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PLEASE HELP: How can I get the most out of my Mariner (Mercury) 50hp outboard?

J

jaykmar

Member
Hello everyone,

Im a recent boat owner but long time 2 stroker (ive rebuilt many 2 stroke motorbikes in my time). I was never interested in boating until I was offered a boat fairly cheap with an engine that needed TLC. Now, after a fair few hours I have a working engine on a pretty neat boat.
Now, I want to try and get the most bang for my buck.
I've done quite a bit of research and have come to the following conclusions (along with questions). But first, a little about my boat and motor :cool:

  • 3.93m (12' 11") runabout. Width at transfom is 1.55m (5' 1") and length from bottom of hull (centre rib) to outer most part of hull is 0.820m (2' 8"). Not sure if that measurement is needed though! :p
  • Fibre glass. Low profile and fairly light (max 4 person capacity)
  • 50hp Mariner (inline 4) 2 stroker. SN is 6453200 so going by the internet it is a 1983 or 1984 model. Mercury made.
  • Main purpose of the boat is for cruising. Top speed is more important to me than quick acceleration
  • I was hoping to do some skiing from it too (two ski's. beach start, considering the small engine) but after some reading up I feel that the sort of prop and tuning I am hoping to do will render the boat/engine fairly inadequate for skiiing. No problem for me. Id prefer the speed :)
  • I am getting about ~120psi per cylinder. Not the best but not bad either. I am not up for rebuilding the top end any time soon (or at all!) as its just not worth the $$
  • I tuned it recently and replaced the carby floats and main needles (they were worn)
  • Currently, I am getting 50km/h (31mph) with 2 people in the boat (each weighing about 80kg's [176lbs]) at WOT with ~5500rpm.
So my questions:

  1. The existing prop (3 blade, aluminium) has scuff marks and some dings. I want to change it but dont know what to change it to. I cant find the existing figures on it so have no idea on the pitch or diameter. My reading up has said I should change to a slightly higher pitch (or highest pitch possible) for higher top end speed. Is this recommended? If so, which prop should I go for?
  2. I have read somewhere that changing to slightly larger carbies will help with more power (I can understand why). During my repairs of the engine I bought a set of 2nd hand carbies that are off a Mercury with a higher HP rating, but the carbies still fit the same block. I am still yet to do my research on them, but assuming they are bigger carbies (im fairly sure they are) could I replace my existing ones for these and hope to get a performance increase? It really is at no additional cost to me and about an hours worth of work. :)
  3. The hull has chips in the bottom. I read this will add noticeable drag to the boat. I have fibreglass putty and a new orbital sander. I read 600grit paper should be good for this sort of hull. Can someone confirm this is OK to do? I also read that sanding in the direction the water will flow helps too. But does it help so much that I need toss the orbital aside and do it all by hand?
  4. What is the recommended fuel octane level for this engine? Living in Australia we have access to 92, 95 and 98 octane fuels. Im currently using standard unleaded fuel (95 octane) but am happy to increase it to 98. But dont want to damage my engine as this is a cooler burning fuel and drier.
  5. I looked in my workshop manual and cant find an oil filter fitted to this motor. Is there one? If so, is it worth it if I change it to a high flow filter?
  6. I removed the inline fuel filter beside the hand pump on the fuel line (this was extra, not needed) but there is still the fuel filter in the engine and then another gauze/filter at each carby. Im obviously not going to remove these, but is there a high flow alternative I could use? Especially when considering ill be putting on bigger carbies. Would I also need to change to better reed valves (ie. Boyesen) to allow for better fuel flow?
  7. I have replaced the spark plugs with what was recommended by the manufacturer. However, would I get a performance increase if I fit it with high performance spark plugs with 1 heat range designation higher than manufacturer specs?
  8. The motor has a Hydrofoil attached to it. I read a few people saying to get rid of it. Should I? Its a fairly small (light) boat so perhaps this isnt such a good idea for me.
PHEW! Long post. Sorry about the wall of text. I believe the prop and hull repair would bring me most power, but any small thing I can do to help is great.

Many thanks for your help! Much appreicated! :eek:
 
Last edited:
Well---------------------The one question ( #5 ) about an oil filter on a 2 stroke has me puzzled.------------------------I thought a 2 stroke------------- " runs on clean oil all the time "--------------but who knows.
 
racerone said:
Well---------------------The one question ( #5 ) about an oil filter on a 2 stroke has me puzzled.------------------------I thought a 2 stroke------------- " runs on clean oil all the time "--------------but who knows.
Click to expand...

