Mercruiser Bravo 2 vs. Bravo 3

I'm looking for some advice/input regarding a potential project. I'm looking for some data on which Bravo drive would provide the best lift and low (on plane) performance. The project is to covert a twin gas engine 2002 Maxum 2900 SCR express cruiser to a single gas engine. Currently the boat has twin 190hp carbed 4.3l Merc's coupled to counter rotating Bravo 2 drives. The idea is to swap the twins out for a single V8. This boat was available with a single 6.2L 320HP Merc with a Bravo 3. While this combo provides a decent top end, I'm looking for the best combination to keep the boat on plane at the lowest possible hull speed. As I have 2 good Bravo 2 drives, I like to be able to use one of them. However, I hear that the Bravo 3 drives provide a good amount of lift due to the dual props. Anyone with any experience with comparison, please assist.

 

Can't help you with the lift generated by the drives themselves, sorry... but would suggest that a pair of large trim tabs will probably have more effect. The only word of caution being that if they are big and you add too much tab you can induce dynamic instability

 

What do you expect to achieve with this costly project?
If your goal is a lower planing speed you should install trim tabs like Willallison suggested.
If the operation is meant to get better fuel economy, first calculate how much fuel you can buy for the investment, or how many service hours are needed if the improvement in efficiency is 5%.

 

The goal is increased fuel economy, and to decrease the planing speed. The 2 V6 motor & drive packages increase the weight in the transom by 800 lbs when compared to the single small block v8. the horse power is 380 combined. The single V8 is 320 HP, I'd say to give up 60 HP but to loose 800 lbs would be a fair trade off. As far as cost is concerned, I'm pretty sure that I can recouperate the majority of the cost by selling my 2 v6 motors with drives. They're in great running condition, the boat is just a bit of a pig in the cruising department. The boat has trim tabs; I don't know the exact size of them at the moment, Do you think installing a pair or larger performance boat sized tabs would add the additional lift to get her big rear up over the swell & on plane at a lower speed?

 

Sure you save some weight near the transom but I can only guess what that may mean re the fuel bill. The prop slip may turn out to be larger than it is at the present and the difference in planing threshold will hardly be noticeable. Only getting there will be somewhat easier.

Planing speed is a mathematical function of surface area. Large trim tabs, adjusted for minimum planing speed are a much more cost effective solution.

 

That's why I was wondering which drive would be the most effective for this application. I have had experience with a slightly smaller express cruiser (26ft) with a single v8 coupled to a Bravo 2 and the results were impressive. i.e. planing at 2600 & maintaining that with ease. As this 29 is both heavier and wider; I am curious as to if the Bravo 2 would perform similarly. The 2 v6 motors have to run upwards of 3200 RPM to keep this boat on plane. I just have to think that a propperly propped & geared V8 with it's increased tourqe would be ably to keep her on plane at 2800 RPM or so. If I remember correctly the boat burns around 17-18 GPH at 3200 RPM now. I'd estimate that the single V8 turning even 3000 would burn less than that 10-12 maybe better.

 

CDK is correct in saying that by far the cheapest solution would be to install larger tabs. Depending on the brand, you can usually simply buy larger "plates" and use all of the rest of the existing system. You'd need to check with the manufacturuer of course...
You are looking for max surface area... so performance tabs, which tend to be long and skinny are not the best option... better to simply go for the largest plates you can fit on the transom.
Planing speed is actually a function of many things, but weight is definitely a part of the equation. How about start by pulling all the crap that you have on board off (no offense intended... we all fill our boats with crap!). You will then be able to see what sort of a difference making the boat lighter will make.

 

Great thread similar question

Ok, I am looking at purchasing our first boat, it is a 2000 BAYLINER 3055 CIERA LX with twin Merc 5.0L engines and already has Bravo 2 Outdrives. My question is simple, would I gain any efficiency or performance by upgrading to Bravo 3 outdrives? Is there any reason to do so?

 

It really depends on what your ultimate goal is???? That boat is very similar to the 2900 Maxum in hull design and performance. The 3055 is a bit bigger hence the 5.0's as opposed to the 4.3's. My experience has been very favorable with the Bravo 2's. They spin a bigger prop in diameter which resulted in being able to hold plane at a lower speed. I opted away from the Bravo 3's as 1 they are more prone to electrolisis in my exerience, and 2) The simple fact that if you hit bottom with a Bravo 3 you are looking at 2 props per side as opposed to 1. We do alot of distance cruising in unfamiliar waters so this was the primary concern. You will most likely have a higher top end speed with the Bravo 3's however I feel that you will suffer mid range and cruising performance. Just my two cents if you will...Someone may have some additional insight.

 

728,
Thanks for the reply! I am very new to boating, so I have been reading and trying to get a better understanding of all this.

We live in the Pacific NW, our intended use of the boat is going to be some cruising on Lake Washington, eventually we will be looking at doing some over nights to the San Juan Islands, which is about a 4 hour trip from where we are located.

My primary interest is going to be fuel economy at a reasonable cruising speed, I am not really concerned about top end speed, just a comfortable cruising speed at an economical burn rate with the twin engine set up, so if I can cruise at lower RPMs and maintain a plane.

I have just seen good things about the Bravo IIIs in relation to power, performance, maneuverability and economics but have been unable to locate a comparison.

So, your input is VERY helpful as it sounds like our use will be similar to yours, since any water we end up in will be unfamiliar .

 

Welcome to boating and smooth sailing! We seriously considered the 3055 as a replacement vessel to our Maxum. That is a gigantic boat for the money. I think the 3055 was available with Bravo 3's coupled to 5.0 EFI's you may be able to obtain some test data from Bayliner for comparison sake. You will likely see best cruising rpms somewhere around 3,000 RPM with the 5.0's. We cruise the 4.3s a little higher as the boat needs the extra RPM as the power is not there. Are your motors carb or efi?

 

Yes. No.


I disagree with 7228 - there are a myriad of benefits with going for counter-rotating props, but it would simply not be worth the outlay if the B2's are in good condition

 

Please do tell of these benefits... With twin B2's you have the benefit of counter rotation from drive to drive, you have less metal hanging out on the rear there is weight savings. Besides backing down into a slip, and a slightly higher top end where is the benefit?