changed prop. lost RPM

[boatman37]

well banged up my prop when dropping the boat in the water. the PO gave me 2 extras when i bought it. according to him all were the same. i believe it is a 15x17. its a 1986 Sea Ray 250 Sundancer 5.7 260hp. the old prop was in bad shape. it took a long time to plain but i could hit 30mph and about 4600 rpm. but i could not hold plain if i backed off. changed the prop today and plains very quickly and holds it at about 20 mph and 3000 rpm. but i can only hit 3800 rpm at WOT. there were only 2 of us on the boat and about 40 or so gallons of fuel. i am a rookie at the trim tabs and the outdrive trim so i'm sure that could have been beeter.

so is a 15x17 reasonable for this boat? bottom is clean. i will be checking next weekend to make sure i'm actually getting full throttle. i also had trouble keeping the boat 'upright'. it kept wanting to stay leaning whichever way i last turned. had to mess with the tabs to level it off. i definitely need lessons on o/d trim and tab trimming

 

[SkiDad]

look at the markings old and new prop. the new prop is holding the water better so it's dropping the rpm - you didn't state the new speed

i would be looking at getting a 15x15 prop if i was you or getting your engine tuned up some - Looks like your boat is about 5000 lbs and that's unfamiliar territory for me to guesstimate on

 

[boatman37]

according to the PO its about 6500 lbs. loaded, whatever 'loaded' means. the trailer was an additional 1500lbs. but the new top speed was about 33.5 mph going downstream. according the the original paperwork i got with the boat the top speed should be about 31-32 mph. the engine does need a tune-up so i may be jumping the gun here but with the old prop i easily hit 4600 rpm. but there may have been cavitation, etc due to the prop being chewed up. it seems normal cruise speed is good with this prop, as well as 'holeshot' takeoff. just seems the rpm isn't there. maybe a tuneup or getting the trim setting right might help. FWIW-there was alot of traffic on the water and quite a bit of wake/chop. not sure if that will hurt the rpm or not?

 

[jacrabbitt]

Check the bilge for water. what looks like maybe a little when it's at rest, might be quite a bit when underway and all the water through the 25 ft of boat rushes to the back. How much water you carrying in the holding tank or tanks? 6500 lbs is a lot of beef for a 350 chevy w/ alfa IO to push around. so the prop has to be in good working order. I agree with skidad. a 15x15P or a 15 X 16 stainless would be a better choice. Also a little chop on the water will make the boat run a little cleaner

 

[boatman37]

bilges are pretty low. checked the mid bilge yesterday. i pumped it off until it was too low to pick up anymore (maybe 1/2" left). the rear bilge has maybe 1/2 gallon in it. i only have about 10 gallons in my fresh water tank. i'm thinking (hoping) that most of it has to do with the trim not being optimal due to inexperience?

 

[Outsider]

but there may have been cavitation, etc due to the prop being chewed up.

So, basically you're at square one. First, tune up the engine, find out what the WOT range should be. Second, find out how the trim and tabs work, make sure they are 'in and down', you have to have a starting point. After determining what the prop specs are (guessing may back you up two steps), put it on and run it flat out. If the RPM is within range, you have a starting point. If not, you can then determine where to move the scale. In a perfect world, hole shot is great, it will plane at any RPM out of idle, and WOT is in maximum range, but few boating worlds are perfect. Start with the RPM range, when you get there, trim and tab come into play to improve low speed operation, and that's a 'try something and see what it does' affair. If that still doesn't do it, you have an over-weight or underpowered boat (or both). Propping correctly isn't rocket science, but there is method to the madness unless money is no object ...

 

[boatman37]

money is certainly an object...lol. part of my thinking is this is the same prop the PO ran on this boat (he bought the boat new 30 years ago) and according to him it was fine. but you are right, i have some work to do before trying to figure that out

 

[jacrabbitt]

86 model....That Quadra Jet operating correctly? 4600 seems a little weak. 48 to 52 but then again your pushin a lot of weight... 350 has to spin and it's 30. And you don't want it to come unglued. I'd make sure my 4 barrel is operating correctly. which I'm sure you have done. and check timing.When you mash it . it should spin right up if all your ducks are in a row. I had a wire that was grounding to the block intermittently. Check your wires.

 

[boatman37]

i will check all that but according to the manual i think it was 4600 or so max