Wot speed and outboard height

[scoobeb]

So I have a 1648 with a 25hp merc 2 stroke on it. I just switched out the 20hp carb to the 25hp carb but haven't tested it yet but I expect to be near or over 30mph as my 20hp carb got me near 29mph with a 1000lbs + load. My thing is I'm getting this with my outboards anti cavitation plate being 1 to 2 inches below the bottom which is incredible speed to begin with. Now knowing it's low will it hurt anything but causing a bit of drag? I may try to lift it to near or even with the bottom of the boat but I'm trying to figure out the best way to make like a 2 inch transom extender or riser without Making it a huge project. I just don't know if it's worth raising it because any faster for a tiller could get awfully scary but it would run more fuel efficient which would help on my gas bill. Any suggestions?

 

[CMOS]

You could get something like a Mini-Jacker plate to get that 2" height, for right at about $100. However, what I don't know is just how much speed, if any, you will gain from that. I've seen a handful of threads like this before - guys asking if a low set motor is worth the trouble and or expense of doing something to the transom. In most of these threads they have decided to simply leave it as-is.

Curious to see what some others have to say about this.

CMOS

 

[Pappy]

The question here is.....how far are YOU willing to go?
Myself and many others have answered this question several times, in detail. The main reason most folks decide to leave it alone is that it eventually requires you to modify or change the prop and modify the new one, etc. That is where most folks draw the line. Performance gains kind of fall into two distinct groups...low hanging fruit and high fruit.
The low hanging fruit is moving weight around, jacking the engine up until the prop no longer bites, and removing weight.
The higher fruit involves all of the above plus prop modifications, drag reduction work, and engine modifications.

 

[scoobeb]

I guess my main thing is just reducing drag If anything at all. I really don't need to go much faster as 30mph is pretty darn fast on a jon boat,if I happen to pick up a couple mph then awesome, if not at least I eliminate drag. I understand less drag equals better gas mileage, how much I don't know but any little bit helps. I know the anti cavitation plate sits very low in the water due to my 300lbs in the stern and I could probably go up even more than the bottom of the boat with my weight. I have no interest in modifying my props and engine anymore than I have already by going from a 20hp carb to the 25hp carb, that's enough for me as I gained 5hp. I'm not interested in a jack plate that sets the outboard back,I just want it higher up on the transom ,that's all.

 

[Pappy]

Sounds like a good decision to me!

 

[Shaugh]

I'm not positive in all cases, but I believe that a lightweight boat with a motor in that horsepower range actually gains top speed with a motor that's "too low". The low motor helps get more of the boat out of the water and creates a smaller pad that the boat rests on at speed. Which is also why it can contribute to chine walking symptoms like in another thread.

 

[chevyrulz]

the drag from the lower unit is not a factor until your talking +50mph speeds, at 30mph the lower unit drag difference between 2" below bottom & 2" above bottom is not even 1 mph, if you're getting 30mph out of 25hp on a 1648, don't change a thing, i only got 32 out of my 1546 with a 25hp, that was w/ the cav plate dead even to the hull bottom, maybe 3 gal of fuel, 1 battery, & my 170lbs

 

[scoobeb]

With the 20hp carb I'm getting a steady 28.5mph of course depending on wind and tide,that is with over 1000 lbs of total weight in the boat . With just me and regular gear I'm at 30 ish,this is with multiple gps,I got a great deal on a 25hp used carb on ebay,rebuilt the whole thing and cleaned it of course, I mean every piece of this carb is new except for the float because it was perfect. I measured the float and it was perfect.(((( I have a severe smoke issue and i believe this is my issue))))) I just think I may have lost my gallon count by adding gas to my existing gas several times to make sure I was always topped off and I believe I may have over oiled the tank by mistake which would definitely cause the nasty smoke . I know 2 strokes smoke, but she is nearly unbearable, lol. I'm wondering if I can just dump the 6 gallons of mix in my old 95 jeep so I don't waste the gas and then start over with a new batch???. I know you can put like 1oz of tcw 3 oil for every 5 gallons of gas from what I read but I'm sure there is quite a bit of oil in the gas,you need 15.6 for 6 gallons to run 50:1 I bet I'm running like 40:1. I'm sure I could just run this gas through the system and then start fresh again to. I am planning a plug and lower unit oil change anyways so I'm sure running wot I could run through the gas fast.

 

[turbotodd]

Have done all of the above, except engine modification.

It was worth 2.7 mph total. Then the prop (blueprinted) blades were paper thin, and if an object was struck, they'd bend like sheet metal. Then it went back to be repaired. Total cost of all the mods, including the jack plate, somewhere in the $1600 neighborhood. For 2.7 mph. 28 to 30.7.

Unless you're at 150hp+ and 50+ mph, it ain't worth it. I even did a 90 on a Tidecrap, jack plate, prop, a few engine mods. Started out at 53.3 tops. After, I could touch 60 now and then on just the right chop. Once again, lots invested. Hydraulic jack plate, prop mods, tuner, carbs, flywheel, countless hours of jetting, etc. Wasn't worth it on that boat either.

