Just finished a new project using the dimensions of a lake buggy, but it's all aluminum. 13hp predator, 42" composite ultra prop. I can't get it up on plane. Not sure if the pitch is not enough or the diameter is to big and I'm not turning the rpms I should. I have ordered a tach to check that out but I'm not real savvy on airboat propellers.

Hello Whatley and welcome to the site!

Your problem could certainly be either prop size, pitch or rpm but I'm guessing that your expectations may be a bit too high for a direct drive 13 hp power plant unless you're in very skinny water. My first airboat was a 14' aluminum (light) Jon boat with a 40hp JLO direct drive and I had a hard time staying on plane in deep water. (JMO)

I would recommend that you get a tach or better yet buy or borrow one of those hand held optical tachs to see what your top end rpms are. That will tell you if your prop is mis-matched to the engine. If you're running skinny water you should have sufficient power for a decent ride.

Welcome to the site. Photos would help a bunch. I do have a few questions though.
Are you sure you have the blades angled the correct way? The ultra prop can be set up to push clockwise or CC.
What pitch blocks are you using?
Do you have the governor still hooked up? On an airboat we use the prop to govern the motor slowing it down to about 3100 rpm which is the top operation speed of that prop.
How much do you weigh?

The length of the prop sounds OK depending on the pitch.
Don't get discouraged, you have a setup that will work. it won't be a strong running setup but it certainly will work.
When I first started out I had a eighteen hp motor on a 16 foot boat and it would plane in shallow water.
As I said, photos would help a bunch. A photo of the whole boat and one of the prop and engine would do.
Aerokirk has already mentioned weight, if the hull is too heavy it may be a bit under powered.

I would love to put photos, but I can't seem to figure out how to. I have a tach coming in today or tomorrow, so I'll know more then. I weigh around 235, boat complete is around 230 fuel and all. I have 8 degree pitch blocks. I'm not worried about being fast just want it to plane out. Where I fish is not a place you can run wide open anyway. Tell me how to post pics and I'll get some posted.

Here is a link to the tutorial on posting photos.
http://miniairboatassoc.com/forum/showthread.php?2147-Posting-an-attachment-to-your-thread-or-reply

I'm on a smart phone, I guess I can't post a pic from it. I can't seem to find it in the go advanced button. My phone is where all the pictures are. I can email them somewhere but that's about it. I could put them in my profile but I can't seem to get there to update that either. How to I get to edit my profile. Thanks

2430Here a side veiw

2432Here's rear/side veiw

Ok, me and my no computer skilled &$# finally figured out how to download a picture! But there she is. 14 gauge aluminum, two men can pick it up. It has to weigh less than the plywood hull in the lake buggy plans I used for the size. Hope to get a tach tomorrow then I'll know where my problem lies. Thanks for all the input. I started a
on the two man hull this evening. 5' 4"X 11'10". I have a full sheet metal/fab at home with brakes, shears,mig,tig, spool gun. These things I understand , this prop stuff is all Greek to me, but I'm trying to learn it. We fish the matagorda bay with flatstalkers and kayaks but I'm done paddling! I want GAS! Again thanks for the input, it helps me keep my perspective.

The first thing I am seeing is that your prop may be pitched backwards. What direction does that motor turn when looking at the prop?

Counter clockwise. The prop turns the direction the rotation marks indicate. It shows the leading edge to be the wider edge ( like the front edge of a plane wing) and the trailing edge to be the thinner sharp edge? If it was pitched backwards would it still push? My directions show the flat side toward the plane, which would put it toward the back of boat? Correct .

Yes, you are correct

Sounds like your prop is correct for your motor. It does look a little heavy though, you may be a little under powered. How much water is it sitting in when floating? Is it a five foot bottom? When you run it try sitting as far front as you can, on the front edge of the seat. There's not much length there to spread the weight out. Let us know what rpm you are running, that may be most important here.