Im sorry. I meant gearbox oil. oops :rolleyes:
 
1/ Prop can be repaired but if you would prefer a new one,take it along to a shop and they will tell you the pitch.As you have the max RPM's for your engine,you could rise a pitch,this will bring you down aprox 200RPM's,but still keep you within the range and will increase top end a little.
2/The larger carbs will give you more power,about an increase of 2-3mph and 10-15% increase in fuel consumption.I believe it's illegal to change the HP of an engine in the 'States ;)
3/If the chips are filled and sanded,the difference the direction of the sanding is going to make is the same as the amount of hair on the body of an Olympic swimmer,.001 of a sec over 100m,that might be the difference of gold or silver in the pool but little or none on a standard 13' boat's speed.:D
4/92 will do fine.
5/No oil filter
6/If there's nothing wrong with the one's there I would leave them alone. "If it's not broken,don't fix it"
7/See above
8/My inexpert opinion,on a small boat like this,dump them as they put a fair bit of pressure on the cavatation plate and as you aren't looking for hole shot.
Here is a small calculation on the max speed you will get from a 13'.I'm only guessing the hull length at water line.
[FONT="]square root of 11 = 3.316 x 1.34 x 1.15 = 5.11 mph x 7 = 35.7 [/FONT]is what you get,which is 4mph less than you are getting.So no matter what you hang on there that's going to be near your top speed,without making the boat unstable that is.So dump your buddy,keep the beer,a higher prop pitch and there you are.All calculations are guess work and not gospel and I will not be held libel for any accidents,damage to egos,boat,wallets,engines,beer cans,friendships or any other misfortunes that might befall on you :D
And just to spoil your fun what's the max HP rating for the boat? 80% of the max is usually optimal.:)
 
Thanks haventaclue!

Im not sure what the max rated HP is for this boat. Im assuming its 50 or parhaps less. I assume that because the previous owner has reinforced the transfom so im guessing this engine may have been too big for the standard transfom. In any case, its an older enginer so I doubt its really pushing 50 horses.

Ill remove the hydrofoil today and see if that helps. I should be taking the boat out today.
Ill also swap over those carbies on the weekend (no skin off my nose there, I already have them) and pull off to get a match with a higher pitch.

As for sanding the hull. Im glad i can use the orbital :) It would be a matter of an hour or two with that little gem. Looking forward to getting the boat on some sort of block/hoist and getting started :)

Thanks for your help and any other opinions are definitely welcome :)
 
Also, how does this calculation work?
[FONT="]square root of 11 = 3.316 x 1.34 x 1.15 = 5.11 mph x 7 = 35.7

Can you post the forumla please? Thanks
[/FONT]
 
I have only seen transoms reinforced when there is something wrong with the boat.-----------------Most transoms are cored with wood.------------After 20 or 25 years many are rotten to the core.
 
As racerone says,transoms are usually replaced if the core goes rotten. As your 50HP BHP block horse power, is pre '85 when the brains changed from BHP measuring HP at the prop. SHP There is about 10% difference in favor of the SHP,so in effect you have a 45HP.Now that is an aprox estimate.There is a formula,which I haven't got and anyway my poor old guinness ridden brain wouldn't be able to handle it.:D
The above formula I lifted from Graham on this forum.It is simplified for non mathematicians,me. Direct from Graham,


" [FONT="]If you are interested in the math to figure maximum speed for a production "planing" hull it's simply the square root of the hull length at the waterline, multiplied by 1.34 multiplied by 1.15 (the 1.15 part is to convert the answer which comes out as Knots into MPH) multiplied by 7 (7x the hull speed is the factor for a planing hull in most production boats).

So if you take a typical production hull in the 16 1/2 to 18 foot range which has a waterline length of 16 feet (just to keep the math simple enough to do in my head) you would calculate as follows:

square root of 16' = 4 x 1.34 x 1.15 = 6.16 mph x 7

So the hull speed of 6.14 mph (displacement speed) x 7 = 43 mph (43.148) which would be the max speed, regardless of horsepower for the fictitious hull I used."
He says it simpler than I could. Hi Graham,my apologies for lifting your stuff :eek:

[/FONT]
 
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