29 Jul 2005 09:44:26
mjgill
Tideway Speed Limits

Dear All,

As Tideway users will know (?) the speed limit above Wandsworth Bridge is 8
knots. As we are also aware, there is also the requirement to control your
wash, but this is somewhat in the eye of the beholder and open to
interpretation.

The other day I saw a cruiser heading downstream, with the tide, towards the
boat I was coaching. They were kicking up quite a bit of wash, and appeared
to me to be going pretty quickly. I picked up the megaphone and reminded
them there was an eight knot speed limit on the stretch (yes, asking them to
cut their wash might have been simpler). They did appear to slow down, but
on passing me the helm shouted that they were only doing five knots. Maybe
so after slowing, maybe that was what his speedo said they were doing
before. Regardless, the wash still wasn't great, but the moment was lost.

Anyway, questions that subsequently occurred to me were:
1) How do cruisers measure their speed?
2) Is the limit speed over land or over water?
3) If say the stream is flowing at 2 knots, and your speed over land is four
knots, will your speedo say you are travelling at 2 or 4 knots (as if you
were not under power you would still be travelling at 2 knots, and I am
guessing that the speed readout is based on some sort of pressure
differential device in the water, but await a response to point 1 on this
:-) ). If so, whilst travelling against this stream at 8 knots, your
equivalent power would be (in simplistic terms) enough to travel at 10 knots
in still water, with I am guessing a commensurate amount of wash.

None of this of course changes the guidance to minimise wash, but it just
got me thinking as to how the absolute speeds are measured, and what their
implications might be on how the boat interacts with different stream
conditions.

Martin




29 Jul 2005 11:58:37
chris harrison
Re: Tideway Speed Limits

mjgill wrote:
> Anyway, questions that subsequently occurred to me were:
> 1) How do cruisers measure their speed?
> 2) Is the limit speed over land or over water?
> 3) If say the stream is flowing at 2 knots, and your speed over land is four
> knots, will your speedo say you are travelling at 2 or 4 knots (as if you
> were not under power you would still be travelling at 2 knots, and I am
> guessing that the speed readout is based on some sort of pressure
> differential device in the water, but await a response to point 1 on this
> :-) ). If so, whilst travelling against this stream at 8 knots, your
> equivalent power would be (in simplistic terms) enough to travel at 10 knots
> in still water, with I am guessing a commensurate amount of wash.
>


The PLA says, in bye-law 48 (Part 4,
http://www.portoflondon.co.uk/display_fixedpage.cfm/id/12/site/maritime)
the master cannot "cause or permit the vessel to exceed a speed of eight
knots through on or over the water".

I read this as meaning that it is stream dependent (with a 2 knot stream
plus 8 knot speed is ok), that boats need not be aware of their speed
made good over land. Are cruisers obligated to have speed measuring
devices fitted? As a corollary, on the UK's roads you are bound by the
speed limit even if your car doesn't have a speedometer (which most are
obliged to have, admittedly).