To planing flybridge owners: are you aware of how your boat performs...

[MapisM]

...in terms of propeller slip?

I'm asking because while I know rather well what to expect from smallish (20' to 28', outboard or outdrive powered) speedboats, I never saw any prop slip numbers related to 50'+ shafts driven f/b boats.
And I suspect that a comparison between these two categories doesn't make a lot of sense.

Now, I also understand that prop slip is far from being the most crucial number in the context of a seatrial, but since I have all the necessary data (props pitch and g/box ratio, plus the rpm and speed that will be measured), I was thinking to calculate it, out of curiosity - and I was wondering how to compare it with similar boats.

So, over to the many 50'+ planing boat owners around here:
Did you ever check the prop slip of your boats, and if yes, would you mind posting the result?
Alternatively, for those who never bothered doing the math, if you know your prop pitch and g/box ratio, plus your usual (and/or max) rpm and speed, I'd happily calculate it for you.
Btw, at least those of you who are the lucky first owners of your pride and joy should have within your docs the initial engines test made by an authorized dealer, which afaik all engine builders require in order to validate the warranty.
And even if I don't think the prop slip is specifically mentioned in these reports, all the necessary numbers to calculate it must be there.

 

[QBhoy]

Mapism
It's an interesting point you raise. I have thought, pondered and researched prop dynamics as far as possible, to the point of risking my sanity !
That said, only really for inboard outdrive and outboard smallish boats like my own.

I've often wondered what results my fathers boat would produce, being a 40 footer twin screw planing hull, but never went further with it.

I'd imagine that the manufacturers would have such data on file and recorded at initial as built trials.

Out of interest, what kind of slip are you getting ?

I've managed to get my own boat down to an incredible <3% slip. Took a while and lots of pennies though !

 

[MapisM]

THREE percent? Wow, that's unreal.
Are you sure that the nominal pitch upon which you based the slip calculation matches the actual prop?
I'm asking because there are some props (even from stock production, like for instance the Merc s/s 4 blades Bravo One) which are known to have a real pitch slightly different from the nominal one (about an inch less, in the case of Merc B1).
Anyway, the numbers of both my own Fever 27 and a few other similar speedboats I had the opportunity to test in Lake Como were always into 2 digits numbers - somewhere between 11 and 14%.
I can only recall a 20' very light Albatro (almost a racing boat), powered by a 2 stroke Merc 2.5 EFI, which was lower - around 8%.

That said, yep, for some reason only speedboat fans seem to care about prop slip - to the point that the latest high performance engine monitoring displays calculate that in real time, allowing a better fine tuning of trim and tabs (though that's a bit OTT, 'fiuaskme).
In fact, I guess I am now wondering about the prop slip of a 30 something knots 57' flybridge just because I used to care about that with much smaller and faster toys.
Also the fact that nobody answered so far, makes me suppose that among the owners of similar boats there's not much interest, or even just curiosity about it...
Oh, well. I'll report my findings anyhow, just in case someone would like to compare in the future! :encouragement:

 

[Portofino]

Prop pitch and G box ratios are not the sort of numbers I have in my head .
Even to get the slip meaningful accurate not 100 % sure of the rpm / speed on the gps ,depends on fouling ,alters it a bit .
suspect many others in same boat
Sorry can,t do it .

 

[Whitelighter]

Let's say I bother to find out, what can I do about it?
The bloody thing weighs 26T, trim taps do bugger all really so I just plod on.

Did get 26.5knts out of her this week after the relaunch, still 250rpm down on max (2100 vs 2350) but for 23 year old engines I didn't think that was too bad

 

[QBhoy]

3%, yes. 7% is considered good and 3-5 is on the money and as good as it gets. It's taken a while though.
Not your average boat.
With an inch or two pitch either way she loses out by a few %. Same pitch and different props, she looses out. Incredibly lucky to find the right prop. They are elk certainly machines differently. Incredibly, even the same models and pitch have shown different results on my boat !

 

[QBhoy]

All even.

 

[MapisM]

Let's say I bother to find out, what can I do about it?

Well, in theory you could, based on those results, find better props by trial and error and eventually increase the propulsion efficiency - which as I understand is what QBhoy did.
And if (as is often the case) your props are not too far from ideal, a good prop shop could just adapt them, avoiding the need for a (pretty expensive) replacement.

Now, if you ask me if it's worth bothering for a biggish, heavy and not so fast boat, the answer is bound to be no, I reckon.
But if we should only discuss things which are REALLY worth doing, the asylum would lose a major chunk of threads...

