Diference between Mercury Vengence 18P and Laser II 20P?

[twr7cx]

Just bought a new boat - a Haines Signature 520C ( http://www.signatureboats.com/content.aspx?pageid=37 ). The motor is a 2001 Mercury Saltwater series 115HP 4 cylinder 2 stroke quad carby outboard.

Currently it has fitted a Mercury Vengence 18P prop that the previous owner changed to (not sure why). With this prop, the boat achieves a fast top speed, above 75km/h. However, the down low pull is very lacking - it feels as though all though the prop is spinning, it's not biting in a d gripping to propel the boat forward (like a car spinning it's wheels) - is this refered to as cavitation?

Originally it came with a Mercury Laser II 20P prop. Any ideas if this would improve the down low pull?

I am happy to sacrific top speed for down low pull. I really want more off the mark as currently it's lacking for wakeboarding and skiing activities.

Would a hydrofoil help to prevent cavitation?

 

[twr7cx]

Re: Diference between Mercury Vengence 18P and Laser II 20P?

Pictures:

2001 Mercury Salt Water Series 115HP:




18P Vengance prop:






20P Laser II prop:

 

[tpenfield]

Re: Diference between Mercury Vengence 18P and Laser II 20P?

The lower pitch (18 P) should give you better low end performance for water sports, etc. It would seem that the 20P would be better at the top end.

Cavitation is a fancy word for 'boiling'. And yes, the lower pitch prop is probably too little prop for the sensation you are describing. As far as the hydrofoil, you may be thinking of 'ventilation' as opposed to cavitation. Ventilation is when top-side air gets pulled into the propeller. The hydrofoil won't do much for cavitation, which is the more probably of the 2 issues.

You should try a 20 p prop and see what the differences are. The correct prop will get the boat engine within its recommended RPM range at full throttle.

 

[steelespike]

Re: Diference between Mercury Vengence 18P and Laser II 20P?

Your info is pretty good but you should read "read this first" at the top of the page.
Get us the details especially wot rpm and speed. Some people might think 10 seconds is a slow hole shot others a minute.What rpm developes as your coming out of the hole?
Is the speed by gps?As suggested the higher pitch isn't likely to produce faster hole shot.
But you should try both props. I'm not sure but I don't think either prop is considered the best for hole shot I think more for top speed.
Improving hole shot may reduce top end but could improve the ability to stay on plane.
Do you trim down for hole shot and balance the load so not too stern heavy?

 

[Texasmark]

Re: Diference between Mercury Vengence 18P and Laser II 20P?

I don't know about the Vengeance, but the Laser II has adjustable (with rubber plugs of different inner diameters) ports which allow for deliberate ventilation in the hole shot...getting up on plane. It allows exhaust gasses to pass over the blade, adding gas to the water which reduces the load on the prop and allows the engine to spin up faster. Since rpm's are one element of horsepower, the faster you can spin her up, the faster you develop your hp and hp is what turns props.

So, with the Laser II you should get a better hole shot and with 2" more pitch and probably a close proximity to the same general design, the Laser will/should give you the best of both worlds.

Mark

 

[twr7cx]

Re: Diference between Mercury Vengence 18P and Laser II 20P?

I fitted the Laser II 20P. Much better. Not an equal test as yesterday was in a smooth lake, today was in a river that was quiet rough. However, it's added at least an extra 5km/h to the top speed and improved the boats take off acceleration from stand still (which is what I was looking for) - not sure if this is due to the different design of the Laser II vs Vengeance prop (as mentioned above by Texasmark, the Laser II has ports for deliberate ventilation in the hole shot) or due to the different sizing - 18P vs 20P. But I am happier. Thanks.

I am still tempted to try a hydrofoil. I had excellent results with an SE Sport 300 on my previous rig.

 

[steelespike]

Re: Diference between Mercury Vengence 18P and Laser II 20P?

What is the wot rpm and speed with the 20?
I have used foils on 2 comepletely different boats with success.
However while the foil will very likely improve hole shot and ability to stay on plane
it also may reduce top end and in the extreme cause wierd handling.Probably worth a try.

 

[Texasmark]

Re: Diference between Mercury Vengence 18P and Laser II 20P?

I fitted the Laser II 20P. Much better...... or due to the different sizing - 18P vs 20P. But I am happier. Thanks.

In answer to that question, you just told me (whether you realized it or not) that the Vengeance is not ported so you lost the best of both worlds. More pitch, with all else equal means better top end (if within operating specs of the engine/boat and all) but sloppy hole shot...more the pitch the sloppier the shot..but the little holes in the Laser correct the "sloppy hole shot"; if not, open them up (change to a larger inner diameter rubber plug) till you are happy with it.

If you think you are stern heavy and it is causing you problems, then surely you want to entertain the idea of a fin or trim tabs. Cases would be folks sitting in the stern seats, wanting to pull up a couple of slalom skiers, or cruising around on a Sunday's afternoon wanting to stay on plane, but at minimum speed for relaxation and minimum fuel consumption.

But if you are happy with your hole shot and seems you are happy with your top end, then the fin "Whales Tail" or whatever they call it may not be for you; trim tabs, if adjustable could be moved out of the way when not wanted, like during WOT operation in calm waters; or just "diddle" with your prop hole size and forget the rest.

Mark

 

[Faztbullet]

Re: Diference between Mercury Vengence 18P and Laser II 20P?

The Vengeance is non ported and basically a S/S prop cast in the aluminum prop mold as you have found out. The Laser II came 2 ways, new style with the round adjustable venting and old style with non adjustable square vents. I run the old style as they have a little more blade rake which helps on top end and turning.

 

[twr7cx]

Re: Diference between Mercury Vengence 18P and Laser II 20P?

it also may reduce top end and in the extreme cause wierd handling.Probably worth a try.

Reducing top end is not an issue at all. As I mentioned, the 20P has added at least an addition 5km/h to the top speed, which is already faster than I require. The improvement on holeshot for watersports would be a big gain for me. I figure I'll try the hydrofoil, if I don't like it, it can come off...

 

[Texasmark]

Re: Diference between Mercury Vengence 18P and Laser II 20P?

I figure I'll try the hydrofoil, if I don't like it, it can come off...

Seems like the right answer and if you decide to take it off and if the mounting holes in your anti-vent plate bother you, you could fill them with something like JB weld, sand it off and repaint it.

Mark

 

[twr7cx]

Re: Diference between Mercury Vengence 18P and Laser II 20P?

Thanks to all. I have fitted an SE Sport 300 hydrophoil (in addition to changing back to the Laser II prop). The boat gets up and planes very easily and still has a top speed of 45mph at 5,100rpm (and the motor trimmed half way up on the gauge). It got some good run time this weekend as we went for a dive yesterday (so it was loaded with all the heavy dive gear and 4 people) and today we were out for some water sports (5 people).