Prop choice on new tohatsu 20hp

[nfo] [/nfo]
Hey guys! So I recently upgraded from a old Mercury 7.5 2 cylinder. Me and my girls fish every weekend and it takes us about an hour to get to our spot pacing along at 5-6mph was our highest speed. Got a flat bottom Oachita 16ft aluminum jon boat that’s built like a tank lol. My dad gave it to me before he passed, im sure he could solve my problem in a jiffy if he was still here heh! I appreciate any help/suggestions from you guys. I don’t know the weight of the boat, we did a makeover on her built a floor with aluminum and plywood so she’s not very light haha. If I need to try and get a weight I can take it to a scale and guess average trailer weight and deduct that if needed. With the merc we always stayed flat on the water and cruised around fine. Never got on plane or anything pretty much stayed just as flat from a stop to full speed. Bit the bullet and thought if we upgraded to a 20hp we would zoom around double the speed and get to our spot in half the time haha! Didn’t work out that way lol, ugh paid a hefty sum to only increase our speed by 1.5 - 2mph. The prop is an 9.25 x 11, after reading a bit looks like I need to go down for heavier boat/load? Would a 8 or 9 be to drastic? I also bought a tach meter after reading and seen that was pretty much a need to diagnose. The boat load hasn’t changed since the merc replacement, just the motors weight difference which I think is 104-106 pounds vs whatever the merc weighed. I tried 3 different trim holes out and recorded the results. Did 1/2 throttle, 3/4 and wide open. I also had my girls get on the bow area just to test if it made a difference while going and it did which is also noted in the pic. I never hit recommended rpms from the manual at wide open, and never went anything past 8mph. Not looking to be nascar on the lake but it would be nice to go a tad faster and not feel like I wasted a few grand on 1-2 mph more lol. Thanks guys, any suggestions, tips, ideas on where to start or what to change I will be grateful. I am a total newb at all this but trying to learn Ty!

Sounds like your heavy! Working backwards with the numbers, a 9 prop is capable of a top end just over 20 mph, an 8 tops out just under 20. I bet the 8 would work better. Might even get you up on plane!

I’ve used a 20hp Tohatsu on a 16 foot jon boat. It performed nicely with two people and trolling motor etc, was a bit doggy with 3 but still planed out. The boat was bare bones and weighed 300#


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Ty! I’ll try the 8 out! I just installed a se 200 hydrofoil today, wondering if I need to get a jack plate as well. I did the yard stick thing and looks like the outboards cavitation plate is little over an inch below the boat. Do you think I need a jack plate with that distance? How much would this affect speed/Rpms? The pic is right before adding the hydrofoil. Ty very much!!

motor is too low and it's way overpropped for the load based on the rpm

what is the rpm with a light load?

That motor is a bit of a slug on a loaded 16' boat, IMO. I wasn't really that happy with the one I was using. Remember, the hull has as much or more to do with how well the rig performs as the motor does. You might find a more modern hull that will actually plane and run much better/quieter/smoother/dryer than what you have, unfortunately it won't have the character nor the sentimental value.

I haven’t tried it with just me, I can do that next time . Do you think a jack plate would be worth it or am I fighting a losing battle. I don’t mind to not hit top speeds, just a few miles faster would suffice. Ty!!

Your prop change is going to give you a lot more push. If it gets you boat up on step, raising you motor a bit would get you a little more. If your using the clamps to hold the motor you can usually slip a spacer between the motor and the transom that will raise the motor 3/4 - 1, Bolting the motor you can get a little more height. But raising the motor isn’t going to change your performance much till you get up on plane. For instance, if you were to get 18 mph with that 8 prop, you might be able to squeeze 19 out of it by raising the motor. Weight distribution can also make a big difference.


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I gotcha. I got the 8 ordered. Waiting for delivery . Ty so much!

My boat is a 14 ft Lund and has a factory floor and trolling motor battery up front so there is some weight there but not sure how much and my 20 hp Tahatsu will do 22 mph gps. My brother has a 16 ft Alumacraft with a 20 hp Mercury (Tahatsu) and he gets 17 mph His has a floor,rod locker,and livewell so a lot of weight. You should get more speed for sure but sounds more than prop problems? Hope you get it figured out. My brother and I both are running the prop that came with the motors.

it could stand to be lifted on the transom a little. But the problem with that is, how it's done. On mine (before I put a CMC on it), I put a 1.25" thick block of wood between the motor and the top of the transom, then clamped it down tight. That was about right, it wanted to blow out when turning at speed. The other way to do it is use of some sort of jack plate, OR a power tilt/trim plate. This is ideal if you don't already have PT&T. With something like a CMC PT-35 (or bob's, or vance, take your pick there are several brands), you get the benefit of PT&T obviously PLUS when you mount the plate to the boat you can mount it high, and then you have some adjustability with the PT&T plate too. Since they offer built-in set back, 5 1/2" in my case, that alone allows you to mount the motor higher than you could if just mounted to the transom, which lets you run shallower and slightly faster. Basically, it allows your boat to run much more efficient--which also saves fuel (if that's a concern with a 20hp).

And lastly, the 20hp is just going to be slow on that hull. It's a heavy hull and doesn't give much "lift", then you have more people in it and loading, it's just going to be slow. A more modern hull will be much much more efficient. Even with a load, a more modern hull will probably actually plane off which I don't think you are able to achieve with your current setup. On plane I'd venture to guess that you should be 14mph+ in my opinion.

I went through this with a 1448, I think it was a weldbilt...older version of your exact motor (20hp tohatsu) and with two of us in the boat it would barely plane and I mean BARELY. One, wasnt much problem. Sold that hull (on the water mind you) and put the motor on a 1542 Grumman, which was heavier but much newer, and it ran circles around the 1448. Sold that rig, bought a bass boat with a 90hp, then sold that and bought yet another Grumman 1542 with a Yamaha 25 4 stroke, loved it....loved the motor more than the hull, and found a war eagle 548 which I liked better and couldn't pass up. Kept the 25. The Grumman hull was pretty light, around 225 lb as I remember and I ran it mostly bare with nothing but me, the motor, gas, and a few fishing rods/reels. Ran 26 mph with all the "tuning" I could do to it. Put the same motor on the war eagle and had to change props, exact same motor ran 29 mph and change. And the war eagle is roughly 125lb heavier, and has a battery, trolling motor, lots more tackle, cooler, and 2 more rod/reel combos. Adding PT&T (PT35) picked it up another 3 or 4 mph. I have also run this hull with a 15 for a little while and it planed off, slowly, but it did it with two in the boat (myself and my dad). The hull--makes all the difference in the world. And there are even better hulls than what I have; I like what I have and have no plans to replace it anytime soon.

The glass half full view is that you should benefit a lot from a prop change. You are at the bottom of the spectrum where anything that gets your rpms up will also get your speed up since the big battle is trying to somewhat plane out. I'd also say that even if you can't get the speed you wanted, as long as you can get your rpms up so as not to lug the motor terribly you should at least have a quieter ride and hopefully get better fuel economy plus that motor will be nice for trolling around scanning the lake or pulling baits.

I would be interested to see more pictures of the boat and how it's set up.

Ty all for the help! I’ll get more pics of the boat setup. Cabelas was on back order for the new prop but fingers crossed it should be here this week! I did get the hydrofoil fin on and it seemed to help a little bit but nothing to write home about. Think I’m gonna move the batteries to the middle of the boat somehow. TBC :lol: