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riverrat56

Looking for a new prop....

7 posts in this topic

I know this has been beat to death on here, but I need some thoughts.

So my boat (16 ft Lund Rebel) gets about 29 mph with just me, but also porposes (bounces up and down) badly. I can plane it out at 22 and ride fine. 2 people it's no problem and I can hit about 27....

Anyways I have a 40 hp 1991 Evinrude VRO, and it needs a new prop, this one is just wore out, dinged up, missing a small chunk, ect....

Since I am a walleye guy and it's not about how fast your boat goes but how slow, would I be better off getting the prop rebuild (Troll forward down too 2.3, would like to get in the 1.9-2.0 range) or could I slow down somewhat by dropping some pitch in my prop?

As long as I can get on plane still and go about 22-25 I would be completly satisfied.

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Does your motor have a trim pin? If think it does... if so, try lowering it one hole, which should take care of the porposing at high end.

Do you have a tach on your boat? If so, what are you running for RPM?

marine_man

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Ya it's already all the way down, porposing is just going to happen. No tach either.

I'm leaning towards a rebuild...

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What bolt hole is your motor mounted on?

If you're satisfied with the performance I would stick with what you have...

marine_man

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It's mounted on the top bolt hole, I could raise the whole motor up by moving it, but then the motor would not sit on the transom, it would only be supported by the bolts.

The porposing problem is caused by all the weight being in the back of the boat, it is a very light boat by nature, and there is no storage upfront to move anything else, so the gas tank, 1 battery, and my fat but are all back there, plus the livewell sits to the back of the centerline.

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Raising the motor wouldn't help your porposing problem.. just thought I'd see if you were mounted up a ways.

marine_man

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Hiya,

to me, what it sounds like may be a couple of things: First the motor cannot "tuck under" enough, or 2) the prop is too high in pitch, or 3) the trailing edge of the hull right at the transom is peened over too far.

As you had mentioned the trim pin is in it's lowest position so that cannot be removed. What can be done is you can buy some shims that would allow your motor to have more negative trim (the lower unit is "tucked under" more, which would force more thrust upward in the rear, thus driving the bow down a little more when on plane.

When over-pitched, the prop is working too hard to try and keep the bow up and due to lower RPM's is unable to. going down in pitch can take care of that since the motor can run at a higher RPM. Being you have an OMC motor, (and I being a former owner of a 50hp OMC) I'd be interested in knowing what prop you are running on your motor. My 50 ran well with a 17 pitch on a 700 pound boat (not much larger than your Rebel...but there is some difference)

If the trailing edge of the hull is peened up too much, the bow never settles down. I don't think this is the issue since when you put weight up front, everything is fine... It is a thought that ran through my head that could potentially be an issue. When peened over too much, the bow has a very easy time going up, but the trailing edge of the hull is not holding it down...water flows over it too easilly, thus it gets to a breaking point...your porpoise.

I'd first raise that motor...The Rebel has a pretty flat rear portion of the hull and your motor has got to be close to max HP if not already. by moving the motor up, you will essentially allow the motor to gain some rpm, but you will introduce some slip to the prop fins in the water, which I think might help reduce the porpoising (worth a shot anyway..especially if you are over-propped). Don't worry about the motor being up off the transom on top...the main weight of the motor is being supported by those bolts and the boat is designed to have a motor mounted higher...it only weight 200 pounds. I'd go up 3 holes and test it. You might really enjoy the results...and the gain in speed. If the prop lets go on moderate turns, drop it one hole and test again.

I'd bet as your motor sits right now, the anti-ventilation plate is below the lowest portion of the hull which is excess drag... No need for that...VERY inefficient setup in that manner...

Steve

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