Spar diameters

Here is an equation from Skene, rendered by Codecogs. Pretty nice.



I found a Beam Deflection Calculator that uses the same formula for a solid beam. If I put my 14' gaff into it with Skene's rule of thumb of 0.02 L as the diameter I can compare the deflection of a solid gaff with that of various different hollow gaffs.

So Skene says my gaff should be 3.36" max diameter if its solid. This gives a deflection of approx 1.5" for a 10 lb force.

I get about the same deflection for a 3.64" diameter gaff made with 1/2" walls or a 3.38" diameter gaff with 1" walls. So a 10% extra diameter and thin walls sounds like a real plan.

This calculation is perfect for relative strengths but it doesn't help much with deciding if scots pine is OK or do I need douglas fir.

I found a Young's modulus of 13 x10^9 N/m^2 on one site.

TRADA provides the following:

Douglas fir

Strength Properties (N/mm2)		Strength Class
		C14	C18
Bending Strength	f m,k	14	18
Tension Strength	f t,0,kf t,90,k	8 0.4	11 0.5
Compression Strength	f c,0,kf c,90,k	16 2.0	18 2.2
Shear Strength	f v,k	1.7	2.0
Modulus of Elasticity	E 0,meanE 0,05E 90,mean	7 4.7 0.23	9 6.0 0.30
Shear Modulus	G g,mean	0.44	0.56
Density	r kr mean	290 350	320 380

European Redwood

Strength Properties (N/mm2)		Strength Class
		C16	C24
Bending Strength	f m,k	16	24
Tension Strength	f t,0,kf t,90,k	10 0.5	14 0.5
Compression Strength	f c,0,kf c,90,k	17 2.2	24 2.5
Shear Strength	f v,k	1.8	2.5
Modulus of Elasticity	E 0,meanE 0,05E 90,mean	8 5.4 0.27	11 7.4 0.37
Shear Modulus	G g,mean	0.50	0.69
Density	rkr mean	310 370	350 420

Musings

I should be in the garage re-assembling Worm's outboard motor.

I should be in the garage making better rowlocks for Worm.

I should be in the garage.

Funny how centering some text makes one feel like a poet.

I am (sort of) working out the scantlings for Robinetta's new gaff. I started by googling. A forum post led me to Skene's Elements of Yacht Design. Google books has a searchable version and I'm working through the hits on gaff in the book. There are lots of sums and I really want to understand how they apply. On the other hand, the most relevant text is 'The loading of booms is almost impossible to determine and reliance may be placed on the proportions indicated in the diagram.' The diagram says a solid gaff should have a max diameter D = 0.02 times its length. The peak end diameter should be .75 D. The throat diameter should be .93 D. Skene goes on to say 'If the boom or gaff is to be made hollow, a section may be used having the same ratio as that of a circle of diameter indicated for the solid spar.' And there is an equation. I'm going to see if I can get the equation into this blog. Earlier in the book there is some maths about hollow spars. I'll get to that but I'm currently distracted by another hit I got when searching for Skene's book.

One link took me to a review of Skene by Tom MacNaughton. I've not come across him before but he's a Naval Architect in Maine. His website is wonderful and I got completely waylaid by the text on his Yacht Design School. In the FAQ there is a question 'What kind of designs are most often requested?' The answer is this lovely list:

1. Ones too large for the customer to afford.
2. Once with too many features to pack into the requested size.
3. Overly complicated ones.
4. Ones with features incompatible with the chosen service. (Mostly commonly overly shoal boats for offshore use.)
5. Ones that sacrifice good performance to chase fashions that are confused with technological advances.
6. Ones that sacrifice appearance and investment value to ill conceived fashion.
7. Ones for production boat builders who refuse to commit to even minimally safe scantlings.

Cool huh?

Could be worse

The mast is out, so it was time to look at the problems. We undressed the mast and brought all the rigging home. Its a pain to have to remove the VHF and anchor light wiring to get the rigging off, but has to be done.

The mast had suffered quite a lot from shackles pressing in to it, but nothing serious. We must try to avoid that when we dress it in the spring.

The serving I did a couple of years ago on the flexible wire is holding out well. The only serious chafe was on the peak halyard that had pushed its way down the mast. I hadn't got the screws deep enough into the mast on the new fittings last time, but it is not a serious problem.

Brass screws don't seem to suffer zinc leaching this far above the waves which is good to know.

The mast collar was totally rotten and definitely the cause of the mast moving forwards. It all came off easily, with just one broken screw left in the deck. The deck seems solid with no sign of any movement, so that is a big relief. Making a new collar seems very straight forward and would be a good use of whitebeam I think; it is rock hard and wearing very well on Worm where it is varnolled. Or I could use some iroko we have.

Started looking at servicing the engine. Needs a lot of cleaning and we need to bring a battery back to run the engine to warm the oil up.

Alison found lots of grease squeezed out from around the stern gland. Something is wrong there. Must take it apart and repack it.

Year End Totals

Thought it was time to write up our personal logs, which means totaling up Robinetta's log. This year she has traveled 610 nm in 35 days; that's 17 nm per day on board...

The engine was run for 84.5 hours. With the new propeller it takes 1 ltr of fuel/hour on average, which is useful to know, but I MUST get around to measuring the capacity of our fuel tanks!