Prop Slip Calculation

[Manos]

Ich hoffe, daß Sie dieses Interessieren finden

Yamaha 200VMax 2002
Carburated
Cubic Capacity 3100cc
Reduction Gear Ratio 1.81

Optimum rev range 4500 - 5500

@ 4000 revs actual speed 26 knots
@ 5100 revs actual speed 31 knots
@ 5300 revs actual speed 34 knots

Prop Yamaha Saltwater Series II 15 1/4 x 17"

Slip calculation

VT (theo speed) = (revs per min/reduction ratio) x [(pitch x 0.02540) x (60/1852)]
VR (actual speed)

%SLIP = [(VT-VR)/VT] x 100

VT = (4000/1.81) x [(17 x 0.02540) x (60/1852)] = 2209.94 x 0.4318 x 0.03240 = 30.91 knots

% SLIP = [(30.91-26)/30.91] x 100 = 15.88% SLIP @ 4000 revs

% SLIP = [(39.42-31)/39.42] x 100 = 21.36% SLIP @ 5100 revs

% SLIP = [(40.96-34)/40.96] x 100 = 17.00% SLIP @ 5300 revs

[nordy]

Hello Manos,

I suppose the 4000 rpm-slippage is not with full throttle.

But the other seem full throttle values to me.

It would make sense because slippage decreases with less load (or torque).

Am I right ?

Greetings Nordy

[Manos]

Correct Nordi.

You measure slip at the high revs at WOT (full throttle).
But even so the slip is too high at 17%
In my opinion it should be not more than 8-9% more than this and there is a problem.

[nordy]

Manos, slippage mainly depends on deployed torque and driving resistance.

If you have a light rib driven by one person slippage would be around 5 % with correct prop geometry, maybe a little % more or less.
Generally the greater the prop's diameter the less the slippage.

With 3 or 4 persons it would be normal for a light boat slippage would rise up to 15 -20 % with considerable less boat speed but only with a little decrease in engine speed due to the bigger slippage.

Tell me what's the weight of the boat, what kind of boat is it.

If the weight tends to be little with respect to the power, a greater diameter could help.

Greetings Nordy

[Manos]

Thanks Nordy

The boat total weight with 550 lts of fuel, 150 ltrs water and 4 person on board is 2998 kgs.

The boat is a MAR.CO 25 Cabinato Special edition
If you click on my signature you can many photos and videos of this boat.
Right now is very difficult to make a decision to change an engine (which is going to be expenssive) or just change the prop to a 15 3/4 x 15 Ymaha Salt Water Series II?

[nordy]

[QUOTE][The boat total weight with 550 lts of fuel, 150 ltrs water and 4 person on board is 2998 kgs/QUOTE]

The weight is really 3 tons ?

That would result in a rather high weight/power-ratio of 15 kg/PS.
This figure is rather high compared to my boat and maybe the reason for the high slippagge.

I think ratios of 10 kg/PS are more typical for comparable big ribs, aren't they ?

Greetings Nordy

[Manos]

Well yes it is so heavy.
The boat weight with just engines and no water and no fuel is 1998 kgs.
If you add 550 ltrs fuel and 150 ltrs water and say 4 persons about 90kgs each is near to 3 tonnes.
Is a hell of a havy boat .... but I love it

[nordy]

Hallo Ribfahrer,

könnt ihr nicht mal sagen, wie gross die Leistungsgewichte eurer Boote im fahrbereitem Zustand bei einer Person sind ?
Mit 15 kg/PS dürfte Manos sehr schlecht liegen.

Also nur das Fahrgewicht durch die Leistung teilen.

Gruss Nordy

[Manos]

Hallo Nordi

Kann ich brauche eine größere Maschine sein.

Was ich werde machen wenn ich fahre nach Griechenland ich das Boot auf eine industrielle Gewichtmaschine stellen werde, und wird das GENAUE Gewicht mit vollem Kraftstoff, Wasser und 4 Personen finden. Ich weiß dass ohne Benzin, Wasser und Leute an Bord, aber nur die Maschinen (Hauptleitung und auxilary), ist das Gewicht 1998kgs.

**********

May be I need a bigger engine.

