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Airboat Rebuild

First off, we’re new to the forum, glad to be here, and glad to finally own an airboat! I enjoy seeing all the support and input across this forum.

Over the last several months my brother and I have been rebuilding a 2012 20’ x 8' deep hull Pathmaker that we picked up. When we got it, it only had a single seat, cage, 454 BBC with 11 original hours, and motor stand. Guy had the hull built so that he and his son could rig it out together for bow fishing but unfortunately “life” happens and things get pushed to the back burner. Fortunately it wasn't due to any family illnesses. We’ve always wanted one, never owned one, but have been on them numerous times over the years. We were fortunate enough to have the resources to do just about everything ourselves from welding, layout, fitting, painting: cage, platform, seat boxes, and console are powder coated and hull, stand and fuel tank is liquid painted, and wiring, or had good friends in the line of work that we couldn’t do ourselves which saved us no telling how much money.
We have access to a 500 acre flooded river pasture so my brother took it out over the weekend to see how it handled after adding all of the extra weight. As I mentioned earlier, we have been on airboats numerous times but have never owned one, and are by no means experts on handling/operating them although we are pretty mechanically inclined and catch on pretty quick on operating stuff with, or without wheels haha... To my surprise, it handled great. It did not porpus, the bow did not plow/dig any, held true to the stick and didn’t try to walk or pull one way or the other when going straight, and did not feel top heavy when turning. I feel we could use a little more rpm but just have to play with it a little more this coming weekend. Our whirlwind stump pullers are currently set at 20 degrees, and saw 3800 max rpm on the trailer, 4000 max rpm on the water, and cruised around 3500ish from what my brother remembers and felt that it was performing pretty good. He ran it up dry in a small loop to ensure he could get back to water without getting stuck. He did make it, but could tell it wouldn’t just run dry as it is. I’m curious if less pitch and more rpm would help do the trick? I would like to drop them back to 17.5 or maybe even 15 degrees to try and get a little more rpm because from what whirlwind has said and what I’ve read, our props need to be spinning 2200-2700 max rpm. Is that correct?? I know we just need more stick time to really get a good feel for everything and just play with the pitches maybe. Fortunately, we have unlimited access to that River pasture to play and learn on with in fairly shallow to shallow water. We have had a blast building it, didn’t kill each other, (took a couple of deep breaths, but made it happen :slap: lol) Hope you guys enjoy the pics, and am open to any and all comments, questions, Concerns, or ideas. Thanks in advance!!


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Welcome aboard! That's some nice metal work you did, it should make for a sweet bow fishing rig.

As for running dry, there is room for improvement, but don't get your hopes up. Like you said, you added a lot of weight.

Yes you want to get more engine/prop speed and pitch is the first thing to try. Test it at 15 and 10 degrees to see what you get. With each setting, record max RPM on the trailer and then take it out to compare. In reality the boat would probably respond well to a numerically higher gear ratio and you can go to a 2.3 ratio configuration on that box I believe.

Your application and question is very similar to the following thread, which may provide some answers for you.

It would be useful if you had more information on the motor itself. Do you know how it is configured (Cam, compression, carb)? This could all be a loosing battle if the motor has an RV cam that makes torque at lower RPMs but falls off above 4,000.
Sweet! A high lift to pull or mount things. Not so sweet with taco neck looking at 90 degree off pics though?

Question at hand though:
Yes pitch it back if the motor can handle it? Stump Pullers can do wonders if a owner and a torque wrench is willing to try.
Many people wont touch a prop for some stupid reason? There made to tweak to the boat needs or dictate the wrong prop...... but stump pullers have a butt load of room to tweak them.
Just hope who ever tweaks the prop is not the same person that loads 90 degree off pics?
Thank y’all. Slidin Gator, I actually read that post a week ago or so and was surprised how similar it was... we have not done a compression test, it does have a holly 750 carb on it that we had rebuilt (runs like a spotted ape), not sure what cam or heads it has either. My brother used to race dirt track and I wanted to take the motor to who he had build his motors to have it dyno’d since we had everything pulled out of the hull, just never got the time to do it. We will be pulling the heads off to see what kind they are here pretty quick. Before we start getting too deep though we’ll just keep playing with the pitch/RPM. When we first put them on we set them to 25 degrees because that’s what they were at when we bought it. 25 degrees only saw 3000 rpm on the trailer, then we dropped it to 20 degrees which is what the maiden voyage was taken on so it should progressively keep getting better as pitch decreases... is 10 degrees to little though for that size motor?? I feel like 10 degrees just isn’t very much for that size motor but then again I know very little about the technical stuff. I guess every boat is different and you just gotta find it’s sweet spot huh? I will say that the polymer is in good shape, and we did send off the props to be revisited with the higher grade of the few options they offer.
Judging the pictures you guys have seen, do any of y’all feel we are starving the props from air flow by adding the elevated platform, and bigger seat boxes??
Lol Prototype, I have no clue why they’re sideways. I noticed that when I looked at them on my computer. They’re right when you view the post through the phone though
454, all the weight of a big block with most of the power of a small block. :dontknow:

