245 dancer repower,ratio/prop.mph HELP

R

rockynsabrina

New Member
Apr 4, 2008
21
lamar,south carolina
Boat Info
87 255 dancer
Engines
350 Chevy Bette engine
Hello,fellow sea ray owners.I own and LOVE my 245 dancer.Its an 83 model but loved like a 2009.I bought her with out an engine or drive.I installed a carburated 305 with an alpha one gen 2 outdrive.The ratio is 1.68.Im running a 15p stainless prop.......Now here goes for the problem....... The beast jumps out the water,planes right out,tachs out at 4700-4800.But she only runs 21mph skint back screaming. if I change the upper gears which direction do I go.I tried a 17p prop and got the same top speed but only turned 4200 rpms.Please give me some advice,what is the top speed of my dancer,what can I expect from her,what do I need to change.Any advice,help,pointers,or assistance would be highly appreciated.....
 
First of all, how are you verifying speed? Second are you running steel or aluminum props. I would leave the gears, as it is a desireable ratio for all around performance in that boat, and fine tune with a good steel prop.
 
Rocky,

Just doing a little math using you WOT, gear ratio and prop pitch (not including prop slip). You come out to about 40 mph. Throw in prop slip and you would get a figure around 35 mph. The number comes out the same using 4200 rpm and a 17" pitch propeller.
 
Some props may have to much surface area to spin with a low tq engine.
I was told,
Merc Stainless props suck for my boat due to this.
I was told by a prop guy the Quicksilver alum props are good for my boat.
They have less blade area:huh:


Talk to a bunch of good prop guys.
Maybe taking some surface area off your 19P prop
Will let it slip a little getting your desired rpm/speed.
I think some prop shops let you test there props before buying also?
 
thanks for the insight,I havnt went to a 19p yet.I am currently running a 15p. 15p@4700rpms =21 mph on gps. the prop is a stainless steel powertech prop 14 1/4 prop diameter. I cant believe it wont run but 21 mph at that rpm with those gears.Yall are awesome. thanks for the replies.......keep em coming,im taking notes
 
Rocky,

Is your boat truly on a "clean" plane at 21 mph. I would think that a 24' dancer would still be running a bit bow high at 21 mph. Furthermore at anything less than wide open throttle, the boat would start falling off plane. Is this the case?

FYI, here's the math

4800 RPM/1.68 GR =2857 p.shaft rpm*15" prop=42,855"/12"=3571'/5280'=.68*60=40.58 mph

Then you guess what your prop slip factor is. Call it 15% and you end up just under 35 mph.
 
imgoing out this weekend,and gonna bolt on the 17 pitch prop.I was unable to perform last time with that prop due to some timing issues.Thanks for the formula,and all the great info,I promise to post the latest results sunday when I return.
 
Rocky,

Also google prop selector and speed. There are websites that will give you give you a theoretical top speed, based on rpm, type of boat, prop pitch and gear ratio.

Good luck.
 
I had the same boat 1981 with a 350 260 hp carbed engine running a 13 p 17 in I could get up on plane ran to 4200 rpm and would top out at 30 mph cruised at 22 mph 3300 rpm just before the 4 barrels open up. ,It is a heavy boat 5500 lbs dry so with you 305 ypur not going to see much higher than 25 mph once you find the right prop. use stainless the boat will run better and faster.
 
I thank you for that.I truely believe im gonne swap the 305 and move to the 5.7. It will get me by this summer like it is but come this winter,in gonna swap em out. I need to clean the bilge anyway,what better time to do it.
 
rockynsabrina said:
I thank you for that.I truely believe im gonne swap the 305 and move to the 5.7. It will get me by this summer like it is but come this winter,in gonna swap em out. I need to clean the bilge anyway,what better time to do it.
Click to expand...

Let me plant this idea. You've got time to consider this and ponder the possibilities...work up an estimate,etc.

A very nice and educated swap for your boat would be a 7.4/Bravo set up. They can be had reasonably priced and if you take your time and shop around you may find a steal on one.

If you really love the boat the big block would make a real sweet package.....just food for thought. :thumbsup:
 
I'm strongly in Sprink's camp on this. I've done this swap in several early '80's 260's. The big block makes a big difference in the boat. The difference between a 305 and 350 is barely perceptible.
 
Another vote for the big block.
From the research I did.

Every 50 ft lbs of tq is good for 2" more of prop pitch.
Thats Approx 400 rpm drop at cruise speed per 2' pitch increase on a 26' boat.

5.0 300 ft/lbs tq
5.7 350 mag 355 tq
6.2 370 tq
383ci 390 tq
454 ci 440 tq
496 ci 509 tq

Do the math on how much more prop pitch you can push with the high torque engines.
A 350mag spinning 3500 rpm at cruise.
The 496ci would be spinning 2300 rpm at the same speed.
Getting the same or better mpg.
 
