Narrowboats at sea

[carlt]

hmm .... I think if you ignore seakeeping ability and projections such as ballast keels, and stick to monohull types, you will find that a flat bottomed hull is most stable, of course depending on a reasonable CG height. It can be shown by simple geometry.

 

Yes, of course, if you ignore the fact that the sea isn't flat, too.

 

A three legged stool is more stable than a four legged one, but neither will be any good as a sea boat.

[mykaskin]

You couldnt be our navigator though as I have heard you didnt realize where the BCN finishes

 

Sorry, I thought we wanted to save all under-used canals. I will be extremely dissapointed if the Birmingham and Warwick Junction Canal (see even has the name Birmingham in it) isn't included in future years. It sees much less use than the Aston section of the Birmingham and Fazeley Canal.

 

I can't remember who it was who wondered why a boat should be limited to a distance from habour, but it tends to be stipulated in the insurance. Of course you don't need insurance on the wide open waters of the sea!

 

Mike

[carlt]

 

I can't remember who it was who wondered why a boat should be limited to a distance from habour, but it tends to be stipulated in the insurance. Of course you don't need insurance on the wide open waters of the sea!

 

It was me but insurance is not a limiting factor.

 

You just need to find a policy that matches your level of adventurousness, possibly, at the extreme end of the scale, underwriting yourself.

[Morat]

/me wonders if colecraft have ever put a box keel on one of their boats

[Sir Nibble]

 

I can't imagine anyone wanting to be out in a small boat if the wind is much above a force 3 so why buy a category C/D vessel if it's never going to be exposed to the conditions for which it was designed to cope. How many pictures of yachts do you see with reefed sails? Not many because the vast majority of yotties know better than to be out in such conditions. Sure, I wouldn't want to be out at sea in a NB in a force 4 but to be honest I wouldn't want to be out in anything smaller than the cross channel ferry in a force 4.

Ever done any sailing? I would call a force 4 good sailing weather and if I were under any time pressure I would have hoisted the iron sail in a F 2. I have certainly seen boats with reefed sails, sometimes down to the poles! and have vivid memories of an uncomfortable ride laying flat on the foredeck clipping a storm jib on whilst beating into a F 9 in biscay. That 40' Olsen was MUCH more comfortable in high seas than a cross channel ferry.

[max campbell]

The boat that crossed the Atlantic, "I Frances" is probably the narrowboat most heavily modified for sea use but it was specially constructed for the job:

Clicky

 

Thanks for the link. For any given draft, though, you have to wonder whether the CG is lowered or raised by the addition of this keel. I note that of a displacement of 10T (I think it said) the ballast was described as 2T. What is the average displacement of a 50' NB, and typically what weight of ballast is used? Also I think I'd avoid the additional complication of a rig - I'd never be that far away from diesel supplies. Presume power of engine at 8HP is a typo.

 

 

Why do you think boat builders bother with multi-chines, round chines, v-bottoms, at all, if the cheapest and easiest boat to build was the most stable?

 

 

Because they're trying to build in factors other than stability. It is obvious that a flat bottom is inherently the most stable hull form, and a semi-circle section the least stable, with zero form stability - but a lot of yachts have near-semi-circular hulls for reasons of low drag, handling when healed, etc.

[carlt]

 

Because they're trying to build in factors other than stability. It is obvious that a flat bottom is inherently the most stable hull form, and a semi-circle section the least stable, with zero form stability - but a lot of yachts have near-semi-circular hulls for reasons of low drag, handling when healed, etc.

No, it is down to the fact that the sea is not a flat, solid surface.

 

If you stand a round bottomed boat, with a keel, and a flat bottomed narrowboat on a concrete floor, then the keeled boat will fall over. If you put them both in the sea and they are broadsided by a 3m wave then the narrowboat falls over.

 

I'm bowing out of this one because it is obvious that I am just another of those narrowboaters that is too scared to ever go to sea, that you told us about.

[Roger Gunkel]

It is obvious that a flat bottom is inherently the most stable hull form, and a semi-circle section the least stable, with zero form stability - but a lot of yachts have near-semi-circular hulls for reasons of low drag, handling when healed, etc.

