2007 Nissian 18hp 4 Stroke

[OFP1]

Hello Ladies and Gents, I have 3 questions here and I'll keep it simple.

Firstly I have a 11' 2004 Achillies HB-340 RIB and I run 93 octane 10% ethanol
I achieve 25mph with just myself and a full 3 gallon tank for load on a slightly calm bay (8' swells)
20.7 mph with two people, full tank and some gear on flat water (glass ) I average cruise at 19mph for miles and miles no issues.
My issue is this, it is not at wot due to a rev limiter and I hit the rev limiter around 80% throttle. I am told I may need a different prop. (Currently 9 1/4 9"pitch)
I plane very aggressively with no issue, rather quick plane from a hole shot, (boat weighs like 138lbs, motor is like 118lbs and I am 185lbs)
This seem right?

Secondly
Is there a way to make this a 20hp, 25hp or 30hp motor via an upgrade?

Thirdly
What prop would perform best, not being able to use the last 20% of throttle seems awkward to me. Other outboards crank 100% all day long, however I don't like to bounce off the limiter on mine.

Thanks in advance

 

[pvanv]

Assuming your model is NSF18B2; Year is irrelevant, but model/serial is the specification you need.

The correct WOT RPM for your motor is 5000-6000. The limiter engages at about 6250. The final 20% of throttle doesn't add much output... probably only a few hundred RPM. To stay out of limiting, you can slow the RPM by about 500. Probably need to increase effective pitch about 1-2 inches.

The 9 pitch prop came standard on long shaft motors; The short motors came with 10 pitch. There's also an 11.5 pitch prop for your motor. When you load the boat with an extra person and gear, the 11.5 may be a little more than optimum, but will probably be great when you are solo.

Outboards are built in "families" of engines. You have the "big brother" of that family, so no, you are already at the maximum. The 9.9/15/18B was 328 cc. The newer 15/20C is a little bigger, 351 cc, and that is how they get that additional 2 hp. The 25/30 motors are bigger 3-cylinder units.

 

[OFP1]

Thanks Paul for the info I am not familiar with the rpm ranges as I do not have a tachometer gauge however I am guessing the rev limiter is fine then. I have the pitch of the outboard fully vertical.

I know my prop is not the factory prop, what would be the advantages and disadvantages of the 11.5. I understand slowrr hole shot and maybe little more top end but engine wear may be an issue? I am assuming keeping it aluminum, I have read that stainless steel is a better propeller however mainly I read about larger motors assuming not many people use them on small boats and small outboards. I would go and buy a new propeller tonight and install it this week if it would give me a performance increase. Would I be better with a 9" or 10" diameter? I'm just not sure lol

 

[pvanv]

Pitch is not trim. Yes, you want vertical trim. Pitch is the aggressiveness of the angle of the prop blades.
Diameter is unimportant on these.
You never want to over-rev. The limiter helps keep you from destroying the motor.
Stainless flexes less with extreme loads, but isn't usually available for these.
Many will run just as you are, but don't open it up all the way when lightly loaded; No one prop is best for all situations.
1 inch of prop pitch will usually change RPM by about 300, so either the 10 or the 11.5 will probably get you below limit. The 10 may not quite be enough to always keep you below limit when lightly loaded, but will give a better hole shot than the 11.5.
Unfortunately, it's always trial and error to dial in the exact prop.

 

[Sea Rider]

Go for one more pitch size or re pitch current prop at a prop shop to lessen wot revs and throttle fully that extra grip you want without rev limiter kicking in. Hole shot will still be very good and with extra slight max speed increase.

Happy Boating

 

[OFP1]

Fantastic I understand much better now, thanks everyone for the helpful tips, I'm gonna get a 11.5 and I'll switch on occasion depending on what I'm doing. Seems fairly easy to change them. I'm learning quickly this sites the best

 

[OFP1]

One last question. Would the larger 11.5 put extra stress or strain on the motor causing additional wear or anything harsher than the current 9"?

 

[pvanv]

No.

 

[OFP1]

No.

Thanks Paul. You've exceeded my expectations with responses and answers. I wish I could shake your hand and buy you a beer. Thank you sir.

 

[Sea Rider]

Would advise to get a Harline Induction Tach model [FONT=&quot]HR-8061-2, check Iboats, Amazon for them. Why do you need one. OB must run inside their min-max wot rpm range factory stated. Bear in mind that in light applications + 1 pitch will render between - 300 to 500 wot revs. If going towards a 11.5 pitch prop probably wot revs will be lessen too much towards the lugging side which is bad, much worse if would like boating with more souls, weight on board.

With a 10 pitch prop will probably achieve for the rev limiter not to kick in, be capable of full throttle while maintaining max wot rpm which gives excellent hole shot and nice top end speed with added deck weight than with a 11.5 pitch prop.

As an example, my 450 Rib planing 430 Kg and solo boating with a 2 strokes Tohatsu 18 HP over revs + 350 wot revs from max 5800 wot revs with 1 less pitch prop, that's with an 8 pitch, factory delivered prop was a niner and OB achieved only 5600 wot revs.

Happy Boating[/FONT]

 

[OFP1]

I believe the factory prop was 10" actually. I guess I can give a 10" a shot... i often taje + 1 or 2 people besides me and gear so i dont want to lug the engine. Best of both worlds sounds like a 10"

 

[OFP1]

Also just bought (Harline Induction Tach model HR-8061-2) Such a great thing to have. Thanks

 

[OFP1]

No Title

After looking, I notice a 4 blade and a few other 3 blade and then the stainless steel. Assume I don't mind the money as I'm budgeted for this. What is my best choice for this boat and motor?

 

[pvanv]

I use OEM props, made for Tohatsu. You could try the 4-blade, it will act like more pitch.

 

[Sea Rider]

Being such a small inflatable with a large OB better stay with alum 3 blade props. Don't need a 4 blade nor a SS prop for that application. Now with an induction tach will know 100% sure at which max wot rpm is the rev limiter kicking in.

Test current prop on a wot run on flat calm water cond as usually will be loaded. Afterwords can play with slight more prop pitches to dial one that achieves for OB to run towards max 6K wot rpm range without rev limiter kicking in.. Report your findings once tach is correctly installed..

Happy Boating

 

[OFP1]

Roger that I will have the tach this week and I'll try it out and have findings updated next week.