Confusing Prop situation - is this just typical?

After realizing that my 2320 was way under-propped with a 15p, I recently swapped up to a Mirage Plus 17p (3 blade) and took her out for a sea trial on a day with light chop and swell. WOW, what a difference! Previously, I could plane up quickly, but not until hitting about 4500 rpm, and the "cruise" was around 4800 rpm @ 20 mph. Calculated slippage was about 40% at that range, and fuel consumption was averaging around 12-15 gph if everything was trimmed perfectly and with a good sea state. With the new prop, 1/2 tank of gas and two guys with a light load, we were able to cruise at 27 mph @ 4200 rpm and 14 gph, which sounds a LOT more like the target performance most 2320's are running. (I have an OX66 250). "Yeah - I've solved my slow cruise problem, AND increased my gas mileage!" I thought!

A few days later, I load up with four guys, dive gear, and a full tank of fuel, motor out to the harbor mouth and hit the juice..... waiting.....waiting......waiting..... she CREEPS up onto plane and finally feels like she is semi-planing with the throttle absolutely firewalled for about 45 seconds to a minute. I back off the throttle, and begin playing with the engine trim and tabs to get the right position for the load and sea state (moderate chop and wind swell, going up hill). No matter what I do, she feels like she is barely planing, the motor is straining mightily and in order to stay on plane I have to run close to 5000 rpms and this produces a speed around 24 mph, burning 20 gph. WORSE THAN THE PREVIOUS PROP!!!

Basically the performance all day, including the trip home in really calm conditions and 1/3 of the fuel burned off was TERRIBLE!. Worse gas consumption, the motor felt like it was straining the entire time, and generally awful characteristics falling off plane, etc.

I have run the boat with four divers before (on the old 15p prop) and never experienced this MARKED difference in performance between empty and full. Is this typical? If I sea trialed this boat and it performed the way it did on that last trip, I never would have bought it - it felt that bad. The crazy thing is that the performance feels GREAT with just me on board or a light load of fuel, people, and gear. Ideal, I might even say.

So, can these 2320's not handle four guys with a 2-stroke 250 and a load of 2 guys' dive gear plus ice and fuel when "properly" propped?

Do I have to just swap out props when I carry extra weight/dive gear? (that just seems insane)

Should I try a four blade prop? It feels like when the boat is loaded up, this prop just can't bite the water, but the motor is not "spinning" - it sounds like it is grunting against a load - it just doesn't go anywhere...

Here's my motor mounting position - not sure if I might be in the "wrong hole"???



I also have a high speed fairing block type transducer installed about 3' behind where the cabin ends on a typical sport cabin (or immediately behind my cabin on the EC). There is also a strainer there from an old pickup that is blocked off (no longer used).

Despite the performance issues, it was a good day of "fishing"...

Your old prop was like low gear, your new prop is like high gear. If that makes sense.

I have the same problem with mine spinning 3 blade 19p, more guys and more weight = poor performance which is typical.

I want to try 4 balde 17p.

Perma trim might help your situation with the single.

Thanks - I understand the "high gear/low gear" problem, but wow, the boat is barely usable with the new prop, despite the moderate load of guys and gear (full tank gas, 800 lbs of dude, 80# of ice, 2 guys' worth of dive gear, and maybe another 150 lbs of fishing gear).

The stern really felt heavy in the water, but we had almost all the gear up on the bow, and the passengers all in the cabin. The 250 should have enough oomph to push this boat and gear with the 17p - I read here about guys with 225's on 2520s swinging 17p props with apparently good results. This is why I am so puzzled...

It looks like the motor is mounted in the top-most hole on the motor and the bottom (of two holes) on the bracket. Note that it is the "older" style Stainless Marine bracket...

We have a different boat with twins that used to have the stock 3 blade black yamaha props on them. We did not feel boat held plane well, ran kind of squat and control slow speed was poor.

Went with 17p Merc Rev 4's -big difference. They provide more stern lift and really hold the boat on plane even at lower speeds. Also much more control docking etc.

Got them from Ken at prop gods. If you buy from him he will swap out props until you are happy and you only need to cover cost of shipping

My 2520XL has a Yamaha 250 four stroke. It came with the Yamaha Saltwater Series II 15 1/2 x 17p 3 blade stainless prop. I noticed that I would easily fall off plane on a following sea. I had trouble staying on plane at 4200 RPM. I just installed a Revolution 4 17p and what a difference. On plane at 3900 rpm, much more control while docking. Still topping out at 5900 rpm at 43 mph. Took me a month to get the prop, and well worth the wait.

Charlie,
Eden, Maryland

Thanks for the comments. The saga continues.... Today I raised the motor up from the top-most hole to the third hole (raised it two holes). Unfortunately the sea state for the subsequent trial was AWFUL - 30+ mph winds and very confused seas, but I tried it out nonetheless. I'm still getting horrible slippage of this prop as you can see from my charts below:





Unfortunately I did not collect a bunch of data before raising the motor because I was simply convinced that it was AWFUL. Here are some charts from the original 15p Powertech prop:

As you can see, the prop is not getting clean water and is just slipping. You should be aiming for 10% slip numbers.

The Mirage Plus is a great prop and if you are getting those kind of slip numbers, I suspect that you may another issue going on. The next step is to get your hands on a 17pitch Vensura or 17pitch Rev 4 to see if the extra blade will cut your slip numbers in at least half. If you are still having these issues then you have a turbulated water issue coming from your transducer or from the strainer.

What does you cavitation plate look like when running at cruise? Any chance you require a 30inch engine and your 25inch engine is mounted all the way down to try and compensate?

captbone":1q6ccekg said:

As you can see, the prop is not getting clean water and is just slipping. You should be aiming for 10% slip numbers.

The Mirage Plus is a great prop and if you are getting those kind of slip numbers, I suspect that you may another issue going on. The next step is to get your hands on a 17pitch Vensura or 17pitch Rev 4 to see if the extra blade will cut your slip numbers in at least half. If you are still having these issues then you have a turbulated water issue coming from your transducer or from the strainer.

What does you cavitation plate look like when running at cruise? Any chance you require a 30inch engine and your 25inch engine is mounted all the way down to try and compensate?

Click to expand...

While underway with a "moderately heavy" load, the cavitation plate was not visible (it was submerged). Because of this and that the motor was mounted in the top-most hole (all the way down), I raised it two holes before yesterday's test. I made some "quickie" measurements (using my aluminum gaff as a straight edge laid along the anti-cavitation plate) and determined that AFTER raising the motor, the A/V plate is now about 4" above the bottom edge of the transom and about 4 feet 3 inches back from the transom. (1" per foot is what I've heard).