Propellers

[Serenity]

I have a 98 28TE repowered in 2009 with a 315 Yanmar. The boat came with a 3 blade prop. However, some tests I have read, the Yanmar is turning a 4 blade. Is there any significant difference between the two? At 3800, I run at 17-18 knots.

[Cape Codder]

I have no idea if your tachometer is accurate, but our 28TE w/ 315 and an adjusted/accurate tach runs 23+knots at 3800.

[RobS]

The 4 blade is what was used on the Yanmar powered 28TE's. A 19" dia with pitch anywhere from 18 to 19". They were originally going with 19x19 but then began to realize the pitch was to heavy resulting in overloading so then 19x18 became the norm. She needs a med to heavy cup to combat the turbulence of the skeg. My TE had a 19x19 w/heavy cup when I got her and she was a bit overloaded. Had the cup changed to a light cup and got the rpms closer to where they belonged but she did not perform as well or smooth without the cup. So I spent the $ and had the prop repitched down to an 18 which allowed me to put the heavy cup back in without overloading her.

The 4 blade will be smoother than the 3 blade. Plus it is more efficient due to better traction in the form of more blade surface contacting the water, like wider tires on a car. For the same reason, a smaller dia is needed.

What is your WOT rpm and speed?
Have you checked the accuracy of your each with a photo-tach?
What 3 blade prop do you have?

[Serenity]

When I picked up the boat in Southold, I could get the engine up to "5200" flat out. It didn't make sense, We settled on a cruisng speed of 3800 which felt OK. The boat has a Aetna Tach and I am dealing with the factory to determine if the tach is out of whack. The question comes up, how many teeth are on the flywheel? Once that is determined, I can adjust the tach. The prior owner had no issues with the tach. In regard to the wheel, I was curious if there was any benefit to either a three or four blade. Right now I am turning a 18X17L Dymex prop, or so that the paperwork that I have states.

[RobS]

5200! Do you have a Yanmar or a Yamaha! The tach setting is out. Should be a 117 tooth flywheel, if memory serves.

[Serenity]

I agree. I believe the tach is off 20%. I appreciate the info on the teeth. I will contact Aetna and see if that makes sense. Enjoy Bunny Day.

[Serenity]

The Aetna Tach is defective. Sometimes it reads correctly and the next day it does not. The factory has told me that there may be a software issue with the tach., Bottom line I ordered a new tach

[johnmurray]

While my boat is slightly different (A-30FC) it has the Yanmar 315 engine and is equipped with a 19X19 four-bladed prop. I think my tachometer is accurate and at WOT it registers 4250 RPMs and that delivers a top speed of 28 MPH or 24.6 knots. Another A-30FC owner has done some calculations for me and advises that with my 2.04 transmission gear ratio I have a theoretical top speed of 37.47 MPH. Since my actual top speed is 28 MPH that means my prop efficiency is 75% which apparently is very good, and it is unlikely that it would be further improved by any propeller changes.
I wonder if others have similar data for their boats.

[RobS]

.... I think my tachometer is accurate and at WOT it registers 4250 RPMs....

Impressive performance numbers for sure. Just watch that WOT as she is not supposed to turn over 4200

[Cape Codder]

I also have the Yanmar 315 6LP w/ the 19x19 prop.
I've also had my Beede tach "corrected" by the local Yanmar dealer on Cape Cod.

I have never seen top rpm's anywhere near that high. Max rpm according to Mack Boring Tech Support (Chicago) was said to be 3950. I can reach approx 3875-3900, and run 24+ kts.
Top speed I've seen is 26 knots, wide open.

[RobS]

Propped for WOT is as Bob states, 3950. But that's different than 4200 high idle. 3950 with load, 4200 unloaded.

[johnmurray]

Maybe my tach is off. I know the top speed at WOT is correct as measured by GPS in two directions and averaged in flat water. I will ask my mechanic to check the tach.

[Serenity]

I installed a new Tach from Aetna Engineering. The new Tach is reading the engine correctly. At WOT I will hit 3850. At idle I read 745, which is correct. At 3800 I am attaining speeds of 25-27 knts. The tach that came with the boat has been sent back to Aetna under warranty for a little R&R. The warranty for the Aetna tach was 2 years. The installation firm that did the repower was totally useless. "Ain't my problem mon" kind of attitude. The propeller issue is not an issue right now.

[RobS]

Wow, that's great. Are you sure your speed is set to knots and not MPH? Seems a bit fast..

[Serenity]

Wow, that's great. Are you sure your speed is set to knots and not MPH? Seems a bit fast..

Perfect sea conditions, no wind, no rain, outgoing tide and running at 7 pm. Boat runs great. Received the Aetna tach back from the factory and I installed a joy stick to replace the 2 buttons on the bow thruster. Couple of more items to r&r and we will be set for Coecles.