Another Prop Question

[pair of jacks]

Ok, I've got a 175 E-Tec with an aftermarket prop (stainless) 14 1/2 inch by 15 pitch. I'm getting wot at 5400 rpms with 80 gallons of fuel (capacity is 95) and just me aboard- 30 knots. Finding cruise is 24 kts @ 4800 rpms. People at Evinrude are telling me that I could get better performance with an Evinrude prop (either Viper or Rebel) due in part to a more curved blade giving better "grip". By the way, my boat is a 21.

Opinions??

[McGillicuddy]

Specifically, what after market prop? Model?
Where is your anti-cavitation plate in relation to your keel?
Does your bow rise when you take off (hole shot)?
Is the 5400 rpm WOT?

Bushwacker has a PowerTech ELE series 4 blade that here very happy with on his Etec 150. But his is a 20 Seafari with a bracket. 21 is bigger but that shouldn't phase your Etec, it has plenty of power for the 21.

I think your pitch is right but maybe your motor height or prop shape is off.

[pair of jacks]

There is no name or brand on the prop so I couldn't tell you. The anti-cav/ventilation plate is right on top of the water on plane like it should be and appears to be just above the bottom of the keel (but difficult to tell exactly as boat is in water). Bow does rise on hole shot takeoff. WOT is 5400 rpm's.

The Evinrude prop I was looking at was a 15 inch 14 pitch version as opposed to the current 14.5 inch and 15 pitch.

[McGillicuddy]

Originally Posted by pair of jacks

There is no name or brand on the prop so I couldn't tell you. The anti-cav/ventilation plate is right on top of the water on plane like it should be and appears to be just above the bottom of the keel (but difficult to tell exactly as boat is in water). Bow does rise on hole shot takeoff. WOT is 5400 rpm's.

The Evinrude prop I was looking at was a 15 inch 14 pitch version as opposed to the current 14.5 inch and 15 pitch.

I'm not so sure you are under-powered. Right now your prop slip at speed is about 15% that's not so hot.

The bow lift suggests a stern-lifting prop is in order. I will also guess that your current prop has too much cup or rake or both based less than desirable rpm. Also might consider getting in the water and assure the the ant-cav plate is about an 1-1/2 inches above the keel with the 2 planes parallel. This is also a source of rpm loss and possibly the most overlooked aspect of dialing in your ride, epecially on a SeaCraft. A 15 pitch should turn easy with your power. Your gear ratio however is 1.85:1 or so so a bigger diameter wheel is probably not the answer.

Bushwacker's recommendations are on the money. The Rogue is BRP's reputed stern-lifter, although most 4 blades offer that attribute. I don't know the Rogue. I do know the classic merc Black Max is a stern lifting champ. I would ask dealer to try a 14-3/4 x 15 Black Max with an etec hub and go from there. It's an easy to turn wheel which is helpful. Seems Bigshrimpin ran a 15p Mirage Plus on a 175 (might have been a Merc) with good results (nearly 40 mph on a 23), too. Good luck.

[sidelock]

If WOT RPM is higher than the manufacturer's listed RPM range for the specific motor, could it cause any damage to the motor if you run it higher than its RPM range ?

[Bushwacker]

Originally Posted by sidelock

If WOT RPM is higher than the manufacturer's listed RPM range for the specific motor, could it cause any damage to the motor if you run it higher than its RPM range ?

That's much less likely to cause damage than lugging the motor! (running BELOW the recommended RPM range at WOT by running a prop with too much pitch/diameter!) Check out that E-Tec link in my earlier post for more info. Most modern motors like the E-Tec have speed limiters built in to the EMM that will keep you from over revving them.

[Bushwacker]

The OPTIMUM rpm for all 175's is 5500-5600 unless it's a 2007, in which case it's 5300-5500. If motor is newer than 2007, your rpms are a bit low for that light load you're running. Check out this web site for a ton of good info on your motor: http://www.etecownersgroup.com/post?id=5396435

Step 1 before changing props is to make sure the tach is accurate, but if you're running the I-Command tach which gets speed from the ECM, you're good on that. Then make sure the motor height is right (AV plate should be out of the water when you're up on plane). That typically equates to the plate being about 1-1.5" above keel for a transom-mounted motor. Then you'll have to decide what performance characteristics are most important, i.e., do you only want max top end speed and high cruising speed with a light load, or do you care about good low speed performance with a heavy load in rough seas, like being to hang on plane at low speed, getting on plane easily with a heavy load (good hole shot), and good cruise fuel economy? I'm guessing you want the latter, or else you'd be running a 300 on that boat! All props are compromises; no single prop will do all those things equally well.

The Rebel is a great 3B prop with lots of blade area and stern lift that will help low speed planing and cruise fuel economy, but most of the guys on the owners forum running that prop on the small block 2.6L V-6 are running bass boats that are much lighter than your rig. I'm giving up some top speed and fuel economy with the 4B 15x15 PowerTech prop I'm running, although it has an awesome hole shot, will plane at about 12 mph, and will turn 5400-5500 pretty much regardless of load. My boat is quite stern heavy with the 30" setback bracket, so I need a lot of stern lift that the 4B provides. I asked my dealer about the Rebel, thinking it might still have enough stern lift but would burn less fuel with the lower drag of only 3 blades. He said that his experience was that the small block V-6 didn't have enough torque to turn the Rebel on a boat like mine, and that most guys running it on large boats had the big block 3.3L V-6.

As I understand the late model 21, it's more of a shortened 23 with much higher deadrise than a lengthened 20, so that's a pretty heavy boat for a 175. You might be better off with a BRP 4B Rogue or Cyclone, or a 4B PowerTech. The latter has a pretty good web site with a prop selector where you enter existing boat, motor, prop and performance info, and it'll recommend a prop. It's still not perfect though, because they recommended a 15x17p 4B prop that was ~200 rpm too low in max rpm, so they exchanged it for the 15x15 I have now. It's VERY important to do all testing with the type of load you typically plan to carry. I always tested with a full gas tank, and about 50 gallons of sea water for ballast in the live well and in a couple of 70 qt coolers.

[pair of jacks]

Bushwacker- Great info- thanks. Yeah, the E-Tec is a 2012 and I am getting the rpm's off of an I-Command gauge. AV plate is right on top of water like you said. You are right, the 175 is not enough hp for the 21's- I made a mistake on that one- should have gone to a 200 E-tec but I gotta live with it now. Just trying to make the most of it.