I built a boat planning to use a regular outboard, but I got an offer of a free-to-good-home Honda 20 HP GD LT. Since I duck hunt in the marsh and it was free, I could not turn it down. My boat is a 16' flat bottom epoxy ply hull that is 40" across the bottom at the middle station and 30" across the bottom at the transom. I did not really build it for a 200# motor but the squat at rest empty is not too bad. I replaced the carb, fuel pump and PTO key along the way, but I have it running smooth now with what feels like the full range of throttle response. When I first used it, the cav plate dove badly and was exhausting to steer, but after tweaking the cav plate it runs just below the surface like it is supposed to. However with the hull @ 320#, motor @ 200#, me @ 230#, gas @ 30# and maybe 20# of everything else I barely got 10 MPH going down stream yesterday and ~8ish heading up stream. It squatted the transom terribly under power and wouldn't get over the bow wave.

Does this seem right? I know LTs are not speed demons, but I expected more than that. I have my fuel under the forward deck and will add another tank and also the battery with time. Right now I suspect I need 400+# forward to get level flotation with me as far forward as I can safely be. I really hope to seldom carry that much weight.

I have no idea what the pitch or diameter of the prop is. It is a replacement, but I assume it is factory standard. If this was an outboard, I'd increase the pitch and maybe the diameter since the motor isn't working hard, but I don't want to start a prop hunt unless I have a reasonable expectation of getting this rig working together.

With my narrow transom, how can I get some reasonable performance out of this motor? Any suggestions are appreciated.

Thanks,

Nate

I run a 9 x 7 on my 20 HP Kohler long tail. A 9 x 6 or a 9 x 7 is common for a 20 HP motor. measure it across the widest part of the prop. A tape measure is fine for this. Any thing smaller than 8 1/2 inch and I would replace the prop. You need a tiny tach on the motor to monitor RPMs. You should be turning 3,600 + RPM at wide open throttle. I would expect 15 + miles per hour out of that motor.


Mark F. Cheney

A 9x7 seems to big for a 20 hp to turn. 9x7 is the recommended size for a 27 hp and 9x6 for the 25 hp. Go devil recommends a 9x4 for the 20 hp on their website.

Interesting. I hadn't considered a Tiny Tac. That would be good info to have before I start buying props.

Nate

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Dang. A new prop from Go-Devil is $225. Definitely must get that Tiny Tac so I can get this right the firs time.

Nate


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Backh20 makes a good prop a lot cheaper than GD.

^^^ agreed. Bought one for a 16 I had..... I run a 20 now and will do 17-18 me on a 1642 aluminum flat. 8-10 mph, I'd say something aint right

What wooden boat plan are you running it on?

My bet is the boat is wrong for the motor. Your floor tapers from 40" in the middle to 30" at the stern which means it probably has rocker in it right?

Can you post a picture of your boat?

No rocker from the middle station to the transom. I know the builder kinda well, as he is me. I made the hull to float shallow and pole easy like a big flat back pirogue. I planned on a 100# or less motor. However I decided to give this beast of a Go-Devil a shot since it was free. In the second to worst case scenario (the worst case scenario always results in death), I can sell it and get something that fits my hull better.

Nate

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Here's a pic.



Nate

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I hate to say it because it is a beautiful boat. I think the narrowness of the bottom is creating too much white water causing the prop to unscrew and not grab. How does the water look behind the transom when you are running? Have you tried adjusting the cav plate to run really deep in clean water? Like I said it looks like a great boat and I hope I am wrong.

20hp uses a 9x5 prop and should cruise a light load well above 3800rpm in water

Looks like a nice boat and should be able to run around 15 -18. You want the cave plate just slightly under the surface when running. Tiny Tach is a must. You don't know what is going on without rpm. 9x5 will most likely be your prop but get the tach first. Install tach and see what rpm you getting full throttle out of water.
Take a picture of your current prop and post

I have the cav plate running just under the surface. I think it will blow out if I adjust it to run higher and when it ran deeper, I couldn't steer and got no more speed. The water from the sides might be aerating the prop though.

Nate

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That is a slick looking boat!!! Great job on the build!

I'd be curious to see what it would do with a lighter Copperhead surface drive on it. I think the 30" floor is some of the problem with performance, if it was 40" at the stern like the mid ship it would probably run better.

