Weird Boat Speed

M

muskie

Active Member
Joined
4 Feb 2003
Messages
45
Visit site
My boat behave quite strange lately. When I sailed her around under engine, the boat speed sometime just got about 2.x knot and 4.x knot after a while.

1. The reading was came from the GPS.
2. like today, I sailed out from the mooring and was about 2.5 knot, after 15mins or so.... it's running at 4.3 knot (same throttle)
3. Quite confirm that nothing tangled the propeller (checked the propeller last week and was quite clean) and it happened many times already
4. I maintained the same course in the same area (should be not much change regarding the tidal/current)

The only thing I suspected may be... the gland packing is setting too tight??? (The last time I adjusted it was almost 3 years ago)

Is there anyone got the similar issue before?

Thanks!
 
In a word

Tide

GPS measures speed over ground so when going with tide GPS = boat speed + tide speed.

When going against tide GPS = Boat speed - tide speed

I think
 
It may be the GPS. My GPS (handheld Garmin unit) had me going 50kts at one point yesterday.

Not bad in a light breeze.

GPS measures speed by timing you between two points--if you have only three satellites visible, the potential error may be 50 feet; it may think you moved a lot further than you did.

If it was reading consistently that wouldn't be it, though.

Did the wind change direction?
 
I believed not the tide/current issue as I mentioned in my 1st post. And, the reading from GPS was quite constant (at least 15 mins or so yesterday). Wind direction and speed were more lesss the same. Remember, this happened few times already!
 
Certainly not the stern gland as you would notice engine speed drop if it was too tight. I reckon you would know if the water speed changed by a factor of 2 and if it is not tide then your speed indication is wrong. you could try timing your trip between 2 points and compare on different trips or better still find from chart or GPS the distance between the points and calculate your speed. But beware of tidal effects......... olewill
 
[ QUOTE ]

4. I maintained the same course in the same area (should be not much change regarding the tidal/current)

[/ QUOTE ]Unless it was also at the same state of the tide then I'm not sure I understand that statement. It certainly sounds like the effect of tide. Next time why not just let the boat drift for a while and see what speed (and direction) the GPS shows?
 
Could you borrow a handheld GPS and double check the result?

What is concerning is that it takes some time to get from 2+ knots to 4+ knots

Donald
 
Sounds like a "slipping" drive in the gearbox especially if you have a Hurth type gearbox. A bit like a slipping clutch on a car manual gearbox.
 
You must log in or register to reply here.

Share this page

Top