Low speed

[Scott Danforth]

Also, check the whole list of Other low speed items in the stickies

 

[dingbat]

Dingbat this is why I'm on here I have no clue why I'm running so slow heavy boat is best reason

Heavy boat theory makes no sense to me. Something else is wrong.

Looking into it a bit more it would appear that your boat is rated to 200 hp.

Certainly would explain the lack of performance but raises the issue of how are you turning a 17” pitch prop that close to WOT that under powered?

Suggest checking the LU gear ratio as noted above and double check your prop size. Things just don’t add up

 

[procraftguy]

Not sure how too check gear ratio listed as 2.07 is there a number on the lower unit too look for to check ? Thanks for all the help as for water logged theory I opened all inspection hatches it looked dry no sign of any water or rot issues this lower unit idea has me wondering.... I did double check prop it is stamped clearly

 

[WesNewell]

You should be seeing 30-35 mph with that setup. If not then there's one or more problems. Trim your engine down to where the AV plate is in line with hull bottom and take a picture level with the AV plate and post it. An engine mounted too low on transom can cause up to 20-25% loss of speed and fuel efficiency due to increased drag of the lower unit.

 

[jimmbo]

Not sure how too check gear ratio listed as 2.07 is there a number on the lower unit too look for to check ?

Take the cowl off and unhook the spark plug wires. Put the engine in gear, rotate the engine, clockwise when viewed from above, until a prop blade is under the anti-ventilation, mark that blade. Also mark the flywheel. Continue to rotate the engine until the marked blade is again in the same spot. If the Flywheel rotated a bit more than 2 turns, you have a 2.07:1 ratio, if 2 and 1/3 turns, you have a 2.3:1.

115 hp on a 21 ft boat should make low 30s, However with the 2.07:1 gear ratio, a 17 is going to be a lot of pitch. as you are at the bottom of the WOT range for your engine, dropping to a 15" should bring the RPMs up to about 5150, which is much closer to to upper limit.

I feel for a 17 on that setup, the rpms are too high, the tach may be not set correctly, or your engine is mounted too high and you are getting some ventilation which is unloading the engine while also resulting in high slippage