Pretty overpropped.

[Bobbywolf]

I recently repowered a 18ft center console aluminum boat (brand and any model numbers have been painted over).

Installed now is a used, but in great shape 2004 Johnson (Suzuki) 70hp 4stroke. It came with a 13x19 3 bladed prop. First run, the motor runs amazing. Using a BRP engine check tach, it shows right about 4000rpm at best trim and speed. I should be in the 5200-5800 range as far as I can find.

Does it seem reasonable to need to drop to a 13 pitch? A 14x13 prop is available, but I want to ensure I am not shooting myself in the foot. I have no idea what size boat this used outboard came from, but it would need to be pretty small for that aggressive of a prop right?

Thanks for any insight.

 

[racerone]

A 15" pitch might be the correct choice.----But step #1 is to make sure motor is in good condition.---And make sure boat is not waterlogged.-----And I know you said it " runs amazing "

 

[Bobbywolf]

A 15" pitch might be the correct choice.----But step #1 is to make sure motor is in good condition.---And make sure boat is not waterlogged.-----And I know you said it " runs amazing "

Thank you. Yea, I guess that statement should be clarified by stating the old OB was a worn out, and terrible running 1980's Johnson 88hp 2-stroke. So of course a good running 4-stroke would put it to shame.

I guess what I should check is that the motor is indeed getting to WOT. I know on a carb you just need the throttle plate flat. I need to investigate how to know this is getting to WOT.

 

[racerone]

Myself I would stick with a 1980's model 88 / 90 HP Johnson / Evinrude.-----A wonderful , easy to maintain motor.----Of course my opinion does not matter.----I rebuild those V-4 OMC with no need to look at a manual.-----

 

[Bobbywolf]

All my experience is with 2-strokes. I have rebuilt several outboards in the past, both Johnson and Mercury. However, this one was more than I wanted to tackle. Rings let go on 2 cylinders, and every other bolt was seized to the point of breaking them off. The trim motor was on its last legs, and the lower gearbox was starting to make a racket. She's destined to be a parts mule at best.

Plus I had to see what this 4-stroke hype is all about, lol.

 

[Texasmark]

I assume your gear ratio is 2.33 or thereabout. Iboats does list a 14x13 for that engine: http://boatpropellers.iboats.com/Su...3.00&chart=63&engine_id=362&cart_id=316434620

You didn't give your speed at 4000 RPM. Hard to answer without that piece of information. Having run a mid 1980s 70 hp 2 stroke Rude on a 15' very light, fiberglass, almost flat bottom, King Fisher fishing boat, I was running a 13+ x 17P and that was with me alone in the boat.....how do I remember that...another story...with an expensive ending. I had no tach. All I had was how the boat-engine combination felt and it felt good and I have been boating since owning my first boat back about 1957.

So, taking your current numbers, with no speed provided let's play with some numbers. Go fast dot com prop slip calculator can give us some ideas.
First estimate is with a 10% prop slip and I come up with 34 max/28 actual MPH. 28 MPH is a good clip for that much boat and a 70.
Changing your RPM to 5500 which is where you probably should be and dropping to your 13P mentioned, keeping the same slip gets you 29/26. Upping to a 15P gets you to 34/31.

Taking your current engines ability to push that boat, severely over loaded (prop pitch), and theoretically driving that boat to 34/28 (or whatever speed), I see no reason why the 13 1/2 x 15 3 blade alum. that iboats sells shouldn't do the job for you and probably will have no problem getting you up to 30....or whatever speed, slightly faster than current condition. Nothing wrong with 30 MPH with 70 hp running 5500 RPM on an 18' alum. CC. at 25, much less 30.

 

[Bobbywolf]

Thanks for that! Top speed was never my goal here, just to get to where the boat/motor combo is happiest.

Ill be taking it out later this week, and I'll get my gps speed as well.

 

[Texasmark]

Thanks for that! Top speed was never my goal here, just to get to where the boat/motor combo is happiest.

Ill be taking it out later this week, and I'll get my gps speed as well.

Top speed and RPMs are measurements of your boat's performance. Nothing says you have to dwell there once you get it setup.

 

[Bobbywolf]

Ok, its been awhile, but I just had it out again. 25mph is the best she'll muster. With the throttle lever full forward, I disconnected the cable at the motor, and found that there was no more travel for the throttle linkage, so it is indeed maxed out, so I can rule out rigging as a limiting factor. I believe the tach to be correct. The motor just doesn't seem to sing like it should.

The boat certainly leaks, and requires the bilge pump several times in a few hour outing, so I can imagine it is heavier than it should be.

So I am going to order a prop. I'd like to do the big move to a 13 pitch, but I am unsure if the 400rpm per 2" change applies with such a large change. I'll probably take the safer option and go with a 15 pitch and see where that gets me. These turning point props aren't too pricey once you have the hub, which I do.

 

[Bobbywolf]

Who knew there is a time limit for edits...

Anyway, I found a picture of the boat before it was painted. It is a Fisher sv-18cc, rated for a 115hp originally from what I can google.