high rpm's low speed

[dhoptay]

anyone have any suggestions as to why i have high rpm's with low speed. running merc 120 i/o with 17" 4 blade prop with 15 pitch gets me about 4500 at WOT. was running fine but was out last night and hit something in the water not hard but from then on felt like i was dragging something. temp didn't increase and when i got out of the water didn't see any damage to prop. haven't gotten back in yet but just looking for suggestions. also pulled prop and checked hub nothing appears damaged. thanks for any help

 

[Don S]

Re: high rpm's low speed

So, what's the high rpm and low speed numbers you mentioned in the title.<br />Nothing wrong with 4500 WOT. That's about as good as it gets for a 120.

 

[Reel Poor]

Re: high rpm's low speed

Be sure to check drive bell housing and gimble ring for damage.

 

[dhoptay]

Re: high rpm's low speed

only about 30mph. didn't see any apparent damage to either housing or ring

 

[waterone1@aol.com]

Re: high rpm's low speed

Run the drive up and down a few times. Put a stripe across the prop and hub (with fingernail polish or permanent marker). Put boat back in water and run. If runs well, your drive may have been "pushed up" and now all is well. If still reving and not hitting top speed, try to notice if you feel any new vibrations. Pull boat, check line on prop hub, if it is not straight, your hub is slipping. If any vibration, or still not hitting top speed, have prop checked, you could have one or more slightly bent blades.