Aquasport Mechanicals - things that need a wrench, screwdriver or multimeter > Props

1970 191 Gull prop/trim tab question

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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 :salut2:

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?

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