Now – after getting some additional information, the customer informs me the rig is under a heavy load and thus he is looking to increase RPM for enhanced hole shot and mid range acceleration to carry the load. Let’s see what happens when we drop the propeller pitch by two inches. Typically, one inch change in pitch affects engine speed by 150 RPM. In this case – dropping two inches of pitch will increase engine speed by 300 RPM. I plug in the new RPM and pitch size into the app – leaving everything else the same. The engine is running at the upper end of it operating range. This will provide the added thrust needed for the enhanced hole shot and mid range performance the customer desires. Note the top end speed drops by 2 mph.
The customer sometimes runs under lighter loads and he is concerned with it being so close to the rev limit using the smaller wheel. Lets see what happens when we go back up one inch in pitch. I go to the app and change the pitch to 24 and decrease the engine speed by 150 RPM. I again select Actual Speed and find a gain of one mph in top end speed.
Let’s see what happens when we go down one-half inch in pitch. Knowing the engine speed will increase 75 RPM, I change the RPM to 5925 and pitch to 23.5 and select the Actual Speed button.
Top speed is comparable to what is achieved with using the 24″ pitch prop. The higher engine RPM will enhance the hole shot and mid range performance. There is enough of a gap between the actual engine speed to the upper engine operating range to allow for occasional light load applications without worrying about hitting the rev limit. This is the prop I would recommend for this application.
Here I wanted to share a couple of examples. The first is regarding a boat originally equipped with twin 800 h.p. engines coupled to our dry-sump M6 drives. The customer has supplied us with gear ratio, engine RPM, pitch and speed. By inputting our known data, we find the slip to 12.83%.
The customer then decides to have his engines rebuilt and updated to 1,000 H.P. He would like to know the pitch size he could run with the updated engines.
Running a conservative calculation we can expect a 10% increase in speed (1.10 x 103 mph) for a total of 113 mph.
The new engines need to rev to 5800 rpm. I plug 5800 into the App and change mph to 113. Keeping everything else the same and select pitch, which changes to 35.42″ pitch. Here, I would suggest he start out by running his existing 36-inch pitch cleavers. He can break the motors in and then check wide open throttle to see if he can get to the recommended 5800 RPM. If he gets there and finds he has more throttle – he can consider higher pitch props.
Using the old general rule of thumb, it takes 10 additional hp for a gain of 1 mph – for both single and multiple engine applications. Remember – this is the old rule of thumb. In this case – we bumped the power from 800 to 1,000 h.p. or 200 h.p./10 h.p. = 20 mph.
Let’s see what the results are with that. When I change the speed to 123 mph in the slip calculator the pitch comes in at 38.55-inch. Now the customer needs to make a decision to go up to either 38-inch or 39-inch pitch props.
For this example I wanted to share an example of an air entrapment vee-bottom hull and the how it affects propeller performance. The subject boat is a 42-foot Fountain equipped with twin 525 EFI sterndrives. The customer provides me with the baseline information. He tells me his is running 34-inch pitch Lab Finished Bravo I props with a 1.50:1 gear ratio. The 525’s turn 5200 RPM at wide open throttle. His slip is high at 22% – resulting with a top speed of 87.05 MPH.
Stepped hulls such as that featured on the Fountain aerate the water just forward of the propeller blades. This creates slip as the propeller is not large enough overall – or in blade area specifically – to grab clean water. Here I would suggest the customer switch from the four blade Lab Finished Bravo I to the five blade Lab Finished Maximus. Let’s go to the app and see if what we find out.
I know from experience the pitch will go down two inches when switching from the 4 blade Bravo I to the larger 5 blade Maximus. I also know the the slip will go down to approximately 12%. I plug in the constants; 5200 RPM, 1.50:1 gear ratio, and revise the pitch to 32-inches and plug in the 12% slip. The resulting top speed calc is impressive and representative of what the Fountain with 525s is capable of producing. The customer is pleased to learn he will gain five MPH when switching to the Maximus. He’ll also gain that speed in the mid-range offering a great cruising speed.
I hope these examples provide a sense of the various calculations you can perform with our new app. Remember, the calculator is a tool to help us better understand a number of variables. It provides a base from which to test the results for your particular application.