Out of interest how do you tell if you are over propped

BruceK

BruceK

Well-Known Member
Joined
8 Feb 2015
Messages
8,361
Location
Conwy
Visit site
The answer everyone seems to give is if you are not making max revs. But surely the heaviest load is getting out the hole onto the plane. And if you can get out the hole easily enough you should be able to make max revs. So what am I missing?
 
Whitelighter said:
Clean hull, clean props: can’t make max rated rpm = overpropped
Click to expand...

I've had more instances where I struggle to get on the plane and then easily make Max rpm. And also have had a few instances where get on the plane easily but then cant make Max rpm (across a spectrum of experience on boats). I'm wondering if Max rpm is not a bit simplistic.

The reason being I have been playing with props trying to find the most suited. Interestingly based on old Volvo brochures the C4 is slightly coarser than the B5. (At least in relative top end speed terms) However I find the opposite. My B5's are about 150 -200 rpm coarser and rev for knt about 3-4 knts quicker at cruise. When I queried this and was thinking of going down a set to B4 I was told there is about 450 rpm difference in duo props B4-B5. What's even more confusing is a mate who runs B4's on a Princess V40 gets the same speed for less revs on B4's than I did on C4's. So there seems no clear size grading but it all depends on boat.
Me. B5's make me run hotter by 5-10 degrees and where as I make max revs it's a stretch. Yet, the B5's kick me onto the plane now quicker than the C4's ever did. I'd go back to C4 if the anode wear wasn't so diabolical, but if I go for B4 then I worry they will be like my C3 where I rev the engine's guts out and make a paltry 20-21 knts slack off the revs and I fall off the plane.
 
I would suggest, with assets of high value, like your boat engines, you don't wan't to be dicking about with the props unless you know what you are doing. As you identify, its not quite as simplistic as pulling max revs.

Needs a careful eye on EGT for example. EGT too high, you'll have heads dropping off exhaust valves leading to tears before bedtime.

All engine parameters have margin around them, but if you don't understand all the parameters and know what the margins are you're playing a risky game.
 
Yup I depends on the boat , drag and Kg,s and ability to lift and thus reduce drag ar various speeds .
So you can’t compare diff boats eg your and V40 .
Then there’s the torque transmission via the gearbox ratios - another variable.
Fouling of hull and particularly the props another variant that changes each week !

Rpm is the only common indicator available.

Ideally the builder should prop it ( pitchwise ) to get a little over rated engine rpm some say 3 to 5 % .
This ideally is better done fully loaded fluid wise and personal cruising stores etc .
With a smaller boat a few extra 16 stone blokes will be felt , however with bigger boats the fluids kg add up .
Eg we displace about 17 tons but carry 2000 L fuel , 500 L water , bkack tanks 400 L so fully loaded with fluids and av family + tender etc that’s another at least 3 tons so an extra 15 stone bloke makes no noticeable difference .
Fortunately we have load guages and see the effects of the fouling and bunkering .
Rates max rpm is 2250 but I can exceed that by 80 or so if I want which suggests we are slightly under proped which is nice .cruise rpm for me is allways set on the “load “. I try to run around 80 % ——- all day
Speed varies depending on the list ^^^ .

So looking at this
https://imgur.com/a/Gnd2qs1

I cruise just below 1800 say 1780or something ( WOT 2250 ). Never really go over 1900 tbo
That’s 1800 is around 30 knots anyhow plenty fast enough thanks
 
Last edited:
burgundyben said:
I would suggest, with assets of high value, like your boat engines, you don't wan't to be dicking about with the props unless you know what you are doing. As you identify, its not quite as simplistic as pulling max revs.

Needs a careful eye on EGT for example. EGT too high, you'll have heads dropping off exhaust valves leading to tears before bedtime.

All engine parameters have margin around them, but if you don't understand all the parameters and know what the margins are you're playing a risky game.
Click to expand...

Because I can access EGT ,s and see it’s rapid changes and see the effect of fouling ( dirty sterngear)
I ve been quite vocal - ok hands up
But I,am trying to be helpful,passing on info .
Sometimes I feel it’s just falling on deaf ears :disgust:

Thing is just because Volvo Penta arguably the largest engine maker represented on here does not “do EGT.s “ it does not make them insignificant.
More like out of sight out of mind :)

https://imgur.com/a/zD2ms

I run mine 550 in May fresh from a yard visit ,to a bit higher as the season progeress 580,

No higher i back off because I can .
 
burgundyben said:
... but if you don't understand all the parameters and know what the margins are you're playing a risky game.
Click to expand...

