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!

Last Outing

Robinetta is due out of the water on Monday, and with the main sail out of commission there was no chance to sail this weekend. We did want to top up the fuel tanks for the winter though, so motored over to Tollesbury. It was sunny on the way there, although chilly with the east wind, and there was more swell than we expected out by the Mersea Quarters buoy.

By the time we were heading back the wind had risen, and the sun disappeared behind thick cloud making it really feel like a bleak November day. I helmed in both directions, while Julian loosened up and greased the shroud shackles so they would be easy for the yard tomorrow, then I ran the water pump while the engine was running to drain the fresh water system down.

Once back on the mooring Julian got the boom off the mast, then we tied up all the halyards to the mast. Laying up has begun!

New sails

We've wanted new sails for a while, and Mark Butler from James Lawrence came today to have a look at Robinetta. The tides were right to bring her to the hammerhead at West Mersea, and very few people want to use the pontoon in November so we tied up alongside at 0955. Luckily the wind was as forecast, and we were able to raise sail there which made Mark's job much easier.

He came up with a number of suggestions, including lengthening the gaff, and raising the peak angle which will give us a much larger main sail area. We've been thinking along those lines, so decided to go with it. Julian will make a new gaff this winter, having had practice with making Worm's spars.

We need larger fore-sail area to balance the new main, so we've ordered two new jibs, a number one, and a working jib which will be cut very differently to our current one. He also thinks we should abandon our club footed stay-sail, and go with one that overlaps the main to produce a better wind slot; we're not quite ready for that so we've not ordered one although he took the measurements so we can have one cut next season if we decide on it.


While standing back on the pontoon to think about things Mark noticed that the hound which holds one of the peak blocks to the mast was coming loose. This is critical to safely using the main sail, so we've decided not to sail again this season. We need the mast out to work on where it passes through the deck, so it was going to come out anyway.

We had a good long discussion, but all the time keeping an eye on the depth gage. I did not want to be stuck on the wrong side of the Gut! Business was concluded just before half tide, then we nipped back through the Gut sharpish. I helmed while Julian kept an eye on the depth. We got through without noticeably touching, but Julian reported 1 foot on the gauge at one point, so we must have been stirring the mud!

We've paid a deposit, to secure a place in the queue to have the sails made, but they won't be cut for a while. This is not a problem since we really want to check how much work needs doing on Robinetta's hull. We've hired Paul Drake to re-caulk her this winter, and don't know what other essential work this might reveal. We're keeping our fingers crossed!

Unplanned delights

We had no plans for this weekend. Alison checked the weather on Friday and it looked good. On Saturday morning we got up decided not only to go to the boat, but to stay the night.

We didn't want to do anything ambitious, and settled on a gentle sail over to Brightlingsea. The weather looked a bit grim when we got to West Mersea, but it soon became obvious that it was clearing rapidly and before long we had a nice breeze under brilliant blue sky.

When we got to Robinetta the bilge pump fuse had blown again. I replaced it and tested it and it blew immediately. I cleaned the pump as best I could and blew another fuse. We thought it might be best to go back to West Mersea and pick one up from Wyatts but we felt more like getting off so we hoped to get to the Chandlers at Brightlingsea.

We had a fine sail over. Close to Cocum Hills a fisherman was harvesting some very clean and new plastic pots. The flags at each end of the reach were uncharacterically large and clear. He looked like he had a decent catch. The boat was marked CK3 (or 2, I forget). I wonder how many boats have carried that number?

An east cardinal beacon on Cocum Hills marks the wreck of the Molliette. We've never seen the wreck itself, but its good to keep clear.

The big ketch Pioneer was also out. She headed off up the Blackwater.

As we entered the Colne we spotted what looked like a Rayner designed Westerly. She had a sail number 302 but no mark letter. It looped back to take a look at us. I tried to look the sail number up in the WOA yearbook but its not there.

We radioed the harbour master and moored up on the visitor's pontoon and Alison rowed us ashore. Keith was on duty at French Marine and sorted us out a new bilge pump. We chatted about sailing up to the AGM weekend after next. Maryll is still in the water and would enjoy a last trip too.

We still had some light to fit the new pump. It wasn't easy. Keith hadn't had a Rule pump small enough and had sold us the same kind he has on Maryll. Its totally different - long and low. It fitted nicely across the bilge but the hose was tricky to fit. I got it all sorted but crimping the wires in without a proper pair of cutters was not easy and I couldn't screw the new pump down. I want to re-do it once she's out of the water and I have mains electric.

We went to the Colne Yacht Club for a good wash and a drink. It turned out to be their Halloween party. They had Spooky Ale on tap and some good (and not so good) records. We ate there but left before the party really got going. We had a nice welcome chat from the on duty committee member, who happens to live in Bishops Stortford too.

Back at Robinetta we read for a bit and had an early night.

The next morning we had a leisurely breakfast and decided to leave as soon as the mud spit separating us from the town hard was half covered. Luckily, the harbour master came around before it was time to leave, so we got to pay. £9.50 for the night, not bad at all.

I decided to have a go at leaving the pontoon under stay-sail. It worked quite well, but with the tide rushing in, I couldn't get her head around in the space between us and the spit, even with the jib, so I used a little engine. Once headed down river we were just making against the tide so I killed the engine and Alison got the main up. We didn't make it all the way to the Colne under sail alone - once round the corner we were completely headed and after a few goes I put the engine on and we motor sailed out past the cardinals.

We had a pretty uneventful sail back to West Mersea. I sailed up the Thorn Fleet and Ray Channel under stay-sail alone and managed to get onto our mooring without needing the engine. I cooked a tinned pie and some sprouts and potatoes for lunch. It was high water by the time we were ready to leave. The whole spit from the bottom of Ray Island was submerged, making the Ray and Strood channels one.

Alison rowed us all the way to West Mersea Marine, something we can only do at high water. We got Worm on the roof of the car and were home by 5pm.

Its nice to have a completely straight forward sail.