New prop on 21-ft Whaler w/ Merc 225

[Frank O]

Hi all, I recently changed out the prop on the 1994 Mercury Offshore 3.0-Liter 225-H.P. carbureted 2-stroke outboard on my 1995 Boston Whaler Outrage 21. The engine originally came with a 19"-pitch Laser II. I typically run with four scuba divers and equipment (about 1,200 pounds all told). I was finding with this combination I topped out a little over 40 mph at ~4,600 rpm at WOT (whereas the engine is rated for more like 5,800 rpm).

I just changed out the prop for a 15"-pitch Revolution 4 and took it out for an initial test last weekend. There was swell of 4 to 5 feet that was fairly short period, so we couldn't open it up all the way. In a couple of flatter spots we did get it up to 35 mph, though. This was with the usual four divers and gear, plus a newly filled 100-gallon tank. Overall I'd say the speed transitions seemed smoother than with the 19" Laser II, but I need more time in the water under different conditions to understand better how it's different.

I did notice that steering with my manual linkage seemed harder than I remember it being. This made more obvious some play in the wheel and some clicking I heard from under the wheel when changing direction. Can harder steering be a side effect in going to a shorter-pitch, four-blade prop, or is this probably from other causes?

 

[Dhadley]

Re: New prop on 21-ft Whaler w/ Merc 225

That's been known to happen. A 4 blade has more blade area generally speaking. Just another reason 3 and 4 blde props don't usually run at the same X dimension (height). With the 4 blade you should, in theory, be able to raise the motor from the 3 blade height.

I'm pretty sure Walleyehed has been running the Revolutions. Maybe we can get his attention.

 

[Frank O]

Re: New prop on 21-ft Whaler w/ Merc 225

Thanks for the comment. I'll have to research the height issue.

 

[walleyehed]

Re: New prop on 21-ft Whaler w/ Merc 225

No need to research the height issue...you just had a post from the Master of set-ups....Dhadley is your man if you want to research....just ask...he'll dial ya in.
The Rev 4 has a massive amount of blade area..it IS a heavy-load prop, but it was mainly targeting big, heavy, mid to high HP Deep V's.
In one of my discussions with Merc, I mentioned it really hauled a load well, but I couldn't run it as high as I thought I should be able to.
18ft deep-V, 200 rude....they said the only reason That prop would work on my boat was hydrol. steering.....I don't have hydro steering., but I did notice the pull was extreme in both directions depending on speed and where you had it trimmed.
Honestly, the Rev-4 is "really" intended for heavy sterndrives.
I wonder about either a Stiletto Bay Pro or possibly a Turbo O.S. 4-bld as being a better choice.
If you "want" my opinion, this is a mis-matched set-up.

 

[walleyehed]

Re: New prop on 21-ft Whaler w/ Merc 225

I went and done some checking on the selector site......the Rev-4 is not listed as a choice, but I'd stated that above...
The problem I see is for your hull style, I punched in both a 200HP and a 225HP....the reason is the Merc selector is "usually" pretty close if you drop 1 size in engine from what you actually have...problem is...it recommends a 19 as an acceleration prop-no laser listed either-Laser does best on light, fast boats. For Mid-range and best cruise, it lists a 21!!!
For the 225 it says a 23-24 for accel., and a 25 for best over-all.
Now, I know for a fact, the 23-25 is flat-out not gonna work on a 225 on that rig. I also know 25hp will not allow an increase of 4-5" of pitch.
Do you know the weight of this boat?
Do you know the tach is correct? and has it always read the same...4600 at WOT?
I did find a listing for the Rev-4 on an outboard, but it was for Heavy, off-shore Deep-V's with High HP. and a 21ft'r with a 225 still showed a 21" pitch for that..???
If you can list a weight, that will help a great deal....I think we have a problem here somewhere we need to find.