[August Rush]

Hello,

Since I am on the hard for the winter time to tune the props. The boat is a 1999 Tiara 40 Express with Cat 3208 TA 435 engines.

The current prop set up is 24” 3 Blade (I can not recall the pitch at the moment)., The boat is slightly over propped as I can barley hit the rated RPM. I was planning to take an inch of pinch out of the props.

I have a set of 22x24 4 blade props with medium cup. These were in the boat as spares and have not been used.

Aside from the rpm issue, the boat rides very bow high. This is a common complaint for the 40 Express. in part the engines are far aft, as they are v drives,. 

My question is about the benefits of a 4 blade set up vs 3. Assuming they are pitched correctly what changes in performance (positive or negative) could I expect going from the 3 blade to the 4? I was reading that more blades can create stern lift which might address some of the running attitude concerns. 

I am not concerned with WOT speeds. I am a coastal cruiser...

[jclark003]

Yes as I understand it the main differences are 3 blade props tend to be more efficient with less drag and 4 blade props tend to have less vibration and can create a little stern lift. May be worth bringing both sets to your prop shop. Tell them to tune the 4 blades to give you an added 100 RPM over your 3 blade setup (or whatever you are looking for). There is a lot of weight in the stern to lift so not sure how effective the 4 blades will be in that regard. There are a few 40 express members on this site. Maybe one of them can give you a little factual basis for changing.

[Knot On Call]

I spoke with Tiara and Michigan Wheel on 3 vs 4 blade. 3 blade is theoretically more efficient. 4 blade may be a little smoother. I was debating going from 3 to 4 blade on my 2003 52 Sovran Salon. With a purchase cost of (2) 4 -blade props near $10K, the money could be put to better use (like in my pocket). As long as the prop is giving you 97-98% load at WOT and you hitting the max RPMs, you are props are properly speced. Remember, higher air temps and warmer water will definitely affect performance negatively, so stay conservative in cooler weather sea trials, otherwise you can overload your engines and may find overload alarms going off mid-summer. Older boats definitely get heavier so original specs may not apply to the boat anymore espially if you load it up with fuel, water, refreshments, and people. I had to reduce my original pitch from 40 to 39 to keep within performance ranges. Michigan Wheel tech service is a good source to review your performance. They have work sheets to submit with RPM, engine load, and speed. They will also recommend any changes.
I don't know if props will affect your altitude running characteristics significantly. It could be just the boat. My boat runs higher than I would prefer. I use the trim tabs. Still if place the bow too low I will lose speed. It just likes to run a little high most efficiently. Mike