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Where has my speed gone again?

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Some may rember me posting a month or so ago asking the question whether my props were the right ones due to White Lady pulling 38 knots on Breydon compared to Sea Trials in Poole where she averaged 30 in a bit of a choppy sea.

Last weekend she would only pull an average of 31 despite being lighter loaded than the 38 knot run.

The only thing I can think of and it is probably total rubish but when White lady came out for maintenance there was a uniform coating of what looked like baked on mud on the props, this was cleaned back to shiny stainless, Was the mud giving better grip on the water?

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In a word Ian,

No :naughty:

I am no expert on high speed boats but 2 questions immediatly come to mind.

By which method are you measuring your speed? Meaning is it a consistant method as a loss of speed of that much is too much for even my much publicised critisism of using GPS to measure low speed ie river speeds of 4mph etc

and

Has the reduction in weight altered the trim of your boat and she is now trimming by the head?

Rod

edit

Sorry annother question comes to mind.

Is the under keel clearence consitant as a reduced underkeel clearance can have a dramatic effect on the speed of the boat?

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Speed is measured by GPS and padle wheel which tend to agree at speed, there was possibly slightly less water in Breydon this time but not a lot. the reason for me questioning the props was that at anything other then legs hard in they seemed to be slipping in the water, with both engines easily pulling 4750 rpm compared to 4300 rpm that was previously recorded at both poole and previous runs.

When she first came to the broads she would pull around 31-32 on Breydon but through the year the speed increased, if anything she has had weight moved forward progresively through the time in an attempt to keep her on the plane at a lower speed, she did stay on at 19 knots, just. This is a couple less than previous but still higher than others are expieriencing with the same type of boat.

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You can discount the prop theory Ian, thats why the racers polish them :-D

Are the legs trimmed the same, are the tabs (if any) deployed to the same extent and as Rod rightly says, when those two are combined is the ride attitude the same, as reduction of "wetted area" is probably the biggest factor once you get past the low 20s.

How old is the fuel, petrol degrades in a relatively short time, though probably not to the extent to cause a large difference, when did you last have the engines state of tune checked, there are so many factors to take into account.

One other point of interest, you would certainly notice a reduction in speed last weekend, over a cold crisp day with high pressure due to the high temperature and therefore lower oxygen content in the combustion air, but to what extent is unclear.

In any case 30 is fast enough for anybody :naughty::naughty:

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Hi Ian,

One part of your message stands out for me so I'd like clarification.

If I am reading it right you said she pulled 31kts at Poole @ 4300rpm. But last week on Breydon she pulled 31kts at 4750rpm? So you're now requiring an extra 450rpm to achieve the same speed? Please confirm. Also, what would you normally expect her maximum revs to be (I realise petrols are not governed) at WOT? Has that gone up or down? For instance, the 4300rpm at Poole, was that WOT?

With no other information to go on I'd say you're now running smaller / lower pitched props. But I guess if that was the case you'd know it so it can't be. Or can it?

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WOT at Poole and previously on Breydon was around 4300 RPM the max on the engines is 4800 RPM, that is why I say it is as if the props just arn't grabbing the water as well and appear to be slipping, maybe if I had a longer run then the boat would catch up, I did come off the plane at least 5 times on the run accross in both directions but she seemed to have stopped accelerating where as before there was a fairly hearty pull up to the mid 30s.

Trim tabs were fully up so same as before legs were trimmed out by a couple of degrees.

Would the cold day theory not have resulted in more power from the engines so in effect higher RPM than the hot day?

80% of the fuel is less than 2 weeks old, looks like I could be scrapping the weight redistribution but think the biggest part of the weight in the boat centres around the tanks, all midships but have the Holding tank in front of the fuel tank and the water tank above the fuel tank but at the stern end of it. on this run the fuel was full (65 Gallons) but the water was empty (25 gallons) and the holding tank 50% full (20 gallons) as the boat is only 3 tons empty and 26'6 long then I suppose even a slight variation over the 4'6" area that encompasses the tanks could well totally change the attitude of the boat.

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Would the cold day theory not have resulted in more power from the engines so in effect higher RPM than the hot day?

Not a theroy Ian, proven fact, and yes it would, the amount of oxygen in the air makes a real difference, on US boats delivered to lakes at higher altitudes, not massively higher, the carburetion is significantly altered by the commissioning dealer to compensate. Obviously the smaller variations in available oxygen per Cubic foot of air due to changes in local temperature and air pressure are impractical to cater for on a carb motor but it is an interesting phenomenon and possibly when coupled to other things could result in a larger cumulative effect. You could always crack open the valve on an NO2 cylinder near the carbs.

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So the engine max is 4800rpm but previously she couldn't pull more than 4300rpm and did around 31kts at those revs? Now she's able to pull nearly top revs (4750rpm to quote you) but the speed is the same at 31kts. It still seems to point to the props or leg trim to me!

I don't understand where the discussion of air temperature fits in to this - it's not like the engines are struggling so I don't see how engine power can be relevant. The engines are delivering the goods, but for reasons unknown an extra 450rpm is being required just to hit the same speed. To me it still points to a lack of "bite" in the water.

Out of interest, Ian, what are you engines (brand / cc / hp) and any idea on White Lady's weight?

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the rev counter could be fault like you have at the back of a rev counter that little screw that you can alter the revs on start up or in motion.

just a thought :oops:

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Jonny, there are 2 rev counters and it would be unlikely that they have both become wrong by the same amount.

Simon, Engines are 3 litre Mercruisers on Alpha 1 gen 2 legs, giving 135hp each on 15 x 9 props, boat weight is virtually 3000kg dry.

David, that is what I was saying the engines would pull the higher revs on a cold day not the hotter one.

Will find out whitson if a longer run on the plane improves things but both engines are pulling the same sort of revs so it is common to both outdrives, I would think more likely to be a boat issue than a drive one, other than the props I can't think of anything else that would effect both engines.

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Just thought this might be worth a read

Tunnels Can Improve Performance Say Two Industry Veterans - 11/28/2007

A boat builder and a designer take issue with Mike Meyer

Frank Piedra started in the marine industry in 1962 with Winner boats, the first company to build fiberglass boats in a production line. Later he was with Sea Bird and was the president of Phoenix Yachts from 1976 to 1999. He now runs a world-wide marine consulting business. Perhaps the most famous, and certainly the most innovative was the Phoenix 29 “pocket convertible†which offered Volvo Penta TM40 diesels as options. He boat he built:

Mike Meyer's discussion of the Volvo IPS was very interesting but I have two comments:

1. The propeller pockets benefits -- properly designed – not only are shallow draft and have a low engine envelope profile but also improved performance. The work and tests we did in 1974 and 1975 at Sea Bird and later on in the late 70s at Phoenix, under the guidance of Jim Wynne, proved this statement. In one case we tested two identical 29s, same engines, same weight, etc., ran them side by side and even took "movies" of them. Under all conditions the 29 with pockets was faster, had a better running angle, turned sharper and the fuel consumption was identical.

2. Another thing that may have influenced Volvo to stay away from pockets is the Wollard patent (3,469,557). In the mid 80s Sea Ray, Phoenix and two other companies were sued for infringement. Jim Wynne was very much involved in the litigation. (Jim Wynne is credited by Volvo Penta has the inventor of the stern drive.)

Tunnels Can Improve Performance, Says Blount

Bill Blount is President of Donald L. Blount and Associates, a leading American naval architect firm, which is responsible for some of the fastest large powerboat hulls ever built.

DLBA worked closely with Volvo Penta in the early design work of the IPS pod system, and has designed hulls for the IPS pod drives for a number of builders. Blount reports that tunnels are not necessarily less efficient in powerboat hulls. He says that properly designed, with proper propeller blade tip clearance they can improve performance in certain applications.

check out the site aswell

http://www.boattest.com/Resources/view_news.aspx?NewsID=599

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Hi All,

Tunnels are not new and do improve the power available at the prop and are in common usage on Trawlers and Tugs and are known as Kort Nozzles.

They do however have their drawbacks.

By containing all the thrust from the prop into a Nozzle (Tunnel) You lose all Cartwheel effect (heard it also called prop walk) when going astern. While this can be desirable in some instances, where high manouverability is required Korts are not used.

There are advances on this idea with a steerable Kort which will have the same turning arc around the long axis as a rudder and will then take away the requirement for a rudder at all and better still the vessel is totally steerable going astern much like an outboard or a sterndrive.

These gave the idea for the 360 degree podded propulsion which has now by and large replaced Kort Nozzles.

Rod

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:wave this is a good one lol reading the threads the only thing i can come up with is the ammount of drag the boat is producing if you can achieve 31knots at 4300rpm then the same speed at 4750rpm then it can be a drag factor or thinking about it what sort of transmission has the boat is it direct or through hydraulic transmission i would check oil levels and grease bearings etc maybe even filter condition

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:wave this is a good one lol reading the threads the only thing i can come up with is the ammount of drag the boat is producing if you can achieve 31knots at 4300rpm then the same speed at 4750rpm then it can be a drag factor or thinking about it what sort of transmission has the boat is it direct or through hydraulic transmission i would check oil levels and grease bearings etc maybe even filter condition

