posted 03-23-2009 10:13 PM ET (US)        

Today's observations: I used GPS and my tach and performed 7 runs to come up with the following (averaged):

1000 rpm 5.4 mph;
1500 rpm 7.0 mph;
2000 rpm 8.0 mph;
2500 rpm 10 mph;
3000 rpm (nearly planing) 19.5 mph;
3500 rpm (planed at 3300)rpm 30.5 mph;
4000 rpm 30.5 mph;
4500 rpm 34.5 mph;
5000 rpm 38.7 mph;
5300 (highest rpm achieved in 7 runs) ran 41.7 mph (but squirrely and porpoising trimmed out).

Trimmed out, but controlled with light tabs, top speed was 40 mph at 5200 rpm.

Time to plane was about 4 seconds

Questions:
When backing off the throttle (down to 3000 rpm or lower from >4000 rpms) the engine noise reminds me of a truck down-shifting on a steep grade. Is this due to the weight of the prop and its inertia coming off high rpms? Will this hurt my motor? I haven't observed this with my brushed stainless 17P "pre-Vengeance, Vengeance".

According to my manual, the operating range is 5250-5750 for the 150 Opti. From this information am I right in concluding that this prop is too big for my motor/hull or hp/weight? Will operating this motor at 5200 rpms damage it? Will a Vensura spin faster? I need bow lift (stern lift) as my boat seems to be a "tail-dragger".

Finally, during my tests today, my throttle stuck a little coming to a no-wake zone. As I gave it some pressure to go to back off the gas, I jumped over neutral and into reverse. As the gear engaged, it stopped the motor with a shock (and clank). The motor started right back up, but I'm worried that I hosed my gear-case/drivetrain. Has anyone done this before and caused damage to the motor? It was quite a shock/clank when the gear engaged while still moving forward...

posted 03-23-2009 10:56 PM ET (US)            

The revs/mph are very similar to my Merc 150 EFI turning a Mirage Plus 15.5 X 17p prop on a 19 foot fibreglass boat.(nesscraft Magnum)
I get to 5400rpm and my maximum according to the manual is 5650rpm so Im happy with that.

As for hitting reverse while going forwards I doubt whether damage has been done, you would know if there had as you would have been towed in.

posted 03-23-2009 11:26 PM ET (US)            

RPM = 5300
MPH = 41.7
PITCH = 17

calculates to a SLIP of about 8.7-percent. That sounds about right.

If you try the propeller LHG suggests, to hit 45-MPH with a 17-inch pitch propeller, and assuming about 9-percent SLIP, you will need to run the engine up to

RPM = 5,745

That is a gain of about 445-RPM from where you are now. There is a rule of thumb which says that a change in pitch of 2-inches causes a change in engine speed of about 450-RPM. This is a good rule, except that here the two propellers are the same rated pitch. This seem contradictory. However, my experience with the REVOLUTION4 propeller is that its rated pitch is typically 2-inches lower than other propellers. That assessment fits better with your results and LHG's predictions.

The REVOLUTION4 is a very heavy propeller. When you reduce engine speed, the engine shaft slows down. The propeller shaft may spin faster due to the momentum of the propeller and from the flow of water past the blades, which will keep it rotating. Direct injection engines tend to immediately slow down with reduced throttle. So you might be hearing a bit of the gear case ratchet operating as the propeller shaft spins faster than the engine shaft is driving it.

posted 03-24-2009 08:15 AM ET (US)            

quote:

---

motor sits on a six-inch hi-jacker set-back bracket, and the cavitation plate is about 1.5-inch above the keel[/url]

I think many of the rigging experts would say the motor is too low. Given your setback, the engine should be raised.

Plotman had a good explanation:
[quote]Remember, that with a transom angle, moving back perpendicular to the plane of the tansom will DROP the effective mounting height of the engine down a little over 1/4" for every inch of setback, so you need some built in lift to offset this geometry.

---

posted 03-24-2009 09:38 AM ET (US)            

-- The Revolution 4 is a heavy prop, as heavy as any outboard propeller I've weighed. However, there is nothing wrong with using one on a 150 HP OptiMax. Its gearcase is robust and can take it just fine.

If you do not like the shift clunk, use a Flo-Torq IV hub kit in lieu of the Flo-Torq II hub kit.

-- There is not perfect consensus about how far up you go with each unit of setback. See this thread for a good discussion:

http://continuouswave.com/ubb/Forum4/HTML/006479.html

I agree that this motor may be too low, especially for the Revolution 4 which can tolerate a high mounting height. Given that the motor is on an adjustable set back bracket, the first and least expensive change to be experimented with is a higher engine height.

A higher engine height will do two things, increase RPM at WOT and possibly alleviate the porpoising.

-- I agree that 5300 RPM at WOT is not quite where this motor should be. It would be best if you could get closer to the redline of 5750. Raising the motor will help with this but may not be enough.

Jim has his own rule-of-thumb about a 450 RPM gain for each two inches reduction in pitch. In my experience,t his is optimistic. The more common rule-of-thumb is you will see a 300-400 gain in RPM for each two inches reduction in RPM.

Bear in mind this only applies to a given line of propellers. If you are going to switch models of props, all bets are off.

Yes, going to a VenSura (Offshore) of the same pitch will certainly gain you RPM. The difference between a 17" Rev 4 and a 17" VenSura may be as much as 600 RPM.

If you want to try a VenSura, I suggest you try the 19" pitch version.

Now to the hard stuff: What prop is best for this boat? It is very hard to arrive at an answer to this question by merely discussing it. You have to experiment, experiment, experiment if you want to achieve the best possible performance.

I am not sure the REv 4 is the best bet for this boat, but I am not ruling it out either. RAISE THE MOTOR more and see how it does.

The VenSura is worth trying. It will not tolerate a high motor mounting height and likes to remain fully submerged. Try a 19" if you experiment with this model.

The Mercury Enertia is also worth trying. It grips very well and can tolerate a fairly high motor mounting height. I would recommend you experiment with an 18" size.

Also consider the Stiletto Advantage 4.75 or the Mercury Laser II. These are both fast propellers that do not grip as well as the above mentioned ones. For the Advantage, you want to try a 17" and the laser should be a 19".

You could also try the Mercury MIRAGEplus, a tried and true propeller design that is fast and carries a load well. It may be too big a prop for this boat is well worth trying. You would want the 17" pitch.

posted 03-24-2009 04:05 PM ET (US)            

I think raising my motor might help, but I don't think it will achieve the 300 + rpm I need to get in the middle of my operating range with this prop. Remember, 5,300rpm was the max I was able to achieve trimmed way out and at the edge of control (with a following wind and running downhill :) 5,200 was the average (do I consider the max or the average? hmmm).

Everyone (!) seems to agree that the Vensura 17" or 19" is the ticket (although I think I'd give back the rpm gain with the 19").

Tom, do you really think the difference between the Rev4 and the Vensura is 600rpm; both pitched at 17? That's huge! I have followed your advice on this forum and others and I know that you are the man when it comes to props, but that seems too big a difference for like pitched four-blades.

I don't know much about the Enertia, but it is intriguing and that will be my next subject of study.

Looks like I'm selling a Rev4 in any case. Thanks again to all.

posted 03-24-2009 04:25 PM ET (US)            

I am not sure the VenSura is "the ticket" but I do think it is worth trying, as is the MIRAGEpplus