WOT speed on a '83 20' SF w/200
 

I've got an '83 20' SF with a 99 Merc 200. I've been playing with props and setup and can't get it to go more than about 46 mph gps. Shouldn't this be a 50+ mph boat?
Thanks,
Dave

Re: WOT speed on a '83 20' SF w/200
 

The appropriate prop needs to be installed to acquire 100% of the maximum rpm as specified by the motors OEM with a light load. That said, yes, it should be a 50 MPH boat. There are a number of factors that could slow her down (excessive weight, t-top, poor tune-up, hull condition, etc.), but the prop thing is a given.

Re: WOT speed on a '83 20' SF w/200
 

should have got a yamaha 200, you could have a 50+ boat with a 175 and a clean hull.

Re: WOT speed on a '83 20' SF w/200
 

Yamaha makes a good dependable outboard. So does Mercury.

The boat came with the Merc already on it. If I were starting from scratch, I might go with a Yami. If top speed were my only consideration, I'd stick with the Merc. All that being said, a 175 Yami ain't gonna be faster than a 200 2-stroke of any make. Horsepower is horsepower, it don't matter what color the engine is.

Re: WOT speed on a '83 20' SF w/200
 

...ah, welcome to the site.

Re: WOT speed on a '83 20' SF w/200
 

I would think so too, I have a 20 MA with a T Top and 200 Merc and I can get to 49 with the top and really and standard marginal Merc prop. Seems like you should but who knows maybe not just have fun with it.

Thanks for your good input FVMAKO17 it was helpful stop back in when we need more help :D

Re: WOT speed on a '83 20' SF w/200
 

I'm really just trying to get the best efficiency out of the setup. To some extent, that translates to top end speed. Besides, for me, part of the fun is tinkering with it.

I have the '83 post-Potter 20SF made by Seacraft Industries. I know it's 8" longer than a Potter 20SF, and maybe a little heavier, but I don't know if performance is similar. I've read here that the potter 20s are stern heavy. Mine seems to balance more toward the bow, at least at higher speeds. I couldn't get mine to porpoise at WOT to save my life, though it will under 4,000 rpm if it's trimmed up enough.

Re: WOT speed on a '83 20' SF w/200
 

mercury mirage plus 19" prop, hydraulic jack plate, and trim tabs will give you 50+mph.

Lighten the boat up as much as possible by removing any excess gear and get the engine up as high as possible (cavitation plate will be about 3" above the bottom of the hull). You're motor might be too high already if you can't get the bow to porpoise w/ full trim.

Re: WOT speed on a '83 20' SF w/200
 

Not too high, if anything, too low. The prop doesn't break loose until it's trimmed up almost all the way. I've got the cavitation plate about 1" above the bottom of the hull and it's covered with white water when running. Holds real good in turns, too.

I was thinking of raising the motor up one hole. Don't want to spring the $$ for a jack plate. I'm running a Mirage 21 and the rpms are lower than I'd like, closer to 5000 than 5500 at WOT. Maybe I should try a Mirage 19. My wife may throw me out if I add another one to my prop collection. It's getting pretty impressive.

Re: WOT speed on a '83 20' SF w/200
 

FWIW, I had a 1977 20ft Sceptre with a 200 Yamaha and it would run 53-54 @ 5400 RPM and has hit 55 with a light load. I was running a 21 pitch Lazer II Merc prop. It seems that the SeaCraft Industries boats are heavier than the Potter boats. I have owned both. ;)

Re: WOT speed on a '83 20' SF w/200
 

I tried a 21" merc offshore 4 blade prop. It was just too much prop . . . My motor could only turn the prop 5000rpm. 19 mirage plus spins 5700/5800rpm with the motor trimmed.

I think you'll see 50+ once you raise the motor and drop down to a 19 pitch.

Re: WOT speed on a '83 20' SF w/200
 

I think I'll try your suggestions in that order. First raise the motor, then a different prop. I'm running around 5200 WOT now. I'll probably pick up som rpms just raising the engine. I wanna see how much before investing in another prop. I may mess with the timing advance too. I've got it at about 21° now.