 

Stability can be defined in a number of ways depending on the uses of the vessel and as I agreed in a previous post, in flat water there is no question that a flat bottomed boat has more lateral stability. However, the sea is rarely flat, so the requirements for stability at sea change from lateral to vertical. The semicircular hull will offer much more vertical stability in extreme conditions but will roll more than a flat bottomed boat in light seas. In a sailing boat, the addition of sails and a keel will dampen the rolling movement considerably giving a much smoother motion at an angle of heel dependent on the force on the sails. In a power hull, the addition of chines will dampen the rolling, and flatter lines aft will help to reduce drag. A hull designed for above displacement speeds will also likely have a deep vee forward to reduce pounding.

 

So a blanket statement that "It is obvious that a flat bottom is inherently the most stable hull form, and a semi-circle section the least stable" is misleading and only relevent within a certain set of circumstances that only occasionally exist offshore.

 

Roger

[max campbell]

 

If you stand a round bottomed boat, with a keel, and a flat bottomed narrowboat on a concrete floor, then the keeled boat will fall over. If you put them both in the sea and they are broadsided by a 3m wave then the narrowboat falls over.

 

I'm not comparing boats - one with and one without a keel. Just stating a simple fact - a flat bottom is the most stable hull form, and a semi-circular the least - I should have said "all other things being equal". It just interests me that the NB happens to come with the most stable hull form.

 

No-one has yet pointed to a NB that has rolled / capsized because of waves - whatever else about them is unsuitable. This would be my major worry, which is why I'm pushing the point. I'm sure arrangements could be made to fix furniture (and have clean fuel, and all the rest) but what about inherent (lateral) stability?

 

Sorry, Roger, don't really get "vertical stability" - do you mean a semi circle section will pitch less? Is that true? Edit - Yes it is, provided the increase in beam continues above the waterline. If the topsides are vertical (as they are on my round bilge but flattish bottomed sailing boat) then the underwater profile won't affect pitching.

Edited June 2, 2011 by max campbell

[carlt]

Just stating a simple fact - a flat bottom is the most stable hull form, and a semi-circular the least - I should have said "all other things being equal".

It isn't a simple fact at all.

 

At sea, in anything other than mirror flat conditions, the flat bottomed boat is not sat on a flat firm surface therefore it loses the advantage you claim for it.

 

There is no point you pushing the point. You have already made your mind up so the only answer you will accept is "Yes, a narrowboat is ideal for going to sea, that is why they are the offshore boat of choice, for so many leisure sailors...Go for it!"

[Ray T]

It isn't a simple fact at all.

 

At sea, in anything other than mirror flat conditions, the flat bottomed boat is not sat on a flat firm surface therefore it loses the advantage you claim for it.

 

There is no point you pushing the point. You have already made your mind up so the only answer you will accept is "Yes, a narrowboat is ideal for going to sea, that is why they are the offshore boat of choice, for so many leisure sailors...Go for it!"

 

 

 

Having been a life long yotie in the past on this ocassion I agree with you Carl.

[Roger Gunkel]

Sorry, Roger, don't really get "vertical stability" - do you mean a semi circle section will pitch less? Is that true?

 

I'm trying to keep it simple Max. As we are discussing the suitability of hull shapes at sea, and we have been talking about stability in the likelyhood of a seaway, I had understood that rolling was the main discussion rather than pitching. I used the words vertcal stability in the gravitational sense to mean staying upright rather than falling over. It is quite clear that a semicircular hull section will stay more upright in rough seas and recover from more extreme angles than a flat bottomed square section will.

 

As regards pitching, a semicircular hull shape will offer more resistance to wave entry than a square section, which will increase the for and aft vertical pitching motion compared with a square section. However I would feel somewhat safer in a boat that rode over the waves than one which was engulfed by them, unless it was a submarine

 

I am in agreement with Carl though, if centuries of boat development and hull design are not sufficient to convince you of the inherent dangers of taking a conventional narrowboat to sea, then by all means do so, and I will follow your adventures with interest.

 

Roger

[RLWP]

<snip>

I will follow your adventures with interest.

 

Roger

 

On the national news

 

Richard

[max campbell]

the only answer you will accept is "Yes, a narrowboat is ideal for going to sea, that is why they are the offshore boat of choice, for so many leisure sailors...Go for it!"

The answers I'd like to see include - CG with 10mm bottom plate & normal ballast is "x" units of length above the bottom plate. With 6' 10" beam this would imply "x" angle of vanishing stability, this may be reached in "y" sea conditions / wave height. I haven't made my mind up at all, just trying to get to the point.

[Roger Gunkel]

The answers I'd like to see include - CG with 10mm bottom plate & normal ballast is "x" units of length above the bottom plate. With 6' 10" beam this would imply "x" angle of vanishing stability, this may be reached in "y" sea conditions / wave height. I haven't made my mind up at all, just trying to get to the point.

