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Post by JohnV on May 22, 2020 20:11:09 GMT
I expect someone will want to disagree but...its a fact that a 50ft boat has a higher theoretical hull speed than one half its length. I couldn't tell you why, but I'm positive at least one forumite can. However in a way that I also can't explain with some algorithm, from experience I can say with confidence that a 50ft narrowboat will have greater stability than one half its size. Perhaps this is simply due to weight; a 50ft boat will have likely close to twice as much ballast. I think there's more to it than just displacement though. speed of a hull is a function of the square root of the waterline length in feet
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Post by lollygagger on May 22, 2020 20:17:25 GMT
The more ballast you have, the deeper it sits and the less tippy it is? My modern style big headroom 60ft is far more tippy than my 30 year old Gorton shell 50ft boat was. This has half or less of the 3" space under the floor filled with cheap paving slabs, mainly to counteract the one side biased fit-out, the Gorton had about 4-5" and was rammed with slabs.
Honest question - why do almost everybody's narrowboats sit 6-8" higher at the front?
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Post by Jim on May 22, 2020 20:20:22 GMT
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Post by Deleted on May 22, 2020 20:34:00 GMT
I expect someone will want to disagree but...its a fact that a 50ft boat has a higher theoretical hull speed than one half its length. I couldn't tell you why, but I'm positive at least one forumite can. However in a way that I also can't explain with some algorithm, from experience I can say with confidence that a 50ft narrowboat will have greater stability than one half its size. Perhaps this is simply due to weight; a 50ft boat will have likely close to twice as much ballast. I think there's more to it than just displacement though. speed of a hull is a function of the square root of the waterline length in feet I've always been a little bit sceptical about the 1.34 rule for canal boat shaped vessels with vertical sides. I suspect this old formula may have been worked out using boats which have the shape of a boat under the water rather than a brick with pointed ends.
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Post by JohnV on May 22, 2020 20:35:11 GMT
Honest question - why do almost everybody's narrowboats sit 6-8" higher at the front? not sure why it seems to be general but a couple of thoughts come to mind
1) ballast costs and providing the prop is deep enough and the bow not pointing at the sky .... then why spens more
2) it moves the pivot point of the hull aft which makes steering more natural
3) could it be a possible subconscious copying of the trim of working boats when running light ?
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Post by Deleted on May 22, 2020 20:38:36 GMT
I'd go for number 3 on that list.
To be fair a boat will handle better if correctly ballasted and with the prop sufficiency deep in water.
Being stern down allows the prop to be immersed but causes other problems.
Being bow down is going to cause some issues but although it does not look as pretty a flat narrow boat is going to go better.
It seems a bit like the old raked back VHF aerials. It looks good never mind if it works properly.
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Post by Deleted on May 22, 2020 20:39:32 GMT
Might it be because the engine is at the back...? Most modern narrowboats can be trimmed level with the water tank.
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Post by JohnV on May 22, 2020 20:41:29 GMT
speed of a hull is a function of the square root of the waterline length in feet I've always been a little bit sceptical about the 1.34 rule for canal boat shaped vessels with vertical sides. I suspect this old formula may have been worked out using boats which have the shape of a boat under the water rather than a brick with pointed ends. I agree ..... one of my boat building books somewhere has a whole section about the relationship between the block coefficient and the hull design speed
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Post by Deleted on May 22, 2020 20:41:55 GMT
Might it be because the engine is at the back...? Most modern narrowboats can be trimmed level with the water tank. Yes but not 6 inches
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Post by Deleted on May 22, 2020 20:44:52 GMT
I've always been a little bit sceptical about the 1.34 rule for canal boat shaped vessels with vertical sides. I suspect this old formula may have been worked out using boats which have the shape of a boat under the water rather than a brick with pointed ends. I agree ..... one of my boat building books somewhere has a whole section about the relationship between the block coefficient and the hull design speed Oh yes the block coefficient. Cb. For canal boats read "Brick Comparison".
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Post by JohnV on May 22, 2020 20:45:50 GMT
Might it be because the engine is at the back...? Most modern narrowboats can be trimmed level with the water tank. that would sort of fit in with my 1)
an interesting subject
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Post by bodger on May 22, 2020 20:46:26 GMT
I'm sure quaysider and Telemachus would disagree with that. On the subject of stability I wonder if anyone has ever fitted automatic self levelling stabilisers into a canal boat. It seems improbable. in conjunction with Citroen Hydropneumatc suspension - should be a doddle.
............ alternatively do what salty sailors with a fixed keel do - attach legs to keep the boat upright.
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Post by Jim on May 22, 2020 20:47:50 GMT
Might it be because the engine is at the back...? Most modern narrowboats can be trimmed level with the water tank. Yes but not 6 inches Mine goes up and down that far. Water tank is about 750 ltr.
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Post by JohnV on May 22, 2020 20:48:01 GMT
I agree ..... one of my boat building books somewhere has a whole section about the relationship between the block coefficient and the hull design speed Oh yes the block coefficient. Cb. For canal boats read "Brick Comparison". Yeah .... but to be fair, they are vaguely pointed at the front whereas Sabina ........
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Post by Deleted on May 22, 2020 20:50:12 GMT
Oh yes the block coefficient. Cb. For canal boats read "Brick Comparison". Yeah .... but to be fair, they are vaguely pointed at the front whereas Sabina ........ Yes I have seen the bows on that one and must admit I was rather taken with it. Never mind coefficients.
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