"I wanna go fast!" -Ricky Bobby

Long story short, the other day I was caught in a freak rain storm while out boating with my girlfriend. Saw it comming...tried to beat it back to the ramp...got soaked...girlfriend wasn't happy. I always check the weather before I go and it was one of those that just came out of nowhere. This event combined with the top speed thread that was posted lately have me trying to get more speed out of the 18 hp I am running. So far my top speed with 2 people has been 16 mph. I currently run a 68' Evinrude Fastwin on a 14' Starcraft Seafarer with added 1/2" carpeted floor and casting decks (you can see from photos on my build).
I have read on the top speed thread that people have been getting over 20mph with 2 people in loaded boats while running 15 - 20 hp motors. I have been trying to troubleshoot a few things from reading on this site and a few others and here is what I came up with.

1. Checked the spark plugs to make sure both cylinders are going. Minor carbon build up and same amount of carbon on each plug. Does this means that both are running and the same amount?

2. I did have the motor trimmed out to the second hole from the transom. I moved it to the third hole (but I haven't tested it out yet) would I be able to gain 5 - 6 mph from this move?

3. The cavitation plate is about 2.5" - 3" below the bottom of the hull. I know it should be even or a little higher than the bottom of the boat so I tried to raise the motor a little higher on the transom. I was able to split the difference and get it within 1.5" - 2" now, but it's still lower. How much speed would I gain if I installed a jackplate and pulled the cavitation plate even or a little higher than the bottom of the boat. Below is a picture of where the motor currently sits. I have taken off the whale tail because I read that it could slow the top end speed.


4. I have the stock prop that came with the boat. Not sure the pitch or width.

5. Would I be able to install a 20hp carb on this motor to increase hp? I read that the 18...20...and 25 hp from OMC all have interchangeable parts, but a 25hp carb would be too much for the 18. Would a 20hp carb bump it up a little?

I guess what I would like to know is, am I currently running as good as I will get with my setup? Is 16mph with two people good for my motor? I am assuming I can get a little over 20 by myself. How much more could I expect from my rig? I would love a newer 20hp 2 stroke but it's just not in the cards right now. As always any help, input, or advice would be great.

Thanks,
Alex

I just recently got a 1975 Mercury 9.8 for my 14' flat bottom and I've been trying to get more speed too because currently I get 15mph solo and 11mph with 2 average sized guys. I've tried some different trim positions with no noticeable difference. I also moved my cavitation plate from about 2" below the hull to being flush with the hull. That seemed to make no difference as I was still only getting 15mph. My motor has the stock 2 blade prop like yours so I'm going to order a new 3 blade prop very soon to try. I'm not sure if those changes will make a difference to your top speed but I figured I'd share my results.

Bob,

The stock prop that came with the motor is a 3 blade but I am not sure of the pitch. I have some material left over from my build...aluminum and plywood. I am wondering how much I can expect from a jack plate if I would make one. I don't need it for shallow running...just top end speed.

There's no replacement for displacement!

Signature props will help but they don't come cheap. Trim tabs and electric trim for your motor will also pull a few mph.

Bigger carb will give you the opportunity to do more fuel/air=more hp

As for speed I'd say it's almost cheaper to get a newer bigger engine nowadays than to make an older one just as fast.

Slap a little sneaky Pete NOS kit on it from ebay for a quick 0-plane time should be under ATV parts and accessories.

Or just bring a rain coat!

Try putting X3 plugs in, that will give you a better fuel burn and give a slight boost

I'd say get some weight reduction done to it, that will give a few mph faster and it's free.

What are you thinking? The boat is running too heavy and displacing too much water... or under powered and not enough displacement with the motor?

atuck593 said:

What are you thinking? The boat is running too heavy and displacing too much water... or under powered and not enough displacement with the motor?

Click to expand...

Both man both!

Lighter means faster and is free

Bigger engine means faster but there's a balance you've gotta find. You can go too big with an engine and end up with a high hp motor that weighs a ton or you can go too small and not get enough hp.

The merc 25hp is the balance of go juice and weight for me.

