for light airs but would allow safe fast speeds across the Atlantic without reefing every time you got a fresh breeze.
This boat is a trimaran, PAR's ROCKY is the inspiration. I don't yet know how practical my idea is but any advice/criticism would be welcome.mik

 

Based on you SOR, I have prepared the following model.
I am convinced that it very little will look like what you have in mind but I show the picture so you understand how difficult it is to guess what you want unless you provide more information. I hope, nevertheless, that this will serve to at least know what you do not want.