Hey guys just a technical question for the experts. I have a homemade short tail and just took it out for its 2nd test and I could not be happier with the results. The motor is a 6.5hp harbor freight on a 1436 quichita riveted jon boat. My gearing is 1.63.1 , martin timing pulleys, 3/4 wide belt. Anyway the motor pushed me, the boat, extra gas, tools and 80lb lab very well. It plained nice and had plenty of power and torque. The sweet spot seemed to be about 3650 rpm and it got there pretty easy, guessing I was doing about 17 mph or so but will verify with gps soon. Anyway, my question is at times i could reach back and tweak the throttle and my tach would read 3900 to 4,000 rpms while on plain with no noticiable gain in power like the prop was washing out. Do you think I could turn a bigger wheel as the prop is a 9x6 backwater longtail prop and not even a surface piercing prop. Just a question as we as men are all about improving things, but my motor and design seemed to perform awsome and I am not complaining, I just enjoyed the day of sweet success after about 3hrs hard run time and if it aint broke why fix it.

You have a couple of options. 1) If you do not have a cavitation plate, add one. It may help hold some of the water on the top half of the prop. You should be running that prop completely submerged. 2) You could also change your driver sprocket to increase your prop rpm. Depending on your belt center distance and how much adjustment you have, you may be able to get closer to 1.4:1 reduction with some added performance. 3) You could scrap the sub-surface prop and try a 9X9 (I think this what copperhead is running) surface-peircing prop.

Post up some pictures of the motor.

Mag10

Mag, grats on your build. I am currently in the middle of my build. My buddy and I are going with a 1.54/1 on our gearing. We are trying to figure out what to prop as well. I see that the copperhead and the dixie SD are running with a 9x9, so we might be leaning that way. Hope that our build turns out as good as yours.

elecg777,

Are you running 6.5 hp also?

The surface drive props will allow you to run the motor with less outdrive in the water and thus less drag. You also do not have to worry about cavitation, as you would with a sub-surface prop. Gets rid of the absolute need for a cavitation plate. One less thing to fabricate and then possibly damage.

I am currently working on a 16 hp clone surface drive for a buddy.

As I stated above. Post up pics if you have them!

Mag10

Hey mag 10 thanks for the response and advice. I am a big fan of your work and got alot of tips off your early 13hp surface drive. I also agree with the idea of no cav plate. Think I will leave the gearing alone and when I get the duckets may get me a true surface drive prop, they are pretty salty. I will try to get pics up soon of my build on this site. I tryed a 11hp build and had to take 2 steps back as my frame was not beefy enough for the weight of the motor or thrust so decided to build the 6.5hp first. My motor ran so gd while I was out there it was too much fun and I am ready to go play some more. Its not often in life where everything including your hard wrk comes together. In my time and experiences during the build its amazing to me how much every detail matters in the end result. For example, having the right prop, right gearing, right boat, right trim, right shaft length, right drive box length and the list goes on and on.

Mag,

Yes we are in the process of building a 6.5 Harbor Freight Motor SD. I will post pictures with the different stages of it. We just got the fab work done with the CnC. And trying to figure out this prop issue from the start. I am new to Mudmotors and how the prop talk is. So since I am a nub, I am going to ask, how do you tell the difference between a surface and sub-surface prop? By looking on HILL MARINE, they show the weedless 9x9 prop, but is that for a Longtail or a SD? Or does that matter. Teach away if you have time.

By looking at the picture posted on their web site, I would say that it is a surface-piercing prop. Notice how the mass of the blade is moved out from the hub unlike the 9X5 or 9X6 that looks like more of an auger. The true way to know, would be to look at many of the other characteristics of the trailing edge of the blade.

So MAG, what you do with the setup I mentioned to select the correct prop? What prop would you select? Would you go with the HIll Marine or somewhere else? My hull is homemade too, so i was guessing on my weight of it but i think it is pretty close.

Barrelburner- Grats on your build, I am impressed with the 17mph. Did you ever decide to change your prop or gears? Did you leave the Cav plate off? Also the HF 6.5 did you put any MODs on it or you running stock?

Thanks again for both of your guys time and knowledge. I am just a dumb Arky boy down in the bayou

Thanks, I have no cav plate at this time and no plans for one. In addition, I have not messed with the prop or gearing at all yet going to leave things alone for now, definitly would like to get a surface drive prop and try it soon though. Also, the hf engine is the earlier greyhound version 196cc not the predator and is bone stock.

elecg777

I would go for the 9X9. I have a 10X8 Hill Marine prop at home, and it looks to be a very well cast and finished prop. I have not run it yet though. I know that the Hopkins prop that copperhead is using is a proven blade. If you have trouble with getting a quote on a prop from copperhead or Hopkins, call Phowler boats/Illowa Marine. They had some Hopkins props at their shop a month or so ago.

Mag10

Dixie uses Ron Hill propellers. You can buy them on his website or on Ebay. So look at size they use and try that.

Ok- I will go with the first run with the 9x9 on the prop. Thanks for the info.

Ok got some other questions. Would like to run a clutch, is the gearing suppose to have the big sprocket on top or bottom, bc trying make room for the clutch?

big gear on bottom for reduction.

big sprocket/pulley/gear on bottom and smaller on top

Ok barrelburner or MAG10. What gears would you run on the 6.5hp HF. Barrelburner I see you used a 1.63:1. What made you choose this over per say a 1.57:1 or 1:1?

If you go 1.5:1 or a similar reduction you will be able to take advantage of some of the surface drive props that are already out there. If you go 1:1, you will be stuck with a smaller sub-surface prop like those run on the small 6.5 hp longtails. Depending on your setup (boat motor and terrain) you may be able to change your gearing to a smaller reduction like 1.3:1. From what I know, the baseline for these smaller engines has been a 1.5:1 reduction.

Only reason I chose 1.63:1 gearing for my 6.5hp was the fact that I wanted to be close too 1.5:1 and I already had a top pulley to match with bottom pulley and was the closest I could get to make it work out. This ratio does seem to be spot on for my particular rig and load and I am very pleased. Matched with the back water 9x6 prop its the best combination of top speed and low end torque. I was very lucky that I got it right the first time and was not sure at first if the little motor would even turn the 9x6 longtail blade instead of surface piercing blade but it does great. I can't wait to here some feedback on the 9x9 hill prop and may try one in the future. Getting speeds even with the copperhead if not better without a surface piercing prop so I aim to try a sd prop in the future.

Ok got the last of my metal in yesterday. Already have the shaft milled and the lower piece for the gear box welded. My buddy is spending tonite desiging the last two pieces for the transom mount so they can be loaded into the CNC in the morning. Then a full day of cutting stuff out with it and some more fab work. We decided to run a 1:56:1 for the gears. Ordered a Hill Marine 9x9 prop. Should be here on Weds. But got some work to get done before we put it on. Might be next weekend.

pics of the hull , I will be pushing it is 10-6

more to add,

Good looking work so far..

new update.

another pic update

Is that a shortened Duck Hunter, like Brad builds? Cool little boats.

i have a home made surface drive also.
6.5hp honda clone engine with rev. limiter taken out, 18lb valve springs, go-cart clutch on top and like a 3.15 sproket on bottom(have to check). currently has LT prop on it. wanting to get SD prop. need to figure out which gear ration will work best with mods, 14'x32" boat, SD prop(9x7)(9x9) combo.

im going home tonight and figure out my current gear ratio.
will continue to read for help...thanks