Outboard Performance Problems w/ Transom Brackets

[scarab30]

I recently purchased a 1994 30 ft Scarab Sport w/ twin 1994 Yamaha 225 hp saltwater series 76 degV outboards. The motors are mounted on a Stainless Marine transom bracket. The problem I am experiencing is that at intermediate speeds (30-40 mph) the motors tend to "hunt" about +/- 300 rpm. I think the cause is that the motors are slightly ventilating at this speed. I've tried all sorts of different trim angles with not much improvement. At higher speeds (>40 mph) the problem goes away and both motors hold rpms very well. I do not want to lower the motor mounting height as they already settle in pretty deep (nearly up to the motor cowling) when I back down off of plane. I'm considering going from a 3 to 4 blade prop to try to correct this problem. I currently have 23P props and my motors run at about 5,200 rpm WOT. I'm thinking about going to a 22P 4-blade prop of similar diameter. Any comments or suggestions would be appreciated, also, any feel for what the rpm drop would be by going from a 3 to 4 blade prop?

 

[Dhadley]

Re: Outboard Performance Problems w/ Transom Brackets

A lot depends on what prop (design) you have now. It also has to do with your set up. How are the motors set now? Toed in? Out? Straight ahead? Which side is the counter rotating gearcase on?<br /><br />It also sounds like they are mounted too low. They don't have nosecones do they?

 

[jim dozier]

Re: Outboard Performance Problems w/ Transom Brackets

Are you sure they are ventilating? Perhaps you have difficulties in the fuel system such as one or more dirty carbs that are only apparent at certain rpm.

 

[jrod8v92ti]

Re: Outboard Performance Problems w/ Transom Brackets

all the above plus, dual 3/8 fuel lines feeding both engines primary fuel filter/coaleser then fuel lines supplied with engine to the engines (install a temporary vacume gauges on primary filters to test)<br /> 660 mm minimum engine centerline seperation is how i understand the yamaha rigging guide.

 

[scarab30]

Re: Outboard Performance Problems w/ Transom Brackets

Thanks for the responses... The motors are mounted ~26 1/4" apart, which translates to 667 mm seperation. The motors are toed in approximately 1/4". Props are set up with RH rotation on right side, LH rotation on left side. Nothing fancy on the lower units like nosecones. Confirmed that a seperate 3/8" dia fuel line is supplied from the tank to the coalesing fuel filter for each motor. From there its the standard fuel line provided with the motor. As far as the mounting hight, I've looked as several Scarab Sports of similar vintage with Stainless Marine brackets, and they all are rigged pretty similar to mine, same props, motors mounted in the same bracket holes. There may be some merit to the dirty carbs suggestion as the previous owner did not use the boat very much over the last 1-2 seasons (like maybe once or twice).