Centre of Lateral Resistance

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mhd
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Centre of Lateral Resistance

Post by mhd »

So, does anyone have a better way to make a simple *estimate* of the centre of lateral resistance (CLR) than I'm using here. I'm building the FB11 with some changes and wanted to work out how a modified sail plan would change the centre of effort (COE) and where it would be in relation to the CLR. My method currently is to draw a scale cross-section of the hull (including rudder and daggerboard), estimate where the waterline will be) and then make a card model of this and simply balance it (here on a piece of dowel).

I can calculate the COE for different sail plans easily enough but want to know the approximate CLR location in relation to this.

Cheers,

Mick

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FB11 (Designer Evan Gatehouse)
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A human being should be able to change a diaper, plan an invasion, butcher a hog, con a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects. Robert A. Heinlein.

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Re: Centre of Lateral Resistance

Post by jacquesmm »

Well before computers could do it, we used to cut out the lateral area from cardboard and balance it on the tip of a pencil. It is good enough for what you do.
Better, today, draw that same surface in CAD and get the center of area.

I would do it for you but the FB11 is not my design.
The FB11 is not a racing sailboat, it uses an Optimist sail, sailing will be fun but not break any records. You can play with the mast rake and have the mast going through a lost in the partner. This allows you to try different postions and angles.
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Re: Centre of Lateral Resistance

Post by mhd »

jacquesmm wrote: Thu May 12, 2022 10:21 am Well before computers could do it, we used to cut out the lateral area from cardboard and balance it on the tip of a pencil. It is good enough for what you do.
Better, today, draw that same surface in CAD and get the center of area.

I would do it for you but the FB11 is not my design.
The FB11 is not a racing sailboat, it uses an Optimist sail, sailing will be fun but not break any records. You can play with the mast rake and have the mast going through a lost in the partner. This allows you to try different postions and angles.
Thanks Jacques,

Yes, I'm not too worried about performance - this entire build is a test-run for a VG23 (plans already bought and hopefully build to commence over winter). I don't have much space to store a boat hence why I decided on the FB11 - I can hang it nested from the ceiling in the garage and still have room to get the car in.

The boat we ended up building has turned out quite a bit different to Evan's original design I finished priming yesterday and just bolted the two halves together and put in the mast and test bowsprit to measure for sails. The plan is to have a sprit boom extend just beyond the stern and rigged via a rope traveller on the transom. Based on the balance-on-a-pencil test, the CLR would be very close to (or aft of) the COE so a small jib should help with balance and going to windward. At least that is my assumption - I'm still learning as I go.

Sail area would be roughly 50+ square feet for the sprit sail and then another 10+ square feet for jib. I'd be interested in any comments/suggestions on this plan!

Cheers,

Mick

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FB11 (Designer Evan Gatehouse)
VG23 (Designer Jacques Mertens)

A human being should be able to change a diaper, plan an invasion, butcher a hog, con a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects. Robert A. Heinlein.

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Re: Centre of Lateral Resistance

Post by jacquesmm »

Everything is already in place, you can't change much but no changes may be needed.
As a starting point, I would try to get the sail center about 10% aft of the CLR. You may be close.

I would make a sail from cheap polyethylene tarp, glued. It is cheap and fast to assemble. You can play with the sail shape that way. Start with the largest one that fits your spars and trim if needed.
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mhd
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Re: Centre of Lateral Resistance

Post by mhd »

jacquesmm wrote: Thu May 12, 2022 2:57 pm Everything is already in place, you can't change much but no changes may be needed.
As a starting point, I would try to get the sail center about 10% aft of the CLR. You may be close.

I would make a sail from cheap polyethylene tarp, glued. It is cheap and fast to assemble. You can play with the sail shape that way. Start with the largest one that fits your spars and trim if needed.
Thanks for the quick response Jacques! I'll try that.

Cheers,

Mick
FB11 (Designer Evan Gatehouse)
VG23 (Designer Jacques Mertens)

A human being should be able to change a diaper, plan an invasion, butcher a hog, con a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects. Robert A. Heinlein.

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Re: Centre of Lateral Resistance

Post by Evan_Gatehouse »

I was a bit late to this (cruising the Canal de Midi) but also shifting your own weight forward/aft will change the CLR as well. So don't forget that while testing.
designer: FB11/GV10,11,13/ HMD18/
SK17,MM21/MT24

mhd
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Re: Centre of Lateral Resistance

Post by mhd »

Evan_Gatehouse wrote: Mon Jun 06, 2022 3:20 pm I was a bit late to this (cruising the Canal de Midi) but also shifting your own weight forward/aft will change the CLR as well. So don't forget that while testing.
Thanks Evan,

Hope you had a good time! Yes, still tuning up. I'm really grateful to you for your FB11 design though - my first build - it has been a lot of fun and I'm happy with how it has all turned out. Looking forward to getting a full spritsail rigged and testing that out.