And since you mentioned 26.5 @ 2100, if you know also your prop pitch and g/box + V-drive ratios (which obviously are not "variable" numbers, and you can surely find the latter on the g/box & V-drive plates, though the pitch might not be so easy, unless you find it in the boat papers), I can give you the percentage of prop slip if you wish - just a matter of throwing these numbers in a spreadsheet I made years ago.

As an aside, 250 short on max rpm ain't so trivial in my books, even for 23yo engines.
The big V12 lumps that BartW has got in BA are even a tad older than that as I recall, but current pump problem aside, they still push 2300 as they should.
And unless I'm mistaken, they did also before the rebuild.
Was Seralia short on max rpm also upon your initial seatrial?

 

[MapisM]

Slightly o/t, but somewhat related

Btw, am I the only one asking for a used boat the engine manufacturer report from the initial sea trial, when the boat was new?
Coming to think of it, every other week we read of someone worried about invoice and VAT when purchasing a used boat, and nobody so far asked anything about this - imho much more relevant - document...?!?

 

[Orange]

Re: Slightly o/t, but somewhat related

Btw, am I the only one asking for a used boat the engine manufacturer report from the initial sea trial, when the boat was new?
Coming to think of it, every other week we read of someone worried about invoice and VAT when purchasing a used boat, and nobody so far asked anything about this - imho much more relevant - document...?!?

Can you explain how this would be useful for a newbie like me please?

 

[Divemaster1]

L.....

Did get 26.5knts out of her this week after the relaunch, still 250rpm down on max (2100 vs 2350) but for 23 year old engines I didn't think that was too bad

Jez, you should not be down on RPM just because of age .... my DD's are older than yours and still get to 2300 WOT under load .... did you ever install mechanical gauges in engine room to verify what the electro-mechanical ones indicates at the helm ??

 

[MapisM]

Re: Slightly o/t, but somewhat related

Can you explain how this would be useful for a newbie like me please?

Well, it's a bit technical, but in a nutshell every manufacturer of marine engines require an initial sea trial, made with one of their official engineers onboard, to check that the installation is kosher and all the engines parameters, while running through their whole rpm range, are correct. Without this, the engine manufacturers do not validate their warranty.
Therefore, it's also in the interest of boat builders, who must give a full warranty on the whole boat anyhow, to cover their own ass vs. potential issues with engines during the warranty period.
In fact, I never heard of any builder skipping this test - though I am aware of builders who don't give a copy of this report to the first owner of the boat, unless specifically asked (shame on them of course, but I'd rather not name names on that).

Now, the relevance of this document when purchasing a used boat is rather obvious, I think.
Not only you have an evidence at arm's length (being done by a third party - the engine manufacturer specialist, whose interest is to find and highlight any potential problem that might affect the warranty) that the engines installation was done properly, but you also have a detailed report of the operational parameters and performance of the boat when new.
I mean, by knowing all the numbers at any given rpm from idle to WOT (speed achieved, engine and g/box temp+press, exhaust backpressure and temp, load... The list goes on), when you make a seatrial on a used boat after several years, you can easily compare the current results with how the boat performed when new.

But I have a funny feeling that while most buyers are obsessed by the most useless piece of paper associated to the boat - i.e. the original invoice - very few are even aware that the initial test must exist, let alone ask a copy it...

 

[Bouba]

Re: Slightly o/t, but somewhat related

Well, it's a bit technical, but in a nutshell every manufacturer of marine engines require an initial sea trial, made with one of their official engineers onboard, to check that the installation is kosher and all the engines parameters, while running through their whole rpm range, are correct. Without this, the engine manufacturers do not validate their warranty.
Therefore, it's also in the interest of boat builders, who must give a full warranty on the whole boat anyhow, to cover their own ass vs. potential issues with engines during the warranty period.
In fact, I never heard of any builder skipping this test - though I am aware of builders who don't give a copy of this report to the first owner of the boat, unless specifically asked (shame on them of course, but I'd rather not name names on that).

Now, the relevance of this document when purchasing a used boat is rather obvious, I think.
Not only you have an evidence at arm's length (being done by a third party - the engine manufacturer specialist, whose interest is to find and highlight any potential problem that might affect the warranty) that the engines installation was done properly, but you also have a detailed report of the operational parameters and performance of the boat when new.
I mean, by knowing all the numbers at any given rpm from idle to WOT (speed achieved, engine and g/box temp+press, exhaust backpressure and temp, load... The list goes on), when you make a seatrial on a used boat after several years, it's extremely interesting to compare the current numbers with how the boat performed when new.
But I have a funny feeling that while most buyers are obsessed by the most useless piece of paper associated to the boat - i.e. the original invoice - very few are even aware that the initial test must exist, let alone ask a copy it...

