Prop blowout cavitation/ventillation w/115 merc

[jameservin]

My new (used) boat is a mod v flat bottom (6degree rise?) sea ark 18ft 96in beam with a 115merc (boat rating=90-140hp. Weight=750lb + 325(motor)=1100. <br />Having blow-out at holeshot and in turns at most any speed. <br /><br />Found that SS 19P (used-that came with boat)is underproped (above 6000rpm), found 21 alum (std Mich wheel)close at 5600.<br /><br />Still terrible blowout in turns and at holeshots.<br />Motor AV plate is even w/keel. <br /><br />I have come to learn this hull is prone to blowout with bigger motors, but what can i do to decrease ventillation? <br /><br />Trim motor all the way in- theres still ventillation.<br /> <br />What prop type/design/3or4blade(?)should i consider to help fix this??<br /><br />Next plan to take some these props to prop guy and check pitch/rake, etc as they are used-may have been modified.<br /><br />Should I be looking for a high cup/rake to solve this issue?<br /><br />thanks.

 

[ob15]

Re: Prop blowout cavitation/ventillation w/115 merc

How much trim do you use? Is it all the way out, 1/2 way or in all the way? If out all the way, try trimming in a little. A good SS prop should help.

 

[gatorred]

Re: Prop blowout cavitation/ventillation w/115 merc

try a ss prop,alum on my boat needed to be lowered 2 in's on jack plate,19 would blow out,21 would run better, a ss is best 4 blade 25 pitch x 13.25 plus add cup to i found out online seach for bass fishing resource guide, proper props,where the horsepower meets the water think basspro shops where how to drive a bass boat this stuff helped me

 

[walleyehed]

Re: Prop blowout cavitation/ventillation w/115 merc

Over 400RPM between 19 and 21??? at max RPM too....<br />I'd say we have 2 very different blade designs.<br />What Model/make is the SS 19???

 

[jameservin]

Re: Prop blowout cavitation/ventillation w/115 merc

Thing is that I have never setup a big motor on a jon boat before, never experienced blowout before.Worried that this rig wont ever work right(?)!.<br /> My deep v has a 60hp and blowout or slip was never a noticed (or I didnt care)with that small a motor.<br /><br />Talking with people I have found that blowout on a jon is a common problem. <br />Im figuring that a 23 pitch will get me the right rpm range.<br /><br />Q: Are trim tabs any help? <br />Q: Just a matter of finding the right SS prop with right cup? <br />Thanks.

 

[jameservin]

Re: Prop blowout cavitation/ventillation w/115 merc

SS prop 19 wot@6000<br />MW alum 21 wot@5600<br /><br />Both props ventillate bad in the slightest turn-even with motor trimmed in. <br /><br />Plan to have props checked monday as they are used and may have been modified from orig specs.<br /><br /><br />Dont have it straight whether I need high rake,low rake.....<br />Have not figured out which brands are best to solve my vent. issue iether.<br /><br />Want to end up with is a SS prop that has good high end speed little blowout as possible.<br /><br />Thanks.

 

[jameservin]

Re: Prop blowout cavitation/ventillation w/115 merc

 

[walleyehed]

Re: Prop blowout cavitation/ventillation w/115 merc

It's very simple...for some reason air is getting into the prop that either the prop is not equipped to handle, or air is being induced by something mounted too close to the center-line on the transom.<br />I doubt the engine is too high, and a high-rake prop is a bow-lifter, which doesn't work well with a flat-bottom boat, unless porpoising is something you like to do.<br />Verify, if you will, that there is nothing mounted to the transom with-in 12", either side of center...

 

[jameservin]

Re: Prop blowout cavitation/ventillation w/115 merc

Kenny, <br />Only thing at transome is the speed/depth finder.Hmmm...<br />My next trip to lake this week-on calm day I will have more exact results to post, and will remove depth finder and speed for tests and will report back.<br />Thanks. <br /><br />thanks alot.

