Prop Cavitation?

[Dave1972]

I have a 25' Eliminator Eagle XP twin stepped hull running a 502Mag EFI with a 25P Hydromotive Quad IV prop. I'm having some problems taking off from a dead stop. If I give the throttle a push to much more than about 1/2 way, the prop cavitates, RPM's rev, and the boat falls off plane. I haven't had the boat that long so I'm not sure if it's designed to take off that way or if something is wrong. I originally thought that it was the hull bringing in air to the prop but at idle to plane there isn't air being pushed under the boat. And yes, the drive is all the way tucked under the boat! :-) Any suggestions or ideas???
Thanks!!

[25 Outlaw 502]

Dave, since you got back to me I figured I would take a look at your dilema. My buddie has a Donzi 30 ZF with the same problems. Donzi told him it was a combination of his drive height and the stepped hull. Although he has twin 250 o/b's it is probally the same problem. He gets air under the hull and it is conjunction with the bow rises which creates a low pressure region just as the boat passes. The lower pressure region allows the prop to cavitate easier. He minimizes the blowout by using his k-planes and is then pretty much able to hammer it and go, as the boat comes up bring the tabs up and then you can blow past my slow Baja!!!!! Thank god for throttle mounted controls.
25 Outlaw 502

[Dave1972]

Hey Outlaw,
That is too funny....I figured that might be some of my problem....the boat comes off plane very quickly but I have a tendency to get into the throttle a little heavy and I end up having to stop and start over. And yes, I hate to say it, but your Baja needs some tweaking...you should be getting much more speed than that...I have basically the same setup that you do...my boat is probably quite a bit lighter, but you should be pushing low 70's with your boat. Where do you do most of your boating? We're in the Midwest and spend time in IL, WI, and MO! Let me know if I can help with any questions about your Baja and the setup! Thanks for your response.

[25 Outlaw 502]

[QUOTE]Originally posted by Dave1972:
[B]Hey Outlaw,
Dave, boat goes in for a look at the motor to see what the story is. Maybe I can scream real loud and get them to install an HP500, its nice to dream anyway. Once I get it back and hopefully pick up some ponies I would love any advice you may have as far as prop selection. I don't care to much about out of the hole shot as long as I can still plane. I bought this boat to run fast to hop from place to place. If someone wants to ski or wake board then they can buy they own boat and i will be happy to drive it for them!!!!!! I live in Northern Florida and spend my time blasting through the intracoastal waterway and up the St Johns river into central Florida. Also planning to tow the boat down to the keys for the 4th and would like to run from Ft. Lauderdale to the Bahamas some time. Sure wish the ocean and inland waterways weren't so salty. It really makes for a lot of work. Enjoy the midwest, I grew up there but hated the winters. Good Luck with your prop and I will ask one of my other smart boat buddies if he may have any more ideas.
Outlaw

[RiverDave]

Dave1972, Does the boat run Transom exhaust or does it run std mercruiser? If it's standard Mercruiser does the prop have any holes at the base of the leading edge of any of the blades? (I figured I would get the simple solutions out of the way first). To your knowlege did this boat ever have a larger motor in it? (Sometimes they will set the X dimension higher and not change it after a motor swap) Stepped Hulls while improving top end speed are also suppose to shorten the amount of time that it takes for a boat to get up on Plane. If your only able to run 1/4 to 1/2 throttle getting your boat up on plane it kind of defeats the purpose of a step. The most important question is What does Eliminator Say???? I think I would have them take a quick ride in it and ask them if all of them do that! ( I know for a fact they don't) The last question I have is how old is the boat and did you buy it new or used? (Perhaps someone put a new outdrive on and had it shortened or something for higher top speed!) RiverDave.

[Dave1972]

RiverDave,
Thanks for your reply. The boat is running IMCO Powerflow exhaust with captains choice (usually through transom). The boat is a 1996 model. To the best of my knowledge, the motor and drive are original with a nosecone added to the drive. My problem is that I'm in the midwest...there is an Eliminator dealer within 60 miles or so. Perhaps I should take them for a ride and see what they say. The prop does not have any holes at the base of the blades. It doesn't matter whether I run the exhaust through the transom or down through the prop. Good call on the X dimension for top speed. Know of anyone in the midwest (IL,WI,IA) area that I can get in touch with to take a look to help solve my dilemma??? It's not that bad as it does pop out of the water rather quickly, but I would think it would be a rocket if I could give it more throttle!!! Thanks for your help.
Dave1972

[RiverDave]

Dave, did you add that nosecone since you have owned the boat or did someone else? The other question is how hard is it to remove and put back on? If your able I would give it a shot without the nose cone. I know it sounds kinda goofy, but sometimes minor changes make HUGE differences in boats. I think I would definately call Eliminator at the very least and describe the problems and see if they have any solutions as well. I'd really be interested in what Ultra28 has to say. He knows boats far better than I, and he should have some better input to the conversation. RiverDave.

