1970 191 Gull prop/trim tab question

[dwilson1982]

Hi, I have a 1970 191 gull tower boat with raised tampa style transom powered with a 2017 yamaha 115 vmax sho. I recently went from a 3 blade to a 4 blade prop to plane the boat quicker. I am looking for advice on pitch. 4 blade is 13 1/4 x 15 getting right at 6k rpms full throttle. Time to plane fully loaded is around 5 seconds no trim tabs are on my boat.  Should I go down in pitch or just bite the bullet and install trim tabs to solve this? I need to get on plane around the 3 second mark. Thanks, any info will help.  Pics are attached so you can see setup to better help.

[mshugg]

Nice looking boat.  It looks like the max RPM for the 115 Sho is 6300.  You’re close to that.  You could drop down i pitch, but I’d look elsewhere.  Tabs would defiantly help.  Another option would be adding ventilation holes , or opening up the ones you have, might allow the prop to spin up a bit faster.

[boatnamesue]

Beautiful rig man, wow.  Rule of thumb for pitch is 150-200 rpms per inch of pitch.  You be hard pressed to find a 13" for a V4 to gain the extra rpms.  The out of hole jump you're wanting isn't gonna change by 2 seconds though.  Tabs, yes. 

I'm thinking the performance you're wanting isn't prop related, as it's pretty dialed in.  More power is the solution, or perhaps lower expectations if you don't install tabs.  The SHO has the low end torque, but you're asking a 115hp to jump a dry weight hull of 2k on plane in less than 5secs.  Add another 600-700lbs for fuel and gear, another 400lbs for engine.   Thats a lot of weight for a 115hp to jump up as quick as you want.  Just my 2cents.   Tabs be a great addition for that size hull, and I believe you'll be happy with the planing time. 

[Mwar410]

I'd give up on that boat, I might know someone who'd take it off your hands

Have you tried a more cleaver style prop?

[dbiscayne]

That hull should be quite a bit lighter than 2000lbs, the spec'd hull weight was 1300 for the early models but without throwing mine on a scale can't say for sure.  The later models with the different cap were up there though.
I agree though 3 sec. is still asking for quite a bit, a couple ideas - can you shim that porta-bracket & get a better angle to get it up on plane? 
Try another prop, go up in diameter if you can & keep the pitch.
BTW cool boat, any pics of it sitting in the water?

[Fish Head]

 Just a thought, I added a Stingray Hydrofoil to my cavitation plate on the 120hp 19’6 and it made a huge difference in time to plane. Helped with turning also. Cheers T

[dwilson1982]

Thanks for all the input. I agree with these comments. I do get on plane in a decent time but I guess I am expecting to much out of it. I did not want to go to a 150 due to weight. I am already below the scuppers as it is a tiny bit. A 150 would be too heavy. Here is a pic in the water. Trim tabs are going on for sure. I have never seen a cavitation plate on my style boat, not sure that would help?? Thoughts??

[dbiscayne]

cavitation plate is just referring to the horizontal plate on the outboard lower unit just above the prop, maybe you meant the Stingray Hydrofoil?  Seems like those work pretty good for some, I've never used one but would give it a shot.

[Fish Head]

It’s the grey wing above the prop. You can pick one up for around $50/$80. I was going to add trim tabs if the Stingray Hydrofoil did not solve the problem but it did wonders.No issues pulling a couple tubers.I couldn’t tell you if it affected my top speed,not a speed demon. Less holes in the transom the better I say.Cheers T

[boatnamesue]

Install tabs.  Drop'um down when jumping up on plane.  The planing time will dramatically decrease.  You're good to go. 

[GoneFission]

The plate above the prop is actually a "ventilation plate" not a capitation plate , but a lot of folks use the incorrect name... 

Anyway, several of us have tried the fins (Stingray, Doel-Fin, etc.) on Aquasport boats - the general conclusion is they don't work well on these boats.  When properly mounted, the fin is just barely touching the water at speed and it causes some real problems when going over waves, etc.  The fins work well on I/O setups or where the motor is mounted low on the transom - that does not help your skinny water problem! 

Tabs are probably your best option unless you want to add more power.  Another option is a 5-blade prop - they pull like a bankrupt dentist!  The High-5 has the best hole shot of almost any prop and the smaller diameter (13") allows for a little higher engine mount. 

[Fish Head]

Actually the plate above the prop is called the cavitation plate. I could post you more examples if you like. I do not use a hydraulic jack plate on my setup so the cavitation plate with a mounted hydrofoil is lined up directly with the bottom of my boat. Works Awesome! I guess if I was fishing skinny water with adjustable jack plate and lift the cavitation plate higher than the bottom of the boat with a hydrofoil than yes I believe it could cause issues.
Cheers T

[Capt. Bob]

Actually the plate above the prop is called the cavitation plate. I could post you more examples if you like.

I have no doubt you could but.....

That stated, one can call it what one likes but it is designed specifically for the reduction of outside air entering the prop rotation area which results in loss of "bite". Boat props (unlike airplane props) work well in water, not so good in air. Every outboard manufacturer I've ever seen list it as an anti vent. plate.

Cavitation is an entirely different phenomenon which is air bubbles (caused by a pressure differential) forming on the back and tip of the prop. This occurs as the prop spins, creating that difference in pressure. I believe engineers call it "cold boiling" of the water on the prop surface. That cavitation or anti-vent plate (call it what you wish), does not prevent this from happening. It is not why it is molded into the lower unit so technically, it should be named for what it actually does or purpose it serves. Still many, many people interchange those two words. It's been happening since outboards have been in existence.

[TikiDoc]

Capt. Bob is closest to correct.  The plate in question is definitely an anti-ventilation plate, intended to prevent air from being entrained into the fluid stream of water.  Air being compressible, it reduces the effectiveness of the prop because some of the torque goes to compressing the air and not producing thrust. Thus the anti-ventilation effort is a component of the outboard design.

Avoiding cavitation is a goal exclusive to design of the propeller.  The phenomenon is described by the Bernoulli equation, which is one solution of the continuity equation.  Simply stated, it requires that total pressure is conserved.  Total pressure equals static pressure plus velocity pressure.  This means when the velocity increases high enough at one tiny spot on the prop surface, the resultant static pressure drop can be low enough to boil the water at ambient temperature, creating not air bubbles but steam bubbles.  This is what happens when you crack your knuckles.  The synovial fluid is briefly accelerated enough to boil a small amount.  The pop you hear is the bubble collapsing.  The energy in this small pop is insignificant, but a stream of millions of little bubbles from a continuous cavitation process can erode the area of the prop where the bubbles strike.  Thus prop design seeks to avoid cavitation.  It has nothing to do with the anti-ventilation plate.

[Capt. Bob]

Thanks professor.