Re: WOT speed on a '83 20' SF w/200
 

Thanks, everyone, for the feedback.
Dave

Re: WOT speed on a '83 20' SF w/200
 

Sounds like a good plan.

When you get to the point where you're adjusting the timing.
I'm pretty sure the 2.5L Spark Advance acts like a rev limiter and automatically retards the timing once you hit 5800rpm (2.0L/2.4L mercs the spark advace module will retard the timing @ 5600rpm) People will often remove those units and manually set the timing up to 24/25 degrees.

Do a search on screamandfly.com . . . there's lots of good tips for getting the most out of your Merc.

This is a good site too.
http://www.biggerhammer.net/mercury/

Re: WOT speed on a '83 20' SF w/200
 

Just thought about something. Make sure that the engine has good compression and if it is carbureted that the carbs are clean. If it's fuel injected make sure that the injectors are clean. Basically make sure that the engine is in perfect running condition.

Re: WOT speed on a '83 20' SF w/200
 

I already disconnected the timing module, now I'm experimenting with max advance. I Already did this on the 150XR4 on my flats skiff and picked up 5 mph top end and had to go up an inch in pitch on the prop to get the revs back below 6000.

I'm still not sure what is a safe max advance for a motor turning under 6K. The guys advancing 25 deg. are running well over that. One mechanic I talked to said 21, another said 23. We'll see.

Re: WOT speed on a '83 20' SF w/200
 

Ah yes, the engine. That's really what my question about top speed is all about. Compression is good, carbs are clean, plenty of spark, all that's good. But the engine itself is what my mechanic calls a Frankenstein motor.

It was basically thrown together using a bunch of black parts. Not sure who built it, boat was sold to me by a small dealer who had it on consignment. The powerhead is a factory reman '03 200 EFI with carbs and reed block off a 150XR6. I don't know about the mid and lower. I've been researching the differences between the 150XR6 and 200, and it looks to me like the differences are mainly in the carburetion and exhaust tuner.

The XR6 had either large bore carbs like the 175s and 200s, or small bore carbs like the standard 150s, depending on year of manufacture. Mine are the large bore WMV-3s. I hypothesized that if I rejetted the carbs with the same mains and backdraft vent jets as a 200, I would boost horsepower. Nope. No difference. I don't really want to pull the powerhead to check the exhaust tuner. Maybe I'll look at it from the other end when I pull the lower to change the impeller.

The bottom line is that if I'm already getting the performance expected of a 200 on this hull, I don't want to waste any more time trying to squeeze another 4 or 5 mph out of it. If not, I'll keep tinkering. Just don't want to spend a couple bills on 200 HP carbs and not get anything out of it.

Re: WOT speed on a '83 20' SF w/200
 

The xri usualy had the smaller prop shaft and differant gear sets, just a thought. also when you get way out there and need a realy meaty wheel i have a 4 blade 27 pitch ballistic for a 200 merc from my basscat i cant use. long story on why its on the shelf but it is new in the box.


as for the motor choice look on almost every race boat and youll find theres nothing faster than mercury

Re: WOT speed on a '83 20' SF w/200
 

Hey Blue Heron I think once you finish tweeking that 200 you should easily get to 50 mgh. I have a '76 20'MA I just repowered with an Evinrude E-Tec 175 and at 5350 rpm I am at 51-52 mgh according to Garmin with a 21 pitch prop. Good Luck

Re: WOT speed on a '83 20' SF w/200
 

Hey dljohn,
Looks like we're neighbors. I'm in Newberry and work in Gainesville.

Your numbers are kinda what I was expecting. The performance I'm experiencing is more what I would expect from a 150. My expectations are based on my 10+ year old memory of a friend's Potter 20 MA with a 150 on it. I'm still trying to figure out to what extent it's because of differences in the hull, or because I'm running this Frankenmerc on it.

I took it to P&J Marine and they stuck it on their Dyno. The torque value they gave me was 95% of the minimum for a 200, but I don't know what rpm they measured it at, so I can't calculate horsepower. If it's a linear conversion, and merc's minimum torque spec for a 200 is at 200hp, then mine is developing 190 hp.

