Cavitation - 2008 Etec 250 HO

[tbng381]

Cavitation - 2008 Etec 250 HO

Evinrude ETEC 250 HO
S/N: 05227273
Model E250DHLSCS
'08 Year Model
Prop: 3 blade, 14-1/2 x 24", Raker II prop with three 5/8” vent holes (Note: ~Prop to Pad Height = 5")

I just replaced my 2008 Lightening lower unit with the M/2 gear case. The problem is that I cavitate and spin out every time when trying to plane out once I exceed 4500 rpm, before getting up. Never had this problem before. I have a 3 blade, 14-1/2 x 24", Raker II prop, with three 5/8” vent holes, one near the front of each blade. I’ve also lost ~7 mph off top end. Any thoughts on how to fix these issues would be greatly appreciated!

[Travis]

Does that prop have a blowout ring?

[Travis]

Wait....plug couple of the holes with the PVS plugs

[tbng381]

No rings on the Raker2

I believe the PVS plugs are Mercury. No plugs that fit the RakerII.

[tbng381]

I plan to try and plug or cover some of the vent holes on the 24" Raker II prop and see how it works. Then I may try a 26" Raker II that has 3/8" vent holes. Also, will this engine perform and hold up with the M/2 gear case, even if I get a prop that works?

[JR19]

The lighting has always been 2-4 mph faster than the magnum so 7mph does seem excessive.

The Raker II has vent holes but I am not sure what size. You say yours is 5/8" but you also have another Raker II that has 3/8"...question has your prop with 5/8" holes been modified to accept Mercury PVS plugs. The reason I ask is because I had my 24 Raker II vent holes enlarged/modified to be exactly like Mercury PVS vent holes. If that is your case install a couple large plugs and see what happens. If closing the holes help, keep closing them until you find the sweet spot.

FYI vent holes has ZERO to do with top end speed. Your speed loss is to due to your new gearcase because the magnum's are slower than the lightings but at 7 mph that is an excessive loss so some of it might have to do with your prop to pad height. Once you get a prop that bites good on holes shot I would focus on finding the sweet spot for your prop to pad height. Do you know your current PTP.

[tbng381]

Thanks for the reply and information. The 5/8” vent holes on the 24” RakerII have not been modified to fit merc plugs. But I can temporarily plug holes and test it. I do have access to a 26” RakerII w/ 3/8” vent holes. I can try that as well. Would going to a 26” increase torgue and have any negative effect on the power head?

[JR19]

In "theory" the 26 should be more stiff to spool up on hole shot due to increase pitch. I will also add having smaller vent holes on the 26 might be an X factor that plays into you favor because with your current prop your getting too much ventilation causing it not too bite. You really want know how another prop runs until you give it a try.

Obviously spinning a motor over the max rpm is not good for then but I have also been told not spinning them hard enough to achieve max rpm buy lugging them with a prop that is too big is not good for them. Find out your recommended wide open throttle rpm range. If your numbers fall into the range your good to go. If they're low you need to find the correct prop. I have not done the research nor do I have the owners manual for your motor but as a pure GUESS I bet you max rpm range is in the 5500-5800 range. Again pure guess on my part so do some research to find out your recommended max rpm range and run a prop that fits into that range.

[tbng381]

Results of PROP TRIAL after changing from the L/2 to the M/2 Lower Unit (Note: ~Prop to Pad Height = 5")

The 26" Raker II w/ (3) 3/8" vent holes got on plane in 5 sec, and ran 68 mph at 5800 rpm's. VERY HIGH torque at higher speeds (very hard to turn left). This M/2 has no torque tab, only a flat plate. Thoughts?

The 24" Raker II w/ (3) 5/8" vent holes, with 1 hole plugged, got on plane in 4 sec, and ran 63 mph while holding at 6000 rpm's. Likely could have run at 6200-6300 rpm's, yet BRP recommended maximum is 6000. Fairly high torque at higher speeds. Lost second plug or would have tried plugging 2 vent holes. Could plugging 2 vent holes increase top end and/or reduce maximum rpm's, vs plugging only one vent hole?

Any knowledgable feedback appreciated!

[McGinnis_Fishing16]

Someone will correct me if I'm wrong but I believe the gear ratio in your lightning case was 1.71:1 whereas your new M2 case is 1.86:1 so that will make a big difference in your rpm and wot and how the motor spools up on holeshot.

