Prop sizes

Chris Dauth

Chris Dauth

New Member
Jan 30, 2009
255
Hervey Bay Australia
Boat Info
1992, 270 Weekender
Engines
Mercruiser 383 Stroker
Hi People, I'm very new to this site, and hopefully may be able to get some help here.
I have just completed a full restoration of a 1992 270 Weekender. It appears to be the only one in Australia. There wasn't that much work to do as the boat sat in a shed, unused for 14 years.

She is powered by a 5.7L Bravo and drives thru a Bravo 2 outdrive. At the moment, the prop is a 19"D x 21"P. This gives me only 3200RPM at WOT and a speed of only 22 knots. I have tried props from 15"P to 25"P with no real change in performance.

So, I'm wondering if anyone out there has a 270 Weekender and if so, what is their setup.
Many thanks
Chris, Hervey Bay, Australia.
 
Welcome chris. I have a slightly larger boat with a Bravo II spinning a 18 3/4 X 19. I have a 7.4 and cruise at 3200rpm 23mph top out at 4800 rpm 32 mph. Have you compression checked your motor ? I suppose the hull is good & clean ? gimbal bearings , ujoints, drive itself lubed and in correct working order ? plugs,wires,dist. new ? A 270 is a lot of boat for a 5.7 but something is not right.
 
All the above has been checked during restoration. The motor is in excellent condition and has new plug leads and I fitted a set of iridum plugs. Compression checked out at 145psi with only about 2% variation over the 8 cylinders.

If I had an Alpha or Bravo 1 outdrive, the prop calculators say I should be spinning a 14 3/4 x 19P or 21P. It seems the blade area of the Bravo2 props create too much drag for the 5.7l motor.
It seems there are very limited sizes in the Bravo2 props and as you decrease pitch the diameters and blade areas go up. One solution was to have the 19"P I have, cut down by a prop shop, to emulate a Bravo1 14 1/2 D x 19"P. As I have 3 (useless) Bravo2 props in the shed, maybe the cut down would be worth a try !
In the past (1970's) I had a 25 footer, (and a heavy old boat!). It had the old 225 Mercrusier (Ford Winsor 351), and had the old pre Alpha drive turning a 13"D x 13"P and she would cruise all day at 26 Knots and run 38 knots at 4500rpm.
So, like you say, something is not right.
 
Getting only 3200rpm.
You are trying to push too much Prop. (pitch)
Generally for every 1" of pitch change.
That should change engine rpms by 200 rpm.

Every 1" prop diameter changes rpm by 100 rpm

Less pitch will let engine spin higher rpm with less hp (increase rpm)

A smaller diameter prop will let engine spin higher rpm with less hp. (increase rpm)
A prop guy told me a 26 ft Sundancer works great with a 5.7 Bravo 2. Gear reduction and large diameter prop.
Bravo 2 and 3 They run a lower gear ratio in them drives.
That will increase prop shaft output torque to help spin the big props that get a lot of bite on the water.

Gear ratio with V8 Alpha 1 Is 1.47
Gear ratio's with the bravo 2 and 3. Start a 1.81 then gear down lower.

Thats screws you up on a prop search using Alpha 1 for comparison.
Been there, Done that.:huh:

Your current setup 19"D x 21"pitch 3200rpm wot
A 19D X 15"P Should get you near 4400rpm WOT


Check there's no linkage binding or anything keeping the throttle opening up 100%

Let us know what you find out.
Then it may get scary like someone installed the wrong gear ratio bravo 2 on your boat. :wow:
 
First up, my Bravo 2 drive is 2.2:1 and yes, I have been down the track of reducing pitch. The finest pitch available here in Australia is 15"P but the diameter goes up to 19 3/4 " and the blade area is huge.

The scary thing is that someone installed a Bravo 2 instead of the Bravo 1. Mercury say that the Bravo2 is for engines 400HP and up and used on workboats and House boats. Maybe the 270 next to mine had a 454 (or bigger) inside and they bolted my Bravo1 onto it and I got the Bravo 2.

When I tried the 15"P, the engine rpm went down to 3000 at WOTand I could only get 18 Knots. So, that trick didn't work either.

Everything on the motor has been checked, even the ignition timing is OK, idle = 8degrees BTDC, 3500rpm = 26degrees, right on spec. Even thought I might have some stuck balance weights in the distributor but all is OK there. It has only done about 150 hours. (Thunderbolt IV ignition)

I see that the Alpha and Bravo1 props only have about 1/3 the blade area at the same diameters as the Bravo2. I figure that since I have a 2.2:1 box and the prop shaft speed will be lower than the Alpha, I could spin a 23" or 25" pitch once the blade area is reduced.

Tomorrow, I got the 3 hour drive to my prop man. I'll get the 21" cut down. If it works out, it will be better than replacing the bottom gear case with a Bravo1 !!
 
G-Day Dave,

I have the prop chart, and there in lies the problem. If you look accross the line for my setup, ie. 270 WE-IO5.7L (240HP) bravo2 2.2:1,,, the prop column has N/A. I guess that means " Not Available" .. The 17.5 x 23 (next line below) is for the 7.4L (300HP)..

The closest boat in the list is the 270DA with my engine and gear box and is fitted with a 16.5 x 23. The DA is a lighter and shorter boat than the WE, the 270 WE has a length of 28' 6", getting close to the 280SS which run a 7.4L single or various twins.

