WOT

[bobtookmm]

Is anyone actually achieving a WOT of 2400 rpm? Did you have to make adjustments to get there?

[ndutton]

Yeah, I can reach 2600RPM (direct drive) but I don't usually run that high. Keen attention to timing, a clean carburetor and a 10x8 fixed two blade prop is my recipe.

https://www.moyermarineforum.com/forums/showpost.php?p=108158&postcount=6

[edwardc]

Not really a direct comparison, but I run a 3-blade 15x8 fixed prop through a 2:1 v-drive, and I can achieve 3100 rpm in gear at WOT, pushing my 13,000 lb P323 at about 6 1/2 kts.

[indigo]

Higher RPM

There are two main reasons to achieve higher WOT engine RPM with your direct drive Atomic 4.

The first has to do with increasing cruise RPM to somewhere around 2000. Many A4s in the fleet are cruising at about 1500 RPM and as such, the boats are moving along at 5 to 6 knots and the engine is not far from WOT. This is somewhat acceptable on a calm windless day however the problem with this scenario is that the engine is “lugging”. Think of it as driving your car uphill in fifth gear, it does it but the engine is working very hard to do so and it’s not particularly happy as far as cylinder pressures, oil pressure, coolant flow, and alternator output. It would be much better to downshift to a lower gear and get the RPM up. That’s exactly what going to a proper prop can do for your engine. Your boat requires a fixed amount of power to achieve a reasonable cruising speed. You can achieve that amount of power output from your A4 at many different RPMs depending on the prop. Higher RPM, within reason, are much better for the A4.

The second main reason for increasing WOT RPM is to give you A4 the opportunity to produce more power for bad weather situations. If you were able to get your cruise RPM up to around 2000 at 5.5 to 6 knots with the correct prop, you could then go on up to perhaps 2400 RPM when you hit bad weather. By bad weather I mean heavy waves and or heavy wind on your nose so that the boat requires significantly more power to make it through those conditions.

So with a higher WOT, my boat will go a lot faster, right? Unfortunately not. Our sailboats are what is known as a “displacement hull” as opposed to a “planing” hull (like most power boats) and as such, the maximum speed through the water is limited by the hydrodynamics of the hull. In general, maximum theoretical hull speed of a clean hull is calculated as 1.34 times the square root of the waterline length (in feet). To achieve that speed on a calm day requires a fixed amount of power. If you put more power into the water, the hull will simply “squat” and you may gain perhaps 1/4 knot regardless of how much power you apply.

When fighting heavy waves or wind (not current), the boat will slow down considerably and now additional power can allow you to increase speed to get home or out of trouble. Since the boat speed is slower in this scenario, it is to your advantage to have a prop with more blade area or blades to get that extra power into the water without prop cavitation. One very popular solution is the Indigo prop with 3 blades, 10” diameter and 7.4” of pitch. More than 1900 of them are in service on direct drive A4s in boats from 25 to 38 feet in length.

I hope this sheds some light on one of the more frustrating characteristics of the direct drive A4.

Tom Stevens