Boat Drops Off Plane, Change Prop?

[POPS PLAYPEN]

I Have A 1992 Searay Sundancer 270, 7.4 Engine 330hp Boat Has 410 Hours On It. Still Looks And Runs Like New. Very Well Maintained. The Problem Is I Have To Run At 3000 Rpms To Keep The Boat On Plane. 2900 Starts To Fall Back. Is This The Correct Plane Rpms?boat Has Been In The Water For 16 Yrs Except To Come Out For Maintanence Then Right Back In Water Which Is Fresh By The Way. I Pulled The Boat Out This Weekend For Service And The Prop Was Corroded On The Outer Edges, To The Point Some Is Missing And Very Thin. It Is A Three Blade Aluminum (i Guess Original). Don't Know Pitch Or Diameter. I Am Thinking Of Changing To A 4 Or 5 Blade Ss Prop. Will This Aid In Keeping The Boat On Plane At Lower Rpms? And If So What Would Be A Good Pitch To Go With? The Vessel Does Have Trim Tabs. They Don't Seem To Help With This Problem. Any Advise Would Be Appreciated. I Have Every Owners Manual And Documents That Came New With This Boat And I Can't Find Anywhere What The Recommended Prop Is. Thanks

 

[oops!]

Re: Boat Drops Off Plane, Change Prop?

hey pop.....two things


1 change prop.....the choices are many .....

if that doesnt work....do a compression test... if all is good....

2 weigh boat....yup.....it might be a hassel... bit youll know if its gettin waterlogged

 

[POPS PLAYPEN]

Re: Boat Drops Off Plane, Change Prop?

i measured prop today, it is 17" still can't find pitch though.

 

[oops!]

Re: Boat Drops Off Plane, Change Prop?

17 is more likley the pitch

 

[oops!]

Re: Boat Drops Off Plane, Change Prop?

the pitch is the distance travled with one revolution of the prop.....


ie 17 inch pitch= 17 ins travel....(therotical)

 

[oops!]

Re: Boat Drops Off Plane, Change Prop?

it will be stamped on the hub.....17x13....ect

 

[magster65]

Re: Boat Drops Off Plane, Change Prop?

Any corrosion on a prop is too much.
Yes, the numbers would be cast into the back of the hub but it might be corroded away by the sounds of it. My guess is your boat should see around 38 mph @ wot so that calculates out at... 1.5:1 ratio drive, 10 % slip, 4600 rpm... I'd try a 15" pitch first. You need to rev at least 4600 rpm @ wot.
I'd try a 4 blade if you can. These are known as stern lift props and will help keep the boat level more than a 3 blade at lower speeds. 5 blade props are best for ski-boats where acceleration is important.
Keep us posted!

 

[Locke]

Re: Boat Drops Off Plane, Change Prop?

I have a 2000 270 Sundancer also with a 7.4L and a Bravo 3. I usually run it at 3800 to 4000rpm when cruising, what's "normal" or recommended for this engine?

Locke

 

[POPS PLAYPEN]

Re: Boat Drops Off Plane, Change Prop?

I Found Out Pitch From A Local Prop Shop. It Is A 17x23. For The Time Being I Bought A 18x21 Aluminum 3 Blade Brand New For A Good Price They Had Ordered For Someone That Didn't Pick It Up. Haven't Run It Yet, Will Let Everyone Know How It Does This Weekend. Thanks For All The Help.

 

[jaxnjil]

Re: Boat Drops Off Plane, Change Prop?

i have a 454 in my boat that needs proped but the prop that is on there has been repitched and dont know what it is so have to start from square 1
so far i have a 15"x17pitch 3 blade, a 14"x18 pitch 4 blade, and a 15"x19 pitch 3 blade.
any way long story short MAKE SURE YOUR PROP IS RATED FOR 300 HP. i have found in looking about half the less expensive props have a max hp rating of 250 hp. black max is just one example and i ahve talked to 2 differnt dealers and a few other boaters that don't realize this.

 

[magster65]

Re: Boat Drops Off Plane, Change Prop?

I don't know of any prop service that will guarantee the results of more than a 1" pitch alteration so... whatever it was... is close to what it is and your prop guy can measure the pitch (within reason) anyways.

 

[magster65]

Re: Boat Drops Off Plane, Change Prop?

I have a 2000 270 Sundancer also with a 7.4L and a Bravo 3. I usually run it at 3800 to 4000rpm when cruising, what's "normal" or recommended for this engine?

Locke

That's fine as long as you can reach at least 4500 rpm at full throttle. If you can, that indicates that you're not lugging the engine. If you can't reach the rpm... you are lugging and it won't last as long.
Too much pitch stresses things beyond their design limit and the oil will get hot... not good.