Mudboat surface drive prototype

Seems that I have another problem with my boat. A friend of mine designed a drive system that he said was the best thing since sliced bread. Well after my test run, I think that it’s worthless.

It is a mudboat inboard type drive. The shaft comes directly out of the bottom of the transom. It is covered with a piece of 12” pipe, cut at a taper. This tapered pipe creates a tunnel which, in theory, would have forced water to the prop, even is the shallowest conditions.

I wanted to build something better that the surface drive mud motors built. I was told that this was it. Well, the tunnel creates an air pocket. I have a lot of cavitation. I can’t plane very well either, even when not cavitating. It seems that this tunnel sucks the back of the boat down. I tried some trim tabs, about 6” wide and 12” long. This seems to help some.

Perhaps it’s too much weight to power. The boat is 16ft with a 48 inch bottom. Total weight of the hull is about 350 lbs. Engine is a Subaru 65 hp 1600 cc. Weight is about 150 lbs. Transmission is a Borg warner 71C 1:1. Weight is about 150 lbs. With water, fuel, ice chests, people, etc., I estimate that total weight is around 1200 lbs. I draft about 6” in the rear with a full load.

I would like some ideas on how to fix this problem. Even when on plane I think that the boat goes around 18 to 20 mph. I would like more speed. I was running 10X8 prop, but got it worked to a 10X10. I am happier with the 10X10, but I was also told that this prop (Michigan HD Nibral mud prop) was designed a long time ago, and that newer and better mud props are available. I don’t understand how a 35 hp lawnmower engine can push my boat faster and better than a 65 hp car engine. Those engines run at 3600 RPM, and so does mine with the 10X10 prop.

Any suggestions? New prop? Modify drive system? Bigger trim tabs? Perhaps I should sell the motor and tranny and get a worry free tried and true mud motor? I hate to do that. Too much time and money invested in this new design.

 

Hi mudman,

The drive system in the photo is a bit of a hack job, for sure. But it has enough similarities to some successful drives that I'm curious to know more.

Is the prop shown below the one you mention? (Michigan Weedless HD shown).

Your prop isn't cavitating, it's ventilating- sucking air into the prop arc. The drive setup shown looks like it is, in effect, a partial surface-piercing setup. Surface piercing drives need special props designed to operate with the top of the disc out of the water. They look nothing like mudboat props. Allowing a prop designed for fully submerged operation to suck air in from the surface invariably results in terrible performance.

There are some folks on here who have experience with surface-piercing setups and may be able to offer more insight, if you wait a day or three....

 

I can't believe that you called it a hack job!!!! I am offended . Just kidding, I know it is. LOL. I'm gonna hack it up even more!!!!!

Yes, that is the prop. I was thinking of investing in a surface piercing mud prop, or a better mud prop. I just don;t know which one.

I rode in the new surface drive mudboats, and boy do they scream. So for performance, I think that the surface drive is the way to go, but for muscle through the mud, with good reverse capabilities, maybe the submerged prop is what is needed. I'm not sure how well the surface props work in reverse, or if they are capable of turning 3600 rpm. I think that you need to run a reduction drive on the surface prop, but I'm not sure.

When at Idle, I do see eddies form and hear the prop sucking air. The prop, though extended from the transom, still sits under the hull of the boat.

I've got some ideas on how to fix it and I'd like to know everyones thoughts.

1. Cut off tunnel. Put plate even with bottom of boat from transom over the prop. Brace accordingly.

2. Cut off tunnel. Shorten shaft until CL of shaft even with the bottom of the boat. Buy a surface piercing prop.

3. Leave unit as is. Add surface piercing prop. Put larger trim tabs.

Getting super shallow is the key. I mean like running across mud flats.

After looking at other designs, it seems that the slope of tunnel is the wrong way. I don't really know how to get the most out of the boat. All that I know is that I want more. A 16 foot boat that planes sometimes and doesn't run through mud is of no use to me. I have to get to those ducks.

For an idea of the surface drive outboards, google prodrive, go-devil, mud buddy, and gatortail.

