Am I on the right propeller?

yamaha140 · 25 January 2013, 18:22

Dear friends,

I own a Ranieri pathfinder 17.5 footer open boat with a Yamaha V4 140 hp outbard.
As a propeller I have the original factory supplied 21k aluminium prop and with such I manage a top speed of 36.5 knots at 5400 rpm.
I once tried a stainless steel solas branded prop with same pitch of the standard alloy one i.e. 21 pitch and managed 37.5 knots at 5400 rpm that is an additional 1 knot.
Many have told me that with such power on such a boat I should at least do 40 knots.
Can anyone please tell me if I will manage such top end speed by installing a slightly bigger prop pitch maybe?

I thank you all in advance,

Regards,

Yamaha 140.

Locozodiac · 26 January 2013, 02:16

What are the min & max rpm parameteres at wot for that engine ? if higher than 5400, you need a less number pitch prop to raise rpm a bit if in need. Best is to buy a prop that gives sligh past middle rpm between min & max rpm. Nice hole shot with good top speed, Can't have both on a single prop and knowbody runs at wot all the time...

Such a power could be lost if boats is heavy, has a inefficient hull desing that drags too much, improper engine transom height, sea conditions...

Happy Boating

RIB-Teccie · 26 January 2013, 07:28

If I remember correctly, 5500 RPM is Maximum. They will withstand slightly more than this, but then usually people will go for a larger pitch prop. If you raise anti-cav plate 1 inch above bottom of hull by taking engine up a hole, you may get your extra 100 RPM and gain a little speed. Max speed is down to hull design. Try a pair of bolt on doel-fin type attachments. But use the ones from unipart marine as you don't have to drill holes in the engine to fit. Only go to all this bother if you really think its worth it. Never raise engine by more than 1 inch, you will starve the top cylinder of cooling water, if you do this be prepared for a loud bang!!

Locozodiac · 26 January 2013, 11:27

If 5500 is the max and you are runing 5400 could benefit going 1 pitch up next to the one you are currently using to lower rpm a bit. Will have better hole shot while carrying more weight and slight less top end speed.

My actual horse runs max 5800 at wot, with standard prop and correctly engine height tunned produced 5700. Went 1 pitch up, now producing 5580 rpm. The hole shot is much better and faster with very slight top speed loss.

Big 2 strokes engines are real fuel guzzlers, runing full time at wot is not the way to go unless you happen to be a tycoon or work for an oil Company.

Happy Boating

spartacus · 26 January 2013, 12:02

Quote:

Originally Posted by Locozodiac

Big 2 strokes engines are real fuel guzzlers, runing full time at wot is not the way to go unless you happen to be a tycoon or work for an oil Company.

He must be a tycoon and work for an oil company! The price of petrol in Malta is similar to the UK. I struggle to pay for the petrol in my 3.5hp lawnmower these days!

Fantastic harbour... reminds me of Guns of Navarone'.

yamaha140 · 26 January 2013, 16:39

Dear Locozodiac,

Whilst I thank you for the useful info, in my previous post I forgot to specify that my engine's WOT is 5400 rpm according to specs, so i think that with the standard aluminium prop the engine is revving perfectly to specs, even when I tried the stainless steel prop with same pitch of the standard alluminium one the revving was still 5400 rpm and I gained 1 Knot i.e. from 36.5 knots up to 37.5 knots.
So do you think if I try an alluminium prop with two pitche's up i.e from 21 up to 23 I will do harm to the engine since obviously the engine will reduce the WOT rpm's but I might gain another 2 to 3 knots maybe?

Spartacus I dont know about your financial situation but Im sorry if you cant even afford petrol to run your 3.5 hp than I suggest that you should go for a PEDDAL boat lol

Regards,

Yamaha 140

peterwhite · 26 January 2013, 17:58

As a very general rule of thumb you should expect 150-200 rpm change for every inch change in pitch so at 5400 with a 21 you might get 5000-5100 with the 23. Personally I'd stick with the 21 as the last thing you want to do is to overload the engine. Also If I remember right the ranieri 17 is quite a heavy boat. Hope this helps. Peter

Locozodiac · 27 January 2013, 14:44

Yam 140,

Are you runing at 5400 rpm with you alone ? Why do you want to run at 5400 at wot all the time. If you go for a 23, assume it's next pitch to 21 will lessen rpm a bit and have better hole shot, slight top speed loss should be minimum.

Engines should rev between min and max factory stated rpm at wot with no issues at all. Best is slight passing middle for general use. You could have a second spare prop for specific water activities like skiing, outing heavily loaded, etc.

Happy Boating

yamaha140 · 28 January 2013, 20:31

Locozodiac,

The engine runs at 5400 wot with just me on the boat and even when loaded with 4 or 5 persons with the standard 21 pitch prop.
Today I just learned that if i go for a 19 pitchX19 propeller the engine will do better ecconomy and have better acceleration. So I guess you are right when telling me to have a spare props i.e one for heavy loading and better economy and other for better top end speed. thanks a very lot for the info.

May I pls ask you if you do have an msn so we can communicate better pls?

Regards,

Yam 140

Locozodiac · 28 January 2013, 22:29

Yam 140,

Be my guest, click my avatar's name and under homepage will find my email. What's your wot rpm with a 19 pitch and you alone ? You should be over reving 5400 + ?

Regards,
Luis

Happy Boating