is the water different in the East

[ian38_39]

Sounds strange I know but I bought my 255 in poole and on sea trials it topped out at 30 knots and stayed on the plane down to 15, moved the boat to the Norfolk broads and now by both GPS and paddle wheel it will do 38 knots and falls off the plane at 18 empty 20 loaded.
the engines will rev to 4250 rpm against a given max for 3lt mercruiser petrols of 4400 - 4800 but that is the same as when in poole.
when the boat was shipped the previous owner who was helpfull enough to remove anything not listed in the sale particulars, like the anchor, helpfully had the props removed for transport and put loose inside a locker, could they possibly have been knocked enough to change performance that much there is a small amount of vibration at low speed but this disapears above about 2000 revs or could the water inland, IE brackish but more or less sea water and basically flat calm on Breydon Water make this difference.
Then again I suppose it is possible that the props are not the ones it was sea trialed with.

Ian

 

[rich]

When did you last clean her bottem?

 

[Athene V30]

Yes, it is thicker - more suspended mud!

/forums/images/graemlins/laugh.gif

 

[ian38_39]

over last winter all old antifoul was taken off back to gel, 5 coats of epoxy then 3 coats of high quality anti foul, she was put back in in April and not been lifted since but regularly moves between salt and fresh. if it was a dirty bottom though I would expect her to slow down not gain 8 knots.

 

[SnaxMuppet]

Winds are generally westerlies over the UK so perhaps all the air has been blown over to the east and your engines have more power! /forums/images/graemlins/smile.gif

But seriously... are you sure that all the other factors are the same? Is the boat significantly lighter? What about the trim? Was that the same? Same sea state? Temperature and pressure can make a noticeable difference too. For example, I know that with small aeroplanes there can be a difference of as much as 10-15% in engine power if you compare power on a cold winters day in high pressure to a warm, humid summer day in low pressure.

Here is an engine power calculator just to indicate this... if you set up a cold winters day at temp 30F, pressure say 30in, dew point 20F (Dry day) altitude 0 it gives relative power of 108%. Change the temperature to a hot summer day of say 90F and make it humid with a dew point of say 85F and power drops to 96%... a 12% drop. Bring air pressure into this and the differences can be even greater.

Engine Power Calculator

Just some thoughts...

 

[Nick_H]

It could make a difference if it was dirty on the sea trial and clean now, a layer of slime would have taken several knots off the top speed. Also maybe you were against the tide on the sea trial?

What's for sure is you haven't suddenly gained 8 knots by bending the prop or running in less viscous water.

 

[neale]

You can gain speed by running in shallow water. I once heard that Broom Boats always went faster on Breydon water because of this.

It is called something like ground effect and the water gets squeezed between hull and bottom and creates extra lift.

 

[ian38_39]

Bottom was clean but the antifoul was messy and uneven with patches missing for the sea trial, boat was light on the trial but now has all the bits and pieces on so a fair bit heavier, Poole sea trial was run in both directions giving 32 and 28 and the sea was fairly lumpy, have to admit the run accross Breydon was wind assisted, done at slack water in 1 direction only.
Looks like the most likely reason though is that the props supplied wern't the props that were on during the trial in Poole. Boat is used mostly at river speeds so guess it doesnt really make that much difference apart from tickover pushing along a bit to quick for comfortable mooring.

 

[Petrolia]

Makes a change when someone is wondering why their boat gained 8 knots !

Usually it's the other way round. Be happy /forums/images/graemlins/smile.gif

Can you come round & re-prop mine please ?

Not very helpfull I know. Sorry.

 

[Petrolia]

On a slightly more helpful note.

Can't you get hold of any reports on the boat to find out which is correct. The performance you have now or what you had before ?

 

[longjohnsilver]

Expect he's given you some old props that had been changed for the ones you had on your sea trial.

Why not ask him?

 

[nonitoo]

[ QUOTE ]
You can gain speed by running in shallow water. I once heard that Broom Boats always went faster on Breydon water because of this.

It is called something like ground effect and the water gets squeezed between hull and bottom and creates extra lift.

[/ QUOTE ]

It is called 'squat' and the 'lift' is downwards (Bernoulli effect) and can result in grounding in very shallow water. Don't know how it could improve performance.

Tom

 

[ian38_39]

Has been re-engined from Volvo AQ151s to Mercruiser 3lt. if anything the mercruiser should be slightly down at 135hp compared to the 140 of the Volvos but they are slightly lighter.
Not too worried about the performance, she goes faster than we could ever afford to cruise at, was more the disparity between poole and the Breydon water that interested me.
Intresting to hear the ground effect theory, plus I guess the dead flat water helps.

 

[neale]

I bow to your greater knowledge on this /forums/images/graemlins/smile.gif

Just following on from reports I heard that boats running in shallow water often report a higher top speed than when in deep water.

 

[nonitoo]

Neale,

The effect I am talking about is really only relevant in displacement mode, I have no idea whether shallow water would enhance a planing hull's speed.

Curious though.

Tom