Questions regarding prop tuning on 6bta M1 250hp

[Bill Stenger]

Hello you all, as always I am super grateful for all the information and time you knowledgeable guys invest here. This is a 7k lb 28′ sea vee center console with 6bta 250 M1 cpl 1570.
Like a dummy while loading my boat on the trailer I hit the prop on the trailer bunk. And obviously bent the ends of the wheel. While at the local very well regarded prop shop (Bobby Soles). He wanted to run numbers to reprop the boat because the current prop is only .25″ away from the hull. He wants desired rpm to be 2650. Not 2750 like I read here as to not overload the engine. I have read and read here. Everything I read points to 2750 as the desired normal load operation. Just verifying to be firm that I want to see 2750 with the prop guy or not. I feel he s pushing me to be satisfied with 2650. The boat currently has a 19″ x 22 wheel and runs 2850 wot. His specs show we should move to a 17″x 25 prop. That will give the boat 1.25″ between hull and prop. Opinions??

1 user thanked author for this post.

Darren

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[Bill Stenger]

Thank you again Tony! More questions I have read the tachometer calibration chart on here. Just set the open and closed dip switches according to the chart or do you sometimes have to play around with different combinations? Reason I ask is I pretty much have to sit on the turbo to adjust it.

https://www.sbmar.com/articles/beede-tach-settings-cummins-flywheel-tooth-count/

Also currently I only have .25″ max between the prop and the hull with a 18 7/8″ prop and already show some cavitation damage. Prop guy is recommending a 17″ prop which would only get me about 1″ prop to hull clearance. What is the minimum prop to hull clearance thats acceptable? And what are the ill side effects of the lack of clearance? Speed, efficiency, hole shot? My main priorities are safe engine load, fastest cruise speed possible, and efficiency. It gets over the hump with ease at any rpm currently.

[Tony Athens]

CPL 1570

Spec when built are attached.. 2850-2900 NO- LOAD for this engine is the spec & where it should be. GOOD.

Under load, IMO, 2700-2750 would be perfect as to safe engine loading unless you are trying to squeeze another Kt or so more at WOT… That would be your call & should not be the prop guys call.

Believe your photo tach and reset your Cummins tach to match– Use 2400 RPM in neutral at the dock for calibrating it.

Tony

[Bill Stenger]

Thank you Tony. The data plate shows 2600 High Idle Speed. The serial number is 45388094. Using a photo tach purchased off Amazon no load it spins up to almost 2900 rpm. Photo tach was jumping around between 2850 and 2902 if I remember correctly. I got my prop repaired and did some under load testing results below. The tach is not accurate unless the phototach is inaccurate. Now I’m unsure which tach is inaccurate. At no load high idle the dash tach will show 2600. Which I thought was accurate until I got this phototach telling me it isnt.

Below tests are with the boat light no ice, fish, gear, and half fuel level.

Wot under load. 2750 Photo tach. Dash tach reading 2550.
Photo tach @ 2400 Dash tach reading 2250.

[Tony Athens]

6BTA CPL 1570

Let’s start with a serial number as the CPL 1570 came in two fuel ratings.. I do not believe either of them would allow the engine to spin up under load to 2850————–Tach accuracy needs to be verified at the dock in neutral at a steady 2400-2500 RPM and then a blip to see what no-load WOT is..Write it down !!!

Once I look up your engine, I’ll get back.. As to your prop guy making the call, if I don’t even know you true FR #, how could be even make a suggestion??

Tony

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