Marlin prop size

[fishon]

We have a 2000 marlin with twin 03 F225’s spinning a 15 1/2 x 17 power tech prop’s. WOT 55-5600 rpm’s. Corrosion I believe go best of the starboard motor. Decided to repower with new 4.2 F250’s running the same props with the same load we are getting 46-4800 rpm’s WOT. The motors an almost 45lbs lighter per motor. Checked throttles to make sure they’re open all the way. Tech is recommending going down in pitch , why ? Those motors should be spinning those props with a bit more enthusiasm. Any ideas ? Could lower unit gearing have any thing to do with it ?

 

[Hookup1]

Talk to Ken at www.PropGods.com .

RPM's seem a little low. Less pitch doesn't alway mean less speed.

 

[fishon]

I’m less concerned about speed and more concerned about lugging the engines. I just don’t understand how a pair of 3.3 F225’s on the same boat with same props spin at 5500 and then put on a pair of 4.2 F250’s , same boat same props and less load only spin 46-4800. Doesn’t make sense to me.

 

[Hookup1]

Ask an expert. It's a free phone call. Start there.

 

[Ekea]

are the gear ratios in the lower unit the same?

 

[Ekea]

a bit of google searching is pointing to the 3.3L being 2:1 and the 4.2 being 1.75:1. it comes out to about a 14% difference. 4800 x 1.14 = 5485. that seems to be your answer.

i do think you should still be doing better with the 4.2 though, although i do recall one guy saying their 3.3 did better than the 4.2

 

[Ekea]

the props you have are what GW is putting on the boat with twin 4.2L F300s now

 

[DennisG01]

Are the engines mounted the same (height)?

You'd have to drop down in pitch quite a bit to get that amount of RPM back... and that doesn't seem right.

 

[Harrysea]

Those 4.2L F250s are strong 250s and actually have about 50 more HP each than the old 3.3L F225s. Plus more torque. Even with the gear ratio difference, your new motors ought to reach higher RPM with those props. you need another thousand RPM. I would have guessed 15 pitch props for those engines on a Marlin to get as close to 6,000 RPM as possible.

 

[fishon]

The mounting is the same and a 25” shaft. But after doing some research it seems the gear ratio is different in the lower unit and a reduction in pitch maybe the answer. Found the others switch from a 3.3 to a 4.2 had similar issues and going with a 15 pitch from a 17 worked. Ekea seems you hit the nail on head. We pick up the boat in a couple weeks after bottom paint and will run her 65 miles home. Will up date numbers. Thank you guys for your input

 

[Ekea]

the thing is you really want to be as close to 6k as possible when light. assuming everything else is right you get about 150-200 rpm per inch of pitch, so if you dropped to 15" prop, you might gain 400 prm and that only gets you to 5200 at wot (theoretically could be more, but unlikely). so that doesnt fully correct for the different gear ratio in the lower, but it definitely gets you in the ball park of where you were before.

there are really two things to figure out here, the first is trying to get these motors to at least match the old motors performance (if not exceed) as we have been talking about in this thread, but the second is why your wot was only 55-5600 before, unless that was with a heavy load. we are currently aiming for a target that may not be optimal.

it might be a good idea as mentioned above to talk some of this through with ken at propgods. he got me set up very well.

 

[Automated14]

We have a 2000 marlin with twin 03 F225’s spinning a 15 1/2 x 17 power tech prop’s. WOT 55-5600 rpm’s. Corrosion I believe go best of the starboard motor. Decided to repower with new 4.2 F250’s running the same props with the same load we are getting 46-4800 rpm’s WOT. The motors an almost 45lbs lighter per motor. Checked throttles to make sure they’re open all the way. Tech is recommending going down in pitch , why ? Those motors should be spinning those props with a bit more enthusiasm. Any ideas ? Could lower unit gearing have any thing to do with it ?

I'd start with checking the engine mounting height. Antiventilation plates should be right on the water surface while at planing speed. I found the easiest way to figure it out is to take a 15/20 second video and then screen shot it. Trying to stay still and actually snap a pic is nearly impossible lol

Once that's dialed in, It's just trying different props till you get it right. I went through 5 sets of props on a recent repower till I got it where I wanted. the good news is you can usually find plenty of used props on the local FB marketplace and CL . Bad news is 15 pitch props seem to be kind of rare... around here at least they are. Below is pic of mine...mounted as high as possible, no more holes to go.

 

[fishon]

Thank you everyone for your help. I sent an email to Ken at prop god and waiting for a reply but also have a local prop shop that I’m working with and we will be going with a 15 x 15 stainless. They are actually taking by original props that are 15 1/2 x17 and are reducing the diameter by 1/2” and reducing the pitch to 15. This is being done in lieu purchasing a new set of Yamaha props at $1,800. We will see what the final labor cost is. I’ll keep you guys posted. The shop that is doing the work is Wilmington propeller in Southern California.

 

[Ekea]

i picked up new props and hubs for less than $1k. they were solas rubex and were recommended by prop gods. i did not ask for budget props, just asked for the best props for my boat.

why did they want to reduce the diameter. id think that a heavier boat would want a bigger diameter to get more traction/grip on the water

 

[fishon]

Finally an update on our prop choice. Bryan at valley prop in Ventura CA recommended a power tech four blade 15 1/4 x 14. Sea trail gave us a comfortable cruise at 3800 rpm , 23 knots and a fuel burn of 1.5 mpg. 4100 rpm , 26 knots 1.3-1.4 mpg. Max rpm is 5700 rpm in calm conditions. Almost thru with our 10 hour break in period. The four blades feel more responsive during close quarter maneuvering around the marina.

 

[Fishtales]

Yamaha has some performance bulletins and GW used to have them on their website as well (can't seem to find). Propos usually listed.

300-225 HP V6 4.2L Outboard Motors - Yamaha Outboards

The V6 4.2L Yamaha outboard has a range of horsepower: 300HP, 250HP or 225HP. It is designed for smarter, lighter and faster offshore power for your boat.

yamahaoutboards.com

 

[fishon]

Awesome , thank you