I put a tach on it today. I was hitting 3300 rpms , then I change pitch blocks from 8 degrees to 10 degrees now I'm at about 3050. Do I need to disconnect to govenor? I don't know, small block Chevy and tires, I can make scream, but this has me at my wit's end!

You give up a lot with short, flat pitched direct drive props (you probably know that). Also the UP prop is inefficient, partly because it's constant pitch from hub to tip (like a fan blade). I like UP props, in fact I have a 3blade UP set on my PPG flyer, but they are notoriously inefficient thrust producers.

So far we've redesigned the hulls, cut almost 40# off and added wetlander coating to the bottom. We have 4 boats done now. One , the first one, a 13hp, the second two had 18hp and the latest one has a 22hp with a 1.5 reduction. I have a 46" 5 blade on it and it won't plane period. I'm wondering if the flat ultra prop blades don't have the thrust that a twisted prop would? You would think this thing would fly ? But it's a dog so far! Still working on it.

I don't think it's the props. I use an ultra prop for my second prop. They are excellent for power getting out of the water quick but lack cruise speed and top end.

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If ya could look at these pics and let me know if I have the propeller assembled correctly ? The 18 hp boat I can get on plane but only in 6 inches or less. The 22 hp boat won't period, Despite having 40 nm of torque, five blade prop and 1.6 gear reduction verses the 18 the direct drive has. I'm looking for any suggestions on where my problem lies. All four boats especially the 22 hp idles great and starts to take off when you give it some gas but then lugs down. Idle to wide open is only a few mph different. The motors are all counterclockwise rotation. I changed from 12 degree pitch to 15 yesterday but can't see a lot of change . All help would be greatly appreciated!! Thanks

They look really heavy for the hp you are using. Is that all steel angle? I'd like to see one floating. I think you are just too far in the water for the low up to overcome.

The hull is about 70#, the motor is 78# the batteries and all weighs in at around 215 for the 18hp and 243 for the 22hp. It's all gauge aluminum. That's got to be lighter than the wood hull the lake buggy uses?

It's all aluminum.Gauge material. It weighs around 225 with motor battery and all. Hulls only weigh about 70# each. Two guys throw it in the back of my truck. It has to weigh less than the lake buggy wood hull? Doesn't it? It sets about 3" in The water with me on it. The 22 hp weighs in around 243 with fuel and all. Is that too heavy ,I can't get it any lighter.

Don't know anything about airboats, but had a quite a few regular powered boats. If that was a regular boat it would seem as if the "trim" was not set properly. Which leads me to ask what is the normal angle of thrust on an airboat? In other words does the prop blast the air up, down or parallel with the hull? Is it adjustable?

TJM- I think you've hit it. I struggled with my set up for the first couple of months running, and found that my thrust angle was completely backwards. The direction of thrust should be biased toward the water at the rear of the boat. I had mine pointed up, thinking of my power boat, but after a few conversations with Pappee we figured out what each other was talking about and i realized i should add shim to the front of my rigging. After a couple of trial and errors I got a major improvement and am quite happy with how it rides now.

TJM- I think you've hit it. I struggled with my set up for the first couple of months running, and found that my thrust angle was completely backwards. The direction of thrust should be biased toward the water at the rear of the boat. I had mine pointed up, thinking of my power boat, but after a few conversations with Pappee we figured out what each other was talking about and i realized i should add shim to the front of my rigging. After a couple of trial and errors I got a major improvement and am quite happy with how it rides now.


I'm very glad to hear that. Make me feel good! Thanks.

Whatley if you are still having trouble with the 13hp I would try adding more pitch to get it down around 2800rpm I've noticed the ultra prop tends to top out there you don't gain much past that.

I've decided that I have a lift problem. The 22 hp will do 40 mph once you get it on top. The 18 hp will do about 25-30 on top of the water. The problem is that I need three to four inches to do it. We fab all this so I'm making a pontoon that is 8"x16" and 4' wide , if I'm doing my math right this should produce a considerable amount of lift. We'll see in a week or two . Right now the weather is too pretty to be boat building . Thanks for all the input.