Anyway, thanks for the input. Every extra data point helps. I'm gonna keep tweaking. BTW, how high is your motor mounted? Where is the cavitattion plate in relation to the bottom of the hull at level trim? Mine is about an inch above.
Thanks,
Dave

Re: WOT speed on a '83 20' SF w/200
 

Max torque will be at a lower RPM than max HP.
Max torque is the RPM where the motor has the best volumetric efficiency.

Torque and HP are mathmatically related: HP equalls (torque (in #/ft) times rpm) divided by 5600.

If you look at the torque - HP plot from a dyno, you'll see they always cross at 5600 rpm. There's more ft/lbs of torque than HP up to that point, and then there's more HP than torque above that.

If the motor doesn't turn that high, then there'll always be more torque than HP.

If you're getting 95% of the torque spec for a 200, you're probably getting close enough to 95% of the minimum HP spec.

They aren't linearly related, but they're close enough for what you're figuring.

Even if you had the rpm where they measured the peak torque, it would only give you the HP at that rpm, not the max HP.

Darlene

Re: WOT speed on a '83 20' SF w/200
 

Thanks, Darlene,
As I understand it, the dyno they used is programmed and all they do is enter the model number for the engine they're testing, run it up to max recommended WOT rpm (5500)and see if the torque curve matches up. I don't think they calculate the max HP or do any further analysis on the numbers, but I'm just guessing. I don't know if the torque value they gave me was max torque, or torque at 5500 rpm.

I got most of what I know about the relationship between torque and HP here:

http://www.revsearch.com/dynamometer...orsepower.html

Re: WOT speed on a '83 20' SF w/200
 

That's a good page on HP/torque. It also made me realize I was thinking of something else when I posted 5600, it is 5252 rpm. (and it's too late to edit my original post to correct that)

Torque is usually given as the peak torque number, just as HP is given as the peak HP. Both are usually referred to at specific RPMs in knowledgable circles, but often just the HP is used in general reference.

For a "collection of black parts", you seem to have at least a reasonable facsimile of a 200 merc.


Darlene

Re: WOT speed on a '83 20' SF w/200
 

My '75 SF20 with a new (14 hours) Johnson 2S carbed 175 just touches 50 mph (GPS)on flat water with a light fuel load, a T-top, bottom paint, and an aluminum 14 1/2 X 19 prop.

Re: WOT speed on a '83 20' SF w/200
 

Dave, I did not mount the engine on my boat. I will look at the cav plate the next time I am at the boat. While working on the trailer I have it in a boat house on the St Johns river. My transom is an original 20 incher and with the new saitwater series E-Tec being a 25" shaft I also run a CMC hyd jack plate. Also I do not have a T Top and I lower the bimini while running.

Re: WOT speed on a '83 20' SF w/200
 

"horsepower, it don't matter what color the engine is."
Shame on you Blue Heron; you obviously know better.
For one thing Horsepower ratings are as much marketing hype as engineering (and all indications are that Mercury is more of a sales organization than an engineering one)
Then there is this business of power curves where power and torque vary with engine design for any given RPM.
There are known occurrences of horsepower being overrated 20% or more by marketers so a spread of 40 HP DOWNWARD is certainly possible.
In which event would be inclined to trust Yamaha's (or just about anyone's) veracity over Mercury.
As you can tell, have gotten a pretty low opinion of Mercury Marine.

Oh, and BTW, is the cam on your FrenkenMerc engine in sync with the other volumetric components (valve lift versus area, versus open (or close) time)? Have you checked breathing?
Regards, HBH

Re: WOT speed on a '83 20' SF w/200
 

Phnx, last time i checked, 2 stroke engines don't have cams, or valves with measured lifts. As far as measured hp, most two strokes are under rated, the SAE measured hp ratings used by outboard manufactures allow 10% varriance in the claimed hp ratings. Most of the big V6 2 strokes, by all manufactures excede the ratings on the cowl. I have personally seen 225 Opti's make 245-250 hp on the dyno. I've also seen 225 fourstokes barely make the 10% under. There is no accross the board statement other than each engine has to make within 10 % of the cowl sticker when new. Right now Yamaha could sell their 150 four stroke as a 175( it makes 165-170 hp, but they decided not to because they got there but kicked when they introduced the 225 four stroke( it barely made 205hp).

