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Engine question???

TexasRob

Member
I've been searching and reading through the forum trying to get some answers on what engine I need on my airboat. What I have now;
sbc 383,stinger 2.38, Sensenich 82" r 3 blade, pushing a 20' Maxcraft full deckover (3300lbs no one in it),poly fair shape bottom, hull little beat up. The motor is blown. I run lakes and rivers bowfishing in Tx. The boat would jump pretty quick on a plane in deep water but going over ground was out of the question. The combination with the small block I'm not happy with. I have a 489 bbc not put together, would this be any better? Don't want to keep trying different combos, don't really want to buy another gear box and prop. Any help would be great! Thanks
 
A healthy BBC with the 2.38 will definitely be a good combo. The Sensenich website states max RPM at 2,800 but if I remember correctly most people are shooting for around 2,200 - 2,400 RPM max with the R. With that gear, you are looking at max engine speed of ~5,250 - 5,700 RPM. Choose your cam and other components accordingly. I'm sure others with far more knowledge than me will weigh in.
 
Ok since motor is not put together yet boring it out to 60 over gives a little more torque, don't know if its enough to worry about. Does the full torque need to pull in at the 4400 to the full 5700rpm range? It's going to take a custom grind on cam right? What I have for this motor right now is machine work on '75-454 truck block for stroker scat crank, puts bore around I think or at 4.3 at 30 over, 6.385 h beam rods, Icon forged dome with valve reliefs, Dart Pro 1 325cc aluminum heads 2.28/1.88, air gap dual plane intake, 850 dp holley , msd dist/6al,I have a roller Comp cam kit but not right cam for this airboat-bought this setup from a guy that needed the money it was for a race car build, couldn't turn down for $2500.
 
Main question is still will I ever be able to run dry no need to climb a hill with this combo- the boat is a "tub" it's made like a tank? I can't find anyone else that can tell me anything on a Maxcraft hull-know it was made in Fl. of the 20' 181/2 is in water,
 
Alright I know I have a lot of questions just trying to figure out where I need to be. The 383 ran at 4900 rpms at wot put prop speed at at 2058rpms. Prop set at 2 on the setting. Would I get more push turning less pitch to get higher prop rpm? Not worried about boat speed just push and not really worried about gph- most bowfishing time is at idle.
 
TexasRob said:
Main question is still will I ever be able to run dry no need to climb a hill with this combo- the boat is a "tub" it's made like a tank? I can't find anyone else that can tell me anything on a Maxcraft hull-know it was made in Fl. of the 20' 181/2 is in water,
That's a lot of boat, running dry takes some power. 3,300 lbs now, add say 200 to move to the big block, depending on aluminum vs. iron heads. Add 3 guys +gear and you are over 4,000 lbs for sure. For go any where, run the hill capability you are looking to make at least 2,000 lbs of thrust. More is always better, but it would run easy ground with less.

This curve is for a 4 blade R at 83.5" diameter (2.5 pitch) and does have some testing basis. To make 2,000 lbs of thrust takes 750 Hp. That 489 is capable of around 600 Hp and would put you around 1,700 lbs of thrust.

4-blade-R-Thrust-800x605.jpg


The 82" diameter you are swinging is about right. The gear ratio will work if you have the motor to push it. With that ratio you want make max Hp at 5,700 RPM (or less) and peak torque in the 4-5K range. The 3 blade at full pitch may be enough to handle 600 Hp with that ratio, but you may need to upgrade to a 4 blade set up, I can't say for sure.

A 383 is not enough motor for that big of a boat. You will definitely see improved performance with the 489. It will run some ground for sure, but you might have to make everyone get off and walk some times. To go anywhere, you would need even more motor and prop.

Given that you have the motor, gear and probably the prop needed, building the 489 gives you the most bang for the buck.

Here are some comparisons of what different motor and gear combos might make with the 4 blade R:

4-Blade-R-Thrust-Compare-800x425.jpg


Hope this helps, good luck! :usa:
 
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