So, you can see why I'm going towards the custom prop. Have the 19 x 19 cut down to 16.5 x 19. May be this will put me in the ball park ??? Dropping from 23" P to 19" P may just compensate for the extra weight and length the 270WE has over the 270 DA.

The "N/A" on the prop chart has always had me worried, maybe the boat should never been fitted with a 5.7L motor or maybe it was a special order with a 5.7L with a Alpha or Bravo 1 drive ??? And it could be the reason the boat sat in the shed for 14 years un-used !

Anyway, I'll get this sucker sorted out, even if I have to drop in a new 6.2L MPI !!!
 
Chris

Since the information for your motor does not appear in the prop chart, you may also want to try emailing Sea Ray to see if they could tell you what size prop came on your boat.

Dave
 
I tried getting that from Sea Ray 12 months ago but as the boat was built back in 1992, they couldn't tell me. As the boat only has 150 hours on it, I guessed the prop on it was origional, a 17 3/4 x 25P.

Anyway, yesterday, Monday, I did the 6 hour drive and took my 2 spare props to the prop shop. After lots of calculations, we decided to cut down the 19P to 16 1/2D x 19P with the view to do a second cut if necessary and make it 16D x 19P.

I'll have the prop back on Wednesday, so I'll stick it on the boat and give it a try. I'll let you guys know how it goes. All the calculations show that at 3500RPM I should get 25 knots and if she'll go to 4600 I'll get 33 knots. so far, only math on the computer !
 
Bravo 2 needing 400 HP is nonsense. IMHO
That drive is for houseboats and heavy fishing boats.
Meaning its strong and can push a lot of weight.

My local saltwater prop guy said he has set up a few 5.7 with the Bravo 2.
Thats what he recommends I repower with.
He did say that some props do have too much blade area.
That will cause poor performance.
Even when the Pitch/Dia specs out right.

Sorry about getting all detailed on prop pitches BLAH BLAH BLAH.
You tried a bunch of props and something should have worked.

I knew a guy with a 24' skipjack. 5.7 with a Bravo 3

The boat one day would not tack over 2400rpm one day.
He said he checked everything.
He even replaced the engine. It did not cure the problem.
The Jerk was making money taking Internet fishermen out.
Even with a bad fuel leak. :wow:
The boat caught on fire, Burned a passenger badly.
Nobody has heard from that captain since.
So never found out why his engine would not Rev.


Long shot if the electronic ignition has a faulty rev limiter.
You have a real stumper there.
I just read your other post. It revs to 3500rpm with no load.
My alpha with a 1.47 GR is turning a 16Dx14P Alum prop
I can also turn a 15Dx19P about the same. I get about 3 mph faster cruise turning 3200rpm with the 15x19
Seems what you read on paper and the way they Actually perform is two different animals.
 
Last edited:
well, I have the 17 3/4 D x 19P back, cut down to 16 1/2D x 19P. It has made one big improvement !.
Now, keep in mind the bottom of the boat is Very dirty, marine growth here in the tropics, in summer, is unbelievable. In the next week or so she'll be pulled out of the water and given a cleanup.

So, with all this growth, the hull speed has dropped to about 18 knots with the 17 x 21 prop. This prop did give me 24 knots at 3000rpm (WOT). Now, with the 16 1/2 x 19, I'm still getting the 18 knots at WOT, but now the engine is up to 4200rpm.

Once the bottom has been cleaned, I'll be able to do some more (meaningful) tests. Looks promising so far. I'll get back here again and let you guys know how it works out.
 
Very cool, the problems almost cured. :thumbsup:

I said before a Bravo 2 should not need 400 hp. IMHO

With the huge props that come with the Bravo 2.
You probably due need big block power.
Or get a custom cut prop like your doing.

I spin a 16D x 14P Alum very easy.
Was looking at a 15.75D x 15P Stainless.
Wanted to drop my RPMs 100 or 150 rpms. That should increase cruise speed a couple of kts.

Prop guy said I could not spin that Stainless prop.
It has way too much blade area.
That was on a Quicksilver thunderbolt.

Boats sure get sensitive to performance mods when your not pushing huge power.
 
Chris Dauth said:
All the above has been checked during restoration. The motor is in excellent condition and has new plug leads and I fitted a set of iridum plugs. Compression checked out at 145psi with only about 2% variation over the 8 cylinders.

If I had an Alpha or Bravo 1 outdrive, the prop calculators say I should be spinning a 14 3/4 x 19P or 21P. It seems the blade area of the Bravo2 props create too much drag for the 5.7l motor.
It seems there are very limited sizes in the Bravo2 props and as you decrease pitch the diameters and blade areas go up. One solution was to have the 19"P I have, cut down by a prop shop, to emulate a Bravo1 14 1/2 D x 19"P. As I have 3 (useless) Bravo2 props in the shed, maybe the cut down would be worth a try !
In the past (1970's) I had a 25 footer, (and a heavy old boat!). It had the old 225 Mercrusier (Ford Winsor 351), and had the old pre Alpha drive turning a 13"D x 13"P and she would cruise all day at 26 Knots and run 38 knots at 4500rpm.
So, like you say, something is not right.
Click to expand...

Stay away from Iridium plugs, just regular NGK's will give you better performance. Did anyone checked the Ignition timing at idle and 3000 RPM's?
 
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