reviving old seacraft (propeller question)

[eggsuckindog]

Not sure on Dia but its a 19 and no I haven't added any cup, to do that would bea 1/3 of a new one. 5600 rpm

[Fr. Frank]

Dixon, a couple more questions:

Do you have a negative trim limiter, such as a rod extending from one side of the transom mount to the other, which limits how far down the engine will trim?

With the motor at neutral trim (cavitation plate exactly parallel to keel line), how high above the keel is the bottom of the cavitation plate?

I'm trying to figure out if you're cavitating or ventilating, or both. The fix is different depending on the cause of the air on the blades, because the cause affects where/how the air enters onto the blade, and produces the most vacuum.

If you're cavitating (or mostly cavitating), the fix is to change the angle of attack and departure angles on the blades by greater progressive rake, and/or increasing diameter of the prop, and/or increasing cupping, and/or increasing blade tip size.

If you're ventilating (or mostly ventilating), the fix is to increase the ability of the thrust side of the prop to hold water while surfacing, which is accomplished with increased cupping, and/or smaller blade tips with lots of cupping and reduced blade flex.

I'd like to actually see your boat out of the water in person, and have several props to try at once. I can bring my 15.25" x 16.5" PowerTech. You probably have the 14.5x19 SS Black Max, which is a mid-range propeller.

I can get a couple of different props to try from AAA Outboard, in different style and pitch. Just gotta set up a date and meeting point that works for both of us, if you're interested. And no, you don't have to buy any props. It's just nice to know what works best.

Maybe I can bring them to Long Point, unless you want to do this earlier?

[eggsuckindog]

It will trim negative a bit I think, I only get cavatation when I try to trim the bow for maximum top end. The prop just doesn't pull the bow up much then looses grip, its not bad except for attaining top end which I know could improve. I will probably be able to find a couple Mirage's at Longpoint to try maybe, as they a readily available on the used market, mine is just fine vs $450 for a new one.

[bitsamonkey]

It's been a while, but it's taken me this long to get the old Merc 200 running right (new stator, trigger, switchboxes, rectifier, a bunch of tuning, timing, syncing, and a set of much nicer carbs off of Ebay)!
For those who recommended the 19 pitch prop...right on the money. 43/44 knots @ 5500. I went with an inexpensive Michigan Wheel aluminum 3 blade.
Now my next issue is the chine walking. The cavitation plate has one of those doelfins on it. Good or bad? It seems overkill as the boat jumps on plane with the engine trimmed down. Is there anything I can do besides trim tabs to help the boat ride level? It seems to want to fall off onto its port side a lot. Thanks for your input.

[Bigshrimpin]

Bits . . . lose that prop. Put on a Mercury Mirage "Plus" 19p or 17P. That will fix the light bow problem. I don't know what the deal is with some of the michigan props . . . they give these seacrafts really squirrelly handling problems. I've been through this before with my boat with the same motor. I also liked the Highfive, but the cruise and top speed was not as good as the mirage plus . . . a fun prop none the less. You can find them on ebay frequently for under $200.

As far as having a level ride . . . you can try to pull of the doelfins and adjust the zinc trim tab on the lower unit. I have a very small low profile one. I honestly think the prop will go a LONG way toward fixing your problems.

[Fr. Frank]

Tim is right, the prop is the place to start. Look on ebay. For example, a 19" Mirage Plus auction ended yesterday at under $200 on ebay.

Bitsamonkey, aQuestion: If you are using a steering cable and not hydraulic steering, can you achieve neutral steering pressure at cruise? By this I mean that, at cruise rpm in flat water, the boat does not want to turn left or right. Many times a percieved tendency to "fall off" to one side or the other is caused by a misaligned torque tab, or the wrong torque tab, especially with Mercurys, for some reason.

With different types of props sometimes you need to change the torque tab. Running a Mirage+ or PowerTech Offshore usually requires a long/deep torque tab that is offset to the rear in order to achieve neutral steering torque over more than a very small rpm band.