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Old 25 May 2012, 08:27   #1
Country: USA
Town: Northport
Boat name: Zazu
Make: Nautica
Length: 7m +
Engine: outboard
Join Date: Apr 2012
Posts: 37
Twin Yamaha 150's on RibCat

I posted this on another forum and then realized that Ribnet might be a much better place to search for this advice --

About a month ago I posted asking for help and advice on a new to me used boat that I was buying. Well it took a good deal longer than expected to get it back home and then a while to ‘fix’ a number of things before we got to try it out. Here is the original post with some edits for the boats correct data:
“I have just purchased a large used RIB with existing props that we sea trialed a couple of weeks ago. I am now in need of selecting other props and would like to ask what choices may make sense for my application. I have read a bunch about props so I likely know just enough information to get myself in trouble here but I will give it a shot anyway. My goal is to select reasonably priced SS props for all around use to include cruising, and towing skiers with 2-6 people on board.
Boat is a 2010 Nautica RIB , Catamaran (shallow hulls), 23' long, 8" 6" beam (tube to tube), total weight rigged to is 3,250#'s, (hull 1,790, engines 960, fuel etc 500). The hulls are 36” apart at the bow and shallow along the length, they are asymmetric.

Current props are ‘stock’ Yamaha black painted SS, 3 blade 19” pitch.

Motors – Twin 2009 Yamaha F150 4 stroke, 2.00 lower, counter rotating, in good condition, tachs are Yamaha digital. WOT range is 5,000 - 6,000.

Mounting - Mounted with jack plates that are 5” adjustable set at 3rd hole down from top on the jack plates. There are no dole fins, trim tabs, or other 'things' back there.”

On the one day that we were able to test it here we unfortunately had pretty heavy wind and 2-3 foot plus mixed and choppy seas the entire time. There are too many variables which are completely new to me which is making the ‘tuning in’ of any adjustments much more complicated. I have never had twin outboards, never had jack plates, and never had a catamaran hull before either.
Given the time and conditions we had with the boat so far we have recorded these numbers (jack plates down with trim fully in):
2900 rpm 21 mph 19% slip calc
3400 rpm 28 mph 10%
4000rpm 33mph 8%
4400rpm 37mph 8%
Hit 5,400 only briefly as it was a bit rough and bow steer was an issue as well. I do not think there was much throttle left as the bow was pretty much buried by the trim well ‘in’. Trim was all the way in as the boat began porpoising above the 30 mph mark and it grew with speed. A few things showed up at that test which I have now fixed including a problem with side to side weight loading and a ‘loose’ steering situation that certainly complicated the tests. We did get a chance to do a few WOT starts from a dead stop and video recorded the gages on one run that I will soon get some time to review but generally the acceleration is very fast and it hits near 30 knots within a few seconds. I would rather not go about this in an unorganized fashion but as I posted above many of these things are new to me so that is the reason for asking for help in planning the next tests

Some of my questions/concerns include:
  • Boat is very stable up to about 35 mph but is prone to porpoising above that speed. I have read that raising the engine is one method used to help this (there is one hole left) but I have also read that lowering an engine which is ventilating will also help that trait (there are two holes left). It is interesting to note that it appears the engines were originally mounted lower down by the two holes based upon marks that are left in the brackets. Since I do not have a specific ‘keel’ and one blade on each prop is still shadowed by the outer hull even in the lowest position how do you determine which way to go to help porpoising?
  • Currently with the jack plates all the way down there is about 3” height of ‘shadow’ from a corner of the leading hull below the ant ventilation plates. How do you determine if this is causing any issues at varying speeds?
  • What props if any are better at controlling porpoising and for this type of boat?
  • Do you adjust the jack plates 1st or the trim?
  • Can you take someone along and just try and ‘look down” at the motors to see if the vent plates are covered with water?
  • When mounting twin outboards should the engine both face exactly forward or is there a reason to tow them either ‘in’ or ‘out’?
I will likely not be able to test things out again for another week and I was hoping to get some ideas that will let me make a plan ahead of time. It would be great if that plan led me to making further adjustments to the engines height, potential prop selection and eventually to key up top speed and reduce the porpoising.
Thank you

smitty477 is offline   Reply With Quote
Old 25 May 2012, 11:52   #2
Country: USA
Town: Seattle
Boat name: Water Dog
Make: Polaris
Length: 4m +
Engine: Yamaha 60hp
Join Date: Apr 2007
Posts: 1,152
I would continue to use the stock props for quite awhile. If you hit 5400rpm in them under those conditions they are good enough for now.

The propoising was due to lack of weight in the bow and excessive speed for conditions (the waves helped it get started). It has very little to do with the selected prop. The trim and/or jack plate might have hurt or helped its hard to know without seeing it and being tehre or at least having video.

Once you have a season's (or more) worth of experience on the boat you'll be in a much better position to decide if the props are suiting your needs or if you want different ones to optimize something else.
captnjack is offline   Reply With Quote

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