what revs are best

[diverjohn]

I have just purchased a new Yamaha 115 4stroke, its done 8 hours now almost run-in and I want to prop it.
The Rib is used for diving and needs to carry when full 8 divers quite a load, we run the boat at a maximum of 23 knots so speed is not what we need, we dropped the pitch 1” from standard to help get on the plain easer, what revs will be best for engine and economy? It was suggested we run it at 4,000 to 4,800rpm. What is best?

[BaileysBoat]

It's in the owners manual. You have to prop for the upper end of the operating range at wide open throttle, with a "normal" load.

If you don't the engine will lug and make oil. 5000-6000 is the range for your motor. Once you have the right prop it will be correct for any speed you choose to run at.

[diverjohn]

more info

Thanks for your reply, the only data I have is the first working trip for the boat with 4 divers in it ran smooth at 4,000 revs at 23 knots; we have yet to put 8 in the rib, maybe this weekend? Are you saying that fully loaded it should cruse at the speed we want at 5,000 to 6,000 rpm?

[99yam40]

No he means that at WOT it should be able to hit about 6K.
But you can cruse at what ever RPM would care to.
Look at Yamaha performance

Performance Bulletin Home

[diverjohn]

what do you mean by WOT? what revs would be best for the engine and fuel eco?

[99yam40]

WOT means wide open throttle.

Look at the link I posted and see what a rig like yours got during the test.
It shows all the info

[diverjohn]

thanks

Thanks for the help, my problem is that I don’t need to exceed 25knots, but will take 8 divers in the Rib that’s about 950 klos, this is a replacement engine, the old 115HP Yamaha did 3,860 hrs and was using a bit of oil at the end. it would struggle to get on the plain but do 35 knots, I was hoping to swap speed for takeoff power we dropped the pitch 1" to get more revs from standing but I am not sure what revs are good for cursing, I thought 4,500rpm is that too high?

Thank you for the help.

[BaileysBoat]

we dropped the pitch 1” from standard to help get on the plain easer, what revs will be best for engine and economy? It was suggested we run it at 4,000 to 4,800rpm. What is best?

Couple of things, there is no such thing as a standard prop. Quick calculation tells me you probably have a 17P. For economy most planing hulls are best just off of plane.

Second, this is not about hull speed but about efficiency. For the engine to run efficiently at cruise or any throttle setting the engine has to be able to hit the high end of the WOT spec.

So, on a calm day, load the boat with your divers and gear, throttle up to WOT and measure the RPM. If you hit 5800 to 6100 RPM you have the right prop. You won't make oil and your hole shot should be good

After you have the right prop you can experiment to find the most economical cruising speed. Let us know and good luck.

[diverjohn]

Thanks again,
so I do need to get max revs and a higher speed on a calm day. Looking at the data you pointed me to best mpg comes around 4,000. we are trying a 17" Pitch and that is looking good, the old engine had a 18" P and if you left the throttle alone and hit a rough patch the speed would decline and not recover forcing you to add throttle to get back up to 23K but the smaller Pitch we are trying recovers after slowing to the required speed. I will report back when we have done fully loaded tests.
John

[diverjohn]

test

Tested the new prop out today with 5 divers (about 500 Klos), no trim used, from standstill to 25Knots it took 22 seconds at WOT kept WOT till we reached the maximum speed it would do that was 30K at 5,600 rpm. It was not fully loaded we carry 8 divers max.
What do you think?

[BaileysBoat]

Why aren't you trimming up? After you come out of the hole you should trim up for maximum rpm without ventilating. Less drag, more efficient.

You are getting close, 5600 RPm isn't bad. I would try a 15P. You are going to need it when you load up other 3 divers.

[diverjohn]

We left the trim off as a bench mark to test other pitches against, we would normally run with 3 or 4 bars of trim showing, we think the 16P aluminium prop we are trying is an improvement on the 17P S/Steel one we used before, it is smother and if the swell slows you down it builds back up to the cruising speed again without applying extra throttle.

Than you all for your help we will try a 16P soon and report back.
John

[99yam40]

You need to be trimmed out with the load you are running to check the WOT RPMs.
If not it is not a true test of WOT RPM for that prop