Validation and the Use of PropExpert for Small High-speed Powerboats

A HydroComp Technical Report
Report 116

Product link: NavCad, SwiftCraft, PropExpert

HydroComp PropExpert is a program for the selection and analysis of propellers for motor yachts and workboats. While it was developed with the inboard propeller in mind, this report validates its use for small high-speed powerboats - both recreational and commercial - and provides guidance on some of the more common pitfalls and concerns.

Predicting vessel weight

Accurate operating vessel weight is important. Weight is used to make predictions on potential vessel speed and other vessel data. Unfortunately, all too often the true operational weight of the vessel is not known.

First, it is important to understand what is - and is not - meant by the vessel's operating weight. It is the total weight of the vessel underway, including hull, power plant, fuel and consumables, passengers and any cargo. The following are things that are not weight:

Hull weight or "light ship" weight - This is the weight of the hull alone. It is typically found by weighing the hull on a scale.

Gross or net tonnage - This is particularly confusing to those not working with merchant cargo vessels. This is actually a measurement of the volume of usable cargo space. (The name came from the British "tun", a barrel of 100 cubic feet.) Regulatory agencies typically use gross or net tonnage to determine taxes.

Vessel

Length OA

Weight

Ratio

Top speed

BHP

Mariah 180

18'-0"

2125 lbs

223

53 mph

180

Carver 23

23'

5400 lbs

272

35 mph

185

Regal 23

23'-1"

3700 lbs

184

55 mph

330

Fibreform Continental

24'-2"

5175 lbs

224

29 mph

---

Blackfin 27

27'-9"

9850 lbs

282

46 mph

500

Deep-V patrol boat

35'

8500 lbs

121

38 mph

---

Sport fisherman

41'

12000 lbs

107

46 mph

---

West Coast Aquabus

50'-6"

32000 lbs

152

27.5 mph

540


In PropExpert, we have provided a non-dimensional relationship of weight and length - the Ratio field. To the right is a plot of some sample values from real vessels taken from the table above. The lower curve is from recreational powerboat data from a publication by Mercury Marine. The upper curve is typical of commercial working vessels. You can use these curves to find realistic figures for your vessels.

Vessel details

For sterndrives, we recommend the following values:

Wake fraction   0.03   Relative-rotative efficiency 1.00
Thrust deduction 0.00 Shaft efficiency 0.97

Evaluating top speed

Evaluating speed is always best using Prior trial data. To determine if the trial data makes sense, you can use the following two relationships. The first is a prediction of expected power based on weight and speed. The second is a "best vessel" minimum power curve. It is unlikely that a vessel would do much better than this best vessel curve.

BHP = Total installed engine power (hp)
W = Total operating vessel weight (pounds)
V = speed (mph)

Average vessel

BHP = (W * V / 4900) + (W0.833 * V2 / 11100)
Best vessel

BHP = W * (0.026 + 0.004*V)2

The following table shows predicted average and best horsepower versus the actual installed power.

Vessel

Weight

Top speed

Actual BHP

Average

Best

Mariah 180

2125 lbs

53 mph

180

173

120

Carver 23

5400 lbs

35 mph

185

175

144

Regal 23

3700 lbs

55 mph

330

298

224

Blackfin 27

9850 lbs

46 mph

500

496

434

West Coast Aquabus

32000 lbs

27.5 mph

540

566

592

Propeller parameters

Analysis of small sterndrive-style propellers suggest the use of the following definitions:

    Flat-faced (stock) propellers - GawnAEW, No cup, T factor = 0.99, P factor = 1.02

    Progressive pitch (high-performance) propellers - GawnAEW, Very light cup, T factor = 0.93, P factor = 0.95

    For additional cupping, raise the amount of cup two or three levels (e.g., from Very light to Light medium).