Author Topic: Share your Gt150 setup  (Read 2451 times)

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Offline 1973GT150

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Share your Gt150 setup
« on: July 21, 2019, 06:30:53 PM »


So I finally splashed my boat after a long three year restoration. Im currently running a 1984 90hp in-line merc six cylinder mounted in the second hole from the top and I tested it with Mercury’s spitfire aluminum four blade prop 17 pitch. It as a amazing hole shot and turns up 5000rpm trimmed up with one person and 12 gallons of fuel (I installed 2-6 gallon fuel tanks under the floor.). With a lot of trim I still get 8-9 psi of water pressure. Now I know max wide open throttle on these engines is 4500-5000 but most guys I speak to say they are safe running closer to 5500. So what setup are you guys running on your gt150 as far as engine height,horsepower,and prop(aluminum-stainless,3-4 blade,pitch). I plan on running the same style prop in stainless.

Offline Hyperacme

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Re: Share your Gt150 setup
« Reply #1 on: July 21, 2019, 07:37:16 PM »
Some of the other GT150 owners should post ..
I know Tim was getting some pretty good speed out of his GT150 / Merc 115 HP.

I have a CV16 that has same basic handling and performance as the GT150's.
I was getting about 50 mph/gps with motor down on transom and a 20P Laser II prop.
Rebuilt transom, lifted motor to one hole up from bottom with cavitation plate is about two inches above hull bottom, installed Baystar steering and have a 22P Laser II .. I'm getting about 53 to 54 MPH / GPS with half tank of fuel and good water.
My water pressure is still good at about 15 PSI at that speed. I haven't moved to bottom hole yet, but will soon.
Your biggest problem when lifting will be steering set up.
You'll have to cut a new hole to run cable or go with a Baystar.

I think there was a "GT150 performance" thread on CGOA forum ..
Search thru some of "Mark S" threads on CGOA.
Joe's GT150 thread is here …
http://forum.cgoamn.com/index.php?topic=4313.0

Offline 1973GT150

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Re: Share your Gt150 setup
« Reply #2 on: July 22, 2019, 06:40:03 PM »
I already installed hydraulic steering I wanted nothing to do with the cable setup. I’m running uflex steering not seastars setup. What engine and horsepower you running on your boat and what wide open rpm’s are you seeing with that 22p Laser prop.

Offline Hyperacme

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Re: Share your Gt150 setup
« Reply #3 on: July 22, 2019, 10:14:08 PM »
1976 CV16 / 1989 Mariner 115 HP / 22P Laser II.
My motor is first year for the I4 115 HP Merc's with the 2+2 thing. Runs on two cylinders until 2000 rpm's then the other two cylinders kick in. Weird .. But motor runs good .. When it was released it had a 5500 rpm red line but after having crank problems they drop max rpm's to 5250 rpm's max.

I heard lifting to bottom hole was fine .. as far as water psi go's .. Wanted to be safe so only went to second from bottom.
I think it would be OK to drop to bottom, if not and water PSI drops, you can put some screws in top holes of water intakes on LU.
Don did it on one of his fancy race boats.
When I lifted motor the tilt ram blocked my splash well drain hole so I added T&H marine transom wedges to get clearance.

My cavitation plate is about two inches above bottom of hull, bottom hole would be about three inches above bottom, give or take ..
At my motor's max rpm's of 5250 rpm's, with the 22P, me and half tank of fuel I can get 53 to 54 MPH/GPS.
At 54 mph I'm get close to 5300 rpm's .. Maybe a bit more ..
I have a 18 gal. bow mounted fuel tank. If I add a 200 lbs. passenger I lose 2 to 3 MPH top speed.

I though all I6 Merc's ran 5500 to 5600 .. But after doing some searches I see some are only 5000 rpm's.
20P Laser II's are pretty easy to find and if it won't work are pretty easy to sell, or bring to a prop shop and lower or raise pitch ..

With props it all guessing until ya put it on the boat and try it ..
« Last Edit: July 22, 2019, 10:25:43 PM by Hyperacme »

Offline Hyperacme

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Re: Share your Gt150 setup
« Reply #4 on: July 22, 2019, 10:38:58 PM »
Jeff took the hook out of his GT150's hull and extended the inner chine's to stern.
Won't know how well it preforms for awhile .. He got a Timmi and shelved the GT .. LOL

Offline 1973GT150

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Re: Share your Gt150 setup
« Reply #5 on: July 29, 2019, 06:28:04 AM »
i know a lot of manufacturers purposely put hook in the bottoms to compensate for prop torque to keep the boat riding level, kinda like a built in trim tab. so i am now up on the fourth hole and still testing with the four blade spitfire. so water pressure was ok but did lose grip in the turns until i trimmed in all the way. still i only picked up 200 rpm with a lot of trim. going to test next with a standard blackmax 17p aluminum 3 blade. im thinking maybe the 90hp inline just doesn't work well with this prop, maybe it needs to have that slip of the 3 blade prop. i will test again and report my findings.

Offline Hyperacme

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Re: Share your Gt150 setup
« Reply #6 on: July 29, 2019, 09:43:18 AM »
Yep .. Sharp turns with motor jacked will blow out prop if not trimmed in .. Kind of a surprise the first time I did it ..
I've heard 4 blades are good for heavy boats, like bass boats. I've never tried one so can't say for sure how they preform.
I did try one of Dave's old 2 blade bronze racing props, racing prop's had a cup. I think it was 21 or 22P.
It had very good speed and handling, was with in one MPH of my 22P Laser II prop's top speed and low torque steer.

A 17P Alum. seems to small .. Maybe your LU was replaced at some point ?
What are you getting for speed's with the prop's ?

Offline 1973GT150

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Re: Share your Gt150 setup
« Reply #7 on: August 01, 2019, 08:57:10 PM »
Hard to say I need to download a speed app to my phone to record speed. I have been so focused on getting closer to 5400-5500 rpm. I know merc called for lower rpm but everyone I speak to says the 90hp is a bit better closer to that rpm. I am running it saltwater as well. Gonna try the 3 blade prop this weekend if possible.