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The Nav Station...


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Yanmar Pleasure Boat Marine Engine Help

Boat performance

My apologies, this site does not cater for multi-hull or high performance monohulls

Firstly, " MEASURE THE RULER " most misconceptions are arrived at because a measurement used in the calculation was not correct. A guess was made at the boat weight, the waterline length, the 'measured' mile marker was missing so a guess was made as to it's position. The log was not calibrated, etc, etc. "My friend's boat does 25 knots, the GPS says so and it cant be wrong!" Wrong! First up, measure the rulers!

"An owner's expectation is often falsely generated by the sales pitch of the designer, the boat builder, the engine salesman and the owners acquaintance, who always 'knows' so little about so much, or was that so much about so little?"

Displacement hulls

A classic displacement  curve on this Davidson 42 built by Lloyd Stevenson. Click the picture for the full details.
Displacement hull: a hull designed to displace the water as it moves forward.

Displacement hull speed, simply put, is dictated by the waterline length. The longer it is the faster she'll go. When the vessel reaches 'hull speed' it will not go any faster unless horsepower is applied 'exponentially' to climb the bow wave.

The formula for working out theoretical hull speed is: = boat speed in knots.
e.g. The square root of a waterline length of 25 feet = 5 x 1.34 = 6.7 knots.

How much horsepower do I need for my displacement boat? Rule of thumb is 1 hp per 500lb total weight. or 1kW per 304kg. This gives adequate performance in most wind and tide conditions.

Planning hulls

Photo of Elite 14M launch
Planning hull: a hull designed to climb up and sit on the bow wave as it moves forward.

Planning hull speed, simply put, is dictated by the weight of the boat and the amount of horsepower available. When the vessel reaches planning speed it will climb on the bow wave. Planning speed appears to occur, on average, between 10 and 12 knots.

Planning hull speed formula: SHP = shaft horsepower at the propeller, WT = total laden weight of the vessel in long tons (2240lb) (1016.04kg) and K = factor for different hull shapes and loaded waterline length, in feet.

Values of 'K':
Loaded waterline length in feet Soft chine, round bilge, flat
at the transom
V-bottom, hard chine
20 2.25 2.75
25 2.40 2.90
30 2.60 3.15
35 2.80 3.4
40 3.05 3.65
45 3.24 3.85
50 3.34 4.00
55 3.45 4.10
60 3.53 4.20

So, a 'V' bottom, hard chine boat with a 30ft waterline length, weighing in at 10 tons with twin 230hp engines may do about 21 knots at full power. A reasonable cruising speed for this combination is about 17 knots.
The same hull with 170hp engines may do 18 knots at full power and cruise at 14 knots, just above planning speed. To go faster you have to remove weight or install bigger horsepower engines.

How much horsepower do I need for my planning boat?

As you can see, it depends how fast you want to go!

Many thanks to Len Gilbert of Dieselcraft Evaluations, Auckland, NZ for the information.

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