Incognito90 said:
...when we first put them on we set them to 25 degrees because that’s what they were at when we bought it. 25 degrees only saw 3000 rpm on the trailer, then we dropped it to 20 degrees which is what the maiden voyage was taken on...

...Our whirlwind stump pullers are currently set at 20 degrees, and saw 3800 max rpm on the trailer, 4000 max rpm on the water, and cruised around 3500ish from what my brother remembers and felt that it was performing pretty good.

... is 10 degrees to little though for that size motor?? I feel like 10 degrees just isn’t very much for that size motor..

All of this is counter intuitive at first, but here goes.
10 degree may very well be too little pitch, all you can do is try. You gained 800 RPM from 25 to 20 degrees in the RPM range of max torque for your motor. Going to 15 degrees will net somewhat less, perhaps getting you to 4,500 on the trailer. 10 degrees might take you into the 5,000+ range, putting you at/near max prop speed. Your actual RPM gains with each pitch decrease will give you a good idea of where max torque and Hp lives on your engine. Max thrust will be made at max engine HP RPM and you will have peak torque available at lower prop speeds for improved acceleration.

Reading some of the boating forums, the consensus seems to be that the typical factory Marine 454's were rated at 330 Hp at around 4,200-4,600 RPM. Other versions (425 Hp engine) are made to produce max power upwards of 5,400 RPM. On the other hand, a motor with an RV cam would fall off rapidly above 4,000. Testing with different pitch settings will tell you a lot about your motor power curve without a true dyno. I'm not the kind of guy that's gonna pull heads off just to see what they are.

Incognito90 said:
Judging the pictures you guys have seen, do any of y’all feel we are starving the props from air flow by adding the elevated platform, and bigger seat boxes??

Back to your tests so far, the prop is getting air, otherwise it would not be holding the motor RPM's down. If the prop were to run in a vacuum, it would have no load and would run away. If you had significantly impacted intake air flow, you would be asking about adding pitch to keep the revs down. Any chance you did a max RPM test when you first bought it? If the REVs went up (at the same pitch) after your rebuild, then you would have impacted air flow. If they went down, you actually improved the intake air flow!

Finally, your fuel burn is going to go up with less pitch. Given that your primary purpose is Bow Fishing, at some point you will want to consider if the added thrust is worth the added fuel. But first find max performance, find out what she will do and post some video from the pasture, especially any good wreck scenes. :cheers:
You haven't said (or I didn't see) what exact flavor/configuration your 454 is. There a number of possibilities.
That missing bit of information may preempt part of what follows, but here are a couple generic thoughts anyway.

While pulling the heads would shed some light on what you have to work worth, were it me,
I would be inclined to first take a little time and identify what it has in it as far as the cam goes.

It would be less intrusive and possibly more telling to do so. While I agree that pulling the heads is not a big deal, I also tend to agree with Slidin' Gator, in that I would be hesitant to pull apart an engine with < 15 hours just to satisfy my curiosity. I would like to think the heads could be identified without having to pull them.

Given a degree wheel, a dial indicator and a pulled valve cover you should be able to determine the cam specs close enough to profit from the effort. With those numbers in hand, you should not only be able guesstimate your max RPM, but also have a point of reference for making the determination if a cam swap would benefit your cause, or maybe a gear change would better utilize the current cam.

All of that aside, with the size boat and load you are working with, I would expect a pulley change to a 2.3 ratio would ultimately yield the biggest improvement toward the end you are wanting to attain, given the correct cam. IF the working range of that blade set starts at 2200, that looks like it may work well. If your luck is better than mine, maybe that is what you already have.

