1990 SCR-27, Merc 7.4l Bravo, prop and drive questions

[ImRich]

Hi all!

I'm new here today. I have an older boat, 1990, I just purchased, it's going to be a slow restoration project. I haven't figured out how much will be restored yet.

So here's the first question: How do I tell if I have a Bravo one or Bravo two drive? When I check other online sites, they show pictures, but they all look the same to me! My serial number also matches the range for both the Bravo 1 or 2 (I know it's not a Bravo 3 as I only have one prop).

The engine is a 1990 vintage 7.4L Mercruiser. The labels on the carb cover simply says "Mercruiser 7.5L Bravo".

I seem to only get to about 4,000 RPM at WOT. At this point I don't know if I have weak compression (I have to get it checked), need a tune up (engine seems to run smoothly and sounds good), or just too much prop. The boat wouldn't go on plane at WOT. I tried the trim tabs up/down/all around. The boat only got to about 11 MPH!! I backed off the throttle to about 1800 or so and the boat was still at about 10 MPH.

At this point I'm questioning the three bladed aluminum prop as it was reconditioned twice, last time in 2011. I called the reconditioning company as it had one of their serial numbers on it, and they told me the prop was an 18x24. Wow, this doesn't match any of the data I can find online. It seems most online sites recommend about a 14.5 x 24 prop for this engine/drive combination.

So obviously I need to measure the diameter to be sure what's on the boat before I try another pitch. The reconditioning company listened to my speed/plane issues and said I should go to a 19 inch pitch, since my current prop is at 24 pitch, as my engine may have lower compression due to it's age and use.

Before I try to buy another prop, I will check the diameter, but here's my second question for all of you: If you have the same boat/engine, what prop are you using (diameter/pitch)? Have you had to go to a lower pitch prop after many years of use of your engine?

If I do actually have an 18 inch diameter prop, perhaps I should try a 14.5 in a 24 pitch, or should I try something in between in pitch (perhaps a 14.5 x 21)? How the heck do you find the right prop without buying more than one?

I know that this is all variable depending on the engine output, weight of the boat, etc., but if everyone is running about a 14.5 diameter, then a 18 diameter is certainly too big.

This is enough typing for my first post here! Thanks in advance for any help or advise!

[shrew]

Hi Rich. Welcome aboard. It's nice to see another Granite Stater on the forum. something certainly doesn't sound right here. 4K from a 7.4 isn't horrible, however not exceeding 11mph @ 4K is WAY off. Some things to think about; (bottom condition, weight (including tankage (gas, freshwater, blackwater)), conditions. However, unless you had full gas, water, holding tank and your company softball team on the boat WITH the beer keg, then something is not right.

Call Mercruiser Customer Support/Technical Support with your serial number and they will be able to tell you exactly what model and gear ratio your drive is.

I'm not sure about the BI & BII drives. BIII don't have a prop hub, however what you're describing sounds like a spun propr hub. Someone here may be able to clarify whether that model has one. Even Merc. customer service should be able to answer that question. If it does have a propr hub, I would have it looked at.

[shrew]

Yes, the profile says Ct, however that is where the boat is located. I live in NH, and have my entire life. Where do you plan on boating? Portsmouth or Winni?

[ImRich]

Thanks for the advice. I'm up at Winni.

So is a 18 inch diameter prop correct for this engine/hull combination? Most of the online calculators list about a 14.5 inch diameter.

[ImRich]

Thanks for the advice to call Mercruiser. I was on the phone with them for a while and they don't think it's the prop hub. He said if the hub was spinning, the RPMs would probably be higher as there would not be much of a load on the engine as the prop was spinning.

He suggests to check the carb to see that the secondary is opening, check fuel filters, etc.

He also confirmed that I have a Bravo-2 with a 2.00:1 gear ratio. So an 18 inch diameter prop may be acceptable.

[ImRich]

Intersting turn of events, I spoke to the dealer and he said that they were out testing the boat today (the trim tab was leaking oil which they had to repair) and they had the boat up to 35 MPH, and it went on plane with no problem. He didn't know what caused the increase in performance.

If this is the case, I'm back to dealing with other smaller things that need maintenance and updates.

[ImRich]

I finally picked up the boat. After a long saga (isn't this always the way?), the dealer found that the oil cooler was leaking (after that first test ride, we found a small oil sheen behind the boat). During the long (expensive = hours) search for the oil leak, in addition to replacing the oil cooler (which was definately leaking, luckily we cought it before it was a major leak) they ended up rebuilding the carb (said it was dumping a lot of raw fuel into the intake), and found that the ignition timing was off a couple of degrees.

The boat now easily gets on plane and at WOT the RPMs are 4900 or so. The speed is good (I forget if we got 35 MPH, or closer to 42 as I was simply excited that it was working so well).

So the answer to my own question is that the 18 x 23 prop is just perfect for the 1990 7.4L Bravo engine with the Bravo 2 sterndrive that has a 2.00:1 gear ratio.

[ss3964spd]

Gald the boat is running better, Rich. You might want to double check the MAX RPM at WOT though - I don't think the 7.4's are supposed to be at 4900 at WOT.

Dan

[ImRich]

I'll double check it, thanks for the heads up.

I had called Mercruiser and they thought that 4900 was about a good target, but looking at the manual that I found, it looks like WOT should be in the 4200-4600 range.

I'm just estatic that I'm no longer plowing at 11 MPH at WOT! So now it's time to pay attention to all the details!

[ss3964spd]

4200-4600 sounds much better Rich. If you get the right prop on it you will be able to cruise at the same speed with less RPM.

Those big blocks (7.4/454) do not like to spin that fast unless really built for it (mostly the valve springs need up-grading) so be sure you don't run it at 4900. I'd say 4500 max.

Dan

[ImRich]

I confirmed I'm running at about the right RPM. I thought the red-line on the tach was 5,000. But when I looked again, I realised it was more like 4600 (or maybe it was 4700?), but certainly not the 5,000 that I thought it was. Thanks again for the heads up on this.