don't care much about cruising speed.
those engines MUST be propped so they can turn 5800-6000 RPM with the given fuel,gear and passenger load THAT day.

I have an F150 on a 25 skiff at the shop now.
it was propped with an 18 gallon gas tank and a center console.
it showed back up with seating for 14 and a wakeboaed tow pole and holes in 3 out of 4 pistons.

The appropriate propeller is one that has the pitch which will allow the motors to run at or near their redline RPM (6000 RPM in your case) when the motors are at the wide open throttle position, with the boat loaded and with the motors trimmed up to the most efficient cruise trim angle (the trim angle that provides the most speed for a given RPM). That takes care of propeller selection.

After getting the right propellers installed, you want to find the most efficient operating speed. You do that using the economy function of your Command Link Plus display. It is the RPM that will provide the most miles per gallon. Usually with a twin engine set up it is typically in the 3500 ~ 4000 RPM range.

Given that it appears you are getting 5400 RPM at WOT with a 19 pitch propeller, you need to go up about 400 RPM's at WOT. Rule of thumb is 150-200 RPM's per inch of pitch change. I would suggest that you try the 17 pitch.

I feel that it is better to be slightly under propped (too little pitch) than slightly over propelled (too much pitch). You have command of the boat and the motors. If there is ever a point in time where the motors will go over 6000 RPM (say when you are lightly loaded) it is your job to mind the throttles and not let this happen.

Well now with pitch 19 am getting 5,900 rpm

Shall I keep it then ?

If the boat was loaded when you did the test, and you are happy with the hole shot (time to get onto plane) then I would say yes.

What is the temperature over there now? Remember that in the high temperatures and humidity of summer that the motors won't produce as much HP. You will lose some WOT RPM.

With the 19 pitch on I reached the 5,950 RPM,,,
Was with 300 Gal fuel with 4 people on board; ,,, the planning is no good in either propeller tried ... over 14 sec ...

Temperature is HOT its below 104F now.... was 110 will be reduced to reach 85F;

Question is with pitch 21 at 4000 my cruising was higher than with 19,,,, maximum speed I obtained with pitch 19 and not 21,,,,

to better understand Yamaha in the emirates, please look at this email ....


From: Simon Olivio Rodrigues <srodrigues@alyousuf.com>
Date: Monday, October 28, 2013 at 3:51 PM
To: rasem <rasem@italianprojects.com>, Rengeu YAMAHA <rmathew@alyousuf.com>
Cc: Saber Ibrahim Elgehiny Elaboudy <selaboudy@alyousuf.com>, Mohammed Elatar <melatar@alyousuf.com>
Subject: RE: Reply to thread 'F350 with 4 batteries and the Link-Plus LCD display'

Dear Boss,

We would recommend the 21” propeller if you are ok with the speed.
Engine serial are as below
Port 6AX 1005121
Starboard 6AW 1008249

Still awaiting for a reply from Japan about how to make your LCD sub battery working.


Best Regards

Simon Olivio Rodrigues
Service Manager
Abu Dhabi
Div: AYM
Al Yousif Motoros

The Yamaha dealer in the UAE is a case,,, if you want to know more ,, I tell yu another amazing story, not limited to propeller and LCD not working and wrong wiring to dry the house batteries...

I wrote to weelcraft scarab (brand of my boat is the scarab tournament) informing them what are best engines to the boat since I want to replace the 3 X 250 Yamaha 2 stroke... The Wellcraft USA, kind enough they answered me with a full detailed performance report, advising to install the 2 F350, to have the boat planning in 4 sec and amazing top speed as well as mid range !
Specification of the engines, propeller size and exact engine positioning;

I took the paper to yamaha asked if available gave the downpayment and took the appointment;

They installed the engines, and I felt that the engines were simply to near to each other!

I asked if it is as per the wellcraft they said no, its better here ,,,,

I noticed that the deep V of the boat will not allow the engines to be really trapping water ! I questioned, they say lets try and see;

First sea trial was a disaster ! boat not even moving!
We found out that they installed 25inch shaft and not the advised 30 !

Engines are used now, they did not allow to return them, I paid already! and,,, NO 30 inch engines in stock, no counter rotation,,, neither 300's not even 250's !
That after I sold my old engines I was without engines and first engines delivery will be in April 2014 !

To Make it short they advised to distance the engines to an accepted level of water flow since engines brackets are not available @ Yamaha UAE and maybe its not advisable to be use fixed brackets on a V8 350 engines...

Then house battery was dried and I was about 35 mil offshore, then the LCD display never worked then the propeller is not really ideal...

If I look at the wellcraft performance data with the 2 350's its a huge difference under all expects,,, planning 4 sec against my 14 top speed is less by 15% and mid range - cruising speed is less by about 18% ... !!

Thats Yamaha exclusive agent in the UAE..... imagine my frustration ?

If the motors are set up properly and propped like the performance set up they sent you and you cannot get the performance they did then the motors are not running properly

The differences in the engines position are minor, are 4-5 inch max
Surely if I had the chance/possibility to have the 350's with long shaft (30) I will surely put those instead of the 25 inch short ...

So with pitch 19 siam 15 1/4 am reaching the 5,900 rpm, with fully loaded water tank, fuel and people;

The other possible options are:

Pitch 17 diam. 16 1/4
Pitch 16 diam. 16 1/4

Based on the above any of the above 2 propeller can imp roe the boat performance / planing - speed?
Or there is no way to found out but to install and try ?

Cheers

Rasem

porta-jack actually makes a scissors style jack plate or try bay engineering for a 5" shaft extension kit.

sounds like a dealership doesn't have a clue.

also look for the technical bulliten pertaining to flywheel rotors.

HEED that bulliten.

In my opinion with the wrong shaft length motors mounted , you are not going to get the listed performance because the mounting height is not getting low enough.
Changing props is not going to fix it

Might look into converting the 25s to 30s so they can be mounted properly like Rodbolt said

Hard to believe they messed up that bad and are not going to do it properly, but it is not Yamaha that did it , it is the dealer you are dealing with

:-)
Exactly hard to believe,,,, but guess what ??
AM dealing with Yamaha EXCLUSIVE and sole dealer in The United Arab Emirates :-) and guess what ? he is also the Suzuki dealer.... and he is the worest dealer aver,,,, and I know that Both, Yamaha and Suzuki are in big legal case to remove him,,,, since over 5 years !

They don't have ANY 30' shaft, boats here are mainly 25' and they go with 30' only if centre mounted (3 engines) so they do have 1 30' 300 but in that case I had to get 3 x 300 with extra $$ and and and... or to wait for about 8 months until new engines to be ordered and paid by me in advance...

Hi,
In fact am looking (online) for a fixed engine brackets that can handle the big V8 350 engine,,, any other advice please?

Yamaha are selling a shaft extension from 25 to 30,,, but, honestly, regardless of the BIG coast of the kit (X 2) I do not trust Yamaha dealer over here to do such a job on a brand new engines...