RPM issue

[RocheH]

Good day fellow member’s, I hope someone can give me some advice with a problem I’m having with the new (second hand) outboards motors I recently fitted to my duck. The detail of the boat taken off manufacturer website are as follows: Predator duck Model: 4.9M, Length 5M, Beam 2.060m, Mass 290KG, recommended HP 1x90HP or 2x40HP. When I bought the duck it had a single 85HP Yamaha year ’95 fitted to it, the motor had a hard life but was still running well, it got on a plane fairly quick and the maxed out top speed was 55KMH with a 15 pitch prop. I recently decided to get the boat ready for CAT C so the whole boat was restored and fitted two Yamaha motors 2006 40VEO with 11 3/8 12pitch props which are in pristine condition with only 80 hour on them and a full service history done by a Yamaha dealer ship. I took the boat out on Sunday for the first time with the new motors and they run very quietly and got on a plane much quicker than the old 85 but they do not rev higher than 4000RPM on WOT while the boat was carring two people and 4 x 25L cans of fuel. We played around with the tilt of the motors toe-in and toe-out and all the throttle cable are set right so the carburettor flaps open fully. I started doing some resursh on RPM’s and most of the research seems to point to the props. What I understand is for every pitch I go down I’ll increase the RPM by roughly 200RPM so from a 12 pitch down to the smallest pitch prop I can find which is a 9 Pitch I will only get the revs up by approximately 600RPM which is still far below Yamaha’s recommended RPM range for the outboard which is 5500RPM. Any suggestions on what else can I do to get my outboards RPM up would be greatly appreciated.

 

[johnnygjr]

Are your engines mounted at the right height on transom in relation to keel,lowest area of hull. Wait for other replies from the prop gurus

 

[RocheH]

I did adjusts the motors by the mounting bracket and had no RPM increase.

 

[deepsea21]

Not familiar with that boat but if it has a V-bottom and not a flat bottom twins get mounted several inches higher than a single would on the same boat. You want the twins mounted high enough that the anti-ventilation plates that reside just above above the props are NOT underwater when on plane. That plate should have the water coming off the botttom of the stern running just under it, not over it. Any deeper than that and the lower units on the engines are creating alot of drag. With the engines down and the anti-vent plate parallel to the bottom of the hull you should be able to look from behind the props forward under that anti-vent plate and have the bottom of that plate line up with (or slightly higher) than the bottom of the stern hull portion just in front of them.