Boat speeds Centaur N2.14 Nanni

[Iliade]

Please can anyone confirm the actual maximum, and comfortable cruising boat speed attained on their Centaur fitted with a Nanni 2.14, or Beta equivalent?
How vulnerable was the speed to headwind and chop? i.e. could you make good headway?

My N2.10 was rather disappointing, just about making 5kt in a flat calm and slowing considerably in a headwind/slop, but it was a used engine and I have my doubts that it was producing anything like 10hp. I'm now considering a new motor and whether to go to the 14hp or up to something nearer the original 25hp, which was superb.

 

[charles_reed]

Please can anyone confirm the actual maximum, and comfortable cruising boat speed attained on their Centaur fitted with a Nanni 2.14, or Beta equivalent?
How vulnerable was the speed to headwind and chop? i.e. could you make good headway?

My N2.10 was rather disappointing, just about making 5kt in a flat calm and slowing considerably in a headwind/slop, but it was a used engine and I have my doubts that it was producing anything like 10hp. I'm now considering a new motor and whether to go to the 14hp or up to something nearer the original 25hp, which was superb.

Except to agree "probably" it's impossible with the limited information to give any valid opinion.

I would expect, however well the engine has been installed, that it would be able to move the boat at much above 4.7kts (square root of LWL). So your vague indications suggest nothing wrong with the poor little engine.
However, top probability is that it's over-propped (unless the prop was changed with the motor), if it achieves 3600 rpm flat out that is unlikely - but would be the 1st point of test.
Even the original was only good, on a clean hull in still conditions for about 6.8 knots.
Unfortunately a fixed pitch prop is only optimised for one boat/engine speed. All other combinations are increasingly inefficient as one diverges from that mean.
As to changing the current for a replacement engine - it's hard to make a viable economic case for it.
Especially as there is probably nothing wrong with it.

 

[Moonbeam]

Hello Steve

Our old Centaur had a Vetus (Mitsubishi block) 25hp with a fixed three blade prop.

It had an impressive amount of grunt and would make 5.5 knots with ease at cruising revs of 2400-ish against a mild amount of tide, headwind or sea etc. Top revs of around 3400/600 it would see 6 knots plus.

So I would go for something approaching the original 25hp spec. You'll probably get to the Scilly Isles a little quicker that way next time you go

 

[Seajet]

I crew quite regularly on a Centaur re-fitted with a 3 cylinder Nanni; the engine works just fine, - turbine smooth compared to the original Volvo's - the prop' stays immersed etc, as one would expect with such a well proven boat.

No particular problems, but I crew on 2 boats with Nanni's, and think the instrument panels a bit lacking ( all installations are different ) - if I had one I'd want something like the NASA exhaust temperature alarm, with the display at eye level and hooter easily audible.

I seem to remember a cruising speed of 5 knots + with a lot in reserve - I discovered long ago that the old ' square root of waterline length X 1.4 ' doesn't work.

 

[jwilson]

Hello Steve

Our old Centaur had a Vetus (Mitsubishi block) 25hp with a fixed three blade prop.

It had an impressive amount of grunt and would make 5.5 knots with ease at cruising revs of 2400-ish against a mild amount of tide, headwind or sea etc. Top revs of around 3400/600 it would see 6 knots plus.

So I would go for something approaching the original 25hp spec. You'll probably get to the Scilly Isles a little quicker that way next time you go

I don't think 25hp was what the designer intended for the Centaur - rather that Westerly got a very good deal on a big batch of 25hp Volvos, so fitted them instead of the 10-15 hp that the hull was probably designed for.

I've been onboard Centaurs with 13hp Volvos and 30 hp Watermota engines, all as sold by Westerly, and to be honest there wasn't much difference except that if you gave the 30 hp full throttle you produced a lot of wash. Didn't actually go much faster.

 

[lw395]

I think a starting point would be to check whether the existing engine is getting up to full rpm.
Fouling matters.
exhaust elbow could be choked.
Prop size etc.

My 28ft Impala used to do over 5knots with a 1GM10 which wasn't really 10hp IIRC.
But any bad fouling resulted in a lot more smoke and a lot less progress.

 

[Iliade]

Please can anyone confirm the actual maximum, and comfortable cruising boat speed attained on their Centaur fitted with a Nanni 2.14, or Beta equivalent?
How vulnerable was the speed to headwind and chop? i.e. could you make good headway?

