Should I remove this foil on my outdrive?...update

[ntheyer]

I have a 1974 Rally 17' made by Mirrocraft that I recently repowered with a 1995 Mercruiser 3.0 4cyl making 120 HP due to using an old manifold and carb from a 120. I have a pretty banged up three blade 17P 15 3/4 diameter prop on it now and make around 34MPH at 4300 RPM WOT with two on board and 18 gals of gas. I would like to replace the prop as it is pretty beat up, but don't need a really fancy prop. Mostly will be used for running around with some waterports towing (skiing, wakeboarding, tubing). Top speed is not a major concern at all since the boat is so short, I can hardly ever go that fast!!<br />My question is, is there any benefit or drawback to going down in diameter and increasing the pitch? Right now it is about as big as it can be without hitting the gearcase. If I went to a 14" for instance, how much does that decrease in diameter usually need to be compensated for with increased pitch? For instance, one inch diameter difference equals one inch pitch??? Any recomendations on a prop for me? It is going on a pre Alpha leg with 1.98 ratio. I used the Mercury prop selector and it came up with a 13 1/2" diameter 23 pitch Vengeance or a 14"dia 23 pitch Black Max? 23" pitch??? That seems pretty agressive to me for this weak of a boat? <br /><br />Thanks

 

[ntheyer]

Re: Should I remove this foil on my outdrive?...update

 

[Silvertip]

Re: Should I remove this foil on my outdrive?...update

Prop diameter (within limits) has far less effect on performance than does pitch. Diameters are limited by engine design (clearance between trim tab and/or AV plate). You will find that as pitch increases, diameter decreases slightly. Conversely, as pitch decreases, diameter increases slightly. The prop design and blade shape dictates these small changes. In other words, a 13-1/2x23 and a 14x23 would likely yield nearly identical results on the same boat, with the same load. Any difference would be due to blade shape. Select prop pitch that places the engine at the top of its RPM band with normal load and diameter will take care of itself. Too much pitch you lug the engine. Too little pitch you overrev. Most boaters should really have two props. One for cruising (higher pitch) and one for heavy loads & water sports (less pitch).

 

[ntheyer]

Re: Should I remove this foil on my outdrive?...update

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[walleyehed]

Re: Should I remove this foil on my outdrive?...update

Yea, that prop needs replaced....

 

[stevens]

Re: Should I remove this foil on my outdrive?...update

What is that huge thing bolted on to the anti-ventilation plate?<br /><br />Oh, I have a 120 hp diesel on an Alpha-drive on my 21' boat.<br />It came standard with a 23" Black Max alu prop.<br />I've tried many others in the 21-23" range, and the original prop seems to be a very good choice.<br /><br />However, the Mercury prop selector is apparently prone to give too high a pitch. Here's what Walleyehed said once (hope you don't mind):<br /><br />

Just a bit of info here...I test props, many, many props...I've compared what the Merc selector says to what my reality test data shows, and 99.9% of the time, the Merc selector recommends TOO HIGH of pitch for the correct RPM.<br />I run on lakes from sea-level to 6200ft.<br />If the Merc selector says 12" pitch, I can almost 100% assure you, you are over-pitched.<br />

 

[ntheyer]

Re: Should I remove this foil on my outdrive?...update

That monster on the antiventilation plate came with the boat. Should I get rid of it? I ended up buying a 15" dia x 19" pitch three blade last night for 20$ delivered. For that price, I can't go wrong. If I need more pitch later, I'll get it. Should I remove the monster metal plate? It sure kicks up a lot of spray when up on plane.

 

[stevens]

Re: Should I remove this foil on my outdrive?...update

I am just waiting to hear the chorus of people telling you that huge plate will do you no good...<br /><br />Good price for the prop, but can you physically fit it in? I thought Alpha-drives needed props smaller than 14,5" ? You may also need to replace that trim anode with a flat one.

 

[ntheyer]

Re: Should I remove this foil on my outdrive?...update

I know it will fit because my current prop is 15 3/4" diameter. This new prop is 3/4 inch smaller. Do you think I should remove the plate, and do I need to fill the bolt holes then?

 

[stevens]

Re: Should I remove this foil on my outdrive?...update

I have no personal experience with this, but the topic of foils have been up a number of times and the verdict seems to be to take them off. Also, the remnant bolt holes have also been discussed. Maybe you could try a search on this forum?

 

[ivar]

Re: Should I remove this foil on my outdrive?...update

As for the plate, try to take it off to see what difference it makes... some boats seems to benefit from it, some not.

 

[beauff]

Re: Should I remove this foil on my outdrive?...update

I had a busted Hydro trool on the back of mine. I took it off and left the holes alone. Frist time I took it out in the water the bow was up too high and no go. Kind of new at this. This is my frist boat. So I got a new one and put it on. Much better.So what I'm trying to say is maybe a better foil will work for you. That is if taking it off dos not benefit for you.

 

[umblecumbuz]

Re: Should I remove this foil on my outdrive?...update

That picnic table - why not remove it and test the results.<br />You can always put it back.<br /><br />If you leave it off, fill the holes. For this, you can use normal auto bodyfiller, sand it flush, and spray over with a good aerosol. Key the AV plate with fine grade, say 600, before doing so. Do it well and it'll last years.

 

[ntheyer]

Re: Should I remove this foil on my outdrive?...update

I removed the 'picnic table' and it didn't affect top speed more than 1MPH, but REALLY did affect time to plane and bow rise. I will reinstall. The 19" pitch prop was better, but I could use a 21" or 23" possibly four or even five blade for waterports holeshot. Tope speed was up about three to four MPH at WOT, but WOT was a bit too high at 4850-4900 RPM. Anyone have a four or five blade 21" prop?