A COUPLE OF PROBLEMS

willtaylor

New member
Joined
7 Aug 2001
Messages
73
Location
Devon
From: willtaylor

Problem one:
My 20ft Signet is powered by a longshaft Honda 5. The motor is a delight except for a tendency to break
shear pins when changing into gear, especially reverse. Clearly this can be inconvenient, even dangerous. I
do throttle right back (indeed it is not possible to change gear on this motor unless you do). I also realise
that I could replace the brass shear pins with stainless steel. That would solve the problem but would
endanger the prop (and gears!) a shear pin is intended to safeguard. Has anyone solved this problem? Is
there an intermediate material between brass and ssteel that would add strength but retain some protection?
Given that the boat draws 3ft and the prop should not hit anything I would rather risk the steel pin than find
myself as I did yesterday needing to hoist sail on a (slightly) lee shore and falling tide having gon alongside to
drop visitors.

Problem two.
I have a new NASA Duet. The echo sounder seems to be fine. The speed and distance log has been
intermittent. Howevr we seemed to have freed it quite well and it is now recording realistic seeming speeds
and distance.
I also have a Garmin 12 GPS. They are both set for nautical units. There is quite a large discrepancy between
the speeds shown on the GPS and the log. I realise that speed through the water and over the ground are
different, and are affected by tidal drift, leeway and all sorts of variables. Even so I suspect the impeller
driven log to be under recording. My principal question therefore is - can I trust the speed shown by GPS? If
that is accurate then I have a benchmark and can proceed to check the impeller log against that.

I cannot see how the GPS can be inaccurate at showing speed over ground, but I am put into doubt by a
chance remark I remember when I first saw one in use when sailing across the Channel with a very much
more experienced friend. He said the speed shown by the GPSwas too fast. I didn't think about it at the time,
now I have doubts because of that seemingly irrational remark.

I shall appreciate any helpful suggestions.

Will Taylor
 

Twister_Ken

Well-known member
Joined
31 May 2001
Messages
27,585
Location
'ang on a mo, I'll just take some bearings
GPS speeds used to be inconsistent in the short-term because of the way that absolute accuracy was degraded. Since Bill Clinton turned SA off, the speed reading has become very reliable, though.

Best way to check speed instruments still remains a measured mile. Is there one near you?
 
G

Guest

Guest
Your GPS is (as near as dammit) accurate for speed over ground - fractions of one percent. It would be good indeed if the speed log measured with anything like that accuracy. You friend was mistaken - speed shown by a gps has always been either very accurate or (now) very accurate indeed. Unlike clocks or watches there aren't any "slow" or "fast" ones - they either work accurately, or they're scrap.

The shear pins are a bit of a pain. The tickover speed should be right down. I wd've though you'll be okay with a steel one - you get a dink on the prop if you touch the bottom, but a broken shear pin for more serious grounding. Are there any lighter prop options? Plastic for 5hp praps?
 
G

Guest

Guest
The GPS is absolutely spot on, end of subject.

My Honda 5 has a Stainless shear pin and has had since new. Maybe they don't expect us ozzies to hit as many things here.
 
B

bob_tyler

Guest
GPS. Remember that it shows the speed in a straight line from A to B. If you have travelled further on a wiggly course it will be slightly under reading your speed through the water. This is presuming no tidal stream or current involved
 
G

Guest

Guest
Re: gps speed

just to be clear, gps speed shows exactly that - the speed over ground. If you drive around in small circles at 20.0 knots, it'll show 20.0 knots.
Your VMG (Velocity Made Good, if available on a given gps brand/type) when on a trip from A-B may well be higher, or lower than the actual speed.
 
B

bob_tyler

Guest
Re: gps speed

Sorry, I was referring to VMG. Yes, you are correct.
 

andrewhopkins

New member
Joined
16 May 2001
Messages
172
Location
West End, Surrey, UK
measured mile

If you read the manual with your log, it will show how to calibrate it.

First find a measured mile on the charts, you'll see two lines on the shoreside marked on the chart and outside you'll find there are orange triangles or sticks, etc to mark it.

You then go as fast as you can in a straight line a hit the start button as you reach the first mark and stop at the 2nd. You then repeat the excercice backwards to compensate for tide.

Press a couple of buttons and hey presto, the log and consequently the speed is calibrated.
 
G

Guest

Guest
Re: gps speed

yes agreed. Is gps usage yet on any RYA coursework? Or. like calculators, are they still frowned upon as "cheating"?
 
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