Re: WOT speed on a '83 20' SF w/200
 

Sorry `spares' was not clear to me franken was a 2-stroke, is she?

Re: WOT speed on a '83 20' SF w/200
 

Spareparts beat me to it. PHNX - 150XR6 is a Merc 2.5L two stroke . . . Same block as the 2.5L 200hp.


That might be true today, but no outboard company put more resources into engineering, testing and research than Mercury. LakeX, Placida Test Facility, mercury racing => 1960 - 80's . . . Merc built the fastest motors around. Their props are great too. I can't comment on the new motors other than the weight of the verado :) But their old two stroke motors and inline motors are/were very good and strong.

Yamaha - Yamaha's 2.6L motor is one of the most reliable engines, but the block was a masterfully engineered copy of the early OMC v6. 1983 lawsuit.

Re: WOT speed on a '83 20' SF w/200
 

they closed lake X

Re: WOT speed on a '83 20' SF w/200
 

they closed lake X!
Bllue Heron, back to the subject at hand. First, i have not had a lot of luck with the mirage on an outboard, too much blade surface. Now the Mirage plus works much better than the original Mirage on an outboard, but if the boat were mine, i'd probably try a Laser, its designed to give more bow lift, and you can run it higher on your plate, if a laser is not available, try a simular prop with some rake to it. And before you go any farther, put a straight edge on the hull bottom, make sure you don't have a lot of hook in it, if the boat was ever on a roller trailer, or on a trailer that wasn't set up right, it could have hooked the hull, don't be suprised if you see a little hook, its probably normal, but any dips or excessive hook could explain the whole problem. And by the way, what are you using to check your speed?

Re: WOT speed on a '83 20' SF w/200
 

Spareparts - They closed placida too back in the early 90's. My point is that Mercury has contributed more than any other outboard company to the engineering and developement of the carb two stroke outboard.

Re: WOT speed on a '83 20' SF w/200
 

i guess i should have said it like this: they closed lake X :(
i aleays wanted to go there, i knew a coupel of guys they went there during some testing, said it was like going into a secret government operation

Re: WOT speed on a '83 20' SF w/200
 

Lake X is alive and well.
In Brevard County, Florida
Just off I-95
From what I understand it is still used for testing on
occasions by Sea Ray which has a big plant on SR-528
by Port Canaveral.
See ya, Ken

Re: WOT speed on a '83 20' SF w/200
 

last time i was in Mercuiser School, they told me the owner of the land, didn't want to renew with Brunswick, wanted to leave the place natural, so he kicked Brunswick out, I think the lake you're talking about is a differnt lake( they had facilities on over 6 lakes in Fl alone). The preoperty Lake X was on is said to be one of the best places for hunting in FL, only problem is, no hunting is alowed

Re: WOT speed on a '83 20' SF w/200
 

I've tried two different mirage 21p props, same part number, but an inch difference in diameter (one 14 1/4", the other 15 1/4"). Neither is optimum for this boat/motor combination. I've got a prop that I think is a laser 22p, that I was running on my flats skiff, and it performs a little better. I don't have a jack plate on the Seacraft, but this prop didn't mind surfacing on the other boat. I'm gonna raise the engine another inch and give her a try.

Hull's pretty straight. Only about 1/8" of hook in 4'.

GPS

BTW, you sound like you know a little about these black motors. I've disconnected the timing module. What's the maximum advance I can safely run if I keep the WOT rpms under 5600?

