Wind and tide affect on speed??

[Ian MacAulay]

I seam to be down on speed around 8 knots. But yesterday was windy and strong tides. Just how much of a effect does wind and tide have in poole area and would this rob me 8-10 knots as that seams a lot?


Thanks


Ian

 

[Happy Dayz]

You slso have to take account of tidal currents, havd yoj checked if your prop is fouled with old rope or tarp.

Alan

 

[rafiki_]

You slso have to take account of tidal currents, havd yoj checked if your prop is fouled with old rope or tarp.

Alan

Or prop and stern gear fouled?

 

[scottie]

Are you talking SOG or boat speed through water

 

[Ian MacAulay]

Everything is clean and yes GPS speed so Sog wind was around20mph and heading straight in to it and tide flow I would not be sure on how fast.

Thanks

Ian

 

[oldgit]

Wave conditions will slow you down,glass smooth is best of course.
On recent club trip one member was on my case because we (actually me) had decided to do entire trip at 8 knots or so.
About an hour into the trip the club member doing the whinging was well behind, a dot on the horizon but dead easy to spot due to the destroyer like cloud of black diesel smoke surrounding him.
After some considerable micky taking,we slowed up and when finally we arrived at our destination,somewhat late, a diver was summoned.
His hull was fairly clean but with a splendid amount of barnycules covering his props.
A quick scrape and all was well.
Boat had been lifted out in spring but had not gone anywhere since.

 

[boatmike]

Wind and tide will both have a substantial effect on speed. No one who has experience if sail, or even sailboats with small auxiliaries would doubt that but often Mobo owners seem to think they are driving something that is oblivious to conditions. The extent to which either will affect you depends on the boat and the conditions. No one can answer your question definitively but you can easily find out for yourself. Just try holding the boats head to wind/tide and just open your throttles wide enough to stand still. You can either watch a land transit or GPS. This will tell you how much power you are using to go nowhere! You can also reverse your course by 180 degrees and see how much faster you go. Half the difference between the two speeds (by GPS) will be the wind/tide effect.

 

[tillergirl]

I have a MAB wooden yacht - heavy, slow. So I tend to look at things that make it slower. If I am motoring direct into the wind clearly though I have the power at say 5 kts in reserve I can see water speed through the water can knock back 0.3/0.5 of a knot. Obviously there is drag from the mast, stowed sails, rigging etc. I guess the drag on a Mobo will depend on the profile. As to the tide, I suppose logically displacing through water 'ought' not to have a lot of difference - or does it. When say at 5 kts in still water, there will be friction (actually I am seriously thinking of taking off the bilge keels). If there is a one knot tide against me, the drag from friction will increase. I cannot quantify that or if it happens but going 5kts under power through the water always seems to be slightly slower than simple sums i.e. 5kts through the water less 1 knot of tide oppose ought just to be 4 knots over the ground but it ALWAYS seems to be a little slower. I can't quantify that but with 25 years experience of the MAB (dearly loved albeit MAB) I am certain it is always a little slower.

And then there's waves. The impact of waves on yachts is always more obvious than Mobos except when you have to come off the plane. When sailing to windward as the wave motion increases so drive from the sails can be reduced as the yacht pitches. When I am pitching badly the impact of the waves is obvious; when the waves are small and the drive of the sails are winning, the fun is great - but it is obviously having an impact. Wave motion clearly has a force. There is a good lesson for sailors. Sailing downwind in strong winds can suffer a broach - where the sails overpower the rudder and rule the direction of the yacht; if the wind is over tide, wave size will be higher. In strong winds the action of the stern of the yacht can locally heap up the waves with the tide and the power of the rudder is reduced. Ugh - broach when the sails win. Only suffered it once. She broached, heeled dramatically to the lee rail down under, keel stalled and she did a sudden right-angled turn out of control (best not doing that in front of something). Of course much of this is irrelevant to a Mobo but I think it demonstrate that waves have a force. I think the point that there are three activities at work - drag from the air, drag from the water and a force from the waves. Not scientific of course and I haven't got a clue whether that would have the impact to the extent of 8kts but I would guess if you are planing it could easily have that effect.

 

[Tranona]

The effect of tide/current on SOG is entirely predictable as all it is doing is moving the boat over the ground. In general the tidal effect in Poole Bay is relatively small, less than 3 knots at maximum springs, but most times less than a knot. However there are local spots where it is substantially faster at LW +/- 2 hours where it can reach as much as 5 knots. Typical areas are off Old Harry and through the Haven, the former caused by the change in bottom and the meeting of flows out of the harbour meeting the main E/W flow on the ebb. The Haven is very constricted so concentrating the flows. If you come round the East Looe channel into the main channel at near low water the outflow can virtually stop a sailing boat - not helped by the confused seas in the normal S/SW winds.

