1999 4100 SCA Prop size

[njlenman]

Looking for the correct prop size and pitch for a 1999 4100 SCA with cummins 6bta 370hp. Also 3 or 4 blade ???

Thanks
Lenny

[MAXMAXUM]

For the 4100 SCA with Cummins 6BTA 370s, the “typical” setup from the factory was 22” x 23” 4-blade Nibral props.
That said, the right prop depends on how your boat is loaded (fuel, water, gear, dinghy, etc.) and whether you want max cruise efficiency or top-end speed.

Rule of thumb:
4-blades give you better hole-shot, smoother ride, and improved mid-range efficiency (most owners prefer them).

3-blades can give you a touch more top speed, but you’ll usually lose efficiency and bite.

Make sure at wide open throttle you’re hitting the Cummins spec (around 2800 rpm WOT). If you’re under-revving, you’re over-propped; if you’re over-revving, you’re under-propped.

Best bet: start with 22x23 4-blades as baseline, then fine-tune with a prop shop after a sea trial.

[njlenman]

Sorry i didn't respond right away... That said I have my boat out of the water while I'm on vacation to have the props tuned.

While put on the bench (comp), the shop told me that they were not 22x23, but rather 24x26. Then I did a search on google and it also said the props were supposed to be 24x26. Can anyone verify what is the correct prop size and pitch for this boat.

At sea trial I was able to reach 2900/port and 2800/strb WOT rpm's. Now I can't get past 2700/port and 2600/strb WOT, and that's with all running gear and bottomed cleaned. I can't understand the change in 2 month's time and I'm not sure what I should ask the shop to do with the props...

[shrew]

Originally Posted by njlenman

Sorry i didn't respond right away... That said I have my boat out of the water while I'm on vacation to have the props tuned.

While put on the bench (comp), the shop told me that they were not 22x23, but rather 24x26. Then I did a search on google and it also said the props were supposed to be 24x26. Can anyone verify what is the correct prop size and pitch for this boat.

At sea trial I was able to reach 2900/port and 2800/strb WOT rpm's. Now I can't get past 2700/port and 2600/strb WOT, and that's with all running gear and bottomed cleaned. I can't understand the change in 2 month's time and I'm not sure what I should ask the shop to do with the props...

Diameter is RARELY played with. There are some calculations for estimating diameter. The key is around the amount of space between the blade tip and the hull. Too small of a gap and you start to get cavitation. The evidence of cavitation can be on the gel coat or paint where the prop spins and on the prop blades themselves. Any evidence of wear and you're getting cavitation.

Number of blades and pitch is where you're going to tune the Max RPM. Likely the difference between the sea trial and now is the weight of gear and fluids. The sea trial was likely done empty (when boats perform at their best) and now it is loaded with gear.

What is the recommended max. rpm and continuous operation RPM for that engine?