How to reduce slip on this setup?

[ipman]

Hi

I just got me a (swedish) cabin boat (because I'm a swede living in Sweden) that is 21 feet, about 3600 lbs with normal load. Powered by a Mercruiser 3.0 (135HP) with an Alpha Gen II (2.0:1 gear ratio), 4400-4800 rpm WOT.

Boat was delivered with a 14 1/2x19P Black Max, 3-blade alu, which it didn't pull through to more than 4100 rpms. Switched to a 15x17P Black Max 3-blade alu and gets about 4500-4700 WOT and 31 Mph, which equals about 17-18% slip. Cruising at 3900 rpm gives me 24 Mph, which equals 24% slip!

So I figured I try out the 4-blade Propulse that came along with the boat as a spare, to see if I could improve cruising rpms (and slip) for better economy. I don't really care about some loss at top end, it's a family cruiser. It turned out that the Propulse needed to be set down to 15P for the engine to rev about 4400 rpm, and then I had a speed of about 25-26 Mph WOT. Nothing to use in other words....

Now is the question if anybody experienced can suggest what to try next to lower slip at cruise and gain some rpm for better economy?

Should I try a 4-blade that's not cupped but with some rake for better bowlift (lesser wet surface) and if so what pitch/rake? The boat runs ok with a pretty fast plane, but it does run a bit low on the bow, even trimmed out at max. (It starts to move up and down at WOT when trimmed out max, needs to be trimmed in a notch or two to make it stop)

Suggestions are welcome, this is fun even though I'm not planning to go propoholic on this vessel...

/ipman

 

[steelespike]

Re: How to reduce slip on this setup?

I think you may have a couple of things going on.By the seat of my pants I would say about a 19" prop would be normal for the most speed and would reach at least
the 4,400 at speeds about 40.A regular speedo can be off up to 4 to 5 mph.
A gps is best for speed. What was the speed with the 19?A good load might pull it down below the rpm rating.What is boat weight and typical load.Aluminum or fiberglass?Does the boat seem to respond to trimming up?
Your motor may be a little tired or in need of a tuneup.Check that the throttle plates open all the say.Bottom clean and smooth.Timing right and advancing as it should.Slip numbers are easily affected by the input and other than an indication something is up I wouldn't put to much into the typical slip calculation.

 

[ipman]

Re: How to reduce slip on this setup?

Just got back from the boat, I took me a serious thought about the situation last night and came to the conclusion that the engine is not delivering what it should!

Well, started from scratch and finally discovered that some moron set the timing to 1* BTDC without putting EST in service mode That means that the timing is 8* off to the low side, no wonder I couldn't get things to add up...

Now it runs like a new engine and my prop calculations are back in known territory. Overrevs 200 rpm with 17" so I might give a 19" a test, even tested the 4-blade Propulse at 17" and slip decreased, gained a knot in cruise and lost 3 at top, lowered WOT by 400 rpm....

 

[steelespike]

Re: How to reduce slip on this setup?

There you go. Its allways nice to see a success.
If your 200 over a 19 could get you in a nice comfortable spot.
You may pick up a little speed but hole shot may be a little slower.