New engine for boat. Which prop is optimal? Alicraft 18 + Mercury 125.

[GreedyMuffin]

Hi!

Last setup: Alicraft Norwind 18 w/ Mariner 90HP two-stroke 2001. Perfect prop was E.Chance 13.25x17 which gave me about 5500 RPM.

The 90HP had a 2,33:1 gearcase.

Same boat, same everything expect that I upgraded to a 125HP. Got that engine cheap. It is a 2002 modell in even better condition (compression is over 9 bar on all cylinders).

That engine got a 2,07:1 gearcase I believe. Is it possible to calculate which prop will be the best one?

I have the caviation-plate about (lower UW-house plate) 2.5 cm higher than the bottom of the boat. I did that because I wanted a fourblade prop as my goal is the best allaround/economical speed.

I was thinking that a Spitfire 19P fourblade would be nice? Or an Amita 4 19P?

 

[GreedyMuffin]

)

 

[Sea Rider]

Need to tach current prop installed on new OB on a wot run, flat calm water cond and as usually loaded. Can go from there maximizing other pitch prop or going per 4 blade props. Each boat/OB set up is unique..

Happy Boating

 

[GreedyMuffin]

Talked to a guy on my forum. He could send me his Solas Rubex HR4 4x13x19. I could buy it if it fit and was optimal. I had to pay shipping and buy a new hub though.. That is the only cost..

The prop seems like it could be an optimal one. Should I take the chance?

 

[Sea Rider]

No one will know if optimal till you test prop on water at wot and check achieved wot revs, will know if at least around middle to max wot factory range for that new OB. Everything else will be pure tech speculations, wild guesses, free talking..

Happy Boating

 

[GreedyMuffin]

Tested the 19P. Got 5400 RPM. I found a 21P for a great price (60 bucks new) so I'll test that next week. I do not know if the carbs are opening 100% yet though.

And about some of the myths surrounding this engine..

The "kick" from going to 4 from 2 cylinder is so rought that it kicks off people skiing behind etc - IS PURE BS!

The engine idles bad and is very rough - BS. The engine was so calm, and it did not vibrate bad at all. Very little vibration.

The noise was also pretty good being a two-stroke. At least compared to 30, 50, 60 and 90HP Mariner/Mercury from the early 90s-early 2000s. And compared to Yamaha two-strokes (40-60 and 70 HPs) from early 90s-early 2000s.

I am very impressed with the engine.

I will need to test if the carbs are opening 100% though. But the speed seemed to be great.

 

[Sea Rider]

What's the min-max wot rpm range factory stated ?

If at wot as usually loaded achieved 5400 wo rpm with a 19 pitch, a 21 pitch will lover max wot revs badly, not the way to go...

Happy Boating

 

[GreedyMuffin]

4750-5250 RPM.

The carbs does not fully open as well. I was at the boat yesterday and tested, but I did not bring the tools needed to change/adjust it.

Bought a Mercury 3.5 HP fourstroke from 2007 for 59 bucks, did not run. Cleaned the carbs and it seems to be running again. These cost 830 USD new..

 

[Sea Rider]

If already at 5400 from max 5250 would leave the beast alone....

Be aware than in some same CC powerheads say 50-70 HP just to mention any number, there will be a carb/timing restriction on the less model, it's a tech way of detuning an OB.

Know guys that have done it with no avail whatsoever. Didin't know that some parts were entirely different between both models althought being same CC.

Happy Boating

 

[GreedyMuffin]

But the carbs are not opened fully.. So I will get even more RPM

 

[Sea Rider]

What's next size to a 125 HP, i'll guess it's a 150, right ? If so, the 125 is a detuned OB from a 150 with carb/timing advance restriction if both happens to be same CC powerheads,

GF, you have the equation all mixed up,if running already at 5400 wot revs, why do you need more top end rpm for ? OB is already slight over revving from max wot 5250 rpm. With more rpm will achieve better out of this world hole shot while losing best top end speed.

If wanting more top end speed need to lower max wot rpm to say 5000 rpm, try one pitch more from current one, bear in mind that hole shot will suffer a bit.

See this "carb not openoing fully" on Tohatsu's 25-30 / 40-50 and higher HP while being same CC, the lower HP are the restricted versions. Leave that carb/timing as it is....

Happy Boating

 

[flyingscott]

But the carbs are not opened fully.. So I will get even more RPM

Do not overrev that motor they do not like it. Prop it to 5250 and be done, I doubt you will gain more power by adjusting the carbs as it is already running good.

 

[Sea Rider]

+ 150 wot rpm gain is not that wow over revving, if like running full throttle can throttlle tad less or add more weigth to combo to compensate. Personally would stay right there..

Happy Boating

 

[GreedyMuffin]

But these 125 does not like over-reving as Scott is saying, and that is true.

But... My father bought a Shetland 570, with trailer and a Evinrude E-tec 90 from 2010 (with a fuel injector problem) for a ridiculous amount of money. (Less than my 125HP is worth alone). Only freshwater used, 140 hours. 85% of the time on idle/low gass.

So since I am 17, it seems like I am selling my boat, and he says that I am free to use his as my own, that is why he bought it in the first place. So I will replace the engine, but might keep the Alicraft Norwind 18, instead of the Shetland 570.

I am waiting on a 21P prop to arrive in the mail. That should fix the over-reving.

 

[Sea Rider]

When achieved max 5400 wot rpm, how many passsengers were there ? If lightly loaded, expect more rpm drop off with a 21 pitch than with previous ? pitch prop. To reduce 150 over revving rpm just need + 1/2 to 1 pitch max re pitch reduction.

Unless being an Oil Company Tycoon, no body runs wot all the time, so going + 150 wot revs once in a while won't be an issue. Has somebody blown for sure same 125 HP-OB while over revving slightly ?

Just for the record post whatever rpm OB dropped with a 21 pitch prop...

Happy Boating