My N2.10 was rather disappointing, just about making 5kt in a flat calm and slowing considerably in a headwind/slop, but it was a used engine and I have my doubts that it was producing anything like 10hp. I'm now considering a new motor and whether to go to the 14hp or up to something nearer the original 25hp, which was superb.

Thank you all for answers thus far...

Just to confirm, my N2.10 is now painted in Norwegian Blue. i.e. it is dead, an ex-engine, ceased to be, expired and gone to meet its maker. I could spend several hundred pounds and a lot of time fixing it, or I could just re-engine with something else. The N2.10 I had was, to be honest, inadequate in a seaway and only just enough to butt the ebb back upstream to my mooring. The question is whether the 25hp originally fitted was indeed mostly a wash generator and 14 hp is perfectly adequate or whether the 14hp would also prove a disappointment or if it was just that I was only getting 6hp from the N2.10 and 10hp is perfectly adequate? Theoretical speeds are not useful but real world experience much more so.

Clean hull and prop are presumed.

I also now realise that another 10' of waterline length may not be out of my reach, so an alternative solution could just be to sell the Centaur as is with the engine removed and a nice fully-prepared empty space into which the new owner could readily drop a new Nanni/Beta of their choosing.

 

[lw395]

I've known a few people 'over engine' their boats, all have been happy with the outcome.


I would suggest that if you want to sell the Centaur as-is, make haste and present it as a package with a firm quote to get the work done.
Anyone wanting to hit the water for April in a new (to them) boat next year needs to crack on with it.

 

[Moonbeam]

I also now realise that another 10' of waterline length may not be out of my reach, so an alternative solution could just be to sell the Centaur as is with the engine removed and a nice fully-prepared empty space into which the new owner could readily drop a new Nanni/Beta of their choosing.

I think you have your answer. What did you have in mind for a next boat? One hull or two?

 

[ghostlymoron]

There's no doubt that the hull speed is proportional to the square root of the LWL but the conversion factor varies from boat to boat. 1.4 is a bit high, probably more like 1.3.

I crew quite regularly on a Centaur re-fitted with a 3 cylinder Nanni; the engine works just fine, - turbine smooth compared to the original Volvo's - the prop' stays immersed etc, as one would expect with such a well proven boat.

No particular problems, but I crew on 2 boats with Nanni's, and think the instrument panels a bit lacking ( all installations are different ) - if I had one I'd want something like the NASA exhaust temperature alarm, with the display at eye level and hooter easily audible.

I seem to remember a cruising speed of 5 knots + with a lot in reserve - I discovered long ago that the old ' square root of waterline length X 1.4 ' doesn't work.

 

[Seajet]

Plenty of doubt about the waterline length formula here, how come then my boat - 19'3" on the waterline - AVERAGED 7 knots from Guernsey to Salcombe in a beam -on ( so not surfing ) F4-5 ?

The chum who was crewing then is sometimes on here and is an experienced offshore racer, he can confirm it as we chatted about it recently.

It's not unusual for boats - including Centaurs - to go significantly faster than ' theoretical hull speed ' while still not planing or surfing.

Consider frontal and wetted area resistance for a start; is a thin keeled boat like a Listang really restricted to the same speed as a Snapdragon of the same length ?

 

[sailorman]

The OP might consider the Outboard in a well option :ambivalence:

 

[lw395]

Hull speed is just an arbitrary point on the drag vs speed graph.
If you scale up the hull, the drag curve (or wave making component of it) scales by sq root (waterline length)
Different shapes have diffferent factors. 1.4 is a popular average for postwar yachts.
Tudor warships were more like 1.1 or even 1.0

The same hull loaded heavily will have a lower hull speed.
Most yachts will go over their 'hull speed' given a decent breeze on a reach.

 

[Tranona]

A 14 would be marginal. With the correct prop would give just under 6 knots in flat water and cruise at just under 5. A 3 cyl 20 would be just right and not a lot of extra money plus plenty of space to fit it.

Real dilemma. £5k+ to fit a new engine or £3k for a good used one such as a Volvo 2020 or a Yanmar. However, an engineless Centaur is pretty worthless given you can get a functioning one with a modern engine for around £5K!

 

[sailorman]

Hull speed is just an arbitrary point on the drag vs speed graph.
If you scale up the hull, the drag curve (or wave making component of it) scales by sq root (waterline length)
Different shapes have diffferent factors. 1.4 is a popular average for postwar yachts.
Tudor warships were more like 1.1 or even 1.0

The same hull loaded heavily will have a lower hull speed.
Most yachts will go over their 'hull speed' given a decent breeze on a reach.

