Prop Help

J

jtaggart

Seaman Apprentice
Joined
Jun 20, 2018
Messages
30
I repowered this past winter and I am looking for some help dialing in the prop. The boat is a 1985 Starcraft MR220V with a 2018 Yamaha FS115XB, currently it is propped with an Aluminum Yamaha Prop 3x13-5/8"x14". I get about 24-25mph with the motor at 5000-5200 RPM, anything beyond that I lose grip and the boat begins to come off plane. I'm not familiar enough with all of the physics to determine what variable to change, prop size or pitch, up or down, I've tried adjusting the trim angle of the motor and that hasn't resolved the issue. Any advice is appreciated.
 
Scott06

Scott06

Admiral
Joined
Apr 20, 2014
Messages
6,428
jtaggart said:
I repowered this past winter and I am looking for some help dialing in the prop. The boat is a 1985 Starcraft MR220V with a 2018 Yamaha FS115XB, currently it is propped with an Aluminum Yamaha Prop 3x13-5/8"x14". I get about 24-25mph with the motor at 5000-5200 RPM, anything beyond that I lose grip and the boat begins to come off plane. I'm not familiar enough with all of the physics to determine what variable to change, prop size or pitch, up or down, I've tried adjusting the trim angle of the motor and that hasn't resolved the issue. Any advice is appreciated.
Click to expand...
What is the motor height - how high is the cavitation plate in relation to the hull bottom ? Thinking motor is set too high.
 
J

jtaggart

Seaman Apprentice
Joined
Jun 20, 2018
Messages
30
It's on a jack plate, same height as the old motor, never adjusted. I don't have a good picture on my phone I can take one later today.
 
Scott06

Scott06

Admiral
Joined
Apr 20, 2014
Messages
6,428
jtaggart said:
It's on a jack plate, same height as the old motor, never adjusted. I don't have a good picture on my phone I can take one later today.
Click to expand...
Even better if it’s on a jack plate , play with dropping it some

yiu can look at pics if you google it. Cav plate ( lower of the two plates) should be even with bottom of boat. If you run at higher heights or are on a power cat you can run a prop made for areated water like Revolution 4 , or something with double cupping.

is it a powered jack plate or manual
 
Bondo

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,953
Ayuh,..... It looks to me, that you have a long shaft motor on a short transom,....
 
Q

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,342
jtaggart said:
Thinking I need to drop the motor a few inchesView attachment 396586
Click to expand...
For a big boat to engine ratio like you have…I’d say you are running that engine height a little high there. You look about 2-3 inches above the lower hull line. That’s fine on a high performance boat with the power to lift the hull with trim and a stainless prop to hold onto the water. But maybe not ideal with what you have. I’d be closer to the lower line of the hull with the lower plate. Ideally an inch or two lower maybe. That will help things lots.
Did she used to be an inboard powered boat by any chance ? That sometimes doesn’t help the dynamics of such a thing, if she’s been converted to take an outboard.
 
J

jtaggart

Seaman Apprentice
Joined
Jun 20, 2018
Messages
30
I lowered the motor a hole over the weekend and was able to get out for a test run yesterday. I was able to rev the motor out all the way without blowing out, picked up 5 mph over all. The goal was 27 or 28 mph, hit 30.5 last night turning 6000-6100 rpm. Debating stepping the prop up a step currently running a 13 1/2 x 14. Overall I am happy with the performance.
 
J

jtaggart

Seaman Apprentice
Joined
Jun 20, 2018
Messages
30
QBhoy said:
For a big boat to engine ratio like you have…I’d say you are running that engine height a little high there. You look about 2-3 inches above the lower hull line. That’s fine on a high performance boat with the power to lift the hull with trim and a stainless prop to hold onto the water. But maybe not ideal with what you have. I’d be closer to the lower line of the hull with the lower plate. Ideally an inch or two lower maybe. That will help things lots.
Did she used to be an inboard powered boat by any chance ? That sometimes doesn’t help the dynamics of such a thing, if she’s been converted to take an outboard.
Click to expand...
This was an outboard. It still had a 1985 Johnson on it when I bought it and I ran that up until November of last year when the lower end went.
 
Q

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,342
jtaggart said:
I lowered the motor a hole over the weekend and was able to get out for a test run yesterday. I was able to rev the motor out all the way without blowing out, picked up 5 mph over all. The goal was 27 or 28 mph, hit 30.5 last night turning 6000-6100 rpm. Debating stepping the prop up a step currently running a 13 1/2 x 14. Overall I am happy with the performance.
Click to expand...
Excellent
 
jimmbo

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
13,446
What is the WOT Rpm range for your Motor? My Guess is 5000 - 5500. If so, you are over revving at 6000 - 6100 and will need about 3 inches more Pitch, so a 17 will be in the Ballpark
 
A

airshot

Vice Admiral
Joined
Jul 22, 2008
Messages
5,080
Bondo said:
Ayuh,..... It looks to me, that you have a long shaft motor on a short transom,....
Click to expand...
Me thinks you typed that backwards...short shaft on a tall transom Mabey ?? He needs to get that lower cav plate closer to the bottom of the hull.
 
Texasmark

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,666
Lot of verbiage here but I see a deep V, high dead rise at the transom boat meaning that the engine height is OK, if not a couple if inches too high....but that might not be a problem or may be.
My first thought is pull the aluminum prop, throw it in the trash and install a good quality SS with good rake and cupped blade tips. Pitch would depend on the rated engine RPMs with the normal load in the boat. Assuming that the operator knows how to use his PTT to his support, not to his/her detriment.
 
You must log in or register to reply here.
Top