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Where's Rosie & Jim's narrowboat?


twocvbloke

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As was pointed out, loaded trad boats didn't have much freeboard when loaded, but also drew a lot of water (up to 3'6" or more). Tugs designed to pull them were heavily ballasted so they also sat quite deep in the water to allow them to use a large diameter propeller for best pulling power.

 

Most modern boats draw much less than either because poorly dredged canals today mean a deep-draughted boat can have difficulties getting around some canals, but since the internal height still needs to be tall enough for people they sit less deeply in the water, usually not much more than 2' draught though some are deeper -- so a lot more of the hull sticks out of the water, they look more like a lightly-loaded trad boat.

 

Modern tug-style boats which are high out of the water look wrong (because in the old days tugs didn't have much freeboard) so they're often more heavily ballasted than non-tug-style boats which means they draw more water. Look "sleeker" and lower -- a bit like a sports car -- but can be a bit less practical because of this (also like a sports car)...

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2 hours ago, Mad Harold said:

I am curious as to why some boats (like Rag Doll) don't have much freeboard.

There was a post on here recently (which I don't know how to find) if memory serves,said the average distance from baseplate to gunwale is 2'-6" (or was it 3'-6")

The reason I am asking is because I am shortly going to view a boat properly that looks rather low in the water.The stated draught of this boat is 2'-9".

I would post a link,but I don't want someone on here diving in first!

Any thoughts welcome.

Assume you want a total height of boat (hull and cabin) that you can stand up in for convenience. You have some choice over how much of that is in the air and how much in the water. If you put more in the air then there comes a point where you can't get through bridges. More under the water and you hit things in the undredged canal system. You can of course change the balance between how much of that height (preferably the bit in the air!) is hull and how much is cabin, which is more down to looks.

 

The total mass of the boat determines how low in the water it will go. If you assume a canal boat is slab-sided, so you have a total cross-sectional area which remains constant as the boat goes deeper in the water, the relationship between weight and draft is linear (coincidentally for a 72' boat it is almost exactly 1 tonne per inch). If you start with the hull and cabin, the thicker the specification, the deeper it will sit. The engine and fit-out then takes it down the rest. That's the minimum draft. If that isn't enough because you want/need a bigger prop, you need to add ballast. That defines how deep it -has- to sit. You can then make it sit deeper still with ballast. This may be a combination of choice and handling - for example, if you have a lot of weight towards the stern and add no extra ballast towards the bow it won't sit anywhere near level front to back. This combination can result in a boat having a surprisingly deep draft for its length by the time everything is levelled up. Ours for example (admittedly unusual) is round bottomed so draws more per tonne for the first foot or so, is built in nominally 3/8" plate but some of that is actually 3/4", with extra weight for lapping, frames and rivets, and has a heavy engine. That gives it a draft in the region of 2"10 on a 38' boat with no ballast at all. Our freeboard would not pass the RCD...!

 

Alec

 

 

Edited by agg221
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5 minutes ago, IanD said:

 

Modern tug-style boats which are high out of the water look wrong (because in the old days tugs didn't have much freeboard)

 

And there are some modern tug style boats out there that have shallow depth hulls to give the low sleek look with moderate draught, but then have improbably tall cabins on them to provide sufficient internal headroom, so still don't look right!

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2 minutes ago, agg221 said:

If that isn't enough because you want/need a bigger prop, you need to add ballast. That defines how deep it -has- to sit. You can then make it sit deeper still with ballast.

But you have to put that ballast under the floor, and if you are using bricks or paving slabs as ballast, you may need a fair depth under the floor to get the necessary weight in, which in turn raises the floor level, reducing the internal headroom, so then you need to rethink the hull or cabin height...

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13 hours ago, Sophie56 said:

“The Ragdoll” is currently living in faversham Kent. My Step-mum walks passed it daily and knowing I was a massive fan in my

younger years has captured a recent pic. 

FEC33AF8-B3D3-4E0C-9CB3-453372145D0B.jpeg

 

Rag Doll looks to me to have been in some difficulty recently, judging by the waterline mud marking. 

 

Looks as though the bow has been 8" or 10" deeper in the water than it is now. Nearly sunk, perhaps. 

 

 

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32 minutes ago, MtB said:

 

Rag Doll looks to me to have been in some difficulty recently, judging by the waterline mud marking. 

 

Looks as though the bow has been 8" or 10" deeper in the water than it is now. Nearly sunk, perhaps. 

 

 

Faversham Creek is tidal, so Ragdoll probably sits on the mud at low tide. As the tide goes out and the stern starts resting on the bottom, the bow will dip down lower in the water.

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1 hour ago, MtB said:

 

Rag Doll looks to me to have been in some difficulty recently, judging by the waterline mud marking. 

 

Looks as though the bow has been 8" or 10" deeper in the water than it is now. Nearly sunk, perhaps. 

 

 

That might be true.

The boat I am looking at has the same grey "tidemark"but it has been two pack blacked,and after a year or so,two pack  turns grey so I am told,don't know if that is true.

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  • 1 year later...
On 15/01/2022 at 15:50, Mad Harold said:

.....after a year or so,two pack  turns grey so I am told,don't know if that is true.

 

On 15/01/2022 at 17:17, agg221 said:

Yes, it is true.

 

Alec

My Jotamastic 90 didn't, except where it had been scraped. 

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