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Hi, last year I hit a rock with my propeller and caused some slight damage to it so I changed it out for an Evinrude aluminum 13.25x17 propeller. The damaged one appears to be original, stainless (black) with 13x19 marked inside the hub. On flatwater conditions, I notice that with the new propeller, my boat now porpoises quite a lot above 25mph even with a couple of adults in the front and the only way to reduce this is to trim all the way in, but at that point the steering is so stiff ( if you let go veers off to the right real hard), that it is not that enjoyable . With the previous prop, I was able to trim out almost halfway (on the gauge) before porpoising and then just trim down a bit and it would run nice and smooth. Steering was light and drove straight. Since the only change has been the prop, it leads me to believe that the cause of the porpoising would be that new propeller. I even put the damaged prop back on today to rule out possible gear case damage, and no porpoising. So now I am not sure what propeller to purchase and what would be the cause within a propeller that would cause this change in ride:

-stainless vs aluminum?
-pitch 17 vs pitch 19?
-shape of the prop (the damaged one has smaller more oval blades than the aluminum one which is more rounded)?

I understand that a great prop for this boat is the Stilletto 13x15 but since they are no longer offered new, which prop would now be recommended that would potentially keep the porpoising down?

thanks

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There are many factors in prop selection but before going there, lets get some additional information.

Where is the engine mounted? How many empty bolt holes above the top set of bolts? It should be 1 or 2 holes up (bolt in third hole counting down from the top)

What is the weight distribution in the boat? Where is the battery? Fuel tank? anchor, heavy gear and passengers?

What is the WOT RPM with the black colored 13x19?

As for prop selection, prop blade design is not universal like shoe sizes. A 13x19 from Company A will not perform the same as the one from Company B so don't go by size alone.

The recommendation from owners with the same model of hull and engine recommend a OMC 13 x19 prop if lightly loaded and a 13x17 otherwise.

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Thanks for the response, my answers:

-engine mounted 1 hole up (av plate +/-1.75'' above bottom of transom)
-battery in original location at stern
-fuel tank under rps, 18gallon Moeller
-usually one passenger (100lbs) on front cooler
-minimal gear in console (tool kit, paddle)
-never really took it to WOT, but I know RPM at 30mph (GPS) = 4000

I am just wondering if it is more the material (stainless vs alum) or the shape that would cause this difference in ride. Unfortunatly I have no way to try another prop without buying it. I was thinking of maybe moving motor one more hole up because there is spray out to the sides from the av plate.

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Hi, I was able to snap a photo of my av plate while on a plane, ( engine mounted 1 hole up) and can see the av plate and small tab completely out of the water (tried but could not upload the photo). Seems high since the small tab at back of plate is not in the water and thus not really serving a purpose, could this be cause of porpoising?

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porpoising is typically caused by the engine being mounted too low and/or wrong prop and/or poor weight distribution.

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Lucas -

[There is a video showing the engine at speed that is good to review https://www.youtube.com/watch?v=gcbj7...cbj7mGZXkc ]

Based on the video, the spray is an indicator you can go up one more hole.

The one factor is consider is sea state. If you ride in downright snotty conditions then raising the engine will need to be tempered.

Also note that when the hull starts to porpoise, often trimming OUT a bit and increasing speed helps reduce it. Try it.

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Lucas:

Unless you're building a Montauk "tuner"(!!!), I go with solid wisdom for small runabout.

1. Stick with <$150 aluminum wheel...the standard OEM wheel that it came rigged with. The aluminum is also a cheap circuit breaker when you hit something, instead of bending a shaft or cratering the entire engine!! My 1999 Montauk just got its third aluminum wheel to swap out a few chips and chunks missing from it. I just feel like I owe the boat a fresh prop for all the great service and fun it's provided!!

2. If the boat and motor are original, it was likely rigged by an experienced Whaler dealer. if you don't have an option to "test" props, and the boat tachs in the normal range for the way you load and operate it, and there aren't any other compelling issues....then put the same prop back on. Stick with what works and don't go crazy with over-priced derivatives.

My boat has the 100Merc and came with a 19p. It would over-tach if light and perfect if heavy...the 100 has great torque. My last and third prop, the home dealer marina put a 21 on it!! That wheel causes a little port list at low speeds because of all the pitch...and runs 47MPH on GPS....and runs a little fast at idle since I can't slow the motor down any more than it goes!!! Moral of the story...the 19p was the right prop for my specific configuration...and it went nearly as fast with RPMs a little in the "yellow".

Here's a picture showing how low the 100 is mounted. It can't get any lower. I think the 100 has a 25-inch shaft.
https://www.dropbox.com/s/xkzsfeeggvpnb1n/IMG_0286.JPG?dl=0

Nick

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Lucas: You can buy another Stainless propeller like the one you had originally. The OMC/Evinrude/Johnson propeller you had was probably serial number 389510, 13X19. You can buy another good condition used one for $129. here: https://www.ebay.com/p/Boat-Propeller...1918871285

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Thanks a lot to all of you for all of the replies, it gives me a lot of options to play with, I appreciate it. Yesterday I raised the motor to the third hole (weird seeing hooks of motor about 2'' above transom) and gave it a go, noticed right away (with the aluminum OMC 17 pitch prop) that the rpms were climbing a lot faster and got it up to 6000 (did not have my sonar hooked up so don't know speed), did not hear any prop ventilating (water smooth as glass), but did keep going back to check temperature of water coming out of exhaust ports to make sure not too hot - all seemed fine. Had one adult move front, back, middle during ride to test, porpoising did continue but I was also playing with trim and speed a lot too to see if I could get it down. There were times when boat was cruising nicely without being trimmed so far in that steering veered when let go, so there is progress. Moving the person around did not seem to have that much impact though. Will order the 19pitch prop next and possibly a four blade 17 pitch and continue testing. It does appear that there is definitely a learning curve on how to properly use the trim for the conditions, speed, loading.

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Lucas

I have a 1966 Nauset with a brand new 90 HP Evinrude E-Tec. I researched all the Evinrude prop info and because I was worried about stern weight, decided on the Stainless Rouge 13x19 4 bladed prop, because according to the literature, the cupping that is possible on a stainless prop, as well as the configuration of this wheel provides for a certain amount of stern lift. I couldn't be happier. Just something to consider..