[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]

anyone?

[ob]

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]

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]

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.whaley I would appreciate if you would check into for me

[ob]

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]

Buy a bigger motor if you want to go faster.

[snooks2005]

ob thanks for your time.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. 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]

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

[ob]

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]

I forgot to mention that my wot should be 6000 rpm

[ob]

What's the lenghth and approx weight of boat?

[snooks2005]

16' flats boat weighs about 900lbs

[ob]

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]

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]

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]

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

[ob]

Even if motor is trimmed in all the way?

[snooks2005]

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

[ob]

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.

[snooks2005]

yeah everybody seems to be out of ideas what tuneups would you suggest?

[BillP]

Two suggestions...reduce the prop diameter and use a 4 blade prop on jackplates...pitch way up if you are mostly running with the jackplate up. Put a Doelfin on it. The Doelfin let me get approx 5 mph faster just by letting me trim higher...expecially so if you are using a 3 blade. Don't forget the low water pickup and presure gage when getting this high.

[snooks2005]

BillP.....I have already tried the smallest diameter 4 bladed prop that power tech makes which is a 12 3/4 x 12 pitch. So what you are saying is to have the diameter of this prop reduced?

[BillP]

You need less diameter and lots more pitch to make approx 6000 at full throttle while jacked up. If you can't find a stock 4 blade look for a 3 and size for around maybe 18 pitch. This stuff is really trial and error. Math only gets you to the starting gate. Your 3 blade may be much better if you try the Doelfin...I would do that before anything else. The one I added made a whopping big difference at top end. It reduced cavitation/ventillation a huge amount when trimmed up. You have another 400 rpms to play with...presuming the max is 6000? I currently have a Mariner 50 4s with 2:0 gearcase and the max is 6000...3 blade propped at 5600 on a 20' pontoon.I ran in a hardcore flats boat that was pitched at 27 (yes 27)on a 4 blade. This one had a 175 Merc and all the go fast goodies for shallow running. It literally screwed through soft bottom in ankle deep water at 45mph with 2 up...it ran 65 in clean water. It was a 20 "Crossover" (650lbs kevlar)and made in your neck of the woods.