What I will do when I go to Greece I will put the boat on a industrial weight machine and will find the EXACT weight with full fuel, water and 4 persons. I know that without petrol, water and people on board, but only the engines (main and auxilary), the weight is 1998kgs.

[DieterM]

Zitat von nordy

Hallo Ribfahrer,

könnt ihr nicht mal sagen, wie gross die Leistungsgewichte eurer Boote im fahrbereitem Zustand bei einer Person sind ?
Mit 15 kg/PS dürfte Manos sehr schlecht liegen.

Also nur das Fahrgewicht durch die Leistung teilen.

Gruss Nordy

Peter, das hatten wir doch schon, schau mal hier

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Ich finde das Leistungsgewicht bei dem großen RIB von Manos auch nur gerade ausreichend. Überraschend für mich ist, das dieses Boot nicht mit einem Innenborder ev.Diesel-Innenborder ausgerüstet worden ist, wie es bei den großen MARLIN RIBs der Fall ist. Bei MARLIN werden ab diesem Jahr alle Innenborderversionen auch mit Diesel-Innenborder geliefert werden können. Die MARLIN29' Version ist sogar mit Doppel-Innenborder lieferbar!

Ich finde in dieser Größe ist ein Outboarder oder auch zwei Outboarder die falsche Lösung.

Vielleicht sollte sich Manos eine Doppel-Motorisierung zulegen, das geht aber auf den Sprittverbrauch. In einer Zeit, wo die Spritpreise nur mehr steigen, wäre er besser bedient mit einem oder doppelten Diesel-Innenborder, dann hätte er auch den Vorteil recht billig Diesel in Griechenland tanken zu können.

Hi Manos,

please have a look to this side in UK
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These italian RIBs (I'm driving the smallest MARLIN16'!) have a very active agent in UK as well a smaller one in Greece.
To powerup your actual MARCO 25' RIB, I would suggest to study a twin outboarder equipment with the manufacturer in Italy, although the fuel consumption would increase. 200 HP Outboarder for upto 3 tons boatweight is poor, sorry. 2 x 200HP would be more reasonable.

The change to a RIB with cabin and an Innboarder-Version in Diesel would the better solution and save by the way a hell of fuel costs.

[OLKA]

Zitat von nordy

Hallo Ribfahrer,

könnt ihr nicht mal sagen, wie gross die Leistungsgewichte eurer Boote im fahrbereitem Zustand bei einer Person sind ?
Mit 15 kg/PS dürfte Manos sehr schlecht liegen.

Also nur das Fahrgewicht durch die Leistung teilen.

Gruss Nordy

ca 5 kg/PS

[Manos]

Thanks Dieter

I am aware that the 200bhp is too little for the boat.
MAR.CO recommnds 2x150. I am thinking to put 2x175 SUZUKI ... I do not like inboards on a RIB although as you very correctly say they have less consumption than the outboard.

[DieterM]

Hi Manos,

I'm quite sure Peter is also agreeing with that 2 x 175 HP Suzuki. It will be the result for a better perfomance. Just a small recommendation, make it only if you have a good Workshop near to. A special service attention w'll be needed whilest the adjustments are sensible to run a twin smoothly.

Cheers
Dieter

[nordy]

Hello Manos,

I conclude the slippage values of your heavy rib are quite normal.

Does that mean your prop efficiency's inevitably bad ?

A converted automatic car with no lockup clutch has similar slippage values

resulting in slip-proportional loss.

Contrararily prop loss is not directly associated with slippage.

Prop efficiency depends on advance coefficient.

See here:

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You must align advance ratio by aligning diameter or pitch so it's under the
efficiency's maximum.

Nevertheless slippage may be 18 % or so.

See the difference to a car with 18 % that would never drive with optimum
efficiency.

Greetings Nordy

[Manos]

@ Dieter .... The problem with a reliable service engineer and spares with the SUZUKIS exists in Greece. This is the reason I am waiting to see if it will improv in the next year or so. If not I will go for Yamahas

@ Nordi ... Thanks very much .... Is as you say and I completely agree with you.
I may try a prop I just bought Yamaha Slatwater Series II 15 3/4 x 15. If this doew not work I will try any prop 14 and something x 17 see if this is better.

WIll let you know guys what happens.

Many Thanks fpr all your help