I don't leave nothing alone. I'm a hotrodder. I told myself when I picked up the "new" (to me) boat that I wouldn't mess with it. Only thing I'd done was to raise the motor 1 3/4" on the transom, and LEFT IT. The hotrodder in me keeps wanting to try a little more but I'm out of transom to clamp to, and I'm already blowing out on tight wide open turns. It runs beautifully with what it is, and that's all I want out of it. To start/run when I need it to. If I were boat racing, first off I wouldn't be trying it with a flat bottom jon, and two, it would probably not be a 4 stroke-since I dont' see much support for them in teh 25hp range anyway.

I agree. If you're seeing 30 mph out of your current setup with a 1648, with 1000 lbs in it, I'd be absolutely tickled-and the ONLY thing I'd do would be to keep it in tip-top condition, and ENJOY IT, as-is.

What hull are you using? Any hull mods?

 

[scoobeb]

1648 flat bottom alumacraft, I have a pvc floor in it and that's all I have. It's pvc board I got from home depot, no rot ,awesome stuff. With me 300lbs,friend 200lbs,boat is 330lbs,engine 115 lbs,gas,tackle,rods and 2 coolers full of ice I'm probably pushing well over 1000 lbs near 29 with the 20hp carb, I would expect to hit at least 30+ with the 25hp carb but the torque I gain out of the hole is what is priceless to me. That was with a 13 pitch prop to,I now have a solas aluminum 14 pitch prop on it so I may get a few or more mph out of it. I tell you this 20hp outboard amazes me being 24yrs old,insane power and speed for this merc. It will be extremely hard for me to ever get rid of it,insane want a 20hp 4 stroke Suzuki which I had before but this merc is one of a kind and so clean you can eat off the whole engine ,I just have to get her tuned perfect.

 

[Crazyboat]

If you want to raise your outboard up 2" try mounting some 1/2" plywood on both the out and inside of the transom, the gap in between you can fill with whatever size wood will be required.

Mount your outboard and your good to go. First you should see how wide your hand clamps will open before adding the plywood, you may need to use 3/8" if it will be too wide.

I did this on an old boat when I was a kid, I had a long shaft 15 so I raised her up 5", it worked well, gave the wood a thick coat of paint for protection and never looked back.

On to your reported top end, I'm finding it hard to accept your WOT numbers with 1,000 lbs to push along. I looked up some Yamaha numbers last night and only 1 boat/motor package came close to your reported speed and that's with light gear and 1 person.

 

[stinkfoot]

I had a friend fab me a jackplate. Didn't make an appreciable difference at ANY setting apart from a lot more spray due to the motor being a couple of inches aft. I ended up replacing my transom and making it both stronger and taller for my small jet OB

 

[scoobeb]

If you want to raise your outboard up 2" try mounting some 1/2" plywood on both the out and inside of the transom, the gap in between you can fill with whatever size wood will be required.

Mount your outboard and your good to go. First you should see how wide your hand clamps will open before adding the plywood, you may need to use 3/8" if it will be too wide.

I did this on an old boat when I was a kid, I had a long shaft 15 so I raised her up 5", it worked well, gave the wood a thick coat of paint for protection and never looked back.

On to your reported top end, I'm finding it hard to accept your WOT numbers with 1,000 lbs to push along. I looked up some Yamaha numbers last night and only 1 boat/motor package came close to your reported speed and that's with light gear and 1 person.

Once I get my boat back in the water here soon I will prove my gps speed. Remember whether u believe this or not my merc and most mercs will outrun a 2 cylinder 25hp yamaha. I've done this plenty of times. Mercs have more power lb for lb then a 25hp Yamaha,now I love yamaha just as much as merc but even the 15hp merc will outrun a 15hp yamaha, these are facts. I average near 28.5 and hit 29mph a few times with the 1000 lbs of weight, I have nothing to gain by fibbing or making these numbers up. Every merc I have ever owned has always been more powerful than the same size yamaha up to like 25hp. That is just my experience and many others on tons of other forums I've read on. Once I get on the water I will record my mph. I expect to be near 30 or over with the new 25hp carb.

 

[scoobeb]

This older merc also has more power than a 2002 25hp I just sold which amazed the heck out of me. This 93 25hp has insane power to weight ratio, it amazed me,believe that.

 

[turbotodd]

Is yours a SeaPro 25 or a twin cylinder? What year?

 

[scoobeb]

1993 ,non sea pro,the sea pro is a Tohatsu. Mine is pure merc.

 

[scoobeb]

Also my merc has a standard 13 pitch prop and now I'm going to spin a 14 pitch prop which is a massive prop for a 25hp engine imo,but it will still have plenty of get up out of the hole even pushing a prop that large. I know people running a 15 pitch prop on these mercs and getting near 35mph with one person on board a light boat. Like I said I couldn't believe the torque this motor possess, more than likely due to it's 2.25 gear ratio in the lower unit, that definitely helps. With just me in the boat and no gear,just boat,motor and floor I hit 30+mph but it porpoises so bad that by myself I have to add weight to the front so I add 150lbs of blocks in a 150 qt cooler to weigh the front down or it's literally unsafe and that brings me back a bit to like 28.5mph steady and if it's really calm I can get near 29 with the 150lbs of weight that is. Like I said, once I get it finely tuned I will take gps readings with my 13 pitch prop and 14 pitch prop to see what difference I get.