prop for better fuel consumption

[johndeerenut]

my 70 hp evinrude has i guess a stock pitch prop it is a 13-1/4x17 i want to get a little more top end and not totally kill my lower end but what im really wanting is that when im cruising to the next fishing spot at 28-30 mph i can get the rpm down a little to help fuel consumption as when im cruising i dont like to run wide open at full throttle my boat runs around 36mph. i just want to be able to throttle back to my regular cruise speed at 28-30 and maybe get some lower rpms is this the right thinking on this and what would be an all around prop that would do this. i dont pull tubes or skis but not to say i wont at some point so would like to be able to get out of the hole still. my boat is an 82 starcraft aluminum v-hull ss 160 16ft thanks

 

[kenmyfam]

Re: prop for better fuel consumption

You should really post this in the prop section.
What is your current RPM at WOT ????

 

[johndeerenut]

Re: prop for better fuel consumption

didnt pay attention to the prop section i dont know the rpm i just know it is in the stock ranges the carbs have all been gone through by a marine mechanic and synched and timing was adjusted i dont have a tch on the boat its just bare bones

 

[kenmyfam]

Re: prop for better fuel consumption

didnt pay attention to the prop section i dont know the rpm i just know it is in the stock ranges the carbs have all been gone through by a marine mechanic and synched and timing was adjusted i dont have a tch on the boat its just bare bones

Hard to offer any advice without knowing the RPM at WOT. You could either be over revving or lugging your motor. You can get a mini tach at a reasonable cost to check it out.

 

[johndeerenut]

Re: prop for better fuel consumption

you talking about the little digital tachs by chance?

 

[kenmyfam]

Re: prop for better fuel consumption

I will look for a link for you. You could use the search function for yourself as well.

 

[johndeerenut]

Re: prop for better fuel consumption

i will search it if thats what it is the local lawn and garden store sells the tiny tachs

 

[kenmyfam]

Re: prop for better fuel consumption

Here it is.

http://www.tinytach.com/tinytach/index.php

 

[johndeerenut]

Re: prop for better fuel consumption

i was just gonna post back and say i found tiny tachs website and it shows a tach for marine engines for like 36.00 i think i will be ordering one soon. thanks for your help

 

[kenmyfam]

Re: prop for better fuel consumption

i was just gonna post back and say i found tiny tachs website and it shows a tach for marine engines for like 36.00 i think i will be ordering one soon. thanks for your help

Best thing you can do in my opinion. You will know exactly where you stand then.

 

[johndeerenut]

Re: prop for better fuel consumption

yup guess thats my next step im in no hurry i still fish all winter but am doin fine with what i got so it gives me some time to do lots o research

 

[kenmyfam]

Re: prop for better fuel consumption

yup guess thats my next step im in no hurry i still fish all winter but am doin fine with what i got so it gives me some time to do lots o research

Looking forward to seeing what you have currently. Please post your results when you have them.

 

[tashasdaddy]

Re: prop for better fuel consumption

what year is your 70? propping a motor is so you get the correct wide open rpms. this is so that when you run at slower speeds, the motor does not lugg. which is also better for the lower unit.

 

[Silvertip]

Re: prop for better fuel consumption

You own a boat which by it's very nature is not an economy champ and cannot be modified to be one. Next, every engine manufacturer specifies a maximum wide open throttle rpm range for their engines. That range is achieved by installing a prop with whatever pitch achieves that range. Anything other than that and you either over rev the engine or you lug it (strain it). Over reving is easy to understand. Lugging is a little less intuitive. Ever ride a multi-speed bike? Did you notice that when on flat ground it is relatively easy to keep a good pace in a higher gear but when going uphill you need to downshift at which time pedaling gets easier but you don't go as fast. Well -- the prop for your boat is like the gearshift on a bike however you only have one gear on a boat. So if you increase pitch (higher gear) it takes more power to spin it. More power means wider throttle opening which means MORE fuel, not less is needed to get out of the hole AND to maintain speed because a boat is essentially going uphill all the time. Water friction is tremendous. So select a prop that puts the engine in the proper rpm band at wot and economy and overall performance will take care of itself.

 

[steelespike]

Re: prop for better fuel consumption

Marine engine fuel use is measured in GPH (gallons per hour)In your case approximately 7 GPH at WOT.The idea is to get the motor propped close to its max rpm range at WOT(wide open throttle)The next thing is to try to get on plane with the least amount of throttle.This is usually done with careful prop selection,careful motor setup (usually motor height)and perhaps trim tabs.An outboards WOT GPH is measured as "about" 10% of its rated hp.
If you get us your boat size/weight, specific motor year/model present max rpm and speed(by gps is best)We can get an idea where your at.

 

[johndeerenut]

Re: prop for better fuel consumption

the boat is a 16ft starcraft aluminum it runs around 35 mph wot by my gps.not sure of the weight and i havent got the rpms yet im runnin low on cash and need to get a tiny tach for it.im guessing it sounds like i should maybe leave it where its at or could i go up one prop size to get a little mor on top i talked to some one at i boats in early summer and they said i could probably go to a 19 and it would maybe drop the rpms by 400 the motor is a 1975 70 hp evinrude. the carbs have been cleaned and floats set and all throttle linkages synched it runs great

 

[steelespike]

Re: prop for better fuel consumption

While waiting for a tach you could try raising the motor.If you have no venting(cavitation) issues you could raise the motor a hole at a time to a point just before venting becomes a problem.Under ideal conditions you could gain a couple of mph
with a slight improvement in fuel consumption hole shot etc.Careful prop selection
will allow ideal height.

 

[Tacklewasher]

Re: prop for better fuel consumption

the boat is a 16ft starcraft aluminum it runs around 35 mph wot by my gps.not sure of the weight and i havent got the rpms yet im runnin low on cash and need to get a tiny tach for it.im guessing it sounds like i should maybe leave it where its at or could i go up one prop size to get a little mor on top i talked to some one at i boats in early summer and they said i could probably go to a 19 and it would maybe drop the rpms by 400 the motor is a 1975 70 hp evinrude. the carbs have been cleaned and floats set and all throttle linkages synched it runs great

I doubt you can properly turn a 19p. I have an 18' Starcraft with a 115 hp and I turn a 15p. Gets me 40mph at about 5800 rpm.

But get the tiny tach and post your numbers. And do what Steelespike is saying as well. I gained 2 mph by lifting my motor to the top holes. I just have to be careful to trim down when turning or I ventilate.

 

[ezmobee]

Re: prop for better fuel consumption

the boat is a 16ft starcraft aluminum

and get ye down to the Starcraft section and post some pics up of this fine craft!

 

[Silvertip]

Re: prop for better fuel consumption

You seem to have the cart before the horse. Wait until you get a tach and find out where the engine rpm currently is. Until you know that, everything else is pointless and potentially expensive since you may end up buying a prop you don't need.