Two vs three blades

[mr 88]

Currently running a two blade 21" @ 6 grand. If I switch to a 3 blade 21" will I lose rpm's ? and if so how much. 15' tunnel hull with a 150 Tower,total weight incl driver 8-900lbs.

 

[steelespike]

Re: Two vs three blades

You don't see 2 to 3 blades change very often. But adding a blade usually is figured at 100 to 200 rpm. reduction.
Only question is sometimes a 2 blade is tweaked for faster performance. and are said to be best for speed
on a light boat.
If you can tell us your wot rpm and speed with the 2 blade we may be able to tell more about your change.

 

[mr 88]

Re: Two vs three blades

I will report back after I borrow a GPS .Will be putting on a bronze two blade that was cupped for me.Previous data from 2 blade no cup. I am breaking free in the stern with that set up when hitting some small chop. Was suggested I go with 3 blade to minimize that .So I am trying to figure out what my next step will be if it continues to get loose,thinking a 20" 3 blade will keep me in the 6 grand range.Set up new this year and I do not have any 21" 3 blade props lying around to try out.

 

[Bamaman1]

Re: Two vs three blades

It's like you're keeping an antique boat going. I don't think I've seen a bronze 2 blade prop since the early 60's.

 

[mr 88]

Re: Two vs three blades

It's a 1966 Power Cat so I guess that fits your antique boat category and the motor is a Frankenmerc so there is no true year on that.Wrap makes it out to be a 1978 blue band,I did that so it matches the topside paint scheme on boat.

 

[steelespike]

Re: Two vs three blades

"breaking free in the stern" do you mean it becomes unstable?

 

[Texasmark]

Re: Two vs three blades

Back in the 50's Merc only ran 2 blade props. When they changed paint to Phantom Black they started using 3 blades. All the hot dog engines that were racing at that time on 3 pt. hydros were on Merc racing lower units on their Mark 55s and only had 2 blades. Bite better and do better on a light fast boat but more vibration...but on that rig, who cares.

I remember watching them get on plane. They had a spring loaded throttle that you had to squeeze to get the rpms up. The guys would squeeze the throttle, the little 7" bronze 2 bladed props, with probably a 30" pitch, would shoot a rooster tail 30 feet and the engine would scream. The boat would move forward slightly. It took them half a dozen times nursing the throttle up down, up down, to finally get that little sucker hooked up and blasting off, but when they did, bye bye.

Mark

 

[mr 88]

Re: Two vs three blades

"breaking free in the stern" do you mean it becomes unstable?

Starts to "walk" side to side,yes it is unstable,no problem on smooth water,hit some cross wakes at speed and your throttling down real quick. I have been told a few things,single Telflex steering should be changed to dual or hydraulic system.Solid motor mounts.Go from two blade to 3 blade and that will give it enough lift so that the motor is not trimmed too much . The skeg is/may be at such a angle that it is almost useless,I have a 1" spacer on the transom so that adds to the lack of bite in the steering,I think.Thought the easiest thing to do now since it is middle of summer is to try a cupped prop,then a 3 blade unit and see if any of that cures it,if not then steering and mounts would be next.

 

[steelespike]

Re: Two vs three blades

Called chine walk. Yeah can be dangerous.
I'm sure you realize that at that speed a boat can kill in a second.
You probably wear a pfd and your killswitch lanyard.
There is a video on you tube of a bass boat crash crossing a wake.

 

[mr 88]

Re: Two vs three blades

It's more like when you see a motorcycle on some crash video wobbling then going into a barrel roll.Feels like if you did not back out of it you would catch a front sponson that would throw the hull into a roll.When I hear chine walking I think of a hull slapping up and down along with left-right rocking on the chines but still headed straight.Mine starts to swerve quickly in a left right manner sort of S shape under certain conditions.

 

[Texasmark]

Re: Two vs three blades

I bought a Ranger fisherman back in '89 new. Engine was 115 tower and boat had a pad hull similar to their bass boats but deeper V. It had like a 24p Laser as I recall prop on it. First time out, after a very long hole shot, I got up to 55 mph and approaching that speed, the boat started rocking side to side, chine to chine touching the water, continued to steer dead ahead. That is what I call chine walking. I hit the benchmark and cut her back, never to go that fast again.

I don't remember the rpms but it was obvious that the engine was over propped and the boat unstable at the higher speeds. I took it back and got a 21P Laser (I don't think I went down to the 19) and the engine was happy and my top speed was reduced to the upper 40's and all was well.

It was a very uneasy feeling and I didn't like it at all.

Mark