This is an interesting subject that is not always understood. Correct prop sizing for meeting Engine manufactures specifications. Most boat owners rely on the boat manufacturer to make the right call and prop the boat correctly.On average the boat manufacture does a good job. When the manufacturer designs a boat part of the design is how much power is going to be needed to push it. Normally the designed cruise speed should be no more than ( 80% load). The manufacture Sea trials the boat with a light load and confirms it meets the WOT specifications. Examples D3 220hp is rated @ 4000 rpm, minimum 3900rpm recommended, Max RPM 4150. D4 Volvo 260 is rated @ 3500RPM minimum 3400 recommended max rpm 3600. The marine engine is always under a load and propping properly is a very important component of maintaining the longevity of these high HP small displacement engines.
Quote from Tony Athens:
Proper propping of a vessel cannot be stressed enough, will typically only come some time after the vessel has settled in to “its” normal running conditions, and then only with an understanding by the vessel operator of the engine’s operating requirements published by the engine manufacturer. But in addition to that, is the understanding that adding some conservatism, “PRUDENT OPERATION”, to these operating conditions by not running “on the edge” of these requirements, can mean the difference between 500 hours between catastrophic failures, and 5000 hours of relatively glitch free operation for the same engine.
The easiest and most proven way to accomplish this “conservatism” is by propping the vessel at full working weight to rated RPM plus 100 to 200 rpm** when using these types of diesels that develop upwards of 50-60 HP per liter.( D3 220 hp 2.4 L = 91 hp per liter)( D4 260hp 3.7L = 70 hp per Liter and D4 300hp = 81 hp per liter.) It’s actually very simple when you understand the whole picture of vessel operation and all of the variables that occur on the ocean over time.
When we purchased our C26 and Sea Trialed, the boat performance was as advertised 4150 20.5kts Full fuel full water but we had not moved in yet so really no equipment on board. As equipment has been added and more supplies added for extended cruising, heavier batteries, dingy, outboard, extra" this and that" I'm down to 3980 rpm WOT good sea's, now add 2' sea's and some wind and I'm going to be below 3900 rpm. I don't run WOT but The WOT rpm gauges the load on the engine through out the rpm range. You can't down shift in a boat. A boat power plant is always climbing a steep hill. When it is propped right it can maintain climbing for years. If the max RPM is lower then the manufacturers specification the hill just got steeper and the need to down shift is present. Down shifting is only achieved by propping in a boat.
I like Tony's rule, prop the boat the way you have it equipped give some extra room in the rpm range for the times you are in worsen sea conditions or extra passengers on board. The engine manufactures provides max RPM specifications for a reason.
I'm going to finish this season with the present sized prop. I normally advance to WOT for a short period of time each time out to see max rpm. I will have the prop tuned this winter based on my data or purchase a spare with different specs. I'm shooting for 4100 WOT loaded the way we cruise.
