Anyone running a Alumacraft 1436 with a Tohatsu 6HP? Top speed?

[fwman1]

Long story short, I have a virtually stock Alumacraft 1436 and a Tohatsu 5HP. I'm looking at spending $200 to upgrade to the 6HP carb.
Currently, I'm able to run about 12.25 mph.

If you have this setup with a Tohatsu 6HP, what is your top speed with just you in the boat?
I don't want to spend the $200 if it is only going to buy me 1 mph!

Thanks for the help!

 

[fwman1]

For the kind and curious who have looked at this thread. I have ordered a Solas 7.8 x 7 prop for my engine. After going to the end of the internet three separate times I found where one poor soul added this to his Jon boat after trying a cheap no-name prop. He stated that he gained a few MPH. It never hurts to have two props, so I decided to take a chance. I'll post results here when it arrives and I have a chance to test it out.
Roger

 

[GYPSY400]

Yeah you'll probably only get 1mph. The rule of thumb is double the horsepower gets you half the speed. For instance: if I have a 20hp that does 20mph, a 40hp gets me to 30mph

Sent from my SM-A526W using Tapatalk

 

[fwman1]

GYPSY400, You pretty much called it.

I purchased a SOLAS 5011-078-07 Amita 3 Aluminum 3-Blade Propeller. This seven inch pitch is the next pitch down, but not a Tohatsu brand prop, obviously. I took the boat out after swapping the prop, and maximum RPMs went from 5400 to 6000. It appears the engine has an built in rev limiter, as RPMs pegged at 6000. Speed is up to 13.5 - 13.8 MPH. The boat now has a more linear response to throttle, utilizing the full throttle range. Previously, once you hit about 80% throttle, there really was not much change in the speed, or RPM.

 

[GYPSY400]

Good to know! 1.5mph out of a prop change on a 5hp is impressive.

Sent from my SM-A526W using Tapatalk

 

[fwman1]

It turns out that my Tach was set to max RPM of 6000. I changed the upper limit and found actual max rpm was 6200 rpm. Since the recommended rpm range for my engine is 5500-6000 rpm, I continued my search for a best prop for my application.

Drag is likely my biggest issue. I'm considering that since my boat does not come out of the water between uses, the bottom needs a good cleaning.

I purchased a Solas 7.8 x 8" prop and took it out for a spin. The Solas rep I spoke with expected around a 200 rpm drop. That was a low estimate. I lost about 400-450 rpm. On the new prop, rpms are 5760. Top speed is between 14.3-14.5 mph. The engine sounds happy enough. It doesn't seem bogged down, although it is no longer singing at those 6200+ rpms. My Tohatsu manual states 5500-6000 rpm in the specs. My engine has only about 17 hours now. Hopefully I'll gain a bit more as the engine completes its break in.

I will continue with the 7.8 x 8" Solas prop. I'll save the Solas 7.8 x 7" for trips with another angler. With another person, the 7" pitch would likely be about right.

For those who come behind me.
2021 Tohatsu 5hp
1436 Alumacraft flat bottom Jon Boat. All up weight about 500 lbs.
Stock Tohatsu 7.8 x 8" prop: 5300 rpm WOT and 11.75-12.15 mph

After 6 HP aftermarket carburetor swap
Stock Tohatsu 7.8 x 8" prop: 5380-5400 rpm and 12.25-12.5 mph
Solas 7.8 x 7" prop : 6200 rpm and 13.8-13.9 mph
Solas 7.8 x 8" prop : 5760 rpm and 14.3-14.5 mph