I'm still not convinced Dave. The 4300rpm Ian quoted was WOT so the engines were giving it everything they had (and previously achieving 31kts). If the hull increased drag for whatever reason then the net result would be a loss of speed certainly, and probably a slight loss of revs too as the engines would be labouring a bit. But in this case the engines have magically found an extra 450rpm but the overall speed has dropped. Clearly the props are spinning faster in the water but the overall speed is reduced. In order to spin the props faster the engines either need to have suddenly and inexplicably gained extra power, or the resistance being fed back to the transmission has somehow been reduced (e.g. by less aggressive props) so allowing the engines to spin a little faster. As we're talking legs here, I imagine the transmission is direct via U/J's so no hydraulics (except maybe in the lift / trim).

There is one other possibility, of course Ian. The legs have been taken off and rebuilt, have they not? I realise it is highly unlikely to have occurred, but in theory a change of gear ratio would exhibit similar symptoms? i.e. the props aren't really turning faster at all, but the engines are having to rev quicker to turn them at the same speed. And since we don't know exactly what the power curve of the engines is, it might tail off slightly at nearly 5000rpm so the engines aren't quite able to deliver as much grunt as they are at 4300rpm hence the overall drop in speed?

One last thing, you didn't say whether the legs are DP (duo prop). 15x9 seems like a very shallow pitch for a standard prop. Silver Dream runs 15x17 long hubs props and even that, on paper, would have been a little light when the engines were new (she originally ran 16x16 which are more like 15x18/19 but these are no longer available). I know the figures are slightly different for petrol engines but even so. In fact the 15x17 we have are petrol engine props, another Mirage owner, with twin 150hp turbo diesels, runs 16x21. Unfortunately I don't know anything about DP's.

Hark at me, 2 years ago I'd never even heard of an outdrive. :lol:

So on that basis ignore everything I've said....

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Hi All,

As to the outdrives there is a counter rotating and a standard rotation drive but only the one leg has actually been removed so it is not possible that they have been put back on the wrong way round.

A posibility would be that the props are on the wrong way but I was carefull to ensure they went back on the right way round, if they were wrong I would have thought the drop off in performance would have been a lot more, maybe someone could clarify this?

the pitch is 15 x 19 so sorry about that Simon missed the 1 and they are single props.

as only one leg has been messed with then the gearing would have only altered one side and the engines are both running to the same revs.

It would certainly appear to be the props not creating as actual push rather than mechanical fault.

Big groan as I put this because it should have been in from the start. with the legs trimmed fully in the engines still only pull 4300 RPM it is when they are trimmed out slightly that the increased revs happen without any increased speed, previously you needed to run a couple of degrees to get the speed. Would just run legs in but she then has a habbit of digging in and putting her self at a very alarming angle of heel.

It appears that the props are cavitating a bit but the mystery is why would they suddenly start to do it.

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I reckon if you put counter rotating props on the wrong legs you would loose a lot of speed Ian, boats tend to go a lot slower astern :naughty:

Has something changed that may have caused a bit of turbulence ahead of the props, can't think that would be the case but just another thing to consider. Another posibility of course is hub slip.

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Now hub slip is something I had not considered David but again how likely is it that both hubs would slip to the same degree?

Anything breaking the water flow would have to be well forward or repeated in front of both props.

Only thing I can think of that has altered below the water line is the thickness of the antifoul as it has had another coat on for this year, could this really cause a major difference?

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It's not likely that both hubs would spin Ian, that's why I had discounted it up to now, but then I realised I'm not sure what effect just one spinning would have either, it may be that it would allow the other to spin up to revs without any appreciable speed increase. The extra coat of underseal wouldn't make any difference at all.

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Big groan

Indeed. I'll be invoicing you for a pint of something approved by David for all the time spent chewing over wrong information! :lol:

So the prop pitch now makes sense and sounds reasonable (15x17 on my 106 diesels, 15x19 on your 135 petrols). Also at normal trim your engines still pull the same revs which rules out smaller props or different gear ratio and since we know that was always unlikely that all now adds up and can be safely ignored. The fact that it turns out only one leg was worked on just underlines that all the more.

As David says, reversing the props (LH and RH) would certainly slow you down as you'd go backwards. I should know, the engineer that did our legs last time did exactly that - though fortunately I spotted the problem before she went back into the water and after much swearing the engineer (at himself, not me) the engineer changed them back.

So at the end of the day, the only thing which has changed is revs when running with a slight trim on the drives. The only thing I could possibly think of (turbulence ahead of the props already being discounted) would be that she's running at a slightly different angle which negates the advantage of trimming the legs slightly. I can't think what might cause that other than a slightly different weight distribution and I assume you've already investigated and discounted that?

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