Slidin Gator said:
454, all the weight of a big block with most of the power of a small block. :dontknow:
. . . .This is cute, hadn't seen that before . . . :)
Thanks guys. We’ve got a few more minor things to do tomorrow, work load permitting, but I’ll keep ya posted if we’re able to get on the water tomorrow at some point. I really appreciate yalls input and suggestions.
Well our testing/playing fell real short on Saturday after finally making it out. We had to shut it down because we thought we had some issues with the gear reduction. After getting a little time yesterday to look at it, there turns out to be a loose bolt inside the bottom pully just bouncing/rolling around. No clue how it got there, there are no holes inside that pully for it to have backed out of, and the gap between the housing and pully/belt is no where near big enough for a bolt of that size, probably 5/16 x 3-3.5” to have fallen in there from the outside. I do know that the previous owner had American Airboat Corp put a new belt on there just shortly before we bought it because of a dirt dobber nest that built up on the belt that he wasn't aware of and when he fired it up one day it damaged the belt. So maybe the guys at American Airboat Corp accidentally laid one in there and forgot to take it out? Not too sure, but please know I am not bashing them, things happen and sometimes slip through the cracks. Although it doesn't have many hours on it since the new belt, I'd say around 5 on the high end, I just find it hard to believe that it has never got caught on the housing to create the grinding as it did Saturday, or make any other noises. :dontknow:


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Hey guys sorry for taking a while to get back with you. We finally had some time to pull that lower bearing off and take that bolt out. We got everything back together this week and were able to test the RPM's at 15 degrees on the trailer yesterday afternoon. Saw a nice increase in RPM's from the 20 degree mark, as we were able to hit 4600RPM WOT. Which was better than before where 25 degrees was 3000 WOT, 20 degrees was 3600 WOT, and now the 15 degrees was 4600 WOT. Planning on running it tomorrow to see what kind of trouble we can get in to haha.


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You are on the right track now! Your earlier post said 3,800 RPM at 20 degrees, which makes the most sense (gained 800 RPM from 25 to 20 degrees and another 800 RPM to 15 degrees, now at 4,600 RPM). Now this is very much seat of the pants and rule of thumb here, but your motor is running as expected, which is pretty flat torque through the 4,000's. You have another 400-800 RPM worth of power remaining in the motor that you can tap with a 12-10 degree pitch.

Go run it just like you have it, then change the pitch to 10 degree to see how much better that is. If it were me, I would probably settle on pitch that nets around 4,800-5,000 RPM on the trailer and call it a day.
We'll keep you posted on what happens tomorrow. I wish I could post a video of the one I took yesterday showing how quick the RPM's climbed but it was a solid steady climb.
Test run went pretty good yesterday. Boy what a difference 15 degree mark made. We saw 4750-4800 on water and were hitting 32 mph, although it is the lightest the boat will prolly ever be, was just me and my brother, not hardly any gear, and very little gas... got a couple hours of stick time between the both of us which was great, what a difference in comfort behind the stick that short amount of time made (still respecting it though), and man are mud flats fun. We dropped the blades back to 12.5 and was able to pick up just a little bit of rpm, right on 4900. We ran out of time and were not able to test the boat at 10 degrees though.


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Excellent! You are getting the expected results, but it does not really make sense until you see it in action! You are starting to home in on max power for your motor. You gained 800 RPM for 5 degrees each between 25-20 and 20-15. Then you gained 300 RPM from 15 to 12.5. The standard issue marine 454 makes peak torque around 3500 RPM and then it is pretty flat into the high 4,000's.

Torque is starting to drop now as you approach max power, so your gains decrease, you might get another 100-200 RPM out of a 10 degree pitch. You didn't mention top end speed before, but I am sure you have found that the boat jumps from a start and runs the flats much better (yep, the flats are fun, that's why we own these things :slap: ) and you probably gained some top end speed. You are now at the point with the 12.5 and 10 degree pitch points where top end speed will limit out or even decrease while your ability to run ground has still improved a bit. This is where the trade offs lie and you determine what is best for your needs.

First, you noted that this is about the lightest you will normally be, that tips the trade off towards less pitch. Get some coolers and fill them with water to add some weight. Load her down and try it next time testing 10-12.5-15 degree pitch settings. By the time you get to 10 degrees, you will notice that the boat jumps from an idle like never before, making it easier to leave Lunch Island. But you have a bit less acceleration available to throw at it when you are at speed (like when you need to steer at speed).

