Prop Slip Calculation

[Manos]

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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