You can get a good idea of the tidal flows from the Tidal Atlas, or many chart plotter programmes have the data in real time.

The effect of wind and waves is more difficult to predict as it depends on the windage of the boat and the size and shape of seas, but an example of where these two plus tidal flows could have an effect of the magnitude described is going over Christchurch Ledge into a 20 knot SW breeze. However, worst on the ebb so actually the tide flow is favourable, but the wind and seas more severe.

 

[BobWad]

Collected my new (to me) boat last weekend and set off from Poole heading east (long story).

Anyways, she had been out of the water, hull completely as new, engine (260 hp) fully serviced etc etc.

Running at 3000 rpm was lucky if I was making 8-10 knots SOG
Two days later in calm conditions, 3000 rpm got me 20-25 knots SOG

So yes, I can state categorically that wind/tide can have a considerable effect on SOG !!!

 

[CharlieAlpha]

Collected my new (to me) boat last weekend and set off from Poole heading east (long story).

Anyways, she had been out of the water, hull completely as new, engine (260 hp) fully serviced etc etc.

Running at 3000 rpm was lucky if I was making 8-10 knots SOG
Two days later in calm conditions, 3000 rpm got me 20-25 knots SOG

So yes, I can state categorically that wind/tide can have a considerable effect on SOG !!!

I think your dramatic difference for 3000rpm was due mainly from being on the plane (or not), the conditions may have stopped you getting on the plane but if you had helped it over the hump with some trim and possibly more throttle (just to get over the hump) and then back to 3000rpm the speed difference would have been much closer

 

[jac]

I think your dramatic difference for 3000rpm was due mainly from being on the plane (or not), the conditions may have stopped you getting on the plane but if you had helped it over the hump with some trim and possibly more throttle (just to get over the hump) and then back to 3000rpm the speed difference would have been much closer

Although tide may have had an impact.

Springs around Hurst can be over 4 - 4.5 knots. If you were actually doing 14 knots through the water then - 4.5 takes you to 9-10 knots SOG. + 4.5 takes you to 18-19 knots SOG.

 

[BobWad]

I think your dramatic difference for 3000rpm was due mainly from being on the plane (or not), the conditions may have stopped you getting on the plane but if you had helped it over the hump with some trim and possibly more throttle (just to get over the hump) and then back to 3000rpm the speed difference would have been much closer

Good point - I was trying the above but it was tiring and I felt I was compromising stability/safety just a bit too much to keep it up all the time. I need a bit more experience I reckon.

 

[CharlieAlpha]

Good point - I was trying the above but it was tiring and I felt I was compromising stability/safety just a bit too much to keep it up all the time. I need a bit more experience I reckon.

8-10kts may have been the ideal speed for the conditions at the time , but because at 3000rpm you may well have pushed up a big bow wave (for the boat to climb over) you might well have got almost the same speed with say 1800rpm and saved a whole load £££ of fuel in the process. If you want to go slower because of say big waves the trick is to get the boat on the plane first say 22knts, lower the trim tabs and throttle back and just keep the boat on the plane and thus more efficient (on a boat with a planing hull) this is roughly about 17knts (very much boat dependant) and you need to experiment to find the sweet spot. If conditions dictate 10knts settle for displacement speed 8-10knts which is just before the bow starts to rise.

Rough Guide: boat flat in the water displacement speed or on the plane, bows high you are climbing over the bow wave and not very efficient.

Yes on my first boat trip taking it back to Portsmouth from Lymington I went at a cautious 12knts with no trim because I had not be told in enough detail about the above,

And yes I'm still learning

 

[BobWad]

8-10kts may have been the ideal speed for the conditions at the time , but because at 3000rpm you may well have pushed up a big bow wave (for the boat to climb over) you might well have got almost the same speed with say 1800rpm and saved a whole load £££ of fuel in the process. If you want to go slower because of say big waves the trick is to get the boat on the plane first say 22knts, lower the trim tabs and throttle back and just keep the boat on the plane and thus more efficient (on a boat with a planing hull) this is roughly about 17knts (very much boat dependant) and you need to experiment to find the sweet spot. If conditions dictate 10knts settle for displacement speed 8-10knts which is just before the bow starts to rise.

Thanks for the tips - I'll give it a go next time !

Rough Guide: boat flat in the water displacement speed or on the plane, bows high you are climbing over the bow wave and not very efficient.

Yes on my first boat trip taking it back to Portsmouth from Lymington I went at a cautious 12knts with no trim because I had not be told in enough detail about the above,

And yes I'm still learning