Cheers,

Mick
FB11 (Designer Evan Gatehouse)
VG23 (Designer Jacques Mertens)

A human being should be able to change a diaper, plan an invasion, butcher a hog, con a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects. Robert A. Heinlein.

mhd
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Re: Centre of Lateral Resistance

Post by mhd »

Evan_Gatehouse wrote: Mon Jun 06, 2022 3:20 pm I was a bit late to this (cruising the Canal de Midi) but also shifting your own weight forward/aft will change the CLR as well. So don't forget that while testing.
So, I estimated a new CLR based on the estimated waterline with two people in the boat (from photographs). I then drew up a draft sail plan, based on a spritsail with jib set flying on the short bowsprit, and calculated the COE. My plan was to have the mailsail just overlapping the stern and for it to be sheeted via a rope traveller from the two eyes that are on the corners of the outboard side of the transom. Even with a mainsail that overlaps the stern, and without setting the jib, the COE will be forward of the CLR, so I'm guessing the boat will have some lee helm. Only one way to test it though. I'm hoping to make progress on making the mainsail in the next few weeks. If there is a large amount of lee helm, I could probably build a longer daggerboard to improve the balance.

Cheers,

Mick

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FB11 (Designer Evan Gatehouse)
VG23 (Designer Jacques Mertens)

A human being should be able to change a diaper, plan an invasion, butcher a hog, con a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects. Robert A. Heinlein.

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Re: Centre of Lateral Resistance

Post by jacquesmm »

This is a lot of calculations for something that will change all the time but I admit, it's part of the boatbuilding fun to speculate about those things. :D

The CLR is 2D but not all parts of sail pull the same way, for that reason and
For even more fun, I recommend to add an ingredient: lead. Lead like leading, not the heavy material.
Ted Brewer explains what it is here:
https://goodoldboat.com/proper-helm-balance/

He shows that the CE should be forward of the CLR by a certain distance and the ratio of that distance to the WL is named lead. In a small boat like this, I like a lot of lead. One of the reason is that I will often row against the wind and then come back under sail with the wind. That is how I use my small sailboats where I live.
The article above mentions how to change the lead.

PS, a lead of 10% in this small boat will be 1", moving it up to 20% is 1" difference. It is really nothing but we can split hairs (or the waterline length) for fun.
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Re: Centre of Lateral Resistance

Post by mhd »

jacquesmm wrote: Tue Jun 07, 2022 3:15 pm This is a lot of calculations for something that will change all the time but I admit, it's part of the boatbuilding fun to speculate about those things. :D

The CLR is 2D but not all parts of sail pull the same way, for that reason and
For even more fun, I recommend to add an ingredient: lead. Lead like leading, not the heavy material.
Ted Brewer explains what it is here:
https://goodoldboat.com/proper-helm-balance/

He shows that the CE should be forward of the CLR by a certain distance and the ratio of that distance to the WL is named lead. In a small boat like this, I like a lot of lead. One of the reason is that I will often row against the wind and then come back under sail with the wind. That is how I use my small sailboats where I live.
The article above mentions how to change the lead.

PS, a lead of 10% in this small boat will be 1", moving it up to 20% is 1" difference. It is really nothing but we can split hairs (or the waterline length) for fun.
Thanks Jacques. I'll check out the links. As you can tell, I'm learning as I go (or trying to).

Yes, as you might have guessed I like to understand the how and why - that is indeed part of the fun - learning new things - even if, as in this case, they aren't of massive importance. I''ve really quite limited sailing experience from back in the UK (mostly laser dinghys - no formal training) but have been hooked over the last few years in preparation for building myself. That was after walking around the harbour in Reykjavik and seeing all the boats deciding I would build one myself. I've learned a lot - I think. Now the FB11 is out of the way (mostly) I'm thinking more about the VG23. We're both booked in for 101/103/104 in San Diego in the autumn for a little more experience and will probably start building in the winter. Before then it is a question of trying to get more experience, and learning as much as possible in preparation.

I've been making a list of some more questions about the VG23 but no rush on those. I'll not bother you now but save the questions, and maybe figure out if they are actually stupid questions or not over the next few months.

Cheers,

Mick
FB11 (Designer Evan Gatehouse)
VG23 (Designer Jacques Mertens)

A human being should be able to change a diaper, plan an invasion, butcher a hog, con a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects. Robert A. Heinlein.

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