Heeling = longer LWL = more speed

 

[ghostlymoron]

That gives a 'k' factor of 1.59. but I'm not sure if LWL is straight line length or curved length along the waterline as a later contributer suggests.

Plenty of doubt about the waterline length formula here, how come then my boat - 19'3" on the waterline - AVERAGED 7 knots from Guernsey to Salcombe in a beam -on ( so not surfing ) F4-5 ?

The chum who was crewing then is sometimes on here and is an experienced offshore racer, he can confirm it as we chatted about it recently.

It's not unusual for boats - including Centaurs - to go significantly faster than ' theoretical hull speed ' while still not planing or surfing.

Consider frontal and wetted area resistance for a start; is a thin keeled boat like a Listang really restricted to the same speed as a Snapdragon of the same length ?

 

[Seajet]

The snag with the longer ' heeling waterline ' - as used by the long overhang boats like J's - is that one gets much more wetted area drag, and usually asymmetric handling leading to at least slight rudder drag as well.

The times I've found my boat going ' faster than she ought to ' have usually either been when she's completely upright - off the wind, without even spinnaker - or heeled right over but with so much wind she seems to overcome the doubtless extra drag.

However when we averaged 7 knots across the Channel ( it was before yot Decca or GPS and we didn't believe the trail log, so landfall was a bit of a surprise ) it was a beam reach and the boat wasn't heeling much at all, just trundling well but she is quite stiff.

 

[lw395]

7 knots on an A22 with a wl of say 20ft? That would be a ratio of about 1.56?

From wikipedia,
https://upload.wikimedia.org/wikipedia/en/2/27/Speed-length_vs_weight-resistance.gif
This suggest that a typical true displacement hull needs about 3x the driving force to make a factor of 1.56 vs 1.3
3x the drive is not unbelievable on a beam reach.
And an A22 is not the oldest of hull styles, so the curve may not be so steep,

Then again last time I averaged a good speed across channel, the north going tide coming up from the channel islands had a bit to do with it.

Anyway, we digress.
More power will get you a bit more speed. Plus more in hand for fighting wind and waves.
Beyond that get a good book on yacht design and work out some numbers instead of generalities.

 

[Iliade]

I just bought (eBay :-0 ) an allegedly good VP 2003. I'll fit it, sea trial, then try to sell boat with a view to getting something considerably longer.

I have seen over 13kt on the GPS, not all of which was tide, and I have seen well over 7kt with no significant tide, but both of these required vast amounts of wind power and are unlikely ever to be bettered on a Centaur with a rational engine fitted. I understand that 'The Captain' tried the hull with some vast motor and it achieved lifeboat-like performance without folding in half, but I will not be fitting anything quite that powerful. I can do the maths until the cows come home, but nothing beats actual experience, hence the request for specifics :0)

I really want a moderately sized mono, but am often tempted by multis, until I see that they are either powered by petrol outboards or are over £50k... And in both cases don't tend to have enough teak, bronze and lead for my liking ;0) For passage making, the 50kt cruising speed of the Telstar and it's kith & kin really does appeal, but the accommodation somehow tends to leave something lacking.

Thank you all and goodnight.

 

[Seajet]

7 knots on an A22 with a wl of say 20ft? That would be a ratio of about 1.56?

From wikipedia,
https://upload.wikimedia.org/wikipedia/en/2/27/Speed-length_vs_weight-resistance.gif
This suggest that a typical true displacement hull needs about 3x the driving force to make a factor of 1.56 vs 1.3
3x the drive is not unbelievable on a beam reach.
And an A22 is not the oldest of hull styles, so the curve may not be so steep,

Then again last time I averaged a good speed across channel, the north going tide coming up from the channel islands had a bit to do with it.

Anyway, we digress.
More power will get you a bit more speed. Plus more in hand for fighting wind and waves.
Beyond that get a good book on yacht design and work out some numbers instead of generalities.

I'm fine with design, numbers and navigation thanks, that's how I can confidently quote figures on here which some might query; allowing for tides - I always think that a good idea - and going by quartz watches, we made right on 7 knots, not 6.9 or 7.1

As for design, I've always felt wetted area drag is the most overlooked factor in casual talk about boat designs.