HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

[snooks2005]

I currently have a 1998 Mariner 50 4 Stroke Big Foot with a 2.31:1 lower unit and would like to change to the standard lower unit with 1.83:1 or the 2:1 lower unit. Does anyone know what model year lower would bolt up to my current engine. thanks

 

[snooks2005]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

anyone?

 

[ob]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

Snooks,I smell a need for more speed.Don't think motor will perform any better as these puppies are engineered to operate with the ratios provided .Best thing you can do is play around with prop pitches.

 

[orca]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

G'Day. I think that you the Merc 50 BF 4 Stroke uses a differant crankshaft to the standard model as the driveshaft uses differant splines, could be wrong though, I will check.

 

[snooks2005]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

Thanks guys...ob yes you do smell a need for more speed. But I have tried just about every prop that exists for this motor for the application I am using it for which is on a flats boat.<br /><br />whaley I would appreciate if you would check into for me

 

[ob]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

Snooks , What rpm are you able to attain at WOT in relation to what the motors recommended WOT range is?Is motor mounted at optimal height(as high as you can get it) on transom without cavitation?

 

[forgetaboutit]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

Buy a bigger motor if you want to go faster.

 

[snooks2005]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

ob thanks for your time.<br /><br />ob currently I am getting 5450 with a power tech 13 3/4 x 15 3 blade SCD model prop and as I raise the jackplate I can get alittle more rpm to about 5600 but after that the prop starts to slip and then it blows out. I am getting about 32 mph with this prop. <br /><br />I also have a power tech 12 3/4 x 12 XSB model 4 blade prop which I can raise a full five inches on the jackplate and get maybe 5500 rpm at about 5 inches of lift but nothing changes once I reach about 3 - 3 1/2 inches of lift in either the rpm range or speed range. With this prop I am getting about 29 mph.

 

[snooks2005]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

mr. forgetaboutit....thank you for your informative response...If I could sell my current engine I would not hesitate to repower.

 

[ob]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

Sounds like you've got your propping and motor height under control .Only thing that would concern me ,and I'm sure you, is that changing gear ratios to closer tolerance will obviously put more strain on motor,particularly out of hole performance.In theory it would go faster, but I'm not sure if your motor will have the juevos to get it there.Sort of like increasing prop pitch.

 

[snooks2005]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

I forgot to mention that my wot should be 6000 rpm

 

[ob]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

What's the lenghth and approx weight of boat?

 

[snooks2005]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

16' flats boat weighs about 900lbs

 

[ob]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

Snooks,that is a relatively heavy rig.Don't know what your added gear weighs,but here is a comparison example.I run a 15ft whaler that weighs 850 lbs. pushed with a 95 70 hp. rude at 6000+ with a 131/4 x17 omc prop and get 42 mph.That four stroke your running also likely is heavier than it's 2 stroke counterpart and when you match a relatively low hp to boat weight ratio every pound begins to have a significant effect on performance.In short it would be my guess,and just that,my guess ,that changing gear sets in lower unit would be an excersize in futility,not to mention the cost.

 

[snooks2005]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

well the boat weighs about 500 lbs the 50 four stroke weighs 228 lbs, the jackplate weighs 37 lbs and I also have a 20 gallon fuel tank in the bow.

 

[ob]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

That changes things a bit.Off hand it sure seems that motor would rap further than 5400 trimmed out with a 15" pitch prop.

 

[snooks2005]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

oh I forgot to mention also that it porpoises when the jackplate is raised also.

 

[ob]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

Even if motor is trimmed in all the way?

 

[snooks2005]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

no just when I have trimmed out the boat and have started to raise the jackplate is starts to porpoise.

 

[ob]

Re: HELP w/1998 Mariner 50 4 stroke Big Foot Lower Unit

Sounds ok as long as you can adjust it out.Man Snooks ,I am fresh out of ideas besides a good motor tuneup for your rig.