Can someone explain please

[Gludy]

Went out this weekend for the lomgest trip yet.

Must have been the only one who had bad weather, om my return to Swansea, they had experienced great weather - that is because I was away! It probably explains why the south coast had such hot weather - I went west and brought the bad weather with me!

Anyway down to my question - I was cruising at about 20 knots, then without the revs altering at all speed would drop to say 15 knots over two minutes or so and then return again to 20 knots. There was a bit of a westerly swell on but nothing that I thought could lift ad drop the props to change the speed that much. It would gradually drop, gradually return and at time even increase to 23 knots - all without changing revs.

If the revs do not change, the prop revs do not change - can a swell cause such a change in speed!

Paul

 

[ccscott49]

Simple answer, Yes.

 

[hlb]

Elongation of Colins answer is. Yes it does.

No one can force me to come here-----------
----- I'm a Volunteer!!!

Haydn

 

[DepSol]

Imagine driving in a car on the flat in 5th gear at 3000 revs doing say 60mph, then you meet a hill and go up it at the same revs, same gear you will slow down. Climbing the swell is like cimbing the hill once you reach the top and drive down it then you will pick up speed like going down hill. Thats the fun bit surfing he wave and seeing how long you can hold on o it for. Did that all the way to Dielette once and saved myself a fortune on fuel.

Dom

My boats going back in the water on Thursday ;-)

 

[Robin2]

I would not rule out misbehaviour by your log either. If swell is the problem the irregular speed should be synchronised with the swell - hardly lasting as long as 2 minutes at one time.

 

[Gludy]

Surely, when you go up hill in the car, the revs drop!

Paul

 

[Gludy]

Can't be the log as my gps speed over ground, dropped and rose proportionally - there was a few knots of current with me, so normally the SOG was higher, but it also dropped by the proportional amount.

When we went faster, the bow seemed to drop, when we slowed down the bow rose. Maybe that piece of information will cast more light on the matter. Could it be someting to do with the trim tab settings?


Paul

 

[ccscott49]

You are quite right, as the connection between the wheels and the engine is a positive one, as opposed to a boat, where its not, its a "fluid" coiupling, clever eh! fluid/water, oh well never mind!

 

[ccscott49]

Could well.

 

[stuartw]

Additionally to the answers already supplied, it does depend on how your speed is being measured. If your speed indication is derived from a hull mounted paddle wheel type log, then the water speed past your hull will increase if you meet an on-coming swell or tidal stream. Your true ground speed may not alter much at all. If on the other hand you are going with the swell or tidal steam then the effect is to slow the impeller down and thus give you an apparent loss in speed.
So as Depsol says, you can get amazing speeds (indicated) virtually going down hill on a large swell, & vice versa going uphill.
However, a more accurate measurement is to use the GPS speed indication, which will, within the accuracy of the system, give you an accurate ground speed, and you will probably find much less variation.
To illustrate the problem, I recently went to Guernsey, via the Alderney race, heading into an on-coming tidal steam. My hull speed indicated 21 knts, the GPS indicated 16knts, so in effect the tidal steam had slowed me down by 5 knts.
Conversely on the return journey, with the tidal steam going with me, the hull speed showed 18 knts and the GPS showed 22.4knts, so I was flying and saving fuel.
I hope this helps.
rgds
Stuart

 

[hlb]

Gludy. The bows dropped cos you were going down bonk. With following sea. Then you get in the trough and nearly stop and bows go up. Then you wallow about going nowhere for a bit. Eventualy you climb back to the top of a wave again, then zoom down it. All very normal. Or you can try surfing and stay on top all the time. (Like in Australia at the beach) Just wiggle about sideways on top of the wave. (Watch some more telly). Once had my last boat doing 25 knots surfing. It only did 16 normaly.

No one can force me to come here-----------
----- I'm a Volunteer!!!

Haydn

 

[Gludy]

Is that right? Are all shaft driven boat props connected with a fluid coupling that allows the props to slow down whilst the negine does the same revs?

Paul

 

[burgundyben]

Are you being sarky?

or do you really have that big a lack of knowledge about how your boat works?

I think what CCS meant was that the fluid coupling is between prop and the water its in unlike wheels on car and tarmac.



I am gonna spend it on wine, women and song, and blow the rest.........

 

[Gludy]

Re: Are you being sarky?

Sorry, I misunderstood.
My post was a question with an intended follow up depending on the answer.



Paul

 

[tico]

No.... the weather wasn't that good in the haven, overcast and humid, but no sun. Then 20 miles inland on the way home last night.... brilliant sunshine (much gnashing of teeth)

seriously tho, sounds like you were only just on the plane and the swells were knocking you off it. Revs and speed will drop off as the load increases and rise as the load drops.
My old boat used to be very susceptable to this with a following sea. The only way around it was to slow down to the speed of the swells (difficult)
but ... yes it happens

 

[Gludy]

Tico - how did you know I was in the Haven?
Do you by any chance own a new Princess?

Back to the point - the revs were not altering at all - that was the most confusing thing for me.

Paul

 

[ccscott49]

No yer' pillock! but they are connected to the ground with a fluid coupling! the water!

 

[adarcy]

Dear Gludy

At the risk of upsetting the authors of he previous replies which "might" be a bit cryptic - your engines throttle setting drives the engine and (as there are not different gears or a torque converter) the props at the set speed. The props then suck/thrust the water (don't get into that) giving aroughly constant thrust driving the boat forwards. As you go up a swell or large wave, the boats slows losing kinetic energy but gaining potential energy (height) and vice versa going down the wave/swell. The thrust (and so revs) does not change on a largeish heavy boat unlike the accelerating and slowing one gets with a lightweight boat which does change its revs in the same situation.

Anthony

 

[jfm]

Agree all the above (except Depsols ). But in addition the effect is exaggerated when you happen to be travelling at a speed near where the boat would normally get over the hump and onto a sweet plane. So when the uphill part of the swell slows you down as others have described, your boat falls markedly off the plane, thus causing you to slow down several kts, not just 1 or 2.

A possible solution, not tried it yet, is a cruise control unit

 

[ToMo]

Oops hasn't anyone told him yet?... Lord & Master Blair has cancelled Thursday cos it might interfere with Friday and that's non PC!

TôMö