Short of motor modifications or a different (bigger) prop, the low pitch points with your present gearing is the optimum configuration for the best ground running. What you loose is top end performance, fuel consumption and thrust at idle. The last point is probably important for your stated purpose of bow fishing. With the lower pitch settings you will have more ability to creep slow, but then you will have to rev the motor to chase, I'm not sure what will be best.

Like I said, if low end push (particularly with a load) turns out to be your highest priority, then you have the set up, just find your best pitch. You could change to the 2.3 ratio on your drive, which will let you add pitch back to the prop while keeping the motor near 5K RPM for max power. Doing that will somewhat decrease your low end push in favor of more top end performance (like on the mud flats 8)). Given your results to date, I do not think the gear change is warranted, at least not unless you decide to do some motor work (and start turning that motor towards 6K RPM), or decide to put on a larger prop for even more low end push.

Enjoy, I wish I had my own private 500 acre playground!
We will definitely load it down next time and see what all happens. I wish I could tell you what all speeds we saw, but we didn’t have the gps mounted until this last time out. I do know it would only run 23mph when we test drove it and that was just with an app on my phone, that was nothing but the hull, cage, motor, 1 seat, and 3 people. I want to say we did mid 20’s at 20 degrees but that was with an app also and going across deep water which I know changes the way it handles/performs... As I mentioned, we could tell a huge difference in hole shot, and acceleration with the new pitch, but still would not take off dry. Would run a solid 40 yards across dry ground from water to water at WOT, not take off dry on a harder bottom, but would jump up and go no problem in that soupy muck bottom. We took our 6 wheeler for this exact reason haha. It would move left and right, but couldn’t go forward, with a slight tug from the buggy it would get up and go. It would start to spin in a circle if one of us gave it a solid push up on the bow.... leading the two of us to believe that it just needs a liiiittle more motor to not necessarily run dry, but get you out of a bind should you ever run up on one. But again we’re not airboat experts either do I could be wrong. What about shimming he motor any?? Changing that should give a little bit of bow lift huh? Or would that not really help our case... I don’t think I’m leaving anything out from yesterday. thanks again for all the support from you guys. Means a lot we don’t have to try and pioneer this on our own.
Stuff like this though it ate up alll day Long with no trouble at all.


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Yep, it don't get no better than that soup for an airboat, if it don't run that you got a problem for sure. You guys got the bug now, so drifting dirt tracks ain't got nothin on that!

You will get a bit more push off dry if you go ahead and set it to 10 degrees. If you have not run into any porpoising so far, there is some capability to gain by shimming the motor to angle the aft higher. Go ahead and shim it until it starts porpoising bad empty and it will probably be about right with a load. But go back to my first response:

Slidin Gator said:
As for running dry, there is room for improvement, but don't get your hopes up. Like you said, you added a lot of weight.

At best case at 10 degrees you will get that motor up to making 400-425 Hp at 5,400 RPM. Figure 10% gearing loss and you will be good to make 380 Hp at the prop, and that is pedal to the floor, valves close to start floating. That is about enough power to run a 2,600 lb boat dry all day, but your boat has to be over 3,000 lbs for sure! Figure you need 1 Hp for every 4 lbs you weigh over 2,600 lbs to get to where you really want to be. Having a racing background, you guys aren't done, my magic 8 ball says there is a 700-800 Hp motor in your future. You have a big heavy boat and a decent motor, it is what it is, use it that way for now!

Now, let's start talking about that bow fishing trip, I'm tuning up my gear now.
That was kind of the mind set we had, do what we can with the motor we got, run it until something happens, then put a motor in there that gives you a $#!+ eating grin. We will try the 10 degrees and see what happens and hopefully have time to go run it next weekend... as far as the porpusing, the first time my brother ran it, at 25 degrees, he had 3 other people in the boat and said he doesn’t remember it porpusing. Yesterday, it would start to porpus in the deeper channel we had to run in, but would not on the mud flats. So should we not play with motor shims? My buddy started adding everything up that he had cut out for us but don’t remember what he said, for some reason 1,000 lbs keeps sticking to me. I know we have about 200 lbs in lights, about 100 pounds in liquid paint, 45-50 lbs in powder coat, 80 lbs in speakers and amps, 60 lb generator. We’ve got a couple of scale houses we can run across to get out total weight and just subtract out the trailer and truck so we know exactly what we’re working with... as far as slinging some arrows, you find yourself in SE Texas area you let me know