90 horse rude dialed in and still only 5000 rmps

[tvpear12]

ok guys, as you may have read in earlier post,I installed a jack plate on my tournament 1600-TF with my 1992 rude 90 horse and checked my cavitation plate while on plane fully trimmed out,the water is touching the bottom of the plate but not the top. seems as the motor is right in the highth it should be in. she runs 48 mph at 5000 rpms and running a 13x19 prop.now understanding what i read here my rpms should be around 5800 rmps. will a 14x17 prop get me where i need to be or is there more to do. this is with me and my son total of about 350 pounds and 10 gallons of fuel.link and sync has been performed and timing is on the money. engine is rebuilt and has aready been broke in. Thank you guys for the help in advance.

 

[Walker]

Re: 90 horse rude dialed in and still only 5000 rmps

Two thoughts. Are you sure the tach is accurate and I think you could raise the motor 1/2 to 1 more inch.

 

[tvpear12]

Re: 90 horse rude dialed in and still only 5000 rmps

no i am not totally sure the tach is accurate and how can i tell? there is only about 1 inch between the prop and the bottom of the cavitation plate. will raising not cause me to ventilate. ventilate is the reason i put on the jack plate to lower the engine. it used to be like 4 inches above the bottom of the hull and vetilated badly. I think this boat was designed for a 25 inch shaft and mine is only 20 inch. do you think i would gain 800 rpms by raising 1 inch?

 

[Paul Moir]

Re: 90 horse rude dialed in and still only 5000 rmps

Something's off - your 90hp has a LU gear ratio of 2:1 right? At 5000RPMS with a 19" prop, the theoretical maximum speed is 45mph - realisticaly 40-41mph. Make sure the switch on the back of the tach (if present) is set to 6p.<br />Is the speed by GPS?

 

[tvpear12]

Re: 90 horse rude dialed in and still only 5000 rmps

but does the weight of the boat have a bearing on the wot speed? also it did run at 45 mph untill i installed the jack plate, and that is when i achieved the 48 mph. it used to ventilate and now it is biting good. i will check my tach. setting tomorrow. no gps just the transom mounted speedo.

 

[Paul Moir]

Re: 90 horse rude dialed in and still only 5000 rmps

The theoretical maximum speed is with no resistance - ie no boat. Imagine the engine travelling through the water by itself and with no wind resistance. <br /><br />Theoretical Maximum Speed = RPMS x LU_Ratio x Prop_Pitch x 0.000947<br />45MPH = 5000 x (1/2) x 19 x 0.000947<br /><br />Add a boat to it, and it's going to be under that number. The lighter and slicker a boat is, the closer to that number you can actually get.<br /><br />Are you saying you're going by a speedometer that works off a water pickup? If so, they're often inaccurate so that may be where the error is coming from. Best to compare it to a GPS sometime.

 

[tvpear12]

Re: 90 horse rude dialed in and still only 5000 rmps

so you are saying you think i may only be cruising at like 40 mph? ok lets say that is accurate. what to do to get those rpms up? If the tach is set on 6p. and yes speedo is water pick up.