Power Sensibility index

[Kerry]

So lets derive a "Power Sensibility index" (for want of better wording at this time) based on some combination of the typical boat/vessel components such as

Length
Beam
Weight
Horse power
Other ??

Open to suggestions if the high or low index figure depending on the most appropriate "index combination" adopted from the panel.

Note: "index combination" is a more appropriate term than the word formula which has some bad under tones in this instance)

So "index combinations" please if you would
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[opti]

i take it you me for outboard power

[stevej]

Length 5m
Beam1.9m
Weight 400kg
Horse power 70hp
make ally runabout

ideal top speed 50kph

[Deepsix]

Waterline length (metres)x waterline beam (metres) hull only draft (metres) x (desired cruise speed in knots, squared). Then divide that by 10 to get the "sensible" horsepower for the vessel.

[Kerry]

[Deepsix]

Correction ( missing "x")

Waterline length (metres)x waterline beam (metres) x hull only draft (metres) x (desired cruise speed in knots, squared). Then divide that by 10 to get the "sensible" horsepower for the vessel.

[lodka]

[Deepsix]

What is the "standard equation" ?

[frozenpod]

That formula cant be right, gives a suggested HP of 300 for our 520 and 418 for our 650.

/by 20 instead of 10 and it comes out about right.

[frozenpod]

Standard is below.

MAXIMUM POWER CAPACITY
2.6.1 Determination of maximum power capacity (outboard installations)
The maximum power capacity of boats, other than pontoon boats, shall be stated in kilowatts and shall be determined by the following method (see Table 2.2):

(a) Calculate the factor for use in Table 2.2 by multiplying the overall boat length in metres by the transom width in metres. The transom width shall not exceed the widest measurement of the transom in that part which is below the point of ingress of water. If spray rails act as chines or part of the planing surface, they may be included in the transom width, but otherwise fins and flare shall be excluded. Where a boat has a rounded stern, for the transom width substitute the maximum width below the static
float line measured at a point one quarter of the boat length forward of the stern.

(b) Use the factor to determine the corresponding maximum power capacity direct from Table 2.2, or, if the factor is over 5, calculate the maximum power capacity by multiplying the factor and subtracting as shown in the table. If the factor is over 5, and the calculated power capacity is not a multiple of 5, the calculated power capacity may be raised to a multiple of 5 to accommodate power ranges of stock engines. For flat-bottomed hard chine boats, the maximum power capacity shall be reduced by one increment (of power capacity in Table 2.2) for factors below 5.

(c) The power capacity determined in accordance with Items (a) and (b) above shall be adjusted in accordance with Clause 5.2.3.

TABLE 2.2
DETERMINATION OF POWER CAPACITY
Type of boat Factor Power capacity, kW
≤2.25 1.5
>2.25 ≤3.3 3.0
>3.3 ≤3.6 4.0
>3.6 ≤3.9 5.25
>3.9 ≤4.2 7.5
All types (except flat bottom, hard chine boats)*
>4.2 ≤5.0 12.0
Remote steering and 500 mm transom or equivalent >5.0 [(16 × factor) – 67]†
No remote steering or transom less than 500 mm or
equivalent—
Flat-bottomed hard chine boats >5.0 [(4 × factor) – 11]†
Other boats >5.0 [(6.5 × factor) – 20]†
* For flat-bottomed hard chine boats with factors below 5, the power capacity is reduced to the next lower
value.
† Power capacity is increased to next multiple of 5.

For most fishing boats I think the follow should be used.

Max power in kw (16*lenght*width)-67

For our 520 this gives 107kw which is about 150hp, max manufacture rating is 150Hp.

For our 650 (using factory spec beam which I think is slightly wider than the transom water line width) this gives 181kw which is about 250hp, max manufacture rating is 225Hp hence probably the result if you use the actual water line beam.