Motor rpm questions?

[npauls]

I have been looking into getting a new motor for my boat since mine is just a little 40hp 4 stroke motor that is very underpowered for my boat. I am trying to figure out a way to get it to its full potential. My motor is rated for 6000rpm and I can't get it any higher then 4000rpm. Does this mean I should lift my motor up one notch or get a lower pitch prop? I have been talking to a few guys about motors and for me to get a new one will be about a month or so away and have alot of fishing planned in between that time. Any help is greatly appreciated.

Last fall I bought this boat and it had a 13 pitch prop on it and I had very little gear in the boat the few times I got on the water with it. I had alot of trouble with prop blow out so I swapped to a 12 pitch and lowered the motor to the last notch on the bolt holes. The problem is that since that time I have added 2 batteries for my 24 volt trolling motors. I have a bow mount and a vantage hanging off of the back. I have also added some more gear to it that is going to help in the weight. Should I just raise my motor back up to the notch it was on and keep the 12 pitch prop on to get more rpm's out of this motor? Or should I drop to a 10 or 11 pitch prop and leave the motor where its at?

Thanks,

nate

 

[QC]

Re: Motor rpm questions?

Welcome aboard nate!! Take a look at the stickies above. We're gonna need a little more info to help you get this close.

Boat size, type, weight? Current speed info with the 13 and the 12. Model of OB. Mo stuff, betta ansah . . .

 

[npauls]

Re: Motor rpm questions?

sorry about that.


I am running a 1999 alumacraft magnum 165 sc with a 99 merc 40hp 4 stroke. It is very underpowered I know but I can't afford to buy something right now and have to save up all my tourney winnings to get a new one for next year hopefully. I had my dad in the boat with all our gear yesterday and hit 19 mph with 4000rpm the max. The guy that had the boat before me had the motor up a notch on the mounting plate and had a 13 pitch prop on it. My marine shop fixed up the 12 pitch and put it back on. I was still having trouble with the prop blowing out before ever getting to wide open throttle or max rpms so I was told to drop the motor to the last mounting plate notch. Since then I haven't had any prop blow out but still can't get to max rpms or anywhere near that. I have added 2 24volt trolling motors, the batteries, A bigger fish finder, more rods,reels, tackle bags, anchor with chain and 100 ft. of rope and a few other things. When I am alone I can hit around 22mph but still only see around 4500rpm and with 2 guys I can hit 19mph with a wind pushing us and 4000rpm. I did have 3 guys in the boat last weekend and seen a whopping 11mph and way down on the rpm's. I know I do need a new motor bad but can't afford a new one and can only find some 80's style johnson 115 hp 2 strokes in my area for sale. I was thinking of buying one of these since I could get it for $400 and it was fully rebuilt last year but that would put me way over my max hp of 80hp.

Sorry if this is so long. Just trying to give as much info as possible to see max potential out of my boat and motor until I can get the proper power on it.

Thanks,

Nate

 

[steelespike]

Re: Motor rpm questions?

small 4 strokes do struggle with loads and your boat dry is 860lbs.
What was the rpm and speed with the 13" you alone?The 13 probably has some cup so won't be an exact comparison to the 12 but should give you an idea of your approximate rpm change per inch. Smaller motors vary from the 200 rpm per inch.
estimate.You could see if the prop shop can take 2" of pitch out of the 12,if its cost effective.Probably won't be enough but better than the 12.I would eliminate as much weight as possible? Why 2 trolling motors? 4 batteries? Weight placement is critical Motor trim as well.A lightweight anchor like a Danforth.Maybe shorten the chain a little.Maybe less fuel.The 40 will use about 4 gph(gallons per hour).at wot(wide open throttle).Whether making 11 mph or 22.Plastic paddle instead of wood?
live well 1/2 full or empty till needed.Evaluate your gear and eliminate any duplicates or not used.A few pounds and their placement could be the difference
of getting over the hump on to plane.1/2 the normal amount of ice and shade the cooler with a white damp cloth, aluminum instead of glass bottles. You get the idea.

 

[hwsiii]

Re: Motor rpm questions?

Nate, Spike is right in everything he is saying about trying to get your total weight down as much as you can, and you are way underpowered from the numbers you are giving us. Although you only have a prop slip of 14% with you by yourself, and I question that.

