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First airboat project

Barefootduck

Well-known member
First off want to thank this forum for giving me information on good starting points to set up my first boat.
When I was growing up my dad had a 12ft fiberglass with 500 caddy DD and paddle prop. Only boat I ever have ridden on. So I have been wanting one of my own and found a good starting point. I had a sbc 305 figured from information I gathered on this forum that it would do ok as a DD with a small hull. So I picked up a ttailer and 12x7 Fiberglass hull with poly and all rigging besides the cage, rudders, engine, and prop. $450 first weekend I changed the engine mounts over to accept the 305 I guess The guy had a Cadillac North Star from the previous owner. Then got started on the cage, I’m actually an electrician so felt confident in bending the conduit lol. I used 3/4 for the main two hoops and lower rudder mounts cross bar. And 1/2 on the rest. I wanted a kind of hybrid cage to have access to the engine but looks of a full and also cover the exhaust. Think it turned out pretty good and from first few rides it doesn’t flex or anything. Picked up some rudders near where I live for $100 and made mounts for those and made linkage to hook to the stick. From info on here I got a starting point for the distance of the rudders to prop and how far apart roughly. Seems to work just fine. So basically I had a warped 72x36 paddle I used for mock up and decided to buy a new sensich 72x36 stick prop to allow me to switch to a 350 and be ok. First test rides and it revs to 2300rpm max which pushes the boat great cruise in deep water at 1800, and runs dry steady as long as I am moving, but doesn’t like to break loose if stopped. I’m hoping if I put a cam for low end in the 305 it will give me a few hundred more rpm to get closer to the 2700 max and that that will be enough to help with braking loose.
Like I said thanks for all the info I’ve gathered from various posts and let me know what you guys think of my set up and how you think that low end cam will help with the rpms. All in all I’m surprised at how good it actually does considering the only thing I know the 305 has is an Edelbrock performer intake. Also my exhaust is temporary untill I get some headers and flex. Thanks!
 

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You did a hell of a job. Nice work for someone who's never done it before. Get a good coat of slick bottom on that thing and you likely wont have a problem. If you do, a cam will help some. Welcome to the addiction. start by either updating your location or posting it in this post. Plenty of knowledgeable folks on here that love to ride and will be happy to ride with you and give you the benefit of they're expertise.
 
radtech said:
You did a hell of a job. Nice work for someone who's never done it before. Get a good coat of slick bottom on that thing and you likely wont have a problem. If you do, a cam will help some. Welcome to the addiction. start by either updating your location or posting it in this post. Plenty of knowledgeable folks on here that love to ride and will be happy to ride with you and give you the benefit of they're expertise.

Thanks radtech,
It has 3/8 poly on it do you put slick bottom on top of that? And ok I’ll go ahead and update that info. Thanks again
 
no, you don't put slick on poly. Most fiberglass hulls that don't get beat up much have slick bottom. It reduces friction better than poly. If you didn't put the poly on new, check it for abrasions. Poly causes some resistance to slide while protecting the hull. If it is badly scratched, it will hinder slide even further. There is a product made to spray on poly to help slide, but in my opinion, it's not worth the money. It doesn't last long. You can invest in a sprayer and spray the last 20 ft before you stop and it will help in breaking loose. Most use dawn soap as it is eco-friendly
 
radtech said:
no, you don't put slick on poly. Most fiberglass hulls that don't get beat up much have slick bottom. It reduces friction better than poly. If you didn't put the poly on new, check it for abrasions. Poly causes some resistance to slide while protecting the hull. If it is badly scratched, it will hinder slide even further. There is a product made to spray on poly to help slide, but in my opinion, it's not worth the money. It doesn't last long. You can invest in a sprayer and spray the last 20 ft before you stop and it will help in breaking loose. Most use dawn soap as it is eco-friendly

Gotcha, yea it already had the poly on it when I bought the hull, it’s got some scratches in it, so I’ll keep that in mind, thanks
 
Got a timing curve spring/weight kit for the distributor so we’ll see if it might bump me up 100rpm or so on Saturday.
Also I still had the vaccume advance hooked up and been reading that airboaters don’t use it and just use the mechanical advance with the curve set as soon as possible basically, so I will disconnect the vac advance and set my static and then curve and max
 
Barefootduck said:
Got a timing curve spring/weight kit for the distributor so we’ll see if it might bump me up 100rpm or so on Saturday.
Also I still had the vacuum advance hooked up and been reading that airboaters don’t use it and just use the mechanical advance with the curve set as soon as possible basically, so I will disconnect the vac advance and set my static and then curve and max
You want to keep the vacuum advance, but you probably do need to adjust it. Assuming you tried adjusting your timing to find maximum RPM, tweaking your timing curve is not going to improve your max RPM, but it will improve starting, idle and throttle response (prop acceleration). Start with timing locked (vacuum advance disconnected and mechanical advance locked), find the timing that gives you maximum WOT RPM's. You will probably find optimum somewhere between 34-38 degrees.

Setting the rest of the curve is a bit more extensive, but you do want to use vacuum and mechanical advance to optimize your overall timing curve. Here are some timing figures I ended up with for a Chevy 350 with a RV cam to give you a general baseline:

Idle 32
1000 RPM PT (Part Throttle) 47
2000 RPM PT 49

1000 RPM WOT 36
2000 RPM WOT 38
3000 RPM WOT 40

Achieving a good map over the full range requires some testing and trial/error. Use the locked distributor to test for optimum setting at various speeds and throttle on your engine first, then use vacuum and mechanical to set your curve to best match the goals.