Thanks. If I'd know I was gonna put a mud motor on it, I wouldn't have tapered it to the transom, nor would I have decked the stern like I did. Curving the sides and clambering the deck adds rigidity to the hull without adding stringers or chine logs, but it sure decreases the volume of the stern. I planned on a jack plate and a 20" shaft 9.9 HP MercHatSan outboard. Now I have to mount the GD on the jack plate. It is higher than I like, but I have to have the height to tilt the motor due to the deck. From what I can tell, it was made for a 20" transom anyway. The tab is level with or a degree below the cav plate and it is running subsurface without blowing out. If the tab is up at all, the cav plate dives like a crank bait and squats the stern worse.

I wish the 6.5 HP CH would work. I like that engine, but I think it is a too small for my purposes. A CH or Dixie 10-12 HP SD would be nice, but you don't see many used 20" transom models and I can't afford a new one at this time.



Once I am rich I'll just buy a aluminum mud boat for the GD and keep an outboard on the skiff. Until then, I'll dork around with this setup.

Nate

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Well crap. You all set me straight. I just measured the prop. I just assumed that the previous owner put the stock prop back on. I was wrong. The dang prop is only 8" and it is fairly new. That is what they use on 8-9 HP Go-Devils! I am not sure why they under propped it. I'll probably have to try a second prop after I tach the new one. At least I have a clear point to start from. I was gonna take a pic but wasn't coordinated enough because I was using my wife's fabric tape measure in the rain.

Thanks guys,

Nate

I have another question. I am usually a bone stock kinda guy, BUT if there are some high benefit/low risk aftermarket upgrades I should consider, please let me know.

Nate

Nice boat, but IMO your boat bottom is way too narrow. Others are probably spot on about it sitting too low in the water and causing your prop not to dig in because of cavitation. But it's hard to say.

I run a 18 hp Go devil LT on a light weight, riveted Lowe 1640 jon boat (~250# hull) and it pushes me about 15 MPH when I'm WAY overloaded with bowfishing gear, front deck, lights, generator, a couple shooters, etc. It gets real interesting when we're in real shallow water and have shot a 55 gallon barrel with fish. We have to offload to get the boat floating again. LOL. I haven't ever GPS'd it without a load but it scoots when I'm not carrying all that stuff and have just another person in the boat.

A couple years ago it was floundering slowly so I replaced the prop (I think with a 9x7) and it's now like a new motor, and it's 25 years old.

I forgot to mention. There's an USCG equation for figuring hp rating of boats under 20 feet in length. It is as follows.

For square chined boats: Length of boat (in feet) x widest measurement of transom (in feet) x .5 - (minus) 15 = hp

For rounded chined boats: Length of boat (in feet) x widest measurement of transom (in feet) x .8 - (minus) 15 = hp.

May not mean anything, but then again it might ......

Oh, I know it is too narrow for the motor. I am just hoping to limp by for the time being.

I used that formula. It comes out to 10 HP. I was going to listen even though I thought it was conservative, but a free motor blurred my perspective. Since boats of similar dimensions are rated to 25 HP, I gave it a shot. It turned out not to be a perfect fit, but it presents an intriguing problem in the mean time.

Nate

Where are you located that you still have ice like that?

BrownDog; I can't blame you for wanting to use that motor. After all, it was free.

I got that equation off the USCG boat builder's website. It's usually fairly accurate. I TRIED to read through and understand how they come to the load capacity for a given boat, but it's fairly complicated and I didn't want to even think that hard about anything.

Flatontheplatte; At least BrownDog has open water to run in LOL... Our rivers have finally thawed out enough and the launches are now accessible (in places) to get into them but our lakes are still froze over. But spring is just around the corner. It was 40 degrees today and I can finally see bare ground in my yard, in places. Come on rain! Keep falling...LOL..

[quote="one click willie"]BrownDog; I can't blame you for wanting to use that motor. After all, it was free.

I got that equation off the USCG boat builder's website. It's usually fairly accurate. I TRIED to read through and understand how they come to the load capacity for a given boat, but it's fairly complicated and I didn't want to even think that hard about anything.

Flatontheplatte; At least BrownDog has open water to run in LOL... Our rivers have finally thawed out enough and the launches are now accessible (in places) to get into them but our lakes are still froze over. But spring is just around the corner. It was 40 degrees today and I can finally see bare ground in my yard, in places. Come on rain! Keep falling...LOL..[/

Sorry DP

I had a 16hp go devil with a 9x4 prop on it and I was getting 14 with me and another guy on a 16ft war eagle boat not the best so I uped to a 23hp go devil!