outboard / bracket set up

I have two questions one regarding outboard set up on a bracket.
The boat is a 36ft open cockpit design powerboat made in Venezula.
Its pretty heavy, i would guess at something close to 8/9 tonnes. Currently it is powered by twin V8 300HP Johnsons, which i intend to change for triple 200HP Yamaha outboards this month.

Currently the engines cavitation plates are pretty much in line with the bottom of the hull, which is correct as I understand. However, I find it very difficult to trim the engines. Almost as soon as I start to trim I get cavitation, this means that in order to push the boat i have to have the motors in almost the completly down position. As you know this means that propulsion is now aim down into the ocean, doing two things, 1. giving me stern lift and 2. pushing the bow down. Obviously this is not a good scenario in a following sea as the boat tends to nose dive.

I believe that if i had the ability to trim the engines properly in the other direction I could reverse my situation, but the cav plates are already at the correct height.

I read something that said the engines should sit 2 inches higher for every 1 foot of set back from the transom, which in my case means the cav plates should be as much as 4 inches above the bottom of the hull line.

Surely that will only increase cavition on the props? or is there any advice on going lower than the bottom of the hull?

Can anyone help?

Secondly, which side should the counter rotating engine be mounted? Port or Starboard?

Thanks in advance.

 

Wouldn't happen to be a former drug-running boat, by any chance? The cops in your part of the world seem to catch and auction off an awful lot of those.

Have you considered putting the motors on hydraulic jack plates? This would give you an extra degree of freedom in the movement of the drive units, allowing you to move the engines up and down as necessary to avoid the ventilation problem and adjust more easily to different loads.

 

Jack plate

Thanks matt, not sure about the first question you asked about, cause i bought it via a boat yard, but I had thought of Jack plates and its a good idea, but i bet they are expensive, any idea of cost?
Mark

 

Rule of thumb: When the boat is on plane, the water will flow out from under the boat and rise about one inch for every 12 inches behind the boat . Your cavitation plate should sit where the front of the plate is at the surface of the water. For example, if your boat has a bracket that is out of the water when the boat is on plane, and the bracket has a 30 inch setback (the engine is 30 inches behind the transom), then the cavitation plate should sit 30/12 = 2.5 inches above the bottom of the hull. To find this height at your motors, put a long straightedge (straight piece of lumber or steel channel, etc.)on the bottom of the hull in line with an engine, sticking out back to where the engine should sit. The cav plate should be 2.5 inches above the straight edge.

 

Jackplates: Hydraulic at www.bobsmachine.com and Electric at www.cook-mfg.com. I don't know which may yield better better results and offer advantages like less maintenence, reliability, longevity, etc.

IMHO, Counterrotating engine should be on the Port Side to (slightly) reduce pivoting to starboard.

Good luck with your proyect.

 

4 blade props

Thanks, can anyone tell me about 4 blade props, I believe there main use is for bow lift , trimability and enhanced hole shot, which is what i need. Bearing in mind the length and weight of my boat can anyone suggest a model, pitch and rake ? I am now re rigging and decided to use twin Yamaha 250hp instead of triple 200hps. Speed is not my need.

thanks
m

 

hi. could anyone tell me whether a boat with an outboard coversion from a sterndrive have proplems planing? I've heard of cases where boats which have been converted and installed with twin 200hp outboard don't plane?

 

Conversion

Hi

I did a boat with a single outboard on a bracket , the boat was a 24ft Formula LS that previously had a 454 V8 with an Alpha one stern drive. The engine I put on was a Yamaha 200hp, on a set back of about 30 inches and it ran fantastic, better than it ever did with the inboard, because of the weight factor. I have also known of many conversions that were a complete success. In fact I have never heard personally of one that didnt work. But its all in the correct set up for the individual hull. Sometimes you get a bit of bow jumping, but thats easily rectified with trim tabs and weight distribution.