[JR19]

Results of PROP TRIAL after changing from the L/2 to the M/2 Lower Unit (Note: ~Prop to Pad Height = 5")

The 26" Raker II w/ (3) 3/8" vent holes got on plane in 5 sec, and ran 68 mph at 5800 rpm's. VERY HIGH torque at higher speeds (very hard to turn left). This M/2 has no torque tab, only a flat plate. Thoughts?

The 24" Raker II w/ (3) 5/8" vent holes, with 1 hole plugged, got on plane in 4 sec, and ran 63 mph while holding at 6000 rpm's. Likely could have run at 6200-6300 rpm's, yet BRP recommended maximum is 6000. Fairly high torque at higher speeds. Lost second plug or would have tried plugging 2 vent holes. Could plugging 2 vent holes increase top end and/or reduce maximum rpm's, vs plugging only one vent hole?

Any knowledgable feedback appreciated!

Vent holes have ZERO effect on top end speed or rpm. Their sole propose it to push exhaust gas out the vent hole during hole-shot, thus creating air bubbles which in return crates a small amount of slip allowing the prop to spin up faster. Too much venting/air bubbles and you will not get any bite and blow out. On hole-shot when you give it that sudden rush of rpm exhaust will exit the vents. Once the prop is hooked up all the exhaust exits out the back of the barrel hence zero effect on top end speed or max rpm.

I have no idea what hull you have, only the HP size of your motor and the props you mention so I don't know if 68 mph is slow, just right or fast. I am going to assume its a bass boat hull so with that being said your numbers are decent. "most" and I use the word most loosely but most bass boats run at a prop to pad near 3.5" with the exception being performance hulls like Allison, Bullet, Stroker, Gambler and a few other hulls. So with out knowing your hull I am going to assume its not a high performance hull. With that being said your deep at 5". Raising the motor should free up the prop some on hole-shot and also see a little extra top end rpm and possibly some speed gain. Running a motor deep creates resistance which can create poor hole-shot and poor top end speed...now with that being said a motor too high also has bad results but since your no where near being too high there is no need of talking about what being too high does...My next move would be set the prop to pad at 3.5"and see what happens.




Here is a link to correctly check prop to pad.






Although you don't have Mercury PVS plugs this video explains how vent holes work...FYI you can have your prop modified to accept Mercury PVS plugs. I done this with my 24 Raker II so I could fine tune my venting.

[tbng381]

Thanks for the information and feedback. I’m running this ETEC on the ‘08 Ranger Z520. Is there anyway to get to 6000 rpm max with the 24”?

[JR19]

My personal opinion is you want to be closer to 5800 than 6000 for the long term health of your motor. Based on the information you posted so far the 26 has better rpm and speed numbers. At 5" your deep. My personal opinion is raising to 3.5" should help with the torque, possibly gain some speed and possibly a a few rpm's as well as help with hole shot...anyone with set-up knowledge agree or disagree...a second opinion would be welcome.

IMO the 24 is not an option based on what you have posted. You can get it pithed up 1 but that will not be enough. You can only go up or down 1 in pitch and the 24 is too far out to get it where you need it.

[2manytoys]

Try removing the plate of the lower. I bought a evinrude 225 and m1 lower with the plate missing. So I put one on and had blow out issues. Remove plate and no more blow out. Went from 3 blade to 4 and still had blow out. There is a hole for more water to cool hub so I had to add a cooling line for that. Something to try.

[tbng381]

ok. Thanks for all the feedback!

[Wmoody]

I have the same motor on a tr21x. Tritons like the motor shallow. Mines 2 1/2 ptp. That being said I too, changed to the heavier gear case. Mines brand new and I lost speed and rpms. I lost about 10 mph and can only get 5200 rpms. I was going to drop down from 14 1/2 x26 to a 14 1/2x 25 this weekend to see if that helps. If you figure it out, please let me know. I will do the same. When I first bought the boat, the motor was where yours is about 5inches. Everyone on the Triton page told me to raise it. I put it at 2 1/2 and still have plenty of water pressure. It drives so much better & even helped with some of the chine walk. Good luck

[tbng381]

Thanks. I’ll sure let you know. I plugged all the vent holes in the 24” prop . Good hole shot yet still over revving up to 6300 rpm at only 63 mph. Backed off to 6000 rpm with 60 mph. Plan to weld vent holes closed and re-pitch to a 25”.
Then tried a 26” prop and ran 5800 rpm at 67 mph, then quickly spun the prop or hub.
Learned the exhaust overheated through the prop and that heat caused spinning of the hub or prop. Plan to evaluate exhaust and water throughput in an effort to prevent overheating and to achieve proper cooling. Comments welcome.