Nissan nsd 90b Question about Rpm's

[gvnitup]

Cleaned the pontoon up yesterday and took it out. Since this was the first time to try the engine out I just want to say I love the engine, smooth, quiet, and it does actually get up and go.. My Question is, I have a 4 blade 15p solas prop. The engine rpm's never go above 4200 rpm's. The owners manual says safe operating range is 5150-5850 rpm's. I don't want to damage the engine but would I be safe going to a 17p 3 blade prop? Biggest question is I don't want to over-rev the engine. Please help guru, I know you may not be able to tell me what pitch I want.. But I am sure you can tell me a safe rpm to run her at.. Last but not least, Since this is a 2 stroke fuel injected engine, does it have a rev limiter?
Thanks in advance. Michael

 

[gvnitup]

Re: Nissan nsd 90b Question about Rpm's

I satnd corrected, it has a 13 pitch solas 4 blade prop. I know that 4 blade props are mostly used on bass-speed boat applications. Not real sure why someone would put one on a pontoon. But am open to suggestions. The boat is at wot it is only turning 4200 rpm's which tells me I should be able to go to a 19 pitch 13.25 3 blade prop. Any disagreements?
Also there is a small 1/4 inch hose coming from the foot up through the center of the engine exits out the center of the transom mount. It has a 1/4 plastic connector that is capped off. Is this for a speedometer? Sorry for the ignorant questions, i am simply trying to get the boat perfect before I put it in wet-dock for the summer. Thanks to all..

 

[rogerwa]

Re: Nissan nsd 90b Question about Rpm's

I would be very surprised if you could get a 19P to spin in the proper range while hanging off a pontoon. What size boat are you pushing?

I have a 25' pontoon that I have an 11p Solas on that is in just about the right range. While I am guessing that I have a bigger load, a pontoon generally requires lower pitch props as they do not plane.

I could be all wrong, but I would be very surprised.

BTW, when you calc prop sizes the guideline is it takes an inch DECREASE in pitch to gain 200 RPM. I think you might have caluculated backwards.

You might need to go down on pitch to get into a good range.

post some specs on your rig..

 

[gvnitup]

Re: Nissan nsd 90b Question about Rpm's

Boat is 24 ft, I agree this thing is a tank. I have never had a boat that would not operate in the rpm range it is optimized for. Right now, I have a solas 4 blade 13.25 13 pitch prop. I thought for every 2 degrees of pitch you normally change the engine rms 300-400 rpm's.. If I understand you correctly your saying I should go down in pitch rather than up?

Reason I ask is it would be hard to find a 9 pitch prop. Just fyi, the boat moves along pretty well. But at 4200 rpm's that just dosen't seem optimal.
I have a second pontoon boat, same leangth with a force 40 on it. It turns 5200 rpm's same load, same amount of children loading the boat. It has an 8. I don't want to sacrifice any speed by changing props. Naturally, I am trying to pick some up..

 

[pvanv]

Re: Nissan nsd 90b Question about Rpm's

Less pitch will give more RPM. Yes, you can turn more than 4200 on a 90 at WOT without a problem. A prop shop can reduce your present pitch some, or Marc at Solas can advise you (see the number on their website), or TG may have a prop on the shelf.

 

[rogerwa]

Re: Nissan nsd 90b Question about Rpm's

I would first try a 13P three blade and see what you get. YOu may also look at how the motor is mounted. It may be too deep and causing drag.

As I said, I am actually performing quite nicely with my 11P on my Nissan 90hp. I have a 25' pontoon that I normally have loaded with 10 people. I don't usually run it in WOT as it tends to get kinda wet, but I can turn right inside the WOT recommendation with the 11P.

It performs better with the 11P, WOT and speed wise, than it did with the 12P Nissan prop which I think is slightly steeper than a 12P prop.

Different props will yield different results, thats why you need to try different ones.

But the key message here is that you need to reduce pitch in order to raise RPM. I wish it was the other way, but it is not.

 

[gvnitup]

Re: Nissan nsd 90b Question about Rpm's

Well, to give a little better info. This boat is heavier than my other 24 ft. It has a few more seats, and for some crazy reason the deck is 1" marine plywood. Combine that with myself 230pds, wife uhh hehe over 150, father 260, 15y-o 135, 4 other brats at roughly 50 bucks a piece. Then all of the fishing gear, coolers, etc we have a pretty good amount of weight. I agree with you, I think a three blade would yield better results. Just for all thats holy. I think I am going to order a prop from here in an 11 pitch. Is there a rev limiter on these engines? I really like my engine, I have only ran it for about 4 hours. But, it was smooth and honestly as far as I am concerned ran just as good I think better than my four stroke does. I think Nissan-Tohatsu has done a jam up job..

 

[TOHATSU GURU]

Re: Nissan nsd 90b Question about Rpm's

Yes, it has a rev limiter and theory says you should be running around a 9 pitch. The lowest Tohatsu offers is an 11....I've run a lot of 24' pontoons with 90's and found that a 13 pitch turns 5400 too 5600 on virtually all of them. You might be better off determining what is causing the unusually low RPM before you change the prop.

And you average about 200 RPM difference per 1" in pitch....Down in pitch means up in RPM.