Prop Questions......

Here is the problem I have an 18' I/O....bad wake bad top speed. It has a 5.7L OMC cobra motor, I think its around 275HP. It has a great hole shot but still a bad wake and tops out at about 30mph. The prop is an aluminum 4 1/4" dia x 21 pitch. What prop can I get to improve both wake and top speed?

Call acme or go to their website. I emailed describing my boat, weight it had in it, what I wanted the prop for and they emailed me right back with the prop info.

www.acmemarine.com

fyi, prop won't fix your wake. Oh and you will sacrifice the hole shot for top speed so make sure top speed is really what you would prefer.


(Message edited by hudd007 on April 10, 2007)

What RPM are you running at top speed? Your speed should be much higher than 30. If you trim you boat up the steering gets a bit stiffer but you'll pick up several MPH. Maybe you are already doing that. If the prop has been repaired lots it might not be good anymore. You might get a vibration if that's the case.
Run a four blade stainless prop with plugs at the blade bases that can be used to let exhaust escape at slow speeds. The four blade doesn't hurt top speed as much as a High-5 prop but gives you much more surface area to push water than a three blade. The stainless is thinner and more efficient than the aluminum so you pick up 1-2 MPH with the same pitch. The holes in the plugs you size to let exhaust out at slow speed that effectively let the prop slip on your hole shot. That gets the RPM up faster just like slipping a clutch on a hard start with a car. The water flow at higher speed stops the exhaust leak at high speed so you don't lose anything there.
Find out the RPM and make sure you are trimmed for max speed before you make the investment in a new prop.
The wake will be very little affected by the prop but you can solve the other problems.

Oh, and check your speed with a GPS. Some speedo's are way off.

(Message edited by rallyart on April 10, 2007)

(Message edited by rallyart on April 10, 2007)

Ok, I do trim the boat up correctly. I'm not for sure what the RPM's are but I know that it is rite around the half-way point of the gauge and to me it seems like it should be more but I guess I'm not real sure about that. Is that the normal range for RPM's?????

So to be clear, at half way on the RPM gauge you're running 30MPH? If so, that's pretty normal. I know on the Merc Alpha One, redline is at 4200RPM but that's a limitation of the drive, not the engine. The Cobra may have a higher redline. I'm almost certain it's not lower.

And not to knock Art's analysis, but if you're already getting good hole shot I'd skip the 4 blade. If anything, it will only further decrease your top speed. If you want top speed, you can sacrifice some hole shot by moving up on the pitch of your prop. As Art pointed out, proper trimming will also help. On plane, you want to be trimmed as high as you can without porpoising for best top speed.

The only things that will help your wake are weight, trim, and hull. Only two of those can be altered without buying a new boat.

When you say move up the pitch of the prop you mean go from the 21 to a 22 or 23? that should give me more top speed with out destroying my hole shot? And yes at half way on the RPM gauge i'm running at 30 MPH which is also full throtle.

Also my prop is pretty dinged up (small dents and bends on the very very outside of the blades), would that have any effect? I have no idea how damaged one would have to be to start hurting its performance.

Yes, I did mean up as in a higher number. Now if full throttle is only putting you at half you RPM gauge and a 5.7L with a 21p prop, your RPM gauge goes very high, your boat weighs an awful lot for an 18'er, and/or your engine/outdrive isn't delivering the power it should. Without knowing real numbers for the RPMs, it's hard for me to say.

If you can see the dings and bends in the prop, they are enough to make a difference in performance. Even a slightly bent up prop can damage the prop shaft over time. That's a risk I wouldn't take on an outdrive like the Cobra that is so hard to find parts for nowadays.

Hmm, I'm pretty sure that its not a weight issue....Its a really low profile boat, almost sits in the water like a bass boat and the hull isn't a deep V at all! I've also pulled other boats with my truck and judging by how the pull feels its much much lighter than most also. And as far as the Tach....Its the stock one that was in the boat so I wouldn't think that it would go much higher than it would need to. Wouldn't you think that it should go faster? how fast? As far as the prop I'll def. change it out for a new one. How would I tell if the prop shaft was damaged?

Your Cobra sterndrive has an overall gear ratio of 1.49 which means your prop spins 1.49 times for every engine revolution. The Mercruiser drives for engines with that kind of HP have a raio of 1.47 so the infor you get about a mercruiser set up should be valid for your boat as well.
I expect Dante's max design RPM is bang-on for the engine so you must get an accurate tach reading to make a good guess on propping. I'm just guessing but I think your pitch is too high and your engine might need to be tuned. That is, you are down on power a bit and you are trying to push too much water with that pitch so the engine is loaded to high to get to max revs.
You might have to got to a 17" but you need to know the RPM first.

I think your boat should definitely go more than 45 MPH. With that power, if it's light and shallow your should go well over 50.
Good Luck

This doesn't sound at all like a prop issue- a 21 turn prop on that boat should get you easily to 50 mph. I'd get it into the shop and see what's happening. Could be anything from a gear issue in the outdrive, tuning the engine, or possibly a stripped drive for the prop? This is wierd. I had a Wellcraft 19 ft. with a similar engine and prop and got it to go over 50- I went to a stainless 19 turn and top speed was about the same with better hole shot. It could be your prop is too large as Art said- and dropping down to a 19 could get your RPM's up ( if you're not redlining) therefore increasing the speed. Good luck.

Is there anything that I could check without taking it to the shop? And if it is one of these problems how expensive of a problem could it be?

Borrow a GPS for speed, verify the RPM, listen for noises and vibrations and under what conditions those things happen. That gives you the best information to provide to someone working on it. A normal service would fix or find most problems.

Could it be just from the damage on the prop? I'd rather try a new prop and take it to the shop as a last resort cause its not running bad or making noise or anything, I'd just like a little more speed but its not 100% needed cause someone is usually in the water at all times.

Find a prop dealer that will let you test drive the prop before you buy it and start with a 19" pitch.
The speedo on my old sterndrive was never accurate until I was over 40MPH and I was in a new Cobalt that had an inaccurate speedo at any speed over 30. So take the time to find out what your real speed and RPM are.

It is possible you have a mechanical issue such as restricted exhaust that would hold the engine back at higher RPM. Also could have a partially clogged fuel filter, both of these scenarios would not cause a driveabillity issue other than losing top end.