3 Blade to 4 Blade?

[jimmbo]

looking at the video, it looks like the engine is hardly trimmed in, it does look like it is trimmed out. Since the CMC unit is separate from the engines tilting system, which hole is the engines trim/tilt rod in, the one closest to the transom?

 

[gholmesjr]

No Title

I took some pictures this morning on my way out the door.

 

[Sea Rider]

In real world boating pics shot on dry land doesn't say much. When OB is on water needs to be 100% operative and healthy, with spott on transom height, along proper trim & deck weight balance. Is it possible to have someone to pull his head out transom and check what's going on at lower leg when on plane ?

What need to do is match lower leg passing throught middle water flow at plane at a correct height. For that will need to perform some water runs till correct height is achieved. Sitting OB at any of the height holes seen won't say much either.

Happy Boating

 

[Sea Rider]

Independently which prop you select would suggest going for a OB/transom height check out & height optimization :

- If with manual trim, adjust trim to a hole that achieves OB sitting on transom t o 90? while combo floats alone.
- Distribute deck weight evenly or bit forward to compensate OB offset weight as a jack plate out transom is being used.
- If with electric trim, trim as usual for hole shot and move trim to a position that will make combo ride parallel to water level when on plane.
- Have someone check at which lower leg height is water flow exiting middle hull.
- If wanting best combo performance under all water cond, this is the ideal lower leg height you must dial.



- Test on flat calm, no wind water cond..
- If water flow bangs against X ! at wot will experience water splash out transom or intrusion splash over transom.
- Match jack plate height (hole) for water flow to skim right under green line at wot while OB rides at 90?.
Will need to run some water trials before that ideal height is matched.
- Ignore (X2) anticav plate, was not invited to the water party.
- Right after, go for the prop of your preference to achieving running at max wot revs with current load.

Note: Don't waste precious boating fun dialing best OB/transom height while going for the aereation/non aereation idiotic sound method, visually checking what's going on at lower leg will dial a better height adjustment than by cavitatation/aereation sound. The issue with old dinosaurs is that distance from small upper plate to anticav plate is very short to play with compared to new OB's which comes with extended lower plates distances.

Happy Boating

 

[MTScott]

I'm no expert, but I've been researching props myself... given the other chatter taking place, perhaps a new prop isn't what you need, but here is what I've learned about 3 vs. 4 blade props:

When going from a 3 blade to a 4 blade you generally drop 2" in pitch to achieve the same WOT RPM. I'm sure there are exceptions to the rule, but that would be per the prop manufacturer. You will likely lose some top end speed at the same RPM. The trade off is better holeshot performance - across the board when I've read reviews on such a swap the holeshot performance is night & day.

If you are going from stainless to aluminum, or vice-versa, pitch behaves differently as well (aluminum flexes more), so you generally need more pitch in an aluminum prop to equate a stainless one. Again, I'm sure there are exceptions. The prop manufacturer is probably the best source.

Generally speaking, the 4 blade will give you more stern lift, thus helping you plane faster. It seems that the solution to better holeshot is almost always to go to a 4 blade if you're running a 3 blade. The high-five merc prop is something to consider as well. The reviews are pretty positive when discussing holeshot and control, but it is a smaller diameter prop, and has too much slip on heavier boats; so, it doesn't work for me (my boat weighs 4500lbs dry). It might work for you though.

 

[gholmesjr]

Thanks for your thoughts guys, I'm going to get the boat out this week and take a look at the leg while at wot.