Propeller reccomendations for 1988' 268DA Sundancer 7.4L alpha outdrive.

[pessemist]

Hello all.

Have a question. I have a 1988' 268DA Sundancer 7.4L with alpha outdrive.

Presently it has a stainless prop which I think is a bit over-sized. Can't tell what exact prop size it because she floats low in muddy water. lol

I finally got some ignition problems fixed and a bad alternator replaced so now I am finally able to take her out and open her up. The motor was rebuilt and has less than 100 hours on her. Carb also rebuilt. So I am assuming (for now) she is running good.

Trouble is the engine is only turning 6,800rpm's at WOT. She is making, at best mid 20mph speed at top end and this after tweeking the trim tabs and outdrive trim

I should be turning closer to 7,400rpms at WOT.

I should note that I have not run at WOT since replacing the alternator. The guy at the boat store told me I could safely go up to 65amp from orig. 55amp alt without melting anything. But I ran with abtteries charged so I don't expect that to be the problem?

I need a spare prop because I cruise the notoriously shallow and obstacle ridden Maumee river in Toledo, OH.

I know what prop size came with the boat but doesn't say if it was in stainless or aluminum?

Can you guys with this model Sundancer and motor combo tell me what prop and material (stainless or aluminum?) works best for you? Also what rpm/top speed are you getting with WOT?

Thanks in advance for any help. I love this forum.!

Doug

 

[KC_Sundancer]

Trouble is the engine is only turning 6,800rpm's at WOT. She is making, at best mid 20mph speed at top end and this after tweeking the trim tabs and outdrive trim

I should be turning closer to 7,400rpms at WOT.

Doug,

I'm not sure what kind of work was done on the engine rebuild but your rpm's seem way too high. The prop pitch is probably way too low which would explain the low speeds and the engine is not working hard enough. Normal operating rpm ranges for 7.4 L's are 4,200 to 4,600 for the base model and 4,600 to 5,000 for the MAG models. I'm sure someone will chime in with good recommendations. I'm thinking try a 23" pitch for starters :huh:

 

[pessemist]

Doug,

I'm not sure what kind of work was done on the engine rebuild but your rpm's seem way too high. The prop pitch is probably way too low which would explain the low speeds and the engine is not working hard enough. Normal operating rpm ranges for 7.4 L's are 4,200 to 4,600 for the base model and 4,600 to 5,000 for the MAG models. I'm sure someone will chime in with good recommendations. I'm thinking try a 23" pitch for starters :huh:

Sorry, had a brain fart. I meant RPM's top out at 3,800rpm while the engine is rated at 4,500rpms. This is the base model 7.4 liter Mercruiser.

I'm not sure what you mean by '23" pitch"? The SEARAY site lists a 15.25"x15" for this boat but not material it made out of and I know that aluminum props differ from stainless in amount they flex so not sure which to go with. I think she has a stainless 15.25"x17" on her now.. hard to read with crap on hub and under murky water.

 

[KC_Sundancer]

Sorry, had a brain fart. I meant RPM's top out at 3,800rpm while the engine is rated at 4,500rpms. This is the base model 7.4 liter Mercruiser.

I'm not sure what you mean by '23" pitch"? The SEARAY site lists a 15.25"x15" for this boat but not material it made out of and I know that aluminum props differ from stainless in amount they flex so not sure which to go with. I think she has a stainless 15.25"x17" on her now.. hard to read with crap on hub and under murky water.

Whew - that sounds more realistic. If you don't already know the second number is pitch, first diameter. Based on your numbers the 15" would get you closer to what you should be running at with rpm's. Material makes a difference but usually only within a few hundred rpm's.

There is a stainless prop manufacturer on your side of town we used when racing, (Hydromotive, Twinsburg, OH) Might be worth talking to. They will tweek their prop for you too if needed to fine tune the rpms. Just a thought :huh:

Maybe Getaway will chime in, he has a similar set-up. Motors hopped up a bit but he can probably get you close numbers.

 

[Willie V]

I have a 250DA with 5.7 l I went to a 15x17 prop, it raised my rpm up to 4200 and 33 WOT on calm day 30-31 in heavy chop. Was running 19p and only got 26-27 @3800rpm

 

[pessemist]

Thanks. I just think I should be going significantly faster and have higher RPM's at WOT then I see currently.

I am going to try a run with the new alternator tonight and see if any difference.

I also need to pick up a spare prop. Any suggestions besides what Searay lists as standard? 15.25x15

Thanks!

doug

 

[Willie V]

going to buy 1 of those and compare to 17p whatever i like the least will be my spare

 

[BonBini]

Hey Doug: We’ve got similar boats so I would expect your performance should be close to mine.

Last year I ran a 15x15 early in the season. Hit 42mph @ close to 5000rpm with light load on cold water.

Running a 15x17 this year; can hit 40mph @ 4500ish normal load. This last weekend loaded with 6 adults, 2 big kids, all there stuff and ½ tank gas, hit 38mph @ 4400rpm. Few weeks back went on a long cruise with bigger boats. Just us two on board, normal load, full fuel; 22mph @ 2800rpm… 30mph @ 3200rpm.

I think in your case: First you need to know your upper gear ratio for sure. Should be 1.32:1.. That means for every 1.32 turns of your engine, your prop will turn 1. Then the stock props and the 7.4’s 330 hp should move your boat as expected… Second is your bottom. Needs to be clean. My boat had a fouled bottom when I first got it and I had very disappointing performance just like you have now.

fwiw: take it easy on that lil Alpha drive. If your trying to power through a dirty bottom with that 4 barrel open for very long @ 3800! That 7.4 makes a lot of torque right there!!

 

[pessemist]

Thanks Ron-

The bottom had a little rough paint but was smooth. Apparently a bad alternator makes some difference although I am not getting your performance.

I am turning a stainless 15.25x17 prop now. Not sure of gear ratio as boat is at the dock in a very muddy river.

Yesterday with full fuel and 2 people I cruised at 24-25mph at 3500rpms. At WOT I got only 3900rpms and hit 29-30mph.

The bottom is painted and could use a touch-up but definitely clean & not fouled. Alpha drive has cooling shower installed.

I am thinking something is wrong. Will check outdrive ratio as soon as possible.

thanks

doug

 

[pessemist]

Poking around the boat at the dock at lunch I noticed the ignition spark-plug coil (ground side) wire was real loose. Tried to tighten it up and noted the nut was stripped although the coil looked brand new. I am getting a new coil later tonight and swapping it out to see if that helps.

The motor sounds to me like it is purring along. Yet I am not getting full rpm's from the engine. I know some of you are using the 15.25x17" prop and getting closer to max rpm (4,500) and much higher speeds (high 30's as opposed to 29-30mph for my setup)

Going to make a concerted effort to figure out the problem this weekend. Any suggestions on things to check? Could a not-snugged-down-tight coil wire affect things? I noted a dead alternator swap perked things up a bit?

The Alpha outdrive is almost new and the guy was pretty motor savy so I doubt he put an overdrive in with the wrong gear ratio...but I'll check by rotating the engine and looking for a 1.5:1 rotation.

Supposedly the carb has been rebuilt along with the engine 100 hours ago. She starts and idles like a new engine should.

Any suggestions on things to check?

Thanks in advance for any help in this matter.

doug