I'm not going off piste here. The boat was delivered from factory with B5's but that was 20 years ago. I bought the boat with C3's on and that was grossly under propped. A drop from B5 to B4 to lighten the load to me makes sense. But to loose 400 rpm plus is lightening the load a bit to much. The boat cruises nicely at about 24knts (faster if you give her welly but she runs out of rpm quickly). So dropping 400 rpm is a lot That would mean thrashing the revs because a cruise of 3200 of 3900 would then put me in the 18knt range and about to fall off the plane
 
BruceK said:
I'm not going off piste here. The boat was delivered from factory with B5's but that was 20 years ago. I bought the boat with C3's on and that was grossly under propped. A drop from B5 to B4 to lighten the load to me makes sense. But to loose 400 rpm plus is lightening the load a bit to much. The boat cruises nicely at about 24knts (faster if you give her welly but she runs out of rpm quickly). So dropping 400 rpm is a lot That would mean thrashing the revs because a cruise of 3200 of 3900 would then put me in the 18knt range and about to fall off the plane
Click to expand...

Just for clarity
When you say you loose 400 rpm -fitting B4 ,instead of B5s do you mean it can,t reach factory WOT which say is around 3900rpm ?
Or it revs 400 rpm over the 3900 .?
Cos if it’s lost 400 rpm with the B4s down to around 2500 @ WOT then it’s overloading as B.B. says above .

Which way is the pitch going from B5 to B4 ?

Sorry I,am a bit confused.
 
Last edited:
Edited

B4 gives you 400 more revs at WOT. i.e if you make 3.6 on B5's you'll make 4.0k with B4

Not that it quite works that way either in practise hence my questions
 
Last edited:
Actually, you know what. I'm not really sure come to think of it. 400 rpm plus decrease per grade (i.e. B4 to B5 etc) is what is commonly bandied about. Whether that is for given speed or rpm / hp I am now not entirely sure.

Certainly from C3 to C4 saw a 9knt performance increase but cruising speed of 20kts at 3900 rpm went to 24 knts at 3200 rpm. 400 rpm didnt come into it.


:disgust::disgust:
 
Last edited:
BruceK said:
Edited

B4 gives you 400 more revs at WOT. i.e if you make 3.6 on B5's you'll make 4.0k with B4

Not that it quite works that way either in practise hence my questions
Click to expand...

You are after ideally just bit over “ rated “ rpm .
This “rated “ is different to your WOT I think .
Rated for me means what the enginemaker says eg 3900 for you 2250 for me .
At WOT both boats ideally fully loaded should just go slightly over the “ rated “ @ your WOT .
So your B4 ,s which make 4000 rpm seem correct in my book .

As said early more so with smaller sub 7/ 10 ton boats a few extra kgs - % wise make a big difference in performance.
So with B5 not making the 3900 to me that’s too much pitch and likely to lead to the overload phenomenon or very close running all the time in excess of 90% .

You need ( in the absence of EGT / load guages ) calculate your back off fast spirited cruise from cruise rpm from the B4
Reaching 4000 say set at 3400/ 3500 max .if that preferable when I had a vp KAD 300 I sat them at 3100 to 3200 no more
This is what VP mean by 400 or so less .400 less than a correctly propped boat .Not 400 from 3900 irrespective of which props .Add a bit of fouling drop it by another 100 .
Where as with the B5 on a new hull lightly loaded a pair of midgets - Ok faster for the journos :) it’s 300 rpm short .
Needs to make 3900 not 3600 .So setting cruise at 3400 is potentially harmful EGT wise .


https://www.sbmar.com/articles/is-my-boat-over-propped/

https://www.sbmar.com/articles/engine-life-vs-engine-loading/

Go to main menu for other related articles like EGT etc see “ Tony’s tips “
 
Last edited:
you may be overthinking this, or looking in the wrong place
volvo expect an engine in good condition, with EVERYTHING working to spec and the best prop selection, to hit their rated WOT.

If it's over-proped it can't do it, much like if the hull is fouled - the engine can't give the extra power required as the boat accelerates to max rpm. In effect the fuel pump can't/is not allowed to pump in even more fuel to overcome the loading.
If the boat is operated either a bit over-proped or driven hard with fouling on the hull, the fuel injection pump puts in more fuel than the design expects to maintain the engine revs for an extended period of time.
As Porto said this will make exhaust gas temps go too high and you risk engine damage.
Volvo expect this to happen occasionally but not for extended periods - e.g you hit a big wave going fast, fuel pump compensates momentarily, on you go.
In the user manual Volvo make it very clear that max cruising speed should 10% less than the achievable WOT rpm. NOT the design or signed off rpm under clean/healthy conditions.