Re: WOT speed on a '83 20' SF w/200
 

i don't like advancing them any more than factory spec, in fact i usually back the timing down 2 degrees just to make sure they live. the fuel has changed a bunch in the last several years, this alters the fuel mix, if I was running a light boat(bass boat or go fast), i would play with the timing some( you pay attention to weight on a fast rig), i would probably add some octane booster, and or run higher octane fuel,but for a fishing boat( where the average weight and load change from one extreme to another), i would tend to err on the side of caution( i also step the jets up one size). On that rig, if you want more speed( assuming you get the current setup figured out) step up to more HP, try a bonafied 200/225 XRI( not to criticise your frankenmerc, i've had a couple run pretty good for what i had in them), or step up to a 3.0 225/250. I still think you have something left in the rig you have. If you have the oil injection pulled off, go ahead and spin that thing a little more( keep it under 6000), we used to twist the crap out of those motors, I'd rather have one that turns, then one that lugs. the mirage props you tried, were they Mirage or Mirage plus? We used to have some luck out of Turbo props also, go up one # in pitch over the pitch you have now, stay away from the turbo four blades( never had any luck). I can't remeber if you posted whether you weighed it or not

Re: WOT speed on a '83 20' SF w/200
 

just thought of a couple of cheaper and easier things to try, get someone to take a picture or video of your boat running at speed, take a look at how wet you're hull is( how much is in the water), check the angle of the t-top( is it acting like a parachute?). Sometimes a picture will save a lot of steps or open your eyes to something that you never thought of. Second thing, have you thought about putting a manual jackplate on it, shop around, you can probably pick up a used one for $100, they make it easy to set the engine height, its not as sweet as the hydraulic ones, but it still helps, it should bring the bow up a little as well, depending on the setback. You could really fine tune the setup, a jackplate is cheaper than another prop.

BTW, what kind of skiff do you run?

Re: WOT speed on a '83 20' SF w/200
 

Hey, Spareparts. Thanks for the suggestions. I wanna touch on all your points, so this is gonna be a long post.

First, I'm not sure which factory timing spec to use on this motor. XR6? 200 carb? 200 EFI? (that's what the powerhead is). They range from about 18 deg to 26 depending on model year and the timing module they came with. I'm just looking for the maximum I can safely advance the timing with the module disconnected. One mechanic told me 21 degrees, another said 23

I rejetted the WMV-3 carbs with the same mains and backdraft vent jets called for in WMV-18s. I left the idle jets alone. It runs fine, in fact, there was no noticeable change in performance. I'm thinking the bigger mains give me a little safety margin on timing advance. Am I on the right track?

I'm not really trying to make a race boat outa this thing, just wanna get the full potential outa the engine I've got. Though it might be cool to get 50mph outa the thing, I don't go that fast very often. My skiff will hit mid fifties, but I usually cruise at 30-35.

Oil injection was not installed when this motor was thrown together. I'm thinking of adding it back. I have a complete system off a friend's merc V6. His ate the gear on the crank.

The Mirage props I've tried are numbers 48-13702-A40-21P and 48-13702-A41-21P. Not sure if either is a Mirage Plus. I've looked for a Laser II on Ebay and haven't found one that fits the large gearcase in the right pitch. Between the Seacraft and the flats skiff, I have quite a prop collection from my ebay shopping.

You're right about the hull running wet. I can't seem to get the bow out of the water even with a lot of trim angle. I was hoping someone on the board has a Seacraft Industries 20SF hull and we could compare notes. I suspect my hull is more bow-heavy than the Potter 20s. I don't have a t-top, so that's not slowing her down.

My flats skiff is an '87 Action Craft Flatsmaster with a '91 XR4. It's got a polling platform, hydraulic steering, hydraulic jackplate, fiberglass push pole, and trolling motor. It's old, but it goes just fine. It cost me about a quarter of what it would for a new rig set up with the same options.

Re: WOT speed on a '83 20' SF w/200
 

Blue Heron;
Ours is an SF20 but till we've repropped we're out of the water, anyway, at this point it would be the blind leading the blind - in fact you sound like you know a hell of a lot more than we.
Don't know how much land based performance tweaking translates to the water, but when we were trying to get the last inch of speed and power out of a custom engine - or even relatively stock - we'd standardize fuel and stick with it, set the cams with a degree wheel, choose plug heat range CAREFULLY, and advance spark till she pinged and then back it off a hair. Such empirical methods have the advantage of dealing with the engine's `personality' there are no two exactly alike. HBH