[RiverDave]

Sorry for the double post, but I had one more question for you Dave. When the boat is on the trailor, (providing you have room) and you lower the outdrive completely, where in relavence is the centerline of the prop to the bottom of the boat? My last question is have you tried any other props???? RiverDave.
(The reason I know a lil bit about this is my father had some similar problems with one of his boats. Spent months before we finally got it dialed in!!!)

[Dave1972]

RiverDave,
I took the boat out this weekend..tried taking off with exhaust open and closed...didn't make a difference at all. I haven't tried any other props and that might be another option...I'll throw a three-blade on there to see if there's a difference! The centerline of the prop to the bottom of the boat...not very far...I "eyeballed" it and appears to be about 6 inches or less. Thanks for your help...I'll be on the phone with Eliminator today!
Dave

[RiverDave]

Dave1972, I was referring more to the height of the prop relative to the bottom of the boat. Someone in the boating industry named that the "X" Dimension. This plays a major factor in prop cavitation as well as top speed. I'd like to know what Eliminator said as well!! RiverDave

[Dave1972]

RiverDave,
I just got off the phone with Eliminator. They weren't much help. They told me that the Hydromotive prop (cleaver style?) will have the tendency to cavitate and in order to reduce the problem I should switch to a three blade cupped prop. I'm not sure who I spoke with but I'm not real happy about that answer as he told me that I will sacrifice top end performance. The "X" dimension that you talked about...didn't I give that to you? Centerline of prop to bottom of the boat or from the top edge of a blade at it's highest point to the top of the boat??? Sorry for the confusion. Perhaps I should try a four blade cupped prop versus cleaver style? Or maybe just a three blade cupped? Unhappy with my answer from them!!!
Dave1972

[Dave1972]

RiverDave,
I just got off the phone with Eliminator. They weren't much help. They told me that the Hydromotive prop (cleaver style?) will have the tendency to cavitate and in order to reduce the problem I should switch to a three blade cupped prop. I'm not sure who I spoke with but I'm not real happy about that answer as he told me that I will sacrifice top end performance. The "X" dimension that you talked about...didn't I give that to you? Centerline of prop to bottom of the boat or from the top edge of a blade at it's highest point to the bottom of the boat??? Sorry for the confusion. Perhaps I should try a four blade cupped prop versus cleaver style? Or maybe just a three blade cupped? Unhappy with my answer from them!!!
Dave1972

[Dave1972]

Oops...double post...needed to revise when talking about "X" dimension! Said top of boat when I should have said bottom. Sorry for the confusion!!!!!!!!!!

[formula93]

Try a Bravo 4 blade prop, more blade surface area & it should not effect your speed. I get 75 mph out of mine. However, with a lot of torque, ie-1/2 to 2/3 throttle it's going to be tough to get any prop to hold. You ARE pumping air to the prop on "launch" due to hull rise and roll over to plane & also the steps. The steps are to help break the surface tension of the water in order to carry to bow higher. The steps do not help getting a boat to roll over onto plane. Your boat is not a drag racer & never will be. Heavy throttle application is extremely hard on outdrives also.
Try 2500-3000 rpm with the tabs full down, drive tucked way under. Apply throttle and as the boat starts to rise & roll over, immediately bring the tabs up, then trim the drive. It works for me in a 252SS Formula (roughly 6000# wet)with at least 600 hp. I set the throttle at 25-2700 rpm and never touch the throttle again until fully trimmed up, at which time I am cruisin' at about 4000 rpm. You can Rock-n-Roll at that point. I come up nice & flat with that procedure and pretty quickly also ( i'm not a drag boat either !!). http://free.***boat.net/ubb/smile.gif Good Luck !

[25 Eagle]

Dave...call Kevin at R&R Prop Shop in Henderson,NV. I have the same boat as you and Kevin blueprinted my Bravo 26P 4 blade. Seems to come out of the hole fine, but a few weeks ago I had some "weight" in the back and it seemed to take longer. He's a great guy and should be able to help you out or get you looking in the right direction. My boat runs 72mph on a GPS. Good Luck
[This message has been edited by 25 Eagle (edited July 05, 2001).]

[Dave1972]

25 Eagle,
Thanks for the post! I'll try and give Kevin a call sometime this summer! Sounds like the Bravo 4 blade is the way to go!

[MrHavasuCat]

The cleaver is your problem. I wish I could run one on my cat but there is no way the boat would get on plane. Because of my high x dimension and big prop (merc lab 32" 4 blade) I have problems planning as it is. I have to feather the throttle to get enough bite to get onto plane.