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Prop Performance Question

nebraskaairboater

Well-known member
I am running a 74" Whirl Wind Stump Puller. I bolted an alunimum plate across my transom to help stop swamping when trailering. Now my prop goes below the top of that plate 6-8 inches (or more) Will this affect my performance? I re-pitched the prop to turn 2600 rpm tons more push lower cruise rpm but in shallow (1 inch or less) it wants to push mud if I am not on plane. It didn't seem to do this as much before re-pitching. Wondering if it is because the prop is pushing more on the top of the swing rather than all the way around. Below is a link to a picture as I haven't figured out how to post a picture yet (and get it to work)

IMGA0353.JPG
 
From your analysis, you have it about right. As a general rule of thumb I used to set the props to cut below the transom no more than the length of a soda can (beer can if ya choose).

Its good to have them cut slightly below the edge so they can bail some water as it comes in if it does. We set them forward of the transom the same amount.

This doesn't sound very scientific but we found it worked for our Buick DD boats just fine.

Anytime you block the prop you of course lose push right where it is blocked off. You keep the same push where it is not blocked off.

You can experiment by putting a little bit of down tilt at the prop end of your motor or taking some out. Main thing is you dont want to go the other direction with up-tilt in it. We set ours at 1/4 bubble with a level and never had one to push it's nose in the mud at idle speeds. Quite a few folks set theirs to be dead level. I'd do some experimenting in the motor mounts and see if it helps you any.

Hope this helps some.

Scotty
 
Mine is set dead level. I was thinking about taking a plasma torch and cutting the aluminum in a curve to allow more air and still protect on the corners. My problem came when I would run up on my trailer rail and the corner would dip and I was done. Happend 3 times and that was enough. That is why I put that aluminum in it. I just figured if it didn't affect the push I wouldn't go thru the trouble of TRYING to make a nice arch cut by hand. It pushed on the nose pretty hard after I built the boat but I used the bottom jacks to bring the nose back up. It runs perfectly on plane so I don't want to mess it up too much but I will try some small (fixable) stuff and see what happens. Wouldn't I tilt the back of the engine up in the back to bring the nose up? Or is it opposite of what seems logical? Thanks for the advice.
 
Yes but up, if its way down, could mean less down. Problem is, if ya go up above level you may be getting into dangerous territory at planing speeds. Ya sure dont want the bow to get squirrly in winds or under heavy power.

You can try going up above level but be ready for things to get hard to control. These things scare me cause its trial and error and developement and experimentation can get out of had real fast. Just be real careful.

Sice it is running fine on a plane now, maybe the solution is in redistributing the weight not the thrust. Since you have bottom jacks, try getting the weight so it runs right at idle then reset the jacks so it planes like you want it to. Low end is much more sensitive to weight shifts than planing speeds are cause at plaining speeds you have jacks to make it right. The jacks will have little if any effect on idle handling. So you can do a balancing act between weight redistribution and bottom shape.

Scotty
 
I went through the same problem and found you want as much as possible even air flow from top to bottom. if you have air flow blockage on the bottom that will cause your boat to push on it's nose and loose push also. Keep an even air flow will keep your boat even too'

Geoff
 
There is some pretty good discussion in the Pushing on its nose thread. If nothing else you may get an idea of how to proceed with this vexing problem.

Keep us posted with your progress, each one of us can learn something new at any time.

Scotty
 
It is still pushin nose first at idle in shallow water. Not pushing nose at all on plane. I think I'll go ahead next time my boat is on the trailer and cut the alum plate to allow more air. It does make sense for an even flow of air top and bottom. That was my main question. I know the tip of the prop doesn't create much push if any but I am beond just the tip being blocked. I went riding last night again and was going to measure just how much but forgot.

Off topic but my 2 1/2 year old son caught his first fish last night. Wasn't a keeper but he was sure excited about it.... kept hearing "daddy, I want to catch another fish!" It was a great time.
 
Dan, he'll never forget that fish ..... mine was a Bluegill about 4" long that I caught off of a dock. I felt like I had just won the Bass Masters :)
 
From looking at the pic you posted the back (transom) of your boat looks just like mine (12 ft classic) I had the exact opposite problem, it was rideing with the nose high and the back corners DANGEOUSLY low in the water . Didn't make sence to me at first but by lowering the back (prop end ) of the motor it helped to bring the nose down and the back up in the water( along with bending the hull lip on the sides ever so slightly ). . Still have a problem with it but after some trial and error I think I got it the best I can get it for now. DON'T DO WHAT I DID and jump out into deep water after adjusting sumthin (sunk it right in front of a tour boat loaded with tourist just going out) I know my problem is that I don't have enough hull but the reason I'm posting this is that a friend has the same hull and motor set up and has the opposite problem of me ( same as you ) pushing the nose. Every boat is unique in its own set up. I tried jacking it - pitching the motor and bending the back lip of the hull. Found that I needed a combination of all three got me to get the boat to perform the best I can get it. Lots of trial and error but now I almost sink it only about 3 or 4 times every time I go out and not ( almost sinking it ) with every turn. Just my 2 cents on this one.
SAFARI RICK
 
Interesting topic and some good ideas but here goes my two cents worth.
The backsplash guard will work if you put it on hinges and springs so that when you let off the gas it closes and when you give it gas it opens. thus allowing the air flow but stopping the back wash water flow. next the tip of the prop below the transom reverses the air flow and creates a lot of that unwanted sound that we all hear about now in a negative fashion. as for the bailing water with the prop I have found that two bilge pumps are a lot cheaper than a new prop. Now for the nose down going slow. If you are like me then check how many emergency supplies you have in the bow storage box. Reconsider what you really need to carry up their. You are not going to sink the boat bow firs going slow but on plane is another story --- then the nose can be change by bending the bottom 1" that sticks out from the back of the transom --up to bring thee bow up or down to bring the bow down. no more than 1/16" at a time. As for the water over the transom when loading on your trailer. If the ramp is steep then back the trailer further into the water so when the bow is at the tie down it can still float but pull out slowly so you can line the boat up with the trailer.
 
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