5knots missing -where's it gone?

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julians

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So, when my boat was new, as part of the pdi, berthon (the supplying dealer) conducted a sea trial and recorded all the vital statistics, at the point (in 2006) it managed 42knots at 3600rpm, trimmed out correctly,in the solent, with two people aboard,a quarter tank of fuel and an empty water tank,and no other kit.

Now,10 years later I can't get past 37knots at 3400rpm, also trimmed out correctly, in mallorca, that's with full fuel, full water, and various bits and pieces of kit (tools,spares etc) on board. The props are clean, as is the hull. I reckon the boat is probably 300kg -350kg heavier than when tested by berthon.

So is that drop of 5 knots about right for a heavier boat in presumably warmer weather? Or do I have an engine related problem.

Engine is a Volvo d4 260, boat is a windy 25.
 
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Weight is a killer for planning boats- you increase your wetted area = drag more of it for a given Hp .
Interesting to ever find out the EGT ( Exhuast Gas Temp @ 3400 rpm -suspect scary figure ? )
 
Sounds fair to be honest. Can you actually get to 3600rpm unladen? The 200rpm would account for a couple of knots at least.
 
I don't have an egt gauge so can't see how hard the engine is working,but VP docs says acceptable wot rpm is between 3400 and 3600rpm, so am hoping it's OK.

Haven't really ever had the opportunity to test in as similar conditions to those that produced 42 knots, always got family aboard on our way out for the day, I guess I really need to do that though to be certain. Although there is never much difference in my top speed between the full fuel tank and say a third of a tank,which I dont tend to go below.
 
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julians said:
So, when my boat was new, as part of the pdi, berthon (the supplying dealer) conducted a sea trial and recorded all the vital statistics, at the point (in 2006) it managed 42knots at 3600rpm, trimmed out correctly,in the solent, with two people aboard,a quarter tank of fuel and an empty water tank,and no other kit.

Now,10 years later I can't get past 37knots at 3400rpm, also trimmed out correctly, in mallorca, that's with full fuel, full water, and various bits and pieces of kit (tools,spares etc) on board. The props are clean, as is the hull. I reckon the boat is probably 300kg -350kg heavier than when tested by berthon.

So is that drop of 5 knots about right for a heavier boat in presumably warmer weather? Or do I have an engine related problem.

Engine is a Volvo d4 260, boat is a windy 25.
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I'd say your weight estimate could be out 3/4 fuel (225 Ltr) and full water (70Ltr) would account on it's own for 300kgs, plus " a few"tools, all the cr*p that one accumulates over the years, plus people - you'd be surprised at how weight kills performance. Also after 10 years - how many engine hours? probably lost a few ponies in that time.
 
My weight estimate could be on the light side. Engine has done 457 hours.

So what we're saying is that the speed is probably about right consideringthe weight difference,temperature difference and 457 hours worth of use?
 
julians said:
My weight estimate could be on the light side. Engine has done 457 hours.

So what we're saying is that the speed is probably about right consideringthe weight difference,temperature difference and 457 hours worth of use?
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450 hours is not much. Might be just about run in.
Med water is more dense than solent water, allegedly.
Med air temperature probably warmer than solent (just guessing). Cool air is best .
25ft boat doesnt take much weight to affect performance. But you do have ample power . Lose some weight and try again closer with the weight in the solent test.
Also guess we are all assuming the bottom is clean and smooth similar to the solent test and no damage to props.
 
I suspect it may be more to do with drag than weight. Was the boat on the hard immediately before the original sea trial? If so, the props and hull were probably spotless (as opposed to just clean), and this makes a big difference at those kind of speeds. Also, some reckon that the antifoul absorbs water over time and adds drag, Coppercoat state this as the reason that boats go a bit faster with their paint.

I think warmer air does have an effect, we always seem a knot or two slower in high summer, though i'm a bit confused by this as i'm told the air leaves the turbo at getting on for 200 degs C anyway, and is cooled by the engines closed water system which itself is running at 80 degs.

When we sea trialled our Khamsin on a cold day in March, just after lifting in from the hard, we hit over 45 kts, but struggled to get much over 40 knots at any point after that once the boat had sat in the marina for a month or two.
 
julians said:
I don't have an egt gauge so can't see how hard the engine is working,but VP docs says acceptable wot rpm is between 3400 and 3600rpm, so am hoping it's OK.

Haven't really ever had the opportunity to test in as similar conditions to those that produced 42 knots, always got family aboard on our way out for the day, I guess I really need to do that though to be certain. Although there is never much difference in my top speed between the full fuel tank and say a third of a tank,which I dont tend to go below.
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If Volvo Penta are prepared to accept WOT of 3,400 rpm for 3,500 rpm engine they must be on drugs!