You will have to give up some top speed when you are by yourself if you intend on being able to carry three people at times and get on a plane.

Right now your motor is so loaded that the life expectancy of your motor is extremely low compared to being able to turn the 6,000 RPM you should be turning.

If you will buy this prop and move your motor up at least 1 notch on the transom, but try two notches first, then you will be in much better shape. And the acceleration will be much higher and prop slip will go down and your RPM will be much better than they are now, and the motor will still be in good shape when you get another one next year.


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Propeller Specifications:
SKUrop #992401Hub #114
Manufacturer: Michigan Wheel
Brand: Vortex XHS
Material: Aluminum
Diameter: 12?
Pitch: 8?
Blades: 4



H

 

[npauls]

Re: Motor rpm questions?

I run the 3 batteries due to having 24 volt trolling motors. I have the bow mount on the front and a vantage for back trolling walleyes on the back. I run a 20lbs anchor with a short 3 foot anchor chain and 100 ft of rope. I have about 12 rods and reels in the boat for different situations and conditions. I also have 3 life jackets and don't fill my live well up at all. I also don't really carry a cooler in the boat just use one of the front compartments to keep my lunch and drinks in. I have 2 small aluminum and plastic paddles that are pretty light weight and one big jig box and a tackle bag with all my cranks, spoon, spinner and lindy rig stuff and all my power baits.

My motor was mounted one hole higher already and the cavitation plate was to high according to some people which was causing the prop blow out I was seeing very early in accelaration. I am not to worried about top end speed with me in the boat since I rarely fish alone but there is usually one other person with me so I would like to get a set up that can handle and plane at a decent rate with 2 people and my gear in.

Would a 12 diameter x 10.5 pitch prop work best for this or the 8 pitch that you recomended me to buy?

I have also found a 1980's style johnson 2 stroke 115hp motor with a brand new prop and 3 extras that was fully rebuilt last year that I can buy for $400 and would have to pay about $200 to get it installed on the boat. I am just a little iffy about this due to it being quite a bit higher in hp than rated for my boat.

Thanks for taking the time to help me out and try to solve my problems and I hope to hear from you guys again real soon. I don't need any kind of speed boat I would just like something that I can reach max rpm's while plaining well and get a decent speed. In southern alberta it gets very windy so I need to be able to get over the waves and also need a solid anchor like the one I have to anchor up and be able to stick solid so I can't really go to a super light anchor.


Nate

 

[hwsiii]

Re: Motor rpm questions?

Nate, you are only getting 4,000 RPM with two people right now and you need to at least get over 5,000 RPM and a 10 1/2" pitch won't do that by any means. Also the fourth blade will help you a lot when you have more than 2 people to get on plane and yet still give you a reasonable speed and acceleration.



H

 

[npauls]

Re: Motor rpm questions?

Where do I get this prop from? Is it through you or a company online?

I have been trying to find our credit card all day today because I know I will need it to order a prop online but can't seem to find it.

Also do you think I should try my motor with this 8 pitch 4 blade prop sitting where it is at or should I try and move it up a spot or 2?

Also if the prop is available through you would you be albe to ship it collect and I can pay for shipping and the cost of the prop when I pick it up from the post office? As of right now that will be the best option for me until I can find my credit card or get a new one sent to me.


Nate

 

[hwsiii]

Re: Motor rpm questions?

Nate, I forgot to tell you that iboats has this prop available, I don't sell props,I just help people find better props than they have now, because I like helping people.

I would move it up at least 1 notch.



H

 

[npauls]

Re: Motor rpm questions?

oh ok sounds good.

I won't have prop blow out like I was with the 12 and 13 pitch props I was running before if I raise the motor back up?

Nate

 

[steelespike]

Re: Motor rpm questions?

Correct;The 4 blade will resist venting and run higher than the 3blade.
The less you drag through the water the better.Will help get the stern up
will also work better in tight manuvers.H has an excellent program for prop selection.

 

[npauls]

Re: Motor rpm questions?

Ok sounds good to me. I am new to all of this and I guess we will see how it works with it mounted on the motor and leaving the motor height where it is at right now and then when I get the chance I will test it with the motor raised one notch and see which is better for performance all around. I am just hoping that my boat will be able to get me where I want to go while not bogging out my motor and getting on plane faster. Would like to see between 20 and 25mph with 2 of us in the boat if that is at all possible also.


Nate