# Prop Blowout



## birdyshooter (Sep 17, 2019)

I'm still working on tweaking out my skiff project. I just recently ditched the hydrofoil and installed Bennett self-leveling trim tabs. Man, I should have done this first!! Major improvement over the hydrofoil in hole shot and overall ride/handling. Only issue now is I'm getting prop blow out on turns. Wide to mild turns are fine, but try and take a sharp turn and instant blowout. Motor is a Mercury 20hp with factory black max 13p 3 blade prop. Anti-ventilation plate is set 1" above hull line... lowest I can go using this jack plate without major modifications to the transom. I adjusted the trim tabs to their weakest setting as well. I believe the tabs are inducing a lot of white water into the prop on turns because I never had blowout issues before installing them. So what are my options here? Modify the tabs to channel water away from the prop(angle aluminum riveted to the tabs) or will switching to a 4 blade prop help?? Let me know what you guys/gals think. Thanks!!


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## BassFlats (Nov 26, 2018)

The motor might be too high. The hydrofoil may have been acting like a large anti-vent plate keeping water around the prop. Since it will be a pain to lower the motor, you can try a prop with more cup (bite). What make skiff is that?


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## birdyshooter (Sep 17, 2019)

It's a 14' semi-v made by _Susquehanna boats. _ Apparently not many made and they must have not been in business for long. I picked it up on a trade knowing it needed some work(improvements). The 2003 Mercury 20hp was like brand new with maybe 5hrs on it and all the factory paint on the prop. So it was a no brainer. The hull is basically the same cut as an aluminum semi-v. I'll try to get some photos of it this evening.


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## Salty Dawg (Mar 10, 2020)

It looks like your jack plate can be lowered one more hole.


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## birdyshooter (Sep 17, 2019)

Salty Dawg said:


> It looks like your jack plate can be lowered one more hole.
> 
> View attachment 150331


It can. The issue is that the motor bracket hits the transom if I move it any lower. I would have to shave 2" off the top of the transom to move it any lower. I've thought about it, or just going without the jack plate and hanging the motor on the transom as intended. I really like the setback though, gives me more room on the rear deck.


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## JC Designs (Apr 5, 2020)

I question whether the “jackplate” motor bracket is really needed if the motor needs to come down an inch.


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## birdyshooter (Sep 17, 2019)

Here are some photos to show y'all what I'm dealing with on the jack plate transom, and some others to show the layout. The final goal is to fiberglass in a balsa back deck(this one is a temp plywood) so hanging the motor on the transom as designed really won't work.


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## Shallows (Mar 29, 2020)

Bobs Mini Manual Jack plate - should have plenty of setback adjustment to clear your clamps off the transom.


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## Vertigo (Jun 3, 2012)

Why not just add a spacer plate of some kind between the jack plate and the transom?


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## birdyshooter (Sep 17, 2019)

Ok, I finally got around to doing some "engineering" on these trim tabs. Now I'll be able to pull them up fully while under way. That's the theory anyway. Tomorrow will tell.


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## jay.bush1434 (Dec 27, 2014)

Put the cav plate foil back on. Tabs and cav plate work together but are for different reasons. The cav plate can act like a big trim tab but its primary purpose is to keep water on top of the prop. Tabs primary job is to level the boat fore/aft and side/side


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## birdyshooter (Sep 17, 2019)

jay.bush1434 said:


> Put the cav plate foil back on. Tabs and cav plate work together but are for different reasons. The cav plate can act like a big trim tab but its primary purpose is to keep water on top of the prop. Tabs primary job is to level the boat fore/aft and side/side


My engineering seems to have solved my issue perfectly. Once I'm on plane, I grab the ropes and pull the tabs up. The issue was the tabs were always forcing the stern up. Problem solved!!


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## DuckNut (Apr 3, 2009)

birdyshooter said:


> The issue was the tabs were always forcing the stern up.


Close. In reality they were forcing the bow down giving you more running surface. That running surface is probably close to 7 degrees. Now that you lift the tabs you are running probably 12-15 degrees lowering the prop in the water.

Any way you slice it - glad you got the mystery solved.


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## Steve_Mevers (Feb 8, 2013)

4 blade props help to eliminate ventilation in turns.


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## Smackdaddy53 (Dec 31, 2012)

Steve_Mevers said:


> 4 blade props help to eliminate ventilation in turns.


So do compression plates and heavily cupped three blades with the correct shape.

Here are common misconceptions the internet can be given credit for...

Four blades = hole shot and grip in turns 

Three blade = speed


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## Steve_Mevers (Feb 8, 2013)

Smackdaddy53 said:


> So do compression plates and heavily cupped three blades with the correct shape.
> 
> Here are common misconceptions the internet can be given credit for...
> 
> ...


With all due respect I would disagree with your conclusion. 4 blade props due help with hole shot and more importantly grip in turns. We had several boats in our fleet that slipped badly in turns and once we changed them to a 4 blade prop it eliminated the problem. I have an Action Craft that with a 3 blade prop the bow raised high with putting it on plane, and it porpoised and ventilated in turns. Bought a Powertech 4 blade prop and it raised the stern which eliminated the porposing and help with hole shot, and it stopped ventilating in turns. I bought a Powertech 4 blade for a tunnel hull boat I had and it was night and day difference in performance. Admittingly, I could care less about speed, I am looking for performance.


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