Howdy, I just aquired a new rig and got to run it today. It is a GD 1644 running a 20hp GDLT. I did not expect alot of speed, but I am only getting about 9.4-10.1 mph with the GPS. It is turning around 3300-3800 until the rev limiter kicks in. These speeds are by myself OR with two people; it made no difference. I am around 265 and my buddy is 160 or so. GD recommends a 9x4 prop but the motor has a new 8x4 on it now. Could this difference in the prop size make that big of a difference? This may be a stupid question but this is the first mud motor I have owned. Thanks for all the great info and inputs on the site. Clent

wrong prop, an 8x4 is what they put on those 9 h.p gdlt, you need you that 9x4. trust me big difference..

will give you a call soon

Something is definitely not right but I find it hard to believe that an inch would make that much of a difference. I would say with a 20 hp run a 9X5 minimum. I run an 8X6 on my 13 and get about 16 mph average on a 14X36. And that's with the prop in the water, surface driving I'm getting 18.1, so I would say with a motor that size to go with at least an 9X5, I don't think you'll be disappointed. An 8X4 is what Beavertail uses on their 13 motors. Don't think it's so much the diameter of your prop as it is the pitch. Just so you know a lot of people on here will give advice on your post and then ask the same question you asked a month later, trust me it happened to me, take a lot of information on here with a grain of salt, much of it is B.S.

Speck, go wear your prop down to 7" and report back.

That extra half inch on each blade will not affect performance that much. I have a friend with a 7X5 on his go devil 6.5 hp and he gets 13.5 out of his motor, so as I said before pitch makes more difference than diameter. Been there, done that. Oh and my friend has had that same 7X5 prop onthere for 4 years so who knows what it's wore down to.

how much you want for that 8x4 prop

My 20 LT came with a 9x6

I put a 9x5 on my 16hp and it made a WORLD of difference!

Well, from the PM's, phone calls and post, I believe I am going with a 9x5. I have got suggestions equally across the board for 9x4 and 9x6. I want to stay somewhere in the middle and keep the torque and gain a little speed. Hopefully the 9x5 will do the trick. Thanks MMT. Does anyone have a 9x5 for sale ?

No, but check on Ebay. Thats where I got my last prop. Its a Ron Hill Marine Prop. for the price, they cant be beat..