replaced lower unit, need prop?

jaz4150

Recruit
Joined
Aug 1, 2013
Messages
3
so here's the deal, I got a boat on the cheap and I am bringing it back to life, it's a 1990 19 ft fisher aluminum center console with a 90 hp mariner two stroke with carburetors and oil injection, the lower unit would not shift into gear when I got it, and I got a deal on a used lower unit that came off a 115, it bolted up fine and works well, the boat comes up on plane well, and runs fine at speed, but I'm running well below wot rpm (around 1000 lower), so is the rpm drop simply a product of the gearing in the lower, or do I need to re-prop, as I said it comes up on plane easily with 2 people, coolers and fishing gear, so I don't think loss of torque is an issue (very light boat). is there any way I can calculate what prop I need or do I need to just experiment? or do I need to work on pulling the original lower apart and fixing it (it was used in salt water and not well maintained so getting apart may be tricky). I'm not sure of the gear ratio in either lower (not at home to check), the prop is original to the boat/motor I'm pretty sure it's a 13" pitch (not at home to check right now) 3 blade stainless.
Any help would be greatly appreciated.:confused:
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: replaced lower unit, need prop?

My sketchy information indicates the 90 and 115 have the same ratio,2.00:1You could remove the plugs and mark flywheel and prop and count flywheel revolutions for one prop revolution.A 13" seems to me close to what you would find on a pontoon.
Assuming I'm correct on the ratio the boat should leap on plane and over rev.
A lightly loaded test run with prop size,wot rpm and gps speed will help evaluate your set up.Some cells have a gps app.
If the tach is accurate you would need to drop about 5" in pitch to regain about 1000 rpm Not possible with a 13" prop.
 

jaz4150

Recruit
Joined
Aug 1, 2013
Messages
3
Re: replaced lower unit, need prop?

My sketchy information indicates the 90 and 115 have the same ratio,2.00:1You could remove the plugs and mark flywheel and prop and count flywheel revolutions for one prop revolution.A 13" seems to me close to what you would find on a pontoon.
Assuming I'm correct on the ratio the boat should leap on plane and over rev.
A lightly loaded test run with prop size,wot rpm and gps speed will help evaluate your set up.Some cells have a gps app.
If the tach is accurate you would need to drop about 5" in pitch to regain about 1000 rpm Not possible with a 13" prop.

Thanks for the help, I'll have to check the ratio when I get home, I'm overseas right now and trying to plan my attack for when I get home in a few weeks. it doesn't over rev, it actually runs very well for am old boat that wasn't taken care of properly. I was expecting far more to get it up and running (the old man I bought it from said he hadn't been able to even get up into it in 5 years) I actually have not had the motor gone though yet... I did it myself, but I want to have a professional go trough it, set the timing and adjust everything etc, before I get too much further. I have a GPS on the boat as I recall she ran 40/45 mph at wot, it popped up on plane like a cork (but it's light) and ran smooth after that, I had left it in the water (brackish canal) while I was experimenting and after a few weeks it had so many barnacles and mini clams on it that that it wouldn't plane at all so I had to pull it to scrape it (won't make that mistake again), then I left to go overseas. so now I'm about to attack it again and want to maximize my limited home time, I'll have to check the accuracy of the boat's tach with my shop tach, but I think I did so last summer and it was dead on, can't hurt to re-check though (need to start keeping notes:facepalm:). I'm making an assumption that the ratios are different, the stickers a long gone on both, and the original wont go into gear to verify. if the 90 and 115 do use the same ratio though I can eliminate that as a problem, I'll have to wait to check though...grrr
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: replaced lower unit, need prop?

If you can give us your actual rpm and and gps speed, best you can recall, not 40-45 try to pick a number.
rpm within about 100 rpm.
I can tell you its not a 13" pitch at 4500 that would be about 24 mph.40 at 4500 would be about a 21 or 22" pitch.
I think your prop will probably be about 17" I don't think the motor can pull a 21 and pop on plane.
At 40 a 17" prop would be turning about 5600,so your tach may be off.Not unusual.
It does appear the motor is reasonably sound.
 

jaz4150

Recruit
Joined
Aug 1, 2013
Messages
3
Re: replaced lower unit, need prop?

If you can give us your actual rpm and and gps speed, best you can recall, not 40-45 try to pick a number.
rpm within about 100 rpm.
I can tell you its not a 13" pitch at 4500 that would be about 24 mph.40 at 4500 would be about a 21 or 22" pitch.
I think your prop will probably be about 17" I don't think the motor can pull a 21 and pop on plane.
At 40 a 17" prop would be turning about 5600,so your tach may be off.Not unusual.
It does appear the motor is reasonably sound.

guess that means I'll have to spend some time on the water testing the boat.... gosh that'll be HARD, what a lousy way to spend the day :joyous: well I'll have to get the motor tuned, I'll have the Mercury dealer check the tach while there at it, then take some speed runs to check it, just have to wait till I get home to verify the prop pitch.
 
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