Calulated propeller efficiency

[7228sedan]

Luhrs was a longtime, large production builder. Presumably they knew how to select the props for their boats. Is the boat which is the subject of this thread in the original configuration? Does anyone know the original propeller size. If so did all the Luhrs 280 have high propeller slip?

The boat was originally powered by a 225 HP Chrysler small block. I am not sure of the ratio of the original Paragon transmission. I got the boat with this set up, and this year repowered with a fresh 454. Now that the motor is up to the challenge, I'd like to optimize the installation as best I can. With the hull shape, beam, displacement, and relatively small prop I don't see how there would not be high slip . That being said, I think there must be some improvements to be had without reinventing the wheel.

 

[7228sedan]

You are probably not going to like this answer because it is kind of pricey, but I don't think you are going to fix this problem without throwing the strut away and fitting a new one. It is quite hopeless for the arrangement you show. It needs to have about four times the meat on it that it has, and a longer cutless bearing would help (say, twice as long). Now you can go back and look at that slightly larger prop, with 4 blades, and a more sensible pitch.

Funny you mention this... I think the previous owner who originally installed the first 454 had modified the current strut with the wedge that is installed between the hull and mounting plate. I assumed that was done to fit the current larger prop over what was there previously. The solution you propose would likely be the ideal solution. It would also likely require a new transmission ratio, longer shaft etc. not sure I love her that much... Honestly, I am looking to get the best bang for the buck with the current configuration to list her next season. Time to move on to something easier to move my disabled son on. He's getting too big for me to continue lifting him up to the flybridge.

 

[7228sedan]

Wanted to post an update after putting some hours on her this year.
While I was focused on a fresh paint job for the hull, I didn't get to implement any of the hull enhancements yet.
I want to run the findings thus far by the forum to see if any of the previous recommendations no longer apply.

I had the prop worked to include a fairly decent cup. Upon review from the prop shop, the diameter was actually 16 while the prop was originally a 17.
So now, I am running a 16X16 cupped 3 blade.

Right off the bat, there appears to be a significant increase in efficiency.

I don't have the full range of speeds per RPM, however at 3000 I picked up 2 knots (13.4 to 15.4) Top end RPMs are now limited to 4250 (expected based on the cup).
However at that RPM I am now seeing 24 knots. 25 knots is not out of the question based on conditions.

One other item of note, the boat no longer needs the trim tabs as much to maintain the best speed. As I have experienced with other boats of my past, I now must bring the tabs in to hit the best speed at a given RPM conditions permitting. Prior to the prop work, the boat needed full trim tab at anything under 3400 RPM to get the most speed at that RPM. It would begin to drop off plan and slow significantly if the tabs were not extended fully.

Into the wind slightly at 3000 RPM with tabs at mid range resulted in the 2 knot increase .

This all leads me to the effective lift of the hull and prop combination.
The lack of trim tab needed due to the prop work leads me to assume the increased efficiency. As the prop is providing more thrust at a given RPM, the hull is providing the appropriate lift needed at the speed to negate the need for the tabs?

All this and my fuel burn seems to not have increased much at all. I was concerned that an increase load at RPM would result in a significant increase in fuel consumption.
So far, I appear to be burning nearly the exact rate as prior years.

Never thought this old girl would hit 24 knots...

 

[75luhrs280]

I know this post is older but just recently picked up the same boat as on here and am very curious myself how to prop it correctly. It has a seized Chrysler 360 in it the previous owner upgraded from the 318. It looks like he also upgraded the prop to a 4 blade and a STAINLESS shaft. I will be dropping a FWC 315hp VORTEC 5.7/350 in it. HOPEFULLY the prop will be ok with the new motor. I'm assuming 4 blade is better anyway..
1975 LUHRS 280 model
Ed at the NJ shore.

 

[7228sedan]

Ed, I'm in Bricktown if you ever want to chat. I sold the boat last year and would be happy help you out with any questions you may have. My email is blueoval66@yahoo.com name is Rob