Rooster Tail

[PaulKim]

Second time taking out my new 90 HP ELPTO, had the motor raised 2 notches b/c water being kicked into the motor well. No water is now being kicked in. But not sure I'm getting the performance that I was getting with my 1984 75HP Merc. The 75 HP Merc was giving me rooster tails but this New 90 HP Merc isn't. I'm getting about 6000 RPM's at WOT. I plan on going from a 19 Pitch to a 21 to bring it down. Will this increase my performance? Did they raise my motor too high?<br /><br />Thanks

 

[LubeDude]

Re: Rooster Tail

You are probably going to have to go to a 23 to get you Rs down to an acceptable limit. Are you able to trim completly out without blowing out?

 

[Laddies]

Re: Rooster Tail

If your not getting cavitation Your probably alright. For top end speed the saying is THEN IN DOUBT UP AND OUT, but on a pleasure boat should be mixed with good judgement also---Bob

 

[ufm82]

Re: Rooster Tail

What is your recommended RPM range? Remember that each pitch will effect RPM by about 200. If you go from a 19 to a 21, you'll drop about 400 rpm. <br /><br /> Are you running stainless? That can help as the cupping and tip design will grab better at high engine settings. Were you throwing a tail from the 75 from the prop of just from the anti-cav plate? Any bad handling characteristics?<br /><br /> The 90 should make a noticeable difference in performance. Your set up has to be wrong if it isn't working like the 75 did. Where is the anti-cav plate in relation to the keel? Slightly above it? Below it? <br /><br /> Need more info. <br /><br />UFM82

 

[walleyehed]

Re: Rooster Tail

My concern would be water pressure first of all....If you're going to raise any higher, I'd put a gauge on it.<br />6k now would put you at or near 5600 with a 21, which would be "perfect".<br />Your engine should be high enough that you don't have to use alot of positive trim to get the lift you need, and part of that is prop choice.<br />As for the rooster-tail, that's not a sign of higher performance if it's a high tail, it's a sign of improper trim angle with a few other small items.

 

[PaulKim]

Re: Rooster Tail

The cavitation plate is even with the keel. The recommended RPM is 5600. The prop is the same one that I used on the 75 HP, it is an old aluminum one.

 

[walleyehed]

Re: Rooster Tail

If the old aluminum even had cup, it may well be worn. AV plate even with bottom is usually our starting point, and in most cases, unless you have a flat-bottom boat, you may be able to go up 2 more holes which will be 1.5", but test one hole at a time.<br />The AV plate may also have slightly different dimensions, holding more water down on the 90, which will work in your favor eventually.<br />I'm concerned as to how much wear there is on the aluminum prop...it may have lost alot of it's bite (Cup).<br />Do we know the gear ratios are the same from the 75 to the 90???<br />Also, this may be a good time to look into another prop when we determine the gear ratio, but I feel after hearing your AV plate is just even, we have some lifting to do first.

 

[ufm82]

Re: Rooster Tail

Absolutely agree on the anti-cav plate. Even with the keel is too low. The bottom of the plate should be at least an inch above the bottom of the keel. You've got too much lower unit in the water. The plate may even be riding underwater when you are at speed and that is a MAJOR drag on things. <br /> Yes, the engine needs to go up. Water pressure probably isn't an issue with the engine being low right now- you have room. <br /> Also, I have to agree with the prop issue. That aluminum off the 75 is probably wrong for the 90 and you will need to review your selection. I'm thinking a Laser II would be a great pick for that boat. <br /><br /> Get the set up right first and then go from there. Moving things around is free. <br /><br />Oh, one more thing. Rooster tails, when applicable, can show you a few things. On a pleasure boat, rooster tails are usually a sign of wasted energy. Even on a hi-perf bass boat, it shouldn't be any higher than your cowling. <br /><br />UFM82

 

[walleyehed]

Re: Rooster Tail

well said...

 

[LubeDude]

Re: Rooster Tail

Originally posted by UFM82:<br /> Even on a hi-perf bass boat, it shouldn't be any higher than your cowling. <br /><br />UFM82

Yes, well said, but boy they sure do look cool. With my High five, it doesn slow me down at all, but with the turbo 4 blade, I actually went faster with it trimmed down some. I do beleive that I need to raise my engine one more hole though.