 

The point is that you are unlikely to find anyone on here willing to put the time and effort into working out the engineering data you require for what is basically a pointless excersise to prove what we already know - it's dangerous! You could though go to a naval architect, who I am sure could provide you with the relative engineering data for a fee.

 

Roger

[carlt]

The answers I'd like to see include - CG with 10mm bottom plate & normal ballast is "x" units of length above the bottom plate. With 6' 10" beam this would imply "x" angle of vanishing stability, this may be reached in "y" sea conditions / wave height. I haven't made my mind up at all, just trying to get to the point.

I suggest you buy "Elements of Boat Strength" by Dave Gerr which has loads of maths in it.

 

Also "Lifeboat, Design and Development" by Eric Fry, so you'll know why the boat coming to your rescue is so much more stable, than your narrowboat.

[max campbell]

What is the average displacement of a 50' NB, and typically what weight of ballast is used?

[RLWP]

If it helps, Tawny Owl is 70', displacement 1' 10", ballast - well the whole boat is about 20 tons, so perhaps a couple of tons all within 2" of the baseplate

 

Richard

[magpie patrick]

If it helps, Tawny Owl is 70', displacement 1' 10", ballast - well the whole boat is about 20 tons, so perhaps a couple of tons all within 2" of the baseplate

 

Richard

 

And Ripple was 62 foot, weighed 18 tonnes, 3 of which were ballast

 

Juno is 800kg, not flat bottomed, but whereas I needed extra insurance cover (and they insisted on a pilot) on Ripple to even do the Sharpness-Bristol run (which I never did) Juno is automatically insured for up to 12 miles off-shore.

 

Juno is a lot more tippy than Ripple (I am, after all, 10% of her weight) but I know which one I'd rather be in in a rough sea with an engine failure. One thing Laurence Hogg doesn't mention about "Progress" is I beleive she always travelled with a Dory as a safety boat

[carlt]

My boat displaces 26tons. I'm not sure how much ballast there is but the cast iron internal keelson accounts for over half the boat's weight.

 

Oh it's round bottomed too and more stable, in a force 6, than my narrow boat was, when a Willow Wren party boat went by.

 

Weebles wobble but they don't fall down.

[RLWP]

My boat displaces 26tons. I'm not sure how much ballast there is but the cast iron internal keelson accounts for over half the boat's weight.

 

Oh it's round bottomed too and more stable, in a force 6, than my narrow boat was, when a Willow Wren party boat went by.

 

Weebles wobble but they don't fall down.

 

To be fair, those figures won't help Max in calculating the performance of a narrowboat at sea. "Sarah and Peter Blake" is a little different to your average clonecraft

 

 

Richard

[nb Innisfree]

Our 10.6.3 plate 60' nb with 22" draught has 2.25t of ballast, if it had a 6mm top it would need 2.8t.

[carlt]

"Sarah and Peter Blake"

Ladies first in your posts, I see.

 

The point I was making is that sea boats not only have their ballast low down but also have lots of it concentrated in the lowest part of the bilge, rather than spread evenly along a flat bottom.

 

This increases the stability, if your interpretation of "stability" is "stopping upright".

 

You have a 20 ton boat with 10% of its weight as ballast, spread out along the whole width of the boat.

 

I have a 26 ton boat with around 50% of the weight as ballast, concentrated into a massive iron bar, that is just in the very bottom of a round bilge.

[RLWP]

Ladies first in your posts, I see.

 

The point I was making is that sea boats not only have their ballast low down but also have lots of it concentrated in the lowest part of the bilge, rather than spread evenly along a flat bottom.

 

This increases the stability, if your interpretation of "stability" is "stopping upright".

 

You have a 20 ton boat with 10% of its weight as ballast, spread out along the whole width of the boat.

 

I have a 26 ton boat with around 50% of the weight as ballast, concentrated into a massive iron bar, that is just in the very bottom of a round bilge.

 

If you are arguing that narrowboats are unsuitable for use at sea, I'm with you. As Max seemed to want proof, I thought I'd feed him some info from a narrowboat to put into some equations. Then he can scare himself with the results

 

Richard

 

If he understands them.

 

Your boat must have a terrific scrap value these days

[carlt]

 

Your boat must have a terrific scrap value these days

Less than yours. The other half is mainly wood, though the copper lined engine room would fetch a bob or two.