Should I have prop tweaked or buy used one with more cup/ pitch?

Hi folks,
As most of you know I have sea trialed my new (home built) aluminum 25ft double eagle power boat a bunch of times now and am generally happy with results.
But, I would like to know if things could still be improved or especially made more efficient at cruise speeds of say 18 to 20kts.

With the Cummins 4bt 150hp engine, velvet drive 1:1 gearbox, 3 blade 15 x 14 prop I am getting the following numbers.

I cleaned and tightened tach pickup on bellhousing as it was going wonky at around 2200 rpms -
in neutral / WOT - I am getting about 3300rpms
High idle for the engine is supposed to be 3100rpms so tach may be off a bit, or it revs about high idel rating...but idle is 800 so that seems correct.

In gear, I get:
4.5 kts at idle 800 rpms
14.5 kts at 2200 which is where it hits peak torque
About 17 kts at 2500rpms which is max rated rpm for continuous use
About 19 or 20 kts at around 2800 rpms which is max rated rpm intermittent use.
23 kts full out which is around 3200 or 3300 rpms.

Looking for input but a local marine diesel mechanic said they usually load them to hit about 2700 rpms in gear full out.

current 15 x 14 used prop was bought cheap on Ebay - with pitting, so I could try boat out and get an idea if more pitch was needed, etc.
i could perhaps go to a 16' prop, but have limited clearance under the prop (skeg/ lower rudder support) so prefer to stay with a 15" prop.
I doubt if current prop has any cupping- looks flat to my novice eye....

A fellow builder ran some numbers for me and this is his thoughts:

If you turning 3200 with a 15x14, then theoretical speed is 42mph, given a modest slip of 9% that yields 38mph.
If you turning 3200 and getting 23mph, then your prop is acting like a 15x9.
If you turning 3200 with a 15x14 getting 23mph, then your slip is 54%.
I think this prop is grossly cavitating, or tach is off, or prop marked wrong"

So, he thinks I have excessive slip..

As you can see, I don't know what I don't know..yet....
but, what I do think is that I need to load the engine more, so that I only get about 2800rpms WOT, as opposed to getting about 3200 or 3300rpms now....and I thought about adding pitch and cupping to current prop, or buying a used one with closer specs, ie. something like a 15 x 17 with cupping....

This is a full keel powerboat... so I am not expecting to make it super fast... My goal is to cruise at about 2200- 2500rpms at speeds that now make me push the engine to 2800- 3200 or so, if possible..

If we can arrive on what changes I should make, I can then decide if it's worth paying to:
- modify my old, cheap pitted prop
- buy another cheap prop with closer numbers
- buy a new prop -

My current prop seems to work well / don't notice vibrations other than a vibration just above idle that I assumed was the 4 cyl diesel. It smooths about above that small rpm range and is smooth all the way to WOT of about 3200rpms.

See pics below. I got the current prop cheap so it had that pitting when I bought it. Could that pitting cause above vibration in limited low rpm range?

With current props pitting, I am hesitant to spend money on cupping and repitching. But, best deals on new ones are about 500 bucks US and with our Canadian doller at about 75 cents US, it would be about $750 to get it home with taxes.
That could get me shot! My wife has seen enough hours and cash going into the boat.

Thoughts?
here's a link to the boat running and tach readings in neutral to find high idle.. and some pics

https://www.youtube.com/watch?v=BN9DheHXZbU#t=41.223437

https://www.youtube.com/watch?v=vS2q5x1X488

 

Sorry it's so longwinded - but I would really appreciate of someone could advise if adding cupping/ pitch or buying another prop is the best approach, please!

 

quick update after having prop reworked.

old numbers:
4.5 kts at idle 800 rpms
14.5 kts at 2200 which is where it hits peak torque
About 17 kts at 2500rpms which is max rated rpm for continuous use
About 19 or 20 kts at around 2800 rpms which is max rated rpm intermittent use.
23 kts full out which is I think around 3100 or 3200 rpms - have not been out again since cleaning sensor.


new numbers after adding cupping and 2`of pitch to prop.
5.2 kts at idle 800 rpms
17 kts at 2200 which is where it hits peak torque
About 20 kts at 2500rpms which is max rated rpm for continuous use
max out at About 21 kts at around 2600 rpms which is 200 rpms less than max intermittent use rating due to load on engine.

hindsight is 20-20 - i am happy with results as I reach a cruise speed at a lower, more efficient rpm range- however, if I had it do to over again - I should only added the cupping, or perhaps cupping and only 1 inch of pitch, as I am now slightly overpropped, where previously I was underpropped.
One thing I did not consider, but now I assume would have made a difference- when going by rules of thumb - like 200 rpms for every inch of pitch - most guys run 1.5:1 or 2:1 gears where I have a 1:1 gear - I assume than my pitch change would therefore have more impact as it will turn more times per minute....but that`s an uneducated guess.

 

Nice job. Will your boat be getting heavier or is it totally complete? The over-propping seems OK at its current weight but might be bad if you add more fitout.

 

I suggest getting a photo tach and try to obtain the complete list of specs for your engine. As mentioned above, is your boat complete? Full fuel, full water, all gear, people etc. Once your at full weight do four runs north south east and west unless it's dead slack tide and do the math taking the average. If your engine is rated for 2800 wot you really want to see at least 2850. Also check the exhaust temp at the turbo, and compare to specs. Don't know if you just put a beater engine you had laying aroud but if you want it to last under propped is the way to go. After that your speed is what it is.

 

Hi Folks - thanks for the interest and the responses!

Season is nearing an end here. I'll enjoy using the boat a few more times, in the next few weeks, as time and weather allows - and then pull for the winter.
I am not done, and will add some weight, in (hopefully) adding a small walkaround or walkthrough cabin this winter, likely out of 1/8" aluminum.
Hope to have time for below changes as well, but most don't add weight:
- using current 1/2" osb floors as templates and welding in 3/16" aluminum floors ( have spare laying around from hull build and weights are near equivalent)
- build new engine box from 1/8" aluminum, which should be lighter than current borrowed, heavy grp box with wood added to make necessary dimensions.

- engine is old and has 4000 hours but I do plan on keeping /using it - not trying to kill it prematurely.... I think as long as I can hit a decent rpm, say 2600rpms- it would not kill it in the sub 100 hours per year I put on it...sub 50 hours really.... if mods bring rpms lower than that, I will likely repitch.

 

The Auto folks have EGT gauges that are about $125.

There is no way to know if the engine is fully loaded with out knowing the EGT's.

When you know how hard your engine is working at your desired cruise RPM, you can decide if you can make it work harder.