Proper Prop??

left_turn56 · July 27, 2004, 03:04:47 PM

Do any of you know how to determine what prop to run? I think I am running a prop with too much pitch...feels like I am taking off in second gear and the motor never gets into the power. Just one of many questions to come...
Left_turn56

DundeeMike · July 27, 2004, 03:29:36 PM

What kind of boat, size,etc. What kind of motor, name, horsepower? What is your wot rpms?

Mike Cork · July 27, 2004, 03:37:18 PM

As Dundee asked Left Turn, There are many things that go into determining the right prop. With some info we can steer you in the right direction...

left_turn56 · July 27, 2004, 03:41:20 PM

Mike,
I have a 15 foot glass shallow v hull with a 1967 Merc 500 (50 hp). I do not have a tach so I dont know what RPM at WFO...I am a motorcycle mechanic by trade and the only way I can describe the problem is it acts like it is overgeared.
Thanks guys!!
Left_turn56
(Joel)

Mike Cork · July 27, 2004, 03:44:52 PM

I guess my next questions are

1) Does it take you more than 10 seconds to get on pad?

2) Do you feel like you need to shift weight or move a person forward to get on pad?

3) Does the rpm sound right when you are at WOT?

4) What pitch prop are you running now?

left_turn56 · July 27, 2004, 03:55:00 PM

Mike,
1-2. It takes about 30 sec with me in the boat by myself. If I have a passenger it will barley plane (same as pad?) if I send them to the bow, but then it seems too much of the boat is in the water...\
3. The motor sounds like it is really working hard at WFO.
4. I think it is a 13 pitch

it also seems like it used a ton of gas for the amount I was out ( I am very familiar with fuel consumptions of 2-cycles)

BIG PAPA · July 27, 2004, 10:45:07 PM

Sucking gas no power, sounds like a reed valve problem, but it has been many years since I have fooled with that .

papa

Mike Cork · July 28, 2004, 12:32:58 AM

The gas issue maybe normal the thrust to weight ratio of a motorcycle to a boat is quite differnt.

Plane and pad are the same thing.

30 seconds is too long so I would suggest a smaller pitch prop, BUT is it stainless steel or aluminum. I would guess aluminum with it's age. Aluminum will flex during high load situations and cause your motor to work harder. Stainless steel is much better (but doesn't give at all and can damage your motor if you hit something but they have hubs that are supposed to give). Running at high rpm's at wot would suggest that you could go up in pitch to bring your rpm's down. But taking so long to get on pad suggest that your pitch is too high for that motor. So I would suggest trying a 11 pitch stainless steel. If you go to a prop shop or boat dealer that deals in props they will usually have some test props that they will let you take and play with for a deposit on a new one. If you can find a place like this it is great. A stainless is much more expensive than the aluminum but in my opinion well worth the money.

Next question, does your motor have tilt and trim? Also can you adjust the hieght that the motor sits on the transom? These both can play a big factor also. If your motor is to high it will create planing problems and also cause your RPM's to be high. In planing the motor doesn't have the mechanical advantage necessary to achieve a quick plane and when running wot your prop could be sucking air from the surface preventing it from biting the water very well. So if you can try lowering your motor 1/4 inch at a time and see if it starts getting better.

If you don't have tilt and trim then you should have a rod that the motor rests against when it is lowered this rod should be adjustable and you need to make sure it is the very lowest motor position possible to start and then raise it up one position at a time until you start to loose performance, this will give you the optimum fuel economy.

Let us know

left_turn56 · July 28, 2004, 10:49:55 AM

Mike,
Thanks for taking the time to help...the boat does have a rod to adjust the trim, I will try that. As much as I would love to get a SS prop, I think an AL will have to do for now. When I said that the motor was working hard I ment that it was almost luging, not overreving...my fault. I think that I may have to re-jet the carb for the altitude...it looks easy enough. Papa, thanks for the tip on the reeds, but the idle is fine, and there is not a miss, pop, or hesitation...so I don't think that is the problem (like Iv'e never been wrong before...just ask dutches!!)
Thanks for your help and time!!!!
Joel

Mike Cork · July 28, 2004, 11:02:40 AM

Very good point Left Turn, I completely forgot about the alltitude, when I fished in California we would go to a lake that was about 4 thousand feet to fish for small mouths and the ol' boat acted just like you are discribing. You may have hit it on the head.....

left_turn56 · July 28, 2004, 12:19:02 PM

I am glad something besides me is getting hit on the head (stinking garage door opener, again!) I think that a combination of prop and jetting will make things right. Thanks again for all the help!
Joel

DundeeMike · July 28, 2004, 03:07:42 PM

Left turn, after you get situated for the altitude, when running your boat at full throttle, your rooster tail should be the height of the top of the motor. Your trim would be correct then......