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PROBLEM RUNNIG DRY

acslammer27

Active member
I HAVE A 14FT COMBEE FIBERGLASS HULL WITH A 0470 ANGLE VALVE , SOME FORM OF SLICK BOTTOM , A 72 IN POWERSHIFT PROP SINGLE REAR AND DOUBLE UP FRONT MY MOTOR SEEMS TO BE LEVEL TO HULL AND THE BOAT RIDES WATER GREAT BUT IT JUST DOESN'T RUN THE DRY HARDLY AT ALL. I HAVE SET MY PROP THE THE MOST AGGRESSIVE PITCH I CAN . I THOUGHT OF UPGRADING THE MOTOR TO THE 520 CONVERSION BUT KINDA PRICEY RIGHT NOW . ANY IDEAS WOULD GREATLY BE APPRECIATED.
 
i had the same thing
what helped me was LESS pitch get the engine to run 3000rpm wide open
she'll move then mine cruses at 2250 and goes great at 2850=35mph with 4 people in the boat

our set up is almost identical
 
make sure your prop is getting to its target RPM, mine wouldnt run dry either-then i found out my prop needed to go 600 rpms faster.......we shall soon see, good luck and keep us posted- do you have a tach on your rig???
 
Have someone else drive your boat on dry ground and you stand back and watch the boat look at the entire boat. t
watch how much the rear of the boat lifts.
The reason i suggest this is i really did not know what me boat was doing untill i had someone ekse drive. My motor was 1/4 buble low with the hull level at the first setting but after i watched the boay i found the rear of the boat lifting 3" and pushing down on the nose and i did not run dry very well. raised the motor and ran dry better.
hope this will help.
 
Good advice here from these guys. The minute I read your post the first two things that popped in my head was jackin the motor and takin out pitch. Big pitch will make her fly, but low pitch will give her grunt.

Felber
 
You just need to get your pitch right , i just recently switched to a 72 in. Q- series and i dont have a tach so Bobby set it by ear at Diamondback and he knows his props he said it was right at 3 grand and i took it to Loughmans and it run dry like a beast !! but i couldnt leave well enough alone and took a protracter and pitched it 2 more degress figuring it would put me right at 2800 and get max push , wrong !! It ended up being like 4 degrees and it was a dog on the dry and i lost my top end.. i just got it pitched back to 3000rpm and took it to Gardners saturday with me and another feller on the boat and went any where in Gardners Marsh we wanted, its back to running dry great again.. It dont take but a 1/4 turn to make a Big diffrence in performance ive been learnin that the hard way...
 
great post from our fellow airboaters here. not from a point of view but from actual experiences.

the thing is to match the prop with the engine. when you are already set at the engine rated power at the rated rpm then you you're ready to go. in some cases, like aircraft engine powered airboats, repitching the prop (less pitch) above the rated rpm will result to more thrust, say 100 to 200 rpm more. this is because a/c engines have a rating called take-off power which is greater to a certain amount than the rated power.

on the other hand, we pitch our props more until a point we arrive at the rated rpm, usually 200 to 400 rpm less than the max rpm at static depending on the set-up. pitching more beyond this point will hold the engine down to a lesser rpm, resulting to less thrust. you loose thrust in two ways. one is that you loose thrust simply because you held your engine to less than rated rpm where it produces less power. the other is that you loose thust by simply adding pitch, more pitch less thrust. you can only gain additional thrust from more pitch if you feed your prop more power.

generally, fine pitched props have greater thrust than coarse (aggressive pitch) pitched props. the trade off is that fine pitch is better in push and coarse pitch is better in speed.

technically, to match the engine, get a prop wide enough just to get a fair amount of pitch. this is way better than a narrow prop with very high pitch. btw, consider factors such as redrive ratio, tip speed, etc.

examples:

- say 72" wide prop with 36 inch of pitch spinning at 2300 rpm is just about to loose bite at 55 to 60 mph.

- another 72" prop with 48 inch of pitch, on the same engine, much narrower so as to get the same 2300 rpm is just about to loose bite at 70 to 80 mph.

.... the second prop may be able to bite up to 70 mph BUT can your engine provide enough power to get that speed? you also suffer low end performance. the first prop is ideal because it is pitched just enough and perhaps, 55 mph is already too high for you unless your up for speed and you have a monster engine to provide you the power to do so.
 
I wouldn't do anything major until you re-did the slick bottom. IMO..those coatings are good for about a year in the best circumstances...after that, they loose their "slick" and start having the opposite effect. Slick bottom applications are best left to the experienced. Once your boat is "slick" and the bottom is flat, and the engine is pitched correctly; then you can start making the slight corrections that bring top performance.
 
Dose poly work better say uhmw than paint on types. Just wondering my friend has a problem in sand my boat use to but now it doesn't with poly? or is just wear on the paint on slickum.
 
Steel-Flex, Slick Bottom & etc. are ok for short periods or if ya don't operate very much. I like polymer cause of its replacability, not that ya can't repain Slick Bottom on. There is some new stuff out I just heard of in another thread thats worth a look. Good tech info on it too.

check this: http://www.southernairboat.com/phpBB2/viewtopic.php?t=8691

This could be the next Polymer.

Scotty :idea:
 
just left the shop in clearwater that makes the poly, pretty cool process, the big sheets for our boats actually come from overseas, the new gatorguard is on its way,same poly,just with a lubricant in it, maybe it will keep it cooler over long runs. Super nice people over there, give them a call with any questions, 727-456-2890
 
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