 

[jameservin]

Re: Prop blowout cavitation/ventillation w/115 merc

Heres what I have found re rake:<br />(coppied fron a prop site online)<br /><br /> Blade Rake<br /><br />"Higher rake normally improves performance in ventilating or cavitating situations (high engine elevations and high trim angles). Additionally, higher rake can provide higher bow lift, which will frequently improve speed. Low rake blades are typically used on motors with propellers running fully submerged, typically carrying moderate to heavy loads." <br /><br /><br />So, <br />high rake=high bow lift----Dont want that. My jons already bow light.<br /><br />Low rake= work boat------Aint me, Im out for play. Boats light.Mine definitly isnt fully submerged-esp. on plane.<br /><br />Looking at medium rake blade.

 

[jameservin]

Re: Prop blowout cavitation/ventillation w/115 merc

Said anyother way- I have read I-boat info re rake and see that high rakes are typ used on performance hulls (bass boats,etc...), but as i search online for props they never address just what the rake angle is(!!!). How does a guy know? <br /><br />Guess I am also at a loss to find good info online that addresses blowout issues/prop selection using high HP on light hulls (jons). Thats why I am wining here!

 

[Dhadley]

Re: Prop blowout cavitation/ventillation w/115 merc

OK, lets try this again....darn computers!!<br /><br />Rake is the angle that the blade is at in relation to the hub or barrel of the prop. The more the blade angles back the more rake it has.<br /><br />If the trailing edge is 90 degrees to the hub, then it has no rake or 0 degrees of rake.

 

[Dhadley]

Re: Prop blowout cavitation/ventillation w/115 merc

I tried to make a little "drawing" but the computer wont leave it in order. Email me and I'll try to send it that way. dhadley50@aol.com

 

[ob15]

Re: Prop blowout cavitation/ventillation w/115 merc

Dhadley, If you need help with the drawing, let me know, I'll try to help. You have my email

 

[jameservin]

Re: Prop blowout cavitation/ventillation w/115 merc

Thanks guys, that explanation makes it clear. I appreciate it.<br /><br /><br />(edited) I also see on this iboat page top left corner of screen under "propeller basics" good info re this.

 

[jameservin]

Re: Prop blowout cavitation/ventillation w/115 merc

Got to the lake midday today. Nice day- medium wind.<br />I had done some rough(quick) testing per my orig post- think I just didnt explain it well.<br /><br />First thing I did today was stop by prop shop and verify what I have,which is 3 props:<br />1-SS (mich Wheel)19P no cup<br />1-SS (brand? heavy used prop)21p no cup<br />1-Alum (mich Wheel)21P no cup<br /><br />(edited) note the "heavy used" prop has had parabolic portion ground down either intentional(?)or from repair(?). Approx 1/2 removed. <br /><br />SS(mich Wheel)19P WOT@6000 had throttle left<br />SS (brand? heavy used prop)21p WOT@6000<br />Alum (mich Wheel)21P WOT@5600<br />All props ventilated horribly!<br /><br />(edited-At this point have not verified tachs accuracy and since learned that it is common for older ons to be "off")<br /><br />So, had Alum (mich Wheel)21P prop cupped real fast. Went back and tested it.<br /><br />Alum (mich Wheel)21P WOT@5400<br />WOW! Not much venting (still some).RPMs right.<br /><br />So figure to sell the 19P and having SS (brand? heavy used prop)21p WOT@6000 double cupped.<br /><br />Also bought a 23P 4-blade turbo and will receive next weeek.

 

[Dhadley]

Re: Prop blowout cavitation/ventillation w/115 merc

Well, you just solved the venting problem if it was indeed prop related (most likely was).

 

[jameservin]

Re: Prop blowout cavitation/ventillation w/115 merc

Seems like with this hull/motor setup and current prop a guy could almost do w/o tilt/trim as there is very little trim out allowed (before it porpoising). Other than for testing hole shots, motor runs +/- in vert position.

 

[ob15]

Re: Prop blowout cavitation/ventillation w/115 merc

Getting a prop with some nice cup may allow some more trim.

 

[Texasmark]

Re: Prop blowout cavitation/ventillation w/115 merc

So what are you going to do with the turbo? Hope that it will increase your mph and allow the revs to come up also due to efficiency?.....cause your cupped 21 Alum was already at your max rpm's at WOT.<br /><br />On the mamal thing, why don't you shift a little weight forward. May stop the bobbling without hurting your mph; who knows it may allow you to run on up.<br /><br />Also, I didn't know they made ss props without cupping.<br /><br />Mark