Engine Selection, Propeller sizing for a boat repower


Repowering a boat using the boat speed calculator to calculate speed of boat

Boat Specifications:

  • Boat Type : Heavely Twins Catamaran 27 ft
  • Length at water line : 21.5 ft
  • Max Displacement : 7250 lb
  • Current Displacement : 6000 lb (approximately)
Heavenly Twins 27 catamaran

Initial Calculations

Using the boat speed calculator, David Gerr’s formula, I’ve calculated with the above figures, that the vessel has a theoreticalSL Ratio of 1.37 as opposed to a traditional monohulls SL Ratio of 1.34. The reason that this boat can feasibly go beyond the traditional displacement hull speed is that it is lighter than the traditional monohull. This gives a maximum feasible hull speed of 6.33 Knots. This speed would however require around 17HP.

Looking at the hull speed calculator’s power required chart, the first thing that jumps out is the power required rises almost exponentially as speed increases. For example 5.6 Kts (88% of top speed) only requires 10HP! You need to more than double the HP to get that last 20% of top speed, hardly seems worth it. But that of course, that depends on the particular application and requirements of the owner. For a number of reasons however, in my case, it is not.

boat speed chart 20 hpboat speed chart 10 hp

Current/previous engine installation

I’m not sure who installed the motor, or what they were thinking, but it’s almost a perfect example of how not to do it. The motor was sized and mounted incorrectly and the propeller was completely wrong. A 30HP, regular go fast two stroke outboard, hanging off the transom, which incidentally wasn’t designed to hold a motor of any size. The result was an extremely inefficient and problematic arrangement. By the results, a motor with 1/3 of the HP and using less than ¼ of the fuel pushes her at the same top speed!! This is simply due to a more appropriate gear ratio (a go slow outboard) and propeller selection.

Engine Selection

I’ve determined from the calculations that a 10HP engine is sufficient for my application. The reasons I am not putting in the full 20HP engine are: Firstly, being a catamaran she is very weight sensitive and I would like to take her on extended cruses which means more water, provisions etc., I could use the extra 100lb in engine weight and the weight of the additional fuel needed. Of course, some would argue that the additional 10HP is for safety, so I could put a 20HP engine in and only use 10HP but then the propeller would be inefficient at ten because it’ll be sized for twenty and knowing myself, if I had twenty I would probably use it. Secondly, cost, I managed to get my hands on a new, old stock, 9.9 Yamaha high thrust four stroke for a reasonable price. I would of course have preferred a diesel but this with installation and the myriad of marinization parts would have been cost prohibitive (almost double).

The high thrust (HT) label on the outboard is of key importance in this application, simply for the slower shaft speed that it provides, which allows for a larger and more efficient propeller. A standard two stroke of the same size would have provided very different results. Under around 35 knots, which is where most of us spend our time, bigger is better… that is to say, a larger diameter propeller is more efficient, but in order to swing a larger diameter prop you need a lower gear ratio i.e. slower shaft speed hence the “High thrust”. Considering that on displacement hulls propellers are generally around 55% efficient and often less, propeller diameter is the single biggest determinant of overall efficiency.

Major outboard manufacturers these days rate the horse power at the shaft, so I don’t have to compensate for friction losses in the gearbox or exhaust back pressure like I would with a diesel. This 9.9HT engine should give me close to 10HP at the shaft or SHP.

Yamaha High Thrust 9.9

Propeller selection (see David Gerr’s propeller handbook, chapter 6)

Using the Bp-Delta charts produced from propellers of various sizes, I’m able to determine if the stock outboard propeller is sufficient or if I need to go find a closer match. The stock propeller is an 11.25 inch diameter and 9.75 inch pitch. First, speed of propeller advance is different to boat speed through the water. Due to the fact that friction of the boat and its appendages with the water drags water along and the propeller moves slower than the boat through the water. The wake factor can be calculated from the block coefficient. In this case however the propeller is below the center cockpit between the hulls as opposed to behind a hull so will be moving much closer to boat speed though the water. Probably around 99%? Using the power factor and pitch ratio on the Bp-Delta charts, gives me an efficiency of around 45% and a speed of around 5.5 Knots which is reasonable but not optimal. As always, a larger diameter prop with a slower shaft RPM would be better but with this being an outboard it is not possible to change the reduction gear. In addition the propeller has a little too much pitch, so the propeller power curve will cross the engine power curve at around 90% which means the pitch will actually bring the top RPM down overloading the engine a bit. But it’s the closest fit available from Yamaha for this motor, so it’s going to have to do, and the results are satisfactory.

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Field test

The test results are promising with the current displacement approximately 6000lb on a sunny morning with a touch of wind on Ensenada harbor:

  • At ½ power (5HP) 4.6 Knots
  • At ¾ power (7.5HP) 5.4 Knots
  • At full power (10HP) 5.8 Knots

The calculations using the boat speed calculator with estimated current displacement (6000lb) are pretty close 4.6, 5.35 and 5.83 Knots respectively. The calculations assume 55% efficiency at the prop, so they are pretty damn spot on. Fully loaded, she should max out around 5.5 Knots which is good enough for a snail, even a psycho one.