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npauls
11-17-2009, 08:26 PM
I just bought a 99 alumacraft 165 cs magnum boat with a 40 hp merc 4 stroke on the back. I have had it out a couple times and have had a few issues with the way it runs. Now I know this motor is pretty small for the boat but I have been told it should perform alot better then it has so far. I am just wondering if anyone on here knows anything about adjusting the motor at the mounts and which pitch I should possibly have on this boat. right now I just have the stock prop and when I trim up after getting on plane the motor seems to "blow out" where the rpm's go through the roof so I have to back it off until the rpm's go back down to normal. If anyone can tell me what I should do to try and cure this problem it would be greatly appreciated. I would like to adjust some things and get it out on the water and test it out this weekend before the ice hits down here in southern alberta.

thanks,

Nate

jet boater
11-17-2009, 11:46 PM
Sounds like your cavitating. The prop is drawing air along with the water. You need to either move the motor down lower or adjust the angle of the motor.

npauls
11-18-2009, 07:56 PM
I can drop the motor one more set of mount holes. Will this be enough to make a big difference. What would I have to do to change the angle of the motor?


Nate

jet boater
11-18-2009, 09:56 PM
You said that it only happens when you trim it after you get on plane. There is a limit on how much you can trim after on plane. you may be trimming too much. Each boat and motor combination are different so you may just have to try different combinations. As for lowering the motor, when you have the motor completly down and on plane does it cavitate. If it does then you have to drop the motor.

npauls
11-19-2009, 05:48 PM
If I leave the motor trimmer all the way down it won't cavitate but doesnt move very fast at all. The motor will only trim up so far on this boat when going full speed. From what I have found out on the net is that the motor trim is set up so that when maxed out it should be going full speed not blowing out the prop. I think I may try dropping the motor to the last bolt holes and get a stainless steel 10 or 11 pitch prop over winter and see how that works out. I would like a new motor but have to fix and sell my old boat before I will have the money for that.

jet boater
11-19-2009, 08:46 PM
Good luck

Tredeb
11-20-2009, 02:21 PM
I am not sure a stainless would benefit you much on a 40 hp motor. the big benefit of stainless is that it doesn't flex under power. There are some benefits realized as manufacturers of props can get a little more creative in their design with the stronger stainless, but I am not convinced that you will be able to see the difference with a 40 hp motor.

You will not be able to trim your motor all the way out. Depending on you speed, prop, and the depth of your motor, you will be able to trim up more or less. Think of your trim as fine tuning. It is the last thing to worry about when setting up your boat.

First, make sure that your motor is at the correct height. When your boat is on plane, your anti ventilation plate should be right at the water's surface. This is the big plate at the bottom of your leg. The last one before the prop. You don't have to be at at the absolut top speed of your boat just well onto plane. If this plate is below the surface, raise your motor up a notch and check again.

FWIW it is very rare that you would have to lower your motor. Lowering your motor will lower your RPM and lower your speed as you are dragging more leg in the water. Also, most boats are set up with the motor too low. A low motor prevents your motor from ventilating/blowing out on turns. If your motor is set up properly, you should have to trim down to make turns without blowing out. Many people see this as undesireable, but think of it this way; If you have trimmed(fine tuned) your motor to the limit when going straight, why do you think this would be a good setting when you are turning.

Once you have your motor at the right height, you need to size your prop correctly. Load up your boat with a typical load that you would have in everyday use. If you always have a 40lb tackle box, a 20lb cooler, and a 250lb buddy, make sure you have them in your boat when doing your test.

Bring your boat to wide open throttle(WOT) and start to trim up your motor. when it ventilates, trim it down a hair until it stops. Read your RPM and your speed. Preferable use a GPS for speed. You need your RPM to be at the top of the Manufacturers recommended WOT range. If your RPM are low, go to a lower pitched prop. Vice Versa if your RPMs are high. You will usually gain/lose around 200RPM for each inch of pitch. Once you have the WOT RPM correct, it is time for fishing.

FWIW, my dealer assured me that the setup was correct on my boat. I ended up raising the motor one hole and going down 2" in pitch. I gained about 700 RPM at WOT and didn't lose any top end.

my boat: Alumacraft Trophy 185 w/ 150hp etec

Tredeb
11-20-2009, 02:40 PM
What is the max horse power you can put on your boat. I couldn't find one for the 165 magnum, but the 165 classic cs is 75hp max. If your max hp is the same, you may just have to live with the performance you have as you would be quite a bit underpowered.

npauls
11-20-2009, 06:42 PM
My boat has a max hp of 80hp. I know the boat is well underpowered right now and I am saving my pennys and trying to sell my old big boat to get the rest of the money for my new motor. I am hoping that I will have enough cash by next year to get a new motor strapped to the back of the boat but until then I have to run what I have, so I would like to get the best performance out of what I have for now. I did lower the motor one spot tonight and on sunday if the weather is decent I will see how it goes. If it makes the performance worse then I will try moving the motor up one notch higher then what it was set at to begin with and test it again. I can't go any lower on the pitch of my prop since I am at a 12 pitch already. I was talking to ken at propgods.com and he said that he could put more cup on a 12 pitch 3 blade stainless for my boat and it would make a world of difference. He said if I matched that prop with my motor down one notch farther it should push this motor to pretty much the best performance I will see out of it. so I am thinking if moving the motor down one notch makes it better then I will order a stainless with more cup then normal and hope it makes it even better. If not then I just ship it back to him and get a full refund.

Thanks for the advice though. I will take it into consideration if this first move doesn't work any better.


nate

Tredeb
11-23-2009, 10:08 AM
You really need to see where the anti ventilation plate is with respect to the water's surface to know if your motor is at the proper height. My boat gained 300 RPM by adjusting the motor height one notch. Don't underestimate the effect this has on your boats performance.

You should be able to get your motor to blow out by raising the trim. Just because it does not blow out, does not mean you are getting better performance.

You are on the right track WRT getting a new motor. It may be a case of simply not enough horsepower to get satisfactory performance.

Waxy
11-23-2009, 10:27 AM
IMHO, the money spent on a custom stainless prop would be better put toward repowering the boat. That's kinda like putting a bandaid on a bullet wound. I don't think you'll ever see "satisfactory" performance from a boat that is so underpowered.

Waxy