Legal emissions injector durability requirements mean you can be pretty much assured you that engine output will be to spec, in fact modern engines tend to grow slightly in output with age. If your CAC has been regularly serviced power will be up to snuff.

Windy 25 spec sheet displacement is 2.2 tonnes is that correct? Sea trial done in U.K. seawater summer ambient max 18 Degrees C moved to Med seawater temp 24/27 Degrees C.

On a smaller boat speed will fall dramatically even with small increase in displacement.

No a V.P. person but I was under the impression that % engine load was available on all electronic engines had % engine load read out, it is used by the base calibration do V.P. not display it?
 
There's no such display (%engine load) on my boat. It's an early d4, and as such I just have revs,water temp,oil pressure and batt volts.

I wasn't at the original sea trial,but I suspect it was done with a spotless hull/sterngear, and possibly no antifouling, ie just bare clean hull.

Sounds like most on here would expect this sort of drop in speed as due to the difference between a real world use scenario and a perfect world scenario.

Interesting thoughts from everyone,keep em coming, especially the viewson whether 3400 rpm (assuming accurate gauge) is too low for wot
 
julians said:
There's no such display (%engine load) on my boat. It's an early d4, and as such I just have revs,water temp,oil pressure and batt volts.

I wasn't at the original sea trial,but I suspect it was done with a spotless hull/sterngear, and possibly no antifouling, ie just bare clean hull.

Sounds like most on here would expect this sort of drop in speed as due to the difference between a real world use scenario and a perfect world scenario.


Interesting thoughts from everyone,keep em coming, especially the viewson whether 3400 rpm (assuming accurate gauge) is too low for wot
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The % engine load is simply derived from the fueling look up table which all electronic engines use regardless of manufacturer. It is more than a little perverse if Volvo Penta not to display important data which engine is actually generating in order to run.

The ability to make rated speed is not a matter of opinion it is a matter of fact..If you are showing 100% engine load at 3,400 rpm (If your engine displays it)however throttle set for maxim rated speed 3,500 rpm engine is attempting to fuel for 3,500 rpm but lacking boost, diesel engines are excess air machines so exhaust gas temperature is going above the comfort zone and remember 100 rpm overload is severe as it is not linear, it is by the exponent of the propeller demand curve in your case by the exponent of around 2.5 so true engine load will be way over 100%.

It looks as if the D4 has a very generous governor droop of 100 rpm so no excuse for over propping.
Caterpillar are very careful on engine application sign off, engine MUST make rated speed no ifs or buts and they are not happy signing off anything much over 85% load, half tanks and clean. Likewise Cummins have limit curves printed in red on their electronic engine power curves, fail to prop correctly and engine is outside the limit curve your warranty is not activated. Volvo Penta engines do not defy physical principals which apply to other engine manufacturers.

Engine rpm displayed on electronic motors is accurate as the datalink is displaying the engine rpm which the engine calibration is using to run the engine.

Out of curiosity how many times have your CAC's been serviced?
 
Interesting info re the engine load.

Cac were cleaned 2 years ago along with the heat exchanger,as I was experiencing the typical d4 Overheat over 3k rpm. Not getting any Overheat at the moment,but maybe it needs cleaning again.
 
Update to my thread, i got my speed back,now seeing 42knots again at wot.

Cant pin it on one particular thing,but heres the list of changes since last year when i last tried.

Props are now spotless
Hull is freshly antifouled ( but to be fair i had scrubbed it clean last year when i tried for top speed,so i dont think theres any difference here)
Engine is freshly serviced,so new oil,filters etc
Charge air cooler freshly cleaned
All belt pulleys replaced ( super charger belt was squeaking due to worn tensioner)

I guess the most likely candidates for 'fixing' the problem is the props being absolutely spotless and the charge air cooler being cleaned.
 
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That's great news. The condition of the hull can have a surprising impact, even if the difference seems marginal.

Presumably the revs are back at 3,600? I think that is over-revving very slightly, although prob best for it to do this when lightly loaded so as to still hit peak revs once fuelled and loaded.
 
Its actually slightly under at 3550 rpm, so im guessing my speed is actually ever so slightly lower than my reading,or the original 42knot recorded speed was slightly inaccurate, either way (revs at 3550 or speed at 42knots)its close enough to the original recorded speed/revs that im happy i dont have an issue.
 
3550rpm is a good place to be for WOT on a D4-260.
On mine, two of them can only manage 30.6 kts, so the overall experience is not quite as exciting :nonchalance:
 
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