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Hornblower
Hornblower

Posted: Wed May 18, 2016 12:06pm

Post Subject: hull speed

As the narrowboats are quite long they must have a relatively high (theoretical) hull speed. That won´t be reached with the common 50 hp engine, so how much power would be necessary and are there any other problems that prevent reaching the hull speed? Regards Hornblower

Tony-B
Tony-B

Posted: Wed May 18, 2016 2:12pm

Post Subject: hull speed

Actually they are no so far out. If we take a 55 to 57ft boat then that equates to around about 16 metres waterline length. There will be no designed hull constant for a narrowboat but if we take 1.3 rather than 1.35 often quoted because of the short swims and often upturned base plate at the bow then the sq. rt. of the W/L length multiplied by the constant gives a maximum speed of 5 to 6 knots. Now I have clocked my 54ft Colecraft hull with a 35HP engine at 6 knots using a sat-nav on the Thames where there is plenty of deep wide water. On restricted waterways like canals the speed is not limited by wave making a sit would be on wide, deep waters but by the boat pushing water ahead of itself and throwing water out from under the boat by the prop action. This means the boat is trying to climb a hill and the water trying to get past the hull to the back of the boat increases the true speed through the water but of course not over the ground. These two things combine to prevent limit the speed over the ground. Not only that but the effective reduction is water under the hull can easily lead to the hull dragging over the bottom and creating even more friction than normal with clear water under the hull. On modern cruising hull with the very limited prop diameter a loss of prop efficiency at higher speeds will not help matters either. In short I believe a 50 bhp engine, correctly propped for the engine and gearbox reduction will get to or very close to the "hull design" speed on open deep water. If you have ever seen an old working boat charging up river with maybe an 18 HP engine you will see what I mean. The bow wave trough does reach the stern.

Hornblower
Hornblower

Posted: Thu May 19, 2016 7:36am

Post Subject: hull speed

Thank you for your quick answer. I can easily imagine that a narrowboat can reach 5 - 6 kn in deeper waters. When I was searching for "hull speed" Wikipedia gives this formula: v = âl x 2,43 That leads me to a hull speed of about 10 kn (with 16 m length of waterline). So isn´t that the theoretical hull speed?

Tony-B
Tony-B

Posted: Thu May 19, 2016 8:58am

Post Subject: hull speed

Red faces here - sorry, I mixed my metric and imperial units and used the water line length in meters, not feet as I should have done. As some kind of engineer I have no idea how much power would need to be installed or how to calculate it. A narrowboat is "designed" - if that's the right word - for a very specific job where the hydrodynamics of the environment normally severely limit the maximum speed so trying to achieve the theoretical maximum speed is a bit pointless unless you intend to take such a boat onto waters it was not rally designed for on a regular basis. If that is the case then it would be wise to consult a naval architect to ensure it is modified to make it as fit as possible for such waters. You are correct that its about 10 knots but as to how much power is needed to achieve that I have no idea. I suspect the virtually horizontal prop shaft combined with the base plate's up sweep at the bow found on many narrowboats will combined to force the boat into a bow up position as the speed increases. This will limit the maximum speed. As I implied in my reply I also suspect that the prop efficiency achievable with a 20" vertical stern post plus the stern swim shape will also conspire to limit the maximum thrust developed by the prop whatever the shaft horse power. In short I have no idea how to even start to answer your question in a scientific manner.

Tony-B
Tony-B

Posted: Thu May 19, 2016 9:15am

Post Subject: hull speed

While trying to get a better answer I found that http://www.frontierpower.com/library/hulltypes.h tm gave the hull constant for a barge as 1 rather than 1.35 (WLL in feet). This gives the maximum speed for a 55ft boat of about 7.4 knots which is closer. I also came across references to the boat's weight having an effect and suspect that narrowboats tend to be far more heavily ballasted than say a GRP cruiser or yacht. The need for so much ballast is to do with minimising the air draft while maximising the interior headroom. If you built a narrowboat with a proper V bottom reaching to the chines and scarified the flat floor at the sides then I suspect you would improve the prop efficiency and make a whole lot lighter boat. http://www.youboat.net/boatpower3.aspx indicates that the power required curve to achieve the hull design speed looks as if its exponential so you may never be able to install enough power to reach the theoretical hull design speed but you may get close to it. That graph if extrapolated for a 55ft boat suggest that maybe 100 bhp or more may be required. I hope this helps.

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