Prop Cavitation

Last year I installed a bracket on the boat but I think I installed it about 1/2" or more to high.... I have a problem with slight cavitating in medium to heavy chop. I don't want to go through the headache of repositioning the bracket, and the motor is all the way down. So I was wondering if changing to a 4 bladed prop would help. I currently run a 15 X 17" 3 blade stainless prop. If I did change..... What size prop would I go to? Thanks in advance. <br /><br />Don

Re: Prop Cavitation

Moving to "Prop Questions and Topics".

Re: Prop Cavitation

"....and the motor is all the way down" confuses me. Normally trimming all the way in solves thrust problems that could be encountered on high transom mounts in turbulence.Had you not said that I would have answered the question this way:<br /><br />When you jack up the engine, you can force the prop to run partially out of the water. That means that for a given pitch propeller, you have less grab per rpm. So to improve performance, you go to a higher pitched prop and possibly a different design with more emphasis on blade tip retention, like cupping or straight trailing edge, or combinations. I realize that most SS props are cupped and you did mention that your current unit is SS. <br /><br />Then, with the drag reduced by the elevation, and increasing the pitch, the performance is enhanced proportionally.<br /><br />Increasing blade count improves your water contact surface area for hole shots, but can add to cavitation at higher speeds/rpm's.......most racing props that run high rpm's and half out of the water are small diameter, very high pitch, 2 bladed.<br />--------------------<br /><br />You must have something else affecting your engine that lifts the rear, or diverts water in front of the lower unit in a chop. You didn't give much information so it's impossible to do much more for you for now. But as a rule of thumb, I personally wouldn't raise the engine without a corresponding increase in pitch.....how much......depends....on what......the rig and conditions.<br /><br />Best I can do.<br /><br />Mark

Re: Prop Cavitation

Mark, I wasn't thinking of raising the motor, I was thinking of lowering the bracket about 1". This should put the prop lower into the water, thus keeping it more in contact with the water. Thats why I was thinking 4 blade.... to keep more prop in the water. When I first got the bracket I installed it using the supplied instructions. Well that put it to low in the water and caused the motor to plow (even with the motor installed using the highest setting I could get it on plain). Well I raised the bracket about 4" and have put the motor down to the lowest hole setting. I guess it's all part of the fine tuning. I was just thinking that a 4 blade would save me about 2 weeks worth of work.... Thanks again for the reply.<br /><br />Don

Re: Prop Cavitation

"....and the motor is all the way down" confuses me. Normally trimming all the way in solves thrust problems that could be encountered on high transom mounts in turbulence.Had you not said that I would have answered the question this way:<br /><br />When you jack up the engine, you can force the prop to run partially out of the water. That means that for a given pitch propeller, you have less grab per rpm. So to improve performance, you go to a higher pitched prop and possibly a different design with more emphasis on blade tip retention, like cupping or straight trailing edge, or combinations. I realize that most SS props are cupped and you did mention that your current unit is SS. <br /><br />Then, with the drag reduced by the elevation, and increasing the pitch, the performance is enhanced proportionally.<br /><br />Increasing blade count improves your water contact surface area for hole shots, but can add to cavitation at higher speeds/rpm's.......most racing props that run high rpm's and half out of the water are small diameter, very high pitch, 2 bladed.<br />--------------------<br /><br />You must have something else affecting your engine that lifts the rear, or diverts water in front of the lower unit in a chop. You didn't give much information so it's impossible to do much more for you for now. But as a rule of thumb, I personally wouldn't raise the engine without a corresponding increase in pitch.....how much......depends....on what......the rig and conditions.<br /><br />Best I can do.<br /><br />Mark

Click to expand...

I'm sure you expected a response, so here it is...<br />Don't change pitch until you've maximized what you have...in this case, it's very simple..you need a prop with a lower slip ratio, ie-more cup.<br />we don't raise the engine to vent, we raise the engine to decrease gearcase drag, this results in more RPM so we then add pitch or blades to bring the R's back to spec.<br />Better take another look at the fastest boats running...they don't use 2-bld.<br />when eliminating gearcase drag, the propshaft is going up...the blade number (in racing situations) goes up...4, 5, maybe even 6 blades when the propshaft is at the max heigth allowed..maybe 1-1/2" under the water depending on what we're running.<br />in the case of Beenthere, we might go to a 4-blade such as a Turbo because the Turbo 3-bld will have 3/4" less diameter, and if we can't lower the engine, we need more bite.<br />This also would depend on the type of boat and what it's used for.....<br />Beenthere, could you give a bit more info on your rig???

Re: Prop Cavitation

Tried to get the pic but I'm not a computer wizard. lake-link.com <br />Fortress racing won the World Cup offshore racing event with a BBlades Professional Propellers, LLC, customized prop........but wait. The picture only shows 2 blades.....Hmmmmmm<br /><br /> http://www.lake-link.com/jumpto/gotosite.cfm?Site=http://www.bblades.com/ <br /><br /><br />Mark

Re: Prop Cavitation

One more hit. Per the experts, the fewer the blades, the better the blade efficiency. Said one is best but can't tolerate the vibration. So obviously next is 2 which balances out the vibe problem. And, for the purists, the more the blades, the smoother it runs -vibration- but the less efficient. But I'll give you the fact that you can up the pitch and make up for losses in efficiency caused by multi blades that will give you a fast hole shot.<br /><br />Wow, and I thought rocket science was fun.<br /><br />Mark<br /><br />Reference: http://www.mercurymarine.com/chapter_4_-_propeller_technology

Re: Prop Cavitation

Mark, I don't need to argue any of this with you..in fact, I've helped re-design one of the sites you've linked....I realize you've been here just a short time, so all of this jibber of single-blade and 2-bld crap is old....VERY old news to me.<br />I know where I stand, what I have and what I know...I shouldn't expect you to know all about me in 6 days....<br />Oh yea, Call Brett Anderson and ask him what props they ran on the Fortress...the picture is a tricky one........

Re: Prop Cavitation

ps when running a jackplate your going to get tork stear or what makes vib's in stearing, cmc manual has foot control switch could be used for jackplate control for better stearing like two hand's when posible <br /><br />just a little though

Re: Prop Cavitation

Hey Gregg, they've got a piece out there called the torque-tamer...bolts to the side of your skeg...keen little piece.<br />eliminates alot of problems when running with no trim-tab or high engine heights....really helps keep the lower unit straighter than with just a tab...neat deal...

Re: Prop Cavitation

Thanks everyone for the advice. I think I'm going to try to find me a Prop shop that will allow me to test before buying. If I can get the results I'm looking for with an easy prop change then that the way I'm going to go. Thanks Again<br /><br />Don

Re: Prop Cavitation

Wish you well on the loaner from the prop shop. Since props are so succeptable to damage, would imagine that loaners would be hard to come by, but who knows.<br /><br />Mark

Re: Prop Cavitation

As you well know, Mark, A "good" prop-shop has loaners and many of them...some Marinas never took the time to get into most any manufacturers program for loaner props, and missed the boat.