Which prop to get?

[iggyw1]

I have a 1991 Nissan 70 H.P. outboard, model number is: 70A2. I want to get a new prop, and I am considering a (4) blade aluminum as apposed to a (3) blade aluminum. Will I gain any top speed with the four blade prop instead of the three blade prop without increasing the RPMs or will the four blade prop increase the RPMs? What I am after is faster top end speed without increasing my RPMs. The hole shot does not matter to me, but just after top end speed. I am going 35 MPH now pushing 5,600 RPM. Will a four blade prop help me go faster? Thanks for any input.

 

[ondarvr]

That's not how it works, and while there are general statements on what a style of prop may do, you won't know until you actually try the prop on the exact boat and motor combo and use it in the way you to.

​Four blade props tend to run slower due to the extra drag, but may allow a better hole shot or load carrying capacity.

​As for increasing or decreasing the RPM's, that depends on the pitch of the prop you buy, higher pitch = fewer RPMs, lower pitch = more.

​Without knowing what the recommended peak RPMs are for that motor, it sounds like you're about maxed out now, so there may not be much more speed to be had.

​With a SS prop you may gain 1-2 mph, and that's just maybe, you could spend $500 and get nothing more.

​If you want more speed, get rid of all the extra stuff in the boat, and that means no cooler with ice and beer, or passengers. You may also be able to gain some speed by dialing in the motor height.

 

[iggyw1]

That's not how it works, and while there are general statements on what a style of prop may do, you won't know until you actually try the prop on the exact boat and motor combo and use it in the way you to.

​Four blade props tend to run slower due to the extra drag, but may allow a better hole shot or load carrying capacity.

​As for increasing or decreasing the RPM's, that depends on the pitch of the prop you buy, higher pitch = fewer RPMs, lower pitch = more.

​Without knowing what the recommended peak RPMs are for that motor, it sounds like you're about maxed out now, so there may not be much more speed to be had.

​With a SS prop you may gain 1-2 mph, and that's just maybe, you could spend $500 and get nothing more.

​If you want more speed, get rid of all the extra stuff in the boat, and that means no cooler with ice and beer, or passengers. You may also be able to gain some speed by dialing in the motor height.

Thank you sir. I should have mentioned that I know that the pitch would change the RPMs. I should have asked about the four or three blade props "with the same pitch as each other" for a good comparison. I found a good article comparing the props on line, and it did state as you said that with a four blade prop,I would lose tope end speed but gain the hole shot speed to get on plane quicker.

As for the SS prop, I know it would be at least $300 more for the three blade prop, and I may only get 1-2 MPH more, and like you said, I MAY get it or I MAY NOT get more speed. I think I will stay with the prop that I have. My RPMs are running about 5,600 and the most the motor is built for is 5,800 RPMs, so I am maxed out right now. Did not see anything in the article I read about if the four blades would increase or decrease the RPMs. Do you know what it would do for the RPMs if it's the same pitch as the three blade prop? Thanks again.

 

[ondarvr]

Same pitch normally drops it a couple hundred

 

[iggyw1]

Same pitch normally drops it a couple hundred

Thanks again! I am going to stay with what I have on there. I am almost at the max on RPMs. If I change the pitch, it may go over the max. You are probably correct in saying I got all the speed I am going to get out of it. My boat is rated for a 90 H.P. motor. I have a 70 on it. I guess if I want more speed, it's go to the max. sized motor then. Again, I will keep what I have as long as I can get parts for the ol' Nissan motor. It's a 1991 model and I bought it two years ago with about 65 hrs. on it. (like brand new). Still has low, low hours since I just use it to get away from shore or up river a few miles, than use my trolling motor or drift fish. Parts are getting scarce!

 

[Sea Rider]

Tohatsu OEM 3 blade alum props performs very well, At 5600 wot revs OB is just 250 wot rpm from max 5850 rpm range if that's a 2 strokes OB. If wanting running towards max range ones less pitch should do the trick, will achieve a better hole shot but top speed will be slightly decreased. Personally would stay with current factory delivered prop as can't have the best of both worlds in a single ?prop.

Happy Boating