There are plenty of articles out there on engine timing, here is one worth reading:
https://www.hotrod.com/articles/set-ignition-curves-create-optimal-performance/

As for your present RPM's changing the cam will help some, but it is not going to give you 400 more RPM. You have too much prop to get maximum out of your motor. The single best thing you can do to significantly improve performance is switching to a better matching prop. Did you buy the prop new or used? Sensenich is pretty good about swap outs if what you bought does not work out well. Further, carbon fiber props are not cheap, but they are adjustable so you can change the pitch to match what you have now and then re-pitch if you ever upgrade the motor, they are usually well worth the money.

Do some reading on adjusting the HEI distributor and then feel free to ask more questions, it takes some head scratching to understand it all. I use a spreadsheet to keep track of adjustments and results when tweaking in timing curves so I know what each change does and provide a road map to the best setup.
 
Thanks for the info, I’ve read that link before but was reading a lot on here that most airboaters didn’t use the vac? That’s why I was going to disconnect it, but if I should leave it on I will. And yea I bought the prop new becuase I wanted to be able to switch to a 350 in the near future, and from what I’ve read the 72x36 stick is standard on 350 DD boats? I already had this 305 so I’m not trying to get it to perferon the best with the 305, already does better than I thought it would. But yea I’m going to play with the timing some this weekend so I’ll see what I can come up with. Thanks again
 
Duck,
The reason a lot of folks don’t run vac is because most marine engines don’t have it. You can get away without it and it is easier to tune the timing. But it does offer some idle and mid range advantages.

As for a future 350, find the casting numbers for the heads on your 305. Depending on what heads you have, they may be useful in building a higher compression 350.

What Carb are you running now?
 
Thanks Gator,
I did a lot of reading on the vac timing about a year ago between port and manifold vac, and I had better results at starting/idle at manifold because it offered the advance at idle so should I try to stick with manifold vac on airboat or do ported? I did set the timing on this engine when it was in the truck but can’t remember what I set the initial timing at or the max advance etc. it turns the prop over just fine so I wouldn’t think I have it set to far advance initial.. not sure I’ll check it out when it’s not pouring rain like it is now. Lol. And yea I will defiantly check those numbers sometime becuase that would be nice if they would help me out on a 350, and it has an Edelbrock 1405. And Edelbrock performer intake. The rest of this engine is stock as far as I know. I resealed it and put a timing set along with the complete metal oil pump drive because it had the plastic drive end. It is also and 85 block I believe I will double check that in a few mins. Got rained out at work today, and getting rained out at my house for airboat work. Lol
 
And I went with a woody prop because it is my first boat and I wanted to keep the cost down to get into it. I have less then 2k in this whole setup including the brand new prop. Figure I can have some fun with this boat get some more experience along with having the build expenerice. Then maybe later on sell this boat and get one that’s a little more capable. Either way Ive learned a lot already and am satisfied with how it performs as is, like I said I wasn’t sure how it would perform becuase I have never built one before and had to set up all the cage work and rudder mounts etc. it was fun building.
 
https://www.youtube.com/watch?v=D7OrnETUyOY

Thats a cellphone qaulity video of the first time i tried to run ground with it. i was maybe at 2k rpm

[youtube-video]https://youtu.be/D7OrnETUyOY[/youtube-video]
 
Barefootduck said:
Thanks Gator,
I did a lot of reading on the vac timing about a year ago between port and manifold vac, and I had better results at starting/idle at manifold because it offered the advance at idle so should I try to stick with manifold vac on airboat or do ported? I did set the timing on this engine when it was in the truck but can’t remember what I set the initial timing at or the max advance etc. it turns the prop over just fine so I wouldn’t think I have it set to far advance initial.. not sure I’ll check it out when it’s not pouring rain like it is now. Lol. And yea I will defiantly check those numbers sometime becuase that would be nice if they would help me out on a 350, and it has an Edelbrock 1405. And Edelbrock performer intake. The rest of this engine is stock as far as I know. I resealed it and put a timing set along with the complete metal oil pump drive because it had the plastic drive end. It is also and 85 block I believe I will double check that in a few mins. Got rained out at work today, and getting rained out at my house for airboat work. Lol
Stick with Manifold vacuum for a good idle, that's one of the primary reasons for keeping Vac. Advance. Driving a prop, these motors don't really use the part throttle, high advance portion of the curve anyhow, a reason for ported advance. The manifold vacuum advance let's you get a good idle and then retards the timing under high load acceleration off idle. Then you want your mechanical all in by 2,000 RPM or less given your max RPM.

You do have too much carb, you would be better served with a 350 CFM 2 barrel, which would be the optimum size for a DD 350. You don't need to change the intake, just get an adapter, the added height helps make torque. In the mean time, try tinkering with the linkage to limit how much the secondaries open, you might pick up some RPM's there.

Can't beat a $2K airboat that slides and sips fuel! Slide safe!
 
10-4 yea I thought it was loading up a little after I was riding dry then idling it acted like it was loaded up then after a min or so idle would come back to normal. Good idea on messin with the secondaries I’ll give that a shot, thanks!
 
Took it out a few more times and it does good. I did the timing curve and played with the secondaries some to have them not open as much and it actually did pick up some power. Will break free on ground now with two people so that’s great
Couple pics
 

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Enjoy your toy, be safe, and don’t let the green sticky sticky discourage you. February through June are the stickiest months for airboating around here.
 
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