As an example on a KAD type engine:
In perfect condition, clean hull, right props, WOT is say 3800rpm so max cruise should be=< 3800 - 380: call it 3400rpm. Volvo tested their engines under that level of load and are happy it won't go bang. The boat designers played with props etc and pick a diameter pitch that will achieve the design max rpm under clean and healthy conditions.
However with fouling/extra load etc if achievable max rpm is now only 3600rpm, then max cruise should be =< 3600- 360; call it 3200.
If you then deliberately run your engine(s) at the higher 3400rpm rather than the less stressed 3200rpm, the EGT will be too high and something may well go bang. Again a few minutes won't matter Volvo know what to expect, and design in some headroom for ignorant/stupid operators. But do it for hours and you will damage something. Short term the turbo exhaust impeller will start to burn away - shrinking blades, bits missing, worst case as Porto said, you could melt the exhaust valves.

If the boat is under-proped then the engines will exceed max design rpm. As Porto says 100rpm over max design rpm gives a margin for fouling /boat load through the season and room as the engine ages. If the engine clearly hits the rev limiter really quickly it's very under-proped. Volvo do care about EGT but expect the boat operator to respect the engines limitations, as described in the article Porto referenced. Basically if the hull is fouled don't thrash the engines.

So before you start worrying about props be sure your engines are healthy, If your KADs are 20 years old it could well be worth changing the turbos as they do fade away even if they look OK. The difference between a good turbo and a tired one is not always visible. I had one die at only 100 hours but after 14 years doing next to nothing. We took it apart and it looked OK. Put a new one on, the difference was dramatic. Best way is to get boost pressure measured. If you cant measure boost then you can measure the temperature of the compressed air pipe into the charge cooler and see if they are the same. If temps are different one /both turbos is tired. Either way if one turbo is weak showing a lower compressed air pipe temperature, the other one won't be far behind so change both so both engines are working equally hard.

Make sure your engines are right before playing with the props. Use the original design prop and see what happens.

EDIT - if you have fuel flow restrictions the engines will exhibit over-propped behaviour too, as the boat tries to go over the hump that takes extra effort at a lower rpm. If fuel flow is restricted it will struggle, and be a bit gutless hovering on the brink. The same will happen with airflow restriction or supercharger issues. A sports boat with KADs should fly over the hump without hesitation. My 9 ton flybridge with KAMDs accelerates like a rocket over the hump. However it took a bit or sorting re turbos and fuel supply to get it spot on. I get 3800 WOT and run at 3200 to give the engines an easy ride.
 
Last edited:
I am no expert but have recent experience of this - which I don’t need to detail on here. However we found out we were under propped, and to determine this we had the local Volvo guys on board with their Vodia tool. Clean everything and freshly serviced, and we couldn’t achieve over 2,900 rpm. The Vodia tool, showed that at 68% throttle the engines were giving 100% of output, and the engineer told us that the software was restricting the engines, and stopping effectively a self destruct. For the technical answers above, I guess that this was the screen based, electronic version of the EGT answer.

Long story short, change of props, and no need for Vodia, as 3,500 revs achieved immediately. Based on this, I would be less concerned about how the boat behaves, more what the under / over performance could be doing to the engines...
 
Thanks for your input guys. Having KAD42's I dont have ready access to things like EGT, Boost, load etc. But a lot of very interesting points raised that have given me pause for thought. Not really sure where to go from here. Maybe go to B4 or back to C4. I suspect that the B5's may be marginal in loading the engine. Bit disappointed as B5 was supposed to be slightly finer than C4. Anybody interested in B5's that have maybe 5 hours on them?
 
Two other factors which have not been discussed yet are propeller material and propeller shape. I think the OP is comparing an Ali prop-set (B-series) with a SS prop-set (C-series). An ali prop will behave (and perform) differently from SS prop, even if they are the 'same' size. I've not seen an ali-duo prop set up side by side a SS duo-prop set up but I would reckon that the shape of the blades will be different, eg cupping, thinner blades...

I've quite a lot of experience experimenting with various SS props on my rib (Scorpion 6.5m, 150hp Optimax Pro XS). Vengence, Revolution 4 and Laser2. Each prop, even if it is the same diameter and pitch behaves very differently both in terms of acceleration (hole shot), WOT revs and top speed.

So, to compare one size ali with a different size SS, is a bit apples and pears.... Unfortunately, a SS set is about £3k a pop, so quite expensive to experiment with.

FFIW - we settled on the Laser2, based on top speed achieved with the boat with full fuel tanks and 2 people on board. Pulls 5350/5400 rpm which is within the 5250-5750 max rpm range. There are other factors but these were the primary factors.
 
You must log in or register to reply here.

Share this page

Top