Mercury 90 / Laser II questions

[longshanks]

Hi guys

I recently acquired a '03 Mercury 90 2-stroke motor for my boat, a 15' aluminum skiff. The whole rig weighs about 1400lbs loaded, with no passengers. I have tested the rig loaded with 4 adult passengers, as well as by myself, and surprisingly there seems to be no variation in what RPMs it will spin. It tops out around 5000 RPM, around 40-41mph (works out to about 9% slip).

The motor came with a jack plate and it is set as high as it will go before spinning out. No problems with water pressure, but my boat bottom is quite flat and it has a small tunnel in it, which I suspect is causing some turbulence. Bottom line is that I feel the prop is working really well where it is right now, but it only will go to 5000-5100 max.

I am using a brand new Faria 7K tach, and the propeller is a mercury laser II 22 pitch. Although this motor is recommended to spin 5000-5500 I think it would be better for it over the long run to spin closer to 5500. I intend to use it for cruising/fishing, with some water skiing and tubing. I'd like it to be responsive, but I don't want it to kill my present fuel economy.

Does it make sense to stick with the Laser II? This prop seems to perform very well but it wouldn't hurt my feelings to go with something lighter that wouldn't give so much clutch dog rattle. The only issue is that my local shops here in the boonies have nothing in stock, so no matter what I get, I buy, and can't exchange or return. I'd like to get it right first time.

Any advice on whether I should go for a 21P or a 20P or??

 

[hwsiii]

Re: Mercury 90 / Laser II questions

Shanks, find the BEST prop repair shop in your area and have them take 2" of pitch out of the prop you have now and you will be right at 5,500 RPM.
Just be sure and tell them NOT to take it out of the cup in the prop. That will be your best and cheapest route to go.



H

 

[longshanks]

Re: Mercury 90 / Laser II questions

Hwsii -

I really do live in the boonies! I have done some checking around and not found any prop shops in the area. The nearest one is several hundred miles away, I know nothing about their reputation, and they said that minimum cost is going to be $250. I'm somewhat leery of having a perfectly good (and re-sellable) prop worked on by somebody that I know nothing about. I expect I could get good $$ for the prop that I have, and direct it towards a new one. Are you suggesting that a 20" Laser II (or mine re-pitched to 20") should be very close to 5500?

thanks for your help

 

[walleyehed]

Re: Mercury 90 / Laser II questions

Although I agree with possibly trying a lower pitch, I would not recommend "taking pitch out"...Literally, this can't be done without removing the blades from the hub so it ends up getting some of the progresive nature (Read Efficiency) taken out, which will in-turn increase your slip.
As for gas mileage, running at 5000 isn't going to save you any bucks...if you set it up to turn 5500 with a normal load, this is where you will see best economy "over-all".
The Laser is know for it's ability to carry a Light boat very good, and it's very odd that the rpm remains the same loaded or unloaded.

 

[longshanks]

Re: Mercury 90 / Laser II questions

thanks for your response Walleyehed. makes sense to me.

I was surprised also by the fact that the rpms didn't change. I would estimate that I added an additional 500lbs to the boat, and the rpms were the same, at around 5000-5100. I remember noticing this with the motor I had on this hull previously, a 50 horse. The only explanation/theory I have is that because it is so heavy and such a shallow vee, adding additional weight doesn't sink the hull that much deeper in the water, and the amount of drag is similar. Typical vee or pad boats that plane out on such a small surface, I can see them decreasing in rpms as the weight drives more of the hull into the water. This effect seems to be less on the hull I have. Not sure if that makes sense, but this has puzzled me in the past.

Am I on the right track then to hunt for a 20P Laser 2? I need to gain at least 400-500 rpms, and I've read that these motors can run more rpm than 5500...

 

[walleyehed]

Re: Mercury 90 / Laser II questions

If you are at the max height you can go without venting then yeah, sounds like a drop in pitch will be needed. The problem I have is I don't know your hull design well enough to say how a possible blade design might change it. The Laser has proven itself on light, pad-bottomed typed hulls, so I don't know what effect, for sure, that the Laser is producing on that hull.
Dhadley may have some knowledge of this design...maybe we can stir him on this...

 

[4JawChuck]

Re: Mercury 90 / Laser II questions

Just seen this post and had some thoughts.

The Laser II is a bow lifting prop and with your flat bottom hull design may not be the best choice for your application and since you have a jackplate you likely do not need more lift anyway.

I agree a 20/21" is the place to start and the cheapest alternative would be an aluminum factory style prop, I have used and tried various Turning Point props and they are relatively inexpensive to purchase ($110 CDN +hub cost). They typically run a little faster than a heavy cupped SS prop so I would try a Legacy or Hustler prop in 21" first, if you buy the Legacy brand from this company you can reuse your Merc hub saving some money.

They are decent lightweight props and are well designed, most dealers in Canada carry them...call around. I used a 13" Hustler on a 50HP Yamaha and it worked great, I intend to buy one as a spare for those "oh oh" moments.

Ocean Pacific in Cambell River, Marine Manifold in Langley and Poco Marine in Port Coquitlam carries this brand.

Give them a call.