Why don't you play around with your high speed jet on your carb, and give the o'l boat a diet. It's free and you might pull 2-3mph quicker.

The higher up your motor is the faster she will go. Too high your bucking like a bronco and too low you're going slow.

Lift it up until it porpoises and then go down one pin. Anything after that can be adjusted by weight.

Well Ricky Bobby, I was running an almost identical motor (66 Evinrude 18hp) on my 14' mod v and mine topped out between 21-24 mph with two people, depending on how much of a load I was carrying. My boat didn't have much plywood, just two small decks, so my boat was probably lighter than yours. I had a 1" spacer on my 17" transom, I found this produced the highest speeds without ventilation. If you do build a jackplate, I recommend incorporating a few inches setback into it, this will allow you to raise the motor higher, reducing the amount of drag

I just don't see why you'd put a jack plate on a Jon boat. Is there really that much performance you're gonna get from it? And isn't that making the tiller handle further back a hassle?

If it makes it go another 2-3mph I just might get one but I'll have to see it to believe it.

I don't know this to be a fact but as I understand it, a jack plate it's self will not make a boat any faster. But, if your motor can not be raised to the proper height to achieve minimum drag, then adding a jack plate would facilitate raising the motor and making the boat faster. So in this case, raising the motor is what made the boat faster, and the jack plate was the method of raising the motor. One thing that a jack plate does do is to move the motor back which allows you to raise the motor even more and that allows a boat to operate in shallower water.

If you added weight to the front wouldn't it lift the engine up higher without so much modification?

I'm not in shallow water anyways so I'm not worried with it, once I get the transom cut properly and the friction plate fixed I'll worry about top speed later. Until then my engine is 8" too far to the right because the previous owner didn't know how to cut a transom correctly, so I'm not willing to attempt another high speed run until those problems are addressed. No wonder why it was so hard to hold a straight line

Buddychrist said:

I just don't see why you'd put a jack plate on a Jon boat. Is there really that much performance you're gonna get from it? And isn't that making the tiller handle further back a hassle?

If it makes it go another 2-3mph I just might get one but I'll have to see it to believe it.

Click to expand...

Well, I'd like to have one on my junk... Not for top speed, so much as bottom clearance. The less the motor hangs down, the shallower I can run.

What I'd really like to have is a jet drive, but nobody makes one that will work on my little Valco U-12. So absent of that, I'd like a transom bracket that sets the motor back from the stern of the boat and lets me run it with as little of the lower unit in the water as possible. Right now, I've got the same issue the OP has with respect to my cavitation plate being below the bottom of the boat by 1.5 to 2 inches or so. With the transom bracket, I might be able to go 1.5 above the bottom of the boat with the cavitation plate.

An extra three inches of bottom clearance might not matter in anyone else's boating, but it matters in mine enough to be worth the hassle to get it.

T-C

Adding weight to the bow would most likely make the stern rise but that would not give the desired effect. The amount of motor that is in the water from the bottom of the boat to the bottom of the motors foot is what you want to reduce as much as possible. That is where you get a lot drag from when the boat is on plane so the more you can reduce that, the faster the boat should run. Also, moving more weight forward is a balancing act. You want to get the best top speed but you also have to maintain the ability to get the boat on plane. If you move too much weight forward you could end up just plowing through the water.

Tele-Caster, there are at least 2 threads that I've seen on here about home built jack plates. You might want to do some searching for those type threads if you haven't seen them.

Here is what I built. It doesn't offer any set back, but did raise the motor to the correct height, which is 3" higher. My speed went from 29 down to 28mph, but I am running with 130# or so extra weight. I think I would have gained 2-3 mph, if not for the extra weight.

https://www.tinboats.net/forum/viewtopic.php?f=3&t=26279

Well I have no doubt that a jack plate will help if you would like to run in shallow water or skinny water in rivers, but 99.5% of the time I run in lakes and dams. Thats not a huge concern for me, but the idea of having less of the motor in the water to reduce drag is what caught my attention with a jack plate. I like what Apex did to raise his motor and close that 2" - 3" gap without having the setback and making tiller handle harder to reach.