Position of Twin 250 HP outboards on dive boat

[galaalice]

Hello all.

We are in Ecuador on the Galapagos islands and had our boat made here.
We bought 2 x 250Hp outboards and have been running in clam seas perfectly for the last 2 months.
Now the weather has changed and we have more chop and the engines are taking in air and not catching the water.
The back of the boat is a V - 20degrees.
We have just moved the engines so the anti-vent plate is in line with the bottom of the V at 25 inches, but we are still getting air.

Previously the engines were further apart and a little higher.
We have also added extra weight into the back of the boat so it's sits further in the water, but it doesn't seem to help.

Does anyone have any ideas? The Suzuki guys who sold us the engines are on the mainland Ecuador and are not being of much help.

[ChrigelKarrer]

Hi galaalice,
i own 2 dive operations and have Suzuki engines installed on 3 of my 4 boats.
1 29ft Panga with a 2012 DF200 + Suzuki 18.5'' 3 blade prop (Costa Rica)
1 29ft RIB with a 2014 DF300AP (Italy) + Solas Titan 19'' 4 blade prop
1 34ft Boat with 2016 twin DF300AP + Mercury Mirage Pro 18'' props (Costa Rica)
All 3 of the boats had issues with ventilating when i changed from Yamaha or Mercury to Suzuki.

My personal experience is that some or a combination of them can/will help
1° Lower your engine one or 2 holes (this WILL help)
2° Use Suzuki (or maybe another brand) 16 inch wide propeller, the wide diameter seems to help to keep grip
3° use 4 blade propeller if you find a 16 inch wide one, 4 blade props have usually more grip and less ventilation in choppy water than 3 blade propellers
4° use a set back plate or make a temporary one of a 5-10 cm thick hard wooden block
5° check that no transducer, water pickup or other protrusion is in front or near the propellers, this will disturb the water flow and lead to ventilation
6° check your weight distribution
Some of my boats ventilate only on a certain RPM/speed range when they are between dislocant and planning speed with the bow high up.
After a lot of tests it is my believe that too much weight on the back of the boat create some turbulence in the water flow when the boat is bow up and this disturb the propeller.
Try to move more weight forward and see what happens

On all of the 3 boats i need to trim the engine in (anti ventilation plate not parallel to the water surface), if i try to trim up they start to ventilate.

As Galapagos is far from main land and you will have limited access to parts i suggest:

If you need a quick fix that works: lower the engine one or 2 holes, this WILL work but you will loose a few knots (usually not a issue on a dive boat)
If you need a quick fix that may work:
Change your weight distribution, more weight forward
If you can get a wooden block and longer bolts: make a set back plate
If you can try other propellers: test different types, they may help
If you want the best, most flexible but expensive solution: Install a hydraulic jack plate, this allow you to rise and lower your engines as they need to not ventilate.

Chris