prop/rpm/mph on a da250

Atis

Atis

Member
Aug 6, 2017
37
New Bern, NC
Boat Info
1987 Sea Ray Sundancer 250, Garmin Chirp GPS
Engines
260 Mercruiser w/Alpha I
so I finally had a nice day to get her out and run her after the throttle cable failed on my maiden. I think the previous owner switched props but put on the wrong prop.

I am spinning 4400rpms at 20mph (gps) and thats a little shy of full throttle. I am running 3300 rpm at 10mph. i never made it completely on plane as I read the max rpm on the engine is suppose to be 4200rpm.

so does that sound like a prop issue and if so any suggestions on prop size/manufacturer? At least half my cruising will be at around 10mph as we have alot of shallow water here in the rivers as we head out to the intercoastals.
 
forgot to add....v8..260hp with alpha 1 drive
 
Yes....and no. You could have a prop issue but before you go there, what is the condition of the bottom? Is it clean? How is the health of the engine? Tuned; good fuel; fuel filter clean or better, new; does the throttle open all the way; is there extra crud in the boat adding unnecessary weight? When you've addressed these issues then take it for the ride and take it up to wide open throttle briefly to see what it does. Your boat's listed dry weight is 4690 lbs. that's a bit of a load for just 260 ponies but you should be able to plane. You are looking for 4200-4600 RPMs at wide open throttle and you should be able to plane at 3000 RPMs.

From what you are describing it sounds as if you are under-pitched. Find a local reputable prop shop and get to know them well.

Did you recently buy this boat? Did you have it surveyed? These are items that should have come up during survey.

Let us know how things turn out.

Shawn
 
i bought the boat about 2 months ago...trailered her home and cleaned her. Bottom is clean. Engine runs smooth, fuel filter is new. Trim tabs work and tab pump replaced. boat had 4 adults and a 4yr old on board....life vests, and a small cooler so weight wasnt an issue. Also, 3/4 full fuel amd water/grey water empty.

my issue is the 10mph at 3200rpms. i should be at 22 to 25mph and on plane at those rpms.
 
i did get on plane at 20mph...but at 4300rpms.
 
I have a 14.5x17 3 blade aluminum on mine and max at 4000RPM and about 33-34MPH. The max RPM range for our boats is 4200-4600. I am thinking of trying a 15x15. The manual says max speed should be about 32MPH on this boat with this engine. You may have a spun prop hub. My boat new should have came with a 16x16 according to my manual.

One other thing to check if all else fails...is it the right outdrive? Our boats should have the 1.5 ratio drives. Is your tach accurate? Speedometer accurate?

I can plane with 4 adults pretty easily. I typically cruise at about 3300RPM at about 27MPH and can stay on plane down to about 24MPH I think about 2600RPM but can't remember)
 
Did you happen to run it across scales when it was on the trailer?
Have you confirmed your tabs work and are being used correctly?
Our boat is a 245 but usually runs around close to 7000 lbs loaded on the water. It has no problems getting into the low 40's. With the old leg it was closer to 45 at 4450 with the 16x16. It did take a little bit to get her up with that weight and prop though.
If you can confirm your outdrive ratio (hasn't been changed) and go on the Mercruiser site, they have a prop selector that has been bang on whenever I have used it. You should be able to get some numbers off of your prop to see what you have for comparison.
 
5.7 260 merc should be 4400-4800 WOT my remembrance.

What size is your prop if you can see it?
 
I tried to confirm prop when I had her on trailer but couldnt locate numbers. I didnt run her across the scales so weight is unknown. MPH was based off the Garmin GPS so it should be accurate.

How do I confirm my outdrive ratio?
 
The gear ratios in the lowers in the alpha 1 gen 1's are the same. The upper gear ratios change with engine size. If someone has replaced it or it's gears from a smaller engine like a 5.0 liter, the ratio will be greater. You will get the feel of being under propped. As mentioned the 260 has a 1.5:1 ratio. Meaning the engine turns 1.5 times for every rotation of the prop. The 5 liter is close behind at 1.65:1 so it will be a bit tough to see the difference. A quick way to check is to get an assistant to hold a slight resistance to the prop in gear and mark the positions of the prop and crankshaft pulley with a paint pen. Bar the engine in forward direction and see how far you have to go to make one full revolution of the prop. If your results are questionable, use multiple prop rotations and divide that number into your engine rotations.
Any damage to the prop can give the same sensation. Before replacing it though you should confirm the ratio. Part of the joys of buying used boats.
Have you confirmed your leg and trim tabs are functioning? Should be tucked in and all down taking off. When it is climbing on top, bump the drive out a few times and then then raise the tabs slightly as required. While doing that you will need to ease the throttle back.
Hope this all makes sense and helps.
 
trim tabs work. i juat replaced the HPU and tested them before going into the water. motor trim works as well.

i keep focusing on the prop because the guy had 4 boats...so if he damaged the prop he probably would have grabbed an old one from another boat. Its SS so I know its not original as I believe stock was alumn. He didnt take it out very far or for long runs so getting on plane wasnt a concern .
 
The dimensions for the prop should be on the hub of the prop. As I said earlier....find a good prop shop and make friends. If there are no numbers on your prop the shop can usually scan it and tell you what you've got. One last thing, on the back of your tach (see below) is a dial that looks like a screw head. This is the selector to set the tach for different engines such as 4, 6 or 8 cylinder. Yours should be marked as such. This is an old tach I had on a bench. Make sure yours is set for "8 cyl". And while your back there, take a small screwdriver and turn that dial back and forth a few times as this will clean the contacts on that switch which can cause problems as well.
Tach -back.JPG


Keep digging away at this and you'll get there. This is a process of elimination.
 
sadly, no prop shop nesr me....mostly sail boats and a ton of bass boats but there is a good overall boat repair shop I will stop in and talk to. thanks for the tip on the tach. I will try and get to thr back of her and check.
 
if you can find a 14.5x17 to borrow then try it out and compare to my numbers. not that mine is spot on but it is pretty close. and i run with my tabs full up except to get the boat level. when it is just the 2 of us i don't even put them down to get on plane.
 
that would be nice. i had to use tabs to get on plane and then brought them up a little amd did a little motor trim to stay level amd drop rpms. I know the book calls for a 15.25 x 15 prop but dang...try and find one. I would even settle for a 15x16.
 
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