prop venting

ssiredfish · #1 07-31-2011, 02:29 PM

So sometimes when I get upset I sit down on the dock and discuss life's problems with my prop and feel a heck of a lot better. But then when I take her out and when I cut sharp turns, she starts venting. It typically only rises about 100 or 200rpms at most but yesterday she kicked in the turn(similar to slipping) and you could feel the prop re-bite. I told the installer and I brought the boat back and we measured everything in the back and determined that the placement on the bracket was correct (even had a second party confirm placement). I don't mind the the subtle rise in RPM's but she can't be kickin like that.

Anyone have any ideas or similar situations????? Its a Yammy F150 with a 14 1/2 x 17SS.

Shrek · #2 07-31-2011, 05:59 PM

Time for a four blade prop.

reelapeelin · #3 07-31-2011, 10:44 PM

Can we assume the sharp turns are with the motor trimmed all the way down?...

ssiredfish · #4 08-01-2011, 07:07 AM

Ive tried the turns both with the engine level and trimmed all the way down. Its not as bad with the engine trimmed all the way down but it still does it. I meant to correct my post the other day but the comp. was slow. The sublte rise in rpm's will come on regular turns(turning 90deg over 100/150yrds) not necessarily tight turns. But the kicking came in on tight turns.

What is the ideology behind a 4-blade keeping this from happening? I've never had one.........

Shrek · #5 08-01-2011, 10:34 AM

Think of a prop as you would a machine screw. Course thread holds well, but fine thread holds better, more surface area to grab onto. More surface area = better bite. The harder you turn, the more slip angle increases. This can be mitigated by trimming the motor down or getting the prop as deep as you can with a jack plate (i.e getting the prop into clean water). However, the nature of prop bite is linear, and as soon as you begin asking it to side load (i.e. turn), slip angle increases and the prop becomes less efficient. Adding another blade gives the prop 20% more surface area to "grab onto" and thus masks some of the inherent inefficiency of the turn.

The upshot is better hole shot and quicker onto plane along with more stern lift and likely the ability to stay on plane at lower RPMs and speeds (read greater midrange performance and economy). The theoretical trade-off is less top end due to drag, however, prop design has come a long way in the last 20 or 30 years and the actual compromise will likely be undetectable for you considering hull shape and power available and speeds likely to be achieved.

ssiredfish · #6 08-01-2011, 12:22 PM

Thanks Shrek.......

You know it really wouldn't even be a trade-off. I can count on one hand how many times I can run WOT, and still don't even when conditions allow.

This was probably the result of the F150 adding weight because she didn't do it when I had the 2-stroke on there.

She is Heiney-heavy and switching would probably be the best bet since we ruled height out.

Now, what to do with that shiny prop I have now.........

Shrek · #7 08-01-2011, 03:16 PM

Sounds like you get a shiny spare

RidgeRunner · #8 08-01-2011, 04:19 PM

I have had a few boats over the years. I have a bracketed V-20 now. The motor height is one of those trial and error deals that can cause ventilation issues if too high. The best advice I can give is have someone at the wheel while the boat is on plane while you climb to the back of the boat and see where the rushing water is with respect to the anti-cavitation plate. Right now my motor is sky high for top end. I can clearly see water rushing across the lower unit 2 inches below the cav plate. with neutral trim. Typically you want the cavplate riding at the surface of the water. Even at that height I do not have any buffeting nor is the prop loosing bite on the turns. If I stick the bow into the face of a wave it is possible to hear a little prop slippage. Mine is maxed out high IMO. I tried a 17 x14.33 a 19x13.75 a 21x 14.5 and a 23x14.25 all three blade and none of them lost bite in the turns.
I agree that a 4 blade prop has more bite than a three blade. From what I read the 4 blade props typically reduce RPM when compared with the same pitch 3 blade. Depending on your setup you may need to drop the pitch down going to the 4 blade. To avoid having two super shiny wall hangers, try one before you buy it. Ken at Prop Gods is a wealth of info on props and can facilitate just that. Ferm helped me to find him. BTW- I love the way you post, you make me smile. Glad to know I am not the only one talking to the boat.

And Shrek, that is one of the best explainations I have ever read.

ssiredfish · #9 08-02-2011, 07:26 AM

Thanks, I'll take a look at it next time I'm runnin around and see where the water is. I think Ken was one of the guys I gave my heights,etc too to look at for me, very nice guy.

RR- thanks!! I was beginning to think no one caught the "venting" pun