I have am '83 225CC with the 305 merc. My current prop is 15x17 3 blade alum. and i top out about 35 gps. Your boat is a bit heavier, but should be in the same range. The 350 (260hp) would be nicer...
 
In 83 the SR245DA with 260Merc was rated with 15.25 x 15 c prop. Prop sizing takes into account many things. Props work best under load and once you pass your RPM/HP scale...the prop is just spinning after that.
Thigs to know to help rate a prop are engine HP @ engine RPM, shaft HP, engine torque, prop slippage, gearbox ratio, # of bearing in gear housing, weight displacement of vessel and the surface area of your boat hull at waterline.

The NET has several sites that have programs to help you calculate the prop size. It is not an exact science, but it will get you there close enough and you can fine tune the props once on water.

There's a good program with Shareware. I used it and it came darn close to what I have on my boat. I recently fine tuned the props with help of a prop shop which added a bit of cup to both.

Best of luck to you. Even tho you may repower, you can see how best to run with the engine you already have.

Your boat is heavy and the hull has a tendancy to plow. i know that from ahving an 1985 DA. ghuskin has a point, he is running a 3 blade prop at 17....which would be much different at high speed with a 15 cupped. your advantage to lower pitch prop is that you need the power to get you out of the water and then a cupped prop will help you acheive a better top speed. But again, all factor may not be the same such as ger ratios, etc, etc.
 
Last edited:
ok felllows. I went fishing this weekend and put the 17 pitch stainless prop on.I found a slight timing issue b4 I left to go,so after setting that,Im topping out @ 26mph and cruising @ 3600 @ 20 mph. I take this boat off shore a lot,so speed is somewhat important.I love the small alpha drive vs the bravo.I think im gonna build a 383 with roughly 300hp and 450 ft/lbs of torque.I gotta crunch the numbers and see just whats possible.The 305 just aint cutting it. I thank all of you so very much. I also added thru hull exhaust last week,Sounds bad ass. Just wanted to throw that in....Thanks guys
 
Through hull - sweet! I don't think your alpa one will last long at all with a 383 made to really crank. I would figure the cost of going to bravo with the build cost. Better to have a slow boat than one with a sweet engine and a smoked outdrive! The way I see it you are going to have the cost of the motor and the outdrive no matter what, might as well figure on both and wait to do the upgrade until you can do both at once. That way you don't have to deal with being stranded with a smoked outdrive - and you don't have any down time in your boating season!
 
rockynsabrina said:
ok felllows. I went fishing this weekend and put the 17 pitch stainless prop on.I found a slight timing issue b4 I left to go,so after setting that,Im topping out @ 26mph and cruising @ 3600 @ 20 mph. I take this boat off shore a lot,so speed is somewhat important.I love the small alpha drive vs the bravo.I think im gonna build a 383 with roughly 300hp and 450 ft/lbs of torque.I gotta crunch the numbers and see just whats possible.The 305 just aint cutting it. I thank all of you so very much. I also added thru hull exhaust last week,Sounds bad ass. Just wanted to throw that in....Thanks guys
Click to expand...

Blueprint engines makes a hot 383ci 400hp/450 tq
US engines in Washington has some nice 383ci also

310 hp and 440 tq is about what a mild 454ci puts out
 
rockynsabrina said:
ok felllows. I went fishing this weekend and put the 17 pitch stainless prop on.I found a slight timing issue b4 I left to go,so after setting that,Im topping out @ 26mph and cruising @ 3600 @ 20 mph. I take this boat off shore a lot,so speed is somewhat important.I love the small alpha drive vs the bravo.I think im gonna build a 383 with roughly 300hp and 450 ft/lbs of torque.I gotta crunch the numbers and see just whats possible.The 305 just aint cutting it. I thank all of you so very much. I also added thru hull exhaust last week,Sounds bad ass. Just wanted to throw that in....Thanks guys
Click to expand...

BIG BLOCK.....BRAVO ONE...broader, flatter torque curve. Frig the horsepower...we need torque...low RPM torque. I have built both engines. The closest I ever got to big block torque was with a 406 (.030 over 400 small block) but it was in a light car ('68 Camaro). These boats are heavy...TORQUE...period! The 454 has over 400 ft/lbs fro 1800 rpm to about 4000. You don't get that low end with a 383. A 383 would be a stump puller in a 20'~23' boat...but that ain't what you got.:thumbsup:
 
You must log in or register to reply here.

Forum statistics

Threads
113,176
Messages
1,427,952
Members
61,086
Latest member
MrWebster
Back
Top