I thought that kind of sea trial was the reserve of superyachts. To be honest I would hate it to happen to à boat of mine, one of the reasons I like to buy new is because I'm fastidious about running the engine in according to the manufacturer's specs. I don't even like the 'handover' stage when the dealer revs it up to show you how it fast it goes.

 

[MapisM]

Jez, you should not be down on RPM just because of age

+1.
Btw, behind the situation Jez described, as Latestarter teached us (apropos, it's been a while since I read a post from him - I hope he's fine! Has anybody heard from him?), the engines could be running overloaded, with the governor trying to squeeze more juice in the engines to increase the rpm, but the engines not capable to comply, for whatever reason.
And in turn, that could mean too high EGT and potential engines damages, as I recall.
But happy to stand corrected if LS (or anyone else) know better!

 

[Portofino]

Re: Slightly o/t, but somewhat related

I think you are flogging a even more dead horse with this ,than my "hull form " post .
Mine was just visual eye --easy to do -just look at it
Yours goes a few steps further , g-box ratios , prop pitch -- speed ,rpm -- et al . If you don,t have the originsal slip no,s -- pretty pointless .

Best of luck for you --

 

[Portofino]

+1.
Btw, behind the situation Jez described, as Latestarter teached us (apropos, it's been a while since I read a post from him - I hope his fine! Has anybody heard from him?), the engines could be running overloaded, with the governor trying to squeeze more juice in the engines to increase the rpm, but the engines not capable to comply, for whatever reason.
And in turn, that could mean too high EGT and potential engines damages, as I recall.
But happy to stand corrected if LS (or anyone else) know better!

EGT,s guages are std on certain MAN engines ,my 2003 versions have them .
Yup agree elivated EGT,s are a diesel killer .
A dirty prop elivates them .
A overloaded boat too many Kg,s
A dirty hull elivates them

If you try to do the same speed .

On an old engined boat thats a keeper ,worth fitting .

 

[MapisM]

Re: Slightly o/t, but somewhat related

I don't even like the 'handover' stage when the dealer revs it up to show you how it fast it goes.

That's precisely what the initial test is NOT meant for.
Checking max rpm/speed is of course one part of the test, but there's much more than that - one of these reports that I just got is made of 7 pages full of numbers, go figure!

Btw, the fact that you say "the dealer" makes me suspicious.
The dealer you bought the boat from might well be onboard of course, but the test I'm talking about has to be made by an official engineer of the engine manufacturer technical assistance network - which must also sign it.
Definitely not something you can only expect when buying a superyacht!

 

[MapisM]

Re: Slightly o/t, but somewhat related

Mine was just visual eye --easy to do -just look at it
Yours goes a few steps further , g-box ratios , prop pitch -- speed ,rpm -- et al . If you don,t have the originsal slip no,s -- pretty pointless

Well PF, good for you if it worked out.
I understand that you didn't ask a MAN engineer to survey your engines?
Or even if you did, you didn't ask him to collect the initial test that MAN must have kept in their database - otherwise you would have all the original numbers that you are mentioning.

Nothing wrong with that, mind.
I understand that you are happy with your purchase, and that's the only thing that matters, at the end of the day.
Me, I'd rather do in advance all I can in order to be reasonably sure beforehand that I will be happy too...

 

[MapisM]

EGT,s guages
...
On an old engined boat thats a keeper ,worth fitting .

That's already in my upgrades list! :encouragement:

 

[BartW]

....Also the fact that nobody answered so far, makes me suppose that among the owners of similar boats there's not much interest, or even just curiosity about it...

I don't think its a lack of interest, but most posters have something else to do between 2 and 9am on a Saturday morning, than lurking on the forum


back to your OP,
perhaps you remember that we have tried to calculate the slip of BA's prop but the result was a unrealistic number,
so one of data we used was probably not correct,
now the point is that indeed we (me) don't have compare-able data from similar boats,

on the other hand, for me, the reference that is significant for me is:
how many knots does the boat at 2000rpm (fast cruising speed)
or how many rpm do I need to achieve 20kn
AFAIK this is in exact relation to the prop slip.

this ratio is influenced by:
- change of total weight of the boat (adding big permanent items like swim platform, scuba gear, or variable items fe fuel, water , guests, ...)
- weight distribution on the boat
- adding more drag (fe Stabs)
- fouling of the hull and props
- wind and sea state

in each situation, I check what speed I get at 2000Rpm
and biggest problem in doing realistic comparisons is that several of the paramaters above are different in other situations

this influence on speed is compensated in more or less prop slip (I think),
so yes, prop slip is relevant and important for me,
but I use another "value" as a reference point; rpm/speed ratio.