 

not for certain of the design but i do know that in boats with tunnel the prop neeeds cup put in it so it wont create cavitaion, i have a regular tunnel in my boat and a prop without cup creates a bad chatter that aggravates me. if you add the proper cup and loose as much cavitaion air as you can you will get more clear water pushin back where its supppose too

 

I've decided to abandon the fixed surface type drive system. I have a guy building an outdrive for the boat. It is very simmilat to other production surface drives, except it is designed to run through the mud.

The prop will have extra heavy cup in it to run as a surface prop.

This is a pic of his design.

http://www.blizzarddesigns.com/mudboat/mudboat.html

 

The surface drive you constructed seems to be a sturdy piece of work, abandoning it because the first attempt failed would not be my choice.
Because the tunnel is tapered, near the prop there is hardly any ceiling left, so the prop can freely draw in air. In my design, extensively discussed on this forum, I use half-circle tunnels without any tapering and still encountered problems with ventilation.
Your concept will work, but probably only with a special, cleaver style prop with enough blade area to compensate for the ventilation. A 2-blade prop will run very rough because of the asymmetrical load, even if the blade area is sufficient to obtain the desired speed.
Before cutting off this nice contraption I would do a simple experiment with a tunnel that does keep the air out and use your current prop. Use the construction you already have as a backbone to attach the tunnel to.

 

did you ever get something to work?

 

I've attached pics of the latest design. I took it for a test run yesterday and I topped out at 4000 rpm, 35 mph. Verry happy!!! I'm gonna take a little pitch out of the prop. Regular mud prop with extra cup that is run as a surface prop. Homemade arsenon drive. Rooster tail about 5 feet high!!!!

I have a little trouble getting on plane in deep water, but in about 2 to 3 inches the boat will pop right up. Too much weight in the back. I draft about 6 inches at the stern and when trying to get on step the back wont pop up. I mean it plows through the water and will not get up until I get to the shallows. Wide open throttle only at 2000 rpm until I get up. After that, I'm fine and can stay on plane. Any suggestions? Trim tabs, water wings, or pods? I'm thinking pods. The boat is 4 feet wide and 16 feet long. With me, the engine and tranny, I suppose that there is about 500 lbs in the back, and nothing in the bow. I got my wife on the bow and that helped out alot, but I'd rather the boat run great no matter how I load it, since I will be loading it up many different ways during duck season.

 

Wow, what a cool project!

The idea of 25 knots in 2" of water sounds damn scary to me though.

 

I have constructed trim tabs and the boat really performs alot better. Getting on plane is not a problem now. I took it for a run on Sunday and popped right up in the deep water. Finding the shallows was tricky, but I found some about 2" deep in a nearby pond. I ran right through that without missing a beat, but it does get slippery. The boat won't lean that shallow and more or less slides. Start turning about 50 feet before you really want to turn.

I am very happy with the design so far. Total cost for just the drive unit was about $2k. The engine was $150 and the tranny was $150 on ebay. Rebuild kit was $400.

This design seems to work very well on a craft up to about 18 feet. It is simple and effective, but needs to be properly maintained. Getting through the mud is a South Louisiana duck hunter's biggest problem, and I think that this design will help us get to where we need to be, but more importantly, get us back home.

 

mudman I am in the beginning process of building what you already have and i have some questions too many to list here can u contact me at gevans@renhcs.com so we can talk thanks gregg evans

 

sup, new here . i own a prodrive and it is da bomb diggity! this "super go-devil" yall are tryn to build sounds like it needs a bigger prop. if it is cavitating that bad u need some more cup in dat prop or a bigger prop overall. my motor is a 36 vanguard on a 16x48 semi-v polar kraft. it turns up 3970 and does 27.8 on a gps wit two people. now, im swingn a 12.25" prop wit my 36hp turning up 3950... u wanna guess what size wheel ya 65 hp subaru is gonna have to swing to grab water??? all in all da super go-devils and surface drives are the poop due to the ability to run the prop more parrallel to the water surface.