Blowout or Cavitation??

[5string]

New to makingchanges to setups still so I have a question or two a out my new to me boat. 1999 TR21 with a 225 efi Mercy. Slow out of the hole as compared to my last boat, a TR18 and about the time it begins to plane out it blows out or cavitatates. If itry and compensate by letting off of the throttle it helps but still does this.

Do I have motor height issues prop.issues as in wrong prop which would be the best place to start.

Or is it just seat time.in a new rig?

Not real sure what the prop is, it does not have any brand name markings on it but has the number SPI6822Aor the last digit may be a 4 I suspect and it is just a light stamping. Three blade.

Can anyone tell me what this prop is and should it not be more like a 25 or 26 with this setup?

Right now I am running 68 top speed gps turning 5800 rpm
As I said earlier this boat is new to me and I have posted a similar question a while back but did not have a lot of info at the time and I have a little more seat time in it now
Thanks for the input in advance.


Can anyone tell me what this prop is and if it is a 24 should it not be more like a 25 or26

[ReelDoc]

Shoot an email to member "Jiggernaut" - http://www.bbcboards.net/zerou...d=491. He has the same boat and motor and should be able to help you.

[Wheels]

If it is blowing out on breakover, and given the speed at 5800 rpms, more than likely its too much prop. Best way to determine the pitch is to take it to a prop shop and have them measure it. The prop may have been worked and have a different pitch than it had when it was new. Get with Jiggy on that setup.

[Cabela10]

Seems like you don't have your motor trimmed all the way down at takeoff for that heavier boat. And out of everyone here, I probably have the least knowledge of this situation, but I know I could duplicate your issue if I trimmed my motor up a little more than it should be at takeoff.

[Jiggernaut]

Hmmmm....where do I start? I think first thing is you should make sure motor is sound. Check compression on all cylinders, tune it up with spark plugs, fuel filter, etc.. if it needs it and make sure the throttle body butterflies are opening all the way prior to tweaking your setup. You are really not that far off. The next thing I would do is the prop. I have run a 25 trophy, a 26 tempest, a 25 powertech and a 26 fury on my rig. For your rig stock, a 25 powertech TRO-4 is an excellent all around prop. good holeshot, decent top end, great lift, but not as fast as a tempest or a fury. the 4 blade also is more forgiving for the novice driver and handles well in rough water. A 25 Trophy worked over by a prop expert is dang good, too.

You will need to learn how to measure propshaft under pad distance and set your motor at 3" under pad to start. 8" of setback seems to be the sweet spot for this rig, in other words an 8" jackplate or in my case I added 2 inch spacers to my 6" hydraulic plate. Run it with one of the above first 3 props mentioned at 3" under and note your numbers. Then raise the jakcplate 1/4" and run it again. Your numbers should gradually improve and at some point they will level off and your rpms will be creeping up on you. Also watch your water pressure for when it starts to drop off. Your motor at that height may require the top 3 or 4 holes of the water inlets on the LU be plugged with stainless steel allen screws (tap the holes first). Once your performance tails off, back the jackplate back down 1/4 or so and you should be set. I think you will find that the best spot is around 2 3/8" to 2 3/4" under pad depending on prop. Also note your rooster tail when going WOT and trimmed all the way out. It should not exceed the height of your motor cowling.

I benefitted to the tune of 1 to 2 mph by adding trim extenders to my rig. $35 part that takes 5 minutes to install. I now have more trim than I need but didn't have enough before. Once I am trimmed all the way out I bump it back down a little and I am set. This is my best recollection of this setup as I am not quite stock anymore. At the time I ran around 72 to 73 gps. Since I have dropped on a set of Simon heads, a set of reeds, a tuner, and a 300 ecu and I turn a 26 fury to the rev limiter so she's really got some bark now. I think you can reasonably expect low 70's out of your setup if tweaked right. It seems like most have been in that neighborhood with a few excpetional hulls that go faster and some slower. Hope that helps....

[ReelDoc]

Dang!!!!! Too bad Jiggy couldn't give you any helpful information!!!!

[5string]

Exactly Doc,

Thanks Jason for the input, it sounds like I have some work to do or a direction to work toward. The compression was checked at the time I purchased the rig and all cylinders were pretty close to the same in the neighborhood of 72 to 75psi. Not being educated enough before I purchased it I was told that these were good numbers and took there word for it, hopefully I was not too neieve, Are these numbers OK? The numbers on my last engine were higher,@ 90psi but it wss a 150 Johnson and thought the comparison was correct considering the 150 was a smaller bore engine. I will start by checking the butterflies and other simple things first. Do the plugs have to be indexed on my model of engine?

Again thanks for all of the input from everyone!!!

[Jiggernaut]

I have always heard a lower compression number may not be a bad thing as long as they are very close to one another, but 72-75 sounds very low. My last check, I think mine were around 120. I would really question the test cause like I said if you are getting 68 out of it at 5800 you're not too far off. Was that just you running light or were you loaded down with tackle, fuel, etc..?

[5string]

Loaded with nearly half a tank of fuel. Both livewells nearly full.