Ok my first post so be kind. I just build a 14hp predator Mud Motor and did a test run. It did bad in about 6" of water. Kind of OK in deeper water . It was pretty sluggish. This was My first build and didn't know long to make the shaft so I made it 69". I have a 20" transom and now I know it needs to be longer. How long? Im only getting 7mph. I have a 1652 riveted flat bottom. Is this the fastest for this motor and boat combo? Also I dont know about the cavitation plate. Should the plate be past the prop? Im about 2" short. I have a prop from Blackwater recommended for my motor.


https://youtu.be/QKvjORYQsv4
https://youtu.be/0cBbEzdltjE

That transom height is to too tall, the boat is too wide/big for a single cylinder engine as well. In the video, looks like you had 3 people (2 grown people + 1 kid). You got some things working against you, 7 mph seems reasonable with those issues.The overall quality of that build actually looks pretty good, think you need some more HP and either a longer shaft or different angle. At WOT you want the prop to just be under the surface, and spinning somewhere around 3600 RPMs.

Did the prop dig or prop out on yall at all?

No it really didnt pop up or down. Whats got me is that the prop will kick in and push the boat pretty good then quits. Almost felt like sucking air and loosing power. I made a longer cavitation plate today. what do you think? Any better? I put at 6 deg.

What size is that prop?

8x4

I run that predator motor with a 8 x 6 prop on a 72" driveshaft. You will lose a bit with that tall of a transom but should still do better than your getting with that motor. The easiest thing to do would be to get the right prop for your motor - much simpler than lengthening the driveshaft.

I matched the prop to motor at backwater. Do i have the wrong one?

I would suggest a survey on the mmt site to get what others are using. I have found hill marine to recommend props that my prop engineer does not recommend. so my point is that not all prop makers specs are necessarily equal. You might check with several makers to get some kind of consensus of specs. I have no experience with Back water's specs - good or bad. I wouldn't expect a prop from my maker that was 8 x 4 to run well with that motor.

Mark F. Cheney

Thanks

Yes, I use 8 x6 props on 420 cc predators. I have a shop full of 14 HP Robins that I use them on also. I am currently putting a 14 HP Robin on a Ebads from Four Rivers that I am mounting a 8x6 0n as well.

Mark F. Cheney

Thanks. Well I decided to make a 22hp with a longer shaft. Maybe 85". Can you recommend a prop for that one? Thanks again

I ran a 22 predator on a backwater long tail. I tried a 9x6, 9x7, and 9x8.

9x7 outperformed the 9x6 in everyway.
9x8 had a little more top speed when solo, but didnt carry a load like the 9x7.

Thanks for everybody's help. Here is my 3rd build. 22hp vtwin 95" shaft.
https://youtu.be/KVXjCS8BOfw
https://youtu.be/GIsnMEVnlFs

Lookin fancy. Have you gotten a chance to put it in the water yet?

Not yet. I will this weekend.

Took it out and couldnt be happier. Ran great. Still have to figure where the sweet spot is and keep it there. Any tips on that? We figured 900lbs in total weight and it ran 17mph. For doing nothing to motor I think that is really good. If I did some mods to the
motor, how much more hp could I expect to get?
https://youtu.be/XKzmw2BXv8k

I tweaked the governor and could maintain 4000ish RPMs in water with the 9x7. I would start there. The motor has over 130 hours on it now and no problems. The only problem it has developed is the starter not fully throwing the bendix and engaging the flywheel. May check to see if a Honda starter will bolt up.

The carb and intake is a big drawback on these. Open the filter housing and remove some of the excessive sheet metal. Gasket match and relax all the intake pieces. Clean up the casting marks. I did this and opened the idle circuit jets 1/2 a turn.

My hull weight around 300 pounds and I'd carry about the same 900 you mentioned and run 18-20 with the load.

Have you added an electric pump or at least replaced the factory fuel pump yet? I've noticed working on several of these motors it won't keep up past 3600 RPMs. The motor will bog down and die. If you keep the primer bulb pumped it will run. Adding the e-pump or a better vacuum pump will solve this.

Yes I installed fuel pump. I will look into the mods. Not sure on how to adjust the govener. Thanks

If I push down on the handle I can feel the boat getting faster. Does that mean I should bend the cavitation plate down?

I tightened the govener spring so I could get about 3800-3900rpms. I was only getting about 3200-3300rpms. Do you thinks that will help?

Cant hurt

It will make a pretty significant difference. Let us know!

Well I took it out. Now with the cavitation plate bent all the way up I really had to pull up to keep the shaft in the water. Enough that I can't live with. And then a little later my cavitation plate sheard off. The weld didn't break it sheard the metal right in half. I might of Used a little to thin of metal. Also without the cavitation plate i could barely get it to move. So how does other people straight shaft run without one? When i make a new plate, should i make it have a litrle more angle to begin with? The last one was at 7 degrees.

https://youtu.be/AcV93EDCJ8Y